PolymerizationofOlefins andFunctionalizedMonomers...
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Polymerization of Olefins and Functionalized Monomers with Zirconocene Catalysts Von der Fakult¨ at f¨ ur Mathematik, Informatik und Naturwissenschaften der Rheinisch-Westf¨alischen Technischen Hochschule Aachen zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften genehmigte Dissertation vorgelegt von Diplom-Chemiker Holger Frauenrath aus Aachen Berichter: Universit¨ atsprofessor Dr. rer. nat. Hartwig H¨ ocker Universit¨ atsprofessor Dr. rer. nat. Wilhelm Keim Tag der m¨ undlichen Pr¨ ufung: 25.1.2001 Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verf¨ ugbar.
Transcript of PolymerizationofOlefins andFunctionalizedMonomers...
Polymerization of Olefins
and Functionalized Monomers
with Zirconocene Catalysts
Von der Fakultat fur Mathematik, Informatik und Naturwissenschaften der
Rheinisch-Westfalischen Technischen Hochschule Aachen zur Erlangung des
akademischen Grades eines Doktors der Naturwissenschaften genehmigte Dissertation
vorgelegt von
Diplom-Chemiker
Holger Frauenrath
aus Aachen
Berichter: Universitatsprofessor Dr. rer. nat. Hartwig Hocker
Universitatsprofessor Dr. rer. nat. Wilhelm Keim
Tag der mundlichen Prufung: 25.1.2001
Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfugbar.
Die Deutsche Bibliothek - CIP-Einheitsaufnahme Frauenrath, Holger: Polymerization of olefins and functionalized monomers with zirconocene catalyst / Holger Frauenrath. - Als Ms. gedr. - Berlin : dissertation.de, 2001 Zugl.: Aachen, Techn. Hochsch., Diss., 2001 ISBN 3-89825-338-4 Copyright dissertation.de – Verlag im Internet GmbH 2001 Alle Rechte, auch das des auszugsweisen Nachdruckes, der auszugsweisen oder vollständigen Wiedergabe, der Speiche- rung in Datenverarbeitungsanlagen, auf Datenträgern oder im Internet und der Übersetzung, vorbehalten. Als Manuskript gedruckt. Es wird ausschließlich chlorfrei gebleichtes Papier verwendet. Printed in Germany dissertation.de - Verlag im Internet GmbH Pestalozzistraße 9 10 625 Berlin URL: http://www.dissertation.de
I
Die vorliegende Arbeit wurde unter der Anleitung von Herrn Prof.Dr. Hartwig Hocker
am Institut fur Technische Chemie und Makromolekulare Chemie der RWTH Aachen in
der Zeit von Dezember 1997 bis Januar 2001 angefertigt.
Teile der Arbeit sind veroffentlicht bzw. zur Veroffentlichung eingereicht worden:
[1] “Polymerization of 1-Hexene Catalyzed by Bis(cyclopentadienyl)-
zirconiumdichloride/Methylaluminoxane; Effect of Temperature on the Molecular
Weight and the Microstructure of Poly(1-hexene)”; H. Frauenrath, H. Keul, H.
Hocker, Macromol. Rapid Commun. 1998, 19, 391–395.
[2] “Coexistence of Two Active Species in the Polymerization of 1-Hexene Catalyzed
with Zirconocene/MAO Catalysts”; H. Frauenrath, H. Keul, H. Hocker in W.
Kaminsky (ed.), “Metalorganic Catalysts for Synthesis and Polymerization: Recent
Results by Ziegler-Natta and Metallocene Investigations”, Springer-Verlag, Berlin,
1999, 283–293.
[3] “Stereospecific Polymerization of Methyl Methacrylate with Single Component
Zirconocene Complexes - Control of Stereospecificity via Catalyst Symmetry”; H.
Frauenrath, H. Keul, H. Hocker, Macromolecules 2001, 34, 14–19.
[4] “Single Component Zirconocene Catalysts for the Stereospecific Polymerization of
MMA”; H. Frauenrath, H. Keul, H. Hocker in R. Blom, A. Follestad, E. Rytter, M.
Tilset, M. Ystenes (eds.), “Organometallic Catalysts and Olefin Polymerization”,
Springer-Verlag, Berlin, 2001, 97–108.
[5] “Deviation from Single-Site Behaviour in Zirconocene/MAO Catalyst Systems. 1.
Influence of Monomer, Catalyst, and Cocatalyst Concentration”; H. Frauenrath, H.
Keul, H. Hocker, Macromol. Chem. Phys. 2001, in print.
[6] “Deviation from Single-Site Behaviour in Zirconocene/MAO Catalyst Systems. 2.
Influence of Polymerization Temperature”; H. Frauenrath, H. Keul, H. Hocker,
Macromol. Chem. Phys. 2001, in print.
[7] “First Synthesis of an A-B Blockcopolymer with Poly(ethylene) and Poly(methyl
methacrylate) Blocks Using a Zirconocene Catalyst”; H. Frauenrath, S. Balk, H.
Keul, H. Hocker, Macromol. Rapid Commun. 2001, in print.
III
Danksagung
Ich mochte mich bei Herrn Prof.Dr. Hartwig Hocker fur die Themenstellung und
die Betreuung meiner Dissertation sowie bei Herrn Prof.Dr. Wilhelm Keim fur die
Ubernahme des Korreferats bedanken.
Mein Dank gilt allen Mitarbeitern am Institut fur Technische Chemie und Makro-
molekulare Chemie, allen voran Dr. Helmut Keul fur die wissenschaftliche Betreuung
meiner Dissertation. Seine weisen Ratschlage und unsere zahlreichen, oft heftig gefuhrten
Diskussionen haben mein wissenschaftliches Denken und Arbeiten maßgeblich gepragt.
Bei Rainer Haas weiß ich gar nicht aufzuzahlen, wofur ich mich bedanken soll. Ohne
ihn ware “der Laden langst zusammengebrochen”. Bei meinem Laborkollegen Thomas
Hovetborn mochte ich mich fur drei angenehme Jahre bedanken, insbesondere auch fur
die regelmaßige Synthese von Kaffee. Peter Pilgram verdient allen Dank als ruhender Pol
bei der gelegentlich nervenaufreibenden Betreuung der GPC-Anlage und Thomas Fey fur
den standigen NMR-Service. Allen Autobesitzern des Arbeitskreises gilt mein Dank fur
die (viel zu regelmaßige) Uberlassung ihres Autos, wenn ich einmal wieder irgendetwas
vergessen hatte.
Der erfolgreichen Diplomarbeit von Dipl.-Ing. Udo Klapperich verdanke ich die Exi-
stenz eines Glasautoklaven fur Polymerisationsreaktionen mit allem Drum und Dran.
Mit meinen zwei linken Handen ware es mir wahrscheinlich nie gelungen, ihn aufzubauen.
Christoph Schoof und Luuk Duisings mochte ich fur den Einsatz bei ihren Forschungsar-
beiten danken.
Ein besonders herzliches Dankeschon geht an Herrn Dr. Wolfgang Schupp von der
hs-GmbH fur die Entwicklung eines Peakfitting-Moduls fur die WinGPC Software. Ohne
seine tatkraftige Unterstutzung vor allem beim Endspurt in der Doktorarbeit und seinen
Rund-um-die-Uhr Service ware ich wahrscheinlich noch kurz vor der Ziellinie verzweifelt.
V
Table of Contents
List of Abbreviations IX
Summary XI
Zusammenfassung XIII
Lebenslauf (Curriculum Vitae) XV
1 Introduction 1
1.1 The Development of Polymer Industry . . . . . . . . . . . . . . . . . . . . 1
1.2 Metallocene Polymerization of Olefins . . . . . . . . . . . . . . . . . . . . . 5
1.2.1 Chain Propagation Reaction . . . . . . . . . . . . . . . . . . . . . . 6
1.2.2 Chain Transfer Reactions . . . . . . . . . . . . . . . . . . . . . . . 9
1.2.3 Stereoselectivity of Olefin Polymerization . . . . . . . . . . . . . . . 10
1.2.4 Tailoring of Polyolefins . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2.5 Generation of the Active Species . . . . . . . . . . . . . . . . . . . 15
1.2.5.1 Methylaluminoxanes (MAO) as Cocatalysts . . . . . . . . 15
1.2.5.2 Synthesis of Cationic Alkyl Zirconocene Complexes . . . . 19
1.2.5.3 “Good” and “Bad” Cocatalysts . . . . . . . . . . . . . . . 22
1.2.5.4 The Role of Ion-Pairs . . . . . . . . . . . . . . . . . . . . 23
1.2.6 Polymerization Kinetics . . . . . . . . . . . . . . . . . . . . . . . . 24
1.2.6.1 Rate Law of Propagation . . . . . . . . . . . . . . . . . . 24
1.2.6.2 Catalyst Deactivation . . . . . . . . . . . . . . . . . . . . 25
VI
1.2.6.3 Deviation from Single-Site Behaviour . . . . . . . . . . . . 25
1.2.7 Copolymerization with Functionalized Monomers . . . . . . . . . . 27
1.2.7.1 Sterically Hindered or Protected Comonomers . . . . . . . 28
1.2.7.2 Noninteracting or Weakly Interacting Functional Groups . 29
1.2.7.3 Late Transition Metals . . . . . . . . . . . . . . . . . . . . 30
1.3 Metallocene Polymerization of MMA . . . . . . . . . . . . . . . . . . . . . 30
1.3.1 Group-Transfer Polymerization of MMA . . . . . . . . . . . . . . . 30
1.3.2 Other Zirconocene Based Catalyst Systems . . . . . . . . . . . . . . 32
1.3.3 Samarocene Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . 34
1.3.4 Stereospecific MMA Polymerization . . . . . . . . . . . . . . . . . . 36
1.4 How to Make Ends Meet? . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2 Target and Specific Aims 41
3 Results and Discussion 43
3.1 Synthesis of Zirconocene Complexes . . . . . . . . . . . . . . . . . . . . . . 43
3.1.1 Preparation of Ligands and Complex Precursors . . . . . . . . . . . 43
3.1.2 Preparation of Zirconocene Dichlorides . . . . . . . . . . . . . . . . 45
3.1.3 Preparation of Dimethyl Zirconocenes . . . . . . . . . . . . . . . . . 46
3.1.4 Preparation of Cationic Methyl Zirconocenes . . . . . . . . . . . . . 47
3.2 Olefin Polymerization with Zirconocenes . . . . . . . . . . . . . . . . . . . 48
3.2.1 General Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.2.2 Effect of Monomer Concentration . . . . . . . . . . . . . . . . . . . 49
3.2.2.1 Reaction Order in Monomer Concentration . . . . . . . . 49
3.2.2.2 Monomer Concentration, Molecular Weight and MWD . . 50
3.2.2.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.2.3 Effect of Catalyst and Cocatalyst Concentrations . . . . . . . . . . 53
3.2.3.1 Reaction Order in Zirconocene Concentration . . . . . . . 53
3.2.3.2 Zirconocene Concentration, Molecular Weight and MWD . 56
VII
3.2.3.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.2.4 Polymerization Kinetics . . . . . . . . . . . . . . . . . . . . . . . . 65
3.2.4.1 Zirconocene Concentration and Catalyst Activity . . . . . 65
3.2.4.2 Kinetics at Different Polymerization Temperatures . . . . 67
3.2.4.3 Polymerization Kinetics, Molecular Weight and MWD . . 71
3.2.4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 76
3.2.5 Effect of Polymerization Temperature . . . . . . . . . . . . . . . . . 76
3.2.5.1 Molecular Weight and MWD . . . . . . . . . . . . . . . . 77
3.2.5.2 Mathematical Modelling of Temperature Effects . . . . . . 86
3.2.5.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.2.6 Effect of Polar and Nonpolar Cosolvents . . . . . . . . . . . . . . . 93
3.2.6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 98
3.2.7 The Nature of the Active Species . . . . . . . . . . . . . . . . . . . 98
3.2.8 What Has Been Learnt Concerning Copolymerization? . . . . . . . 102
3.3 MMA Polymerization with Zirconocenes . . . . . . . . . . . . . . . . . . . 103
3.3.1 Kinetics of MMA Polymerization . . . . . . . . . . . . . . . . . . . 104
3.3.2 Mathematical Modelling of Polymerization Kinetics . . . . . . . . . 107
3.3.3 Control of Molecular Weight and MWD . . . . . . . . . . . . . . . 110
3.3.4 Control of Stereospecificity . . . . . . . . . . . . . . . . . . . . . . . 113
3.3.5 Proposal for a Polymerization Mechanism . . . . . . . . . . . . . . 119
3.3.5.1 Mechanism of Chain Propagation . . . . . . . . . . . . . . 119
3.3.5.2 Mechanism of Stereocontrol . . . . . . . . . . . . . . . . . 121
3.3.5.3 Generation of the Active Species . . . . . . . . . . . . . . 123
3.3.5.4 What Makes the Difference? . . . . . . . . . . . . . . . . . 123
3.3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
4 Experimental Part 127
4.1 General Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
VIII
4.2 Preparation of Ligands and Reactants . . . . . . . . . . . . . . . . . . . . . 128
4.3 Preparation of Zirconocene Complexes . . . . . . . . . . . . . . . . . . . . 130
4.3.1 Preparation of Zirconocene Dichlorides . . . . . . . . . . . . . . . . 130
4.3.2 Preparation of Dimethyl Zirconocenes . . . . . . . . . . . . . . . . . 131
4.3.3 Preparation of Cationic Methyl Zirconocenes . . . . . . . . . . . . . 134
4.4 Polymerization Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
4.4.1 1-Hexene Polymerization . . . . . . . . . . . . . . . . . . . . . . . . 136
4.4.2 MMA Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . 149
Appendix 153
A Mathematical Equations 153
A.1 Equation 3.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
A.2 Equation 3.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
A.3 Equation 3.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
A.4 Equations for Activation Parameters . . . . . . . . . . . . . . . . . . . . . 154
A.5 Equation 3.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
A.6 Equation 3.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
A.7 Equation 3.7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
A.8 Equation 3.8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
A.9 Equation 3.9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
A.10 Equation 3.15 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
A.11 Equation 3.16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
A.12 Equations 3.18 and 3.19 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
B NMR Spectra 163
References 173
IX
List of Abbreviations
Ac acetyl
b broad (in NMR)
9-BBN 9-bora[3.3.1]bicyclononane
CIP contact ion pair
COC cycloolefin copolymers
CL caprolactone
Cp cyclopentadienyl
CpH cyclopentadiene
DE diethyl ether
DMSO dimethyl sulfoxide
d duplet (in NMR)
DFT density functional theory
DVB divinyl benzene
EA ethyl acrylate
ESI ethylene-styrene interpolymers
Et ethyl
Flu fluorenyl
FluH fluorene
FT-NMR fourier transform nuclear magnetic resonance
GPC gel permeation chromatography (size exclusion chromatography)
GTP group transfer polymerization
HPLC high performance liquid chromatography
HV high vacuum
Ind indenyl
IndH indene
i-PP isotactic polypropylene
m multiplet (in NMR)
X Abbreviations
MA methyl acrylate
MAO methyl aluminoxane
Me methyl
MMA methyl methacrylate
Mn number average molecular weight
MO molecular orbital(s)
Mon monomer
m-PE polyethylene produced with a metallocene catalyst
m-PP polypropylene produced with a metallocene catalyst
Mw weight average molecular weight
MWD molecular weight distribution(s)
NMR nuclear magnetic resonance
OSIP olefin separated ion pair
PBB tris(2,2’,2”-perfluorobiphenyl)borane
PE-ULD ultra low density polyethylene
PE-VLD very low density polyethylene
Ph phenyl
PMMA poly(methyl methacrylate)
Pn number average degree of polymerization
PNB tris(2-perfluoronaphthyl)borane
PP polypropylene
PS polystyrene
q quadruplet (in NMR)
s singulet (in NMR)
s-PP syndiotactic polypropylene
s-PS syndiotactic polystyrene
t triplet (in NMR)
THF tetrahydrofuran
thf tetrahydrofuranyl
Thind tetrahydroindenyl
TIBA triisobutyl aluminum
TMA trimethyl aluminum
UV ultra violet
VE valence electron(s)
wt weight
XI
Summary
The subject of this dissertation is the application of zirconocene catalysis to the poly-
merization of nonfunctional 1-olefins and functional olefins, such as methacrylates.
The polymerization of 1-hexene with zirconocene/MAO catalyst systems is studied in
order to elucidate the nature of the active species on the basis of kinetic experiments and,
more particularly, by using molecular weight determination as a sensitive probe.
It is concluded that the general classification of such catalyst systems as single-site
catalysts is an oversimplification. In zirconocene/MAO systems, more than one active
species is present under the applied reaction conditions, which may cause bimodal MWD
of the obtained polymers. It is concluded that if the active species are in an equilibrium
at any point of time, then it must be slow on the time scale of chain growth. From the
experimental data some thermodynamic and kinetic parameters of the active species are
determined. The observed deviation from single-site behaviour may be part of a major
problem in olefin polymerization, i. e. the high excess of cocatalyst necessary in order to
generate a catalytically active system.
The addition of polar cosolvents to the reaction mixtures is shown to have a deleterious
effect with respect to polymerization. The experimental data is interpreted in terms of
the polar cosolvent and the olefin monomer competing for the vacant coordination site
of the polymerization catalyst. The results indicate that copolymerization of olefins and
functionalized monomers by means of zirconocene/MAO catalyzed olefin polymerization
is unlikely to be successful.
The application of chiral zirconocene catalysts to the polymerization of MMA is shown
to be a promising tool for the rational control of polymer microstructure. Thus, with
polymerization of MMA with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 yielding highly isotactic
PMMA and polymerization of MMA with Me2C(Cp)2Zr(Me)(thf)+BPh−4 yielding syndio-
tactic PMMA, the first example of a tight correlation between catalyst symmetry and
polymer microstructure in MMA polymerization is presented. On the basis of kinetic
XII Summary
experiments, a polymerization mechanism is proposed which is distinct from the known
zirconocene catalyzed group transfer polymerization of MMA and may offer a perspective
on the sequential copolymerization of 1-olefins and methacrylates by means of zirconocene
catalysis.
Finally, all results presented in this thesis lead to the conclusion that a (block) copoly-
merization of nonfunctional 1-olefins and of functional monomers may be possible, but
only if the order of reactivities towards the electrophilic catalyst is respected. This means
that the olefin must first be polymerized with a suitable catalyst system, following which,
an active site transformation may allow for the polymerization of the functional monomer.
XIII
Zusammenfassung
Das Thema der vorliegenden Dissertation ist die Verwendung von Zirkonocenen als
Katalysatoren bei der Polymerisation von nicht-funktionalisierten 1-Olefinen und funk-
tionalisierten Olefinen wie Methacrylaten.
1-Hexen wird mit Katalysatorsystemen auf der Basis von Zirkonocenen und MAO
polymerisiert, um die Natur der aktiven Spezies auf der Grundlage von kinetischen Ex-
perimenten naher zu untersuchen. Die Molekulargewichtsbestimmung wird dabei als
empfindliche “Sonde” genutzt.
Aus den experimentellen Befunden laßt sich schließen, daß die verallgemeinerte Klas-
sifizierung der verwendeten Katalysatorsysteme als “single-site” Katalysatoren eine grobe
Vereinfachung ist. Vielmehr sind unter den gegebenen Reaktionsbedingungen mehrere
aktive Spezies vorhanden, die eine bimodale Molekulargewichtsverteilung verursachen
konnen. Daraus laßt sich folgern, daß diese Spezies (wenn uberhaupt) in einem Gleich-
gewicht in Beziehung zueinander stehen, dessen Einstellung auf der Zeitskala des Ket-
tenwachstums langsam ist. Auf der Grundlage der experimentellen Ergebnisse werden
einige thermodynamische und kinetische Parameter der aktiven Spezies bestimmt. Die
beobachtete Abweichung von einem Verhalten als “single-site” Katalysator ist als eine der
Ursachen fur den großen Uberschuß an Cokatalysator anzusehen, der notwendig ist, um
ein katalytisch aktives System zu erzeugen.
Die Zugabe polarer Cosolventien zeigt einen auf die Polymerisation schadlichen Ein-
fluß. Die experimentellen Befunde konnen im Sinne einer Konkurrenz des polaren Cosol-
vens mit dem Olefinmonomer um die freie Koordinationsstelle am Katalysator gedeutet
werden. Eine Copolymerisation von Olefinen und funktionalisierten Monomeren auf
Grundlage von Zirkonocen/MAO Katalysatoren ist wahrscheinlich nicht moglich.
Die Verwendung chiraler Zirkonocen-Katalysatoren bei der MMA-Polymerisation stellt
sich als wirkungsvolle Methode zur gezielten Kontrolle der Polymermikrostruktur heraus.
So erhalt man bei Verwendung von Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 hochisotakti-
XIV Zusammenfassung
sches PMMA, wahrend die Polymerisation mit Me2C(Cp)2Zr(Me)(thf)+BPh−4 syndio-
taktisches PMMA ergibt. Dies ist das erste Beispiel fur einen engen Zusammenhang
zwischen Katalysatorsymmetrie und Polymertaktizitat bei der Polymerisation von MMA
mit Zirkonocenen. Auf der Grundlage von kinetischen Experimenten wird ein Polymeri-
sationsmechanismus vorgeschlagen, der sich als grundsatzlich verschieden von dem bereits
bekannten Mechanismus der Gruppentransfer-Polymerisation erweist und auch eine Per-
spektive fur eine sequentielle Polymerisation von 1-Olefinen und Methacrylaten bietet.
Schließlich kann man aus allen in der vorliegenden Dissertation prasentierten Ergebnis-
sen schließen, daß eine (Block-) Copolymerisation von 1-Olefinen und funktionalisierten
Monomeren mit Zirkonocen-Katalysatoren moglich ist, aber nur, wenn die Reihenfol-
ge der Reaktivitaten bezuglich des elektrophilen Katalysators berucksichtigt wird. Das
bedeutet, daß zuerst das Olefin mit einem geeigneten Katalysatorsystem polymerisiert
werden und dann eine Umwandlung der aktiven Spezies stattfinden muß, bevor das funk-
tionelle Monomer polymerisiert werden kann.
XV
Lebenslauf (Curriculum Vitae)
Name Holger Frauenrath
Eltern Hanneli Frauenrath, geb. Gogarn, Realschullehrerin
Dr. Herbert Frauenrath, Universitatsprofessor
Geburtsdatum 27.10.1972 in Aachen
Familienstand ledig
1979-1983 Besuch der Gemeinschaftsgrundschule Bruhlstraße, Aachen
1983-1992 Besuch des Stadtischen Einhard-Gymnasiums, Aachen
20.6.1992 Tag der letzten Abiturprufung (Allgemeine Hochschulreife)
1.10.1992 Einschreibung im Studiengang Chemie an der RWTH Aachen
10.5.1995 Tag der letzten Diplomvorprufung im Studiengang Chemie
28.4.1997 Tag der letzten Diplomprufung im Studiengang Chemie
2.6.1997 – 21.11.1997 Diplomarbeit im Arbeitskreis von Herrn Prof.Dr. Hartwig Hocker
am Lehrstuhl fur Textilchemie und Makromolekulare Chemie
der RWTH Aachen zum Thema “Polymerisation von 1-Hexen
und funktionalisierten Olefinen mit Metallocen-Katalysatoren und
MAO als Cokatalysator”
21.11.1997 Diplom im Studiengang Chemie
1.12.1997 – 25.1.2001 Doktorarbeit im Arbeitskreis von Herrn Prof.Dr. Hartwig Hocker
am Lehrstuhl fur Textilchemie und Makromolekulare Chemie
der RWTH Aachen zum Thema: “Polymerisation von funk-
tionalisierten und unfunktionalisierten Olefinen mit Metallocen-
Katalysatoren”
25.1.2001 Tag der mundlichen Promotionsprufung
Aachen, den 10.9.2001
1
1 Introduction
1.1 The Development of Polymer Industry
The development of polymer industry in the past four decades is an illustrative ex-
ample of the growth of industrial production and, at the same time, of the limited scope
of perspectives. Since 1950, polymer materials have experienced a rapid development
from cheap, low-quality surrogates to materials that allow for new applications or replace
established materials such as metal, wood or glass because of their better properties. The
growth of polymer industry has greatly outperformed the growth of other industries, in-
creasing from a production of 1.7 Mt a−1 in 1950 to 31.5 Mt a−1 in 1970, 158 Mt a−1 in
1998, 168 Mt a−1 in 1999 and an estimated 180 Mt a−1 in 2000.1 Remarkably, the growth
in polymer production is only slightly behind the predictions of the 1970s (Figure 1.1).2
An
nu
al p
oly
ole
fin p
rod
uct
ion
in M
t a
-1
1950 1960 1970 1980 1990 20000
50
100
150
200
Year
Figure 1.1: Annual global polymer production and prediction of 1970 (dashed red line).
2 1 Introduction
1975 predictions for 2000 1995 production figures
high performance plastics
engineering plastics
standard plastics
< 1%
12 %
88 %
Figure 1.2: Distribution of high performance, engineering and standard plastics.3,4
0,01 0,1 1 10 1000,6
0,81
2
4
6
810
20
40
60
Mar
ketp
rice
inD
Mkg
-1
World consumption in Mt a-1
LCP
PS
U, P
ES
,PA
R, P
EI
Fluoro-polymers
PP
S
PET (engin.)
PC
/PB
T
PB
T
PC
PC
/AB
S
PO
M
PA
PMMA
AB
S
SA
N
PP
(en
gin.
) PE
TP
S-H
I
PS PP PE
PV
C
high performance plastics
engineering plastics
standard plastics
Figure 1.3: Annual world consumption and market price of selected plastics in 1999.5
1.1 The Development of Polymer Industry 3
Fuelled by innovations in the field of polymer materials, predictions from the 1970s also
envisioned that engineering plastics and even high performance plastics would progressive-
ly replace cheap standard plastics (Figure 1.2).3,4 Consequently, a large amount of invest-
ment was dedicated to the development of new, specialized, “smart” polymers. However,
reality has seen the course of development taking the opposite direction. Despite some
growth in the absolute production of high performance and engineering plastics, these
have been continuously replaced by standard plastics, which accounted for approximately
80− 90% of polymer production in 1999 (Figures 1.2 and 1.3).3–5
The divergence between the predictions and the real course of development is not due
to a general misestimation of the growth in polymer production. However, it is remarkable
that in 1970 – on the basis of an expanding world economy – the only limiting factor in the
growth of polymer production was seen as the technology itself, the capability of the in-
dustry to provide appropriate production capacities.2 In hindsight, with the “world-scale”
plants now producing up to 500 Mt a−1 each, this turns out to be a gross underestimation.
What has significantly changed since then is the general economic environment, accom-
panied by a paradigm change. A rapid expansion of production capacities far exceeding
the growth of the polymer markets led to a continuous decrease in average production
margins, forcing producers to further lower production costs with new production plants,
thus fuelling the process.6 The need for low-cost materials, the standardization of poly-
mer applications and the increasing performance of standard plastics led to a refocussing
of research on the latter.6 Technological improvements have been achieved, however, by
further developing already established processes and applying cheap monomers in new
processes. In summary, the real course of development has not led to a replacement of
cheap, low-value materials by more expensive, more valuable materials, but rather to a
specialization by developing and improving the cheapest possible polymer materials, the
polyolefins.
Polyolefins have increased their share of total polymer production from approximately
30% in 19707,8 to 45% in 1998,9 with their absolute production increasing from 9.2 Mt a−1
in 1970 to 71.1 Mt a−1 in 1998, 75 Mt a−1 in 1999 and an estimated 79 Mt a−1 in 2000.
Major leaps in the development of process engineering, Ziegler-Natta polymerization and
metallocene polymerization have made a portfolio of polyolefin products with a broad
range of properties accessible (Figure 1.4).10,11
Polyolefins substitute other polymers in their fields of application. The tailoring
of polymer properties already in the polymerization process allows for the synthesis of
4 1 Introduction
105
104
103
102
0.9 1.0 1.30.8
PE-HD
PE-LLD
PE-VLD
PE-ULD
EPT
TPX
PPs-PP
r-PP
COCSPS
engineeringplastics
PB-1
ionomer
EVA
PE-LD
PVCsoft
Density in g cm-3
Mod
ulus
in k
g cm
-2106
105
104
103
20 40 60 80 1000
poly-1-olefins(viscosity agents)
i-PPs-PPs-PS
PP-waxes
telechelslubricants
COC
EPM
PE
-VLDP
E-L
LD
PE
-HD
PE
-UH
MW
PE-waxes
Comonomer content in %
Mol
ecul
ar w
eigh
t
Figure 1.4: Portfolio of polyolefin products accessible today and properties of selected
polyolefins; metallocene products labelled in colour.
“high-performance polyolefins” with virtually new properties or unusual combinations
of properties. Examples are the reactive blending of polyolefins (Catalloy, Hivalloy
by Himont/Montell),12 “reverse monomer incorporation”,13 and morphology control by
means of catalyst particle design.12,14
1.2 Metallocene Polymerization of Olefins 5
The polyolefin market itself is subject to similar trends. Since the early 1980s a
rapid replacement of Ziegler-Natta polymerization by the growing field of metallocene
polymerization has been predicted. However, ever more polyolefins are produced by means
of Ziegler-Natta processes today. The advantages of 30 years of development render it
impossible to replace. Nevertheless, metallocene polymerization of olefins paves the way
to new materials, such as “very low” and “ultra low density polyethylene” (PE-VLD,
PE-ULD), syndiotactic polypropene (s-PP), cycloolefin copolymers (COC), syndiotactic
polystyrene (s-PS), or ethylene-styrene interpolymers (ESI).13 It is in these fields that the
metallocene polyolefin market will grow in future and successfully replace other polymers,
including other polyolefins. Only a few years after commercialization, metallocene PE
and PP materials (m-PE and m-PP) have become established products summing up to
a production of 0.615 Mt a−1 in 1997 and an estimated 2.37 Mt a−1 in 2001.13 It seems
that the “cannibalization” of other materials, including polymers, by polyolefins has only
just begun. However, the production of ethylene-CO copolymers provides a similarly
illustrative example. It took slightly more than a decade from the discovery of highly
active catalysts in 1984 to industrial production in 1996.15,16 However, production on
a scale necessary in order to meet the product margins of other polyolefins raises the
question as to whether production will be continued beyond 2001.
A thorough understanding of these developments is crucial for further innovation. On
the one hand, the described processes have technological limits. A completely new genera-
tion of functional materials will require chemical functionality at a molecular scale going
far beyond the scope of hydrocarbon based materials such as polyolefins. The incorpora-
tion of functional groups into otherwise nonpolar materials offers control over important
material properties such as toughness, adhesion, barrier properties (membranes), sur-
face properties (paintability, printability), solubility, compatibility/miscibility with other
polymers, and rheological properties (processing). However, as long as profitability deter-
mines what is produced, new polymers will basically have to be achieved with established
processes and/or well-known monomers.
1.2 Metallocene Polymerization of Olefins
As a consequence of the success of group IV metallocene polymerization catalysts,
many other transition metal centers have been investigated with respect to polymeriza-
tion catalysis.17 However, so far only zirconocenes have found broad industrial applica-
6 1 Introduction
Si Zr
RH
RPol
H
Si Zr
Pol
H
HR
R
R
Si Zr RH
R
Pol
HR
Si Zr
Pol
HHR
R
R
Pol
RH
+
+ monomer+ monomer
migratory1,2-insertion
migratory1,2-insertion
β-H-transfer+ monomer
Si Zr
HR
H
activ
e sit
e ep
imer
izatio
n
"cha
in ba
ck sk
ip"
Scheme 1.1: Mechanism of chain propagation and chain transfer in zirconocene catalyzed
olefin polymerization.
tion. Therefore, the following sections will focus on zirconocene catalyzed polymerization.
Reviews by Muhlhaupt et al.,18 Brintzinger et al.,19 Bochmann et al.20 and recently by
Resconi et al.21 highlight the tremendous research efforts and the progress in this field.
The generally accepted mechanism of the insertion polymerization of olefins catalyzed
with group IV metallocenes is outlined in Scheme 1.1.
1.2.1 Chain Propagation Reaction
It is proposed that the active species in zirconocene catalyzed olefin polymerization
is a highly electrophilic, cationic alkyl zirconocene complex with 14 VE and a vacant
coordination site. It is electronically and coordinatively unsaturated and hence capable of
complexing an incoming olefin molecule as a ligand, being converted into a 16 VE complex.
1.2 Metallocene Polymerization of Olefins 7
The coordinated monomer is then inserted into the growing polymer chain in the sense of
a “migratory 1,2-insertion”. In a concerted reaction, the alkyl group (the growing polymer
chain) migrates to the carbon atom C-2 of the monomer, while the carbon atom C-1 of
the monomer is attached to the zirconium center. Experimental evidence from kinetic
isotope effects22,23 indicates that there is an α-H-agostic interaction that facilitates the
insertion and fixes the conformation of the polymer chain (Scheme 1.2). The reaction
product is a 14 VE complex like the initial species which is temporarily stabilized by a
γ-agostic interaction. Most likely, a rearrangement to a more stable β-agostic interaction
takes place subsequently, which may also be a sort of a resting state before the next
insertion step.20,21, 24 Compared to the initial situation, the alkyl group (the growing
polymer chain) and the vacant coordination site have switched their positions. Subsequent
reactions include the coordination and insertion of the next monomer, or an “active site
epimerization” (“chain back skip”) prior to the next polymerization step.
L2Zr
Pol
HL2Zr
Pol
H
HH
HH
R R
R
L2Zr
Pol
H
HH
R
R
L2Zr
Pol
H
HH
R
R
Scheme 1.2: Mechanism of chain propagation in olefin polymerization.
A different concept presented by Green and Rooney25 involves an oxidative hydrogen
shift from the α-carbon of the growing chain to the metal centre, the formation of a four-
membered metallacycle and the final reductive elimination opening the metallacycle. Of
course, this mechanism is ruled out for d0 systems such as zirconocenes, but it has been
modified by Brookhart and Green26 in assuming α-agostic interactions. All flavours of
mechanistic explanations agree that (i) polymerization is a two-step process, (ii) there
must be a vacant coordination site available to the incoming monomer, (iii) insertion
occurs by cis-opening of the double bond, (iv) the polymer chain migrates and changes
its position at the active centre and (v) α-H-agostic interactions are present that facilitate
the insertion and fixate the conformation of the polymer chain.19,21
Olefin coordination and insertion reactions at cationic zirconocene complexes are well
investigated from the view of theoretical chemistry. An alkyl group attached to a {Cp2Zr}fragment mainly interacts with the 1a1 orbital and is oriented off-axis, thus allowing
for two possible orientations and resulting in a pseudo-tetrahedral instead of a trigonal
environment of the metal centre (Figure 1.5).27,28 An incoming olefin molecule mainly
interacts with one lobe of the 1b2 orbital and with the 2a1 orbital. Good overlap is also
8 1 Introduction
Mt Mt
180° 160° 140° 120°
-6
-7
-8
-9
-10
-11
θ
Ene
rgy
in e
V
θ θ
a2
b1
2a1
b2
1a1
e1g
a1g
e2g
Figure 1.5: Frontier orbitals of a {Cp2Mt} fragment as a function of the Cp-Zr-Cp angle.
obtained for a 90◦ rotation of the olefin. This implies that the electronic barrier to olefin
rotation is small, which is calculated to be below 15 kJ mol−1.21,28 Olefin uptake energy
in the absence of solvent molecules ranges between −60 and −120 kJ mol−1, which is
reduced to approximately −20 to −40 kJ mol−1 when agostic interactions are taken into
account.21,28 It is worth noting that the situation changes fundamentally, when the more
stable ion pair with the counter ion (section 1.2.5.4) is considered in place of the “free”
cationic complex.29 Olefin uptake is essentially barrierless which implies that the olefin
molecule can easily dissociate despite the exothermic nature of coordination.
Insertion proceeds almost synchronously via a nearly planar four-center transition
state with bond lengths similar to that of the reaction product. An important feature of
d0 systems is the fact that they do not have high energy, occupied d orbitals available
1.2 Metallocene Polymerization of Olefins 9
for back-donation to the empty π∗ orbital of the olefin. This stabilization would be lost
towards the transition state of insertion imposing a substantial barrier on insertion.27,30
Suitable polymerization catalysts from dn systems may be envisioned,31 but they must
comprise ligands that accept d electron density in orbitals orthogonal to the π∗ orbital,
or low energy d orbitals like those of late transition metal complexes.
The insertion barriers in d0 systems are calculated to be approximately 20 kJ mol−1
for ethylene and 30−45 kJ mol−1 for propene.21,30 Regioselective 1,2-insertion of propene
is consistent with the Markovnikov rule and favoured by approximately 15 kJ mol−1. The
small insertion barriers in zirconocene complexes are explained by the ligand framework
forcing the attached alkyl chain into an almost planar arrangement, thus requiring only
minimal deformation to attain the transition state. The only obstacle appears to be the
rupture of β-agostic interactions.30 The extremely high catalytic activity of group IV
polymerization catalysts is well reflected by the fact that the whole ethylene insertion
sequence takes place in a 70 to 170 fs timescale.20
1.2.2 Chain Transfer Reactions
As the active chain end is a metal carbon bond, the presence of β-hydrogens in the
polymer chain may allow for chain termination via β-H elimination or β-H transfer (Figure
1.3). The β-hydrogen of the polymer chain is transferred to either the zirconium center
itself or to a coordinated monomer, thus yielding a zirconium hydride or a zirconium alkyl
complex, respectively. In both cases the zirconium complex remains capable of starting
a new polymer chain. The resulting process is a chain transfer reaction (although often
referred to as “chain termination”), yielding polyolefins with a limited molecular weight, a
molecular weight distribution (MWD) with a polydispersity Mw/Mn = 2 and unsaturated
(vinylidene) end groups.
L2Zr
L2ZrH
R
H
Pol
RL2Zr
H
Pol
RL2Zr
H
Pol
RL2Zr
H
L2ZrH
R
R RPol Pol
L2Zr
R
HH
L2ZrH
Pol
R R
Pol
R
Pol+
+
Scheme 1.3: Mechanisms of β-H elimination or β-H transfer in olefin polymerization.
10 1 Introduction
From the perspective of theoretical chemistry, chain transfer via β-hydride elimina-
tion appears to be energetically unfavourable, with about +80 to +140 kJ mol−1 for the
complete release of the “olefin-like” polymer, leaving a very unstable cationic zirconocene
hydride complex. As opposed to this, β-H transfer to a coordinated monomer is endo-
thermic with only about +9.5 kJ mol−1.21,32, 33
1.2.3 Stereoselectivity of Olefin Polymerization
In Ziegler-Natta catalysis the control of stereospecificity is limited. According to the
generally accepted Cossee-Arlman mechanism34 the octahedral Ti centres on the crystal
surface of the catalyst particle have only one coordination site available to the olefin. The
growing chain has to skip back to its original position after each and every insertion step.
Therefore and because the environment around the active center cannot be changed at
will, these catalysts will only produce isotactic or atactic polyolefins, depending on the
respective enantiofacial selectivity of the different active Ti centres on different crystal
surfaces.21
The major advantage of metallocene polymerization is the degree of control it allows
in the tailoring of polymer microstructure via ligand geometry, which the application
of ansa-zirconocenes with rigid chelating ligands, first described by Brintzinger et al.,
has paved the way for.35–37 Elements of chirality in the catalyst-olefin complex are the
different non-superimposable orientations of the olefin, the configuration of the tertiary
carbon of the last inserted monomer unit, and finally the chirality of the catalyst it-
self, either arising from a chiral ligand set or from the metal centre being situated in a
(pseudo)tetrahedral environment with four different residues.21 All of these elements will
determine the stereospecificity of a polymerization catalyst. In order to efficiently con-
trol polymer microstructure, one has to control the regioselectivity and the enantiofacial
selectivity (stereoselectivity) of the monomer coordination, as well as the stereospecificity
of the monomer insertion. As the polymer chain – unlike “small molecules” in catalytic
organic reactions – remains connected to the catalytic center, the stereoconfiguration of
the last inserted monomer unit may have an influence on the stereochemistry of the in-
sertion of the next one. If this influence is determining, the mode of stereoregulation is
referred to as “chain end control”. If the ligand set is chiral and overrides the influence
of the polymer chain end, the mechanism of stereoregulation is referred to as “enantio-
morphic site control”.21,38 Both modes of stereoregulation are distinguishable by the
analysis of stereoerror pentads (Figure 1.6).
1.2 Metallocene Polymerization of Olefins 11
MLnPol
MLnPol
MLnPol
MLnPol
m m m m r r m m m m
r r r r m m r r r r
m m m m r m m m m m
r r r r m r r r r r
chain growth
isospecific
syndiospecific
sitecontrol
isospecific
syndiospecific
chain-endcontrol
Figure 1.6: Different stereoerror pentads (red) arising from an erroneously inserted
monomer unit under enantiomorphic site control or chain end control of stereospecificity.
Mt Mt Mt Mt Mt
(A)
(B)
(C)
(D)
C2v (achiral)
homotopic (N,N)
atactic
atactic
Cs (achiral)
diastereotopic (N,N)
atactic
atactic
C2 (chiral)
homotopic (E,E)
isotactic
isotactic
Cs (prochiral)
enantiotopic (E,E)
syndiotactic
atactic/isotactic
C1 (chiral)
diastereotopic (E,N)
hemiisotactic
atactic/isotactic
Figure 1.7: Catalysts of different symmetry (A), relation of the two coordination sites
(B), with and without enantiofacial selectivity (“E”, “N”), microstructure of the polymer
obtained in the absence (C) and in the presence (D) of active site epimerization.
Homogeneous organometallic polymerization catalysts are classified in five symmetry
categories, producing different polymer microstructure according to what is referred to
as “Ewen’s symmetry rules” (Figure 1.7).21,39 When the metallocene is achiral (C2v- or
Cs-symmetric) atactic polyolefins are achieved, although chain end control may allow for
a certain degree of (iso- or syndio-) tacticity at low polymerization temperature. Re-
markably, Cp2ZrCl2 1 yields isotactic PP at low polymerization temperature, while the
12 1 Introduction
sterically more encumbered (C5Me5)2ZrCl2 2 yields atactic (slightly syndio-enriched) PP
and syndiotactic poly(1-butene).40,41 Energetic differentiation is below 5 kJ mol−1 in the
case of chain end control.
Zr ClCl Si Cl
ClZrZr ClCl
Zr ClCl
1 43
5
Zr ClCl
2
6
Zr ClCl
7
Zr ClCl
Figure 1.8: Examples for catalyst precursors of different symmetry.
ZrMe2Si
H ZrMe2Si
Me
R
HMe
ZrMe2SiMe
Me
predominantlysi-coordination
predominantlyre-coordination
no enantiofacialselectivity
Me
Figure 1.9: Dependence of the enantiofacial selectivity of propene coordination to (R,R)-
Me2Si(Ind)2Zr(alkyl)+ on the bulkiness of the attached alkyl group (R = Me, polymer
chain).
In C2-symmetric complexes such as C2H4(Ind)2ZrCl2 3 or Me2Si(Ind)2ZrCl2 4 the two
potential vacant coordination sites are homotopic. Consequently, every newly formed
asymmetric centre in the growing chain obtains the same configuration, irrespective of
the switch in the relative positions of the free coordination site and the polymer chain
in two subsequent polymerization steps. It is proposed that stereocontrol is achieved by
“relaying” catalyst chirality to the monomer by means of the steric requirements of the
growing polymer chain.42 This is supported by an experimentally determined change in
enantiofacial selectivity depending on the bulkiness of the attached alkyl chain (Figure
1.2 Metallocene Polymerization of Olefins 13
1.9)43,44 and by the influence of α-agostic interactions on stereospecificity, as demon-
strated by kinetic isotope effects.22,23 Theoretical calculations reveal that the energetic
differentiation of monomer enantiofaces is below 5 kJ mol−1 in the case of cationic methyl
zirconocene complexes because the propene methyl group is sufficiently far away from the
ligand skeleton. The situation changes for an isobutyl group or even larger alkyl residues
attached to the zirconocene, in which case there is a preferential chiral conformational ori-
entation of the alkyl residue of approximately 17 to 22 kJ mol−1, and hence a preferential
enantiofacial orientation of the monomer of the same magnitude.21
Stereoerrors may arise at an energetic penalty from a “wrong” enantiofacial orientation
of the incoming monomer or more probably from chain end epimerization.23,45 These
errors are statistically isolated, giving rise to rr error triads. The result is a highly isotactic
polymer with mrrm stereoerror pentads, and a ratio mmmr : mmrr : mrrm ≈ 2 : 2 : 1
(Figure 1.6). As the two potential free coordination sites are homotopic, an active site
epimerization does not cause a stereoerror. Energetic differentiation may range up to
20 kJ mol−1. Lower polymerization temperatures will yield higher isotacticity.
In Cs-symmetric catalysts such as Me2C(Cp)(Flu)ZrCl2 5, the two potential vacant
coordination sites are enantiotopic, each leading to an opposite configuration of the new
formed asymmetric center in the growing chain. Thus, if polymerization proceeds at alter-
nating coordination sites in subsequent polymerization steps and active site epimerization
is excluded, a syndiotactic polymer is obtained. The stereo errors observed are rmmr -
pentads from “wrong” enantiofacial insertion, and also rmrr -pentads due to active site
epimerization, producing an m dyad but not affecting the following polymerization step.
The degree of syndiotacticity is thus determined by the ratio of the rates of propaga-
tion and active site epimerization and is therefore a function of monomer concentration
(pressure) and polymerization temperature. At higher temperature and low monomer
concentration (pressure), active site epimerization is expected to be favoured, leading to
a decrease in syndiotacticity and tendentially atactic polymers. In fact, in the case of
Me2C(Cp)(Flu)ZrCl2 5 iso-enriched polymers are obtained at elevated temperatures.
Even more complex polymer microstructures are obtained with C1-symmetric
zirconocene catalysts that offer two diastereotopic vacant coordination sites to the in-
coming monomer. As stereospecificity depends on the frequency and the sequence of
insertion at each of the two potential coordination sites and on their respective enantio-
facial selectivity, the microstructures of resulting polymers do not appear to be generally
predictable. Thus, with Me2C(3-tBuCp)(Flu)ZrCl2 6 isotactic PP is obtained,46–49 be-
14 1 Introduction
cause the polymer chain occupies the free quadrant at the active site. In this case,
the monomer is inserted from the more crowded side with its residue pointing away
from the bulky tert-butylgroup of the ligand and a rapid active site epimerization takes
place after each insertion step. Alternatively, hemiisotactic polyolefins are obtained with
Me2C(3-MeCp)(Flu)ZrCl2 7.39,46–49 Apparently, the steric constraint imposed by the
methyl group is not large enough to be a driving force for active site epimerization after
every insertion step and the different stereoselectivity of the two diastereotopic orienta-
tions yields an alternating isotactic/atactic enchainment of the monomers.
1.2.4 Tailoring of Polyolefins
The degree of control that has been achieved in zirconocene catalyzed olefin poly-
merization is a striking example of structure-reactivity control. Virtually every aspect
of polymer microstructure – molecular weight and MWD, tacticity, comonomer content
and comonomer distribution – is adjustable by means of a suitably substituted ligand set.
Especially the 2- and the 4-positions of the indenyl ligands are sensitive to substituents.
An example of the progress made at Hoechst in the late 1980s is given in Figure 1.10 and
Table 1.1.50–52
Si ClClZrZr Cl
Cl
3
Si ClClZr Si Cl
ClZr
Si ClClZrSi Cl
ClZr
4 8 9
10 12
Si ClClZr
11
Figure 1.10: Design of catalyst structure.50–52
1.2 Metallocene Polymerization of Olefins 15
Table 1.1: Polymerization of propene with ansa-zirconocenes (liquid propene, 50 ◦C).
Catalyst PrecursorProductivity
kg(PP) mmol(Zr)−1 h−1
Mw
1000
mmmm
%
Tm◦C
C2H4(Ind)2ZrCl2 3 188 24 78.0 132
Me2Si(Ind)2ZrCl2 4 190 36 82.0 137
Me2Si(2-Me-Ind)2ZrCl2 8 99 195 88.0 145
Me2Si(2-Me-4, 5-benzind)2ZrCl2 9 403 330 89.0 146
Me2Si(2-Me-4-iPr-Ind)2ZrCl2 10 245 213 89.0 150
Me2Si(2-Me-4-Ph-Ind)2ZrCl2 11 755 729 95.2 157
Me2Si(2-Me-4-naphtyl-Ind)2ZrCl2 12 875 920 99.1 161
“4th generation” Ziegler Natta 20 900 > 99.0 162
Therefore, together with the fact that polymer microstructure is variable as outlined
in the previous section, zirconocene catalysts can compete and even outperform the latest
generation of Ziegler-Natta catalysts. However, the scope of homogeneous catalyst design
is limited by the fact that the influence of different substituents is not incremental, but
synergistic. Hence, new polymers such as “low melting point, high molecular weight PP”
will probably remain inaccessible by means of catalyst design.50–52
1.2.5 Generation of the Active Species
Usually, the catalytically active cationic alkyl zirconocene is generated in situ from the
corresponding zirconocene dichlorides with a cocatalyst. Early attempts involve activation
with simple aluminum alkyls such as AlClEt2 or AlEt3.53 Investigations have revealed
that in these systems a dynamic equilibrium exists between polymerization active solvent
separated ion pairs (SSIP) and “dormant” contact ion pairs (CIP).54–57 Overall, the
inability of these systems to efficiently polymerize propene and higher 1-olefins limits
their utility and has triggered research on other activation methods.
1.2.5.1 Methylaluminoxanes (MAO) as Cocatalysts
Among the attempts to improve the performance of the aluminum activators is the
remarkable finding of Kaminsky et al. that small amounts of water in systems with
aluminum alkyl cocatalysts yield highly active polymerization catalysts, which led to the
16 1 Introduction
discovery of the MAO cocatalysts58 and is the starting point for the industrial application
of metallocene catalysts. With MAO the highest activities are obtained. However, a
high excess of MAO has to be applied, typically with a ratio of Al/Zr� 1000. MAO
is a moderately strong methylating agent and a strong Lewis-acid. In contrast to early
studies on the mechanism of MAO activation, it is now clear that MAO monomethylates
zirconocene dichlorides such as Cp2ZrCl2 1 to yield the corresponding methyl zirconocene
chloride Cp2Zr(Me)(Cl) 13 and then abstracts the second chlorine atom, thus generating
the cationic methyl zirconocene complex Cp2Zr(Me)+ 14. Dimethyl zirconocenes such as
Cp2ZrMe2 15 are definitely not formed.21,24 The detailed mechanism of MAO activation
as supported by recent results (vide infra) is depicted in Scheme 1.4.
Zr MeO
AlO
Al ZrMe
Me Me
MeMe
Me2Al MeAlO
OMeAl
O AlMe O
AlMe
O AlMe
O MeAl Me
AlO
OMeAl
O AlMe O
AlMe2
O AlMe
OAlMe2
Me2Al
O OAl
Al
O
AlO
Al
O OAl
Al
Al Al
O
OAl
AlO
Al O O
Al
AlAl
AlO
OAlOAl O
Al
O AlO
AlAlO
OAl
O Al
O AlO
OAl
Al
OAl
Al O
16 17 18
19 20
Figure 1.11: Structures of MAO clusters and reaction products with zirconocenes com-
plexes; alkyl groups in 16, 17, and 18 omitted for clarity.
The exact structure and function of MAO is not fully understood and is still a matter
of investigation. MAO appears to consist of clusters, the sizes of which are dependent
on MAO concentration and reaction conditions. Recent results indicate the approximate
sum formula to be Me1.3−1.5Al1O0.75−0.85.59 From cryoscopic measurements the num-
ber average molecular weight of MAO appears to be 700 to 1200, which means that
individual MAO clusters consist of 10 to 20 {MeAlO} units.59 The molecular weight
of MAO decreases upon addition of AlMe3 proportional to the reciprocal amount of
AlMe3 added. NMR measurements indicate the existence of tetrahedrally coordinated
1.2 Metallocene Polymerization of Olefins 17
Al atoms, and O atoms in a three-coordinate environment.60 MAO clusters are believed
to form cage-like cluster structures formally derived from prismatic or cuboctahedral
structures such as 16, 17, or 18. One possible structure is the “fused Barron-cage”
[Me18Al14O12] 19. Most likely, there is no exact structure of a distinct minimum of steric
energy, but rather a continuum of highly fluxional structures. Barron et al. have iso-
lated closed cage structures such as [(tBu)Al(µ3-O)]6 16 and [(tBu)Al(µ3-O)]9 17, which
react with Cp2ZrMe2 15 to yield an ion pair that has been spectroscopically characterized
and which is active for ethylene polymerization. Apparently, three-coordinate aluminum
is not a prerequisite for methyl abstraction.61,62 Little data is found in the literature
which allows for conclusions about the interactions of cationic methyl zirconocenes and
their counter ions. Thus, µ3-O contacts are proposed, and Erker et al. isolated the
complex Cp2Zr(Me)(µ3-O)(AlMe2)2(µ3-O)(Me)ZrCp2 20 which is, however, inactive for
polymerization.63
Zr MeMe
ZrMe
Zr ClCl Zr Me
Cl Zr MeMe
Zr Me(µ-Me)-MAO
ZrMe
AlMe
Me
Me
ZrMe
Zr Me(µ-O)-MAO
or
Al/Zr 20
Al/Zr 150
Al/Zr 1000 Al/Zr > 1000
1 13
14
15
21 22
23
Al/Zr 50
Scheme 1.4: Generation of cationic methyl zirconocenes by MAO cocatalyst.
Deffieux et al. and Brintzinger et al. investigated the reaction of C2H4(Ind)2ZrCl2 3
and Me2C(Cp)(Flu)ZrCl2 5 with MAO by means of UV spectroscopy.24,64–67 From the
changes in the UV spectra, the authors conclude that different species are formed depend-
ing on the Al/Zr ratio. A hypsochromic shift at Al/Zr < 30 indicates the formation of the
18 1 Introduction
methyl zirconocene chloride, as assigned by comparison with AlMe3 as the methylating
agent. At 30 < Al/Zr < 150 a bathochromic shift is observed that is claimed to be due to
the formation of some sort of (hetero-)dinuclear complex. For 150 < Al/Zr < 2000 this
absorption band is further shifted to higher wavelengths, probably due to the gradual
transformation of this species into the “free” cationic methyl zirconocene. In the case of
Me2C(Cp)(Flu)ZrCl2 5, with AlMe3, the dimethylated complex is also obtained, but this
is not the case with MAO.66,67 With CH2Cl2 as the solvent, the same absorptions are
observed, however, they are at lower respective Al/Zr ratios (Al/Zr = 20, 50 and 200).65
The activity when using toluene or heptane as the polymerization solvent, at a given Al/Zr
ratio, is also enhanced by preactivation in CH2Cl2.68 From their NMR investigations of the
reaction of MAO with Cp2ZrMe2 15 Babushkin et al. conclude that at Al/Zr < 50 there is
a fast equilibrium between Cp2ZrMe2 15 and a weak complex [Cp2Zr(Me)(µ-Me)-MAO]+
21.69 The formation of a dinuclear species [Cp2Zr(Me)(µ-Me)(Me)ZrCp2]+ 22 is also
observed. At Al/Zr ≈ 1000 new signals appear which are assigned to the heterodinuclear
complex [Cp2Zr(Me)(µ-Me)AlMe2]+ 23 and the cationic complex Cp2Zr(Me)
+ 14, loosely
coordinated by the counter ion. In this case, NMR indicates a slow exchange. From a
comparison of NMR spectra of Cp2ZrMe2 activated with MAO, AlMe3 and B(C6F5)3,
Tritto et al. conclude that at Al/Zr < 20 mono- and dinuclear ion pairs are formed.70
At Al/Zr ≈ 20 the heterodinuclear complex [Cp2Zr(Me)(µ-Me)AlMe2]+ is also observed.
Interestingly, at higher Al/Zr ratios and higher temperatures the mononuclear complexes
are favoured, indicating the endothermic nature of their formation.
This detailed picture is just to illustrate that under typical polymerization conditions
there is a “zoo” of zirconocene species in the reaction mixture, the role of which with
respect to polymerization kinetics and deviation from single-site behaviour (section 1.2.6)
is subject to speculation.
Generally, polymerization activity increases with increasing MAO concentration. How-
ever, for many zirconocenes the relationship of polymerization activity and Al/Zr ratio
yields “bell-shaped” curves. Activity increases with increasing Al/Zr ratio up to a maxi-
mum activity, e. g. at Al/Zr = 11000 for Me2Si(Ind)2ZrCl2/MAO or at Al/Zr = 1300 for
Me2C(Cp)(Flu)ZrCl2/MAO in propene polymerization, and it decreases again at higher
Al/Zr ratios. The activity maximum is shifted towards a higher Al/Zr ratio with increas-
ing monomer concentration (pressure).71–73 In the case of Me2C(Cp)(Flu)ZrCl2/MAO a
decrease in syndiotacticity of the obtained PP due to the promotion of active site epi-
merization is also observed. Increasing solvent polarity leads to an increase in catalyst
1.2 Metallocene Polymerization of Olefins 19
activity, and a decrease in syndiotacticity of the obtained polyolefin in the case of
Me2C(Cp)(Flu)ZrCl2/MAO.66,74 This all indicates that for very high Al/Zr ratios, MAO
and olefin compete for the vacant coordination site (Scheme 1.4).
1.2.5.2 Synthesis of Cationic Alkyl Zirconocene Complexes
The success of metallocene/MAO catalyst systems has fuelled research in the field of
cationic alkyl metallocene complexes since the late 1980s. Starting with the first syn-
thesis of Cp2Zr(Me)+BPh−4 and its application as a polymerization catalyst by Jordan
et al.75,76 chemistry on this field has already filled a number of books.77,78 In order to
create catalytically active complexes, it is important to realize that the counter ions are
potentially Lewis-basic, thus competing with the monomer for coordination to the vacant
coordination site. Cationic complexes which are active for olefin polymerization have
been successfully synthesized by introduction of “very weakly coordinating” anions, such
as BPh−4 ,75,76 B(C6F5)
−4 or MeB(C6F5)
−3 .
79,80 Recently, sterically more encumbered, even
weaker coordinating anions generated from tris(2, 2′, 2′′-perfluorobiphenyl)borane (PBB)
or tris(2-perfluoronaphthyl)borane (PNB) have been described.81–83
Zr MeCH2Cl2
Zr MeMe
Zr MeMeB(C6F5)3
Zr MeO
Ph3CB(C6F5)4CH2Cl2
B(C6F5)3CH2Cl2
BPh4
B(C6F5)4
15
24
25
26
HNBu3BPh4toluene/THF, – CH4
or AgBPh4toluene/THF, – C2H6
– Ph3CMe
Scheme 1.5: Synthesis of cationic methyl zirconocenes.
Synthesis of cationic alkyl zirconocene complexes is accomplished by starting from
the respective dimethyl zirconocenes (Scheme 1.5) via (i) methyl abstraction by a strong
Lewis-acid such as B(C6F5)3,79 (ii) methyl abstraction using a tritylium salt such as
20 1 Introduction
Zr PhPh Zr Ph
B(C6F5)2F
F
FF
Zr FPh
Zr ClCl Zr Zr
FB(C6F5)2
F
FF
F
B(C6F5)3toluene
B(C6F5)3toluene
Mg(C4H6)-MgCl2
∆, -C6H6
27 29
28 30
Scheme 1.6: Synthesis of zwitterionic methyl zirconocenes.
Ph3C+B(C6F5)
−4 ,
84 (iii) protonolysis with a Brønsted acid such as HNBu+3 B(C6F5)−4 ,
85
or (iv) oxidative cleavage of metal alkyl bonds with one-electron oxidants such as
(C5H4R)2Fe+BPh−4 or Ag
+BPh−4 ,75,86 as illustrated for Cp2Zr(Me)
+(µ-Me)B(C6F5)−3 24,
Cp2Zr(Me)(CH2Cl2)+B(C6F5)
−4 25 aand Cp2Zr(Me)(thf)
+BPh−4 26 (Scheme 1.5). The
reaction of so-called “tuck-in” (fulvene) complexes like (C5Me5)(η6-C5Me4CH2)ZrPh
2787,88 or butadiene complexes Cp2Zr(η4-C4H6) 28
89 with B(C6F5)3 yields “zwitterionic”
complexes90 such as 29 or 30 that are also active for olefin polymerization (Scheme 1.6).
The obtained cationic or zwitterionic alkyl zirconocene complexes are very strong
electrophiles and consequently very sensitive. Hence, they are typically stabilized by the
addition of Lewis-bases or by close contact with the counter ion, which both saturate the
vacant coordination site. In most cases this Lewis-base stabilization is inevitably connect-
ed with the synthetic route of cation generation (Schemes 1.5 and 1.6). The stabilizing
ligand must be coordinating weakly enough in order to be replaced by a monomer under
polymerization conditions. So-called “base-free” cationic complexes are described, but in
most cases they are reactive enough to find a pathway for the saturation of the vacant
coordination site.91 The electrophilicity of cationic alkyl zirconocenes also gives rise to de-
composition reactions, e. g. via fluoride abstraction from B(C6F5)−4 , (pentafluoro) phenyl
abstraction from BPh−4 or B(C6F5)−4 , reaction with the solvent via C-H activation and
other pathways.80
In the absence of other stabilizing agents, dinuclear cationic complexes are also formed,
especially with an excess of the dimethyl zirconocene from which the cationic complex is
generated (Scheme 1.7). Thus, a reaction of two equivalents of Me2Si(Ind)2Zr(Me)2 31
with one equivalent of Ph3C+B(C6F5)
−4 in CD2Cl2 quantitatively yields the corresponding
1.2 Metallocene Polymerization of Olefins 21
Si MeMe
Zr
SiMe Zr
Si MeMeZr
Si ZrZrMe
AlMe
Me
Me
B(C6F5)4
B(C6F5)4
CH2Cl2Ph3CB(C6F5)4AlMe3 < 25°C
CH2Cl2Ph3CB(C6F5)4
(0.5 eq. or 1 eq. < 20°C)
31
32
33
– Ph3CMe
– Ph3CMe
Scheme 1.7: Synthesis of dinuclear cationic zirconocene complexes.
dinuclear complex [Me2Si(Ind)2Zr(Me)(µ-Me)(Me)Zr(Ind)2SiMe2]+ 32.92 These dimers
may not only be intermediates, but may be inert to a second equivalent of cation generat-
ing agent, as is the dimer formed from Me2Si(Ind)2Zr(Me)2 below 20 ◦C. The stabilizing
role of dimerization is evident from the fast decomposition of the mononuclear cationic
complex in CD2Cl2 above 20◦C.92 In the presence of AlMe3 the heterodinuclear complex
[Me2Si(Ind)2Zr(µ-Me)2AlMe2]+ 33 is formed, which is also stable and non-fluxional up
to 25 ◦C in CD2Cl2. Polymerization activity is lower as compared to the mononuclear
cationic complex and decreases with AlMe3 concentration. Furthermore, a decrease in
molecular weight is also observed, indicating that the dinuclear species may provide a
pathway for chain transfer to the aluminum centre.20,92
Brintzinger et al. report the formation of dinuclear species in equilibrium
with the mononuclear complex upon addition of Cp2ZrMe2 15 to a solution of
Cp2Zr(Me)+(µ-Me)B(C6F5)
−3 24 in C6D6.
93 Remarkably, two different types of dinuclear
species are identified, which are assigned to a solvent-separated and an associated (con-
tact) ion pair (SSIP and CIP) of the dinuclear cationic complex and a MeB(C6F5)−3
counter ion, the latter being predominant at high concentration. Marks et al. suc-
cessfully isolated and crystallized a series of dinuclear complexes from equimolar mix-
tures of dimethyl zirconocenes and the sterically encumbered cation generation agent
tris(2, 2′, 2′′-perfluorobiphenyl)borane.82 The authors point out that enhanced stability of
the dinuclear complexes arises from the fact that tris(2, 2′, 2′′-perfluorobiphenyl)borane
and Me-PBB− have reduced abstracting and coordinating abilities as compared to
B(C6F5)3 and MeB(C6F5)−3 , so that the unreacted dimethyl complex is favoured as the
22 1 Introduction
stabilizing agent. The dinuclear cationic complexes are active for ethylene and MMA
polymerization, however, probably only after dissociation.
The synthesis of cationic alkyl zirconocene complexes and their application as olefin
polymerization catalysts was a major breakthrough for the investigation of the olefin poly-
merization mechanism. However, they are much more sensitive than systems generated
in situ, due to the decomposition reactions already mentioned. That is one reason why
MAO activation is predominant in today’s industrial polymerization processes.
1.2.5.3 “Good” and “Bad” Cocatalysts
Marks et al. have determined the relative Lewis-acidity of different Lewis-acids from
reaction with crotyl aldehyde by comparison of the shift of H-3 in 1H-NMR spectra. They
have found poor activators such as BBr3 or AlCl3 to be much stronger Lewis-acids than
good activators like B(C6F5)3.81 Apparently, a good activator must be sufficiently Lewis-
acidic in order to quantitatively remove the methyl group. However, a “structural match”
of the activator and the zirconocene is also crucial. Labile nucleophilic substituents are
potential catalyst poisons, steric hindrance must be sufficient in order to prevent strong
coordination of the anion to the cationic complex, and some extent of stabilization by
the anion is desirable, in order to enhance long-time stability of the cationic complex in
solution.
Methyl abstraction is always exothermic, e. g. −101.1 kJ mol−1 for the
system (1, 2-Me2Cp)2ZrMe2/B(C6F5)3, and −45.6 kJ mol−1 for the system
(1, 2-Me2Cp)2ZrMe2/MAO. Assuming entropy-loss to be similar for activation with
MAO or B(C6F5)3 (−22 J mol−1 K−1 in toluene), the equilibrium constant of methyl
abstraction by MAO is Kabstr = 2000 at T = 300 K. This means that at zirconocene
concentrations below 1 · 10−3 mol L−1, methyl abstraction proceeds to less than 20%
completion. This may be a reason for the high excess of MAO necessary.81 However, this
comparison does not account for consecutive reactions, and the assumption of similar
entropy-loss is questionable.
In the case of B(C6F5)3 activation, ion pair dissociation is highly endothermic, so
that separated ion pairs are never observed.80,81, 94 Results reported by Brintzinger et
al. point to the fact that ionic compounds may even exist as higher aggregates such
as ion quadruples in organic solution.95 Ion pair dissociation enthalpy and entropy ex-
perimentally determined from the activation parameters of ion pair reorganization are
∆H = +100.3 kJ mol−1 and ∆S = +17 J mol−1 K−1 in toluene.81,96 These values may
1.2 Metallocene Polymerization of Olefins 23
even be too low, because the reorganization process does not require complete dissocia-
tion. Thus, ion pair separation appears to be inconvenient, but is still far less unfavourable
than is expected from ∆H = +322 kJ mol−1 for the separation of two point charges with
an initial distance of 8 A in toluene. This trend may be continued for MAO activation and
thus explain the superior activating features of MAO. Furthermore, it is important to note
that the role of MAO as a cocatalyst is not limited solely to cation generation. It plays
an active role in the course of the polymerization reaction, and it acts as a “scavenger”,
reacting with and removing protic and many other contaminants from the system. It
coordinates weakly enough to be replaced by the monomer, but it is at the same time a
stabilizing and a solubilizing agent for the cationic complexes. Finally, it is responsible for
the regeneration of deactivated complexes throughout the polymerization period (section
1.2.6). The major drawback of MAO cocatalysts is the high excess of MAO necessary to
generate an active catalyst system. However, the application of zirconocenes adsorbed to
heterogeneously supported MAO greatly reduces this problem.81
1.2.5.4 The Role of Ion-Pairs
While the existence of ion-pairs in catalytic systems with simple aluminum alkyl co-
catalysts and in cationic complexes with borate counter ions is a well-established fact,54–57
the role of such species in MAO activated catalyst systems is still a matter of considerable
discussion and generally neglected in mechanistic and theoretical considerations.
Recent results from DFT studies indicate that the role of the counter ion must not
be neglected. Qualitatively consistent with experimental results, a contact ion pair is
by far the most stable species in solution in any case, with the “free” cationic complex
being destabilized with +203 kJ mol−1 in the case of the system Cp2Zr(Me)+MeB(C6F5)
−3 .
Olefin coordination must start from the contact ion pair, and is slightly endothermic
with +34.3 kJ mol−1, leading to an olefin separated ion pair (OSIP). Again the “free”
olefin complex is destabilized with +75.6 kJ mol−1.29 Similar studies for MAO activation
indicate that in this case complete ion pair separation is laso unfavourable.97 Klesing
et al.98 conclude that in the absence of substituents at the indenyl ligands, both µ3-O-
and µ2-Me-coordination of the MAO clusters to the cationic zirconocene is possible, with
the latter being favoured as the bulkiness of the ligand framework is increased. The
dissociation of a MAO cluster is endothermic in any case. The authors state that it would
be misleading to classify MAO anions as non-coordinating in reaction media with a low
dielectric constant. Apparently, the OSIP is inevitably the most feasible intermediate in
24 1 Introduction
the olefin insertion sequence. This is also supported by the calculations of Fusco et al.99,100
who interpret their results in terms of the MAO counter ion enhancing the polarization
of the olefin monomer in preparation for the insertion step.
1.2.6 Polymerization Kinetics
Conclusions from kinetic experiments are impeded by the fact that neither the con-
centration of the active species [Zr∗] nor the concentration of the monomer [Mon] can be
exactly determined. Catalyst activity is strongly influenced by experimental conditions
such as the Al/Zr ratio (section 1.2.5.1) and is definitely a function of time due to catalyst
activation, deactivation and regeneration reactions (vide infra). In most cases it rises up
to a maximum after a short induction period and then decreases to a steady state level
for the rest of the polymerization time. Some authors refer to catalyst activity as the
maximum activity, others as the average activity and still others as the steady state level.
Monomer concentration in the case of gaseous monomers is mostly calculated from the
solubility constants in the applied solvent. However, polymerization reaction mixtures
may exhibit an altered solubility of the monomer, and, especially at high catalyst activi-
ties, diffusion limitation or heat transfer resistance may lead to erroneous results.101,102
1.2.6.1 Rate Law of Propagation
According to the generally accepted mechanism of olefin polymerization, the rate law
of propagation should obey the simple relation21
rp = kp · [Zr∗] · [Mon] (1.1)
The simple rate law presented above requires olefin coordination to be the rate-
determining step. However, the experimentally determined reaction order in monomer
concentration is often larger than 1. A so-called “trigger mechanism” is proposed, which
assumes that insertion is (must be) triggered by an incoming second monomer molecule.
Consequently, the active site is never vacant, the insertion will proceed more slowly with-
out a second monomer unit, two monomer molecules are involved in the transition state,
and the reaction order will range between 1 and 2 depending on monomer concentration.103
1.2 Metallocene Polymerization of Olefins 25
1.2.6.2 Catalyst Deactivation
From kinetic experiments at different polymerization temperatures Muhlhaupt et al.
conclude that catalyst activation proceeds in a two-step process, the first step leading to
a steady state value of activity and the second, slower process leading to a continuing
decrease of activity over polymerization time at elevated polymerization temperatures.104
The authors claim that in a bimolecular reaction, the active species Zr∗ is converted to an
inactive species Zrinact,1, which can be captured and regenerated with excess MAO before
it decomposes irreversibly to another inactive species Zrinact,2. Similar observations are
reported by Rempel et al.,105 who claim the existence of two equilibria in which the active
species is involved, one with a “dimerized” inactive species, and one with an inactive
MAO adduct (Scheme 1.8). The inactive species may comprise Zr-CH2-Al or Zr-CH2-Zr
structures, the formation of which, accompanied with an evolution of methane, is proved
by Kaminsky et al.106 These species are regenerated with excess MAO, yielding L2ZrMe+
and Al-CH2-Al structures.
MAO
Zr* Zrinact,1 Zrinact,2irreversible
Zr•MAOinactZr*MAO
Scheme 1.8: Deactivation processes in zirconocene/MAO based catalyst systems accord-
ing to the results of Muhlhaupt et al.104 and Rempel et al.105
1.2.6.3 Deviation from Single-Site Behaviour
The participation of the cocatalyst in the polymerization mechanism has been a point
of discussion since the early investigations of the MWD of polyolefins that were explained
in terms of an “intermittent growth” mechanism involving an adduct with the cocatalyst
as a dormant species.107–110 Considering the number of different species formed in the
course of the activation reaction with MAO (section 1.2.5.1), it should be of no surprise
to find one of them playing an active role in the polymerization mechanism under typical
polymerization conditions. There is some experimental evidence which indicates that
referring to zirconocene catalysts as single-site catalysts may be an oversimplification.
However, results regarding that subject have so far been isolated and incoherent, and
have always been subject to a hard response. One must admit that conclusions from all
of the following investigations suffer from a substantial lack of experimental data.
26 1 Introduction
The most complete picture is given in a study by Chien et al.111 A general observation
is that, depending on polymerization conditions, the MWD is broader111–117 than should
be expected for a single-site mechanism, and that it is temperature dependent.111,112 Thus,
for ethylene polymerization with Cp2ZrCl2/MAO, typically a MWD with Mw/Mn ≈ 3
at 25 ◦C and Mw/Mn ≈ 5 at 90 ◦C is obtained.112 Moreover, despite a linear Eyring
plot of catalyst activity vs. polymerization temperature, a non-linear Eyring plot of
Mw vs. polymerization temperature in ethylene and propene polymerization is ob-
served. Thus, a “bell-shaped plot” with a maximum Mw at 0 ◦C is obtained for the
system C2H4(Ind)2ZrCl2/MAO, and no further increase in Mw is obtained for the system
Cp2ZrCl2/MAO below 30 ◦C.111–113 Occasionally, the MWD is bimodal at certain poly-
merization temperatures. Some authors find that the MWD varies with polymerization
time, with the low molecular weight fraction disappearing with polymerization time.115
With the methodology developed in heterogeneous polymerization, Wang et al. deconvo-
lute the MWD from GPC curves, which are broader than Schulz-Flory distributions in the
case of MAO activation, and slightly bimodal in the case of TIBA activation.117 Chien et
al. observe that activity in ethylene polymerization with Cp2ZrCl2/MAO varies both with
the zirconocene and the MAO concentration, although almost all zirconium centers are
claimed to be active. If this is true, the observed phenomenon must be due to the existence
of more than one active species.112 Hamielec et al. have developed a kinetic model in
order to fit the experimental results of ethylene polymerization with Cp2ZrCl2/MAO.
They conclude that two active species are present, the second of which is formed from
the first one in a slow pseudo first-order reaction involving MAO.118 Another indica-
tion of a deviation from single-site behaviour is the finding that PP polymerized with
zirconocene/MAO catalyst systems may be “anisotactic”, which means predominantly
isotactic but fractionable by means of solvent fractionation.73,111 Similar observations are
reported by Soga et al. from ethylene/1-olefin copolymerizations where the two polymer
fractions are obtained and separated via cross fractionation chromatography.119
In contrast to all findings implying more than one active species, Rieger et al. pro-
pose that the observed anomalies are due to the inevitable precipitation of the polymer
during the course of polymerization, leading to homogeneously dissolved and heteroge-
neously “polymer-supported” species.120 Other authors assume that the broadening of
MWD may be caused by mass- and heat-transfer resistance,116 due to the high activity of
the zirconocene catalysts, and the increasing viscosity and heterogeneity of the reaction
mixture, as also pointed out by Lahti et al.101,102
1.2 Metallocene Polymerization of Olefins 27
1.2.7 Copolymerization with Functionalized Monomers
The major limitation of metallocene polymerization catalysts is their lack of tolerance
towards functionalized monomers because of the catalysts’ and the cocatalysts’ elec-
trophilicity. Part of this problem is inevitable, because the activation except the carbon
double bond for a polymerization reaction is achieved by exploitation of its electron densi-
ty. Most functional groups comprise nucleophilic or Lewis-basic centers themselves, thus
naturally competing with the carbon double bond for the vacant coordination site. How-
ever, the problem is increased in metallocene chemistry by the fact that these catalysts
(and also the cocatalysts) are electrophilic in the sense of strong and hard Lewis-acidity,
giving further rise to a preference for coordination of any other functional group but the
carbon double bond. The attempts to circumvent the problems inevitably encountered
with the polymerization of functional monomers are all in the line of one of the following
strategies or a combination of these:
(i) separation of the double bond and the functional group by a long chain spacer,
(ii) introduction of bulky substituents at the functional group, often combined with the
use of sterically more demanding ligands at the zirconocene complex,
(iii) covalent attachment of a protecting group lowering polarity or increasing steric
demand of the functional group,
(iv) complexation of the functional group by pretreatment with an additional amount
of a Lewis-acid such as MAO, or
(v) application of noninteracting (Lewis-acidic) or weakly interacting functional groups,
such as boranes, silanes or halogenides, and further functionalization by post-
polymerization processes.
Some progress has been achieved during the past decade (cf. the reviews by Padwa121
and Novak et al.122), but in general and regardless of the strategy, catalyst activities
drastically decrease with increasing comonomer concentration, and comonomer incorpo-
ration is low. In all cases the comonomers applied are not industrial monomers such as
(meth-)acrylates or vinyl compounds that one would like to copolymerize with olefins.
For the latter monomers, apart from catalyst poisoning an additional obstacle arises
from an “orbital mismatch”.122 MO of polarized double bonds are not suitable for a
concerted insertion reaction. These monomers may be polymerizable via other, rather
polar polymerization mechanisms. However, these are incompatible with the insertion
polymerization of olefins and not a pathway to copolymerization.
28 1 Introduction
1.2.7.1 Sterically Hindered or Protected Comonomers
Waymouth et al., Fink et al. and Muhlhaupt et al. report the polymerization of
sterically hindered dialkylamines 34, silyl ethers 35 and disilylamines 36.123–126 Catalysts
derived from the sterically more hindered (C5Me5)2ZrMe2 activated with B(C6F5)3 or
HNMe2Ph+B(C6F5)
−4 turn out to be more stable towards catalyst poisoning than those
from C2H4(Thind)2ZrMe2 and other stereospecific catalysts. MAO based catalyst systems
are less active and yield only partial conversion. Catalyst activity in copolymerization with
ethylene drastically decreases with comonomer concentration and increases with the steric
demand of the functional group and spacer length. Attempts to homo- or copolymerize
vinyl or allyl silyl ethers using zirconocene catalysts have so far failed.127
R2N
n
34a (R = Me, Et, iPr, iBu; n = 3)34b (R = iBu; n = 2)
R3SiO
n
35a (R3 = tBuMe2; n = 3)35b (R3 = Me3; n = 9)
(R3Si)2N
n
36 (R3 = Me3; n = 9)
O
ROn
37 (R = H, Me, tBu; n = 8)
Figure 1.12: Functional olefins with bulky substituents or protecting groups suitable for
polymerization with metallocene catalyst systems.
The precomplexation of the free 10-undecene-1-ol with MAO is also proposed,128–131
but activity and comonomer incorporation are very low, and the MWD of the obtained
polymers is very broad. The use of the catalyst Me2Si(2-Me-4, 5-benzind)2ZrCl2 9 and
pretreatment of the alcohol with TIBA rather than MAO allows for better results concern-
Mt MeO
O
Mt MeO
O
Mt MeMt Me
TiO
O
Ti
O O
+ +
Cp2ZrMe2/B(C6F5)3C2H4
m n
38
Scheme 1.9: Methyl metallocene protecting groups for (meth-)acrylic monomers (above);
copolymerization of ethylene and titanocene carboxylates.
1.2 Metallocene Polymerization of Olefins 29
ing activity and comonomer incorporation.132 Similar results are reported for undecenoic
acid and undecenoates 37.130,131
A very interesting approach to protecting group chemistry is suggested by Novak
et al. The idea (“if you can’t break them, join them”) is to use appropriate methyl
metallocene protecting groups in order to turn monomer coordination to the catalyst
into a degenerate exchange process (Scheme 1.9).133 So far, titanocene(III) carboxylate
complexes from (meth-)acrylic acid, such as 38, have been studied in copolymerization
with ethylene. Activity is reported to be maintained regardless of comonomer feed in the
mixture. However, no figures concerning comonomer incorporation have been given.
1.2.7.2 Noninteracting or Weakly Interacting Functional Groups
Allylsilane 39 has successfully been homopolymerized and copolymerized with
propene.134,135 The reaction of the Si-H bonds with water is a pathway to the Si-O-Si
crosslinking of polyolefins.135
H3Si
39
X
n
40a (X = I; n = 2)40b (X = Cl; n = 9)
B
n
41 (n = 4, 6)
Figure 1.13: Weakly interacting or noninteracting functional olefins suitable for poly-
merization with metallocene catalyst systems.
Halogenide monomers are polymerizable with a few restrictions. Activated (secondary,
tertiary) halogenides are susceptible to halogen transfer to the catalyst due to nucleophilic
substitution or elimination. Vinyl halogenides comprise altered reactivity of the double
bond, and once inserted are prone to β-halo elimination.122 The stability of halogenide
monomers increases with the size of the halogen atom (decreasing Lewis-basicity), the
strength of the C− X bond (tertiary < secondary < primary), and the distance between
halogen atom and double bond.136 Thus, α-halo-ω-olefins such as 4-iodo-1-butene 40a
have successfully been homopolymerized136 and copolymerized with propene137 using
heterogeneous Ziegler-Natta catalysts. Only recently, the investigation of polymerization
of α-halo-ω-olefins with metallocene catalysts has revealed that only long chain monomers
such as 11-chloro-1-undecene 40b are polymerized due to an intramolecular complexation
of the halogen atom to the catalyst for smaller monomers.138 The obtained polymers are
transformed to polyalcohols by post polymerization processes.
30 1 Introduction
A number of patents and papers by Chung et al. describe the polymerization of borane
functionalized olefins.139–145 Nonconjugated dienes such as 1,5-hexadiene or 1,7-octadiene
are hydroborated with 9-BBN to yield α-(9-BBN)-ω-alkenes 41. These are then ho-
mopolymerized141 or copolymerized with 1-olefins such as 1-octene142 with heterogeneous
Ziegler-Natta catalysts. The poly(borane)s are converted to alcohols, amines or aldehydes
by post-polymerization processes via standard methods, or used for grafting of PMMA
onto the chains via borane mediated radical polymerization. MWD are generally broad
(Mw/Mn > 5). In the case of the copolymerization of ethylene or propene with borane
comonomers using metallocene catalysts, no or very low comonomer incorporation is ob-
tained.143,145 Attempts to homo- or copolymerize vinyl or allyl boranes using zirconocene
catalysts have failed so far.146
1.2.7.3 Late Transition Metals
A summary of late transition metal polymerization catalysis is beyond the scope of this
introduction. The long history of late transition metal polymerization catalysis has been
reviewed by Wilke et al.147 and Keim et al.148 There is considerable evidence that late
transition metal catalysts may be more tolerant towards functional comonomers because
of their comparably soft Lewis-acidity, as highlighted by Rieger et al.149 Recent reports
include copolymerization with functionalized norbornenes,150 MA,151–153 and CO.16
1.3 Metallocene Polymerization of MMA
1.3.1 Group-Transfer Polymerization of MMA
In 1992, Collins et al. reported the first group-transfer polymerization (GTP) of MMA
using a zirconocene based catalyst system.154 The mechanism proposed and investigated
is outlined in scheme 1.14.155,156
The “Collins-type” catalyst system consists of two components, a cationic methyl
zirconocene complex Cp2Zr(Me)+ 14 and a neutral methyl zirconocene ester enolate com-
plex Cp2Zr(Me)[OC(OMe)C(Me)(R)] 42 (R = Me, Et, PMMA). The cationic complex
coordinates an MMA monomer, thus activating it as an acceptor (a3 in Seebach nomen-
clature). The ester enolate is an activated donor (d2 in Seebach nomenclature). Carbon
bond formation proceeds via Michael addition between the monomer and the ester enolate
complex. After the reaction, both catalyst components have switched their nature, the
1.3 Metallocene Polymerization of MMA 31
ZrMe
O
MeO
COOMe
COOMe
Pol
Zr MeO
MeO
Zr Me
O
OMe
MeOOC
MeOOC
Pol
Zr
MeO
OMe
ZrMe
Zr MeO
MeO
OMeO
COOMe
COOMe
Pol
Zr Me
ZrMeO
OMe
OOMe
MeOOC
MeOOC
Pol
+
+
a3 d2
a3d2
+ MMA + MMA
14 · MMA
42 – PMMA
14' · MMA
42' – PMMA
Figure 1.14: Mechanism of zirconocene catalyzed group-transfer polymerization of MMA
as described by Collins et al..
former methyl zirconocene ester enolate complex now being a cationic methyl zirconocene
complex which is loosely attached to the growing polymer chain, and the former cationic
methyl zirconocene complex now being a neutral methyl zirconocene ester enolate com-
plex which is the active chain end. The catalytic cycle is completed by replacement of the
growing chain loosely coordinated to the cation by a new monomer, a new Michael type
polymerization step between the activated chain end and the new activated monomer,
and a repeated switch in the nature of the two catalyst components, which regain their
initial identity.
A one-component mechanism via a cationic ester enolate complex is ruled out from
equilibrium studies, and polymerization using such complexes as initiators afforded inco-
herent results.155,156 Sustmann et al. investigated different potential reaction pathways
for the “Collins-type” polymerization from a theoretical point of view.157 They found a
monometallic mechanism via a cationic zirconocene ester enolate complex to be energet-
ically more favourable than the experimentally determined bimetallic mechanism, apart
from the last step, the replacement of the loosely coordinated chain by a new monomer
molecule.
32 1 Introduction
The catalyst system Cp2Zr(Me)(thf)+BPh−4 26/Cp2Zr(Me)[OC(OMe)C(Me)(R)] 42
(R=Me) is synthesized separately156 or generated in situ from a mixture of two equiv-
alents of Cp2ZrMe2 15, one equivalent of HNBu+3 BPh
−4 and MMA.154,155 The excess of
Cp2ZrMe2 15 transfers a methyl group to an MMA molecule, thus generating the active
catalyst system described above.
The Collins-type GTP of MMA is a highly efficient method to obtain well defined
PMMA. It is a fast polymerization giving a quantitative yield after a couple of minutes.
It shows all the features of a living polymerization, with a narrow MWD (Mw/Mn < 1.2)
up to high conversion, a linear dependence of molecular weight on monomer conversion
and the molecular weight being controlled by the monomer/initiator (i. e. the ester enolate
complex) ratio.154 Chain termination is observed, but only to a very small extent. If the
catalyst system is generated in situ, an initiation period and consequently, a slight broad-
ening of the MWD is observed. It is a point of discussion whether the polymerization is
zero order with respect to the MMA concentration, as described by Collins et al.155,156
or rather first order, because the data presented by Collins et al. may leave room for
interpretation. The discussion may be obsolete, because if the polymerization step is
rate-determining, then zero order kinetics will be expected. However, if the coordination
equilibrium of the monomer is rate-determining, then first order kinetics should be ob-
served. Consequently, the experimentally determined reaction orders may be somewhere
in between, if both rate constants are comparable in magnitude, and they may also be
dependent on reaction conditions.
1.3.2 Other Zirconocene Based Catalyst Systems
While the results of Collins et al. suggest that cationic methyl zirconocenes
alone are inactive for MMA polymerization, Hocker et al.158,159 have shown that
the cationic zirconocene complex Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 is active
for MMA polymerization without addition of the corresponding ester enolate
complex Me2C(Cp)(Ind)Zr(Me)[OC(OtBu)CMe2] 44 as opposed to the achiral
Cp2Zr(Me)(thf)+BPh−4 26. Gibson et al. have reported similar observations.160 Thus,
an equimolar mixture of Cp2ZrMe2 15 and HNEt+3 BPh−4 does not generate a cat-
alytically active system, which is consistent with the findings of Collins et al. and
Hocker et al.. However, the dimethyl zirconocenes Cp2ZrMe2 15, (Cp)(C5Me5)ZrMe2 45,
C2H4(Ind)2ZrMe2 46, Me2C(Cp)(Ind)ZrMe2 47 and Me2C(Cp)(2−MeInd)ZrMe2 48 (but
not Me2C(Cp)(Flu)ZrMe2 49) activated with equimolar amounts of B(C6F5)3 are highly
1.3 Metallocene Polymerization of MMA 33
Zr MeMe
N
ZrSiOO
O
Zr OMe Me
OtBu
Zr O
Zr MeMe
Zr MeMe
Zr MeMe
Zr MeMeZr Me
Me
BPh4
BPh4
46
49
45
51
43 44
15
47 48
Si ClClZrZrSi Me
Me
3150
Figure 1.15: Zirconocene catalysts and catalyst precursors applied in the polymerization
of MMA
active for MMA polymerization. The molecular weights are reported to be too large by a
factor of approximately 2, and yields are not quantitative in some cases. The authors claim
the dimethyl zirconocenes to be quantitatively converted into the corresponding cationic
methyl zirconocenes. However, incomplete reaction and the existence of a “Collins-type”
catalyst system cannot be ruled out. Soga et al. reported MMA polymerization with
catalyst systems closely related to the Collins-type catalysts.161–164 They use dimethyl
zirconocenes such as Me2Si(Cp)(Ind)ZrMe2 50 or Me2Si(Ind)2Zr(Me)2 31 as catalyst pre-
cursors. The catalyst system is generated in situ by application of a cation generating
agent such as Ph3C+B(C6F5)
−4 and a high excess of additional Lewis-acids like zinc alkyls
or aluminum alkyls. Polymerization of MMA is carried out in toluene as the solvent, as
34 1 Introduction
opposed to methylene chloride, which is the case in “Collins-type” polymerizations. The
authors claim that the polymerization is living. However, the polymerization mechanism,
the role of the additional Lewis-acid, the reason for the high excess necessary, and the
nature of the active species remain unclear, and the results reported are somewhat incoher-
ent. Polymer yields vary greatly, but are generally far from quantitative and decrease with
increasing Lewis-acid concentration. Also, control of Mn is poor. In a typical polymeriza-
tion experiment (with Cp2ZrMe2, Ph3C+B(C6F5)
−4 and ZnEt2, Zn/Zr = 1000, Tp = 0 ◦C)
the polymer yield is 38%, Mn = 29000 (instead of 400000), and Mw/Mn = 1.62.161 Gibson
et al. assume that the large excess of zinc alkyls may be due to the sequence of adding
MMA to the catalyst prior to borane addition, which leads to complexation of the borane
by MMA.160
Recently, Collins et al. have published the polymerization of MMA with the cationic
zirconocene ester enolate complex 51 as a single component catalyst.165 Because of the
thermal instability of the catalyst, the polymerization is carried out at temperatures of
−20 ◦C and below. Conversion is not quantitative in all cases, and MWD is narrow only
for very low temperatures with Mw/Mn = 1.72 at 0 ◦C and Mw/Mn = 1.10 at −60 ◦C.
The mechanism proposed is similar to the one described for the isoelectronic neutral
samarocene ester enolate complexes (section 1.3.3).
1.3.3 Samarocene Catalysts
Yasuda et al. investigated the polymerization of MMA with samarocene cata-
lysts.166–171 They found neutral samarocene hydrides and alkyl samarocenes such as
[(C5Me5)2SmH]2 52 and (C5Me5)2Sm(Me)(thf) 53 to be active for MMA polymerization
and prove the mechanism illustrated in Scheme 1.10.166
SmH
SmH
Sm OMe
52 53
Figure 1.16: Samarocene catalysts applied in the polymerization of MMA.
The active species is the neutral samarocene ester enolate complex 54 which is capable
of complexing an MMA molecule. Thus, the ester enolate chain end is activated as a
1.3 Metallocene Polymerization of MMA 35
Sm
O
MeO
MeOOC COOMe
O
OMe
Pol
SmO
MeO
MeOOC COOMe
O
OMe
Pol
SmO
MeO
MeOOC COOMe
O
OMe
Pol
a3
d2
Sm
O
MeO
MeOOC COOMe
O
OMe
Pol
a3
d2
+ MMA + MMA± MMA
54 · MMA
54 · MMA
Scheme 1.10: Mechanism of samarocene catalyzed MMA polymerization.
donor while the coordinated monomer is activated as an acceptor. In other words, the
samarocene catalyst combines the roles of the two components in “Collins-type” catalyst
systems. Carbon bond formation proceeds via an intramolecular Michael addition, yield-
ing a cyclic intermediate with the polymer chain loosely attached to the catalyst. The
enchained MMA monomer has become the ester enolate chain end, situated at the other
side of the catalyst wedge as compared to the initial situation. The catalytic cycle is
completed by the replacement of the loosely attached polymer chain by a new incoming
MMA monomer and another polymerization step analogous to the first one.
Generation of the active species obviously involves a hydride transfer from the
samarocene hydride to an MMA molecule, as also implied by the hydrogenation of MMA
by stoichiometric amounts of [(C5Me5)2SmH]2 and by the end group analysis of PMMA
obtained using [(C5Me5)2SmD]2.166 Yasuda et al. successfully isolated and crystallized
the 2:1 adduct of MMA and [(C5Me5)2SmH]2 52, which is an active catalyst for MMA
polymerization.166 The obtained PMMA is well controlled by the monomer/initiator
36 1 Introduction
ratio, exhibits a very narrow MWD (Mw/Mn < 1.05) and is basically syndiotactic. Also
acrylates are polymerized in quantitative yield with excellent initiator efficiencies.169,170
Block copolymers of MMA, MA, EA and CL with ethylene have been synthe-
sized167,168, 170, 171 with (C5Me5)2Sm(Me)(thf) 53 as the catalyst, by homopolymerization
of ethylene and subsequent addition of the polar comonomer. However, this methodology
just works “one-way down the energetic hill”. The reverse order of monomer addition
does not yield copolymers,122,167 partial chain transfer producing polyolefin cannot be
excluded, and the lanthanidocene catalyzed olefin homopolymerization reported by the
authors does not yield very well defined polyolefins.172
1.3.4 Stereospecific MMA Polymerization
Generally, all possible pathways of MMA polymerization (radical, anionic or
group transfer polymerization) yield basically atactic, syndio-enriched PMMA with ap-
proximately 60 to 70% syndiotacticity (calculated from rr triads) due to chain end control.
Anionic MMA polymerization is known to be stereospecific under certain reaction condi-
tions.173–177 Hatada et al. reported the use of Grignard reagents as initiators for the an-
ionic living polymerization of MMA, yielding highly isotactic PMMA with tert-C4H9MgBr
and syndiotactic PMMA with tert-C4H9Li/AlEt3(Al/Li ≥ 3), in toluene as the solvent
and at low temperatures in both cases. In the case of isotactic PMMA, the mm triad
abundance at −78 ◦C is 96%. In the case of syndiotactic PMMA, the rr triad abundance
ranges from 71% at 0 ◦C to 84% at −40 ◦C, 90% at −78 ◦C and 94% at −93 ◦C.175 This
is the first example of a range of PMMA microstructures being accessible with the same
methodology just by a change in reaction conditions. However, the conditions investigated
are completely empirical and do not allow for a rational control of PMMA microstructure.
Given the success of chiral ansa-zirconocenes with respect to stereocontrol in the
polymerization of olefins, one should expect similar efforts in the field of zirconocene
catalyzed MMA polymerization. However, the use of chiral ansa-zirconocenes in “Collins-
type” polymerizations is only briefly mentioned once,155 and a systematic study has never
been published. Recent results by Hocker et al. show that the “Collins-type” poly-
merization of MMA with the chiral catalyst system Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4
43/Me2C(Cp)(Ind)Zr(Me)[OC(OtBu)CMe2] 44 yields basically atactic, syndio-enriched
PMMA, just as do achiral catalyst systems or any other non-stereospecific method of
MMA polymerization.158 Apparently, the zirconium centres are too far away from the
actual position of carbon bond formation to allow for chiral induction.
1.3 Metallocene Polymerization of MMA 37
However, when Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 is applied alone or in excess,
the PMMA obtained is highly isotactic.158,159 The addition of increasing amounts
of Me2C(Cp)(Ind)Zr(Me)[OC(OtBu)CMe2] 44 results in a broadening of MWD, a de-
crease in isotacticity, and pentad abundance consistent with a blend of iso-enriched and
syndio-enriched PMMA.158 Apparently, the cation Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4
43 is capable of polymerizing MMA with a mechanism distinct from the “Collins-type”
mechanism, and addition of the ester enolate complex appears to promote a parallel poly-
merization via both mechanisms. Similarly, if chiral dimethyl zirconocenes are activated
with B(C6F5)3 as proposed by Gibson et al. highly isotactic PMMA is obtained. In
contrast to this, synthesis of syndiotactic PMMA with Me2C(Cp)(Flu)ZrMe2 49 has not
been successful.160
“Soga-type” catalyst systems based on chiral dimethyl zirconocenes, a cation gener-
ating agent like Ph3C+B(C6F5)
−4 and a high excess of additional Lewis-acids like zinc
alkyls or aluminum alkyls are suitable for the stereospecific polymerization of MMA.
With C1- or C2-symmetric catalyst precursors such as Me2Si(Cp)(Ind)ZrMe2 50 or
Me2Si(Ind)2Zr(Me)2 31 highly isotactic PMMA with a mmmm pentad abundance of 83%
is obtained in both cases.178 Butyl acrylate is also polymerized stereospecifically.179
N
ZrSiO
MeO
O
MeOPol
N
ZrSiO
MeO
O
MeOPol
N
ZrSi
O
MeO
O
MeO
O
OMe
Pol
+ MMA
Scheme 1.11: Isospecific MMA polymerization with 51 involving an associative dis-
placement of the coordinated polymer chain by a new monomer.
38 1 Introduction
Remarkably, the cationic ester enolate complex 51 also yields highly isotactic PMMA
in CH2Cl2 despite the Cs-symmetry of the catalyst.165 Isotacticity ranges from 80.5% at
−20 ◦C to 95.5% at −60 ◦C, and decreases upon the addition of a coordinating solvent
like THF, which may promote active site epimerization. The authors explain isospecificity
with the assumption that the stereoconfiguration at the zirconium centre is the same in
subsequent polymerization steps due to an associative displacement of the coordinated
polymer chain by a new monomer (Scheme 1.11). Hence, the enantiofacial selectivity of
the involved coordination site accounts for the isospecificity of MMA polymerization.
According to Yasuda et al., MMA polymerization catalyzed with achiral samarocenes
such as [(C5Me5)2SmH]2 52 or (C5Me5)2Sm(Me)(thf) 53 yields syndio-enriched PMMA
with increasing syndiotacticity at low polymerization temperatures. The experimentally
determined rr triad abundance ranges from 77.3% at 40 ◦C to 82.4% at 0 ◦C and 93.1%
at −78 ◦C.166 The pentad distribution is consistent with a chain end control mechanism.
Interestingly, acrylate polymerizations appear to be completely unspecific even at poly-
merization temperatures as low as −78 ◦C.169
LaRMe2Si SmR'Me2Si
(R)-55 (R)-56
Figure 1.17: Chirally substituted lanthanidocenes for the stereospecific polymerization
of MMA (R = N(SiMe3)2, R′ = CH(SiMe3)2).
Marks et al. have reported the stereospecific MMA polymerization
using chirally substituted ansa-lanthanidocenes as the polymerization cata-
lysts, such as (R)-Me2Si(C5Me4)(3-(+)-neomenthyl-C5H3)La[N(SiMe3)2] 55 or
(S)-Me2Si(C5Me4)(3-(−)-menthyl-C5H3)Sm[CH(SiMe3)2] 56.180 PMMA microstructure
varies from syndio-enriched in the case of 56 (67 % rr at Tp = 25 ◦C) to isotactic in the
case of 55 (75 % mm at Tp = 25 ◦C), and is neither describable with a pure Bernoullian
nor with an enantiomorphic site control model. MWD is broad in comparison to achiral
samarocene catalysts, with 1.7 < Mw/Mn < 7.9. The mechanism proposed assumes an
active site epimerization reaction within the catalytic cycle described for “Yasuda-type”
polymerizations. Therefore, PMMA microstructure is dependent on the sequence of
the monomer coordination to the two possible coordination sites and their respective
1.4 How to Make Ends Meet? 39
enantiofacial selectivities. A remarkable fact is the obvious inability to obtain highly
syndiotactic PMMA, which may point to the fact that if stereocontrol is achieved at
all, it may rather be due to a mechanism as outlined for MMA polymerization with 51
(Scheme 1.11).
1.4 How to Make Ends Meet?
On the one hand, the zirconocene catalyzed insertion polymerization of olefins paves
the way to new materials by means of tailoring virtually every aspect of polymer structure,
but, so far, it does not allow for copolymerization with functional olefins. On the other
hand, zirconocenes and other metallocenes provide a tool for the controlled polymeriza-
tion of (meth-)acrylic monomers, but with limited possibilities to control polymer micro-
structure.
By comparing the features of the two polymerization mechanisms, they are apparent-
ly incompatible. From a general perspective the Lewis-acidic nature of cationic alkyl
zirconocene complexes renders the reactivity of the carbon double bond ready for poly-
merization. It is the transition metal d-orbitals that “translate” this Lewis-acidity and
make the carbon double bond behave like a bifunctional reactive center, with the C-2
position of a coordinated monomer being activated as an acceptor, and the C-1 position
subsequently being activated as a donor in the next polymerization step. However, if such
a catalyst is exposed to (meth-)acrylic monomers inherently bearing donor and acceptor
functions, the polymerization mechanism will proceed involving these functions rather
than the carbon double bond alone. While at first sight the different reactivity of the
monomers appears to limit the possibility of copolymerization, the general perspective
points to how this limitation may be overcome.
A thorough understanding of both the differences and the parallels in the insertion
polymerization of olefins and the GTP of MMA may point towards new catalyst systems
suitable for both classes of monomers, hopefully allowing for the synthesis of new polymer
materials, be it on the basis of tailoring polymer microstructure in the case of functional
monomers or the copolymerization of functional and nonfunctional olefins.
41
2 Target and Specific Aims
The target of this dissertation is the application of zirconocene catalysts to the poly-
merization of olefins such as 1-hexene as well as to the polymerization of functionalized
monomers such as MMA. The work focuses on
(i) experiments to elucidate the nature of the active species in olefin polymerization, and
to evaluate the consequences with respect to the copolymerization of functionalized
monomers,
(ii) experiments that aim at an implementation of principles of rational catalyst design
in order to control polymer microstructure in the polymerization of functionalized
monomers.
43
3 Results and Discussion
3.1 Synthesis of Zirconocene Complexes
3.1.1 Preparation of Ligands and Complex Precursors
The general strategy for the synthesis of aromatic chelating ligands for ansa-
zirconocenes depends on the nature of the bridge. While dimethylsilyl and ethylidene
bridges are introduced by nucleophilic substitution in Me2SiCl2 and 1,2-dichloroethane,
respectively, in the case of isopropylidene bridges the method of choice is the nucleophilic
addition of a cyclopentadiene derivative to acetone as the {C3} building block and thesubsequent elimination of water.181,182
Thus, Me2C(CpH)2 57 is prepared by reacting two equivalents of cyclopentadiene with
acetone in a one-pot synthesis in THF with NaOH as the base and Aliquat-336 as the
phase transfer catalyst (Scheme 3.1). The product is isolated by vacuum distillation in
30.6% yield as a slightly yellow viscous liquid that consists of a mixture of constitutional
isomers according to 1H-NMR spectroscopy.
HH
HH
HH
2
1. THF, NaOH, Aliquat-3362. acetone
57
Scheme 3.1: Synthesis of Me2C(CpH)2.
In the case of mixed aromatic chelating ligands, a two-step synthesis is applied
(Scheme 3.2). Thus, 6,6-dimethylfulvene 58 is sythesized by the reaction of 1 equi-
valent of cyclopentadiene with acetone, and pyrrolidine as the base. Pure 6,6-
dimethylfulvene 58 is obtained in 40.8% yield as a yellow oil after distillation. Synthesis
44 3 Results and Discussion
of Me2C(CpH)(IndH) 59 and Me2C(CpH)(FluH) 60 is accomplished by deprotonation
of indene and fluorene with butyl lithium, nucleophilic attack at 6,6-dimethylfulvene and
aqueous workup. Me2C(CpH)(IndH) is obtained in 88.2 % yield as a red oil that is a
mixture of cyclopentadien-1-yl and cyclopentadien-2-yl isomers according to 1H-NMR
spectroscopy. Constitutional isomers at the indenyl system are not observed. In the case
of Me2C(CpH)(FluH), the yield is 41.6 % of a light yellow powder that is also a mixture
of constitutional isomers.
1. indene, BuLi2. H2O
59
1. fluorene, BuLi2. H2O
60
H H1. pyrollidine, MeOH2. acetone
58
Scheme 3.2: Synthesis of mixed aromatic chelating ligands.
In all complex syntheses in this study, Zr(NEt2)4 61 is used as the reagent for the
introduction of the zirconium center. Zr(NEt2)4 61 is prepared by deprotonation of
diethyl amine with butyl lithium and nucleophilic substitution in ZrCl4(Scheme 3.3). The
reaction is exothermic, and care must be taken not to exceed 0 ◦C in the reaction flask.
The pure product is isolated by distillation in HV in 71.6 % yield as a colourless viscous
liquid.
ZrCl4 Zr(NEt2)4
HNEt2/BuLiEt2O/pentane, < 0°C
61
Scheme 3.3: Synthesis of Zr(NEt2)4.
3.1 Synthesis of Zirconocene Complexes 45
3.1.2 Preparation of Zirconocene Dichlorides
The zirconocene dichlorides Cp2ZrCl2 1, C2H4(Ind)2ZrCl2 3, Me2Si(Ind)2ZrCl2 4
and Me2Si(Cp)2ZrCl2 62 used in this study have been obtained commercially. Syn-
thesis of the zirconocene dichlorides Me2C(Cp)2ZrCl2 63, Me2C(Cp)(Ind)ZrCl2 64 and
Me2C(Cp)(Flu)ZrCl2 5 is accomplished by reaction of the neutral ligands with Zr(NEt2)4
61 and conversion of the initially formed zirconocene diamides into the corresponding
zirconocene dichlorides in situ by reaction with Me3SiCl after removal of diethyl amine
(Scheme 3.4). Thus, reaction of Me2C(CpH)2 57 yields Me2C(Cp)2ZrCl2 63 (84.4 %,
light yellow powder), reaction of Me2C(CpH)(IndH) 59 yields Me2C(Cp)(Ind)ZrCl2 64
(82.1 %, orange-yellow powder), and from the reaction of Me2C(CpH)(FluH) 60 one ob-
tains Me2C(Cp)(Flu)ZrCl2 5 (26.8 %, red-orange powder).
ZrSi ClCl
Zr ClCl
1. Zr(NEt2)4 612. Me3SiCl Zr Cl
Cl
Zr ClCl64
5
57
59
60
63
Zr ClCl Si Cl
ClZrZr ClCl
1 6243
cm
Scheme 3.4: Synthesis of zirconocene dichlorides.
The methodology described here is superior to the use of ZrCl4 as the reagent for the
introduction of the zirconium center because reactions proceed in homogeneous solution,
no additional base is needed, Zr(NEt2)4 is easily purified by distillation, and the problem
of passivation of ZrCl4 is circumvented.
46 3 Results and Discussion
3.1.3 Preparation of Dimethyl Zirconocenes
Synthesis of dimethyl zirconocenes is accomplished by the dimethylation of the
corresponding zirconocene dichlorides with 2 equivalents of methyl lithium. Thus,
the reaction of C2H4(Ind)2ZrCl2 3, Me2C(Cp)(Flu)ZrCl2 5, Me2Si(Cp)2ZrCl2 62,
Me2C(Cp)2ZrCl2 63 and Me2C(Cp)(Ind)ZrCl2 64 yields C2H4(Ind)2ZrMe2 46 (41.2 %),
Me2C(Cp)(Flu)ZrMe2 49 (65.9 %), Me2Si(Cp)2ZrMe2 65 (73.9 %), Me2C(Cp)2ZrMe2 66
(86.2 %) and Me2C(Cp)(Ind)ZrMe2 47 (59.1 %) as brown powders (Scheme 3.5).
Zr ClCl
Zr MeMeZr Cl
Cl
Zr ClCl
Zr ClCl
Zr ClCl
ZrSi ClCl ZrSi Me
Me
Zr MeMe
Zr MeMe
Zr MeMe
Zr MeMe
MeLi (2 eq.), Et2O
64
5
63
1
62
3
47
49
6665
46
MeMgCl (2 eq.), Et2O
15
Scheme 3.5: Synthesis of dimethyl zirconocenes.
Caution must be taken to avoid an excess of methyl lithium because this will cause
decomposition of the product. In the case of Cp2ZrCl2 1 the weaker Lewis-basic MeMgBr
is used as the methylating agent in order to avoid replacement of the cyclopentadienyl
ligands. Cp2ZrMe2 15 is obtained in 62.1 % yield as an almost colourless solid after
sublimation.
3.1 Synthesis of Zirconocene Complexes 47
3.1.4 Preparation of Cationic Methyl Zirconocenes
The cationic methyl zirconocenes are synthesized from the corresponding dimethyl
zirconocenes by removal of a methyl group via protonolysis with HNBu+3 BPh−4 in a solvent
mixture containing THF as a stabilizing ligand (Scheme 3.6).
Zr MeMe
Zr MeMe Zr Me
Me
Zr MeOSiZr Me
Me
Zr MeO
ZrO
Me
ZrO
MeZrO
Me
Zr MeO
BPh4
BPh4
BPh4
BPh4
BPh4BPh4
MeZr Me
Si MeZr Me
HNBu3BPh4toluene/THF
47 49
66
65
46
14
43 68
70
69
67
26
Scheme 3.6: Synthesis of cationic methyl zirconocenes.
Thus, reaction of Cp2ZrMe2 15 yields Cp2Zr(Me)(thf)+BPh−4 26 (81.9 %, almost
white powder), reaction of C2H4(Ind)2ZrMe2 46 affords C2H4(Ind)2Zr(Me)(thf)+BPh−4
67 (79.8 %, brown-orange solid), from the reaction of Me2C(Cp)(Ind)ZrMe2 47
one obtains Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 (53.4 %, light orange solid),
starting from Me2C(Cp)(Flu)ZrMe2 49 yields Me2C(Cp)(Flu)Zr(Me)(thf)+BPh−4 68
(72.4 %, brown-orange solid), from the reaction of Me2C(Cp)2ZrMe2 66 one obtains
Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 (67.4 %, light beige powder) and the reaction of
Me2Si(Cp)2ZrMe2 65 yields Me2Si(Cp)2Zr(Me)(thf)+BPh−4 69 (32.9%, red-brown solid).
48 3 Results and Discussion
3.2 Olefin Polymerization with Zirconocenes
3.2.1 General Approach
The polymerization of 1-hexene 71 is studied with different zirconocene dichlorides
activated with MAO as the cocatalyst. 1-Hexene is chosen as a model monomer because
(i) as a liquid monomer it is easy to handle on a laboratory scale, (ii) it has a defined
concentration in homogeneous solution, (iii) polymerization is much slower than ethylene
or propene polymerization so that diffusion limitation is much more unlikely, (iv) poly(1-
hexene) 72 is soluble in most organic solvents so that the polymerization proceeds in
homogeneous solution during the whole polymerization time, and (v) polymer character-
ization of poly(1-hexene) by GPC and NMR is convenient because of its solubility in
organic solvents. Therefore, the polymerization of 1-hexene appears to be a suitable tool
for the study of the reaction mechanism details, invalidating the arguments put forward
by Lahti et al.,101,102 Soares et al.116 and Rieger et al.120 (section 1.2.6.3). A similar
approach was chosen by Landis et al. who investigated the polymerization kinetics of
1-hexene polymerization using cationic zirconocene complexes.183
71 CH3n
72
L2ZrCl2/MAOtoluene(cosolvent)
Scheme 3.7: Polymerization of 1-hexene.
It is well known that polymer properties strongly depend on the history of the catalyst
system. Therefore, a standard methodology of temperature control and catalyst activa-
tion is applied. In a typical polymerization experiment, the desired amount of a freshly
prepared stock solution of the zirconocene dichloride in toluene, MAO solution in toluene
and additional reactants or cosolvents are placed in a Schlenk flask. The reaction mixture
is stirred at the desired polymerization temperature Tp for at least 1 h. The polymeriza-
tion is then started by the addition of a gravimetrically determined amount of 1-hexene,
and it is terminated after the polymerization time tp by diluting the reaction mixture
with pentane and pouring it into an aqueous HCl solution. Deviation from this general
procedure, the exact reaction conditions and all experimental data presented in diagrams
are listed in tabular form in the experimental section.
3.2 Olefin Polymerization with Zirconocenes 49
Besides the investigation of catalyst activities, the molecular weight and the MWD of
poly(1-hexene) obtained under the given polymerization conditions are chosen as “probes”
for the nature of the active site, because under a true single-site mechanism, the MWD
of the polymer obtained is expected to be independent of catalyst concentration and
polymerization time, and sensitive with respect to the nature of the active site. A similar
approach has been proposed by Karol et al.184 (Union Carbide), who use the comonomer
distribution in ethylene/1-olefin copolymerization as a “probe” in screening experiments
of various zirconocene polymerization catalysts.
3.2.2 Effect of Monomer Concentration
3.2.2.1 Reaction Order in Monomer Concentration
The reaction order a of the polymerization reaction with respect to the monomer is
determined from series of polymerization experiments with the catalyst systems Cp2ZrCl2
1/MAO and Me2Si(Ind)2ZrCl2 4/MAO via gravimetrical determination of the polymer
yield ∆[Mon] after a comparably short polymerization time (tp = 10 h and tp = 600 s,
respectively) at varying initial monomer concentrations [Mon]0. All other reaction con-
ditions are kept constant ([Zr] = 3 · 10−4 mol L−1; Al/Zr = 5000; Tp = −20 ◦C and
Tp = 60 ◦C, respectively).
A double logarithmic plot of ∆[Mon] vs. [Mon]0 (Figure 3.1) reveals a linear de-
pendence in both cases that allows for a determination of the reaction order. For small
monomer conversion it is (Appendix A.1):
ln∆[Mon] ≈ a · ln [Mon]0 + ln kapp + ln∆t. (3.1)
Reaction orders a1 in the case of Cp2ZrCl2 1/MAO, and a2 in the case of
Me2Si(Ind)2ZrCl2 4/MAO determined by linear regression are
a1 = 0.84 (±0.04)
a2 = 1.06 (±0.08)
Monomer conversion ranges between 15 % and 25 % in all experiments, which is above
the usually applied 5 % conversion that is regarded as a precondition for the approx-
imation. However, a non-linear plot should be observed if the approximation was not
valid. Consequently, the polymerization of 1-hexene with the catalyst systems Cp2ZrCl2
1/MAO and Me2Si(Ind)2ZrCl2 4/MAO is well described as being first order in monomer
50 3 Results and Discussion
-1,0 -0,5 0,0 0,5 1,0 1,5-3,0
-2,5
-2,0
-1,5
-1,0
-0,5
0,0
0,5
1,0
ln(∆
[Mon
])
ln ([Mon]0)
Figure 3.1: Double logarithmic plot of monomer conversion after short polymerization
time vs. initial monomer concentration in 1-hexene polymerization with Cp2ZrCl2 1/MAO
(�), and Me2Si(Ind)2ZrCl2 4/MAO (❍).
concentration within the limits of experimental error. This is in line with the results
reported for 1-hexene polymerization with cationic complexes,183 and in remarkable con-
trast to ethylene and propene polymerizations, in which higher, fractional reaction orders
(typically a = 1.3 − 1.5) are often observed. Apparently, on the basis of the generally
accepted polymerization mechanism (Figure 1.1) one has to conclude that for 1-hexene
as the monomer the coordination of the monomer is the rate-determining step, and no
deviation from this simple picture (e. g. a “trigger-mechanism”) is observed.
3.2.2.2 Monomer Concentration, Molecular Weight and MWD
A plot of molecular weight and MWD of poly(1-hexene) obtained with the catalyst
systems Cp2ZrCl2 1/MAO (Figure 3.2) and Me2Si(Ind)2ZrCl2 4/MAO (Figure 3.3), un-
der the reaction conditions described in the previous section, reveals a slight difference
between the two systems.
With Cp2ZrCl2 as the catalyst, the number average molecular weight Mn of poly(1-
hexene) increases monotonously with the initial monomer concentration [Mon]0 over the
whole concentration range investigated. The molecular weight appears to remain un-
3.2 Olefin Polymerization with Zirconocenes 51
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,51000
2000
3000
4000
5000
Mn
[Mon]0 in mol L-1
1,0
1,5
2,0
2,5
3,0
3,5
4,0
Mw/M
n
Figure 3.2: Molecular weight and MWD of poly(1-hexene) obtained with Cp2ZrCl2
1/MAO vs. monomer concentration after tp = 10 h (�,�) and tp = 5 d (✷,✷).
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,53000
3500
4000
4500
5000
5500
6000
6500
7000
7500
8000
8500
9000
Mn
[Mon]0 in mol L-1
1,0
1,5
2,0
2,5
3,0
3,5
4,0
Mw/M
n
Figure 3.3: Molecular weight and MWD of poly(1-hexene) obtained with
Me2Si(Ind)2ZrCl2 4/MAO vs. monomer concentration after tp = 10 min (�,�) and
tp = 60 min (✷,✷).
52 3 Results and Discussion
changed within the limits of experimental error regardless of partial or complete monomer
conversion after tp = 10 h and tp = 5 d, respectively. The MWD of poly(1-hexene) is not
dependent on monomer concentration for both polymerization times. However, an in-
crease of Mw/Mn is observed as the polymerization is driven to completion.
With Me2Si(Ind)2ZrCl2 4 as the catalyst, an increase of Mn with [Mon]0 is only ob-
tained at low monomer concentration, while at higher monomer concentrations, a satu-
ration behaviour is observed. The MWD is independent of monomer concentration, with
2.5 < Mw/Mn < 3.0 being larger than expected for a Schulz-Flory distribution. Both
Mn and Mw/Mn remain virtually unchanged irrespective of monomer conversion after
tp = 10 min and tp = 60 min.
An increase in molecular weight with increasing monomer concentration, as described
for Cp2ZrCl2 1/MAO, is expected if the propagation reaction is first order in monomer
concentration as determined in the previous section, but the chain termination is zero
order in monomer concentration. This may point to the fact that for Cp2ZrCl2 1 and
Me2Si(Ind)2ZrCl2 4, different chain transfer mechanisms may be predominant, and that
the pathway of chain transfer may depend on monomer concentration in the case of
Me2Si(Ind)2ZrCl2 4.
3.2.2.3 Conclusions
The reaction order in 1-hexene concentration is determined to be a ≈ 1 for both
catalyst systems investigated. No anomalies such as unexpectedly high reaction orders
are observed, as is often reported in ethylene or propene polymerization. Consequently,
the model system chosen is well behaved, and provides a good and simple tool in order
to investigate the effect of different reaction parameters on the nature of the active site.
As a first reaction parameter, the effect of monomer concentration and monomer con-
version on molecular weight and MWD of poly(1-hexene) obtained is examined. The
results may be interpreted as a first indication that, depending on the actual catalyst
system and the reaction conditions, different chain transfer mechanisms are predominant.
A general observation is that MWD are broader than a Schulz-Flory distribution that
should be expected in the case of a single-site propagation mechanism with a chain trans-
fer reaction. This is consistent with occasional reports of other groups (section 1.2.6.3).
The effect of monomer conversion will be examined more thoroughly in the light of poly-
merization kinetics (section 3.2.4.3).
3.2 Olefin Polymerization with Zirconocenes 53
3.2.3 Effect of Catalyst and Cocatalyst Concentrations
3.2.3.1 Reaction Order in Zirconocene Concentration
The reaction order of the polymerization reaction with respect to the zirconocene
catalyst is determined from a series of polymerization experiments with the catalyst
system Cp2ZrCl2 1/MAO and gravimetrical determination of the polymer yield after
a fixed polymerization time at varying catalyst concentrations. All other reaction con-
ditions are kept constant ([Mon]0 = 1.5 mol L−1, [Al] = 1.5 mol L−1, Tp = −20 ◦C).
Two different polymerization times tp = 120 h and tp = 48 h are chosen in order to ex-
tend the investigation to lower/higher zirconocene concentrations at comparably moderate
monomer conversions (Figures 3.4 and 3.5). For matter of reference, the molecular weight
of poly(1-hexene) obtained is also displayed in both cases.
From a comparison of the two experimental series, it is observed that at a low
zirconocene concentration (tp = 120 h, Figure 3.4) a linear dependence of monomer con-
version on the zirconocene concentration is obtained in a first-order plot, while at a higher
zirconocene concentration (tp = 48 h, Figure 3.5) the relationship is clearly nonlinear. The
slope of the regression line in the low concentration case is approximately equal to the
initial slope in the high concentration case (dashed line in Figure 3.5). Over the whole
range of zirconocene concentrations the number average molecular weight Mn decreases
with increasing zirconocene concentration, approaching a lower limit of Mn ≈ 2700.
Assuming that the zirconocene dichloride is quantitatively converted into the active
species, the following equation describing the relation of monomer conversion after a fixed
polymerization time and zirconocene concentration is derived from the first-order rate law
of polymerization (Appendix A.2):
ln[Mon]0[Mon]
= kapp · [Zr∗]z · tp (3.2)
Thus, by keeping the polymerization time tp constant one is able to determine the
reaction order z with respect to the zirconocene catalyst by fitting the experimental data
with a function a · [Zr]b. The values obtained are
z1 = 0.975 (±0.015)
kapp,1 · tp = 0.252 (±0.005)
z2 = 0.482 (±0.057)
kapp,2 · tp = 0.125 (±0.054)
54 3 Results and Discussion
0 2 4 6 8 10 12 14 16 18 200,0
0,5
1,0
1,5
2,0
2,5
ln([
Mon
] 0/[M
on])
[Zr] in 10-5
mol L-1
1000
1500
2000
2500
3000
3500
4000
4500
5000
Mn
Figure 3.4: Monomer conversion after a polymerization time of tp = 120 h vs.
zirconocene concentration (Cp2ZrCl2 1/MAO, Tp = −20 ◦C).
0 20 40 60 80 1000,0
0,5
1,0
1,5
2,0
2,5
ln([
Mon
] 0/[M
on])
[Zr] in 10-5
mol L-1
1000
1500
2000
2500
3000
3500
4000
4500
5000
Mn
Figure 3.5: Monomer conversion after a polymerization time of tp = 48 h vs. zirconocene
concentration (Cp2ZrCl2 1/MAO, Tp = −20 ◦C); black line from function fitting, dashed
black line from regression in Figure 3.4.
3.2 Olefin Polymerization with Zirconocenes 55
Apparently, at a zirconocene concentration below [Zr] = 1 · 10−4 mol L−1, the system
is well described assuming first order kinetics in catalyst concentration (z1 ≈ 1) as is
expected from the generally accepted polymerization mechanism (section 1.2). However,
at higher zirconocene concentrations a fractional reaction order z2 ≈ 0.5 in zirconocene
concentration is observed. The apparent rate constants of propagation are kapp,1 = 5.83 ·10−7 s−1 and kapp,2 = 7.23 · 10−7 s−1, which are equal within the limits of experimental
error. The observed phenomenon may be due to one of the following reasons:
(i) At a higher zirconocene concentration the polymerization mechanism is not first
order in monomer concentration, so that the first order plot in Figure 3.5 is not a
suitable description of the system.
(ii) At a higher zirconocene concentration the conversion of the zirconocene dichloride
is not quantitative, in contrast to the situation at a lower zirconocene concentration.
(iii) At a higher zirconocene concentration a second (active) species is formed in a bi-
molecular reaction from the original one.
In any case, it may be concluded that depending on the zirconocene concentration
different species are formed, irrespective whether or not they are active species or they
coexist in equilibrium. Of course, on the basis of the experimental data presented none
of the possibilities are ruled out. However, (i) is unlikely because the experimental de-
termination of the reaction order with respect to the monomer is carried out at a com-
parably high zirconocene concentration of [Zr] = 3 · 10−4 mol L−1 (section 3.2.2.1), and
because the effect should also be observed at low zirconocene concentration. Also, (ii)
appears to be rather questionable, because even at the highest zirconocene concentration
the catalyst/cocatalyst ratio is Al/Zr = 1670. In the case of (iii), kinetics are described
in the sense of a parallel propagation with two active species, assuming that one active
species [Zr]1 is converted to another (active) species [Zr]2 in a bimolecular reaction and
both species are in an equilibrium (Appendix A.3):
ln[Mon]0[Mon]
=
(2kp,1 − kp,2
2√2Kzr
· [Zr]120 +
kp,2
2· [Zr]0
)· tp (3.3)
From the fitting of the experimental data with the function a1 · [Zr]z + a2 · [Zr] thedetermined values are
z = 0.794 (±0.288)
a1 = 0.225 (±0.087)
a2 = −0.066 (±0.150)
56 3 Results and Discussion
In this case a plot is also obtained which is well in line with the experimental data.
However, it is not a sensible result, because a2 < 0 means that the rate constant kp,2 must
be negative. The standard deviation of a2 may also allow for the conclusion that a2 is 0.
Keeping z = 0.5 fixed yields the values
a1 = 0.257 (±0.033)
a2 = −0.0031 (±0.0046)
This is already very close to the result achieved by the fitting with the function a· [Zr]b,with a2 < 0 and a2 ≈ 0. Of course, when a2 = 0 is kept fixed, then the values determined
for z and a1 are exactly the same as with the function a · [Zr]b. Apparently, this is alreadya reasonable approximation. Consequently, the assumption of an equilibrium between two
zirconocene species, one of which is generated from the other in a bimolecular reaction,
gives a suitable description of the experimental data, but only if the former exhibits a very
low polymerization activity kp,2 ≈ 0. It is worth noting that this result supports exactly
the results of kinetic experiments reported byMuhlhaupt et al.,104 who found a bimolecular
catalyst deactivation process. As previously mentioned, none of the other possibilities are
ruled out, and the results from function fitting must be interpreted with caution because of
approximations in the determination of the respective equation. Nevertheless, the results
indicate that all of the experimental data are explained in a most complete fashion by
this model, because it also includes the linear relation of monomer conversion as the other
limiting case at low zirconocene concentrations.
3.2.3.2 Zirconocene Concentration, Molecular Weight and MWD
The effect of catalyst and cocatalyst concentration on the molecular weight and MWD
of poly(1-hexene) obtained is investigated in series of polymerization experiments (Figures
3.6 and 3.7) with the catalyst system Me2Si(Ind)2ZrCl2 4/MAO, [Mon] = 1.5 mol L−1,
Tp = 60 ◦C and
(i) varying the MAO concentration at a fixed zirconocene concentration of [Zr] = 3 ·10−4 mol L−1 (series A),
(ii) varying the zirconocene concentration and the MAO concentration at a fixed ratio
Al/Zr = 5000 (series B),
(iii) varying the zirconocene concentration at a fixed MAO concentration of [Al] = 7.5 ·10−1 mol L−1 (series C).
3.2 Olefin Polymerization with Zirconocenes 57
0,0 0,5 1,0 1,5 2,0 2,5 3,0
1,8
2,0
2,2
2,4
2,6
2,8
3,0
[Al] in mol L-1
Mw/M
n
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Al:Z
r=
250
Al:Z
r=
60005
000400
0
200
0
100
0
500
Al:Z
r=
125*
[Zr] = 6·10-4
mol L-1
3·10-4
mol L-1
2·10-4
mol L-1
1·10-4
mol L-1
5·10-5
mol L-1
[Zr] = 2·10-5
mol L-1
Mn
Figure 3.6: Molecular weight and MWD of poly(1-hexene) obtained with the catalyst
system Me2Si(Ind)2ZrCl2 4/MAO in series A (❍,❍) and series B (�,�); dashed line marks
identical reaction conditions; ∗ at Al/Zr = 125 polymer yield is below 2 %.
0,0 2,0 4,0 6,0 8,0 10,0 12,00
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Mn
[Zr] in 10-4
mol L-1
1,8
2,0
2,2
2,4
2,6
2,8
3,0
150
00
375
00
750
0 250
0
150
0
Al/Z
r=
750
[Al] = 3.0 mol L-1
1.5 mol L-1
1.0 mol L-1
0.5 mol L-1
0.25 mol L-1
[Al] = 0.1 mol L-1
Mw
/Mn
Figure 3.7: Molecular weight and MWD of poly(1-hexene) obtained with the catalyst
system Me2Si(Ind)2ZrCl2 4/MAO in series B (�,�) and series C (�,�); dashed line marks
identical reaction conditions.
58 3 Results and Discussion
1,02,0
3,04,0
5,0
6,0
0,51,0
1,52,0
2,53,0
2000
3000
4000
5000
6000
7000
8000
9000
10000
11000
12000
[Al] in mol L-1
[Zr] in 10-4 mol L-1
Mn
Figure 3.8: Molecular weight of poly(1-hexene) obtained with the catalyst system
Me2Si(Ind)2ZrCl2 4/MAO at variable MAO and zirconocene concentrations; only ap-
proximate functions from Figures 3.6 and 3.7 are displayed for clarity.
The variation of both the zirconocene and the MAO concentration in series B (Figures
3.6 and 3.7) has a strong effect on molecular weight and MWD of poly(1-hexene) obtained.
Increasing concentration leads to a drastic decrease in molecular weight and at the same
time to a remarkable increase of polydispersity indexes from Mw/Mn ≈ 2 to Mw/Mn ≈ 3.
The latter increase indicates that the system changes its behaviour from one that is ex-
pected for a single-site polymerization with random chain transfer reaction to a behaviour
completely unexpected.
Variation of either the MAO concentration at a fixed zirconocene concentration in
series A (Figure 3.6) or of the zirconocene concentration at a fixed MAO concentration in
series C (Figure 3.7) has a qualitatively similar, yet not so strong effect. As is seen in series
A, the molecular weight of poly(1-hexene) is generally smaller and the MWD is generally
3.2 Olefin Polymerization with Zirconocenes 59
0,0
1,02,0
3,04,0
5,06,0
0,00,5
1,01,5
2,02,5
3,0
1,9
2,0
2,1
2,2
2,3
2,4
2,5
2,6
2,7
2,8
2,9
3,0
Mw/M
n
[Zr] in 10-4 mol L-1
[Al] in mol L-1
Figure 3.9: MWD of poly(1-hexene) obtained with the catalyst system Me2Si(Ind)2ZrCl2
4/MAO at variable MAO and zirconocene concentrations; only approximate functions
from Figures 3.6 and 3.7 are displayed for clarity.
broader than in series B. This is due to the fact that the zirconocene concentration in
this series is constantly high compared to series B. Analogously, in series C the molecular
weight is higher and the MWD is narrower compared to series B, as long as the MAO
concentration is lower than in series B and vice versa. The dashed lines in Figures 3.6
and 3.7 mark the points of identical reaction conditions in the respective series.
It is worth noting that the MAO concentration cannot be changed at will. The lower
limit of the MAO concentration at a given zirconocene concentration is determined by
the minimum Al/Zr ratio necessary in order to generate a catalytically active system.
At Al/Zr ≤ 125 in series A no polymerization occurs. Obviously, the generation of the
active species does not take place under these conditions. The reaction mixtures remain
yellow as are solutions of Me2Si(Ind)2ZrCl2 4 in toluene. Consistent with the results of
60 3 Results and Discussion
UV studies by Deffieux et al.64,65, 67 and Brintzinger et al.,24 a colour change to orange-
red indicates the formation of the active species at higher Al/Zr ratios. Catalyst activity
continues to increase with increasing MAO concentration until it reaches a maximum
plateau at Al/Zr � 1000. Thus, a high MAO concentration is necessary in order to
achieve maximum catalyst activity.
A unifying interpretation of series A, B and C is less complex than it may seem at first
glance. All three series are combined in 3D plots (Figures 3.8 and 3.9), in which, for clarity
reasons, only the approximate functions are shown. Both, the zirconocene concentration
and the MAO concentration determine the molecular weight and the MWD of poly(1-
hexene) obtained. Generally, the lower both the zirconocene concentration and the MAO
concentration are, the higher is the molecular weight and the narrower is the MWD. In
other words, the lower the catalyst and the cocatalyst concentrations are, the more the
system behaves as expected for a true single site mechanism.
In contrast to catalyst activity, molecular weight and MWD are expected to be in-
dependent to the catalyst concentration if the nature of the active species remains un-
changed. Not only do the experimental data reveal that this is clearly not the case. The
data also allow for a further and very profound conclusion. The Molecular weight and
MWD of poly(1-hexene) are obviously affected by catalyst and cocatalyst concentration via
two distinct pathways.
(i) Molecular weight is decreased and MWD is broadened with increasing zirconocene
concentration and at constant MAO concentration, i. e. with decreasing Al/Zr ratio
(series A). This may be due to incomplete conversion into the active species, but
only if the excess of catalyst precursor alters the nature of the active species or it is
considered to be active for polymerization itself.
(ii) Furthermore, molecular weight is reduced and MWD is broadened with increasing
MAO concentration at a constant zirconocene concentration, i. e. with increasing
Al/Zr ratio (seriesC), which means under the contrary change in reaction conditions
as compared to (i). Consequently, there must be a second pathway of altering the
nature of the active species by excess MAO, e.g. complexation of MAO.
Remarkably, the respective reaction conditions and the possible explanations resemble
the pathways of catalyst deactivation discussed by Muhlhaupt et al., Chien et al. and
Remple et al. (section 1.2.6.2).
3.2 Olefin Polymerization with Zirconocenes 61
The most illustrative information is provided by the GPC plots (Figure 3.10) of poly(1-
hexene) obtained at different zirconocene and MAO concentrations with a fixed ratio
Al/Zr = 5000, that correspond to the molecular weight and MWD data of series B
(Figures 3.6 and 3.7).
1015
2025
3035
40elution volume in mL
[Zr] in 10-6 mol L-1
2050
100200
300600
Figure 3.10: GPC plots of poly(1-hexene) in series B.
At the lowest zirconocene concentration [Zr] = 2 ·10−5 mol L−1 the GPC plot reveals a
unimodal and fairly narrow peak. The MWD well reflects the fact that at this zirconocene
concentration a polydispersity index Mw/Mn = 1.98 is determined. However, while the
peak maximum of this main peak appears to remain more or less unchanged, a low mole-
cular weight shoulder develops with increasing relative intensity at increasing zirconocene
concentration. Thus, at a zirconocene concentration [Zr] = 1 · 10−4 mol L−1 and above,
the obtained MWD are clearly bimodal.
This observation correlates with the observed decrease in molecular weight and the in-
crease in the polydispersity indexes of series B. The bimodal MWD can only be explained
in terms of two distinct pathways of chain propagation and chain transfer. Consequently,
either a second active species is formed or, at least, the pathway of propagation and of
termination of part of the active species is altered.
62 3 Results and Discussion
1000 10000 100000
M
1000 10000 100000
M
1000 10000 100000
M
1000 10000 100000
M
1000 10000 100000
M
1000 10000 100000
M
[Zr] = 20 µmol L-1 [Zr] = 50 µmol L-1
[Zr] = 100 µmol L-1 [Zr] = 200 µmol L-1
[Zr] = 300 µmol L-1 [Zr] = 600 µmol L-1
Figure 3.11: Fitting of the bimodal GPC peaks of poly(1-hexene) obtained with
Me2Si(Ind)2ZrCl2 4/MAO at different zirconocene and MAO concentrations (series B).
3.2 Olefin Polymerization with Zirconocenes 63
A peak fitting software module integrated in the GPC software has been developed185
that allows for a deconvolution of the bimodal MWD by fitting with a superposition of two
(or more) appropriate unimodal peak functions (Figure 3.11). The resulting molecular
weights Mn, polydispersity indexes Mw/Mn and the relative peak area Ahigh/Alow of the
constituting peaks are displayed in Table 3.2.3.2.
Table 3.1: Molecular weight Mn, polydispersity indexes Mw/Mn, and relative peak area
Ahigh/Alow of the low and the high molecular weight faction in 1-hexene polymerization
with Me2Si(Ind)2ZrCl2 4/MAO (series B), determined via peak deconvolution.
[Zr]
10−5 mol L−1
Mn
exp.
Mw/Mn
exp.
Mn
low
Mw/Mn
low
Mn
high
Mw/Mn
high
Ahigh
Alow
2 9417 1.98 5180 1.58 19920 1.76 2.77
5 7908 2.18 3890 1.77 17110 1.67 2.48
10 6380 2.36 3110 1.58 14360 1.70 2.28
20 4849 2.53 2690 1.73 12930 1.67 1.46
30 4400 2.68 2470 1.82 12466 1.73 1.34
60 3562 2.90 2311 2.13 12990 1.65 0.75
0 10 20 30 40 50 600
5000
10000
15000
20000
25000
Mn
[Zr] in 10-5 mol L-1
0,0
0,5
1,0
1,5
2,0
2,5
3,0
Ahi
gh/A
low
Figure 3.12: Plot of the experimentally determined (�) and the deconvoluted Mn of the
high (�) and the low molecular weight fraction (�), and relative peak areas (�) of the
deconvoluted peaks in series B.
64 3 Results and Discussion
The results indicate that the decrease in the molecular weight of poly(1-hexene) is due
to both a decrease in the molecular weight of the main (high molecular weight) fraction
and an increasing portion of a low molecular weight fraction (Figure 3.12). However,
while the molecular weight of both fractions appears to be virtually independent of the
zirconocene concentration at concentrations above [Zr] = 1 · 10−4 mol L−1, the experi-
mentally determined molecular weight still decreases slightly, which correlates with a still
changing relative portion of the two fractions.
3.2.3.3 Conclusions
All of these findings imply that the dependence of catalyst activity on the zirconocene
concentration is not just due to incomplete conversion of catalyst precursors into the
active species or to catalyst deactivation reactions, but to a change in the nature of the
active site.
(i) If the active species remained unchanged at high and low zirconocene concentration,
but its concentration was not proportional to the zirconocene dichloride concentra-
tion, and if catalyst precursors are inactive for polymerization, then no change in
molecular weight and MWD should be observed. This is in contradiction to the ex-
perimental results in series A and B, and especially with the observation of bimodal
MWD in the GPC plots of series B.
(ii) Furthermore, the highest possible Al/Zr ratio should guarantee for a complete con-
version into the active species and therefore true single-site-behaviour. However,
just the opposite behaviour is observed in series B and C.
(iii) Incomplete conversion of catalyst precursors into the active species cannot account
for bimodal MWD in the GPC plots of series A if the former are not active for
polymerization.
While it is still impossible to decide whether two active species coexists or just the
reactivity (the pathway of propagation and of termination) of part of the active species
is altered, the results allow for the further conclusion that molecular weight and MWD
of poly(1-hexene) are altered by catalyst and cocatalyst concentrations via two distinct
pathways, one of which is predominant at a low MAO concentration (low Al/Zr ratio),
the other one at a high MAO concentration and high overall catalyst concentration. An
interpretation on the basis of the findings concerning the activation reaction with MAO
3.2 Olefin Polymerization with Zirconocenes 65
(section 1.2.5.1) and the catalyst deactivation reactions (section 1.2.6.2) is that in one
case dinuclear species are formed by incomplete conversion of catalyst precursors into
the active species, while in the other case complexation of the active species with MAO
occurs. At this point neither of these assumptions has been proved. However, in the light
of the results of molecular weight determination as a sensitive “probe” for the nature of
the active site, a common explanation of all experimental findings is given if the proposed
“inactive” species are considered not to be virtually inactive, but rather less active. The
results are unexpected if zirconocenes are regarded as single-site catalysts. Apparently,
they are not.
3.2.4 Polymerization Kinetics
The kinetics of 1-hexene polymerization are investigated with the catalysts Cp2ZrCl2
1 and Me2Si(Ind)2ZrCl2 4. Interpretation of the kinetic data has to take into account
the fact that it is not feasible to take samples from one single polymerization experiment,
but that each data point is a separate polymerization experiment. Slightly deviating
experimental conditions giving rise to larger scatter of data are inevitable.
3.2.4.1 Zirconocene Concentration and Catalyst Activity
The kinetics of 1-hexene polymerization at different zirconocene concentrations are
carried out with the catalyst system Me2Si(Ind)2ZrCl2 4/MAO, at [Mon]0 = 1.5 mol L−1,
[Al] = 1.5 mol L−1 and Tp = 60 ◦C. First order plots of monomer conversion vs. poly-
merization time (Figure 3.13) reveal a linear relationship up to high monomer conversion,
well in line with the determination of the reaction order in 1-hexene concentration (section
3.2.2.1).
From the kinetic plots the apparent rate constants of propagation kapp are determined
via linear regression of the experimental data up to moderate monomer conversion. The
values of kapp as well as the normalized rate constants k = kapp/[Zr] are listed in Table
3.2. Apparently, catalyst activity expressed in k = kapp/[Zr] decreases with increasing
zirconocene concentration. Even if the comparably large deviations of k = kapp/[Zr] of up
to 6 % are taken into account, the overall loss in catalytic performance of approximately
20% over a zirconocene concentration range of an order of one magnitude cannot be
neglected. The approximately linear relation in Figure 3.14 implies that catalyst activity
increases almost exponentially as the absolute zirconocene concentration is reduced.
66 3 Results and Discussion
0 100 200 300 400 5000
1
2
3
4ln
([M
on] 0/[
Mon
])
tp
in min
Figure 3.13: Polymerization kinetics with the catalyst system Me2Si(Ind)2ZrCl2 4/MAO
at Tp = 60 ◦C and [Zr] = 3 · 10−4 mol L−1 (�), [Zr] = 5 · 10−5 mol L−1 (❍) and [Zr] =
1 · 10−5 mol L−1 (�).
1 1 0 100
8,0
8,5
9,0
9,5
10,0
10,5
11,0
11,5
k app/
[Zr]
inL
mol
-1s-1
[Zr] in 10-5
mol L-1
Figure 3.14: Normalized rate constants of propagation k = kapp/[Zr] in 1-hexene poly-
merization with Me2Si(Ind)2ZrCl2 4/MAO (Tp = 60 ◦C) as a function of zirconocene
concentration.
3.2 Olefin Polymerization with Zirconocenes 67
Table 3.2: Apparent rate constants of propagation kapp and normalized rate constants
kapp/[Zr] at different zirconocene concentrations.
[Zr]
10−5 mol L−1
kapp10−3 s−1
kapp/[Zr]
L mol−1 s−1
1 0.105 (±0.003) 10.52 (±0.32)5 0.470 (±0.030) 9.42 (±0.60)30 2.550 (±0.164) 8.50 (±0.55)
Of course, the observed phenomenon may be due to either incomplete conversion of
catalyst precursors into the active species at higher zirconocene concentrations (lower
Al/Zr ratio), to a bimolecular deactivation process as postulated by Muhlhaupt et al.
(section 1.2.6.2), or to the existence of a second, less active species in equilibrium with
the first one, which is favoured at higher zirconocene concentrations.
In any case, if the decrease in polymerization activity with increasing catalyst con-
centration is regarded to be significant, then one has to conclude that the active species
prevailing at low zirconocene concentration is not the only zirconocene species in the
system, or the nature of (part of) the active species unexpectedly changes with zirconocene
concentration, which is consistent with the results of the previous sections.
3.2.4.2 Kinetics at Different Polymerization Temperatures
The kinetics of 1-hexene polymerization with the catalyst systems Cp2ZrCl2 1/MAO
and Me2Si(Ind)2ZrCl2 4/MAO are investigated in series of polymerization experiments
with [Mon]0 = 2 mol L−1, [Zr] = 3·10−4 mol L−1, Al/Zr = 5000 at different polymerization
temperatures.
First order plots of monomer conversion as a function of polymerization time (Figures
3.15 and 3.16) reveal a linear dependence up to a moderately high monomer conversion.
Apparently, the polymerization of 1-hexene is well described as being first order in
monomer concentration irrespective of the polymerization temperature. This is expected
from the determination of the reaction order in monomer concentration (section 3.2.2.1)
and consistent with the behaviour in kinetic experiments at different catalyst concentra-
tions (section 3.2.4.1).
68 3 Results and Discussion
Neither the actual catalyst concentration [Zr∗] nor the reaction order z with respect
to the catalyst is known exactly. Therefore, the rate law for the propagation reaction is
denoted with the apparent rate constant kapp:
−d[Mon]dt
= kp · [Zr∗]z · [Mon] = kapp · [Mon] (3.4)
The apparent rate constant kapp at different polymerization temperatures are obtained
from the slopes of the regression lines up to moderately high monomer conversion and
listed in Table 3.3. Eyring plots of kapp reveal linear relationships in both cases (Figures
3.17 and 3.18).
Table 3.3: Apparent rate constants kapp for 1-hexene polymerization with Cp2ZrCl2
1/MAO and Me2Si(Ind)2ZrCl2 4/MAO at different polymerization temperatures Tp.
Cp2ZrCl2 1 Me2Si(Ind)2ZrCl2 4
Tp
◦C
kapp10−6 s−1
Tp
◦C
kapp10−6 s−1
−20 6.22 (±0.31) 0 85 (±2)−10 20.42 (±0.81) 20 302 (±5)0 46.36 (±1.11) 30 622 (±20)
40 1220 (±65)50 2980 (±60)60 4610 (±486)
The activation parameters of the propagation reaction for 1-hexene polymerization
with the catalyst systems Cp2ZrCl2 1/MAO and Me2Si(Ind)2ZrCl2 4/MAO are de-
termined from the Eyring plots of the apparent rate constants kapp (Figures 3.17 and
3.18) via linear regression. With the assumption that the catalyst concentration [Zr] and
the reaction order z are equal at all polymerization temperatures, the activation enthalpies
∆H‡p and the activation entropies ∆S
‡p (further assuming [Zr
∗] = [Zr]0 = 3 · 10−4 mol L−1,
z = 1) are calculated from the activation energies Ea,p and the temperature factors k∞p
with the appropriate equations (Appendix A.4) and listed in Table 3.4.
These values must be interpreted with caution, because the temperature intervals
used as the basis for the calculation are only 60 ◦C and 20 ◦C, respectively. This is
smaller than a 100 ◦C interval which is usually regarded as a safe basis for an evalua-
tion of kinetic parameters in physical chemistry. Furthermore, the number of data points
is very small in the case of Cp2ZrCl2 1. However, the standard deviations of the rate
3.2 Olefin Polymerization with Zirconocenes 69
0 10 20 30 40 50 60 700,0
0,5
1,0
1,5
2,0
2,5
ln([
Mon
] 0/[M
on])
tp
in h
Figure 3.15: Polymerization of 1-hexene with the catalyst system Cp2ZrCl2 1/MAO at
0 ◦C (�), −10 ◦C (❍) and −20 ◦C (�).
0 50 100 150 200 250 3000,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
ln([
Mon
] 0/[M
on])
tp
in min
Figure 3.16: Polymerization of 1-hexene with the catalyst system Me2Si(Ind)2ZrCl2
4/MAO at 60 ◦C (�), 50 ◦C (✷), 40 ◦C (�), 30 ◦C (❍), 20 ◦C (�) and 0 ◦C (�).
70 3 Results and Discussion
3,6 3,7 3,8 3,9 4,0
-8,0
-7,5
-7,0
-6,5
-6,0
-5,5ln
k app
1/T in 10-3
K-1
Figure 3.17: Eyring plot of the apparent rate constants of propagation kapp for the
catalyst system Cp2ZrCl2 1/MAO.
2,8 3,0 3,2 3,4 3,6 3,8
-6
-5
-4
-3
-2
-1
0
lnk ap
p
1/Tp in 10-3
K-1
Figure 3.18: Eyring plot of the apparent rate constants of propagation kapp for the
catalyst system Me2Si(Ind)2ZrCl2 4/MAO.
3.2 Olefin Polymerization with Zirconocenes 71
Table 3.4: Activation parameters of 1-hexene polymerization with Cp2ZrCl2 1/MAO
and Me2Si(Ind)2ZrCl2 4/MAO; ∆H‡p and ∆S
‡p at 293 K.
CatalystEa,p
kJ mol−1ln k∞p
∆H‡p
kJ mol−1
∆S‡pJ mol−1 K−1
Cp2ZrCl2 1 56.1 (±3.0) 22.8 (±2.1) 53.7 (±3.0) −98 (±17)Me2Si(Ind)2ZrCl2 4 52.7 (±4.6) 21.8 (±1.2) 50.3 (±4.6) −107 (±10)
constants are apparently small, the Eyring plots exhibit good linear relations, and the
resulting activation parameters are reasonable in comparison to the corresponding acti-
vation parameters determined in ethylene polymerization (section 1.2.1). The activation
enthalpies are higher than in ethylene polymerization for both catalysts. This reflects the
fact that higher 1-olefins such as 1-hexene are polymerized much more slowly. Just like
in ethylene polymerization, the stereospecific catalyst Me2Si(Ind)2ZrCl2 4 polymerizes
1-hexene faster than does Cp2ZrCl2 1, due to its lower activation enthalpy. The slightly
more negative activation entropy in the case of Me2Si(Ind)2ZrCl2 4 may point to the fact
that the sterically more demanding ligand backbone imposes higher sterical constraints
to the transition state.
3.2.4.3 Polymerization Kinetics, Molecular Weight and MWD
In order to evaluate the effect of polymerization time, or rather monomer conversion
on the nature of the active species or on the pathway of polymerization, all kinetic ex-
periments are examined concerning the molecular weight and the MWD of poly(1-hexene)
obtained under the respective reaction conditions and after different polymerization times
tp. The experimental data is taken the following series of polymerization experiments:
(i) Polymerization kinetics with the catalyst system Me2Si(Ind)2ZrCl2 4/MAO at diffe-
rent polymerization temperatures (Figures 3.19 and 3.20, compare section 3.2.4.2).
(ii) Polymerization kinetics with the catalyst system Cp2ZrCl2 1/MAO at different poly-
merization temperatures (Figures 3.21 and 3.22, compare section 3.2.4.2).
(iii) Polymerization kinetics with the catalyst system Me2Si(Ind)2ZrCl2 4/MAO at Tp =
60 ◦C at different zirconocene concentrations (Figures 3.23 and 3.24, compare section
3.2.4.1).
72 3 Results and Discussion
0 10 20 30 40 50 60 70 80 90 1000
5
10
15
20
25
30
Mn/1
000
yield in %
Figure 3.19: Molecular weight Mn of poly(1-hexene) obtained with the catalyst system
Me2Si(Ind)2ZrCl2 4/MAO at 60 ◦C (�), 50 ◦C (✷), 40 ◦C (�), 30 ◦C (❍), 20 ◦C (�) and
0 ◦C (�) as a function of polymer yield.
0 20 40 60 80 1001,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
Mw/M
n
yield in %
Figure 3.20: Polydispersity index Mw/Mn of poly(1-hexene) obtained with the catalyst
system Me2Si(Ind)2ZrCl2 4/MAO at 60 ◦C (�), 50 ◦C (✷), 40 ◦C (�), 30 ◦C (❍), 20 ◦C (�)
and 0 ◦C (�) as a function of polymer yield.
3.2 Olefin Polymerization with Zirconocenes 73
0 10 20 30 40 50 60 70 80 90 1000
1
2
3
4
5
6
Mn/1
000
yield in %
Figure 3.21: Molecular weight Mn of poly(1-hexene) obtained with the catalyst system
Cp2ZrCl2 1/MAO at 0 ◦C (�), −10 ◦C (❍) and −20 ◦C (�) as a function of polymer yield.
0 20 40 60 80 1001,0
1,2
1,4
1,6
1,8
2,0
2,2
2,4
2,6
2,8
3,0
Mw/M
n
yield in %
Figure 3.22: Polydispersity index Mw/Mn of poly(1-hexene) obtained with the catalyst
system Cp2ZrCl2 1/MAO at 0 ◦C (�), −10 ◦C (❍) and −20 ◦C (�) as a function of polymer
yield.
74 3 Results and Discussion
0 10 20 30 40 50 60 70 80 90 1000
2
4
6
8
10
12
Mn/1
000
yield in %
Figure 3.23: Molecular weight Mn of poly(1-hexene) obtained with the catalyst system
Me2Si(Ind)2ZrCl2 4/MAO at Tp = 60 ◦C and [Zr] = 3 · 10−4 mol L−1 (�), [Zr] = 5 ·10−5 mol L−1 (❍) and [Zr] = 1 · 10−5 mol L−1 (�) as a function of polymer yield.
0 20 40 60 80 1001,0
1,2
1,4
1,6
1,8
2,0
2,2
2,4
2,6
2,8
3,0
Mw/M
n
yield in %
Figure 3.24: Polydispersity index Mw/Mn of poly(1-hexene) obtained with the catalyst
system Me2Si(Ind)2ZrCl2 4/MAO at Tp = 60 ◦C and [Zr] = 3 · 10−4 mol L−1 (�), [Zr] =
5 · 10−5 mol L−1 (❍) and [Zr] = 1 · 10−5 mol L−1 (�) as a function of polymer yield.
3.2 Olefin Polymerization with Zirconocenes 75
First of all, by summarizing all molecular weight and MWD data in all experimental
series, it is definitely observed that neither the molecular weight nor the MWD of poly(1-
hexene) obtained is a function of monomer conversion. In general, both molecular weight
Mn and polydispersity indexes Mw/Mn are constant throughout polymerization time with-
in the limits of experimental error and the behaviour is as expected, that is molecular
weight increases with decreasing polymerization temperature and polydispersity indexes
are Mw/Mn ≈ 2.
Of course, there are a few exceptions to this general observation and a few peculiarities
that need to be discussed. One has to keep in mind the fact that all data points are
separate experiments, not samples taken from the same experiment. This means that on
one hand deviations within a series may be due to slightly differing reaction conditions.
On the other hand, it also means that if a whole series differs from the expectations, this
cannot be attributed to experimental error.
In all 1-hexene polymerizations with the catalyst system Me2Si(Ind)2ZrCl2 4/MAO at
different polymerization temperatures (Figures 3.19 and 3.20) molecular weight and MWD
are constant, with Mw/Mn ≈ 2, apart from the experiments carried out at Tp = 50 ◦C. In
this case, extraordinarily large polydispersity indexes 3.5 < Mw/Mn < 4.7 are observed,
and the average molecular weight is lower than at Tp = 60 ◦C. It must be said that while
the MWD of poly(1-hexene) obtained at Tp = 50 ◦C is visually broad in the GPC plots,
the MWD of poly(1-hexene) obtained at Tp = 60 ◦C is even bimodal, which is not reflected
in the polydispersity indexes of the series. However, in all experiments at Tp = 60 ◦C and
different zirconocene concentrations (Figures 3.23 and 3.24) the polydispersity indexes
determined are 2.0 < Mw/Mn < 2.6.
Similarly, in all 1-hexene polymerizations with the catalyst system Cp2ZrCl2 1/MAO
at different polymerization temperatures (Figures 3.21 and 3.22) nothing unusual is ob-
served, with the exception of polymerizations carried out at Tp = −20 ◦C. In this case,
at a monomer conversion below 30 % the MWD is broad, while at a higher conversion a
normal MWD is observed, however, the molecular weights are too low. Even if the two
experiments at low monomer conversion are not considered for evaluation, then all other
experiments show deviation from the expected behaviour with respect to the molecular
weight.
76 3 Results and Discussion
3.2.4.4 Conclusions
Consistently with the determination of the reaction order in monomer concentration,
polymerization kinetics reveal that the polymerization is well described as being first
order in monomer concentration. Eyring plots of the apparent rate constants kapp at
different polymerization temperatures show linear relations, and the activation parameters
determined are reasonable. In conclusion, from the view of polymerization kinetics, no
unexpected behaviour is observed, with the exception of the reduced catalytic activity at
increasing zirconocene concentrations.
A broadening of MWD as a function of monomer conversion as occasionally reported by
other groups (section 1.2.6.3) is definitely not observed. Molecular weight and MWD are
generally constant throughout polymerization time, as expected for a single-site non-living
polymerization mechanism. In other words, the polymerization of 1-hexene is generally
“well-behaved” under the reaction conditions applied in this study. From the results
presented so far, there is no reason to take either the generally accepted polymerization
mechanism (Figure 1.1) or the experimental setup into question.
However, an unexpected broadening of MWD of poly(1-hexene) is observed at certain
polymerization temperatures, depending on the catalyst system applied. The general
approach chosen in this study (section 3.2.1) and the reasonable results render it unlikely
to assign these anomalies to the experimental setup. More probably, it is a consequence of
the polymerization mechanism itself. Therefore, the effect of polymerization temperature
with respect to molecular weight and MWD of poly(1-hexene) will be investigated in
detail in section 3.2.5.
3.2.5 Effect of Polymerization Temperature
The effect of polymerization temperature on the molecular weight and the MWD
of poly(1-hexene) is investigated in series of polymerization experiments with the
catalyst systems Cp2ZrCl2 1/MAO, C2H4(Ind)2ZrCl2 3/MAO, Me2Si(Ind)2ZrCl2 4/MAO,
Me2C(Cp)(Flu)ZrCl2 5/MAO and Me2Si(Cp)2ZrCl2 62/MAO.
All experimental conditions, apart from the polymerization temperature, are kept
constant ([Zr] = 3 · 10−4 mol L−1, Al/Zr = 5000, [Mon]0 = 2 mol L−1). Polymer proper-
ties strongly depend on the history of the catalyst system. For this reason, a standard
methodology of temperature control and catalyst activation slightly deviating from the
aforementioned general procedure is applied to all experiments. A freshly prepared stock
3.2 Olefin Polymerization with Zirconocenes 77
solution of the zirconocene dichloride in toluene is added to the MAO solution in toluene
at 0◦C, and the mixture is stirred for at least 60 min at this temperature. Then, the mix-
ture is kept under stirring at the desired polymerization temperature for at least 60 min
before polymerization is started by the addition of 1-hexene. Termination and workup
are carried out as usual.
3.2.5.1 Molecular Weight and MWD
An Eyring plot of the molecular weight of poly(1-hexene) obtained with different
catalyst systems (Figure 3.25) reveals the following observations:
(i) The molecular weight of poly(1-hexene) increases strictly monotonously with de-
creasing polymerization temperature for all catalyst systems.
(ii) In the case of the aspecific catalyst systems Cp2ZrCl2 1/MAO and Me2Si(Cp)2ZrCl2
62/MAO sigmoidally shaped plots are obtained. At the upper and the lower limits
of the temperature interval investigated, the curves appear to approach linear lim-
iting functions with a comparably small slope. Within the transition temperature
interval, a drastic increase in molecular weight with decreasing polymerization tem-
perature is observed.
(iii) In the case of the stereospecific catalyst systems C2H4(Ind)2ZrCl2 3/MAO,
Me2Si(Ind)2ZrCl2 4/MAO and Me2C(Cp)(Flu)ZrCl2 5/MAO the plots are not sig-
moidally shaped. At the low temperature limit of the temperature interval investi-
gated, a linear regime with a smaller slope is observed, but not at the high tem-
perature limit. However, it cannot be excluded that for these catalyst systems the
high temperature limit is situated at temperatures beyond the temperature interval
investigated, which is limited by the boiling point of 1-hexene (b. p. = 67 ◦C).
(iv) The plots determined for the aspecific catalyst systems Cp2ZrCl2 1/MAO and
Me2Si(Cp)2ZrCl2 62/MAO on sone hand are very similar with respect to both
their qualitative shape and the quantitative information on the molecular weight
of poly(1-hexene) at a given polymerization temperature.
(v) On the other hand, the same is true for the plots of the stereospecific catalyst systems
C2H4(Ind)2ZrCl2 3/MAO, Me2Si(Ind)2ZrCl2 4/MAO and Me2C(Cp)(Flu)ZrCl2
5/MAO.
78 3 Results and Discussion
(vi) The molecular weight of poly(1-hexene) obtained with the aspecific catalyst
systems Cp2ZrCl2 1/MAO and Me2Si(Cp)2ZrCl2 62/MAO is generally much lower
than in the case of the stereospecific catalyst systems C2H4(Ind)2ZrCl2 3/MAO,
Me2Si(Ind)2ZrCl2 4/MAO and Me2C(Cp)(Flu)ZrCl2 5/MAO.
(vii) At the same time, the transition temperature interval exhibiting the drastic
increase in molecular weight with decreasing polymerization temperature is
situated at lower temperatures in the case of the aspecific catalysts Cp2ZrCl2
1/MAO and Me2Si(Cp)2ZrCl2 62/MAO as compared to the stereospecific catalysts
C2H4(Ind)2ZrCl2 3/MAO, Me2Si(Ind)2ZrCl2 4/MAO and Me2C(Cp)(Flu)ZrCl2
5/MAO.
For the generally accepted polymerization mechanism of olefin polymerization, a
simple relation of the number average molecular weight Mn to the polymerization tem-
perature Tp is obtained (Appendix A.5).
lnMn ∝ −Ea,p − Ea,t
R · Tp(3.5)
As a consequence of this equation, a strictly monotonous increase of molecular weight
with decreasing polymerization temperature is expected, because Ea,p − Ea,t is always
negative. Otherwise no polymerization would occur. However, in the case of a single-site
mechanism with definite pathways of propagation and termination, the respective acti-
vation energies are expected to be constant at any polymerization temperature. There-
fore, an Eyring plot of the logarithmic molecular weight vs. the inverse polymerization
temperature should reveal a linear relation. This is clearly not the case in 1-hexene poly-
merization with zirconocene/MAO catalyst systems, irrespective of the actual catalyst
applied. Consequently, depending on the polymerization temperature, either different
active species promote polymerization, or at least different pathways of propagation or of
termination (or both) are preferred.
The observed behaviour resembles experimental results in anionic polymerization,
where an equilibrium of contact ion pairs (CIP) and solvent-separated ion pairs (SSIP),
and the absence of termination or transfer reactions, lead to a sigmoidally shaped de-
pendence of the logarithm of the rate constant of propagation on the inverse polymeriza-
tion temperature. A comparable interpretation of the experimental data assuming the
coexistence of more than one active species is strongly supported by the MWD of poly(1-
hexene) obtained at different polymerization temperatures (Figure 3.26). As for the effect
of zirconocene concentration, the most illustrative information on the temperature induced
3.2 Olefin Polymerization with Zirconocenes 79
3,0 3,5 4,0 4,5 5,0
6
7
8
9
10
11
1/Tp in 10-3
K-1
lnM
n80 40 0 -40 -80
400
600
800
1000
2000
4000
6000
8000
10000
20000
40000
60000
Tp in °C
Mn
Figure 3.25: Eyring plot of the molecular weight of poly(1-hexene) obtained with
Cp2ZrCl2 1/MAO (�), Me2Si(Cp)2ZrCl2 62/MAO (�), C2H4(Ind)2ZrCl2 3/MAO (❍),
Me2Si(Ind)2ZrCl2 4/MAO (❍), Me2C(Cp)(Flu)ZrCl2 5/MAO (❍); lines from function
fitting (section 3.2.5.2).
80 3 Results and Discussion
Mw/M
n
3,0 3,5 4,0 4,5 5,00
1
2
3
4
5
6
1/Tp in 10-3
K-1
80 40 0 -40 -80
Tp in °C
Figure 3.26: Polydispersity indexes Mw/Mn of poly(1-hexene) obtained with the catalyst
systems Cp2ZrCl2 1/MAO (�), Me2Si(Cp)2ZrCl2 62/MAO (�), C2H4(Ind)2ZrCl2 3/MAO
(❍), Me2Si(Ind)2ZrCl2 4/MAO (❍), Me2C(Cp)(Flu)ZrCl2 5/MAO (❍) as a function of the
inverse polymerization temperature; lines from function fitting (section 3.2.5.2)
3.2 Olefin Polymerization with Zirconocenes 81
0 5 10 15 20 25 30 35 40
Tpin °C
6010
-10-20
-35-78
elution volume in mL
Figure 3.27: GPC plots of poly(1-hexene) obtained with the catalyst system Cp2ZrCl2
1/MAO at different polymerization temperatures Tp.
change in the nature of the polymerization mechanism is provided by the GPC plots of
poly(1-hexene) obtained at different polymerization temperatures (Figures 3.27 and 3.28).
The observations concerning MWD and GPC plots are summarized as follows:
(i) In the case of Cp2ZrCl2 1/MAO “normal” MWD with polydispersity indexes 1.5 <
Mw/Mn < 2.0 are observed at the high and the low temperature limits.
(ii) The same is true for the catalyst systems C2H4(Ind)2ZrCl2 3/MAO,
Me2Si(Ind)2ZrCl2 4/MAO and Me2C(Cp)(Flu)ZrCl2 5/MAO at the low tem-
perature limit.
(iii) Within the transitional temperature interval between the high and the low tem-
perature limit (0 ◦C ≥ Tp ≥ −40 ◦C) MWD of poly(1-hexene) obtained with
the catalyst system Cp2ZrCl2 1/MAO are broader with polydispersity indexes
2.0 < Mw/Mn < 3.5.
82 3 Results and Discussion
10 15 20 25 30 35 40 4565
6050
4030
20
Tp in °C
elution volume in mL
Figure 3.28: GPC plots of poly(1-hexene) obtained with the catalyst system
Me2Si(Ind)2ZrCl2 4/MAO at different polymerization temperatures Tp.
(iv) Also, the catalyst systems C2H4(Ind)2ZrCl2 3/MAO, Me2Si(Ind)2ZrCl2 4/MAO and
Me2C(Cp)(Flu)ZrCl2 5/MAO give unexpectedly broad MWD with 2.0 < Mw/Mn <
5.5 at polymerization temperatures Tp > 30◦C.
(v) The plot of the polydispersity indexes Mw/Mn vs. the inverse polymerization tem-
perature in the case of Me2Si(Cp)2ZrCl2 62/MAO is qualitatively similar to that of
Cp2ZrCl2 1/MAO, yet the polydispersity indexes are generally smaller.
(vi) In the case of Cp2ZrCl2 1/MAO the GPC plots reveal the existence of a high mole-
cular weight shoulder at a polymerization temperature of Tp = 0 ◦C. The intensi-
ty of this shoulder increases with decreasing polymerization temperature, the two
peak maxima being of equal height at a polymerization temperature of Tp = −20 ◦C.
Finally, at the low temperature limit (Tp = −78 ◦C) the high molecular weight peak
is predominant, exhibiting only a low molecular weight tailing. Thus, at the high
and the low temperature limits of the temperature interval investigated, more or
less unimodal MWD are observed, while within the transition temperature interval
the MWD is bimodal.
3.2 Olefin Polymerization with Zirconocenes 83
(vii) Similar observations are made for the catalyst systems C2H4(Ind)2ZrCl2 3/MAO,
Me2Si(Ind)2ZrCl2 4/MAO and Me2C(Cp)(Flu)ZrCl2 5/MAO at a polymerization
temperature of Tp = 40◦C and above, with a low molecular shoulder increasing with
increasing polymerization temperature.
In conclusion, the analysis of MWD of poly(1-hexene) obtained with catalyst systems
consisting of different zirconocene dichlorides activated with MAO is consistent with a
true single-site mechanism only at some polymerization temperatures, while within a
certain temperature interval an unexpected broadening of the MWD is observed. The
range of this temperature interval is apparently dependent on the ligand geometry of the
zirconocene catalyst. It is situated at 0 ◦C ≥ Tp ≥ −40 ◦C in the case of the aspecific
catalyst systems Cp2ZrCl2 1/MAO and Me2Si(Cp)2ZrCl2 62/MAO, and at Tp ≥ 40 ◦C in
the case of the stereospecific catalyst systems C2H4(Ind)2ZrCl2 3/MAO, Me2Si(Ind)2ZrCl2
4/MAO and Me2C(Cp)(Flu)ZrCl2 5/MAO.
Remarkably, the temperature intervals of both the drastic increase in molecular weight
and the broadening of MWD coincide (Figures 3.25 and 3.26). Thus, the experimental da-
ta are qualitatively described as a superposition of two unimodal MWD, the proportion of
which is a function of polymerization temperature. At lower polymerization temperatures
the MWD with the higher molecular weight is preferred, and vice versa. This effect su-
perposes the general trend towards higher molecular weight at lower polymerization tem-
peratures, thus giving rise to the unexpectedly strong increase in molecular weight within
the transition temperature interval.
From the bimodal MWD obtained within this temperature interval one has to draw
the conclusion that every single polymer chain is polymerized following either one distinct
pathway or another. Once a chain is started, the pathway it takes is determined until it is
released from the catalyst. If that was not the case, broadened, but still unimodal MWD
would be observed. A straightforward explanation for the observed phenomenon is the
coexistence of two active species. If these two species are at all in equilibrium, it must be
slow on the time scale of chain growth. The bimodal MWD are deconvoluted with the
GPC software by fitting with a superposition of two appropriate unimodal peak functions
(Figure 3.29). The resulting molecular weights Mn of the constituting peaks are listed in
Table 3.2.5.1. Unfortunately, the relative peak areas are not reliable, probably due to a
software error. As can be seen from the plots of the deconvoluted peaks, the software is
very efficient in fitting to the comparably broad bimodal peaks. However, in some cases
the peak shapes of the deconvoluted peaks appear to be slightly skewed.
84 3 Results and Discussion
1000 10000 100000
M
1000 10000 100000
M
1000 10000 100000
M
1000 10000 100000
M
-10°C -25°C
-40°C -60°C
Figure 3.29: Examples for peak fitting of bimodal molecular weight distribution of
poly(1-hexene) with Cp2ZrCl2 1/MAO at different polymerization temperatures.
A plot of the experimentally determined number average molecular weights Mn,exp of
poly(1-hexene) and the number average molecular weights Mn,high and Mn,low obtained
via peak deconvolution (Figure 3.30) reveals that the linear regimes at the high and low
temperature limits are both extended towards the transition interval. This allows for
the determination of linear regression functions in both cases, from which the respective
differences in the activation energies of propagation and of termination at the high and
the low temperature limit are calculated:
Ea,t,high − Ea,p,high = 13.4 (±0.4) kJ mol−1
Ea,t,low − Ea,p,low = 2.9 (±0.4) kJ mol−1
3.2 Olefin Polymerization with Zirconocenes 85
2,5 3,0 3,5 4,0 4,5 5,0 5,5
6
7
8
9
10
1/Tp
in 10-3 K-1
lnM
n80 40 0 -40 -80
400
600
800
1000
2000
4000
6000
8000
10000
20000
Tp
in °C
Mn
Figure 3.30: Experimentally determined molecular weight Mn (�), and molecular weights
Mn,high (�) and Mn,low (�) determined via peak deconvolution; lines from linear regression.
86 3 Results and Discussion
Table 3.5: Molecular weight Mn, and polydispersity indexes Mw/Mn of the low and
the high molecular weight faction in 1-hexene polymerization with Cp2ZrCl2 1/MAO at
different polymerization temperatures, determined via peak deconvolution.
[Zr]
10−5 mol L−1
Mn
exp.
Mw/Mn
exp.
Mn
low
Mw/Mn
low
Mn
high
Mw/Mn
high
−60 12969 1.92 7888 1.45 14392 1.60
−50 11331 1.84 6111 1.41 15138 1.62
−40 9427 2.22 3343 1.56 13091 1.63
−30 5670 2.13 2639 1.52 13474 1.64
−25 5229 2.31 2542 1.48 11732 1.54
−20 4831 2.27 2288 1.82 11400 1.57
−10 2932 2.48 1991 2.37 9683 1.57
It appears that the temperature dependence of each of the deconvoluted unimodal
MWD obeys the expected linear relation of logarithmic molecular weight Mn and inverse
polymerization temperature Tp. The straightforward explanation is that each of the uni-
modal MWD is the result of a single-site mechanism, and the experimentally determined
MWD is the superposition of them. In other words, two active species are present in
zirconocene/MAO catalyst systems under the applied reaction conditions, one of which
is predominant at high, the other one at low polymerization temperatures. The bimodal
MWD prove that both species coexist in the reaction mixtures. Surprisingly, the active
species prevailing at low polymerization temperatures exhibits the smaller slope in the
Eyring plot, corresponding to a lower difference of activation energies of propagation and
termination. The higher molecular weight produced by this species is correlated with a
much higher ordinate value of the Eyring plot. This may point to the fact that in this
case either the propagation reaction is entropically favoured, or the termination reaction
is entropically disfavoured, or that the mechanism of either reaction may involve other
reactants. However, the slope of the regression line in the high temperature case may also
appear to be too large because the high temperature limit is not reached completely.
3.2.5.2 Mathematical Modelling of Temperature Effects
Various models starting from the generally accepted mechanism of olefin polymeriza-
tion (Figure1.1) are taken into consideration in order to find an appropriate mathematical
description for the temperature dependence of molecular weight and MWD of poly(1-
3.2 Olefin Polymerization with Zirconocenes 87
hexene) obtained with zirconocene/MAO catalyst systems. For clarity, all determinations
of the equations discussed in the following sections are collected in the corresponding
sections of Appendix A.
One active species in equilibrium with a dormant species. Early investigations
by Fink et al. reveal that the active species may be in equilibrium with a dormant species
(e.g. a complex with the cocatalyst) and chain propagation proceeds via an “intermittent
growth” mechanism. In this case, the temperature dependence of the molecular weight is
determined as (Appendix A.6):
lnMn = −Ea,p − Ea,t
RTp
+ const. (3.6)
Consequently, a linear relation in an Eyring plot of the molecular weight should be ob-
served. The intermittent growth mechanism may account for a broadening of the MWD,
but not for the observed non-linear Eyring plot of the molecular weight.
One active species, multiple mechanisms of termination. More than one pathway
of chain termination may be assumed (e.g. chain termination in the active species, chain
termination in the dormant species, chain transfer to the cocatalyst, etc.) which may also
comprise different reaction orders in monomer concentration. For any reaction mechanism
comprising more than one pathway of chain termination the following general expression
with Ei being some energy value is obtained (Appendix A.7):
lnMn = − Ea,p
RTp
− ln
(n∑
i=1
ci · e−Ei
RTp
)+ const. (3.7)
Thus, a non-linear relation of lnMn vs. 1/Tp may be obtained in this case. However, it is
proved that this function does not have a point of inflection (Appendix A.7). Consequently,
any model that only assumes more than one pathway of chain termination, regardless of
their nature, is not a suitable description for the sigmoidally shaped plots in the case of
1-hexene polymerization with Cp2ZrCl2 1/MAO or Me2Si(Cp)2ZrCl2 62/MAO.
One active species, multiple mechanisms of propagation. In analogy to the pre-
vious section, more than one mechanism of chain propagation may be assumed that may
comprise different reaction orders in monomer concentration. The equation describing
this model is similar to the case of multiple mechanisms of termination (Appendix A.8):
lnMn =Ea,t
RTp
+ ln
(n∑
i=1
ci · e−Ei
RTp
)+ const. (3.8)
88 3 Results and Discussion
Again, a non-linear relation of lnMn vs. 1/Tp may be obtained in this case. However,
analogously to the case of multiple mechanisms of termination it is proved that this func-
tion does not have a point of inflection (Appendix A.7). Consequently, any model that
only assumes more than one pathway of chain propagation, regardless of their nature, is
not a suitable description of the sigmoidally shaped plots in the case of 1-hexene poly-
merization with Cp2ZrCl2 1/MAO or Me2Si(Cp)2ZrCl2 62/MAO.
Two active species, two mechanisms of propagation and termination. In con-
trast to other models, assuming both two different pathways of propagation and termi-
nation affords an expression that is capable of exhibiting a point of inflection and is thus
suitable for modelling the sigmoidally shaped graphs (vide infra).
However, this model does not overcome the profound weakness of all models presented
so far if only one active species is considered. None of the models accounts for the observed
bimodal MWD of poly(1-hexene). In all cases, only a broadening of MWD is expected.
For this reason, the determination of an appropriate equation describing the relation of
lnMn and 1/Tp (Appendix A.9) also assumes that the reason for the different pathways
is the coexistence of two active species Zr∗1 and Zr∗2. The equilibrium is described as
Zr∗1 +A⇀↽ Zr∗2 with the equilibrium constant Kact = [Zr∗2]/[Zr∗1][A]:
lnMn = const.− Ea,t,1 − Ea,p,1
RTp
+ ln
1 + [Mon]a,2−a,1 · [A] ·k∞p,2
k∞p,1
· e−∆Gzr+Ea,p,2−Ea,p,1
RTp
1 + [Mon]b,2−b,1 · [A] ·k∞t,2k∞t,1
· e−∆Gzr+Ea,t,2−Ea,t,1
RTp
(3.9)
In the case that only one active species is present, the resulting expression is equivalent
to the one displayed above (Eq. 3.9), except with ∆Gzr = 0 and [A] = 1. A more
complicated expression will be obtained if the equilibrium is not monomolecular in either
of the active species, but rather a Zr∗1 ⇀↽ Zr∗2 with the equilibrium constant Kact =
[Zr∗2]/[Zr∗1]
a and [Zr∗1] + a [Zr∗2] = [Zr]0. The determination of the respective equation is
beyond the scope of this study.
Equation 3.9 may be denoted in a more general form that is a suitable function for a
fitting of the experimental data with the six parameters c1 to c6:
lnMn = c1 +c2Tp
+ ln1 + e
c3Tp
+c4
1 + ec5Tp
+c6(3.10)
3.2 Olefin Polymerization with Zirconocenes 89
The results of function fitting are summarized in Table 3.6. The corresponding graphs
are displayed as lines in Figure 3.25. Evidently, on the basis of a model assuming two
active species with two mechanisms of propagation and termination, all experimental
data is well described. In the case of the stereospecific catalysts Me2Si(Ind)2ZrCl2 4,
C2H4(Ind)2ZrCl2 3 and Me2C(Cp)(Flu)ZrCl2 5 the parameters c1 = 0 und c2 = 0 must
be kept fixed. This reflects the fact that no sigmoidal curves with linear regimes at the
high temperature limit are observed with these catalyst systems.
Table 3.6: Parameters of the model expression describing the dependence of logarithmic
molecular weight on the inverse polymerization temperature.
Catalyst c1 c2 c3 c4 c5 c6
Cp2ZrCl2 1 3.919 734 5097 -18.0 5189 -20.5
(±2.401) (±82) (±852) (±3.8) (±1329) (±5.0)Me2Si(Cp)2ZrCl2 62 1.145 1647 6634 -26.0 7922 -32.9
(±0.799) (±257) (±1867) (±7.7) (±1933) (±7.7)C2H4(Ind)2ZrCl2 3 0 0 9174 −19.4 8672 −27.7
(±1021) (±3.1) (±891) (±2.6)Me2Si(Ind)2ZrCl2 4 0 0 10449 −22.3 9527 −29.2
(±1523) (±4.5) (±1453) (±4.3)Me2C(Cp)(Flu)ZrCl2 5 0 0 8471 −17.6 7443 −24.4
(±829) (±2.5) (±685) (±2.0)
While interpreting the results of function fitting, one has to keep in mind that the
number of six parameters is high. Therefore, a successful fitting of the experimental data
is not too surprising, and also, other models may result in similar expressions. For these
reasons, an interpretation must be done with caution. From the parameters derived from
function fitting thermodynamic and kinetic parameters of the polymerization mechanism
are calculated (Table 3.7) with the following expressions
A = ∆Gzr + Ea,p,2 − Ea,p,1 = −c3 ·R (3.11)
B = ∆Gzr + Ea,t,2 − Ea,t,1 = −c5 · R (3.12)
C = Ea,t,1 − Ea,p,1 = −c2 · R (3.13)
D = Ea,t,2 − Ea,p,2 = (c5 − c3 + c2) · R (3.14)
If the differences in activation energies EA,p,2 − EA,p,1 and EA,t,2 − EA,t,1 are assumed
to be small, then the values calculated from the parameters c3 and c5 are approximate val-
90 3 Results and Discussion
Table 3.7: Thermodynamic and kinetic parameters calculated from the results of function
fitting; A = ∆Gzr + Ea,p,2 − Ea,p,1, B = ∆Gzr + Ea,t,2 − Ea,t,1, C = Ea,t,1 − Ea,p,1, D =
Ea,t,2 − Ea,p,2.
CatalystA
kJ mol−1
B
kJ mol−1
C
kJ mol−1
D
kJ mol−1
Cp2ZrCl2 1 −42.4 −43.1 6.1 6.9
(±7.1) (±11.0) (±0.7) (±4.6)Me2Si(Cp)2ZrCl2 62 −55.2 −65.9 13.7 2.9
(±15.5) (±16.1) (±2.1) (±2.7)C2H4(Ind)2ZrCl2 3 −76.3 −72.1 − −
(±8.5) (±7.4) − −Me2Si(Ind)2ZrCl2 4 −86.9 −79.2 − −
(±12.7) (±12.1) − −Me2C(Cp)(Flu)ZrCl2 5 −70.4 −61.9 − −
(±6.9) (±5.7) − −
ues of the free enthalpy ∆Gzr of the reaction Zr∗1+A⇀↽ Zr∗2. The values determined appear
to be reasonable, being comparable to e. g. the free enthalpy of dissociation of dinuclear
cationic zirconocenes or the dissociation of ion pairs with MAO (sections 1.2.5.2 and
1.2.5.3). Remarkably, in the case of Cp2ZrCl2 1/MAO and Me2Si(Cp)2ZrCl2 62/MAO the
values determined for the respective differences in activation energies of the propagation
and the termination reaction of the two active species EA,t,1 − EA,p,1 and EA,t,2 − EA,p,2 are
quite close to the corresponding values EA,t,1 − EA,p,1 and EA,t,high − EA,p,high determined
via deconvolution of the bimodal MWD and linear regression at the high and the low
temperature limit in the case of Cp2ZrCl2 1/MAO (Figure 3.30). The slope of the re-
gression line from peak deconvolution at the high temperature limit is apparently larger
than the limiting slope of the graph for 1/Tp → 0. Deconvolution appears to “exaggerate”
the relative portion of the low molecular weight fraction. However, results from function
fitting still imply that the species prevailing at low temperatures (the formation of which
is consquently exothermic) produces the higher molecular weight as compared to the
other species at the same temperature, yet it exhibits a smaller difference of activation
energies EA,t − EA,p. In other words, higher molecular weight poly(1-hexene) is not the
result of a larger difference in the activation energies of propagation and termination,
which may be surprising at first glance. From function fitting and qualitatively from the
3.2 Olefin Polymerization with Zirconocenes 91
Eyring plots of the molecular weight it must be concluded that higher molecular weight
poly(1-hexene) produced by the species prevailing at low polymerization temperature is
rather due to the large ordinate value of lnMn of its Eyring function. An interpretation of
this ordinate value is not straightforward because it is composed of a number of physical
figures (Appendix A.10).
lnMn(0) = ln
(mMon ·
[Mon]ap
[Mon]at· 1
[A]·k∞pk∞t
)(3.15)
Thus, the large ordinate value of the species prevailing at low temperature may be
due to (i) a lower reaction order in the monomer concentration of the chain termination
reaction, (ii) chain transfer not involving a chain transfer reagent A, (iii) a higher tem-
perature factor (i.e. a higher activation entropy) of the propagation in comparison to the
termination reaction. An evaluation of these possibilities is not feasible on the basis of
the experimental data.
Polydispersity indexes. An equation describing the temperature dependence of poly-
dispersity indexes Mw/Mn on the basis of the model assuming two active species is ob-
tained from a more general approach (Appendix A.11):
Mw
Mn
=
(1 + e
d1Tp
+d4
)·(1 + e
d2Tp
+d5
)(1 + e
d3Tp
+d6
)2 (3.16)
The fitting parameters d1 to d6 for all catalyst systems are listed in Table 3.8 for the
matter of completion. The corresponding graphs are displayed in Figure 3.26. Apparently,
the equation obtained for the polydispersity indexes and polymerization temperatures
is also capable of describing the experimental data. However, the fitting parameters
have large standard deviations, and the fitting is very sensitive to the starting values.
This is probably due to several parameters compensating each other. Furthermore, the
parameters d1 and d2 as well d4 and d5 are not distinguishable with respect to their physical
meaning. Only in the case of Cp2ZrCl2 1/MAO, the results of function fitting are regarded
as reliable enough to allow for the calculation of the corresponding thermodynamic/kinetic
figure with the following equation:
∆Gzr + Ea,p,2 − Ea,p,1 = −d3 · R = −54.4 (±21.9) kJ mol−1 (3.17)
Despite the large standard deviation this value is reasonably close to the corresponding
values determined by fitting the molecular weight (Table 3.7). This further supports the
92 3 Results and Discussion
relevance of the fitting of the polydispersity indexes, as well as of the suitability of the
applied model. However, a mathematical expression more suitable for function fitting still
has to be determined.
Table 3.8: Parameters d1 to d6 from the model expression (Eq. 3.16) describing the
dependence of polydispersity index on polymerization temperature.
Catalyst d1 d2 d3 d4 d5 d6
Cp2ZrCl2 1 701 11280 6544 3.3 37.6 22.0
(±225) (±1583) (±2637) (±0.9) (±2.8) (±8.5)Me2Si(Cp)2ZrCl2 62 4320 4181 4228 19.6 15.3 17.3
(±3733) (±7565) (±4886) (±16.9) (±26.2) (±23.2)C2H4(Ind)2ZrCl2 3 3185 4503 1726 23.1 14.5 11.9
(±4937) (±6654) (±2201) (±16.8) (±21.4) (±9.0)Me2Si(Ind)2ZrCl2 4 139 10705 6766 0.02 34.4 20.9
(±155) (±2701) (±3150) (±0.63) (±7.0) (±10.2)Me2C(Cp)(Flu)ZrCl2 5 −310 11429 84556 −1.7 37.1 25.7
(±618) (±5202) (±1798) (±2.4) (±15.1) (±7.8)
3.2.5.3 Conclusions
For all of the zirconocene/MAO catalyst systems applied, non-linear Eyring plots of
the molecular weight are obtained. In the case of Cp2ZrCl2 1/MAO and Me2Si(Cp)2ZrCl2
62/MAO the Eyring plots are sigmoidally shaped. Coinciding with the temperature inter-
vals with the maximum slope of the Eyring plots, MWD obtained at these temperatures
are substantially broadened or even bimodal, and polydispersity indexes Mw/Mn are larger
than expected. All of these findings are a strong indication of a temperature induced
change in the nature of the active species or in the pathway of polymerization.
It is especially the bimodal MWD which indicate that every single polymer chain is
polymerized following either one distinct pathway or the other. So, once a chain is started
the pathway it takes is determined until it is released from the catalyst. Otherwise
broadened, but unimodal MWD would be obtained. A plausible explanation for the
experimental data is the coexistence of two active species, possibly in an equilibrium
that is slow on the time scale of chain growth. Bimodal MWD are deconvoluted, and
the relative portions of the unimodal MWD exhibit a tight relation with polymerization
temperature, supporting the idea of a temperature-dependent equilibrium.
3.2 Olefin Polymerization with Zirconocenes 93
Several models considered for a mathematical description of the Eyring plots are found
to be inconsistent with the experimental data from a mathematical standpoint. Only a
model assuming two different pathways of propagation and of termination provides a suit-
able description of the sigmoidally shaped Eyring plots of the molecular weight. Although
the cause for these different pathways need not necessarily be the coexistence of two active
species, this is surely the most straightforward explanation. Thus, the mathematical con-
siderations do not only afford a consistent model, but additional evidence to the proposed
mechanism.
From function fitting, it is learnt that the pathway (the active species) producing
higher molecular weight poly(1-hexene) does not exhibit the larger difference of activation
energies of propagation and termination. This points to a more substantial change taking
place between the two pathways, e.g. a change in reaction orders, activation entropies, or
the involvement of other reactants in either of the pathways.
Taking all information concerning molecular weight, polydispersity indexes and bi-
modal MWD, the function fitting of molecular weight and of MWD, the deconvolution
of bimodal MWD and the resulting thermodynamic and kinetic parameters together, one
must draw the conclusion that all findings are consistent with the coexistence of two
active species. The experimental data may leave room for other explanations. However,
an artefact due to the experimental setup is definitely ruled out.
3.2.6 Effect of Polar and Nonpolar Cosolvents
The effect of cosolvent addition on the polymerization of 1-hexene catalyzed with
zirconocene/MAO catalyst systems is investigated in series of polymerization experiments
with the catalyst system Me2Si(Ind)2ZrCl2 4/MAO. The solvent toluene is partially re-
placed with varying volume fractions χ of heptane, CH2Cl2, C6H5Cl, Bu2O or THF.
Toluene and the cosolvent are pretreated with the MAO-solution before Me2Si(Ind)2ZrCl2
is added, in order to reduce deleterious effects of the cosolvent or impurities as far as
possible. All other reaction conditions are kept constant, with [Zr] = 3 · 10−4 mol L−1,
Al/Zr = 5000, [Mon] = 1.4 mol L−1 and polymerization temperature Tp = 30 ◦C in the
case of CH2Cl2 and Tp = 50 ◦C for all other cosolvents.
Higher molecular weight and narrower MWD in the case of CH2Cl2 addition is mainly
due to the lower polymerization temperature in this case. Polymerization activity has not
been studied systematically. Polymer yields are determined after a fixed polymerization
94 3 Results and Discussion
0 10 20 30 400
2000
4000
6000
14000
16000
18000
20000
Mn
% v/v cosolvent
Figure 3.31: Molecular weight of poly(1-hexene) obtained with Me2Si(Ind)2ZrCl2
4/MAO and CH2Cl2 at 30◦C (�) and with C6H5Cl (�), heptane (�), Bu2O (�), and
THF (❍) at 50◦C.
3.2 Olefin Polymerization with Zirconocenes 95
0 5 10 15 20 25 30 35 401,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
Mw/M
n
% v/v cosolvent
Figure 3.32: MWD of poly(1-hexene) obtained with Me2Si(Ind)2ZrCl2 4/MAO and
CH2Cl2 at 30◦C (�) and with C6H5Cl (�), heptane (�), Bu2O (�), and THF (❍) at 50◦C.
0 5 10 15 20 25 30 35 400,0
0,2
0,4
0,6
0,8
1,0
pol
ymer
yiel
din
%
% v/v cosolvent
Figure 3.33: Yield of poly(1-hexene) obtained with Me2Si(Ind)2ZrCl2 4/MAO and
CH2Cl2 at 30◦C (�) and with C6H5Cl (�), heptane (�), Bu2O (�), and THF (❍) at
50◦C after tp = 60 min.
96 3 Results and Discussion
time of tp = 60 min. One may speculate that more pronounced effects may be observed
at shorter polymerization times.
Addition of heptane leads to a continuous decrease of molecular weight with increasing
volume fraction χ, while polydispersity indexes increase from Mw/Mn ≈ 2.2 in pure
toluene to Mw/Mn ≈ 2.5 at a heptane volume fraction of χ = 20 % v/v and above
(Figures 3.31 and 3.32). The corresponding GPC plots reveal a substantial broadening of
MWD with increasing heptane content, giving peak shapes similar to those obtained at
high catalyst/cocatalyst concentrations. Polymer yield after a fixed polymerization time
of tp = 60 min decreases slightly with increasing heptane portions (Figure 3.33). At a
heptane volume fraction of χ = 20%v/v the reaction mixture is turbid, and it is obviously
heterogeneous at higher volume fractions of the cosolvent.
For polar cosolvents such as C6H5Cl or CH2Cl2 an increasing molecular weight, a
slightly decreasing polydispersity index and an unchanged polymer yield after a fixed
polymerization time of tp = 60 min are obtained, if the cosolvent volume fraction χ is
small. The maximal molecular weight and polymer yield are observed at 3 < χ/% < 4
C6H5Cl and 1 < χ/% < 2 CH2Cl2. Larger portions of the cosolvent result decreasing
molecular weight and polymer yield in both cases. The observed increase in polymer yield
at very large volume fractions may be an artefact. It is worth noting that polydispersity
indexes decrease slightly when CH2Cl2 is added and GPC plots of poly(1-hexene) are
perfectly unimodal over the whole range of cosolvent content investigated.
It is definitely true that stronger Lewis-basic cosolvents such as Bu2O or THF more
or less prohibit the polymerization of 1-hexene already at low cosolvent content in the
reaction mixture. Molecular weight and polymer yield decrease drastically with increasing
cosolvent content, and the poly(1-hexene) obtained exhibits broad or even multimodal
MWD.
Solvent polarity is obviously a crucial reaction parameter in 1-hexene polymerization
with zirconocene/MAO catalyst systems, as is the Al/Zr ratio and the reaction tem-
perature. Toluene is a nonpolar solvent with a dielectricity constant of εr = 2.34 and
a dipole moment of µ = 0.375 D. Nevertheless, MAO solutions and zirconocene/MAO
reaction mixtures in toluene as the solvent are homogeneous. Remarkably, the solubility
of cationic methyl zirconocenes such as Cp2Zr(Me)(thf)+BPh−4 26 in pure toluene is
drastically lower (typically 3 mg mL−1) than the solubility of the same complexes in
MAO solution, or the solubility of catalyst precursors in MAO solution at a Al/Zr ratio
at which a complete conversion into the corresponding cationic methyl zirconocene is
3.2 Olefin Polymerization with Zirconocenes 97
expected. This points to the fact that the solvating properties of toluene solutions are
substantially altered in the presence of a considerable amount of MAO, which acts as a
“polar cosolvent”.
A lowering of solvent polarity compared to toluene by using heptane (εr = 1.92, no
dipole moment) results in precipitation of MAO (and most likely also of the cationic
zirconocene). It is straightforward to assume that the solvation of both the MAO clusters
and the cationic active species along with the counterion is progressively reduced with
increasing heptane content in toluene/heptane mixtures. One should expect the MAO
clusters to aggregate and any sort of ionic species to first form contact ion pairs and then
larger aggregates. Remarkably, at a heptane content below χ = 20%v/v the reaction mix-
ture still appears to be homogeneous, yet MWD of poly(1-hexene) is already substantially
broadened. This may consequently be assigned to an increasing “heterogeneity” of the
active species, i.e. the “free” cationic zirconocene not being stabilized enough to prevent
the formation of any sort of species (contact ion pairs with the counterion, complexes
with excess MAO, or larger aggregates etc.) that offers a polar enough environment to
the cationic zirconocene and a more lipophilic “surface” to the interface with the solvent,
first on a molecular scale, and finally yielding phase separation.
This interpretation is supported by the effect of more polar cosolvents, such as C6H5Cl
(εr = 2.71, µ = 1.69 D) or CH2Cl2 (εr = 9.08, µ = 1.60D) that are weakly enough co-
ordinating to allow for 1-hexene polymerization. Apparently, toluene is not the “ideal”
solvent. Higher rates of propagation, and narrower MWD are observed in reaction mix-
tures containing small amounts of the polar cosolvent. On the basis of the generally
accepted mechanism of olefin polymerization (Figure 1.1) this may be attributed to en-
hanced stabilization of the “free” cationic zirconocene complexes and suppression of any
form of association or aggregation. As compared to these more polar solvent mixtures,
pure toluene already promotes the trend towards aggregation as outlined for solvent mix-
tures containing heptane. However, increasing solvent polarity is inevitably connected
with the introduction of Lewis-basic centres in the solvent molecules. The “stabiliza-
tion” of the cationic species also means that the solvent is capable of competing with
the monomer for occupation of the vacant coordination site. Therefore, the polymeriza-
tion of 1-hexene is progressively inhibited (i.e. molecular weight and polymer yield are
decreased) with increasing cosolvent content as well as solvent polarity and coordination
strength (C6H5Cl < CH2Cl2 < Bu2O < THF). However, the situation is fundamentally
different from the observed decrease in molecular weight upon addition of heptane. Con-
98 3 Results and Discussion
stant or even lower polydispersity indexes and unimodal GPC elution curves indicate that
in the case of CH2Cl2 no change in the nature of the active species and no deviation from
single-site behaviour is induced. Apparently, just the rate of propagation is affected by
the presence of another coordinating species in the reaction mixture.
3.2.6.1 Conclusions
Increasing polarity from hydrocarbons through aromatic hydrocarbons to chloro
hydrocarbons tends to increase the interaction of solvent and catalyst system. To a
certain extent, this will enhance the catalyst performance by means of stabilizing the
cationic active species and suppressing interaction of the latter with its counterion. How-
ever, the more stabilizing the solvent, the more it is a potential catalyst poison at the
same time, competing with the monomer (and possibly outperforming the latter) for the
vacant coordination site.
In toluene solution, the role of MAO is not limited to the generation of the active
species. It also stabilizes and solubilizes (solvate) the latter. Thus, it is not the non-
coordinating, but rather the coordinating features of MAO that make it superior in this
respect. MAO may thus be considered as a sort of weakly coordinating, polar cosolvent.
Toluene is a “good” solvent but not an optimal one, promoting a slight deviation from
single-site behaviour under the applied reaction conditions. A further decrease in polarity
results in a more pronounced deviation from single-site behaviour, which is attributed to
the aggregation of MAO clusters and ionic components in the reaction mixture. In the
light of these findings, the temperature induced changes in the nature of the active species
may be due to the same cause, if the solvation entropy of zirconocene/MAO mixtures in
toluene is considered to be negative.
3.2.7 The Nature of the Active Species
All of the experimental data presented in the previous sections are kinetic data in
nature, be it activities, molecular weights or MWD. In the latter two cases, it is both
the activation energies of propagation and termination that determine the result. The
thermodynamic statements have been obtained indirectly from mathematical treatment of
the data derived from kinetic experiments. Generally, no mechanistic conclusions should
be drawn from kinetic investigations. Thus, all mechanisms and structures proposed are
to be understood as plausible assumptions consistent with the experimental findings.
3.2 Olefin Polymerization with Zirconocenes 99
The mechanism that is proposed (Scheme 3.8) takes into consideration the following
conclusions from previous sections:
(i) Deviation from single-site behaviour is observed at both high catalyst concentra-
tions (low Al/Zr ratios) and at high cocatalyst concentrations (high Al/Zr ratios),
therefore being the consequence of two different effects. All other investigations are
carried out at moderately high overall catalyst concentrations and high cocatalyst
concentrations.
(ii) Molecular weight and MWD are not a function of monomer conversion but are
generally constant throughout polymerization time. No change in the nature of the
active species induced by a change in monomer concentration is observed.
(iii) Eyring plots of molecular weight and MWD suggest the existence of (at least) two
different pathways of propagation and termination, while bimodal MWD indicate
the coexistence of two active species in a temperature-dependent concentration ratio.
(iv) Decreasing polarity induces similar effects on molecular weight and MWD, as does
changing the polymerization temperature, while increasing solvent polarity appears
to promote single-site behaviour.
On the basis of these findings, the coexistence of two active species Zr∗1 and Zr∗2 is
envisioned which respectfully have the rate constants of propagation kp,1 and kp,2 and the
rate constants of termination kt,1 and kt,2. Both active species are in equilibrium with
the equilibrium constant Kzr. One of the active species is assumed to be a sort of “free”
cationic complex, while the other one must comprise an active site which is altered in
reactivity. Most likely, it is formed in a reaction with excess cocatalyst. The formation
of such heterodinuclear complexes is exothermic, regardless of their nature. This means
that it is the heterodinuclear species that prevails at low polymerization temperatures
and yields higher molecular weight poly(1-hexene). This is not in contradiction to the
“free” cationic species exhibiting the faster rate of polymerization, and possibly the slower
rate of termination. As previously indicated, higher molecular weight poly(1-hexene) in
the case of the active species at low temperatures is not the result of a larger difference
in activation energies of propagation and termination but is a consequence of reaction
orders, transition entropies or the involvement of other reactants (section 3.2.5.2). The
effect of solvent polarity is straightforwardly explained by replacement of the very weakly
Lewis-basic metal fragment with the more Lewis-basic polar solvent, thus shifting the
100 3 Results and Discussion
Si
Zr
HR
H
Si
Zr
RH
RP
ol
H
Si
Zr
Pol
H
HR
RR
Si
Zr
RH
R
Pol
HR
Si
Zr
Pol
HH
RR
Si
Zr
Pol
HH
RR
XA
l
Si
Zr
Pol
H
HR
RR
Si
Zr
RH
R
Pol
HR
Si
Zr
HR
H
XA
l
XA
l
Si
Zr
RH
RP
ol
H
XA
l
(
)
(
)
(
)
(
)
Kzr
+ m
onom
er
k p,1
k p,1
+ m
onom
er
k t,1
– po
lym
er
– po
lym
er
k t,2
+ m
onom
er
k p,2
k p,2
+ m
onom
er
±
XA
l(
)
XA
l (
)
Zr*
1
Zr*
2
Scheme 3.8: Model mechanism on the basis of the experimental results presented in the
previous sections
3.2 Olefin Polymerization with Zirconocenes 101
Si Zr
PolR
H
MeAl
( )
Si Zr
PolR
H
Al ( )
AlOSi Zr
PolR
H
Al( )
R
A B C
Figure 3.34: Possible structures of the proposed heterodinuclear zirconocene complexes.
equilibrium towards the monomeric species and enhancing the single-site nature of the
polymerization mechanism.
However, several questions remain open or unclear. First of all, no conclusions can be
drawn concerning the nature of the heterodinuclear complex, whether it is a complex with
the MAO counterion, with free MAO or with smaller units (e.g. AlMe3), and whether it
coordinates to the zirconocene via a µ2-Me- or via a µ3-O-bridge (A andB in Figure 3.34).
At first glance, one would expect that coordination of the counterion is energetically most
favoured. However, ion-pair dissociation is shown to be far more feasible than in the case
of counterions such as B(C6F5)−4 , and the high MAO content in toluene should further
stabilize, “solvate” and help to separate ionic species in solution. This is the same as
admitting that non-ionic MAO clusters may coordinate to the active site and it is further
supported by the statistical factor that there is only one [Me-MAO]− counterion per active
species, but approximately two orders of magnitude more MAO clusters (e.g. ≈ 200 at
Al/Zr = 5000). This also poses the question, how the generally formulated “free” cationic
active species would exist in MAO solution and whether its existence is likely at all.
Secondly, how would monomer coordination proceed in a heterodinuclear complex?
Homo- and heterodinuclear complexes are found to be active for olefin polymerization.
However, it is not clear whether or not they dissociate prior to monomer coordination.
An associative replacement of the coordinated metal fragment is expected to be electron-
ically unfeasible. Theoretical calculations99,100 even suggest the existence of an “olefin
separated ion-pair” (OSIP) to be the most effective pathway of monomer insertion be-
cause the counterion enhances the polarization of the coordinated monomer (C in Figure
3.34, compare section 1.2.5.4). Without further experimental evidence the results remain
speculative.
Finally, how is the fact explained that if the two species are at all in equilibrium, it must
be slow on the time scale of chain growth? All experimentally determined exchange rates
102 3 Results and Discussion
appear to be much faster, e. g. for the dissociation of homodinuclear zirconocenes or of ion-
pair reorganization in cationic methyl zirconocenes. If one assumes that a heterodinuclear
complex must dissociate prior to monomer coordination, the question is what makes the
dissociated fragment stay close and reassociate? A possible explanation may be diffusion
limitation due to the size of the coordinated MAO cluster. Once the polymer chain starts
to grow, it interpenetrates the coordinated cluster fragment and keeps it from diffusing
away, even when dissociated. A conversion to the “free” cationic species would only occur
when the polymer chain is released from the catalyst. An alternative explanation is that
under polymerization conditions, there is no equilibrium of the active species at all. Both
may be formed from a common (high energy) precursor in proportion to the respective
activation energies of formation but an “equilibrium” between both involves this high
energy precursor and is therefore “frozen” or very slow.
3.2.8 What Has Been Learnt Concerning Copolymerization?
The active species is electrophilic enough to react with any Lewis-basic component
present. In the absence of other alternatives it also reacts with dimethyl zirconocenes
or MAO in order to form stable (hetero-)dimers, although these reactants are considered
to be very weak Lewis-bases. Furthermore, the polymerization experiments with polar
cosolvents provide evidence for the fact that, in the case of typical zirconocene catalysts
that are not extraordinarily sterically hindered, the maximal Lewis-basicity tolerated is
comparable to that of chloro hydrocarbons. Separation of the functional group from
the carbon double bond with a spacer will not significantly change this situation if one
considers the fact that the polar cosolvents are even separate molecules. Steric hindrance
may circumvent the problem to a certain extent but new problems are encountered, such
as low comonomer incorporation and low overall polymerization activity. Complexation
of the Lewis-basic sites by additional amounts of MAO or other Lewis-acids is neither an
attractive alternative given the induced effects on the polymerization mechanism.
After all, olefin polymerization with zirconocene catalysts as a potential starting point
for the incorporation of functional comonomers into polyolefin materials appears to be a
dead end.
3.3 MMA Polymerization with Zirconocenes 103
3.3 MMA Polymerization with Zirconocenes
The polymerization of MMA with cationic methyl zirconocenes as single-component
polymerization catalysts is carried out with the complexes Cp2Zr(Me)(thf)+BPh−4
26, Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43, C2H4(Ind)2Zr(Me)(thf)
+BPh−4 67,
Me2C(Cp)(Flu)Zr(Me)(thf)+BPh−4 68, Me2Si(Cp)2Zr(Me)(thf)
+BPh−4 69 and
Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 in CH2Cl2 as the solvent.
ZrO
Me ZrO
MeZrO
Me
Zr MeO
BPh4
BPh4 BPh4BPh4
OMe
OMeO O
73n
74
L2Zr(Me)(thf)+BPh4–
CH2Cl2
43 68
70 69
67
26
Zr MeOSiZr Me
O
BPh4BPh4
Scheme 3.9: Polymerization of MMA with cationic methyl zirconocenes.
While Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 and Me2C(Cp)2Zr(Me)(thf)
+BPh−4 70
are highly active catalysts for MMA polymerization (Tables 3.9 and 3.10), all
other cationic methyl zirconocenes investigated exhibit virtually no polymerization
activity under the applied experimental conditions. MMA polymerization with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 and Me2C(Cp)2Zr(Me)(thf)
+BPh−4 70 at 0 ◦C is
almost quantitative on the time scale of several minutes in both cases. However, poly-
merization of MMA only occurs at zirconocene concentrations well above 5 · 10−4 mol L−1
in the case of Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 and even 1.2 · 10−3 mol L−1 in the
case of Me2C(Cp)2Zr(Me)(thf)+BPh−4 70. The following observations will be studied in
more detail in the next sections:
104 3 Results and Discussion
(i) Quantitative monomer conversion is only obtained at low poly-
merization temperatures. In the case of MMA polymerization with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 at Tp = −20 ◦C and below, the system
remains active for polymerization and control experiments reveal that quantitative
yields are obtained after 12 h.
(ii) The number average molecular weight Mn is more than a factor of 2 higher
than expected from the monomer/zirconocene ratio [MMA]/[Zr]. This is
well reflected by the catalyst efficiencies I∗ < 0.5. Polymerization with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 affords catalyst efficiencies between 0.35 <
I∗ < 0.50, while they are even lower in the case of Me2C(Cp)2Zr(Me)(thf)+BPh−4
70.
(iii) MWD are narrow with polydispersity indexes Mw/Mn ≤ 1.4 only at low
polymerization temperatures and considerably broader than in MMA poly-
merization with “Collins-type” catalyst systems. This is true especially for
Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 as the catalyst.
(iv) In the case of MMA polymerization with the chiral C1-symmetric catalyst
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43, highly isotactic PMMA is obtained, while
the achiral Cs-symmetric Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 as a catalyst yields
syndiotactic PMMA at low temperatures.
This is the first example for a tight correlation of PMMA microstructure and
catalyst symmetry in MMA polymerization with zirconocene catalysts. Seeming-
ly, the similar ligand structure of Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 and
Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 renders these cationic methyl zirconocenes active for
MMA polymerization, while catalyst symmetry allows for the control of PMMA micro-
structure.
3.3.1 Kinetics of MMA Polymerization
The kinetics of MMA polymerization with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43
as the catalyst are investigated at different polymerization temperatures Tp (Figure
3.35). All other reaction conditions are kept constant, i. e. [Zr] = 4 · 10−3 mol L−1,
[MMA]0 = 1 mol L−1 and total reaction volume 12.5 mL. Approximately 3% wt/wt hexyl
benzene is added to the reaction mixtures as an internal standard for the determination of
3.3 MMA Polymerization with Zirconocenes 105
Table 3.9: MMA polymerization catalyzed with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43
in CH2Cl2 at different polymerization temperatures; [Zr] = 8 mmol L−1, [MMA]0 =
2mol L−1; catalyst efficiency I∗ = Mn(theor)/Mn(exp).
Tp
◦C
tpmin
yield
%Mn Mw/Mn I∗
mm
%
mmmm
%
30 20 54 40500 1.43 0.33 84.3 74.9
20 40 62 45700 1.42 0.34 87.0 79.6
10 60 94 51000 1.42 0.46 89.0 82.1
0 90 97 58800 1.34 0.41 90.4 84.5
−10 180 97 55100 1.30 0.44 92.1 87.6
−20 180 93 55300 1.24 0.42 93.5 n. d.
−30 240 77 57300 1.21 0.34 94.7 91.5
Table 3.10: MMA polymerization catalyzed with Me2C(Cp)2Zr(Me)(thf)+BPh−4 70
in CH2Cl2 at different polymerization temperatures; [Zr] = 16 mmol L−1, [MMA]0 =
2mol L−1; catalyst efficiency I∗ = Mn(theor)/Mn(exp).
Tp
◦C
tpmin
yield
%Mn Mw/Mn I∗
rr
%
rrrr
%
30 20 65 66600 1.64 0.12 69.0 52.1
20 30 61 93000 1.60 0.08 72.9 57.6
10 40 59 104100 1.38 0.07 75.1 59.2
0 60 81 90600 1.32 0.11 78.8 61.9
−10 105 86 123200 1.30 0.09 80.1 67.2
−20 180 71 110500 1.35 0.08 82.4 69.1
−45 1080 98 78500 1.31 0.16 89.0 76.9
relative polymer yield via GPC. Polymer yield, molecular weight and polydispersity index
as a function of polymerization time are determined by taking 0.05 mL samples from the
reaction mixture and GPC analysis. Relative polymer yield in one polymerization experi-
ment is obtained from the ratio of the integrals of the polymer peak and the hexyl benzene
peak. From these data, the absolute polymer yield is determined by termination of the
polymerization experiment instantaneously after taking the last sample and gravimetrical
determination of polymer yield.
106 3 Results and Discussion
0 50 100 150 200 2500,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
ln([
M0]/
[M])
tp
in min
Figure 3.35: Kinetics of MMA polymerization catalyzed with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 at 30 ◦C (�), 20 ◦C (✷), 10 ◦C (�), 0 ◦C (❍),
−10 ◦C (�), −20 ◦C (�) and −30 ◦C (�); lines from function fitting (section 3.3.2).
0 50 100 150 200 250
-30
-20
-10
0
10
20
30
Tp
in°C
tp in min
orange yellow
yellow
polymerization
Figure 3.36: Colours observed in Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 solutions upon
addition of MMA at tp = 0 min.
3.3 MMA Polymerization with Zirconocenes 107
Consistent with the results displayed in Table 3.9 it is observed that quantitative
yields are only achieved at a polymerization temperature of Tp = 0 ◦C and below. Above
0 ◦C the polymerization reaction appears to be terminated within a couple of minutes.
The maximum yield achieved appears to be a function of polymerization temperature,
with higher temperature resulting in lower yield. Evidently, a chain termination reaction
competes with the polymerization reaction. In this case (i. e. in contrast to the use of
this term in olefin polymerization) chain termination seems to be irreversible in the sense
that the active species is terminated and it is not a chain transfer reaction (vide infra).
At lower polymerization temperatures the polymerization reaction is driven to com-
pletion and the dependence of monomer conversion on polymerization time approaches
linearity in a first-order plot. At the same time, a short initiation period is observed
at polymerization temperatures of 0 ◦C and below. This may indicate that the initially
applied cationic complex Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 is not the active species
itself. The active species may instead be generated through an initiation reaction.
Solutions of Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 in CH2Cl2 instantaneously change
colour from yellow to orange upon addition of MMA at Tp = 0 ◦C and above, and after
a few minutes at lower temperature (Figure 3.36). Another slow colour change to yellow
appears after some time. The time interval is a function of polymerization temperature
and roughly correlates with the time needed for quantitative monomer conversion.
Polymerization of MMA with Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 in CH2Cl2 turns
out to be too sensitive for reproducible kinetic experiments. Only experiments
in which no samples are taken from the reaction mixture yield PMMA. This cor-
relates with the higher zirconocene concentration necessary to achieve polymeriza-
tion (Table 3.9) and the lower catalyst efficiencies as compared to polymerization
with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43. Apparently, in MMA polymerization with
Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 the active species is more susceptible to irreversible
deactivation than in MMA polymerization with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43.
3.3.2 Mathematical Modelling of Polymerization Kinetics
It is assumed that polymerization is first-order in monomer concentration. The con-
centration of the active species is considered to be time-dependent due to an initiation
reaction with the rate constant ki and a chain deactivation reaction with the rate constant
kd.
108 3 Results and Discussion
3,2 3,4 3,6 3,8 4,0-9
-8
-7
-6
-5
-4
-3
1/Tp
in 10-3 K-1
lnk
Figure 3.37: Eyring plot of the apparent rate constants kp (�), ki (❍)and (�) kd in MMA
polymerization with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43.
The following expression is obtained for monomer conversion as a function of poly-
merization time (Appendix A.12)
ln[Mon]0[Mon]
= [Zr0]0 ·kp · ki
kd − ki·[1
kd
(e−kd·t − 1
)− 1
ki
(e−ki·t − 1
)](3.18)
The apparent rate constants kp, ki and kd are determined via fitting the experimental
data with the appropriate fitting function f(x) = ab(e−b·t−1)− a
c(e−c·t−1) and summarized
in Table 3.11. The corresponding graphs are displayed as lines in Figure 3.35. All experi-
mental data aeem to be well described with the applied model.
Consistent with the observation that at a polymerization temperature above Tp = 0 ◦C
no initiation period is observed, successful function fitting in this case requires that either
the rate constant of initiation is kept fixed at ki = ∞ or that function fitting is carried
out with the appropriate function f(x) = ab(1− e−b·t) derived from the following equation
(Appendix A.12)
limki→∞
(ln[Mon]0[Mon]
)= [Zr0]0 ·
kp
kd
(1− e−kd·t
)(3.19)
From an Eyring plot of the apparent rate constants kp, ki and kd (Figure 3.37) the
corresponding activation parameters Ea, k∞, ∆H‡ and ∆S‡ are derived via linear re-
gression. In order to determine k∞ and ∆S‡ it is assumed that all reactions are first-order
3.3 MMA Polymerization with Zirconocenes 109
Table 3.11: Apparent rate constants kp, ki and kd at different polymerization tem-
peratures Tp.
Tp
◦C
kp10−3 s−1
ki10−3 s−1
kd10−3 s−1
30 7.05 (±1.01) n. d. 9.10 (±1.54)20 5.45 (±0.51) n. d. 5.85 (±0.61)10 4.08 (±1.55) 8.85 (±2.01) 1.35 (±0.40)0 2.17 (±1.32) 2.13 (±0.71) 0.58 (±0.19)
−10 1.35 (±0.72) 1.58 (±0.43) 0.36 (±0.06)−20 3.33 (±0.05) 1.48 (±0.52) 0.05 (±0.02)−30 n. d. n. d. n. d.
Table 3.12: Activation parameters of the propagation reaction, the initi-
ation reaction, and the deactivation reaction in MMA polymerization with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43; ∆H‡ and ∆S‡ at 293 K.
ReactionEa
kJ mol−1ln k∞
∆H‡
kJ mol−1
∆S‡
J mol−1 K−1
Propagation 28.4 (±2.3) 11.14 (±1.02) 25.7 (±2.3) −6.4 (±0.8)Initiation 12.9 (±8.9) 4.83 (±2.35) 8.5 (±8.9) −58.9 (±4.1)Deactivation 57.2 (±6.9) 22.33 (±2.96) 54.8 (±6.9) +90.7 (±1.1)
in zirconocene concentration, and that the latter is [Zr∗] = [Zr] = 8 · 10−3 mol L−1. With-
out knowledge concerning the details of the polymerization mechanism both assumptions
are admittedly questionable. The reaction order is not known and the concentration of
the active species may be below the originally applied zirconocene concentration, as im-
plied by the catalyst efficiencies I∗. Therefore, the values determined for k∞ and ∆S‡ are
to be understood as a first approximation and they definitely range within the correct
order of magnitude. Unfortunately, no comparable data appears to be available for the
metallocene catalyzed group transfer polymerization of MMA.
110 3 Results and Discussion
3.3.3 Control of Molecular Weight and MWD
The experimentally determined number average molecular weight Mn at any given
monomer conversion is substantially higher than the “theoretical” molecular weight de-
termined by the ratio of MMA and zirconocene concentration. This is well reflected by
the catalyst efficiencies I∗ < 1 (Tables 3.9 and 3.10). This observation may be due to
two reasons. Either the active species is not generated in a monomolecular reaction from
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 so that the concentration of active species and the
originally applied amount of zirconocene are not identical, or just part of the zirconocene
species is converted into the active species.
As mentioned above, no polymerization occurs at catalyst concentrations below a
critical minimum of 5 · 10−4 mol L−1 in the case of Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43
and even 1.2 · 10−3 mol L−1 in the case of Me2C(Cp)2Zr(Me)(thf)+BPh−4 70. Further-
more, experiments with varying time intervals between the addition of CH2Cl2 to the
catalyst and the addition of MMA to this solution reveal that polymer yield decreases
drastically when this interval is increased to more than a couple of seconds. In NMR
spectra recorded in CD2Cl2 a decomposition of Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 occurs
0 20 40 60 80 1000
10
20
30
40
50
60
xp
in %
Mn/1
000
Figure 3.38: Molecular weight of PMMA obtained with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 vs. monomer conversion at polymerization
temperatures of 10 ◦C (�), 0 ◦C (❍), −10 ◦C (�), −20 ◦C (�) and −30 ◦C (�); dashed line
is theoretical molecular weight.
3.3 MMA Polymerization with Zirconocenes 111
0 50 100 150 200 2501,0
1,1
1,2
1,3
1,4
1,5
tp
in min
Mw/M
n
Figure 3.39: Polydispersity indexes Mw/Mn of PMMA obtained with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 as a function of polymerization time tp at
polymerization temperatures of 30 ◦C (�), 10 ◦C (�), −10 ◦C (�) and −30 ◦C (�).
0 20 40 60 80 1001,0
1,1
1,2
1,3
1,4
1,5
xp
in %
Mw/M
n
Figure 3.40: Polydispersity indexes Mw/Mn of PMMA obtained with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 as a function of polymer yield at polymerization
temperatures of 30 ◦C (�), 10 ◦C (�), −10 ◦C (�) and −30 ◦C (�).
112 3 Results and Discussion
within a couple of minutes. This may point to the fact that the deactivation reaction is
caused by reaction with the chlorinated solvent. Possibly, part of the zirconocene species
is already decomposed before it is transformed into the active species.
A plot of the molecular weight of PMMA vs. monomer conversion (Figure 3.38) in
MMA polymerization with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 as the catalyst reveals
a linear dependence up to high monomer conversion at low polymerization temperatures.
This may also be the case at polymerization temperatures above 0 ◦C but, due to fast
polymerization and fast termination of the polymerization, all data points are in a very
narrow range of monomer conversion so that it is not possible to evaluate the relation
seriously.
The linear dependence at lower polymerization temperatures is an indication of the
absence of a chain transfer reaction. However, the presence of a termination reaction is not
ruled out. In this case, single active chains are terminated under irreversible deactivation
of the active species, leaving the rest of their share of the initial amount of monomer to
the still living chains. Consequently, the average molecular weight at a given monomer
conversion is not affected by a termination reaction. However, a substantial broadening
of molecular weight distributions is expected.
Indeed, a slow broadening of the MWD in the course of the polymerization reaction is
observed (Figure 3.39), which means that the MWD broadens progressively with increas-
ing MMA conversion (Figure 3.40). MWD as narrow as in anionic polymerization or in
group-transfer polymerization with and without zirconocenes with polydispersity indexes
Mw/Mn ≤ 1.1 are only observed for moderate monomer conversion at a polymerization
temperature below Tp = 0 ◦C. Even at Tp = −30 ◦C the MWD is substantially broadened
when monomer conversion is driven to completion. At Tp = +30 ◦C polymerization char-
acteristics approach a polymerization process with a random chain termination reaction
and a MWD according to a most probable Schulz-Flory distribution. The corresponding
GPC curves (Figure 3.41) exhibit a broad low molecular weight tailing that increases
to broad shoulders with increasing polymerization temperature. Apparently, there is a
chain termination process, the activation energy of which is large enough to mimic liv-
ing polymerization behaviour at low enough polymerization temperatures. However, at
room temperature and above, it must be fast enough to substantially compete with chain
propagation. The activation energies determined from function fitting (cf. the Eyring plot
in Figure 3.37) imply that the ratio of the apparent rate constants at Tp = −20 ◦C is
kp/kd = 6.7, while at Tp = +30 ◦C it is kp/kd = 0.8, well in line with the GPC analysis.
3.3 MMA Polymerization with Zirconocenes 113
20 25 30 35 40 45 50 55-30
-20-10
010
20
Tp in °C
elution volume in mL
30
Figure 3.41: GPC plots of PMMA obtained with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43
at different polymerization temperatures.
3.3.4 Control of Stereospecificity
As seen from Tables 3.9 and 3.10, MMA polymerization with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 is already highly isospecific at room temperature.
Isotactic mm triad abundance calculated from 1H-NMR spectra ranges from 84.3 % at a
polymerization temperature of Tp = +30 ◦C to 94.7% at Tp = −30 ◦C. Contrary to this,
MMA polymerization with Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 is syndiospecific at low
temperatures, with syndiotacticity on the basis of rr triad abundance calculated from
1H-NMR spectra varying from 69.0 % at a polymerization temperature of Tp = +30 ◦C
up to 89.0 % at Tp = −45 ◦C. In both cases the results concerning stereospecificity and
its temperature dependence compare very well with the results achieved in samarocene
polymerization of MMA.
From the Eyring plots of the ratio of dyad abundances m/r calculated from the
triad abundance determined by 1H-NMR spectroscopy at different polymerization tem-
peratures (Figures 3.42 and 3.43), the difference between the activation energies of syn-
diospecific and isospecific propagation which controls stereospecificity are estimated to
be Ea,r–Ea,m = 12.2 (±0.6) kJ mol−1 in the case of Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43
114 3 Results and Discussion
3,0 3,2 3,4 3,6 3,8 4,0 4,2 4,4 4,61,5
2,0
2,5
3,0
3,5
ln((
2mm
+m
r)/(
2rr+
mr)
)
1/Tp in 10-3
K-1
Figure 3.42: Eyring plot of the ratio of dyad abundance m/r in MMA polymerization
with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43.
3,0 3,2 3,4 3,6 3,8 4,0 4,2 4,4 4,6
-3,0
-2,5
-2,0
-1,5
ln((
2mm
+m
r)/(
2rr+
mr)
)
1/Tp in 10-3
K-1
Figure 3.43: Eyring plot of the ratio of dyad abundance m/r in MMA polymerization
with Me2C(Cp)2Zr(Me)(thf)+BPh−4 70.
3.3 MMA Polymerization with Zirconocenes 115
(ppm)
175.2175.6176.0176.4176.8177.2177.6178.0178.4178.8
— — — — — ——
30°C
20°C
10°C
0°C
-10°C
-10°C
-30°C
mrr
m
mrr
r
rrrr
mrm
m /
mrm
r
mm
rr /
rmrr
mm
mm
mm
mr
rmm
r
—
Figure 3.44: 13C-NMR spectra of isotactic PMMA obtained with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 at different polymerization temperatures.
and Ea,r–Ea,m = −9.5 (±0.4) kJ mol−1 in the case of Me2C(Cp)2Zr(Me)(thf)+BPh−4 70.
Compared to the activation energy of the propagation reaction Ea,p = 28 (±2.3) kJ mol−1
determined in MMA polymerization with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 these
values appear to be large, accounting for approximately 50% and 30% of the activation
energy of propagation, respectively. This is equivalent to a very effective mechanism of
stereocontrol.
Stereo pentad analysis of PMMA obtained with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43
and Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 is carried out using the integration of the carbonyl
peaks in quantitative 13C-NMR spectra (Figures 3.44 and 3.45).
116 3 Results and Discussion
(ppm)
175.2175.6176.0176.4176.8177.2177.6178.0178.4178.8
— — — — — ——
mrr
m
mrr
rrr
rr
mrm
m /
mrm
r
mm
rr /
rmrr
mm
mm
mm
mr
rmm
r
—
Figure 3.45: 13C-NMR spectrum of syndiotactic PMMA obtained with
Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 at Tp = −45 ◦C.
PMMA obtained in the polymerization with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43
reveals mmmm pentad abundance between 74.9 % at a polymerization temperature of
Tp = +30 ◦C and 91.5 % at Tp = −30 ◦C (Figure 3.46). Error pentads are observed with
a ratio mmmr : mmrr : mrrm ≈ 2 : 2 : 1 over the whole temperature range investigated.
Other error pentads are practically absent. Only at 30 ◦C a measurable amount of mrrr
is detected. These results are fully consistent with an enantiomorphic site control of
isospecificity. The chirality of the catalyst induces the configuration of the newly formed
chiral center in the polymer chain and a stereo error, i.e. addition of a monomer in a
“wrong” orientation (producing an r dyad), is followed by the correct addition in the next
polymerization step with high probability (producing another r dyad), thus yielding rr
error triads.
From pentad analysis of PMMA obtained in polymerization with
Me2C(Cp)2Zr(Me)(thf)+BPh−4 70, the abundance of syndiotactic pentads is calcu-
lated to range between 52.1 % at Tp = 30 ◦C and 76.9 % at Tp = −45 ◦C (Figure 3.47).
Error pentads display a ratio of rrrm : rrmr ≈ 2 : 1 over the whole temperature range
investigated. This is consistent with a chain end control of syndiospecificity. Other
3.3 MMA Polymerization with Zirconocenes 117
error pentads such as rmrm are also observed, which is not contradictory to a chain
end control mechanism, but a consequence of the lower degree of control of stereospeci-
ficity in MMA polymerization with Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 as compared
to Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43. Apparently, the length of rn-sequences is
relatively short.
Remarkably, the situation in MMA polymerization with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 and Me2C(Cp)2Zr(Me)(thf)
+BPh−4 70 formally
parallels the situation in olefin polymerization. With chiral C2-symmetric catalysts
such as Me2Si(Ind)2ZrCl2 4, highly isotactic polyolefins are obtained with a comparably
weak dependence of isotacticity on polymerization temperature and a pentad abundance
consistent with an enantiomorphic site control mechanism. In the case of C1-symmetric
catalysts, isotactic polymers are only obtained if one of the two coordination sites is
sufficiently sterically crowded, as in the case of Me2C(3-tBuCp)(Flu)ZrCl2 6. This
enforces a chain back skip reaction after each and every insertion step, which therefore
always takes place at the same coordination site (see section 1.2.3). With prochiral
Cs-symmetric catalysts such as Me2C(Cp)(Flu)ZrCl2 5, syndiotactic polyolefins are
obtained with increasing syndiotacticity at lower polymerization temperatures and a
pentad abundance consistent with a chain end control mechanism. Both results are
uniformly explained in terms of not being due to the overall (a)chirality of the catalyst,
but the local chirality that is presented to the monomer at the free coordination site.
In both cases the monomer is forced into a certain chiral orientation. The difference is
that the free coordination sites in two subsequent polymerization steps are homotopic
to each other in C2-symmetric catalysts, diastereotopic in C1-symmetric catalysts and
enantiotopic in Cs-symmetric catalysts.
A proposal for the mechanism of stereocontrol in MMA polymerization with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 and Me2C(Cp)2Zr(Me)(thf)
+BPh−4 70 must be
capable of explaining the respective results concerning stereospecificity and stereo pentad
distribution in a similarly uniform way. Although the polymerization mechanism and
hence the concrete mechanism of stereocontrol are completely different in MMA and in
olefin polymerization, the formal concepts developed in the latter case may help to under-
stand the results in the case of MMA polymerization.
118 3 Results and Discussion
3,2 3,4 3,6 3,8 4,0 4,20
5
10
70
80
90
pen
tad
(tria
d)ab
unda
nce
in%
1/Tp in 10-3
K-1
Figure 3.46: Abundance of stereo pentads (triads) in PMMA obtained with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 at different polymerization temperatures: mm (�),
mmmm (✷), mmmr (�), mmrr (❍), mrrm (�) and mrrr (�).
3,2 3,4 3,6 3,8 4,0 4,2 4,40
10
20
50
60
70
80
90
pen
tad
(tria
d)ab
unda
nce
in%
1/Tp in 10-3
K-1
Figure 3.47: Abundance of stereo pentads (triads) in PMMA obtained with
Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 at different polymerization temperatures: rr (�), r-
rrr (✷), rrrm (�), rrmr (❍), rmrm (�).
3.3 MMA Polymerization with Zirconocenes 119
3.3.5 Proposal for a Polymerization Mechanism
The proposed mechanism (Scheme 3.10) is closely related to the mechanism proved
in samarocene catalyzed MMA polymerization, as described by Yasuda et al., and to the
recent findings of Collins et al. (section 1.3). It serves as a working hypothesis.
3.3.5.1 Mechanism of Chain Propagation
The active species is assumed to be a cationic zirconocene ester enolate complex
L2Zr[OC(OMe)C(Me)(R)]+ (R = Me, growing chain; A,A′ in Scheme 3.10). It is
isoelectronic to the neutral samarocene ester enolate that is proved to be the active
species in the “Yasuda-type” MMA polymerization. Like the latter, it combines
the functions of the two catalyst components in “Collins-type” MMA polymerization,
activating the growing chain end as a donor (d2 in Seebach nomenclature) and, at
the same time, an incoming MMA molecule as an acceptor (a3 in Seebach nomen-
clature). Recently, Collins et al. have similarly proposed propagation via such a
cationic zirconocene ester enolate complex in MMA polymerization catalyzed with
Me2Si(C5Me4)(NtBu)Zr(thf)[OC(OtBu)CMe2]
+BPh−4 .
Consequently, carbon bond formation proceeds via an intramolecular Michael addition,
yielding a cyclic intermediate (B,B′) with the polymer chain loosely attached to the
catalyst. The enchained MMA monomer has become the ester enolate chain end, situated
at the other side of the catalyst wedge as compared to the initial situation. The loosely
attached polymer chain is then replaced by a new monomer, probably in an associative
mechanism (via C,C′). If the associative mechanism proceeds via a backside attack of
the new monomer, then the initial situation will be reestablished and the catalytic cycle
will be completed.
Also a dissociative mechanism via a species bearing a vacant coordination site may
be possible (B′ → A,B → A′). In this case the polymer chain and the new monomer
interchange their relative positions after every polymerization step, unless active site epi-
merization occurs. However, such a dissociative mechanism is unlikely, because a cationic
ester enolate with a vacant coordination site is considerably unstable and there is no
reason to assume that the comparably stable, electronically saturated and chelated cyclic
structure will break up. Depending on the ratio of the rates of monomer coordination and
carbon bond formation, an active site epimerization may also take place in an associative
fashion (A⇀↽ A′).
120 3 Results and Discussion
L2Zr
O
MeO
O
MeO
O
OMe
Pol
L2Zr
O
MeO
O
MeO
O
OMePol
L2Zr
O
MeO
O
MeO
O
OMe
COOMe
Pol
L2ZrO
MeOO
OMe
Pol
L2Zr
O
MeOO
MeO
Pol
L2ZrO
MeOO
MeO
PolCOOMe
O
MeOO
OMe
L2Zr
PolCOOMe
a3
d2
a3
d2
+ MMA
+ MMA
± MMA
± MMA
B'A'
C'
B
A
C
Scheme 3.10: Proposal for a polymerization mechanism.
3.3 MMA Polymerization with Zirconocenes 121
3.3.5.2 Mechanism of Stereocontrol
The proposed model mechanism allows for a uniform explanation of
the results with respect to PMMA microstructure in polymerization with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 and Me2C(Cp)2Zr(Me)(thf)
+BPh−4 70. It
could also account for similar results in samarocene catalyzed MMA polymerization and
the recent findings of Collins et al. (section 1.3.4).
Analogous to the zirconocene catalysts in olefin polymerization, the zirconocene ester
enolate has two potential coordination sites for a new monomer or the growing chain.
Therefore, stereospecificity must be explained in terms of both the sequence of monomer
coordination to either of the two sites available and the respective enantiofacial selectivity
of either site. The two catalysts applied are substantially different with respect to the
latter and should give some insight into the mechanism of stereocontrol.
Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 is C2v-symmetric, bearing two homotopic coordi-
nation sites, which are both aspecific. In other words, monomer coordination to either
of the two coordination sites as well as re- or si-coordination of a monomer at a given
coordination site are all energetically equal (Figure 3.48). Regardless of the sequence of
monomer coordination, be it always at the same coordination site as a consequence of
an associative mechanism or at alternating sites in subsequent steps, no stereospecificity
should be observed due to the absence of enantiofacial selectivity at both coordination
sites. Consequently, the observed syndiospecificity must be due to chain end control,
which is consistent with pentad analysis. It is worth noting that the lower overall syn-
diospecificity of Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 as compared to the isospecificity of
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 is not the result of a drastically lower energetic
differentiation. Therefore, a strong temperature effect is observed, giving rise to a con-
siderable degree of syndiotacticity at low polymerization temperatures.
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 is C1-symmetric, bearing two substantially dif-
ferent diastereotopic coordination sites at the zirconium center, one of which is expected
to exhibit enantiofacial selectivity, while the other one is aspecific. That is to say, the
latter coordination site does not place any constraint on either the conformation of the
growing chain or re- or si-coordination of a new monomer or of the prochiral active chain
end, while the other one does so with its annealed benzene ring. Apparently, the high
isospecificity of Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 is the result of monomer coordi-
nation always taking place at the same coordination site. It is difficult to conclude at
which one, because the situation is complicated by the fact that both the monomer and
122 3 Results and Discussion
ZrO O
MeO
PolZrO O
OMeMeO
Pol
ZrO O
OMe
Pol
O Zr O
MeOOMe
PolZrO O
OMe
OMe
Pol
O Zr
O
MeOOMe
Pol O
OMe
no enantiofacial selectivity
Figure 3.48: Stereochemistry of MMA polymerization catalyzed with
Me2C(Cp)2Zr(Me)(thf)+BPh−4 70.
ZrO O
MeO
PolZrO O
OMeMeO
Pol
O Zr O
MeOOMe
Pol
O Zr
O
MeOOMe
Pol O
OMe
re selectivity
Figure 3.49: Example for a possible mechanism of stereocontrol in MMA polymerization
catalyzed with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43.
3.3 MMA Polymerization with Zirconocenes 123
the chain end are prochiral, and the new asymmetric centre is actually formed where the
chain end is situated. One possible scenario is that the polymer chain is situated at the
site with enantiofacial selectivity and the monomer on the other side (Figure 3.49). It
may even be possible that only one of the two possible monomer orientations displayed
in Figure 3.49 allows for carbon bond formation because of electronic requirements.
A straightforward explanation for monomer coordination always taking place at
the same coordination site is an associative replacement of the growing chain by a
new monomer. In the case of a dissociative mechanism, a fast and irreversible active
site epimerization reaction reestablishing the initial situation after every polymeriza-
tion step must be assumed. In either case, there must be a driving force leading to
a preference of one coordination mode over the other. Possibly, the bulky polymer
chain is always coordinated to the coordination site without steric hindrance, or the
new monomer can only attach to this site. As a result, MMA polymerization with
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 appears to be isospecific with enantiomorphic site
control, which is consistent with pentad analysis.
3.3.5.3 Generation of the Active Species
The proposed active species, the cationic zirconocene ester enolate complex
L2Zr[OC(OMe)C(Me)(R)]+, may be generated from Me2C(Cp)(Ind)Zr(Me)(thf)
+BPh−4
43 and Me2C(Cp)2Zr(Me)(thf)+BPh−4 70, respectively, via a replacement of the THF
ligand with a MMA molecule and a transfer of the methyl group to the coordinated
MMA (Figure 3.50). The methyl transfer may account for the observed initiation period
in polymerization kinetics.
3.3.5.4 What Makes the Difference?
According to theoretical calculations by Sustmann et al.157 the proposed monometal-
lic mechanism is expected to be only slightly less feasible than the bimetallic mechanism
that is proposed in “Collins-type” MMA polymerization. Furthermore, the “Collins-
type” mechanism gains preference by the concluding back-side attack of a new MMA
molecule on the cationic methyl zirconocene replacing the growing chain as a ligand.
Therefore, it may even be disfavoured, when ansa-zirconocenes are applied, be-
cause the ligand backbone does not allow for the backside attack. The question
is why Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 and Me2C(Cp)2Zr(Me)(thf)
+BPh−4 70
124 3 Results and Discussion
ZrOO
MeO
Me
MeO
Zr MeO
ZrO
Me
Zr MeO
MeO
ZrO
Me
MeO
ZrOO
MeO
Me
MeO
MMA
MMA MMA
MMA
41
68
– THF
– THF
Figure 3.50: Proposal for the generation of cationic zirconocene ester enolate complex
from Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 and Me2C(Cp)2Zr(Me)(thf)
+BPh−4 70.
are active catalysts without the addition of the corresponding neutral zirconocene
ester enolate component under the experimental conditions chosen, while the other
cationic complexes investigated are not. It is worth noticing that Gibson et al.160 re-
port that (Cp)(C5Me5)ZrMe2 45, C2H4(Ind)2ZrMe2 46, Me2C(Cp)(Ind)ZrMe2 47 and
Me2C(Cp)(2−MeInd)ZrMe2 48 (but not Me2C(Cp)(Flu)ZrMe2 49) are active cata-
lysts for MMA polymerization when activated with an equimolar amount of B(C6F5)3
(section1.3).
All of these facts indicate that the activation process is the crucial step which
discriminates different cationic zirconocene complexes under the applied experimental
conditions. Apparently, under the experimental conditions chosen in this study, only
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 and Me2C(Cp)2Zr(Me)(thf)
+BPh−4 70 are capable
of transferring a methyl group to an MMA molecule, thus generating the active species. As
only the ligand backbone changes, it must be its geometry or the electronic consequences
inevitably related with the latter that makes the difference. The most obvious geometric
feature of Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 and Me2C(Cp)2Zr(Me)(thf)
+BPh−4 70
is the large ligand aperture caused by the very short isopropylidene bridge. In complexes
with a methylene or an isopropylidene bridge the Cp-Zr-Cp angle θ is far below 120◦,
while most of the typical bridged or unbridged zirconocenes exhibit Cp-Zr-Cp angles
θ of 125◦ and above (Table 3.13). A small Cp-ZrCp angle θ is equivalent to a large
ligand aperture (Figure 3.51). Possibly, it is the large ligand aperture in the case of
Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 and Me2C(Cp)2Zr(Me)(thf)
+BPh−4 70 that alters
the reactivity to allow for the methyl transfer.
3.3 MMA Polymerization with Zirconocenes 125
Zrθ ligand aperture
Figure 3.51: Relation of Cp-Zr-Cp angle θ and ligand aperture in ansa-zirconocenes.
Table 3.13: Cp-Zr-Cp angles of different zirconocene dichlorides.
Catalyst Cp-Zr-Cp Ref.
θ
Me2C(Cp)2ZrCl2 63 116.6◦ 186
Cp2ZrCl2 1 129.3◦ 187
Me2Si(Cp)2ZrCl2 62 125.4◦ 188
Me2Si(Ind)2ZrCl2 4 127.8◦ 182
C2H4(Ind)2ZrCl2 3 126.9◦ 182
3.3.6 Conclusions
The polymerization of MMA with certain chiral cationic zirconocene catalysts turns
out to be an efficient tool for a control of the stereospecificty of the polymerization reaction
and hence the microstructure of PMMA. It is essential that the polymerization mechanism
is different from the well-known mechanism of the group-transfer polymerization of MMA
with zirconocenes. If the proposed mechanism is proved, then it appears to be possible
to transfer the concepts of microstructure control in olefin polymerization to the poly-
merization of MMA.
What may be even more important is the fact that this mechanism may provide a
pathway to poly(olefin-block -MMA) and related copolymers. Possibly, the initiation will
not only proceed via a methyl transfer, but also via the transfer of a growing polyolefin
chain polymerized with a suitable cationic zirconocene complex. After all, it seems that
there may be a possibility of copolymerizing olefins and functionalized monomers, but
only if the respective reactivity towards the highly electrophilic zirconocene catalyst is
respected.189
127
4 Experimental Part
4.1 General Procedures
All experiments are carried out in a nitrogen atmosphere using standard Schlenk
techniques. Nitrogen is passed over an activated copper catalyst (BASF R3-11) at 100 ◦C,
over molecular sieves (4 A), and over finely dispersed K/Al2O3 for purification. Glassware
is prepared prior to use with repeated cycles of evacuating in HV, heating with a hot-air
drier, cooling to room temperature and filling with nitrogen.
Me3SiCl (Fluka, > 99 %), MeLi (Fluka, 1.6 M solution in DE), BuLi (Aldrich, 2 M
solution in pentane), Na+BPh−4 (Fluka, > 98 %), Aliquat 336 (Aldrich), NBu3(Fluka,
> 98%), MeMgCl (Aldrich, 3 M solution in THF), ZrCl4 (Fluka, > 98%), NaOH, Na2SO4,
NaHCO3, P2O5, are used without further purification.
Toluene and THF are distilled from Na/benzophenone; CH2Cl2 and acetone are dried
over P2O5 prior to distillation; pentane, hexane and heptane are distilled from Na/K
alloy; MeOH is distilled from a solution of NaOMe in MeOH; pyrollidine, HNEt2 and
HNiPr2 are dried over KOH prior to distillation. All solvents and reactants are stored in
a nitrogen atmosphere. All deuterated solvents are degassed by three freeze-pump-thaw
cycles and stored in a nitrogen atmosphere over molecular sieves (4 A). MMA is stored at
0 ◦C over CaH2 and freshly distilled before use. Cyclopentadiene (b. p. 54◦C) is freshly
prepared by thermal cracking of bis(cyclopentadiene) over Fe powder at 180 ◦C.
All 1H-NMR and 13C-NMR spectra are recorded on a Bruker DPX 300 FT-NMR
spectrometer at 300 MHz and 75 MHz, respectively. All chemical shifts δ are given in
ppm with Me4Si as a reference. Quantitative13C-NMR spectra for the anlysis of poly(1-
hexene) and PMMA microstructure are recorded with an inverse gated decoupling pulse
sequence and using NS > 12000, TD = 32768, D1 = 5 s, pulse angle 30◦.
All gel permeation chromatography (GPC) analyses are carried out at room tem-
perature using a HPLC pump (Waters 510) and a refractive index detector (ERC 7515a).
128 4 Experimental Part
The eluent is THF stabilized with 250 mg mL−1 2,6-di-tert-butyl-4-methylphenol. The
flow rate is 1.0 mL min−1. Four columns are applied (l = 300 mm, d = 8 mm, PS-DVB
gel, 5µ, MZ Analysentechnik) with pore sizes of 100A, 100A, 1000A and 10000A. Num-
ber average molecular weights and polydispersities are relative to PS standards (Polymer
Standards Service) for all polymers apart from PMMA, and relative to PMMA standards
(Polymer Laboratories) in the case of PMMA.
4.2 Preparation of Ligands and Reactants
Synthesis of Me2C(CpH)2 57
NaOH (100 g, 2.5 mol) and 10 g of Aliquat-336 are suspended in 500 mL THF. Freshly
distilled cyclopentadiene (66 g, 1 mol) is added within 30 min at 0 ◦C via a dropping
funnel. Then acetone (29 g, 0.5 mol) is added under vigorous stirring within 120 min at
the same temperature. The reaction mixture is allowed to warm up to room temperature.
The solution is decanted from the solid residue, diluted with pentane, washed until the
washing waters react neutral, and dried over Na2SO4. The volatiles are removed in vacuo,
and the resulting oil is distilled in vacuo. Me2C(CpH)2 57 (26.4 g, 30.6 %, b. p. 56◦C in
HV) is obtained as a slightly yellow viscous liquid.
1H-NMR (CDCl3): δ = 1.42/1.43/1.42 (s, 6H, Me), 2.81/2.95 (m, 4H, CpRH/CpH2),
5.95-6.05 (m, 8H, CpH); 13C-NMR (CDCl3): δ = 27.5/28.5/29.3 (Me), 36.9/37.8 (CMe2),
123.2/123.9/124.7 (Cp), 131.0/131.8/133.4 (Cp), 154.5/155.4/157.1/158 (Cp).
Synthesis of 6,6-dimethylfulvene 58
Acetone (116 g, 2 mol) is added to a solution of pyrrolidine (187 g, 2.6 mol) in 600 mL
of MeOH within 30 min at room temperature. The mixture is heated to 40 ◦C, and freshly
distilled cyclopentadiene (66 g, 1 mol) is added within 30 min. The reaction mixture is
stirred for 60 min, before 162 g (2.7 mol) of AcOH are added within 30 min. The reaction
mixture is diluted with 200 mL of water and extracted four times with 150 mL of pentane.
The combined organic solutions are washed with aqueous NaHCO3 solution, then with
water, and dried over Na2SO4. Distillation in vacuo yields 6,6-dimethylfulvene 58 (43.3 g,
40.8 %, b. p. 38 ◦C in HV) as a yellow oil.
1H-NMR (CDCl3): δ = 2.15 (s, 6H, Me), 6.43-6.50 (m, 4H, CH); 13C-NMR (CDCl3):
δ = 23.0 (Me), 120.5 (CH), 130.6 (CH), 142.5 (CR2), 149.8 (CR2).
4.2 Preparation of Ligands and Reactants 129
Synthesis of Me2C(CpH)(IndH) 59
BuLi solution (100 mL, 2 M in pentane) is added to a solution of freshly distilled indene
(23.2 g, 0.25 mol) in 100 mL of THF within 120 min at −78 ◦C. The reaction mixture is
allowed to slowly warm up to 0 ◦C. Then 6,6-dimethylfulvene 58 (21.2 g, 0.25 mol) is
added within 60 min at −78 ◦C. The reaction mixture is stirred at room temperature for
12 h, diluted with 100 mL of pentane, washed with water and dried over Na2SO4. The
volatiles are removed in vacuo. Me2C(CpH)(IndH) 59 (39.2 g, 88.1 %) is obtained as a
red oil.
1H-NMR (CDCl3): δ = 0.97/0.99, 1.25/1.30 (s, 6H, Me), 3.03 (m, ≈ 4H, CpH2), 3.67
(m, ≈ 2H, CRH), 5.97-6.56 (m, ≈ 2H, vin. CpH,IndH), 6.76-7.44 (m, ≈ 4H, IndH);
13C-NMR (CDCl3): δ = 22.8/23.4, 27.0/28.8 (Me), 37.9/38.8 (CMe2), 40.4/40.9 (CpH2),
58.9/60.6 (CpRH), 124.1, 124.3, 124.4, 125.6, 126.3, 126.4, 130.8, 131.6, 137.3, 145.2,
155.5, 157.6 (Cp,Ind).
Synthesis of Me2C(CpH)(FluH) 60
100 mL of BuLi solution (2 M in pentane) are added to a suspension of fluorene (33.1 g,
0.25 mol) in 100 mL of THF within 120 min at −78 ◦C. The reaction mixture is allowed
to warm up to room temperature and stirred for 12 h. Then 6,6-dimethylfulvene 58
(21.2 g, 0.25 mol) is added within 60 min at −78 ◦C. The reaction mixture is stirred at
room temperature for 18 h, washed with water and poured into 200 mL of pentane. The
precipitate is filtered off, recrystallized from CHCl3/MeOH/hexane and dried in vacuo.
Me2C(CpH)(FluH) 60 (22.6 g, 41.5 %) is obtained as a light yellow powder.
1H-NMR (CDCl3): δ = 1.04/1,05 (s, 6H, Me), 3.05/3.06, 3.14/3.15 (m, ≈ 2H, CpH2,
4.11/4.14 (m, ≈ 1H , CpRH), 5.90/6.11 (m, ≈ 2H, CpH), 6.43/6.51, 6.52/6.62 (m, ≈ 1H,
FluH), 7.02-7.35, 7.64-7.73 (8H, m, ar. FluH); 13C-NMR (CDCl3): δ = .
Synthesis of Zr(NEt2)4 61
In a 2 L three-necked round-bottom flask with a magnetic stirring bar, a dropping
funnel and a thermometer 455 mL of BuLi solution (2 M in pentane) are slowly added to
a solution of diethyl amine (66.1 g, 0.905 mol) in 200 mL of Et2O within 120 min so that
a temperature of −60 ◦C is not exceeded. The reaction mixture is allowed to warm up to
0 ◦C and stirred for 60 min. Then a suspension of ZrCl4 (52.64 g, 0.226 mol) in 200 mL
130 4 Experimental Part
toluene is slowly transferred to the reaction flask via a double-tipped needle (gauge 12),
so that the temperature does not exceed 0 ◦C. The volatiles are removed at 80 ◦C at
ambient pressure, then at room temperature in HV. Distillation at 160 ◦C in HV yields
Zr(NEt2)4 61 (61.4 g, 71.6 %) as a colourless viscous liquid.
1H-NMR (CDCl3): δ = 1.24 (t, 12H, CH3), 3.42 (q, 8H; CH2);13C-NMR (CDCl3):
δ = 14.9 (CH3), 42.3 (CH2).
Synthesis of HNBu+3BPh
−4
Aqueous HCl (20 mL, conc.) is added to a solution of dibutyl amine (8.3 g, 45 mmol)
in 200 mL of THF at 0 ◦C. The volatiles are removed in vacuo. The mixture is dissolved
in 200 mL of water and poured into a solution of Na+BPh−4 (14.4 g, 42 mmol) in 100 mL
of water. The suspension is stirred for 2 h. The precipitate is filtered off and dried at
50 ◦C in HV. HNBu+3 BPh−4 (21.2 g, 100 %) is obtained as a white solid.
1H-NMR (CDCl3): δ = 0.92 (t, 9H, CH3), 1.25-1.40 (m, 6H, CH2), 1.50-1.64 (m, 6H,
CH2), 3.00 (m, 6H, NCH2), 6.76-6.84 (bt, 4H, Ph), 6.88-6.98 (bt, 8H, Ph), 7.12-7.25 (m,
8H, Ph); 13C-NMR (CDCl3): δ = 13.9 (CH3), 19.7 (CH2), 25.4 (CH2), 52.4 (NCH2), 121.9
(Ph), 125.6 (Ph), 135.9 (Ph), 163.7 (q, BPh).
4.3 Preparation of Zirconocene Complexes
4.3.1 Preparation of Zirconocene Dichlorides
Synthesis of Me2C(Cp)2ZrCl2 63
A solution of Me2C(CpH)2 57 (4.8 g, 28 mmol) in 30 mL of toluene is slowly added
to a solution of Zr(NEt2)4 61 (10.6 g, 28 mmol) in 80 mL of toluene via a double tipped
needle at −78 ◦C. The reaction mixture is allowed to warm up to room temperature and
stirred for 12 h. It is then heated to 90 ◦C under stirring for 72 h. Toluene and other
volatiles are removed in vacuo, and the mixture is again diluted with 50 mL of toluene.
Me3SiCl (9.0 g, 83 mmol) is added slowly at 0◦C, and the reaction mixture is stirred for
24 h at room temperature. Filtering off the precipitate, washing with hexane and drying
in HV yields Me2C(Cp)2ZrCl2 63 (7.9 g, 84.4 %) as a light yellow powder.
1H-NMR (CDCl3): δ = 1.75 (s, 6H, Me), 5.70 (t, 4H, Cp), 6.61 (t, 4H, Cp); 13C-NMR
(CDCl3): δ = 23.0 (Me), 36.7 (CMe2), 102.0 (Cp), 105.8 (Cp), 121.8 (Cp).
4.3 Preparation of Zirconocene Complexes 131
Synthesis of Me2C(Cp)(Ind)ZrCl2 64
A solution of Me2C(CpH)(IndH) 59 (6.2 g, 28 mmol) in 30 mL of toluene is slowly
added to a solution of Zr(NEt2)4 61 (10.6 g, 28 mmol) in 80 mL of toluene via a double
tipped needle at−78 ◦C. The reaction mixture is allowed to warm up to room temperature
and stirred for 12 h. It is then heated to 90 ◦C under stirring for 72 h. Toluene and other
volatiles are removed in vacuo, and the mixture is again diluted with 50 mL of toluene.
Me3SiCl (9.0 g, 83 mmol) is added slowly at 0◦C, and the reaction mixture is stirred for
24 h at room temperature. Filtering off the precipitate, washing with hexane and drying
in HV yields Me2C(Cp)(Ind)ZrCl2 64 (8.8 g, 82.1 %) as a yellow-orange powder.
1H-NMR (CDCl3): δ = 1.88, 2.10 (s, 6H, Me), 5.62 (m, 1H, Cp), 5.73 (m, 1H, Cp),
6.04 (m, 1H, Ind), 6.41, (m, 2H, Cp), 6.75 (m, 1H, Ind), 6.92-7.30 (m, 2H, Ind), 7.50-7.61
(m, 2H, Ind); 13C-NMR (CDCl3): δ = 24.7, 25.3 (Me), 39.2 (CMe2), 103.3, 105.2, 111.2,
112.7, 119.2, 119.9, 120.6, 122.2, 125.1, 125.7, 125.9, 127.3, 128.0, 130.0 (Cp,Ind) .
Synthesis of Me2C(Cp)(Flu)ZrCl2 5
A mixture of Me2C(CpH)(FluH) 60 (4.63 g, 17 mmol) and 200 mL of toluene is slowly
added to a solution of Zr(NEt2)4 61 (6.5 g, 17 mmol) in 100 mL of toluene via a double
tipped needle at−78 ◦C. The reaction mixture is allowed to warm up to room temperature
and stirred for 12 h. It is then heated to 80 ◦C under stirring for 5 d. Toluene and other
volatiles are removed in vacuo, and the mixture is again diluted with 200 mL of toluene.
Me3SiCl (5.4 g, 50 mmol) is added slowly at 0◦C, and the reaction mixture is stirred for
72 h at room temperature. Filtering off the precipitate, washing with hexane and drying
in HV yields Me2C(Cp)(Flu)ZrCl2 5 (1.97 g, 26.8 %) as an orange-red powder.
1H-NMR (CDCl3): δ = 2.33 (s, 6H, Me), 5.69 (t, 2H, Cp), 6.15-6.28 (m, 2H, Cp), 6.15-
7.27 (m, 2H, Flu), 7.44-7.53 (m, 2H, Flu), 7.78 (d, 2H, Flu), 8.06 (d, 2H, Flu); 13C-NMR
(CDCl3): δ = 28.7 (Me), 101.9, 118.1, 121.7, 122.4, 123.9, 124.1, 124.9, 125.3, 126.4, 128.1
(Cp,Flu).
4.3.2 Preparation of Dimethyl Zirconocenes
Synthesis of Cp2ZrMe2 15
A MeMgCl solution (23 mL, 3 M in THF) is added slowly to a suspension of Cp2ZrCl2
1 (9.6 g, 33 mmol) in 40 mL of Et2O at −30 ◦C. The mixture is allowed to warm up
132 4 Experimental Part
to room temperature and is stirred for 3 h. The volatiles are removed in vacuo. The
residue is treated with 30 mL of toluene, stirred for 3 h, and then filtered off. Toluene
is removed from the filtrate in vacuo. The raw product (6.04 g, 73.1 %) is obtained as a
yellow powder and purified by sublimation (80 ◦C, 3 · 10−3 mbar). Cp2ZrMe2 15 (5.13 g,
62.1 %) is obtained as an almost colourless solid.
1H-NMR (CDCl3): δ = -0.46 (s, ≈ 6H, ZrMe), 6.02 (m, 10H, Cp); 13C-NMR (CDCl3):
δ = 30.1 (ZrMe), 110.7 (Cp).
Synthesis of C2H4(Ind)2ZrMe2 46
A MeLi solution 9 mL, 1.8 M in DE) is added slowly to a suspension of
C2H4(Ind)2ZrCl2 3 (3.37 g, 8 mmol) in 60 mL of Et2O at 0 ◦C. The mixture is stirred
for 2 h at 0 ◦C and for 12 h at room temperature. The volatiles are removed in vacuo.
The residue is treated with 50 mL toluene. Filtration and removal of the toluene in vacuo
yields C2H4(Ind)2ZrMe2 46 (1.25 g, 41.2 %) as a light brown powder.
1H-NMR (CDCl3): δ = -1.47 s, 6H, ZrMe), 3.04-3.30 (m, 4H, Et), 5.89 (d, 2H, Ind),
6.46 (d, 2H, Ind), 6.96 (m, 2H, Ind), 7.12 (m, 2H, Ind), 7.32 (dm, 2H, Ind), 7.39 (dm, 2H,
Ind); 13C-NMR (CDCl3): δ = 27.0 (et), 35.1 (ZrMe), 101.0, 108.8, 111.9, 119.4, 119.8,
121.1, 123.7, 124.6, 127.6 (Ind).
Synthesis of Me2C(Cp)(Ind)ZrMe2 47
A MeLi solution (15 mL, 1.8 M in DE) is added slowly to a suspension of
Me2C(Cp)(Ind)ZrCl2 64 (4.45 g, 13 mmol) in 60 mL of Et2O at 0 ◦C. The mixture is
stirred for 2 h at 0 ◦C and for 12 h at room temperature. The volatiles are removed in
vacuo. The residue is treated with 50 mL toluene. Filtration and removal of the toluene
in vacuo yields Me2C(Cp)(Ind)ZrMe2 47 (2.35 g, 59.1 %) as a light brown powder.
1H-NMR (CDCl3): δ = 1.51, -0.37 (s, 6H, ZrMe), 1.63, 1.84 (6H, Me), 5.40 (m, 1H,
Cp), 5.47 (m, 1H, Cp), 5.83 (d, 1H, Ind), 6.30 (m, 1H, Cp), 6.38 (m, 1H, Cp), 6.70 (d,
1H, Ind), 6.84 (dd, 1H, Ind), 7.14 (dd, 1H, Ind), 7.42 (d, 1H, Ind), 7.56 (d, 1H, Ind);
13C-NMR (CDCl3): δ = 24.5, 25.4 (Me), 29.2, 32.4 (ZrMe), 37.4 (CMe2), 96.7, 102.3,
102.7 (Cp), 103.3, 111.7 (Ind), 112.2 (Cp), 112.7 (Ind), 113.5 (Cp), 117.8, 122.4, 122.5,
122.9, 124.6, 125.6 (Ind).
4.3 Preparation of Zirconocene Complexes 133
Synthesis of Me2C(Cp)(Flu)ZrMe2 49
A MeLi solution (2.3 mL, 1.8 M in DE) is added slowly to a suspension of
Me2C(Cp)(Flu)ZrCl2 5 (1.08 g, 2.5 mmol) in 50 mL of Et2O at 0 ◦C. The mixture
is stirred for 2 h at 0 ◦C and for 12 h at room temperature. The volatiles are re-
moved in vacuo. The residue is treated with 30 mL toluene. Filtration and removal
of the toluene in vacuo yields Me2C(Cp)(Flu)ZrMe2 49 (0.64 g, 65.9 %) as a dark brown
powder. Because of the low amount of the raw product, it is used for the synthesis of
Me2C(Cp)(Flu)Zr(Me)(thf)+BPh−4 68 without NMR analysis.
Synthesis of Me2Si(Cp)2ZrMe2 65
A MeLi solution (3.6 mL, 1.8 M in DE) is added slowly to a suspension of
Me2Si(Cp)2ZrCl2 62 (1.12 g, 3.2 mmol) in 60 mL of Et2O at 0 ◦C. The mixture is stirred
for 2 h at 0 ◦C and for 12 h at room temperature. The volatiles are removed in vacuo.
The residue is treated with 50 mL toluene. Filtration and removal of the toluene in vacuo
yields Me2Si(Cp)2ZrMe2 65 (0.73 g, 73.9 %) as a brown powder. Because of the low
amount of the raw product, it is used for the synthesis of Me2Si(Cp)2Zr(Me)(thf)+BPh−4
69 without NMR analysis
Synthesis of Me2C(Cp)2ZrMe2 66
A MeLi solution (11 mL, 1.8 M in DE) is added slowly to a suspension of
Me2C(Cp)2ZrCl2 63 (3.31 g, 10 mmol) in 60 mL of Et2O at 0 ◦C. The mixture is stirred
for 2 h at 0 ◦C and for 12 h at room temperature. The volatiles are removed in vacuo.
The residue is treated with 50 mL toluene. Filtration and removal of the toluene in vacuo
yields Me2C(Cp)2ZrMe2 66 (2.51 g, 86.2 %) as a beige powder.
1H-NMR (CDCl3): δ = d -0.23 (s, 6H, ZrMe), 1.62 (s, 6H, Me), 5.59 (m, 4H, Cp),
6.65 (m, 4H, Cp); 13C-NMR (CDCl3): δ = 22.9 (CMe2), 27.3 (ZrMe), 35.6 (CMe2), 103.7,
113.7, 123.3 (Cp).
134 4 Experimental Part
4.3.3 Preparation of Cationic Methyl Zirconocenes
Synthesis of Cp2Zr(Me)(thf)+BPh−4 26
A solution of Cp2ZrMe2 15 (5.26 g, 20.9 mmol) in 20 mL of THF is added slowly and
under vigorous stirring to a suspension of HNBu+3 BPh−4 (10.6 g, 20.9 mmol) in 80 mL
toluene at 0 ◦C via a double-tipped needle. The reaction mixture is allowed to warm up
to room temperature and stirred for 2 h. The light yellow precipitate is filtered off and
dried in HV. Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 (10.8 g, 81.9 %) is obtained as an almost
white powder.
1H-NMR (CD2Cl2): δ = 0.70 (s, 3H, ZrMe), 1.75 (bs, 4H, THF), 3.40 (bs, 4H, THF),
6.25 (s, 10H, Cp), 6.91 (t, 4H, Ph), 7.05 (t, 8H, Ph), 7.37 (m, 8H, Ph); 13C-NMR (CD2Cl2):
δ = 24.4 (THF), 42.7 (ZrMe), 75.3 (b, THF), 113.8 (Cp), 120.8, 124.6, 135.0 (Ph), 163.1
(q, BPh).
Synthesis of Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43
A solution of Me2C(Cp)(Ind)ZrMe2 47 (2.04 g, 6.0 mmol) in 20 mL of THF is added
slowly and under vigorous stirring to a suspension of HNBu+3 BPh−4 (2.96 g, 5.9 mmol)
in 80 mL toluene via a double-tipped needle. The reaction mixture is stirred for 18 h at
room temperature. A red oil separates from the solution. The solvents are removed with
a canula. The oil is dried in vacuo to yield Me2C(Cp)(Ind)ZrMe2 47 (2.29 g, 53.4 %) as
an orange solid.
1H-NMR (CD2Cl2): δ = -0.82, 0.58 (s, ≈ 3H, ZrMe), 1.65, bs, 4H, THF), 1.67, 1.180,
1.85, 2.24 (s, ≈ 6H, Me), 3.36 (bs, 4H, THF), 5.29, 5.48, 5.71, 5.77, 5.80, 5.88, 6.03, 6.64
(m, 4H, Cp), 5,64, 5.99, 6.24, 6.44 (m, ≈ 2H, Ind), 6.78-6.88 (bt, 4H, Ph), 7.05-7.19, 7.34-
7.66 (bm, ≈ 4H, Ind), 6.92-7.00 (bt, 8H, Ph), 7.22-7.31 (bm, 8H, Ph); 13C-NMR (CD2Cl2):
δ = 23.1, 23.5, 24.2, 25.0 (Me), 24.8 (THF), 44.0, 45.8 (ZrMe), 38.5, 38.9 (CMe2), 121.1,
125.2, 135.2 (Ph), 163.2 (q, BPh), 103-138 (approximately 28 signals, Ind).
Synthesis of C2H4(Ind)2Zr(Me)(thf)+BPh−4 67
A solution of C2H4(Ind)2ZrMe2 46 (2.39 g, 6.3 mmol) in 20 mL of THF is added slowly
and under vigorous stirring to a suspension of HNBu+3 BPh−4 (3.13 g, 6.2 mmol) in 80 mL
toluene via a double-tipped needle. The reaction mixture is stirred for 18 h at room
temperature. A dark red oil separates from the solution. The solvents are removed with a
4.3 Preparation of Zirconocene Complexes 135
canula. The oil is dried in HV to yield C2H4(Ind)2ZrMe2 46 (3.73 g, 79.8%) as an orange
solid.
1H-NMR (CD2Cl2): δ = 0.0 (s, 3H, ZrMe), 1.21 (bs, ≈ 4H, THF), 3.60-3.76 (m,
≈ 8H, Et,THF), 6.14 (d, 2H, Ind), 6.52 (dd, 2H, Ind), 7.13 (ddd, 2H, Ind), 7.26 (ddd, 2H,
Ind), 7.42 (dm, 2H, Ind), 7.58 (dm, 2H, Ind); 13C-NMR (CD2Cl2): δ = 25.1/25.7 (THF),
28.0/29.1 (Et), 49.5 (ZrMe), 77.4 (b, THF), 105.4/108.5, 114.8/116.6, 120.4/120.6, 123.0,
125.0, 126.4/126.6, 127.1, 127.9, 128.7, 135,7, 138.3 (Ind), 121.6, 125.4, 135.7 (Ph), 163.7
(q, BPh).
Synthesis of Me2C(Cp)(Flu)Zr(Me)(thf)+BPh−4 68
A solution of Me2C(Cp)(Flu)ZrMe2 49 (0.573 g, 1.46 mmol) in 10 mL of THF is added
slowly and under vigorous stirring to a suspension of HNBu+3 BPh−4 (0.720 g, 1.46 mmol) in
50 mL toluene via a double tipped needle. The reaction mixture is stirred for 24 h at room
temperature. The volatiles are removed in HV. The resulting oil is treated with 20 mL of
hexane and stirred for 24 h. The dark orange precipitate is filtered off and dried in HV
to yield Me2C(Cp)(Flu)Zr(Me)(thf)+BPh−4 68 (0.813 g, 72.4 %) as a brown-orange solid.
The product is only soluble in CD2Cl2, which leads to rapid decomposition. Therefore,
no reproducible NMR spectra are obtained.
Synthesis of Me2Si(Cp)2Zr(Me)(thf)+BPh−4 69
A solution of Me2Si(Cp)2ZrMe2 65 (0.576 g, 1.87 mmol) in 10 mL of THF is added
slowly and under vigorous stirring to a suspension of HNBu+3 BPh−4 (0.93 g, 1.85 mmol)
in 50 mL toluene via a double tipped needle. The reaction mixture is stirred for 18 h at
room temperature. The volatiles are removed in HV. The resulting oil is treated with
20 mL of hexane and stirred for 24 h. The brown precipitate is filtered off and dried in
HV to yield Me2Si(Cp)2Zr(Me)(thf)+BPh−4 69 (0.421 g, 32.9 %) as a brown solid. The
product is only soluble in CD2Cl2, which leads to decomposition within a few minutes.
Therefore, no reproducible NMR spectra are obtained.
Synthesis of Me2C(Cp)2Zr(Me)(thf)+BPh−4 70
A solution of Me2C(Cp)2ZrMe2 66 (2.28 g, 7.82 mmol) in 20 mL of THF is added
slowly and under vigorous stirring to a suspension of HNBu+3 BPh−4 (3.87 g, 7.66 mmol) in
80 mL toluene via a double-tipped needle. The reaction mixture is stirred for 2 h at room
136 4 Experimental Part
temperature. The light brown precipitate is allowed to settle, and the solvents are removed
with a canula. The solid residue is dried in vacuo to yield Me2C(Cp)2Zr(Me)(thf)+BPh−4
70 (3.57 g, 67.4%) as a light beige powder. The product is only soluble in CD2Cl2, which
leads to decomposition within a few minutes. Therefore, assignment of the NMR spectra
obtained is insecure.
1H-NMR (CD2Cl2): δ = 0.45 (s, 3H, ZrMe), 1.32-1.95 (m, 10H, THF and Me), 3.40
(br s, 4H, THF), 5.23-6.38 (m, 8H, Cp) 6.84 (m, 4H, Ph), 6.98 (m, 8H, Ph), 7.29 (m,
8H, Ph); 13C-NMR (CD2Cl2): δ = 26.5 (THF), 22.2 and 28.3 (Me), 40.4 (CMe2), 44.3
(ZrMe), 78.8 (THF), 108.3, 112.2, 119.4, 131.9, 130.6 (Cp), 124.6, 128.4, 138.6, 166.5
(BPh4). Laboratory Nr. HF 566.
4.4 Polymerization Reactions
4.4.1 1-Hexene Polymerization
MAO-solution in toluene, the solvent and any other cosolvent or reactant are placed
in a 50 mL Schlenk flask. The reaction mixture is stirred at room temperature for at
least 30 min. Then the desired amount of a stock solution of the zirconocene dichloride in
toluene is added. At a polymerization temperature of Tp = 0 ◦C and above, the reaction
mixture is kept at the desired polymerization temperature and stirred for at least 60 min
at that temperature. At a polymerization temperature below Tp = 0 ◦C, the reaction
mixture is stirred at room temperature for at least 60 min, and then kept at the desired
polymerization temperature under stirring for another 60 min. The polymerization is
started by addition of a gravimetrically determined amount of 1-hexene. After the desired
polymerization time the reaction mixture is diluted with pentane and instantaneously
poured into diluted aqueous HCl solution. The aqueous phase is extracted two times
with 50 mL of pentane, and the combined organic phases are dried over Na2SO4. The
volatiles are removed in vacuo, and the obtained poly(1-hexene) is dried at 50 ◦C in vacuo
to constant weight.
This general procedure is applied to all 1-hexene polymerization experiments with
zirconocene dichloride catalysts activated with MAO as the cocatalyst. The results of
polymerization experiments are summarized in tabular form with the changes to the
general procedure denoted in the following sections.
4.4 Polymerization Reactions 137
Reaction Order in Monomer Concentration
In all experiments 20.0 mL MAO-solution (10 % wt/wt in toluene), 2.0 mL of a stock
solution of Cp2ZrCl2 1 (3.3 · 10−3 mol L−1 in toluene) and varying amounts of 1-hexene
are applied. Thus, the concentrations prior to the addition of 1-hexene are [Zr] = 3 ·10−4 mol L−1, [Al] = 1.5 mol L−1. All experiments are carried out at Tp = −20 ◦C.
Run Nr.tph
1-Hexene
g
Poly(1-Hexene)
gMn Mw/Mn
HF481 10 0.49 0.21 1123 1.36
HF482 10 0.83 0.36 1662 1.61
HF483 10 1.48 0.62 2398 1.89
HF484 10 2.24 0.80 2985 2.92
HF485 10 2.81 0.89 3542 2.29
HF486 10 3.76 1.26 3237 2.08
HF487 120 0.46 0.46 1541 2.40
HF488 120 0.83 0.81 2043 2.62
HF489 120 1.42 1.37 2832 3.02
HF490 120 2.20 2.02 3367 3.22
HF491 120 2.87 2.76 3781 3.13
HF492 120 3.82 3.81 4154 3.20
In all experiments 10.0 mL MAO-solution (10 % wt/wt in toluene), 1.0 mL of a stock
solution of Me2Si(Ind)2ZrCl2 4 (3.3 · 10−3 mol L−1 in toluene) and varying amounts of
1-hexene are applied. Thus, the concentrations prior to the addition of 1-hexene are
[Zr] = 3 ·10−4 mol L−1, [Al] = 1.5 mol L−1. All experiments are carried out at Tp = 60 ◦C.
Run Nr.tpmin
1-Hexene
g
Poly(1-Hexene)
gMn Mw/Mn
HF218 10 0.49 0.13 4019 2.54
HF217 10 0.81 0.24 5540 2.45
HF216 10 1.54 0.73 7433 2.40
HF215 10 2.21 0.77 7366 2.77
HF214 10 2.88 0.95 7839 2.67
HF219 10 3.93 1.11 8029 2.69
HF224 60 0.48 0.22 4138 2.27
HF223 60 0.84 0.46 5242 2.53
138 4 Experimental Part
Run Nr.tpmin
1-Hexene
g
Poly(1-Hexene)
gMn Mw/Mn
HF222 60 1.58 1.16 6951 2.44
HF221 60 2.18 1.68 7577 2.34
HF220 60 2.91 2.17 7543 2.84
HF225 60 3.96 2.67 7533 2.83
Reaction Order in Catalyst Concentration
In all experiments 6.0 mL MAO-solution (10% wt/wt in toluene), and varying amounts
of a stock solution of Cp2ZrCl2 1 (3.0 · 10−3 mol L−1 in toluene) are applied, and toluene
is added, so that the total volume of the reaction mixture prior to 1-hexene addition is
10 mL. Approximately 2 mL of 1-hexene are applied. Thus, the MAO concentration prior
to the addition of 1-hexene is [Al] = 9 · 10−1 mol L−1. All experiments are carried out at
Tp = −20 ◦C and terminated after 120 h.
Run Nr.1-Hexene
g
Zr-Sol.
mL
Poly(1-Hexene)
gMn Mw/Mn
HF095 1.44 0.1 0.10 3721 2.10
HF096 1.41 0.2 0.20 3684 1.96
HF103 1.44 0.2 0.23 3617 1.89
HF097 1.40 0.4 0.38 3638 1.98
HF098 1.43 0.6 0.50 3554 1.90
HF099 1.39 0.8 0.61 3417 1.84
HF100 1.38 1.0 0.69 2994 1.87
HF104 1.42 1.0 0.73 2909 1.82
HF101 1.45 2.0 1.05 2609 2.02
HF102 1.45 3.0 1.23 2084 1.96
In all experiments 6.0 mL MAO-solution (10% wt/wt in toluene), and varying amounts
of a stock solution of Cp2ZrCl2 1 (0.5 · 10−3 mol L−1 in toluene) are applied, and toluene
is added, so that the total volume of the reaction mixture prior to 1-hexene addition is
10 mL. Approximately 2 mL of 1-hexene are applied. Thus, the MAO concentration prior
to the addition of 1-hexene is [Al] = 9 · 10−1 mol L−1. All experiments are carried out at
Tp = −20 ◦C and terminated after 48 h.
4.4 Polymerization Reactions 139
Run Nr.1-Hexene
g
Zr-Sol.
mL
Poly(1-Hexene)
gMn Mw/Mn
HF085 1.42 0.1 0.23 3868 2.13
HF086 1.42 0.2 0.51 4485 1.95
HF093 1.41 0.2 0.65 4132 2.03
HF087 1.44 0.4 0.78 3445 1.99
HF088 1.43 0.6 0.98 2621 2.09
HF089 1.38 0.8 1.08 2918 2.46
HF090 1.43 1.0 0.97 2722 2.44
HF094 1.41 1.0 1.05 2994 2.09
HF091 1.44 2.0 1.14 2595 2.53
HF092 1.47 3.0 1.24 2928 2.86
Zirconocene Concentration, Molecular Weight and MWD
In all experiments varying amounts of MAO-solution (10% wt/wt and 30% wt/wt (∗)
in toluene), varying amounts of a stock solution of Me2Si(Ind)2ZrCl2 4 (3.0 ·10−3 mol L−1
in toluene) are applied, and toluene is added so that the total volume of the reaction
mixture is 20 mL. Approximately 4 mL of 1-hexene are applied. All experiments are
carried out at Tp = 60 ◦C.
Run Nr.1-Hexene
g
tpmin
MAO-Sol.
mL
Zr-Sol.
mL
Poly(1-Hexene)
gMn Mw/Mn
HF493 2.73 1200 10.0 0.2 2.53 7700 2.50
HF494 2.68 180 10.0 0.5 2.52 6500 2.61
HF495 2.66 120 10.0 1.0 2.59 5900 2.68
HF496 2.75 60 10.0 3.0 2.75 5500 2.75
HF497 2.73 60 10.0 5.0 2.66 4800 2.80
HF498 2.73 60 10.0 10.0 2.71 4300 2.84
HF499 2.86 240 1.3 0.2 2.76 9417 1.98
HF500 2.86 180 3.3 0.5 2.77 7908 2.18
HF501 2.88 120 6.7 1.0 2.68 6380 2.36
HF502 2.79 120 13.3 2.0 2.62 4849 2.53
HF503 2.85 120 6.7∗ 3.0 2.61 4400 2.68
HF504 2.85 120 13.3∗ 6.0 2.77 3562 2.90
140 4 Experimental Part
Run Nr.1-Hexene
g
tpmin
MAO-Sol.
mL
Zr-Sol.
mL
Poly(1-Hexene)
gMn Mw/Mn
HF505 2.85 360 0.5 3.0 0.05 − −HF506 2.82 300 1.0 3.0 2.39 6324 2.28
HF507 2.80 240 2.0 3.0 2.52 6093 2.33
HF508 2.84 180 4.0 3.0 2.73 5883 2.40
HF509 2.82 150 8.0 3.0 2.76 5208 2.41
HF510 2.80 150 16.0 3.0 2.74 4643 2.57
HF511 2.85 120 6.7∗ 3.0 2.61 4471 2.67
HF512 2.80 60 8.0∗ 3.0 2.43 3960 2.70
Zirconocene Concentration and Catalyst Activity
In all experiments varying amounts of MAO-solution (10% wt/wt in toluene), 2.0 mL
of a stock solution of Me2Si(Ind)2ZrCl2 4 of varying concentration [Zr]0 and approximately
2 mL of 1-hexene are applied. Thus, the MAO concentration prior to the addition of 1-
hexene is [Al] = 1.5 mol L−1. All experiments are carried out at Tp = 60 ◦C.
Run Nr.[Zr]0
10−4 mol L−1
1-Hexene
g
tpmin
Poly(1-Hexene)
gMn Mw/Mn
HF361 33.0 2.73 2 1.50 5907 2.29
HF362 33.0 2.84 3 1.91 5117 2.42
HF363 33.0 2.80 5 2.05 5670 2.27
HF364 33.0 2.77 8 2.17 5129 2.41
HF365 33.0 2.85 11 2.45 5311 2.31
HF366 33.0 2.85 15 2.66 4628 2.60
HF367 33.0 2.85 20 2.73 4634 2.49
HF368 33.0 2.92 25 2.79 4787 2.58
HF369 33.0 2.78 30 2.75 4839 2.48
HF370 6.7 2.89 10 0.35 10100 1.92
HF371 6.7 2.94 20 0.81 10003 1.98
HF372 6.7 2.77 30 1.07 9586 1.92
HF373 6.7 2.86 40 2.13 7189 1.96
HF374 6.7 2.85 60 2.34 6957 2.02
HF375 6.7 2.86 100 2.70 6432 2.04
4.4 Polymerization Reactions 141
Run Nr.[Zr]0
10−4 mol L−1
1-Hexene
g
tpmin
Poly(1-Hexene)
gMn Mw/Mn
HF379 1.1 2.73 30 0.64 6253 2.37
HF380 1.1 2.85 60 0.94 7139 2.44
HF381 1.1 2.82 120 1.43 7068 2.49
HF382 1.1 2.78 180 1.76 6452 2.56
HF383 1.1 2.83 300 2.32 6755 2.37
HF384 1.1 2.78 480 2.66 5857 2.21
Kinetics at different Polymerization Temperatures
In all experiments 20.0 mL MAO-solution (10 % wt/wt in toluene), 2.0 mL of a stock
solution of Cp2ZrCl2 1 (3.3 · 10−3 mol L−1 in toluene) and approximately 4 mL of 1-
hexene are applied. Thus, the concentrations prior to the addition of 1-hexene are [Zr] =
3 · 10−4 mol L−1, [Al] = 1.5 mol L−1.
Run Nr.Tp
◦C
1-Hexene
g
tpmin
Poly(1-Hexene)
gMn Mw/Mn
HF406 0 2.73 30 0.26 1450 1.41
HF407 0 2.85 60 0.40 1377 1.49
HF408 0 2.89 120 0.79 1430 1.48
HF409 0 2.83 180 1.01 1228 1.47
HF410 0 2.79 240 1.30 1246 1.49
HF411 0 2.74 360 1.86 1089 1.46
HF412 0 2.73 480 2.06 1123 1.47
HF413 0 2.73 720 2.33 1150 1.49
HF175 −10 2.90 60 0.19 2631 1.73
HF176 −10 2.90 120 0.30 2772 1.79
HF177 −10 2.86 240 0.91 3263 2.06
HF178 −10 2.83 480 1.54 2609 2.05
HF179 −10 2.90 960 2.04 2526 1.84
HF180 −10 2.72 1920 2.44 2842 2.16
HF398 −20 2.78 360 0.29 4518 2.71
HF400 −20 2.75 1080 0.84 3655 2.75
HF401 −20 2.80 1320 1.01 2411 2.04
HF402 −20 2.80 1740 1.13 2453 1.95
142 4 Experimental Part
Run Nr.Tp
◦C
1-Hexene
g
tpmin
Poly(1-Hexene)
gMn Mw/Mn
HF403 −20 2.74 2400 1.40 2525 1.95
HF404 −20 2.87 3000 1.90 2012 1.97
HF405 −20 2.77 4320 2.32 2344 1.89
In all experiments 20.0 mL MAO-solution (10 % wt/wt in toluene), 2.0 mL of a stock
solution of Me2Si(Ind)2ZrCl2 4 (3.3 · 10−3 mol L−1 in toluene) and approximately 4 mL
of 1-hexene are applied. Thus, the concentrations prior to the addition of 1-hexene are
[Zr] = 3 · 10−4 mol L−1, [Al] = 1.5 mol L−1.
Run Nr.Tp
◦C
1-Hexene
g
tpmin
Poly(1-Hexene)
gMn Mw/Mn
HF161 60 2.72 2 1.48 7282 2,02
HF162 60 2.86 3 1.90 6785 2,03
HF163 60 2.79 5 2.38 6861 2,03
HF164 60 2.87 10 2.48 6593 2,23
HF165 60 2.83 15 2.68 7201 2,16
HF388 50 2.80 1 0.23 3702 4.61
HF396 50 2.80 3 1.08 4863 3.71
HF389 50 2.82 6 2.10 4950 3.37
HF390 50 2.72 9 2.25 4617 3.44
HF391 50 2.75 12 2.50 4692 4.14
HF392 50 2.81 15 2.54 4277 3.58
HF393 50 2.76 20 2.67 3910 3.75
HF166 40 2.78 1 0.23 9723 2.51
HF167 40 2.74 5 0.95 12778 2.04
HF168 40 2.77 10 1.78 12175 2.00
HF169 40 2.82 20 2.30 12407 1.92
HF170 40 2.76 30 2.41 11883 2.00
HF171 40 2.76 40 2.47 11947 1.93
HF172 40 2.71 60 2.56 10905 2.06
HF173 40 2.77 90 2.72 10728 2.08
HF184 30 2.80 2.5 0.25 12807 2.26
HF185 30 2.84 10 0.86 15536 1.92
HF186 30 2.87 20 1.65 16586 1.79
4.4 Polymerization Reactions 143
Run Nr.Tp
◦C
1-Hexene
g
tpmin
Poly(1-Hexene)
gMn Mw/Mn
HF187 30 2.66 30 1.96 15915 1.80
HF188 30 2.58 40 2.06 15344 1.82
HF190 30 2.76 90 2.65 15117 1.85
HF205 21 2.90 1.5 0.08 5752 3,23
HF206 21 2.78 10 0.34 15738 2,18
HF207 21 2.84 20 0.90 16507 1,95
HF208 21 2.77 30 1.17 16268 1,95
HF209 21 2.76 40 1.42 15983 1,90
HF210 21 2.79 60 1.85 16155 1,95
HF211 21 2.78 90 2.11 16680 1,92
HF212 21 2.85 120 2.21 17360 1,82
HF213 21 2.77 180 2.47 17000 1,80
HF359 0 2.73 30 0.49 26633 1.54
HF352 0 2.72 60 0.78 30078 1.54
HF353 0 2.77 120 1.30 26508 1.51
HF354 0 2.75 240 1.93 24368 1.50
HF355 0 2.73 360 2.06 24794 1.51
HF356 0 2.70 480 2.20 25385 1.53
HF367 0 2.70 600 2.27 25975 1.52
HF358 0 2.78 720 2.46 26906 1.52
Effect of Polymerization Temperature
In all experiments a standard methodology of temperature control and catalyst acti-
vation slightly deviating from the aforementioned general procedure is applied. A freshly
prepared stock solution of the zirconocene dichloride in toluene is added to the MAO solu-
tion in toluene at 0 ◦C, and the mixture is stirred for at least 60 min at this temperature.
Then the mixture is kept under stirring at the desired polymerization temperature for at
least 60 min, before polymerization is started by the addition of 1-hexene. Termination
and workup are carried out as usual.
In polymerization experiments with the catalysts Cp2ZrCl2 1, C2H4(Ind)2ZrCl2 3, and
Me2C(Cp)(Flu)ZrCl2 5 10.0 mL MAO-solution (10% wt/wt in toluene), 1.0 mL of a stock
solution of the zirconocene dichloride (3.3 · 10−3 mol L−1 in toluene) and approximately
144 4 Experimental Part
2 mL of 1-hexene are applied. Thus, the concentrations prior to the addition of 1-hexene
are [Zr] = 3 · 10−4 mol L−1, [Al] = 1.5 mol L−1.
Run Nr. Catalyst1-Hexene
g
tph
Tp
◦C
Poly(1-Hexene)
gMn Mw/Mn
HF008 1 1.47 24 65 1.31 467 1.36
HF006 1 1.45 24 60 0.93 476 1.41
HF004 1 1.43 24 55 1.15 493 1.32
HF00I 1 1.45 24 50 0.83 518 1.35
HF018 1 1.46 24 45 1.25 585 1.38
HF00H 1 1.42 24 40 0.76 619 1.29
HF064 1 1.64 24 35 1.58 508 1.38
HF00G 1 1.45 24 30 0.77 850 1.47
HF002 1 1.46 24 24 1.05 748 1.60
HF076 1 1.79 48 20 1.40 933 1.59
HF067 1 1.53 48 15 1.45 1015 1.88
HF044 1 1.72 48 10 1.52 1218 2.07
HF00F 1 1.61 48 5 1.40 1336 1.90
HF00E 1 1.57 48 5 1.29 1384 1.83
HF025 1 1.72 48 0 0.72 1576 2.11
HF047 1 1.69 48 −5 1.60 2196 2.54
HF00D 1 1.55 48 −5 1.27 2871 3.42
HF026 1 1.63 48 −10 1.54 3329 2.72
HF073 1 1.76 48 −10 1.72 2932 2.48
HF029 1 1.43 48 −15 1.38 4021 2.69
HF00C 1 1.49 48 −20 1.42 3675 2.66
HF032 1 2.02 48 −20 1.93 4831 2.27
HF036 1 1.84 48 −25 0.65 5229 2.31
HF039 1 1.86 48 −30 0.59 5670 2.13
HF00B 1 1.64 72 −35 0.73 7147 2.49
HF00L 1 1.48 72 −40 0.91 9427 2.22
HF061 1 1.57 72 −50 0.57 11331 1.84
HF079 1 1.71 120 −60 0.55 12969 1.92
HF058 1 1.62 120 −78 0.86 16901 1.88
HF00A 1 2.73 120 −78 0.32 17633 1.91
4.4 Polymerization Reactions 145
Run Nr. Catalyst1-Hexene
g
tph
Tp
◦C
Poly(1-Hexene)
gMn Mw/Mn
HF009 3 1.45 24 65 1.45 1989 5.27
HF001 3 1.47 24 60 0.80 2930 3.99
HF011 3 1.47 24 55 1.32 3491 3.12
HF003 3 1.40 24 50 1.26 6561 3.30
HF005 3 1.47 24 50 1.23 7253 3.64
HF013 3 1.45 24 45 1.30 7872 3.13
HF071 3 1.73 48 40 1.62 8593 2.45
HF015 3 1.46 24 35 1.33 10393 2.55
HF065 3 1.59 24 35 1.44 11681 2.72
HF017 3 1.43 24 30 0.92 16721 2.41
HF007 3 1.43 24 25 1.02 19489 2.70
HF019 3 1.43 24 20 0.94 21341 1.62
HF068 3 1.67 48 20 1.65 22314 1.80
HF021 3 1.47 24 15 0.78 29789 1.62
HF045 3 1.81 48 10 1.30 24171 1.59
HF023 3 1.35 24 5 0.42 32336 1.66
HF077 3 1.77 48 0 1.02 23787 1.51
HF053 3 1.75 48 0 1.32 26327 1.57
HF027 3 1.48 24 −5 0.22 28799 1.61
HF074 3 1.72 48 −10 0.56 25412 1.70
HF051 3 1.73 72 −10 0.29 26401 1.59
HF030 3 1.52 24 −15 0.27 26483 1.58
HF033 3 1.84 48 −20 0.26 30568 1.59
HF037 3 1.90 48 −25 0.28 33138 1.59
HF040 3 1.78 48 −35 0.15 36050 1.61
HF049 3 1.58 72 −40 − − −HF059 3 1.68 72 −60 − − −HF048 3 1.71 120 −80 − − −HF010 5 1.46 24 65 1.05 1608 4.67
HF012 5 1.42 24 55 1.16 2735 4.45
HF014 5 1.42 24 45 0.90 6714 2.99
HF016 5 1.44 24 35 1.18 9960 2.40
HF020 5 1.45 24 25 0.89 18523 1.74
146 4 Experimental Part
Run Nr. Catalyst1-Hexene
g
tph
Tp
◦C
Poly(1-Hexene)
gMn Mw/Mn
HF022 5 1.46 24 15 0.69 22604 1.49
HF069 5 1.60 48 10 1.48 27858 1.51
HF046 5 1.76 24 10 0.22 31575 1.57
HF028 5 1.46 48 −5 0.96 37905 1.65
HF034 5 1.78 48 −20 0.66 52903 1.68
HF038 5 1.72 48 −25 0.63 56253 1.68
HF041 5 1.69 72 −35 0.30 63600 1.64
HF081 5 1.73 120 −40 − − −HF063 5 1.64 120 −60 − − −HF060 5 1.56 120 −80 − − −
In polymerization experiments with the catalysts Me2Si(Cp)2ZrCl2 62, and
Me2Si(Ind)2ZrCl2 4 20.0 mL MAO-solution (10 % wt/wt in toluene), 2.0 mL of a stock
solution of the zirconocene dichloride (3.3 · 10−3 mol L−1 in toluene) and approximately
4 mL of 1-hexene are applied. Thus, the concentrations prior to addition of 1-hexene are
[Zr] = 3 · 10−4 mol L−1, [Al] = 1.5 mol L−1.
Run Nr. Catalyst1-Hexene
g
tph
Tp
◦C
Poly(1-Hexene)
gMn Mw/Mn
HF320 62 2.74 6 65 1.04 441 1.15
HF308 62 2.77 6 60 1.39 474 1.25
HF321 62 2.74 6 55 1.33 456 1.20
HF309 62 2.79 6 50 1.89 490 1.29
HF322 62 2.80 6 40 1.54 589 1.23
HF310 62 2.83 6 30 2.02 671 1.29
HF311 62 2.79 6 20 1.93 853 1.35
HF312 62 2.88 19 10 2.40 1153 1.47
HF313 62 2.72 19 0 2.43 1670 1.68
HF314 62 2.78 19 −10 2.53 2225 2.00
HF315 62 2.75 19 −20 2.32 3694 2.16
HF316 62 2.71 48 −30 2.24 6790 2.03
HF317 62 2.78 48 −40 2.15 14130 2.22
HF318 62 2.86 72 −50 0.61 12980 2.23
4.4 Polymerization Reactions 147
Run Nr. Catalyst1-Hexene
g
tph
Tp
◦C
Poly(1-Hexene)
gMn Mw/Mn
HF341 62 2.70 120 −60 0.60 17000 1.28
HF319 62 2.79 120 −78 0.28 20570 1.21
HF305 4 2.78 1 65 2.42 4163 2.48
HF293 4 2.70 0.5 60 2.62 5167 2.34
HF306 4 2.83 2 55 2.61 7885 2.26
HF294 4 2.96 1 50 2.76 9228 2.11
HF307 4 2.77 2 45 2.23 12300 1.82
HF295 4 2.75 1 40 2.58 13510 1.73
HF334 4 2.82 2 35 2.32 17710 1.55
HF296 4 2.82 1.5 30 2.54 17430 1.52
HF297 4 2.77 2 20 2.49 19390 1.52
HF298 4 2.70 3 10 2.22 24980 1.54
HF299 4 2.78 3 0 2.32 28830 1.55
HF300 4 2.78 6 −10 2.21 32480 1.57
HF301 4 2.79 18 −20 1.09 42390 1.58
HF302 4 2.80 24 −30 2.65 36530 1.59
HF303 4 2.70 72 −40 0.26 50560 1.56
HF335 4 2.76 72 −60 − − −HF304 4 2.76 72 −78 − − −
Effect of Polar and Nonpolar Cosolvents
In all experiments 10.0 mL MAO-solution (10 % wt/wt in toluene), 1.0 mL of a stock
solution of Me2Si(Ind)2ZrCl2 4 (3.0 · 10−3 mol L−1 in toluene) are applied, and toluene is
added so that the total volume of the reaction mixture is 20 mL. Approximately 4 mL
of 1-hexene are applied. Thus, the concentrations prior to the addition of 1-hexene are
[Zr] = 1.5 · 10−4 mol L−1, [Al] = 7.5 · 10−1 mol L−1. All experiments are carried out at
Tp = 60 ◦C and terminated after 60 min.
Run Nr.1-Hexene
g
Cosolvent
mL
Poly(1-Hexene)
gMn Mw/Mn
HF445 2.80 − − 2.72 5211 2.23
HF446 2.87 heptane 0.6 2.77 4679 2.19
HF447 2.87 heptane 1.0 2.50 4220 2.25
148 4 Experimental Part
Run Nr.1-Hexene
g
Cosolvent
mL
Poly(1-Hexene)
gMn Mw/Mn
HF448 2.78 heptane 2.0 2.65 3166 2.25
HF449 2.86 heptane 4.0 2.43 2533 2.21
HF450 2.87 heptane 6.0 2.14 2256 2.45
HF451 2.74 heptane 8.0 2.32 1845 2.47
HF458 2.85 C6H5Cl 0.2 2.64 5217 2.34
HF459 2.88 C6H5Cl 0.4 2.60 6274 2.29
HF460 2.84 C6H5Cl 0.6 2.67 6339 2.29
HF461 2.84 C6H5Cl 0.8 2.67 5479 2.27
HF462 2.87 C6H5Cl 1.0 2.53 4856 2.22
HF463 2.85 C6H5Cl 1.6 2.26 4293 2.93
HF464 2.86 C6H5Cl 2.0 2.49 3892 2.57
HF465 2.88 C6H5Cl 4.0 2.00 3413 2.52
HF466 2.90 C6H5Cl 5.0 2.04 2894 2.49
HF467 2.82 C6H5Cl 6.0 2.38 2621 2.92
HF468 2.84 C6H5Cl 8.0 2.15 2427 2.59
HF256 2.82 Bu2O 0.2 2.24 5069 1.98
HF257 2.66 Bu2O 0.4 2.32 4987 1.86
HF258 2.75 Bu2O 0.6 1.03 3806 2.23
HF259 2.84 Bu2O 0.8 0.63 2523 2.08
HF260 2.84 Bu2O 1.0 0.52 2009 2.47
HF261 2.84 Bu2O 2.0 0.10 630 4.32
HF262 2.79 Bu2O 3.0 − − −HF263 2.73 Bu2O 4.0 − − −HF452 2.81 THF − 1.96 5230 2.34
HF453 2.80 THF 0.2 2.12 4167 2.13
HF454 2.86 THF 0.4 0.49 1985 2.77
HF455 2.87 THF 0.6 0.41 1160 3.96
HF456 2.78 THF 0.8 − − −HF457 2.80 THF 1.0 − − −
In all experiments 10.0 mL MAO-solution (10 % wt/wt in toluene), 1.0 mL of a stock
solution of Me2Si(Ind)2ZrCl2 4 (3.0 · 10−3 mol L−1 in toluene) are applied, and toluene is
added so that the total volume of the reaction mixture is 20 mL. Approximately 4 mL
4.4 Polymerization Reactions 149
of 1-hexene are applied. Thus, the concentrations prior to the addition of 1-hexene are
[Zr] = 1.5 · 10−4 mol L−1, [Al] = 7.5 · 10−1 mol L−1. All experiments are carried out at
Tp = 30 ◦C and terminated after 60 min.
Run Nr.1-Hexene
g
Cosolvent
mL
Poly(1-Hexene)
gMn Mw/Mn
HF469 2.78 − − 2.29 17497 1.68
HF470 2.77 CH2Cl2 0.2 2.67 19200 1.60
HF471 2.76 CH2Cl2 0.4 2.58 18060 1.62
HF472 2.83 CH2Cl2 0.6 2.73 16610 1.61
HF473 2.82 CH2Cl2 0.8 2.52 16400 1.60
HF474 2.84 CH2Cl2 1.0 2.22 16180 1.59
HF475 2.78 CH2Cl2 1.8 2.38 15100 1.58
HF476 2.77 CH2Cl2 3.0 1.89 14110 1.60
HF477 2.77 CH2Cl2 6.0 2.31 13270 1.60
4.4.2 MMA Polymerization
MMA (2.5 g, 25 mmol) and CH2Cl2 (7.5 mL) containing approximately 3 %(wt/wt)
hexylbenzene are placed in a 50 mL Schlenk tube with a septum and stirred at the desired
polymerization temperature for 30 min. The polymerization is started by the addition of a
solution of the cationic methyl zirconocene in 2.5 mL of CH2Cl2 so that in all experiments
the total reaction volume is 12.5 mL. The polymerization is terminated after the desired
polymerization time tp by addition of 2 mL MeOH/HCl (aq.)/hydroquinone (90/9.9/0.1%
wt/wt). Workup of PMMA includes precipitation in MeOH, filtration, and drying in HV.
Polymerization with Me2C(Cp)2Zr(Me)(thf)+BPh−4 70
All experiments are carried out with Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 as the catalyst
at Tp = 0 ◦C, and with varying MMA and Zirconocene concentration, as well as varying
contact time tCH2Cl2 of the catalyst in CH2Cl2 prior to polymerization. All experiments
are terminated after 60 min.
Run Nr.MMA
g
[Zr]
mmol L−1
tCH2Cl2
s
PMMA
gMn Mw/Mn
HF639 4.96 16 120 1.53 n. d. n. d.
HF640 5.09 16 10 4.15 n. d. n. d.
150 4 Experimental Part
Run Nr.MMA
g
[Zr]
mmol L−1
tCH2Cl2
s
PMMA
gMn Mw/Mn
HF641 2.47 8 120 0.17 n. d. n. d.
HF642 2.56 8 10 0.36 n. d. n. d.
HF643 2.49 16 10 1.25 n. d. n. d.
All experiments are carried out with Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 as the catalyst
at a zirconocene concentration of [Zr] = 16 mmol L−1, at an MMA concentration of
[MMA]0 = 2 mol L−1, and with a contact time tCH2Cl2 ≤ 15 s of the catalyst in CH2Cl2
prior to polymerization.
Run Nr.MMA
g
tpmin
Tp
◦C
PMMA
gMn Mw/Mn
HF644 2.48 20 30 1.62 66600 1.64
HF645 2.52 30 20 1.53 93000 1.60
HF646 2.42 40 10 1.43 104100 1.38
HF647 2.57 60 0 2.08 90600 1.32
HF648 2.51 105 −10 2.17 123200 1.30
HF649 2.44 180 −20 1.74 110500 1.35
HF650 2.49 1080 −45 2.43 78500 1.31
HF651 2.50 60 20 1.76 101200 1.51
Polymerization with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43
All experiments are carried out with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 as the
catalyst, at an MMA concentration of [MMA]0 = 2 mol L−1, with a contact time
tCH2Cl2 ≤ 15 s of the catalyst in CH2Cl2 prior to polymerization, and at a polymerization
temperature of Tp = 0 ◦C. All experiments are terminated after 60 min.
Run Nr.MMA
g
[Zr]
mmol L−1
PMMA
gMn Mw/Mn
HF610 2.49 0.5 − − −HF611 2.53 1.0 0.10 105000 2.36
HF612 2.49 2.5 0.35 60800 1.40
HF613 2.49 6.0 1.87 53400 1.36
HF614 2.52 10.0 2.32 56700 1.38
4.4 Polymerization Reactions 151
All experiments are carried out with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 as the
catalyst at a zirconocene concentration of [Zr] = 8 mmol L−1, with a contact time
tCH2Cl2 ≤ 15 s of the catalyst in CH2Cl2 prior to polymerization, and at a polymerization
temperature of Tp = 0 ◦C.
Run Nr.MMA
g
PMMA
gMn Mw/Mn
HF616 1.19 0.92 28500 1.35
HF617 2.16 1.67 36300 1.39
HF618 4.05 3.18 40000 1.42
All experiments are carried out with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 43 as the
catalyst at a zirconocene concentration of [Zr] = 8 mmol L−1, at an MMA concentration
of [MMA]0 = 2 mol L−1, and with a contact time tCH2Cl2 ≤ 15 s of the catalyst in CH2Cl2
prior to polymerization. In kinetic experiments (∗) samples of 0.10 mL are taken from the
reaction mixture. Relative polymer yield of the samples is determined by means of GPC
via integration of the polymer peak relative to hexyl benzene as an internal standard.
Absolute polymer yield is determined gravimetically by terminating the reaction instan-
taneously after taking the last sample. Calculating polymer yield in kinetic experiments
has to take into account a loss in yield due to drawing the samples of approximately 8%.
Run Nr.MMA
g
tpmin
Tp
◦C
PMMA
gMn Mw/Mn
HF601∗ 2.57 20 30 1.20 40500 1.43
HF602∗ 2.47 40 20 1.33 45700 1.42
HF603∗ 2.51 60 10 2.06 51000 1.42
HF604∗ 2.53 90 0 2.15 58800 1.34
HF605∗ 2.47 180 −10 2.06 55100 1.30
HF606∗ 2.50 180 −20 1.95 55300 1.24
HF607∗ 2.48 240 −30 1.65 57300 1.21
HF608 2.51 90 0 2.22 52820 1.37
HF609 2.48 60 20 1.41 39600 1.50
152 4 Experimental Part
Polymerization with Other Cationic Methyl Zirconocenes
Polymerization experiments with the catalysts Cp2Zr(Me)+ 14,
C2H4(Ind)2Zr(Me)(thf)+BPh−4 67, Me2C(Cp)(Flu)Zr(Me)(thf)
+BPh−4 68, and
Me2Si(Cp)2Zr(Me)(thf)+BPh−4 69 are carried out at a zirconocene concentration
of [Zr] = 16 mmol L−1, at an MMA concentration of [MMA]0 = 2 mol L−1, with a
contact time tCH2Cl2 ≤ 15 s of the catalyst in CH2Cl2 prior to polymerization and at a
polymerization temperature Tp = 0 ◦C. All experiments are terminated after 60 min.
Run Nr. CatalystMMA
g
Tp
◦C
PMMA
gMn Mw/Mn
HF593 14 2.42 0 − − −HF594 14 2.49 −20 − − −HF597 67 2.58 0 − − −HF598 67 2.51 −20 − − −HF599 68 2.48 0 − − −HF600 68 2.48 −20 − − −HF595 69 2.47 0 − − −HF606 69 2.47 −20 − − −
153
Appendix A Mathematical
Equations
A.1 Equation 3.1
For small monomer conversion, the differential quotient of monomer concentration
and time (the actual rate of polymerization) is linearly approximated by the differences
of monomer concentration and polymerization time.
∆[Mon]
∆t≈ −d[Mon]
dt= kapp · [Mon]a (A.1)
Furthermore, the monomer concentration [Mon] is assumed to be approximately constant
and equal to the initial monomer concentration [Mon]0. Thus, it is
∆[Mon]
∆t≈ kapp · [Mon]0
a (A.2)
ln∆[Mon]
∆t≈ a · ln [Mon]0 + ln kapp (A.3)
A.2 Equation 3.2
The rate equation of the propagation reaction assuming first order kinetics in monomer
concentration and z-order kinetics in the concentration of the active species [Zr∗] is
−d[Mon]dt
= kp · [Mon] · [Zr∗]z (A.4)
−d[Mon][Mon]
= kp · [Zr∗]z dt (A.5)
ln[Mon]0[Mon]
= kp · [Zr∗]z · t (A.6)
154 A Mathematical Equations
A.3 Equation 3.3
Assuming that the active species Zr∗1 is converted to another zirconocene species Zr∗2
in a bimolecular reaction and both species being in equilibrium (2 Zr∗1 ⇀↽ Zr∗2, and [Zr∗1] +
2[Zr∗2] = [Zr]0) affords the equilibrium constant:
Kzr =[Zr∗2]
[Zr∗1]2=[Zr]0 − [Zr∗1]
2[Zr∗1]2 (A.7)
Thus,
2Kzr · [Zr∗1]2 + [Zr∗2]− [Zr]0 = 0 (A.8)
with the solution
[Zr∗1] = − 1
4Kzr+
√1
16Kzr2 +
[Zr]02Kzr
=1
4Kzr
(√1 + 8KZr[Zr]0 − 1
)(A.9)
With the approximations 8Kzr[Zr]0 � 1 and√8Kzr[Zr]0 � 1 it is obtained
[Zr∗1] ≈√8Kzr[Zr]04Kzr
=1√2Kzr
· [Zr]012 (A.10)
[Zr∗2] ≈ [Zr]0 − [Zr∗1]
2=1
2·([Zr]0 −
1√2Kzr
· [Zr]120
)(A.11)
For a propagation via both species and first-order kinetics in monomer concentration, the
rate law of propagation is
−d[Mon]dt
= kp,1 · [Mon] · [Zr∗1] + kp,2 · [Mon] · [Zr∗2] (A.12)
−d[Mon][Mon]
= (kp,1 · [Zr∗1] + kp,2 · [Zr∗2]) · dt (A.13)
ln[Mon]0[Mon]
= (kp,1 · [Zr∗1] + kp,2 · [Zr∗2]) · tp (A.14)
Thus,
ln[Mon]0[Mon]
=
(2kp,1 − kp,2
2√2Kzr
· [Zr]120 +
kp,2
2· [Zr]0
)· tp (A.15)
A.4 Equations for Activation Parameters
For the determination of the enthalpy of activation ∆H‡ and the entropy of activation
∆S‡ from the slope m and the ordinate value k∞ of an Eyring plot the following equations
are applied190
∆H‡ = Ea − RT = −Rm−RT (A.16)
∆S‡ = R ln
[hp�k∞
NAe2k2BT
2
]≈ R ln
[7.8119 · 10−11 ·
(k∞
L mol−1 s−1
)·(T
K
)2](A.17)
A.5 Equation 3.5 155
A.5 Equation 3.5
The number average molecular weight Mn is proportional to the number average degree
of polymerization Pn which is determined by the ratio of the rate of propagation rp and
the rate of termination rt:
Mn ∝ Pn =rprt=kp · [Zr∗]zp · [Mon]ap
kt · [Zr∗]zt · [Mon]at=kp
kt
· const.′ (A.18)
Thus, with an Arrhenius approach for the rate constants kp and kt one obtains:
lnMn = −Ea,p − Ea,t
R · Tp+ const. (A.19)
A.6 Equation 3.6
If the active species Zr∗ is in an equilibrium with a dormant species Zrdorm with the
equilibrium constant Kdorm = [Zrdorm]/[Zr∗], and Zr0 = Zr∗ + Zrdorm, an expression for the
molecular weight affords:
Mn ∝ Pn =rprt=kp · [Zr∗]zp · [Mon]ap
kt · [Zr∗]zt · [Mon]at(A.20)
Mn ∝ Pn =kp
kt
·(
[Zr0]
1 + Kdorm
)zp−zt
· const.′′ (A.21)
There is no reason to assume that the propagation and the termination reaction are not
first order with respect to the active species. Thus it is zp = zt = 1 and hence
Mn ∝ Pn =kp
kt· const.′ (A.22)
With an Arrhenius approach for the rate constants kp and kt it is
lnMn ∝ ln Pn = −Ea,p − Ea,t
RTp
+ const. (A.23)
A.7 Equation 3.7
In the case of different pathways of chain termination, the rate laws for the respective
termination reactions via chain termination in the active species Zr∗, chain termination
in the dormant species Zrdorm and chain transfer to the cocatalyst A are:
rt,1 = kt,1 · [Mon]at,1 · [Zr∗] (A.24)
rt,2 = kt,2 · [Mon]at,2 · [Zrdorm] = kt,2 · [Mon]at,2 ·Kdorm · [Zr∗] (A.25)
rt,3 = kt,3 · [Mon]at,3 · [A] · [Zr∗] (A.26)
156 A Mathematical Equations
Apparently, all rate laws of the termination reaction have the form
rt,i = kt,i · [Mon]at,i · [Zr∗] · ci (A.27)
However, in the case of ci = Kdorm one has to keep in mind the fact that Kdorm is
temperature dependent. Thus, assuming the parallel reaction via n of these pathways
affords:
Mn ∝ Pn =rprt=
kp · [Zr∗] · [Mon]ap∑ni=1 kt,i · [Mon]at,i · [Zr∗] · ci
(A.28)
An Arrhenius approach for all rate constants (and in the case of ci = Kdorm also for the
equilibrium constant) results with Ei = Ea,i +∆Gdorm in the case of chain transfer in the
dormant species, and Ei = Ea,i in all other cases:
lnMn ∝ ln Pn = − Ea,p
RTp
− ln
(n∑
i=1
ci · e−Ei
RTp
)+ const. (A.29)
In a general form this equation and its first and second derivative are denoted as
f(x) = a+ bx− ln
(n∑
i=1
ciedix
)(A.30)
f ′(x) = b−∑n
i=1 cidiedix∑n
i=1 ciedix
(A.31)
f ′′(x) = −∑n
i=1 cid2i e
dix ·∑n
j=1 cjedjx −
∑ni=1 cidie
dix ·∑n
j=1 cjdjedjx∑n
i=1 ciedix ·
∑nj=1 cje
djx(A.32)
The existence of a point of inflection requires f ′′(x) = 0:n∑
i=1
cid2i e
dix ·n∑
j=1
cjedjx −
n∑i=1
cidiedix ·
n∑j=1
cjdjedjx = 0 (A.33)
Rearrangement of the sums resultsn∑
i=1
n∑j=1
cicjd2i e
(di+dj)x −n∑
i=1
n∑j=1
cicjdidje(di+dj)x = 0 (A.34)
n∑i=1
n∑j=1
(cicjd2i − cicjdidj)e
(di+dj)x = 0 (A.35)
This can be rearranged to
1
2
n∑i=1
n∑j=1
cicj · (d2i − 2didj + d2j) · e(di+dj)x = 0 (A.36)
1
2
n∑i=1
n∑j=1
cicj · (di − dj)2 · e(di+dj)x = 0 (A.37)
Because of ci, cj > 0, e(di+dj)x > 0 and (di − dj)2 ≥ 0 this is only true if all di and dj are
equal. However, the solution would then be true for all values of x, which means that a
linear Eyring plot would be obtained. Consequently, the function derived from a model
assuming different pathways of propagation does not have a point of inflection.
A.8 Equation 3.8 157
A.8 Equation 3.8
In the case of different pathways of propagation, the rate laws for the respective
propagation reactions all have the form
rp,i = kp,i · [Mon]ap,i · [Zr∗] (A.38)
Thus, assuming the parallel reaction via some (or all) of these pathways affords:
Mn ∝ Pn =rprt=
∑kp,i · [Mon]ap,i · [Zr∗]kt · [Zr∗] · [Mon]at
(A.39)
An Arrhenius approach for all rate constants results (in analogy to the case of different
pathways of termination)
lnMn ∝ ln Pn = +Ea,t
RTp+ ln
(n∑
i=1
ci · e−Ea,p,iRTp
)+ const. (A.40)
Like in the case of different pathways of termination this equation and its first and second
derivation are denoted in a general form as
f(x) = a+ bx+ ln
(n∑
i=1
ciedix
)(A.41)
f ′(x) = b−∑n
i=1 cidiedix∑n
i=1 ciedix
(A.42)
f ′′(x) = −∑n
i=1 cid2i e
dix ·∑n
i=j cjedjx −
∑ni=1 cidie
dix ·∑n
j=1 cjdjedjx∑n
i=1 ciedix ·
∑nj=1 cje
djx(A.43)
It is obvious, that the precondition for the existence of a point of inflection f ′′(x) = 0
affords the same type of equations and results as in the case of different pathways of
termination. Consequently, also the function derived from a model assuming different
pathways of propagation does not have a point of inflection either.
A.9 Equation 3.9
If the coexistence of two active species Zr∗1 and Zr∗2 with two distinct mechanisms of
propagation and of chain termination is assumed and if both species are considered to be
in an equilibrium Zr∗1 + A ⇀↽ Zr∗2 with the equilibrium constant Kzr = [Zr∗2]/[Zr∗1][A] and
158 A Mathematical Equations
[Zr∗1] + [Zr∗2] = [Zr]0, the expression for the number average molecular weight is:
Mn ∝ P n =rprt=kp,1 · [Mon]ap,1 · Zr∗1 + kp,2 · [Mon]ap,2 · Zr∗2kt,1 · [Mon]at,1 · Zr∗1 + kt,2 · [Mon]at,2 · Zr∗2
(A.44)
=kp,1 · [Mon]ap,1 · Zr∗1kt,1 · [Mon]at,1 · Zr∗1
·1 + kp,2
kp,1· [Mon]ap,2−ap,1 · [Zr∗2]
[Zr∗1]
1 +kt,2
kt,1· [Mon]at,2−at,1 · [Zr∗2]
[Zr∗1]
(A.45)
=kp,1 · [Mon]ap,1
kt,1 · [Mon]at,1·1 + kp,2
kp,1· [Mon]ap,2−ap,1 ·Kzr[A]
1 +kt,2
kt,1· [Mon]at,2−at,1 ·Kzr[A]
(A.46)
Thus, an Arrhenius approach for all rate constants and the equilibrium constant affords
lnMn = const.− Ea,t,1 − Ea,p,1
R · Tp
+ ln1 + [Mon]a,2−a,1[A] · k∞
p,2
k∞p,1
· e−∆Gzr+Ea,p,2−Ea,p,1
R·Tp
1 + [Mon]b,2−b,1[A] · k∞t,2
k∞t,1
· e−∆Gzr+Ea,t,2−Ea,t,1
R·Tp
(A.47)
A.10 Equation 3.15
The exact expression for the number average molecular weight Mni assuming first
order kinetics in catalyst concentration for the propagation and the termination reaction
and possibly chain transfer to a reagent A (in case of chain transfer in the active species,
it is [A] = 1) is (compare also A.5):
Mn = mMon · Pn = mMon ·rprt= mMon ·
kp
kt· [Mon]
ap
[Mon]at· 1
[A](A.48)
An Arrhenius approach with the temperature factors of the propagation and the termi-
nation reaction k∞p and k∞t affords
lnMn = ln
(mMon ·
[Mon]ap
[Mon]at· 1
[A]·k∞pk∞t
)− Ea,p − Ea,t
RTp(A.49)
Thus, the ordinate value is
lnMn(0) = ln
(mMon ·
[Mon]ap
[Mon]at· 1
[A]·k∞pk∞t
)(A.50)
A.11 Equation 3.16
The number of polymer chains ni started by an active species i is proportional to the
respective rate of termination rt,i. The average number of monomer units per polymer
chain Mi produced by this species is proportional to the ratio of the rates of propagation
A.11 Equation 3.16 159
rp,i and of termination rt,i. The total number of monomer units niMi consumed by this
active species is proportional to the rate of propagation rp,i.
ni ∝ rt,i = kt,i · [Mon]at,i · [Zr∗i ] (A.51)
Mi ∝ rp,i
rt,i=kp,i
kt,i· [Mon]
ap,i
[Mon]at,i(A.52)
niMi ∝ rp,i = kp,i · [Mon]ap,i · [Zr∗i ] (A.53)
(A.54)
Thus,
niM2i ∝
r2p,i
rt,i=k2p,i
kt,i· [Mon]
2ap,i
[Mon]at,i· [Zr∗i ] (A.55)
The general expressions for the number average molecular weight Mn, for the weight
average molecular weight Mw and for the polydispersity index Mw/Mn consisting of frac-
tions produced by more than one species i are
Mn ∝∑niMi∑ni
(A.56)
Mw ∝∑niMi
2∑niMi
(A.57)
Mw
Mn
=
∑niMi
2 ·∑ni∑
niMi ·∑niMi
(A.58)
Thus, in case of two active species the equation already determined in the previous section
is obtained for the number average molecular weight Mn
Mn ∝ kp,1 · [Mon]ap,1 · Zr∗1 + kp,2 · [Mon]ap,2 · Zr∗2kt,1 · [Mon]at,1 · Zr∗1 + kt,2 · [Mon]at,2 · Zr∗2
(A.59)
For the weight average molecular weight Mw results
Mw ∝k2
p,1
kt,1· [Mon]2ap,1
[Mon]at,1 · Zr∗1 +k2
p,2
kt,2· [Mon]2ap,2
[Mon]at,2 · Zr∗2kp,1 · [Mon]ap,1 · Zr∗1 + kp,2 · [Mon]ap,2 · Zr∗2
(A.60)
(A.61)
Finally, the polydispersity index Mw/Mn is determined as:
Mw
Mn
=
k2p,1
kt,1· [Mon]2ap,1
[Mon]at,1 · Zr∗1 +k2
p,2
kt,2· [Mon]2ap,2
[Mon]at,2 · Zr∗2kp,1 · [Mon]ap,1 · Zr∗1 + kp,2 · [Mon]ap,2 · Zr∗2
· kt,1 · [Mon]at,1 · Zr∗1 + kt,2 · [Mon]at,2 · Zr∗2kp,1 · [Mon]ap,1 · Zr∗1 + kp,2 · [Mon]ap,2 · Zr∗2
(A.62)
Mw
Mn
=1 +
k2p,2
k2p,1
· kt,1
kt,2· [Mon]2ap,2
[Mon]2ap,1· [Mon]at,1
[Mon]at,2 ·Kzr[A]
1 +kp,2
kp,1· [Mon]ap,2
[Mon]ap,1 ·Kzr[A]
·1 + kt,2
kt,1· [Mon]at,2
[Mon]at,1 ·Kzr[A]
1 + kp,2
kp,1· [Mon]ap,2
[Mon]ap,1 ·Kzr[A](A.63)
160 A Mathematical Equations
Thus, an Arrhenius approach for all rate constants and the equilibrium constant affords
an appropiate fitting function with the six parameters d1 to d6:
Mw
Mn
=
(1 + e
d1Tp
+d4
)·(1 + e
d2Tp
+d5
)(1 + e
d3Tp
+d6
)2 (A.64)
A.12 Equations 3.18 and 3.19
The rate law of propagation assumes first order kinetics in monomer concentration
and in the concentration of the active species Zr∗, and time-dependent concentration of
the latter. Thus, it is
−d[Mon]tdt
= kp · [Mon]t · [Zr∗]t (A.65)
The active species is considered to be generated from a precursor Zr0 in an initiation
reaction with the rate constant ki and to be subject to a deactivation reaction with the
rate constant kd. For the consecutive reactions
Zr0 −→ Zr∗ −→ Zr†
The rate laws of the initiation and the deactivation reaction are determined as
−d[Zr0]tdt
= ki · [Zr0]t (A.66)
d[Zr∗]
dt= ki · [Zr0]t − kd · [Zr∗]t (A.67)
Integration of the rate law of the initiation reaction affords
[Zr0]t = [Zr0]0 · e−ki·t (A.68)
d[Zr∗]tdt
= [Zr0]0 · ki · e−ki·t − kd · [Zr∗]t (A.69)
ekd·t · d[Zr∗]t
dt= [Zr0]0 · ki · e(kd−ki)·t − ekd·t · kd · [Zr∗]t (A.70)
ekd·t · d[Zr∗]t
dt+ ekd·t · kd · [Zr∗]t = [Zr0]0 · ki · e(kd−ki)·t (A.71)
With u = ekd·t, v = [Zr∗]t and (uv)′ = uv′ + u′v it follows
d([Zr∗]t · ekd·t
)dt
= [Zr0]0 · ki · e(kd−ki)·t (A.72)
Integration with [Zr∗]0 = 0 affords
[Zr∗]t · ekd·t − 0 = [Zr0]0 ·ki
kd − ki
· e(kd−ki)·t − [Zr0]0 ·ki
kd − ki
(A.73)
[Zr∗]t = [Zr0]0 ·ki
kd − ki
·(e−ki·t − e−kd·t
)(A.74)
A.12 Equations 3.18 and 3.19 161
Consequently, the rate law of the propagation reaction is
−d[Mon]tdt
= [Zr0]0 · [Mon]t ·kp · ki
kd − ki·(e−ki·t − e−kd·t
)(A.75)
Finally, integration results
ln[Mon]0[Mon]
= [Zr0]0 ·kp · ki
kd − ki
·[1
kd
(e−kd·t − 1
)− 1
ki
(e−ki·t − 1
)](A.76)
If the initiation reaction is too fast to be observed, thus ki → ∞, a simpler expression is
obtained
limki→∞
(ln[Mon]0[Mon]
)= [Zr0]0 ·
kp
kd
(1− e−kd·t
)(A.77)
163
Appendix B NMR Spectra
All 1H-NMR and 13C-NMR spectra are recorded on a Bruker DPX 300 FT-NMR
spectrometer at 300 MHz and 75 MHz, respectively. All chemical shifts δ are given in
ppm with Me4Si as a reference. Usually, the signals of Me4Si and residual proton signal of
the solvent (δ = 7.19 ppm for CDCl3, 5.25 ppm for CD2Cl2) are not assigned in the NMR
spectra. In the experimental part, signals of constitutional isomers that are assignable to
the same H (or C) atom are denoted with a “/”. Integrals are denoted as the sum of all
aforementioned signals.
Figure B.1: 1H-NMR spectrum of Me2C(CpH)2 57 in CDCl3.
164 B NMR Spectra
Figure B.2: 1H-NMR spectrum of 6,6-dimethylfulvene 58 in CDCl3.
Figure B.3: 1H-NMR spectrum of Me2C(CpH)(IndH) 59 in CDCl3.
165
Figure B.4: 1H-NMR spectrum of Me2C(CpH)(FluH) 60 in CDCl3.
Figure B.5: 1H-NMR spectrum of Zr(NEt2)4 61 in CDCl3.
166 B NMR Spectra
Figure B.6: 1H-NMR spectrum of HNBu+3 BPh−4 in DMSO− d6.
Figure B.7: 1H-NMR spectrum of Me2C(Cp)2ZrCl2 63 in CDCl3.
167
Figure B.8: 1H-NMR spectrum of Me2C(Cp)(Ind)ZrCl2 64 in CDCl3.
Figure B.9: 1H-NMR spectrum of Me2C(Cp)(Flu)ZrCl2 5 in CDCl3.
168 B NMR Spectra
Figure B.10: 1H-NMR spectrum of Cp2ZrMe2 15 in CDCl3.
Figure B.11: 1H-NMR spectrum of C2H4(Ind)2ZrMe2 46 in CDCl3.
169
Figure B.12: 1H-NMR spectrum of Me2C(Cp)(Ind)ZrMe2 47 in CDCl3.
Figure B.13: 1H-NMR spectrum of Me2C(Cp)2ZrMe2 66 in CDCl3.
170 B NMR Spectra
Figure B.14: 1H-NMR spectrum of Cp2Zr(Me)(thf)+BPh−4 26 in CD2Cl2.
Figure B.15: 1H-NMR spectrum of Me2C(Cp)(Ind)Zr(Me)(thf)+BPh−4 in CD2Cl2.
171
Figure B.16: 1H-NMR spectrum of C2H4(Ind)2Zr(Me)(thf)+BPh−4 67 in CD2Cl2.
Figure B.17: 1H-NMR spectrum of Me2C(Cp)2Zr(Me)(thf)+BPh−4 70 in CD2Cl2.
173
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