WO2005016980A1

WO2005016980A1 - Cocatalysts

Info

Publication number: WO2005016980A1
Application number: PCT/GB2004/003350
Authority: WO
Inventors: Brian Stephen Kimberley; Sergio Mastroianni; Michael John Taylor
Original assignee: Innovene Europe Limited
Priority date: 2003-08-15
Filing date: 2004-08-04
Publication date: 2005-02-24
Also published as: GB0319200D0

Abstract

The present invention relates to novel cocatalysts comprising (a) a Lewis base and a hydroxy compound and (b) a Lewis acid. Typical Lewis bases are amines and the hydroxy compound is suitably a phenol. The Lewis acid may be tris(pentafluorophenyl) borane. The cocatalysts are typically used in the presence of transition metal compounds for the polymerisation of olefins particularly when supported for use in the gas phase.

Description

COCATALYSTS The present invention relates to novel cocatalysts, in particular to novel cocatalysts for use with transition metal compounds. The novel cocatalysts of the present invention may be used for example as components of supported metallocene catalyst systems and are particulary suitable for use in the polymerisation of olefins in the gas phase. In recent years there have been many advances in the production of polyolefm homopolymers and copolymers due to the introduction of metallocene catalysts. Metallocene catalysts offer the advantage of generally a higher activity than traditional Ziegler catalysts and are usually described as catalysts which are single site in nature. There have been developed several different families of metallocene complexes. In earlier years catalysts based on bis (cyclopentadienyl) metal complexes were developed, examples of which may be found in EP 129368 or EP 206794. More recently complexes having a single or mono cyclopentadienyl ring have been developed. Such complexes have been referred to as 'constrained geometry' complexes and examples of these complexes may be found in EP 416815 or EP 420436. In both of these complexes the metal atom eg. zirconium is in the highest oxidation state. Other complexes however have been developed in which the metal atom may be in a reduced oxidation state. Examples of both the bis (cyclopentadienyl) and mono (cyclopentadienyl) complexes have been described in WO 96/04290 and WO 95/00526 respectively. The above metallocene complexes are utilised for polymerisation in the presence of a cocatalyst or activator. Typically activators are aluminoxanes, in particular methyl aluminoxane or compounds based on boron compounds. Examples of the latter are borates such as trialkyl-substituted ammonium tetraphenyl- or tetrafluorophenyl- borates. Catalyst systems incorporating such borate activators are described in EP 561479, EP 418044 and EP 551277. The above metallocene complexes may be used for the polymerisation of olefins in solution, slurry or gas phase. When used in the gas phase the metallocene complex and/or the activator are suitably supported. Typical supports include inorganic oxides eg. silica or polymeric supports may alternatively be used. Examples of the preparation of supported metallocene catalysts for the polymerisation of olefins may be found in WO 94/26793, WO 95/07939, WQ 96/00245, WO 96/04318, WO 97/02297 and EP 642536. WO 98/27119 describes supported catalyst components comprising ionic compounds comprising a cation and an anion in which the anion contains at least one substituent comprising a moiety having an active hydrogen. In this disclosure supported metallocene catalysts are exemplified in which the catalyst is prepared by treating the aforementioned ionic compound with a trialkylaluminium compound followed by subsequent treatment with the support and the metallocene. Tris(pentafluorophenyl) borane has also been used as a cocatalyst for metallocene systems Yang in JACS 1991, 113, 3623 describes the reaction between bis(cyclopentadienyl)zirconium dimethyl with tris(pentafluorophenyl) borane to prepare a metallocene catalyst system. US 5416177 describes Lewis base complexes formed by the reaction between tris(pentafluorphenyl) borane with alcohols. These complexes when combined with Group IVB organometallic compounds produce catalysts useful for the polymerisation of olefins. For example the complex (C₆F₅) B.2MeOH is formed by reaction with methanol. The reference also describes the further treatment of such Lewis base complexes with bases such as amines. The above complex when treated with triethylamine results in the acidic salt [Et₃NH]⁺ [(C₆F₅)₃ BOCH₃]^" which when combined with a Group IVB organometallic compound may be used for polymerisation. We have now surprisingly found that suitable cocatalysts may be prepared by contacting a Lewis base with a hydroxy compound and subsequently with a Lewis acid such as tris(pentafluorophenyl) borane. Thus according to the present invention there is provided a cocatalyst suitable for use in the polymertisation of olefins, said cocatalyst prepared by (i) contacting a Lewis base with a hydroxy compound, and (ii) addition of a Lewis acid. The Lewis base may preferably be an amine or a phosphine but is preferably an amine volatile under reduced pressure. Suitable amines include long chain alkylamines. The hydroxy compound may typically comprise a phenolic or an alcoholic compound including for example sugars, clays, silicas. The preferred hydroxy compounds are functionalised phenolic compounds. A particularly suitable phenolic compound is pentafluorophenol. Also suitable as phenolic compounds are alkylammonium tris(pentafluorophenyl) (4-hydroxyphenyl) borates described in WO 96/28480. The Lewis acid may preferably be an organometallic compound for example an aluminoxane or a borane and is preferably a perfluorinated borane for example tris(pentafluorophenyl) borane. The preferred cocatalysts of the present invention are those wherein the Lewis base is an amine, the hydroxy compound is a phenol and the Lewis acid is a borane. According to another aspect of the present invention there is provided a cocatalyst suitable for use in the polymerisation of olefins comprising

(a) the combination of a Lewis base and a hydroxy compound, and

(b) a Lewis acid. The cocatalysts of the present invention may be used with traditional polymerisation catalyst components for example transition metal compounds. Thus according to another aspect of the present invention there is provided a polymerisation catalyst system comprising (a) a transition metal compound, and (b) a cocatalyst as hereinbefore described. Suitable transition metal compounds are those based on the late transition metals (LTM) of Group NIII for example compounds containing iron, nickel, manganese, ruthenium, cobalt or palladium metals. Examples of such compounds are described in WO 98/27124 and WO 99/12981 and may be illustrated by [2,6-diacetylρyridinebis(2,6- diisopropylanil)FeCl₂], 2.6-diacetylpyridinebis (2,4,6-trimethylanil) FeCl₂ and [2,6- diacetylpyridinebis(2,6-diisopropylanil)CoCl₂]. Other catalysts include derivatives of Group IIIA, INA or Lanthanide metals which are in the +2, +3 or +4 formal oxidation state. Preferred compounds include metal complexes containing from 1 to 3 anionic or neutral ligand groups which may be cyclic or non-cyclic delocalized π-bonded anionic ligand groups. Examples of such π- bonded anionic ligand groups are conjugated or non-conjugated, cyclic or non-cyclic dienyl groups, allyl groups, boratabenzene groups, phosphole and arene groups. By the term π-bonded is meant that the ligand group is bonded to the metal by a sharing of electrons from a partially delocalised π-bond. Each atom in the delocalized π-bonded group may independently be substituted with a radical selected from the group consisting of hydrogen, halogen, hydrocarbyl, halohydrocarbyl, hydrocarbyl, substituted metalloid radicals wherein the metalloid is selected from Group INB of the Periodic Table. Included in the term "hydrocarbyl" are CI - C20 straight, branched and cyclic alkyl radicals, C6 - C20 aromatic radicals, etc. In addition two or more such radicals may together form a fused ring system or they may form a metallocycle with the metal. Examples of suitable anionic, delocalised π-bonded groups include cyclopentadienyl, indenyl, fluorenyl, tetrahydroindenyl, tetrahydrofluorenyl, octahydrofluorenyl, etc. as well as phospholes and boratabenzene groups. Phospholes are anionic ligands that are phosphorus containing analogues to the cyclopentadienyl groups. They are known in the art and described in WO 98/50392. The boratabenzenes are anionic ligands that are boron containing analogues to benzene. They are known in the art and are described in Orgariometallics. 14, 1, 471 — 480 (1995). The preferred polymerisation catalyst of the present invention is a bulky ligand compound also referred to as a metallocene complex containing at least one of the aforementioned delocalized π-bonded group, in particular cyclopentadienyl ligands. Such metallocene complexes are those based on Group INA metals for example titanium, zirconium and hafnium. Metallocene complexes may be represented by the general formula: LxMQn where L is a cyclopentadienyl ligand, M is a Group IVA metal, Q is a leaving group and x and n are dependent upon the oxidation state of the metal. Typically the Group IVA metal is titanium, zirconium or hafnium, x is either 1 or 2 and typical leaving groups include halogen or hydrocarbyl. The cyclopentadienyl ligands may be substituted for example by alkyl or alkenyl groups or may comprise a fused ring system such as indenyl or fluorenyl. Examples of suitable metallocene complexes are disclosed in EP 129368 and EP 206794. Such complexes may be unbridged eg. bis(cyclopentadienyl) zirconium dichloride, bis(pentamethyl)cyclopentadienyl dichloride, or may be bridged eg. ethylene bis(indenyl) zirconium dichloride or dimethylsilyl(indenyl) zirconium dichloride. Other suitable bis(cyclopentadienyl) metallocene complexes are those . ^' bis(cyclopentadienyl) diene complexes described in WO 96/04290. Examples of such complexes are bis(cyclopentadienyl) zirconium (2.3-dimethyl-l,3-butadiene) and ethylene bis(indenyl) zirconium 1,4-diphenyl butadiene. Examples of monocyclopentadienyl or substituted monocyclopentadienyl complexes suitable for use in the present invention are described in EP 416815, EP 418044, EP 420436 and EP 551277. Suitable complexes may be represented by the general formula: CpMX_n wherein Cp is a single cyclopentadienyl or substituted cyclopentadienyl group optionally covalently bonded to M through a substituent, M is a Group VIA metal bound in a η⁵ bonding mode to the cyclopentadienyl or substituted cyclopentadienyl group, X each occurrence is hydride or a moiety selected from the group consisting of halo, alkyl, aryl, aryloxy, alkoxy, alkoxyalkyl, amidoalkyl, siloxyalkyl etc. having up to 20 non-hydrogen atoms and neutral Lewis base ligands having up to 20 non-hydrogen atoms or optionally one X together with Cp forms a metallocycle with M and n is dependent upon the valency of the metal. Particularly preferred monocyclopentadienyl complexes have the formula:

wherein :- R' each occurrence is independently selected from hydrogen, hydrocarbyl, silyl, germyl, halo, cyano, and combinations thereof, said_. R' having up to 20 nonhydrogen atoms, and optionally, two R' groups (where R' is not hydrogen, halo or cyano) together form a divalent derivative thereof connected to adjacent positions of the cyclopentadienyl ring to form a fused ring structure; X is hydride or a moiety selected from the group consisting of halo, alkyl, aryl, aryloxy, alkoxy, alkoxyalkyl, amidoalkyl, siloxyalkyl etc. having up to 20 non-hydrogen atoms and neutral Lewis base ligands having up to 20 non-hydrogen atoms, Y is -O-, -S-, - R% -PR*-, M is hafnium, titanium or zirconium, Z* is SiR*₂, CR*₂, SiR*₂SIR*₂, CR*₂CR*₂, CR*=CR*, CR*₂SIR*₂, or GeR*₂, wherein: R* each occurrence is independently hydrogen, or a member selected from hydrocarbyl, silyl, halogenated alkyl, halogenated aryl, and combinations thereof, said R* having up to 10 non-hydrogen atoms, and optionally, two R* groups from Z* (when R* is not hydrogen), or an R* group from Z* and an R* group from Y form a ring system., and n is 1 or 2 depending on the valence of M. Examples of suitable monocyclopentadienyl complexes are (tert-butylamido) dimethyl (tetramethyl-η⁵- cyclopentadienyl) silanetitanium dichloride and (2- methoxyphenylamido) dimethyl (tetramethyl— η - cyclopentadienyl) silanetitanium dichloride. Other suitable monocyclopentadienyl complexes are those comprising phosphinimine ligands described in WO 99/40125, WO 00/05237, WO 00/05238 and WOOO/32653. A typical examples of such a complex is cyclopentadienyl titanium [tri (tertiary butyl) phosphinimine] dichloride. Another type of polymerisation catalyst suitable for use with the cocatalysts of the present tinvention in the present invention are monocyclopentadienyl complexes comprising heteroallyl moieties such as zirconium (cyclopentadienyl) tris (diethylcarbamates) as described in US 5527752 and WO 99/61486. Particularly preferred metallocene complexes for use with the cocatalysts of the present invention may be represented by the general formula:

wherein :- R' each occurrence is independently selected from hydrogen, hydrocarbyl, silyl, germyl, halo, cyano, and combinations thereof, said R' having up to 20 nonhydrogen atoms, and optionally, two R' groups (where R' is not hydrogen, halo or cyano) together form a divalent derivative thereof connected to adjacent positions of the cyclopentadienyl ring to form a fused ring structure; X is a neutral η⁴ bonded diene group having up to 30 non-hydrogen atoms, which forms a π-complex with M; Y is -O-, -S-, -NRS -PR*-, M is titanium or zirconium in the + 2 formal oxidation state; Z* is SiR*₂, CR*₂, SiR*₂SIR*₂, CR*₂CR*₂, CR*=CR*, CR*₂SIR*₂, or GeR* , wherein: R* each occurrence is independently hydrogen, or a member selected from hydrocarbyl, silyl, halogenated alkyl, halogenated aryl, and combinations thereof, said R* having up to 10 non-hydrogen atoms, and optionally, two R* groups from Z*

(when R* is not hydrogen), or an R* group from Z* and an R* group from Y form a ring system. Examples of suitable X groups include s-trans-η⁴- l,4-diphenyl-l,3-butadiene, s- trans-η⁴-3-methyl-l,3-pentadiene; s-trans-η -2,4-hexadiene; s-trans-η⁴-l,3-pentadiene; s-trans-η ⁴-l,4-ditolyl-l,3-butadiene; s-trans-η⁴-l,4-bis(trimethylsilyl)-l,3-butadiene; s- cis-η⁴-3-methyl-l,3-pentadiene; s-cis-η⁴-l,4-dibenzyl-l ,3-butadiene; s-cis-η⁴-l,3- pentadiene; s-cis-η⁴-l,4-bis(trimethylsilyl)-l,3-butadiene, said s-cis diene group forming a π-complex as defined herein with the metal. Most preferably R' is hydrogen, methyl, ethyl, propyl, butyl, pentyl, hexyl, benzyl, or phenyl or 2 R' groups (except hydrogen) are linked together, the entire C₅R' group thereby being, for example, an indenyl, tetrahydroindenyl, fluorenyl, terahydrofluorenyl, or octahydrofluorenyl group. Highly preferred Y groups are nitrogen or phosphorus containing groups containing a group corresponding to the formula -N(R^7/)- or -P(R⁷)- wherein R^7/ is Cι-₁₀ hydrocarbyl. Most preferred complexes are amidosilane - or amidoalkanediyl complexes. Most preferred complexes are those wherein M is titanium. Specific complexes suitable for use with the cocatalysts of the present invention are those disclosed in WO 95/00526 and are incorporated herein by reference. A particularly preferred complex is (t-butylamido) (tetramethyl-η⁵- cyclopentadienyl) dimethyl silanetitanium -η -1.3 -pentadiene. The cocatalysts of the present invention may also be used as components of supported polymerisation catalyst systems. Suitable support materials include inorganic metal oxides or alternatively polymeric supports may be used. The most preferred support material is silica. Suitable silicas include Ineos ES70 and Grace-Davison 948 silicas. The support material may be subjected to a heat treatment and/or chemical treatment to reduce the water content or the hydroxyl content of the support material. Typically chemical dehydration agents are reactive metal hydrides, aluminium alkyls and halides. Prior to its use the support material may be subjected to treatment at 100°C to 1000°C and preferably at 200 to 850 C in an inert atmosphere under reduced pressure. The support material may be further combined with an organometallic compound preferably an organoaluminium compound and most preferably a trialkylaluminium compound in a dilute solvent. The support material is pretreated with the organometallic compound at a temperature of -20°C to 150°C and preferably at 20°C to 100°C. Alternative supports for the present invention are non-porous polystyrenes for example divinylbenzene crosslinked polystyrene. Thus according to another aspect of the present invention there is provided a polymerisation catalyst composition comprising

(a) a transition metal compound

(b) a support, and

(c) a cocatalyst prepared as hereinbefore described. The polymerisation catalyst systems of the present invention may be suitable for the polymerisation of olefin monomers selected from (a) ethylene, (b) propylene (c) mixtures of ethylene and propylene and (d) mixtures of (a), (b) or (c) with one or more other alpha-olefins. Thus according to another aspect of the present invention there is provided a process for the polymerisation of olefin monomers selected from (a) ethylene, (b) propylene (c) mixtures of ethylene and propylene and (d) mixtures of (a), (b) or (c) with one or more other alpha-olefins, said process performed in the presence of a polymerisation catalyst system as hereinbefore described. The polymerisation catalyst systems of the present invention are suitable for use in solution, slurry or gas phase processes. A slurry process typically uses an inert hydrocarbon diluent and temperatures from about 0°C up to a temperature just below the temperature at which the resulting polymer becomes substantially soluble in the inert polymerisation medium. Suitable diluents include toluene or alkanes such as hexane, propane or isobutane. Preferred temperatures are from about 30°C up to about 200°C but preferably from about 60°C to 100°C. Loop reactors are widely used in slurry polymerisation processes. The polymerisation catalyst systems of the present invention are most suitable for operation in the gas phase. Gas phase processes for the polymerisation of olefins , especially for the homopolymerisation and the copolymerisation of ethylene and α- olefins for example 1-butene, 1-hexene, 4-methyl-l-pentene are well known in the art. Particularly preferred gas phase processes are those operating in a fluidised bed. When used in the gas phase the polymerisation catalyst systems of the present invention are most suitably supported. Examples of such processes are described in EP 89691 and EP 699213 the latter being a particularly preferred process for use with the supported catalysts of the present invention. Particularly preferred polymerisation processes are those comprising the polymerisation of ethylene or the copolymerisation of ethylene and α-olefins having from 3 to 10 carbon atoms. Preferred α-olefins are 1-butene, 1-hexene, 4-methyl-l-pentene and 1-octene. Thus according to another aspect of the present invention there is provided a process for the polymerisation of ethylene or the copolymerisation of ethylene and α- olefins having from 3 to 10 carbon atoms, said process performed under polymerisation conditions in the present of a polymerisation catalyst system as hereinbefore described. The present invention will now be further illustrated with reference to the following examples: Abbreviations

TEA triethylaluminium TiBA triisobutylaluminium

FAB tris(pentafluorophenyl) borane

Amine A N(CH₃)(C₁₈H₃₇)₂

Complex A (C₅Me₄SiMe₂N'Bu)Ti(η⁴-l,3-pentadiene)

Hydroxy Compound A [N(H)Me(C₁₈-₂₂H₃₇-₄₅)₂][B(C₆F₅)₃(p-OHC₆H₄] Example 1

Preparation of cocatalyst

To a solution of pentafluorophenol (5mMol in 50 ml toluene) was added Amine A (5 mMol in 50 ml toluene). The mixture was allowed to react for 30 mins. To the resultant solution was added tris(pentafluorophenyl) borane (5 mMol of 6 %wt solution in hydrocarbons) and the mixture allowed to react for 1 hour. Supported catalyst preparation

To 3g. of Ineos ES70 silica (previously calcined at 500°C for 5 hours under nitrogen, pore volume 1.55 ml/g) was added a solution made with 2.81 ml of a hexane solution of triisobutylaluminium (TiBA), 0.96 mol/L, and 1.84 ml of hexane. The mixture was allowed to react for 2.5 hours under agitation then dried under vacuum. 1.21 ml of the above cocatalyst solution was slowly impregnated to the above TiBA treated silica and manually agitated until no lumps were visible. The resultant sample was held for 30 mins. 0.7 ml of a solution of Complex A in heptane (9.17 % wt) was then slowly added and manually agitated until no lumps were visible. The sample was then held for 30 mins and then dried under reduced pressure to give [Ti] = 45.8 μmol/g catalyst and [Al] = 0.76 mmol/g of catalyst as measured by ICP. Example 2 Polymerisation data The supported catalyst prepared in Example 1 was tested for ethylene- 1-hexene copolymerisation using the following procedure: A 2.5 1 double jacketed thermostatic stainless steel autoclave was purged with nitrogen at 70°C for at least one hour. 236g of PE pellets previously dried under vacuum at 80°C for 12 hours were introduced and the reactor was then purged three times with nitrogen (7 bar to atmospheric pressure). -0.13 g of TEA treated silica (1.5 mmol TEA/g) was added under pressure and allowed to scavenge impurities for at least 15 minutes under agitation. The gas phase was then composed (addition of ethylene, 1- hexene and hydrogen) and a mixture of supported catalyst (~0.1 g) and silica/TEA (-0.1 g) was injected. A constant pressure of ethylene and a constant pressure ratio of ethylene/co-monomer were maintained during the run. The run was terminated by venting the reactor and then purging the reactor 3 times with nitrogen. The PE powder produced during the run was then separated from the PE seed bed by simple sieving. Typical conditions are as follows: T = 70°C

PC2= 6.5Bars.

C6/C2 (pressure ratio) constant at 3300^"4

SiO2/TEA impregnated used as scavenger. H2 added during the gas phase composition (60 ml).

Polymerisation time = 73 min

Quantity of catalyst injected = 0.085 g

Polymer produced = 6 g. Activity: 8 g/ghbar A slowly decaying activity profile was obtained. The following examples illustrate the preparation of other cocatalysts according to the present invention:

Example 3 Pentane and toluene solvent were purified by passing over molecular sieve and copper catalyst to remove water and oxygen. Approximately 2 litres of each solvent was collected under nitrogen over a sodium mirror and taken into a glove box. Solvents were further purified by slurry with TEA silica (lOg/litre), stirring overnight and isolation of solvent by filtration. Methyl dioctadecylamine was purchased from Aldrich and tris(pentafluorophenyl)borane was supplied by Boulder Scientific. Toluene solutions of methyl dioctadecylamine and tris(pentafluorophenyl)borane were prepared at concentrations of 48.8 and 43.3 μmol/g respectively. Hydroxy compound A was supplied as a 72.8 μmol/g solution in toluene. Davison 948 silica was calcined at 250 °C over 5 hours to yield a silica of 1.56 mmol/g Si-OH. This was treated with excess TEA and washed to yield a TEA treated silica.

Cocatalyst preparation (Step (i)) To 365 umol of Hydroxy Compound A in toluene was added methyl dioctadecylamine (401 μmol) followed by tris(pentafluorophenyl)borane (364 μmol).

1H and¹⁹F NMR spectra showed that a quantitative reaction had occurred. Changes in the nmr spectra were consistent with consumption of the borane and the hydroxy compound.

Supported Catalyst Preparation (Step (ii)) To TEA treated silica, 4.08g, was added the reaction product from step (i) (242 μmol, added as 7.19g of solution in toluene). Addition was drop-wise with agitation of the silica. Agitation was continued until no lumps were visible in the silica. Toluene was removed under reduced pressure until silica out gassing ceased. To the impregnated silica was added Complex A in heptane (195 μmol/g of solution), (240 μmol), followed by pentane, (2.7g). Addition was drop-wise with agitation of the silica. Agitation was continued until no lumps were visible in the silica. After standing for 1 hour at ambient, the impregnated silica was formed into a slurry by addition of 25ml of pentane. Solid catalyst was isolated by filtration and was further washed with 25ml of pentane. Solvent was removed under reduced pressure to yield a green free flowing powder.

Calculated composition, 59.4 μmol/g of boron, B/Ti=l. Example 4 To TEA treated silica (4.08g) was added the reaction product from step (i) of

Example 3, (121 μmo^'l added as 3.56g of solution in toluene). Addition was drop-wise with agitation of the silica. Agitation was continued until no lumps were visible in the silica. Toluene was removed under reduced pressure until silica out gassing ceased. To the borate impregnated silica was added Complex A in heptane (195 μmol/g of solution), (122 μmol), followed by pentane, (3.4g). Addition was drop-wise with agitation of the silica. Agitation was continued until no lumps were visible in the silica. After standing for 1 hour at ambient, the impregnated silica was formed into a slurry by addition of 25ml of pentane. Solid catalyst was isolated by filtration and was further washed with 25ml of pentane. Solvent was removed under reduced pressure to yield a green free flowing powder.

Calculated composition, 29.7 μmol/g of boron, B/Ti=l.

Claims

1. A cocatalyst suitable for use in the polymerisation of olefins , said cocatalyst prepared by

(a) contacting a Lewis base with a hydroxy compound, and

(b) addition of a Lewis acid.

2. A cocatalyst according to claim 1 wherein the Lewis base is an amine.

3. A cocatalyst according to claim 1 wherein the hydroxy compound is a phenolic compound.

4. A cocatalyst according to claim 3 wherein the phenolic compound is pentafluorophenol.

5. A cocatalyst according to claim 1 wherein the Lewis acid is an aluminoxane or a borane.

6. A cocatalyst according to claim 5 wherein the borane is tris(pentafluorophenyl) boron.

7. A cocatalyst suitable for use in the polymerisation of olefins comprising (a) the combination of a Lewis base and a hydroxy compound, and

(b) a Lewis acid.

8. A polymerisation catalyst system comprising

(a) a transition metal compound, and

(b) a cocatalyst as claimed in claims 1 - 7.

9. A polymerisation catalyst system according to claim 8 wherein the transition metal compound is a metallocene

10. A polymerisation catalyst system according to claim 9 wherein the metallocene is represented by the formula:

wherein :- R' each occurrence is independently selected from hydrogen, hydrocarbyl, silyl, germyl, halo, cyano, and combinations thereof, said R' having up to 20 nonhydrogen atoms, and optionally, two R' groups (where R' is not hydrogen, halo or cyano) together form a divalent derivative thereof connected to adjacent positions of the cyclopentadienyl ring to form a fused ring structure; X is hydride or a moiety selected from the group consisting of halo, alkyl, aryl, aryloxy, alkoxy, alkoxyalkyl, amidoalkyl, siloxyalkyl etc. having up to 20 non-hydrogen atoms and neutral Lewis base ligands having up to 20 non-hydrogen atoms, Y is -O-, -S-, -NRS -PR*-, M is hafnium, titanium or zirconium, Z* is SiR*₂, CR*2, SiR*₂SIR*₂, CR*₂CR*₂, CR*=CR*, CR*₂SIR*2, or GeR*₂, wherein: R* each occurrence is independently hydrogen, or a member selected from hydrocarbyl, silyl, halogenated alkyl, halogenated aryl, and combinations thereof, said R* having up to 10 non-hydrogen atoms, and optionally, two R* groups from Z*

(when R* is not hydrogen), or an R* group from Z* and an R* group from Y form a ring system., and n is 1 or 2 depending on the valence of M.

11. A polymerisation catalyst system according to claim 9 wherein the metallocene is represented by the formula:

wherein :- R' each occurrence is independently selected from hydrogen, hydrocarbyl, silyl, germyl, halo, cyano, and combinations thereof, said R' having up to 20 nonhydrogen atoms, and optionally, two R' groups (where R' is not hydrogen, halo or cyano) together form a divalent derivative thereof connected to adjacent positions of the cyclopentadienyl ring to form a fused ring structure; X is a neutral η⁴ bonded diene group having up to 30 non-hydrogen atoms, which forms a π-complex with M; Y is -O-, -S-, -NR*-, -PR*-, M is titanium or zirconium in the + 2 formal oxidation state; Z* is SiR*₂, CR*₂, SiR*₂SIR*₂, CR*₂CR*₂, CR*=CR*, CR*₂SIR*₂, or GeR*₂, wherein: R* each occurrence is independently hydrogen, or a member selected from hydrocarbyl, silyl, halogenated alkyl, halogenated aryl, and combinations thereof, said R* having up to 10 non-hydrogen atoms, and optionally, two R* groups from Z* (when R* is not hydrogen), or an R* group from Z* and an R* group from Y form a ring system.

12. A polymerisation catalyst system according to claims 8 to 12 additionally comprising a support.

13. A polymeriasation catalyst system according to claim 12 wherein the support is silica.

14. A process for the polymerisation of olefin monomers selected from (a) ethylene, (b) propylene (c) mixtures of ethylene and propylene and (d) mixtures of (a), (b) or (c) with one or more other alpha-olefins, said process performed in the presence of a polymerisation catalyst system as claimed in claims 8 to 13.

15. A process for the polymerisation of ethylene or the copolymerisation of ethylene and α-olefins having from 3 to 10 carbon atoms, said process performed under polymerisation conditions in the present of a polymerisation catalyst system as claimed in claims 8 to 13.

16. A process according to claim 15 wherein the α-olefin is 1-butene, 1-hexene, 4-methyl- 1 -pentene or 1 -octene.

17. A process according to any of claims 14 to 16 performed in the solution, slurry or gas phase.

18. A process according to any of claims 14 to 16 performed in a fluidised bed gas phase reactor.