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. 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
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Die vorliegende Arbeit wurde unter der Anleitung von Herrn Prof. Dr. Hartwig H¨ocker am Institut f¨ur Technische Chemie und Makromolekulare Chemie der RWTH Aachen in der Zeit von Dezember 1997 bis Januar 2001 angefertigt.
Teile der Arbeit sind ver¨offentlicht bzw. zur Ver¨offentlichung 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. H¨ocker, 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. H¨ocker 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. H¨ocker, Macromolecules 2001, 34, 14–19.
[4] “Single Component Zirconocene Catalysts for the Stereospecific Polymerization of MMA”; H. Frauenrath, H. Keul, H. H¨ocker 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.H¨ocker, Macromol. Chem. Phys. 2001,inprint.
[6] “Deviation from Single-Site Behaviour in Zirconocene/MAO Catalyst Systems. 2. Influence of Polymerization Temperature”; H. Frauenrath, H. Keul, H. H¨ocker, Macromol. Chem. Phys. 2001,inprint.
[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.H¨ocker, Macromol. Rapid Commun. 2001,inprint.
III
Danksagung
Ich m¨ochte mich bei Herrn Prof. Dr. Hartwig H¨ocker f¨ur die Themenstellung und die Betreuung meiner Dissertation sowie bei Herrn Prof. Dr. Wilhelm Keim f¨ur die Ubernahme¨ des Korreferats bedanken.
Mein Dank gilt allen Mitarbeitern am Institut f¨ur Technische Chemie und Makro- molekulare Chemie, allen voran Dr. Helmut Keul f¨ur die wissenschaftliche Betreuung meiner Dissertation. Seine weisen Ratschl¨age und unsere zahlreichen, oft heftig gef¨uhrten Diskussionen haben mein wissenschaftliches Denken und Arbeiten maßgeblich gepr¨agt. Bei Rainer Haas weiß ich gar nicht aufzuz¨ahlen, wof¨ur ich mich bedanken soll. Ohne ihn w¨are “der Laden l¨angst zusammengebrochen”. Bei meinem Laborkollegen Thomas H¨ovetborn m¨ochte ich mich f¨ur drei angenehme Jahre bedanken, insbesondere auch f¨ur die regelm¨aßige Synthese von Kaffee. Peter Pilgram verdient allen Dank als ruhender Pol bei der gelegentlich nervenaufreibenden Betreuung der GPC-Anlage und Thomas Fey f¨ur den st¨andigen NMR-Service. Allen Autobesitzern des Arbeitskreises gilt mein Dank f¨ur die (viel zu regelm¨aßige) 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 f¨ur Polymerisationsreaktionen mit allem Drum und Dran. MitmeinenzweilinkenH¨anden w¨are es mir wahrscheinlich nie gelungen, ihn aufzubauen. Christoph Schoof und Luuk Duisings m¨ochte ich f¨ur den Einsatz bei ihren Forschungsar- beiten danken.
Ein besonders herzliches Dankesch¨on geht an Herrn Dr. Wolfgang Schupp von der hs-GmbH f¨ur die Entwicklung eines Peakfitting-Moduls f¨ur die WinGPC Software. Ohne seine tatkr¨aftige Unterst¨utzung vor allem beim Endspurt in der Doktorarbeit und seinen Rund-um-die-Uhr Service w¨are 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
1.2MetallocenePolymerizationofOlefins...... 5
1.2.1 Chain Propagation Reaction ...... 6
1.2.2 ChainTransferReactions...... 9
1.2.3 StereoselectivityofOlefinPolymerization...... 10
1.2.4 TailoringofPolyolefins...... 14
1.2.5 GenerationoftheActiveSpecies...... 15
1.2.5.1 Methylaluminoxanes(MAO)asCocatalysts...... 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 TheRoleofIon-Pairs...... 23
1.2.6 PolymerizationKinetics...... 24
1.2.6.1 Rate Law of Propagation ...... 24
1.2.6.2 CatalystDeactivation...... 25 VI
1.2.6.3 DeviationfromSingle-SiteBehaviour...... 25
1.2.7 CopolymerizationwithFunctionalizedMonomers...... 27
1.2.7.1 StericallyHinderedorProtectedComonomers...... 28
1.2.7.2 Noninteracting or Weakly Interacting Functional Groups . 29
1.2.7.3 LateTransitionMetals...... 30
1.3MetallocenePolymerizationofMMA...... 30
1.3.1 Group-TransferPolymerizationofMMA...... 30
1.3.2 OtherZirconoceneBasedCatalystSystems...... 32
1.3.3 SamaroceneCatalysts...... 34
1.3.4 StereospecificMMAPolymerization...... 36
1.4HowtoMakeEndsMeet?...... 39
2 Target and Specific Aims 41
3 Results and Discussion 43
3.1SynthesisofZirconoceneComplexes...... 43
3.1.1 PreparationofLigandsandComplexPrecursors...... 43
3.1.2 PreparationofZirconoceneDichlorides...... 45
3.1.3 PreparationofDimethylZirconocenes...... 46
3.1.4 PreparationofCationicMethylZirconocenes...... 47
3.2OlefinPolymerizationwithZirconocenes...... 48
3.2.1 GeneralApproach...... 48
3.2.2 EffectofMonomerConcentration...... 49
3.2.2.1 ReactionOrderinMonomerConcentration...... 49
3.2.2.2 Monomer Concentration, Molecular Weight and MWD . . 50
3.2.2.3 Conclusions...... 52
3.2.3 EffectofCatalystandCocatalystConcentrations...... 53
3.2.3.1 ReactionOrderinZirconoceneConcentration...... 53
3.2.3.2 Zirconocene Concentration, Molecular Weight and MWD . 56 VII
3.2.3.3 Conclusions...... 64
3.2.4 PolymerizationKinetics...... 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 EffectofPolymerizationTemperature...... 76
3.2.5.1 MolecularWeightandMWD...... 77
3.2.5.2 Mathematical Modelling of Temperature Effects ...... 86
3.2.5.3 Conclusions...... 92
3.2.6 EffectofPolarandNonpolarCosolvents...... 93
3.2.6.1 Conclusions...... 98
3.2.7 TheNatureoftheActiveSpecies...... 98
3.2.8 WhatHasBeenLearntConcerningCopolymerization?...... 102
3.3MMAPolymerizationwithZirconocenes...... 103
3.3.1 KineticsofMMAPolymerization...... 104
3.3.2 Mathematical Modelling of Polymerization Kinetics ...... 107
3.3.3 ControlofMolecularWeightandMWD...... 110
3.3.4 ControlofStereospecificity...... 113
3.3.5 ProposalforaPolymerizationMechanism...... 119
3.3.5.1 Mechanism of Chain Propagation ...... 119
3.3.5.2 MechanismofStereocontrol...... 121
3.3.5.3 GenerationoftheActiveSpecies...... 123
3.3.5.4 WhatMakestheDifference?...... 123
3.3.6 Conclusions...... 125
4 Experimental Part 127
4.1GeneralProcedures...... 127 VIII
4.2PreparationofLigandsandReactants...... 128
4.3PreparationofZirconoceneComplexes...... 130
4.3.1 PreparationofZirconoceneDichlorides...... 130
4.3.2 PreparationofDimethylZirconocenes...... 131
4.3.3 PreparationofCationicMethylZirconocenes...... 134
4.4PolymerizationReactions...... 136
4.4.1 1-HexenePolymerization...... 136
4.4.2 MMAPolymerization...... 149
Appendix 153
A Mathematical Equations 153
A.1Equation3.1...... 153
A.2Equation3.2...... 153
A.3Equation3.3...... 154
A.4EquationsforActivationParameters...... 154
A.5Equation3.5...... 155
A.6Equation3.6...... 155
A.7Equation3.7...... 155
A.8Equation3.8...... 157
A.9Equation3.9...... 157
A.10Equation3.15...... 158
A.11Equation3.16...... 158
A.12Equations3.18and3.19...... 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 DEdiethyl ether DMSO dimethyl sulfoxide d duplet (in NMR) DFT density functional theory DVB divinyl benzene EA ethyl acrylate ESI ethylene-styrene interpolymers Etethyl 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-PEpolyethylene 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 VEvalence 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) BPh4 yielding highly isotactic + − PMMA and polymerization of MMA with Me2C(Cp)2Zr(Me)(thf) BPh4 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 n¨aher zu untersuchen. Die Molekulargewichtsbestimmung wird dabei als empfindliche “Sonde” genutzt.
Aus den experimentellen Befunden l¨aßt 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 k¨onnen. Daraus l¨aßt sich folgern, daß diese Spezies (wennuberhaupt) ¨ 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 f¨ur den großen Uberschuߨ an Cokatalysator anzusehen, der notwendig ist, um ein katalytisch aktives System zu erzeugen.
Die Zugabe polarer Cosolventien zeigt einen auf die Polymerisation sch¨adlichen Ein- fluß. Die experimentellen Befunde k¨onnen 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 m¨oglich.
Die Verwendung chiraler Zirkonocen-Katalysatoren bei der MMA-Polymerisation stellt sich als wirkungsvolle Methode zur gezielten Kontrolle der Polymermikrostruktur heraus. + − So erh¨alt man bei Verwendung von Me2C(Cp)(Ind)Zr(Me)(thf) BPh4 hochisotakti- XIV Zusammenfassung
+ − sches PMMA, w¨ahrend die Polymerisation mit Me2C(Cp)2Zr(Me)(thf) BPh4 syndio- taktisches PMMA ergibt. Dies ist das erste Beispiel f¨ur einen engen Zusammenhang zwischen Katalysatorsymmetrie und Polymertaktizit¨at bei der Polymerisation von MMA mit Zirkonocenen. Auf der Grundlage von kinetischen Experimenten wird ein Polymeri- sationsmechanismus vorgeschlagen, der sich als grunds¨atzlich verschieden von dem bereits bekannten Mechanismus der Gruppentransfer-Polymerisation erweist und auch eine Per- spektive f¨ur eine sequentielle Polymerisation von 1-Olefinen und Methacrylaten bietet.
Schließlich kann man aus allen in der vorliegenden Dissertation pr¨asentierten Ergebnis- sen schließen, daß eine (Block-) Copolymerisation von 1-Olefinen und funktionalisierten Monomeren mit Zirkonocen-Katalysatoren m¨oglich ist, aber nur, wenn die Reihenfol- ge der Reaktivit¨aten bez¨uglich des elektrophilen Katalysators ber¨ucksichtigt 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, Universit¨atsprofessor Geburtsdatum 27.10.1972 in Aachen Familienstand ledig
1979-1983 Besuch der Gemeinschaftsgrundschule Br¨uhlstraße, Aachen 1983-1992 Besuch des St¨adtischen Einhard-Gymnasiums, Aachen 20.6.1992 Tag der letzten Abiturpr¨ufung (Allgemeine Hochschulreife)
1.10.1992 Einschreibung im Studiengang Chemie an der RWTH Aachen 10.5.1995 Tag der letzten Diplomvorpr¨ufung im Studiengang Chemie 28.4.1997 Tag der letzten Diplompr¨ufung im Studiengang Chemie 2.6.1997 – 21.11.1997 Diplomarbeit im Arbeitskreis von Herrn Prof. Dr. Hartwig H¨ocker am Lehrstuhl f¨ur 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 H¨ocker am Lehrstuhl f¨ur Textilchemie und Makromolekulare Chemie der RWTH Aachen zum Thema: “Polymerisation von funk- tionalisierten und unfunktionalisierten Olefinen mit Metallocen- Katalysatoren” 25.1.2001 Tag der m¨undlichen Promotionspr¨ufung
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 1.7Mta−1 in 1950 to 31.5Mta−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
200 -1
150
100
50 Annual polyolefin production in Mt a
0 1950 1960 1970 1980 1990 2000 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 < 1%
engineering plastics 12 %
standard plastics 88 %
Figure 1.2: Distribution of high performance, engineering and standard plastics.3, 4
60 high performance plastics
40 LCP engineering plastics PSU, PES, PSU, PEI PAR,
Fluoro- standard plastics 20 polymers -1 10
8 PPS PBT PC PET (engin.) 6 POM PA
PMMA
4 ABS PC/ABS PC/PBT SAN PET
Market price in DM kg 2
PP PE PS PP (engin.)
1 PS-HI 0,8 PVC 0,6 0,01 0,1 1 10 100
World consumption in Mt a-1
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 9.2Mta−1 in 1970 to 71.1Mta−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
poly-1-olefins 106 (viscosity agents) PE-UHMW
EPM i-PP 5 s-PP 10 s-PS COC PE-HD PE-LLD PE-VLD 4 10 PP-waxes Molecular weight PE-waxes
3 10 telechels lubricants
0 20 40 60 80 100 Comonomer content in %
105
COC engineering SPS plastics -2 PP s-PP 4 10 TPX r-PP
PE-HD
PB-1 PE-LLD PE-LD 103 PE-VLD ionomer
PE-ULD EVA Modulus in kg cm
EPT PVC 102 soft
0.8 0.9 1.0 1.3 Density in g cm-3
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-PEand 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
migratory 1,2-insertion Pol R Si Zr Si Zr R H R H H R H Pol R
+ monomer + monomer
active site epimerization "chain back skip"
Pol R Si Zr Si Zr R R H H
H migratory R 1,2-insertion H Pol R
+ monomer β-H-transfer
Pol Si Zr + R H R H H R
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 M¨uhlhaupt 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 VEand 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 VEcomplex. 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 VEcomplex 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.
H H H H H Pol H Pol H Pol H Pol
L2Zr R L2Zr R L2Zr R L2Zr R H H H H R 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
-6
e1g
a2 -7
b1 2a1 -8
-9 Energy in eV
a1g b2 -10
1a1 e2g -11 180° 160° 140° 120° θ
θ Mt θ Mt
{ } 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,whichis 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.
Pol Pol Pol Pol L2Zr L2Zr L2Zr L2Zr + R H H R H R H R
Pol Pol H Pol R Pol L Zr L Zr R L Zr L Zr H + 2 2 2 H 2 H R H R R R R
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 +9.5kJmol−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
Pol isospecific MLn
mmmm r r mmmm site control
Pol syndiospecific MLn
rrrrmmrrrr
Pol isospecific MLn
mmmm r mmmmm chain-end control
Pol syndiospecific MLn
rrrrmrrrrr
chain growth
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) C2v (achiral) Cs (achiral) C2 (chiral) Cs (prochiral) C1 (chiral) (B) homotopic (N,N) diastereotopic (N,N) homotopic (E,E) enantiotopic (E,E) diastereotopic (E,N) (C) atactic atactic isotactic syndiotactic hemiisotactic (D) atactic atactic isotactic atactic/isotactic 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 21, 39 as “Ewen’s symmetry rules” (Figure 1.7). 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.
Cl Cl Cl Cl Zr Cl Zr Cl Zr Cl Si Zr Cl
1 2 3 4
Cl Cl Cl Zr Cl Zr Cl Zr Cl
5 6 7 Figure 1.8: Examples for catalyst precursors of different symmetry.
R Me Si Me2Si Me Me2Si H 2 H Zr Me Zr Zr Me
Me Me
predominantly no enantiofacial predominantly si-coordination selectivity re-coordination
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 t 46–49 predictable. Thus, with Me2C(3- BuCp)(Flu)ZrCl2 6 isotactic PP is obtained, 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 39, 46–49 Me2C(3-MeCp)(Flu)ZrCl2 7. 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
Zr Cl Cl Cl Cl Cl Si Zr Cl Si Zr Cl Si Zr Cl
3 4 8 9
Cl Cl Zr Cl Si Zr Si Zr Cl Si Cl Cl
1011 12
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).
Productivity M mmmm T Catalyst Precursor w m kg(PP) mmol(Zr)−1 h−1 1000 % ◦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 i Me2Si(2-Me-4- Pr-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 53 with simple aluminum alkyls such as AlClEt2 or AlEt3. 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 21, 24 Cp2ZrMe2 15 are definitely not formed. The detailed mechanism of MAO activation as supported by recent results (vide infra) is depicted in Scheme 1.4.
Al O O O Al Al Al O Al Al Al O Al O OAl O OAl Al Al O O Al O O O Al O Al Al O Al O O O Al Al Al Al Al O Al Al Al O O O Al Al O O Al 16 17 18
Me Al 2 Me Me O Al O Al Me AlMe O Al O 2 Me Me O AlMe MeAl Al O O Zr Me AlMe O O MeAl O Zr O Al O Al Me Me2Al Me Al Me O Al O Me Me Me AlMe2
19 20 Figure 1.11: Structures of MAO clusters and reaction products with zirconocenes com- plexes; alkyl groups in 16, 17,and18 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 59 sum formula to be Me1.3−1.5Al1O0.75−0.85. 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,or18. 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- t t lated closed cage structures such as [( Bu)Al(µ3-O)]6 16 and [( Bu)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
Al/Zr 20 Zr Cl Me Zr Me Cl Zr Cl Me
113 15
Al/Zr 50 Al/Zr 150 Zr Me Me or Zr (µ-Me)-MAO Me Me Zr
21 22
Me Al/Zr 1000Me Al/Zr > 1000 Me Me Zr Al Zr Zr µ Me ( -O)-MAO Me
23 14
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 24, 64–67 and Me2C(Cp)(Flu)ZrCl2 5 with MAO by means of UV spectroscopy. 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 66, 67 is not the case with MAO. 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 68 ratio, is also enhanced by preactivation in CH2Cl2. 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] 69 + 21. 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- 71–73 ing monomer concentration (pressure). 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 66, 74 Me2C(Cp)(Flu)ZrCl2/MAO. 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) BPh4 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 − 75, 76 − − 79, 80 as BPh4 , B(C6F5)4 or MeB(C6F5)3 . 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
B(C6F5)3 CH2Cl2 Zr Me MeB(C6F5)3
24
B(C6F5)4 Ph3CB(C6F5)4 CH Cl 2 2 Me Zr Me Zr Me CH Cl – Ph3CMe 2 2
15 25
BPh4 HNBu3BPh4 toluene/THF, – CH4 Zr Me or AgBPh4 O toluene/THF, – C2H6
26 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 79 Lewis-acid such as B(C6F5)3, (ii) methyl abstraction using a tritylium salt such as 20 1 Introduction
B(C F ) B(C6F5)2 ∆ 6 5 3 , -C6H6 toluene F Zr Ph Zr Ph Zr Ph Ph F F F 27 29 F
Mg(C4H6) B(C6F5)3 Cl -MgCl2 toluene Zr Cl Zr Zr B(C6F5)2 F F 28 30 F F F Scheme 1.6: Synthesis of zwitterionic methyl zirconocenes.
+ − 84 + − 85 Ph3C B(C6F5)4 , (iii) protonolysis with a Brønsted acid such as HNBu3 B(C6F5)4 , or (iv) oxidative cleavage of metal alkyl bonds with one-electron oxidants such as + − + − 75, 86 + − (C5H4R)2Fe BPh4 or Ag BPh4 , as illustrated for Cp2Zr(Me) (µ-Me)B(C6F5)3 24, + − + − Cp2Zr(Me)(CH2Cl2) B(C6F5)4 25 aand Cp2Zr(Me)(thf) BPh4 26 (Scheme 1.5). The 6 reaction of so-called “tuck-in” (fulvene) complexes like (C5Me5)(η -C5Me4CH2)ZrPh 87, 88 4 89 27 or butadiene complexes Cp2Zr(η -C4H6) 28 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 BPh4 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
B(C6F5)4 CH2Cl2 Ph3CB(C6F5)4 (0.5 eq. or 1 eq. < 20°C) Si Zr Me Me – Ph3CMe Me Zr Si
Me 32 Si Zr Me
CH Cl 2 2 B(C6F5)4 Ph CB(C F ) 3 6 5 4 Me AlMe3 < 25°C Me Si Zr Al 31 – Ph CMe Me 3 Me
33
Scheme 1.7: Synthesis of dinuclear cationic zirconocene complexes.
+ 92 dinuclear complex [Me2Si(Ind)2Zr(Me)(µ-Me)(Me)Zr(Ind)2SiMe2] 32. 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 ◦ 92 complex in CD2Cl2 above 20 C. 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 CinCD2Cl2. 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 + − 93 Cp2Zr(Me) (µ-Me)B(C6F5)3 24 in C6D6. 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 81 good activators like B(C6F5)3. 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. −101.1kJmol−1 for the − −1 system (1, 2-Me2Cp)2ZrMe2/B(C6F5)3,and 45.6kJmol for the system
(1, 2-Me2Cp)2ZrMe2/MAO. Assuming entropy-loss to be similar for activation with − −1 −1 MAO or B(C6F5)3 ( 22 J mol K 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 = +100.3kJmol−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 −1 + − being destabilized with +203 kJ mol 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 +34.3kJmol−1, leading to an olefin separated ion pair (OSIP). Again the “free” olefin complex is destabilized with +75.6kJmol−1.29 Similar studies for MAO activation indicate that in this case complete ion pair separation is laso unfavourable.97 Klesing 98 et al. 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 M¨uhlhaupt 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 106 + by Kaminsky et al. These species are regenerated with excess MAO, yielding L2ZrMe and Al-CH2-Al structures.
MAO Zr* irreversible Zr•MAOinact Zr* Zrinact,1 Zrinact,2
MAO
Scheme 1.8: Deactivation processes in zirconocene/MAO based catalyst systems accord- ingtotheresultsofM¨uhlhaupt 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 ◦ ◦ 112 at 25 CandMw/Mn ≈ 5at90 C is obtained. 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 ◦ 111–113 Cp2ZrCl2/MAO below 30 C. 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 M¨uhlhaupt 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
O
R2N R3SiO (R3Si)2N RO n n n n
i i t t 34a (R = Me, Et, Pr, Bu; n = 3) 35a (R3 = BuMe2; n = 3) 36 (R3 = Me3; n = 9) 37 (R = H, Me, Bu; n = 8) i 34b (R = Bu; n = 2) 35b (R3 = Me3; n = 9) 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-
Me Mt Me + Mt Me Mt + Mt Me O O
O O
Cp2ZrMe2/B(C6F5)3 O C2H4 Ti O mn OO
38 Ti
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 X B n n 39 40a (X = I; n = 2) 41 (n = 4, 6) 40b (X = Cl; n = 9) 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
MeO Me Zr Zr Me O + O a d2 Me 3 COOMe MeO Zr MeO O Zr Me 14 · MMA COOMe O
42 – PMMA COOMe Pol MeO COO Me
+ MMA + MMA Pol
Zr Me OMe O Me OMe Zr Zr Me Me Z O r O O MeOOC + d2 a3 OMe MeOOC MeOOC OMe MeOOC 14' · MMA
Pol 42' – PMMA Pol 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) BPh4 26/Cp2Zr(Me)[OC(OMe)C(Me)(R)] 42 (R=Me) is synthesized separately156 or generated in situ from a mixture of two equiv- + − 154, 155 alents of Cp2ZrMe2 15, one equivalent of HNBu3 BPh4 and MMA. 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, H¨ocker et al.158, 159 have shown that + − the cationic zirconocene complex Me2C(Cp)(Ind)Zr(Me)(thf) BPh4 43 is active for MMA polymerization without addition of the corresponding ester enolate t complex Me2C(Cp)(Ind)Zr(Me)[OC(O Bu)CMe2] 44 as opposed to the achiral + − 160 Cp2Zr(Me)(thf) BPh4 26. Gibson et al. have reported similar observations. Thus, + − an equimolar mixture of Cp2ZrMe2 15 and HNEt3 BPh4 does not generate a cat- alytically active system, which is consistent with the findings of Collins et al. and
H¨ocker 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
OtBu BPh4
Zr O Zr O Me Me
43 44
Me Zr Me Zr Me Zr Me Me Me
15 45 46
Me Me Me Zr Me Zr Me Zr Me
47 48 49
BPh4
Me Cl O Si Zr Si Zr Si Zr Me Cl O N O
50 31 51 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) 161 the polymer yield is 38%, Mn = 29000 (instead of 400000), and Mw/Mn =1.62. 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 CandMw/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
H Me Sm Sm Sm O H
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
OMe
OMe O MeO Sm a3 O O MeO Sm d2 O
MeOOC COOMe
MeOOC COOMe Pol 54 · MMA Pol
± MMA + MMA + MMA
Pol Pol
MeOOC COOMe MeOOC COOMe
d2 O O Sm Sm a3 O MeO O MeO
OMe OMe
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 166 obtained using [(C5Me5)2SmD]2. 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- 167, 168, 170, 171 sized 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 H¨ocker et al. show that the “Collins-type” poly- + − merization of MMA with the chiral catalyst system Me2C(Cp)(Ind)Zr(Me)(thf) BPh4 t 43/Me2C(Cp)(Ind)Zr(Me)[OC(O Bu)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) BPh4 43 is applied alone or in excess, the PMMA obtained is highly isotactic.158, 159 The addition of increasing amounts t of Me2C(Cp)(Ind)Zr(Me)[OC(O Bu)CMe2] 44 results in a broadening of MWD, a de- crease in isotacticity, and pentad abundance consistent with a blend of iso-enriched and 158 + − syndio-enriched PMMA. Apparently, the cation Me2C(Cp)(Ind)Zr(Me)(thf) BPh4 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-orC2-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
Pol MeO Pol MeO O OMe O Si Zr O Si Zr O N N O MeO MeO
+ MMA Pol MeO
O Si Zr O N MeO
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 165 in CH2Cl2 despite the Cs-symmetry of the catalyst. 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
Me2Si LaR Me2Si SmR'
(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 − 180 (S)-Me2Si(C5Me4)(3-( )-menthyl-C5H3)Sm[CH(SiMe3)2] 56. 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 the subsequent 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.
H H H H 1. THF, NaOH, Aliquat-336 2. acetone 2 H H
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, BuLi 2. H2O
59 HH 1. pyrollidine, MeOH 2. acetone
58 1. fluorene, BuLi 2. H2O
60 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.
HNEt2/BuLi Et2O/pentane, < 0°C ZrCl4 Zr(NEt2)4
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).
Cl Cl Cl Cl Zr Cl Zr Cl Si Zr Cl Si Zr Cl
1623 4
Cl Zr Cl
1. Zr(NEt2)4 61 63 57 2. Me SiCl Cl 3 Zr Cl
59 Cl 64 Zr Cl
60 cm 5
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%)andMe2C(Cp)(Ind)ZrMe2 47 (59.1 %) as brown powders (Scheme 3.5).
Cl Cl Me Me Zr Cl Zr Cl Zr Me Zr Me
3 5 46 49
MeLi (2 eq.), Et O Cl Cl 2 Me Me Si Zr Cl Zr Cl Si Zr Me Zr Me
62 63 65 66
Cl Me Zr Cl Zr Me
64 47
MeMgCl (2 eq.), Et2O Zr Cl Me Cl Zr Me
1 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 HNBu3 BPh4 in a solvent mixture containing THF as a stabilizing ligand (Scheme 3.6).
BPh4 BPh4
Me Me Zr Me Si Zr Me Zr Si Zr Me Me O O
14 65 26 69
BPh4 BPh4 HNBu3BPh4 Me toluene/THF Me Zr Me Zr Zr Zr Me Me Me O O
46 66 67 70
BPh4 BPh4
Zr Me Zr Me Zr Me Zr Me Me Me O O
47 49 43 68
Scheme 3.6: Synthesis of cationic methyl zirconocenes.
+ − Thus, reaction of Cp2ZrMe2 15 yields Cp2Zr(Me)(thf) BPh4 26 (81.9%,almost + − white powder), reaction of C2H4(Ind)2ZrMe2 46 affords C2H4(Ind)2Zr(Me)(thf) BPh4
67 (79.8 %, brown-orange solid), from the reaction of Me2C(Cp)(Ind)ZrMe2 47 + − one obtains Me2C(Cp)(Ind)Zr(Me)(thf) BPh4 43 (53.4 %, light orange solid), + − starting from Me2C(Cp)(Flu)ZrMe2 49 yields Me2C(Cp)(Flu)Zr(Me)(thf) BPh4 68
(72.4 %, brown-orange solid), from the reaction of Me2C(Cp)2ZrMe2 66 one obtains + − Me2C(Cp)2Zr(Me)(thf) BPh4 70 (67.4 %, light beige powder) and the reaction of + − Me2Si(Cp)2ZrMe2 65 yields Me2Si(Cp)2Zr(Me)(thf) BPh4 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
L2ZrCl2/MAO toluene (cosolvent)
71 CH3 n 72
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 =10handtp = 600 s, respectively) at varying initial monomer concentrations [Mon]0. All other reaction con- −4 −1 ◦ ditions are kept constant ([Zr] = 3 · 10 mol L ;Al/Zr = 5000; Tp = −20 Cand ◦ 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 +lnkapp +ln∆t. (3.1)
Reaction orders a1 inthecaseofCp2ZrCl2 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 [Mon]) ∆
ln ( -1,5
-2,0
-2,5
-3,0 -1,0 -0,5 0,0 0,5 1,0 1,5 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
5000 4,0
3,5
4000
3,0 n
n 3000 2,5 /M M w M
2,0
2000
1,5
1000 1,0 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5
-1 [Mon]0 in mol L
Figure 3.2: Molecular weight and MWD of poly(1-hexene) obtained with Cp2ZrCl2
1/MAO vs. monomer concentration after tp =10h( , )andtp =5d(✷,✷).
9000 4,0
8500
8000 3,5
7500
7000 3,0
6500 n
n 6000 2,5 /M M w 5500 M
5000 2,0
4500
4000 1,5
3500
3000 1,0 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5
-1 [Mon]0 in mol L Figure 3.3: Molecular weight and MWD of poly(1-hexene) obtained with