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Structure - property relationships in early transition metal based olefin polumerisationcatalystsBeetstra, Dirk Johannes

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Structure – Property Relationships in Early Transition Metal Based Olefin Polymerisation Catalysts

Dirk Beetstra

The Dutch Polymer Institute, P.O.Box 902,

5600 AX Eindhoven, The Netherlands,

is gratefully acknowledged for supporting this work under project #102.

RIJKSUNIVERSITEIT GRONINGEN

Structure – Property Relationships in Early Transition Metal Based Olefin Polymerisation Catalysts

Proefschrift

ter verkrijging van het doctoraat in de Wiskunde en Natuurwetenschappen aan de Rijksuniversiteit Groningen

op gezag van de Rector Magnificus, dr. F. Zwarts, in het openbaar te verdedigen op

vrijdag 15 juli 2005 om 16:15 uur

door

Dirk Johannes Beetstra

geboren op 2 mei 1971 te Waskemeer

Promotores: Prof. Dr. B. Hessen Prof. Dr. J. H. Teuben Beoordelingscommissie: Prof. Dr. R. Beckhaus Prof. Dr. J.B.F.N. Engberts Prof. Dr. J. Okuda ISBN 90-367-2306-X (paperback version) ISBN 90-367-2307-8 (electronic version)

Dankwoord Een chemisch onderzoek, vooral in de richtingen waarin moleculen worden

gemaakt, is natuurlijk vooral mogelijk doordat de moleculen van de uitgangsstoffen enige reactiviteit vertonen. Als gevolg van hun reactiviteit worden (over het algemeen) nieuwe bindingen gevormd, en oude verbroken, wat dan resulteert in nieuwe moleculen met weer nieuwe reactiviteiten. Ook al volgden de meeste moleculen braaf mijn beslissing om samen in één reactievat te gaan zitten, en werden ook vaak nieuwe gevormd, leverde dit ook regelmatig niet het (ene) molecuul op, die ik in gedachte had. Toch heeft het samenspel van verbreken van oude bindingen en het vormen van nieuwe mij vaak geboeid.

Dit proefschrift markeert het eind van mijn reactiviteit als promovendus. Naast alle moleculen, zijn er een groot aantal ingewikkelder “reactiemengsels” geweest, die mijn onderzoek mogelijk hebben gemaakt. Van deze wil ik als eerste mijn promotores Bart Hessen en Jan Teuben noemen. Zij hebben mij de afgelopen jaren de vrijheid gegeven om onderzoek te doen in hun groep, en mijn grillen (en die van bovenstaande moleculen) proberen in goede banen te leiden. Het heeft geresulteerd in een aantal leuke resultaten (wat een aantal leuke publicaties heeft opgeleverd), en ook nog een aantal open einden, waar (hopelijk) een opvolger zich op stuk mag bijten.

De verdere lijst met personen, die hun bijdrage aan mijn reactiviteit hebben gegeven wordt aangevoerd door Maaike Wander. Maaike, jou wil ik bedanken voor de tijd die je als ‘mijn’ hoofdvakstudent hebt doorgebracht. Een deel van jouw werk is verwerkt in hoofdstuk 2. Veel succes met jouw promotie, en alle verdere dingen die je doet en gaat doen.

Veel onderdelen van het onderzoek zijn niet mogelijk zonder alle andere mensen in een laboratorium. Van dit personeel wil ik allereerst Daan van Leusen bedanken voor de (hulp bij) syntheses van de (soms ingewikkelde) liganden. Auke Meetsma wil ik bedanken voor het meten en oplossen van de vele kristalstructuren in dit proefschrift. Ook dank voor je enthousiasme waarmee je elk nieuw kristal verwelkomt, en waarmee je zelfs het meten van de meest triviale verbinding tot een feest kunt maken. Oetze Staal wil ik bedanken voor de vele hulp bij de polymerisatieopstellingen, en zijn worstelingen om de opstelling, de oplosmiddelen en monomeren droog te houden, om de druk te regelen etc. etc. Verder mijn dank voor het verzorgen van alle apparatuur op het lab. Andries Jekel wil ik bedanken voor de vele GC-MS en GPC analyses. In eerdere tijden werd de apparatuur op het lab verzorgd door Jan Helmantel, Jan, bedankt voor deze hulp, en de gezellige tijd. Verder gaat mijn dank uit naar al die mensen die er voor zorgen dat de apparatuur in het gebouw werkt, die de doorstroom van chemicalien verzorgen, de post regelen, die van een aantal glassplinters weer een werkend Schlenkvat maken, en die de droge en gecomprimeerde (koude) gassen leveren.

The laboratory has always been filled with a large number of collegues. In the beginning of my research the people from the Hessen/Teuben group, later also the growing Hummelen group, and now very recently the start-up of the Van Koningsbruggen group. I will not even try to name you all, but I thank you for all your help with my chemistry, for sharing your chemistry with me, and for all the other fun moments in and around the lab. I am glad to see that you have not been scared away by all my moments of madness, keep up the good work!

Still, there are some that have been more involved into my (chemical) life, and who have made life in the lab even more pleasurable. Marco Bouwkamp and Timo Sciarone, thank you both for all the help and for all the discussions about chemistry. You have made the lab such a better place to live, chemicalwise, musicwise, fruitwise ánd mentalwise. I also thank you both for the time outside the lab (pubs, dinners, Vera concerts, congresses). Special thanks also to Aurora, Cindy, Coen, Elena, Erica, Marten, Minze, Patrick D., Peter W., Piet-Jan, Sérgio, Stéphanie, Victor, Weidong, and Winfried for the help, interaction and nice time during my years in the lab. Live in (and outside) the “Teuben/Hessen Asylum for the Chemically and Mentally Challanged” is not thát bad.

Buiten het lab zijn er een aantal vrienden die ik wil bedanken voor hun steun tijdens

mijn frustraties. Ik hoop dat na deze periode van schrijven (en schrijven), ik ook eens tijd ga hebben om mij te ontspannen en wat meer tijd aan jullie te besteden. Hier wil ik ook mijn ouders en mijn zus bedanken voor de broodnodige afleiding, de mogelijkheid om Groningen te ontvluchten en dan wat tijd doorbrengen in de Fryske Wâlden heeft vaak een goed effect gehad op mijn rust.

Dirk.

Contents

1 Introduction 1

1.1 Polymerisation of olefins 2 1.2 Mechanisms of homogeneous, catalytic olefin polymerisation 3 1.2.1 Chain transfer - molecular weight and molecular weight distribution 4 1.2.2 Regio- and stereospecificity 7 1.3 Activators 9 1.3.1 Aluminium alkyls and aluminoxanes 10 1.3.2 Lewis acids 10 1.3.3 Brønsted acids / weakly coordinating anions 12 1.4 Homogeneous, single site catalysts 13 1.4.1 Half sandwich complexes with a tetravalent metal 14 1.4.2 Half sandwich complexes with a trivalent metal 16 1.5 Aim and scope of the thesis 17 1.6 References 18

2 Titanium Complexes with Linked Cyclopentadienyl Amido Ligands 31

2.1 Ethylene bridged tetramethylcyclopentadienyl amido ligands 33 2.1.1 Ligand synthesis 33 2.1.2 Catalyst precursor synthesis 36 2.1.3 Olefin homopolymerisation with [C5Me4(CH2)2NR]TiCl2/MAO 37 2.1.4 Synthesis of dialkyl derivatives 39 2.1.5 Olefin homopolymerisation with [C5Me4(CH2)2NR]TiR2 activated with

borane and borate reagents 41 2.1.6 Discussion 43 2.2 Effect of the counterion on polymerisation with [C5H4(CH2)2NR]TiR-

cation 44 2.2.1 Polymerisation activity 44 2.2.2 Counterion synthesis 48 2.2.3 Polymerisation experiments with extra anion 50 2.2.4 Discussion 53 2.3 Conclusions 53 2.3 Experimental Section 54 2.4 References 65

3 Metal(III) Dichlorides of Group 3 to 6 Metals Supported by a Linked Cyclopentadienyl Amine Ligand 69

3.1 Synthesis of cyclopentadienyl-amine M(III) dichlorides [η5,η1-C5H4(CH2)2NMe2]MCl2(PMe3)n (M = Sc, Ti, V, Cr; n = 0,1) 71

3.2 Olefin polymerisation experiments 78 3.3 Conclusions 80 3.4 Experimental Section 80 3.5 References 85

4 Synthesis and Reactivity of Cyclopentadienyl-Amine Transition Metal 2,3-Dimethyl-1,3-Butadiene Complexes 87

4.1 Synthesis and structure of cyclopentadienyl-amine M(III) 2,3-dimethyl-1,3-diene complexes 88

4.2 Formation of zwitterionic/betaine compounds. 91 4.3 Olefin polymerisation experiments 96 4.4 Thermal decomposition of the scandium butadiene complex. 97 4.5 Reactivity of the scandium 1,3-butadiene complex 99 4.6 Conclusions 107 4.7 Experimental section 108 4.8 References 118

5 Synthesis and Reactivity of Cyclopentadienyl-Amine Transition Metal Dialkyl Complexes 123

5.1 Synthesis of cyclopentadienyl-amine M(III) dialkyl complexes 124 5.2 Olefin polymerisation experiments 128 5.3 Decomposition of scandium dialkyls; is α-hydrogen abstraction possible

for group 3 metals? 130 5.4 Conclusions 139 5.5 Experimental section 140 5.6 References 152

6 Samenvatting (Dutch Summary) 155

1

1 Introduction Understanding the fundamentals of reactions that are at the heart of industrial

processes yields information that can be important for achieving improvements in these processes, and can also open up pathways to new processes and materials. One of the topics for which this statement is certainly true, is the area of catalytic olefin polymerisation.1 This subject has been of tremendous industrial importance over the last 50 years, and at the same time has fascinated scientists.2 With the enormous, and still growing, quantities in which polyolefins are produced today, and the increasing number of applications that these materials find, this interest is likely to remain high for some time to come.2c

Catalysis by transition-metal species has given access to an entire family of polymeric materials,3,4 the linear polyolefins, that has had an industrial and societal impact that is difficult to overestimate. Production of polyethene and polypropene in 2000 totalled about 75 Mt and is still growing annually by several percent. The initial breakthroughs in this field were made using heterogeneous catalysts (Ziegler-Natta catalyst, Phillips catalyst), that are relatively difficult to modify and improve by rational design, although empirical approaches have yielded great improvements in catalyst productivity and selectivity. Over the last two decades the use of well-defined molecular organometallic catalysts has come to the fore. In these systems, catalyst structure-property relationships are much more readily explored. This thesis deals primarily with probing catalyst structure-property relationships in a particular family of molecular catalysts.

In this introductory chapter an overview of the area of catalytic polymerisation of (α-)olefins is given. After a short introduction on olefin polymerisation processes (section 1.1), the focus will shift to mechanistic aspects of polymer growth and chain transfer processes and selectivity issues (section 1.2). Molecular catalysts for olefin polymerisation generally require a cocatalyst or activator to generate the active species, and families and action of such activators are presented in section 1.3. In section 1.4, molecular olefin polymerisation catalysts will be reviewed, with a focus on mono(cyclopentadienyl) complexes of tetravalent (section 1.4.1) and trivalent (section 1.4.2) transition metals. The aim of this chapter is to discuss relevant structural and electronic factors of catalysts, and to provide an understanding of the underlying fundamental processes. Those who are interested in a broader overview or want to obtain a deeper understanding of olefin polymerisation are referred to the excellent reviews that have appeared.5

1

2

1.1 Polymerisation of olefins

Until 1953, processes for olefin polymerisation were based on a radical process at high pressures and high temperatures. Polymerisation of ethene under these conditions (2000-3000 bars; 150-230°C) yields low-density polyethene (LDPE, figure 1.1), a low melting, highly branched polyethene, containing both long- and short chain branches.6 With propene only atactic, low molecular weight material can be obtained.7

Ziegler found that ethene could also be polymerised using TiCl4 and aluminium alkyls.8 The process yields linear polyethene (HDPE, figure 1.1) with a high molecular weight. Natta proved that the same type of catalyst also polymerises propene.9 The resulting polymer mixture is predominantly isotactic with additional polymer fractions that are of a lower stereoregularity or atactic. Copolymerisations of ethene with 1-hexene/1-octene with the titanium Ziegler catalysts result in copolymers in which the degree of incorporation of the α-olefin varies over the molecular weight distribution.

Upon reaction of a vanadium compound, e.g. V(acac)3 (acac = acetylacetonato) or VCl4, with an aluminium alkyl cocatalyst a catalyst for the production of EP (copolymer of ethene and propene) and EPDM (ethene-propene-diene elastomers) is obtained.5ac,10 The (homogeneous) system shows high (initial) activity, but is rapidly deactivated.11,12 An important advantage is that the comonomers are randomly incorporated in the polymer over the full range of the molecular weight distribution.

A chromium based catalyst was synthesised by Hogan and Banks at Phillips by impregnating chromic acid on silica (Cr/SiO2).

5k,13,14 A similar system, based on chromocene on dehydroxylated silica, was developed by Karapinka et al. at Union-Carbide.15,16 These systems also show a high activity, but differ from titanium and vanadium based catalysts in that they produce HDPE with broad molecular weight

LDPE

HDPE

LLDPE

Figure 1.1. Examples of polyethenes: LDPE, HDPE and LLDPE (copolymer of ethene and 1-hexene).

Chapter 1

3

distributions without the requirement of a cocatalyst.17 A main disadvantage of this system is that it is incapable to produce polypropene or copolymers. The oxidation state of the active chromium in the polymerisation, the molecular structure of the catalyst and the mechanism of the reaction are still under debate.18

The heterogeneity of Z-N systems and Phillips/Union Carbide type catalysts makes them very attractive for industrial application, and most polyolefin materials are still produced by means of heterogeneous catalysts.19 These heterogeneous systems combine high activity with an easy processability of the resulting product mixture and good polymer particle morphology. The catalyst systems contain various types of active sites with different geometries and activities, which often leads to polymers with broad or polymodal molecular weight distributions or to mixtures of different types of polymers (e.g. mixtures of atactic and isotactic polypropene).20 Many improvements on the classical Z-N type catalysts have been made over the last 30 years, and modern Z-N systems allow a much better control of polymer properties. Most of these improvements were achieved by empirical methods.21,22

In 1957 the first articles on homogeneous titanium-based olefin polymerisation were published by Breslow and Newburg23,24 and by Natta, Pino and co-workers.25 When reacting Cp2TiCl2 with Et2AlCl (DEAC) under conditions similar to those used with Z-N systems, a catalyst that polymerises ethene is obtained. The first homogeneous systems showed a low activity, when compared to classical Z-N systems and were also not active in polymerisation of higher olefins. In contrast to heterogeneous systems, homogeneous catalysts have a single type of well-defined active sites.

Although heterogeneous catalysts are in general industrially more practical, a higher control of properties of the catalyst, and more detailed kinetic and mechanistic studies are possible with well-defined molecular catalysts (“single site” catalysts).26 In industrial research, some of the main goals in this area have been the development of catalysts for production of new materials or for more advantageous production of existing products, as well as obtaining independent patent positions. Academic research has focused more on an understanding of e.g. details of the polymerisation process (stereo- and regioregularity), catalyst structure-property relationships, but also on the design and synthesis of alternative ancillary ligand types. These investigations, combined with the occasional serendipitous findings, have produced a vast amount of catalytically active systems.5

1.2 Mechanisms of homogeneous, catalytic olefin polymerisation

The geometric and electronic structure of the active species affect the properties of the resulting polymer, such as molecular weight, molecular weight distribution, regio- and stereoselectivity (for the homopolymerisation of α-olefins) and the incorporation of other monomers (for copolymerisations). It is now generally accepted that this active

4

species is an electron deficient, preferably cationic metal alkyl species. For heterogeneous systems the active sites are at dislocations and edges of the crystals, for homogeneous catalysts the active site is enclosed by a set of ancillary ligands. The cationic metal species are electronically balanced by a, preferably weakly nucleophilic, weakly coordinating counter anion.

Cossee and Arlman were the first to propose a mechanism for catalytic olefin polymerisation.27 They proposed that the polymer chain is growing via a cis-insertion of the olefin into a metal-carbon bond (migratory insertion mechanism, in which the metal-bound alkyl group migrates to the alkene, scheme 1.1). Various modifications to this mechanism have been postulated (scheme 1.1), in which agostic interactions have been proposed to assist in the ground and transition states.28,29 These agostic interactions mainly stabilise the electronically unsaturated metal centre, but may also play an important role in stereospecific olefin insertion (paragraph 1.2.2) by increasing the rigidity of the transition state.30

1.2.1 Chain transfer - molecular weight and molecular weight distribution

Besides chain growth, chain transfer processes are also important in olefin polymerisation. The rate of chain growth over chain transfer determines molecular

Cossee-Arlman Mechanism (Direct Insertion)

Green-Rooney Mechanism (Hydride Shift)

Modified Green-Rooney Mechanism (Ground and Transition State -Agostic Interaction)α

[M]P

[M]P

[M]

P

[M]

P

Transition State -Agostic Interactionα

[M]

H

P

H

[M]

HP

H [M]

P

HH

[M]

H H

P[M]

H H

P

[M]

H

P

H

[M]

H HP[M]

HP

H

[M]

H PH

[M]

H

P

H

[M]

H

P

H

[M]

H HP[M]

HP

H

[M]

HP

H

[M]

H

P

H

Scheme 1.1.

Chapter 1

5

weight and molecular weight distribution of the resulting polymer, which are important factors for material and processing properties. These rates are determined by the catalytic centre and its surrounding ligand (and sometimes by the cocatalyst), and they provide essential information about the polymerisation mechanism.

For chain transfer several mechanisms have been revealed, which include termination reactions by β-H and β-CH3 elimination,31,32 H-transfer to monomer (scheme 1.2),32a,33 chain transfer to aluminium (e.g. when an aluminium activator or scavenger is used, scheme 1.3),34 and σ-bond metathesis between the M-alkyl bond and a C-H bond of an alkene35 or a solvent molecule.36 Also chain transfer between catalyst active sites has been suggested (in a dual site ethene/1-hexene copolymerisation).37 In industrial polyolefin production, often chain transfer agents such as H2 are added to the polymerising mixture to gain a better control of polymer molecular weights.38,39

In absence of aluminium alkyl the main mechanism for chain transfer is via β-H abstraction. Two different mechanisms for β-H abstraction have been observed (scheme 1.2), which differ in the rate determining step (r.d.s.) of the chain transfer.5s In both cases polymers are obtained with olefinic end-groups. When the β-H transfer to monomer is rate determining (route 1, scheme 1.2), the rate of chain termination increases with increasing olefin concentration (1st order in olefin). Since also the chain growth (rate of insertion) is 1st order in olefin, molecular weights are independent of monomer concentration for these systems. When the rate determining step is the β-H transfer to metal (route 2, scheme 1.2) the rate of termination is independent of monomer concentration (0th order in olefin), and molecular weights increase with increasing monomer concentration.

When MAO or other aluminium alkyls are present in the reaction medium, another mechanism of chain transfer can be observed.40 Especially in MAO with a high Me3Al (TMA) content a considerable amount of chain transfer to aluminium may occur,

LnM H

PLnM H

P

LnM H LnM H

P

LnM H

LnM H

P

LnM H

P

LnM H

P

r.d.s.

+

+ -P

-P

(1)

(2)

Scheme 1.2.

6

although this transmetallation has also been observed in TMA-free MAO.41 As opposed to the mechanisms responsible for chain transfers to monomer, which give olefinic end groups, chain transfers to aluminium give, after hydrolysis, polymers with aliphatic endgroups.42,43

If no chain transfer is occurring, the polymerisation is called ‘living’ and the polydispersity (PDI = Mw/Mn) approaches 1.

44 Living polymerisations allow both precise molecular weight control as well as the synthesis of a wide array of polymer architectures (specifically end-capped polymers, block-copolymers).45 They are therefore very attractive for polymers with special architectures. Although most living polymerisations have to be performed at low temperature,46 recently some examples of high temperature, living insertion polymerisations have emerged.47 The usually low reaction temperatures required, and the fact that only one polymer chain is produced per active centre, make these living systems generally not very attractive from an industrial point of view.

For single site catalysts, where rates of propagation (vP) and termination (vT) are constant during the polymerisation, a molecular weight distribution with Mw/Mn = 2 is found (Flory-Schultz distribution).44 In the case that more than one type of active species is present during the polymerisation (‘multi site catalysts’) a molecular weight distribution with Mw/Mn > 2 (or a truly bimodal distribution) is observed. In general, polyolefins obtained with heterogeneous catalysts have broad molecular weight distribution with large polydispersities (Mw/Mn ≈ 5 – 10),48 whereas most homogeneous catalysts produce polymers with polydispersities close to 2.49

Broadening of the molecular weight can also result from features of the process conditions. This can be observed with non-isothermal runs (e.g. due to the exotherm observed under semi-batch conditions with high catalyst activities). The change of temperature has different effect on the rates of propagation and termination, resulting in a changing ratio of propagation to termination during polymerisation. Temperature and activity of the catalyst also affect the concentration of the monomer. In addition, mass transfer limitation of the monomer may occur, e.g. as a result of insufficient mixing due to the viscosity of the medium (either due to a high concentration of polymer or formation of polymer with a high molecular weight), or coprecipitation of the catalyst with the crystalline polymer formed.

MMe

AlR2

P

LnM

P

MeAlR2 LnM

MeAlR2

P

Scheme 1.3.

Chapter 1

7

The use of lower catalyst concentrations may overcome problems with highly exothermic polymerisation experiments, but for this solvent and monomer(s) of high purity have to be used. Therefore the use of impurity scavengers (e.g. aluminium trialkyls or partially hydrolysed aluminium trialkyls, see also section 1.3.1) is often necessary.50 These may on their turn lead to catalyst modification or deactivation due to interaction with the catalyst (e.g. due to coordination of the aluminium alkyl to the electropositive metal centre, with possible concomitant chain transfer mechanisms). These problems may especially hamper systems with relatively open geometries or with exposed polar metal-ligand bonds. 50b

1.2.2 Regio- and stereospecificity

In homopolymers of higher olefins (propene, α-olefins), the properties are also determined by the microstructure as expressed in the regioregularity (figure 1.2) and the stereoregularity (figure 1.3) of the polymer.5c,q,51,52 These properties of the polymer are determined by the regioselectivity and stereoselectivity of a catalyst. Several techniques for the determination of regioselectivity, tacticity, and degree of stereoselectivity of a polymer are available. The most useful method for determining the stereoregularity and regioregularity of a polymer is 13C NMR. The chemical shifts of the carbon nuclei in the polymer are sensitive to its adjacent stereogenic centers, hence these shifts provide quantitative information about the polymer microstructure.53

The regiospecificity of the enchainment is influenced by the steric environment of the active centre, in combination with the regio-preference of the monomer. In general, simple α-olefins prefer 1,2 insertion (due to steric requirements of the propene methyl group and to the polarity of the monomer in combination with the electrophilic metal in the active species), but this preference is only small. For several catalytic systems it has been recognised that an initial 2,1-insertion can be followed by consecutive 2,1-insertions, resulting in syndiotactic polymers by chain-end control (vide infra).5o,54 Modification of the steric bulk around the active site enhances its regioselectivity.

The stereoselectivity of a catalyst is determined by both the symmetry of, and steric congestion around the active site (as governed by the ligand and the growing polymer

1,3

(B)

2,1

(A)

Figure 1.2. Isolated 2,1- (A) and 1,3-insertions (B) in isotactic polypropene.

8

chain). 5o The tacticity of the resulting polymer (figure 1.3) is conveniently described by the Bovey formalism. An “m” is used for a meso -, an “r” for a racemic relationship between two adjacent stereogenic centres. The tacticity can now be described in terms of triads or pentads (3 resp. 5 subsequent stereogenic centres). An isotactic polymer has a high fraction of mmmm pentads, syndiotactic polymers have a high fraction of rrrr pentads.

isotactic

syndiotactic

atactic

hemi-isotactic

heterotactic

Figure 1.3. Polypropenes of different stereoregularity.

MP

MP

MP

MP

PLnM

PLnM

PLnM

m m r m m

r r m r r

m r r m m

Pm~1

Pr~1

Chain end

Enantiomorphic site

m

rChain end controlsstereochemistry

Ligand controlsstereochemistry

Pm

Pr

MP

Stereoerror

α α~1

Figure 1.4. Chain end vs. enantiomorphic site control (Pm = probability of meso insertion, Pr = probability of rac insertion). (Reproduced from ref. 5o).

Chapter 1

9

If the ligand set overrides the influence of the polymer chain end, the mechanism of stereochemical direction is termed “(enantiomorphic)-site controlled”. If, on the other hand, the configuration of the stereocentre belonging to the last inserted monomer determines the stereoselectivity of the next monomer insertion, the mode of stereochemical regulation is referred to as “polymer chain-end control” (figure 1.4).55 When the stereospecificity of a monomer insertion is controlled by the chain end, a high probability of subsequent meso-insertions (Pm approaches 1) results in a mainly isotactic polymer. When Pr approaches 1 (high probability of subsequent rac insertions), the polymerisation results in a syndiotactic polymer.

As for the determination of tacticity, 13C NMR can also be used to determine the mechanism of stereoregulation. For example, occurrence of isolated r-dyad errors in an isotactic polymer is indicative of an isospecific chain-end controlled mechanism, whereas occurrence of isolated rr-dyad errors is indicative for isospecific site controlled polymerisations (figure 1.4). Simulation of spectra, using statistical methods yields important information on the mechanisms that control a specific polymerisation.56

1.3 Activators

The active catalyst contains an electronically unsaturated (preferably 14 v.e. or less) metal with a (polarised) M-C bond and an adjacent coordination site. In general, higher activities may be achieved with cationic metal alkyl species when compared to neutral complexes. Since cationic metal alkyl species are often prone to deactivation reactions, especially in the absence of monomer,57 it is more convenient to store catalysts in a stable, inactive, pre-catalyst form. Activation of the precatalyst (in general in the presence of olefin substrate) yields the actual catalytic system.5s

Common procedures to generate active catalysts start from metal dichloride or dialkyl catalyst precursors. For dichloride precatalysts, the generation of the active species requires the initial conversion of the dichloride into a dialkyl compound and subsequently the abstraction of one alkyl to form the cationic metal-alkyl compound. This can be achieved by the combination of an alkylating agent with an alkyl abstracting agent (the latter are described in section 1.3.2 and 1.3.3), or with certain activators that combine both functions (especially MAO, section 1.3.1). For activation of pre-formed dialkyl precursors, only alkyl abstraction is required (section 1.3.2 and 1.3.3).5s,58

The choice of activator can have a dramatic effect on activity and selectivity of a catalyst system.42,59 This is reflected in their effectiveness to generate active, cationic, metal alkyl species, as well as in coordinative (and steric) properties of the (generated) counter-anion. In the following sections, some features of commonly used activators will be discussed.

10

1.3.1 Aluminium alkyls and aluminoxanes

Early homogeneous catalysts were activated with aluminium compounds of general formula RnAlCl(3–n) (n = 1,2). The resulting systems were modestly effective in polymerisation of ethene, but inactive in polymerisation of higher olefins. Reichert and Meyer showed that adding small amounts of water increased the rate of ethene polymerisation by the system Cp2TiEtCl/AlEtCl2.

60 The otherwise inactive system Cp2TiCl2/Me2AlCl could also be activated by addition of trace amounts of water, which led to the notion that stronger Lewis acidic compounds like the partial hydrolysis product of Me2AlCl (ClMeAl-O-AlMeCl) are needed as activators for Cp2TiMeCl to display ethene polymerisation.61 Further progress was made by Sinn and Kaminsky, who discovered that adding water to the inactive Cp2ZrMe2/AlMe3-system yielded a surprisingly active catalyst.52,62,63 Direct synthesis of methylaluminoxane (MAO) by partial hydrolysis of AlMe3 and subsequent reaction with Cp2ZrMe2 or Cp2ZrCl2 proved that this compound was indeed an exceedingly successful activator. Furthermore, it was noticed that these MAO-activated metallocene catalysts now could also readily polymerise propene and higher olefins.26a,62b–f

For hydrolysis of AlMe3 various sources of water can be used,64 but lately also

alternative, non-hydrolytic routes to methylaluminoxanes have been reported.65 The high cost of MAO arises from the requirement of sophisticated production equipment and the cost of AlMe3. To circumvent the use of TMA several MAO-analogues have been synthesised based on cheaper aluminium alkyls, e.g. from triethylaluminium (ethylaluminoxane, EAO) or tri-iso-butylaluminium (iso-butylaluminoxane, (I)BAO),66,67 but the resulting catalysts are in general not as effective as MAO activated systems.

The identification of metal dichloride precursors with MAO activator as powerful catalytic systems for olefin polymerisation has interested many research groups in the actual structure of MAO.58b,42a MAO is a mixture of oligomeric cage structures consisting of [-Al(R)-O-]n subunits (n ~ 5 – 20).

5r The many physical, and spectroscopic studies, in combination with carefully designed catalyst studies,68 molecular modelling studies,69,70 and comparisons with simple related aluminoxanes71–74 have revealed more information on its actual structure and role in activation. But despite all the studies to elucidate the structure of MAO, it still remains basically a “black box”. MAO activities strongly depend on the nature of the H2O-source, exact reaction conditions used for the synthesis of MAO, ‘age’ of the solution and amount of free TMA.

1.3.2 Lewis acids

The discovery by Marks75 and Ewen76 in the early 1990’s that strongly Lewis acidic boranes like tris(pentafluorophenyl)borane (B(C6F5)3)

58b,77 promote olefin polymerisation with group 4 metallocene dialkyls opened a new field of activator

Chapter 1

11

studies for olefin polymerisation.78,58 From reaction mixtures several, catalytically active, cationic metallocene complexes have been isolated and crystallographically characterised. Studies on these complexes have given further insight into the nature of the active species, and the mechanisms of catalytic olefin polymerisation.79

The borane (or the analogous alane, Al(C6F5)380) abstracts an alkyl group from a

metal dialkyl species (scheme 1.4), leaving a cationic metal alkyl, active in polymerisation, and an alkyl borate (or alate) anion. Although the anion will always be in proximity of the cationic metal centre due to Coulombic attraction, the degree of actual interaction with the metal centre depends on steric encumbrance and electrophilic character of the metal species and on the nature of the abstracted alkyl group.81 The polymerisation activity of the resulting catalysts is greatly influenced by interaction of anion and cation.82

The two main alkyl groups used in precursors for olefin polymerisation catalysts are methyl (Me) and benzyl (CH2Ph). Abstraction of a methyl from a dimethyl precursor gives a cationic methyl metal species and methylborate anion (scheme 1.4, A).75b,83 The methylborate anion can be strongly coordinated to the cationic metal centre,83a,84 which may hamper the coordination of an incoming olefin.85 In line with this, in several cases inhibition of the polymerisation or significantly lower activities compared to analogous catalysts with less strongly coordinating anions have been observed.

Reaction of a dibenzyl precursor with a borane results in abstraction of one of the benzyl groups and formation of a benzylborate anion. Coordination of the benzylborate anion is dependent on the electropositive character of the metal centre and the steric encumbrance of the metal cation, and several modes of coordination of this anion to electron deficient metal centres have been observed.81a,86 As with the methyl-tris(pentafluorophenyl)borate anion, coordination of the benzyl-tris(pentafluorophenyl)borate may strongly influence catalyst activity.

Much research is aimed at synthesis of sterically encumbered perfluoroarylboranes

+ B(C6F5)3LnM

Me

Me

LnMMe

Me

B(C6F5)3

(A)

(B) (C)

LnM

Ph

Ph+ B(C6F5)3

LnM

B(C6F5)3PhCH2B(C6F5)3

LnM

Scheme 1.4.

12

that yield less coordinating anions.87 Especially the groups of Marks88,89 and Piers90 have developed a number of new and effective activators. Among the boranes that lead to species that are active in olefin polymerisation are BF3,

91 HB(C6F5)2,92

(N-pyrrolyl)B(C6F5)2,93 N3B(C6F5)2.

94 Although the boranes, and anions derived there from, are relatively inert, several reactions have been observed in which the anion is involved. The ion pair may decompose by ways of a) aryl transfer,95 b) fluoride-activation,96,97 and c) activation of reactive groups on the ligand system.81a,98

1.3.3 Brønsted acids / weakly coordinating anions

With the development of very weakly nucleophilic anions99 like [B(C6F5)4]– or

[Al(C6F5)4]–,100 even more active olefin polymerisation catalysts have been obtained.

These anions are mostly delivered as counterion of a cationic Brønsted-acid, a cationic Lewis acid or a cationic oxidator. Brønsted-acids like ammonium (e.g. anilinium, [PhNMe2H]

+),101 phosphonium (e.g. [Ph3PH]+),102 and oxonium cations (e.g.

[(Et2O)2H]+)103 can readily activate M-R bonds via protonolysis (scheme 1.5, route

1).102b,104 The Lewis acidic trityl cation ([Ph3C]+) is a powerful alkide and hydride

abstracting agent (scheme 1.5, route 2),105 but can also be used as an oxidizing reagent.76a Alkide abstraction has also been observed with borate salts of group 15 cations (e.g. TMEDA and diethyl ether adducts of [R2Al]

+ and [R2Ga]+; R = Me,

Octyl).106 Halogen abstraction from mixed metal alkyl halide compound can be performed

with silver or thallium salts (scheme 1.5, route 3).107 Another route to cationic metal-alkyls is by oxidation of a metal-alkyl species in a lower oxidation state (scheme 1.5, route 4),108,109 or the oxidative removal of an alkyl from a metal dialkyl species (scheme 1.5, route 5).107a,110,111 This is in general performed with ferrocinium ([Cp2Fe]

+) salts,112 or with salts of the more noble metals (e.g. Au+, Ag+, Pd2+, Pt2+, Hg2+, or Cu+).107a,113

Although high catalytic performances have been observed with cationic complexes with the very weakly coordinating [B(C6F5)4]

–, these compounds suffer from low solubility in hydrocarbon solvents, reduced thermal stability and crystallisability. This results often in short catalyst lifetimes and it limits the number of tools to characterise these species.114 Therefore, many functionalised fluorinated anions have been developed to overcome these problems.87,115,116 Another route to anions with low coordinating power has been reported by LaPointe,78c Bochmann et al.,117 and Chen et al.118 They synthesised extended anions by linking of two M(C6F5)3-fragments (M = B, Al) by anionic bridges such as imidazolide, cyanide or fluoride. Although these anions are designed for their weakly coordinating behaviour, they may still suffer from activation by the cationic metal centre.107a Moreover, several interactions between C-F

Chapter 1

13

bonds and metal119 or interactions with other functional groups120 have been observed, as well as, decomposition of the anion via C-F or B-C bond cleavage.42d

Problematic for the synthesis of perfluorinated borate anions is often the thermodynamic instability of fluorinated precursors, in particular when delivery of the fourth perfluoro substituent needs more drastic reaction conditions.121 Especially lithiated polyfluorophenyl compounds are known to be prone to explosive decomposition.122 This has encouraged researchers to develop weakly coordinating anions that can be synthesised using less risky reagents.123–125

1.4 Homogeneous, single site catalysts

Many new polyolefin materials have been obtained using specially developed homogeneous catalysts.126 The ligands in these systems determine stability, activity and selectivity of the resulting catalyst. They also dictate electronic properties of, and steric crowding around the metal, which can have tremendous effects on the resulting product properties.127

In this thesis synthesis, reactivity and polymerisation activity of monocyclopentadienyl early transition metal complexes is described. The next sections describe some properties of such monocyclopentadienyl complexes of tetravalent

(3)

(2)

(1)

(4)

(5)

[Cp2Fe][B(C6F5)4]

[Cp2Fe][B(C6F5)

ZrMe

Me

ZrMe

Me

Cl

MeZr

[PhNMe2H][B(C6F5)4]

[Ph3C][B(C6F5)4]

[Ag][B(C6F5)4]

B(C6F5)4

B(C6F5)4TiMe2Si

N CH2Ph

CH2Ph

R

TiMe2Si

N CH2Ph

R

TiMe2Si

N CH2Ph

R

TiMe2Si

N CH2Ph

R

B(C6F5)4Me

Zr

B(C6F5)4Me

Zr

B(C6F5)4Me

Zr

- MeH- PhNMe2

- Ph3CMe

- AgCl

- Cp2Fe

- Cp2Fe- 1/2 PhC2H4Ph

Scheme 1.5.

14

(section 1.4.1), and of trivalent metals (section 1.4.2). Structures shown depict precatalysts (LnMX2, where Ln is the ligand set and X = halogen, NMe2 or an alkyl group); these need an activator to form the catalyst (activation processes and activators are discussed in section 1.3). Where necessary, ligand systems have been simplified using R for alkyl groups, Ph for phenyl and Ar for aryl groups (e.g. 2,6-dimethylphenyl, 2,4,6-trimethylphenyl, 2,6-diisopropylphenyl and the like).

1.4.1 Half sandwich complexes with a tetravalent metal

Historically, metallocene complexes with a tetravalent group 4 metal (especially Zr) have received most attention in olefin polymerisation catalysis. For these catalyst systems many structural variations have been worked out, which have been thoroughly reviewed by Alt and Köppl,5n and Coville.5z It was concluded that not all details that influence the properties of a metallocene catalyst can be understood because of subtle interplay of a large number of parameters. Minor changes in the catalyst system can have dramatic effects on the activity and selectivity of the catalyst.5n

In recent years, many synthetic efforts have afforded a huge number of complexes in which one or both cyclopentadienyl functionalities have been replaced by other (mono or dianionic) ligands.5i,k,m,w,x,ad,ah These so-called “post-metallocene” catalysts show interesting differences in activity and selectivity, and the combined efforts of both industry and academia has resulted in new catalysts for specific polyolefin materials.126

Many examples of complexes in which one cyclopentadienyl has been substituted by another monoanionic ligand have emerged (figure 1.5). Complexes of this type are usually synthesised by simple reaction of a monocyclopentadienyl metal precursor with

MO Cl

ClR

R

MN Cl

Cl

R3P

MN Cl

Cl

NR2

R2N

MN Cl

Cl

N

N

R

R

MN Cl

Cl

R

R

MN Cl

Cl

R

RM

O Cl

ClR2N

B C DA

E F G

Figure 1.5. Cp-phenoxide (A);128 Cp-phosphanimide (B);129d,130–132

Cp-guanidinate (C);133,134 Cp-iminoimidazolidide (D);133,134

Cp-ketimide (E);135,136 Cp-amide (F);137 Cp-nitroxide (G).138

Chapter 1

15

the ligand, which allows a large scope for variation. The electronic and steric characteristics of the non-Cp ligand in these so-called “half sandwich” complexes can easily be tuned by the choice of substituents. These characteristics make these complexes very interesting for the study of steric and electronic effects of the ligand on the polymerisation characteristics of the catalyst. Depending on the steric bulk of the non-Cp ligand, relatively open active sites can be designed, which allows the (co)polymerisation of α-olefins,139 and diene monomers. Especially the phosphinimide and the iminoimidazolidide catalysts are thermally very robust and hence very suitable for high-temperature solution processes for polyethene copolymer synthesis.

Catalysts in which the heteroatom functionality has been tethered to the cyclopentadienyl (figure 1.6) also combine high activity with high thermal stability. Especially the silicon bridged Cp-amide ligand on group 4 metals (A, ‘constrained geometry catalyst’, ‘CGC’) has proven its great value in copolymerisations, and has been applied in industrial practice. These catalysts randomly incorporate higher olefins into the polymer chain, which enables the production of linear low-density polyethene (LLDPE) with a high incorporation of α-olefin (1-hexene, 1-octene). As a result, both industrial and academic groups have explored the effect of cyclopentadienyl and amide substitution, nature of the bridge, incorporated metal and effect of various activators on polymerisation.5

MMe2Si

N Cl

Cl

R

MN Cl

Cl

R

MN

Cl

Cl

R

MN Cl

Cl

R

MO Cl

ClMO Cl

Cl MO Cl

Cl MO Cl

Cl

MQ

O Cl

Cl MMe2Si

P Cl

Cl

R

M

P

Me2Si

N Cl

Cl

R

A B C D

E G HF

I J K

Figure 1.6. Cp-amide: silicon bridged (CGC, A);5i,128o,140–142 C1-bridged (B);143

C2-bridged (C, M = Ti,144 V145); C3-bridged (D);

146 and other bridges;147 Cp-alkoxide: phenylene-bridged (E);148 C1-bridged (F);

149 C2-bridged (G);150

C3-bridged (H);150,151 and other bridges;152–155 Cp-phosphido: silicon bridged

(J);143b,156,157 Cp-analogues:158,159 phosphanyl-amido, silicon bridged (K);160

16

1.4.2 Half sandwich complexes with a trivalent metal

In contrast to the enormous number of catalysts based on cationic M(IV) complexes (vide supra), reports on homogeneous catalysts based on cationic catalysts with metals in the trivalent oxidation state have been scarce. Only recently some well defined, early transition metal based, M(III) cationic catalyst systems for polymerisation and oligomerisation of ethene have been developed. In general these systems are based on group III metals (Sc, Y, lanthanides), and Cr(III). Systems based on Ti(III) or V(III) are scarce in open literature, which is perhaps surprising since simple Ti(III) compounds (e.g. TiCl3/MgCl2) and V(III) (e.g. V(acac)3) are widely used in olefin polymerisation.161

Although used on a variety of metals to stabilise electron-poor metal centres, cyclopentadienyl ligands functionalised with a nitrogen-derived Lewis base (figure 1.7A – E),5l only recently moved into the area of half-sandwich olefin polymerisation chemistry. The first examples were reported by Van Beek and Gruter at DSM as ligands for titanium(III) (figure 1.7A, ‘Low Valency Catalyst’ or ‘lovacat’),172,173 followed by the group of Jolly, who used the ligand to stabilise trivalent chromium species.5k,173–176 These groups also introduced the phosphine analogue of these type of systems (figure 1.7F).172a,d,e,174a,c,177 Later examples include systems synthesised by Christopher et al. at Exxon (M = Sc178) and Kotov et al. (figure 1.7E, aminophosphanyl substituted cyclopentadienyls on vanadium and chromium).175m

MN

XX

MN

XX

MN

XX

ML

XX

MP

XX

ML

XX

MN

R3R2

PRR1

XX

LL

MN

XX

E F G H

A B DC

Figure 1.7. Cp-amine:173 ethylene bridged (A, M = Sc;178 Ti;172 V;175l,162 Cr174–176); phenylene bridged (B, M = Cr);175d,h Cp-quinoline (C, M = Cr);175h

Cp-imine (D, M = Cr);175d Cp-aminophosphanyl (E, M = V, Cr; L = PMe3);175l

Cp-phosphane (F, M = Cr);175i,163 Cp-methoxy and Cp-methylthio (G, L = O resp. S);175d Cp(Lewis Base) H ( M = La, L = THF;164

M = Sc, L = THF;165 M = Sc, L = tBu3P=O;166 M = Cr, L = PMe3, THF;

175d,167,168

M = Cr, L = carbene;167,169,170 M = Cr, L = –171).

Chapter 1

17

The catalytic activity of the resulting catalytic systems is very dependent on ligand framework and on the nature of the Lewis base. The chromium systems in which the Lewis base is not tethered to the Cp-ligand (figure 1.7H) shows significantly lower polymerisation activity.167–170 Catalysts in which the Lewis basic ligand is removed show an even lower activity.171 For most systems, catalyst activity and lifetime increase with higher substitution degree of the cyclopentadienyl. The electronic properties of the Lewis base seem to be of little effect on catalyst activity, but have a large effect on the molecular weight of the resulting polymer (increasing molecular weight with increasing hardness).179 Whereas activity generally increases with more steric bulk on the amino substituent (e.g. Me2 vs. cyclo(C4H8)), the opposite trend is observed for the phosphine substituent (e.g. Me2 vs. Cy2).

179

1.5 Aim and scope of the thesis

The previous sections have presented a general background of catalytic olefin polymerisation, an overview of the ways to generate active single-site catalyst systems, and the possible variations and structure-property relationships in monocyclopentadienyl transition-metal olefin polymerisation catalysts. The effects of activator/cocatalyst and of electronic and steric features of the ancillary ligand are frequently so intertwined that it is difficult to separate the various contributions to catalyst efficiency. Attempts to obtain accurate insight into these effects require careful design and execution of the experiments to limit the number of variables. Information thus obtained can still yield knowledge that is of importance to issues of current interest in olefin polymerisation.

The aim of the research, presented in this thesis, is to provide further insight in the effect of structural and electronic factors on the polymerisation characteristics of monocyclopentadienyl metal catalysts. Specifically, the effect of ligand geometry and of the counterion in cyclopentadienyl-amido titanium(IV) catalysts will be targeted in chapter 2, the effect of the electronic configuration of the metal in isostructural cyclopentadienyl-amine catalysts of Sc, Ti, V and Cr will be studied in chapters 3 – 5.

Chapter 2 describes a general synthesis of ethylene-bridged tetramethyl-cyclopentadienyl-amido ligands and the effect of the ligand geometry on the catalytic properties of their Ti(IV) derivatives. This chapter also presents a study of the effect of the anions on polymerisation behaviour (regio/stereospecificity and molecular weight) of cationic cyclopentadienyl-amide titanium alkyl species. For this purpose the sterically more open derivatives with the unsubstituted Cp ligand moiety (earlier described by Sinnema144) were selected for examination.

Chapters 3 through 5 describe attempts to obtain a series of catalysts, based on trivalent metals supported by a monoanionic, dimethylaminoethyl substituted cyclopentadienyl ligand. In these catalysts the electronic configuration ranges from d0

18

in Sc(III) to d3 in Cr(III), and should allow an evaluation of the effects of d-electron count on catalytic performance in an isostructural series.

In chapter 3, the synthesis, characterisation and polymerisation activity of cyclopentadienyl-amine metal dichloride species is described, using MAO as activator. This approach only allowed a comparison for d0, d2 and d3 centres.

In chapter 4, the synthesis, structure and and reactivity of cyclopentadienyl-amine M(III) butadiene complexes and their derived zwitterionic complexes is described. This approach only gave access to isostructural catalysts with d0–d2 centres. This chapter also contains a study of the reactivity of the first reported scandium 1,3-butadiene complex, which performs unusual insertion and elimination chemistry.

In chapter 5, cyclopentadienyl-amine M(III) dialkyls are described that can serve as precursors to single component olefin polymerisation catalysts for metal centres over the full d0–d3 configuration range. The chapter describes synthesis, reactivity and stability of this class of compounds and a study of polymerisation activity of the cations, derived from these dialkyls. In addition, the chapter details the thermal decomposition of the scandium dialkyl complexes, as these are potential precursors to (as yet unknown) scandium alkylidene species.

1.6 References

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