EP1165635A1

EP1165635A1 - Catalyst system and process for olefin polymerisation

Info

Publication number: EP1165635A1
Application number: EP00902786A
Authority: EP
Inventors: Klaus-Joachim Jens; Arild Follestad; Ulf Palmquist; Ivar Martin Dahl; Richard Blom
Original assignee: Borealis Technology Oy
Current assignee: Borealis Technology Oy
Priority date: 1999-02-11
Filing date: 2000-02-11
Publication date: 2002-01-02
Also published as: WO2000047640A1; AU2452500A; NO990642D0

Abstract

A catalyst system for olefin polymerisations comprising (i) a particulate solid support impregnated with a magnesium halogenide and a chromium compound having at least one organic ligand; and (ii) a cocatalyst selected from an alkyl aluminium or alkyl aluminium halogenide or mixtures thereof; wherein the support is not calcined after impregnation of said chromium catalyst; and wherein the molar ratio between the total amount of aluminium and the total amount of chromium is form 1:1 to 50:1.

Description

CATALYST SYSTEM AND PROCESS FOR OLEFIN POLYMERISATION

The present invention relates to a catalyst system and a process for the (co) polymerisation of ethylene and optionally other α-olefins. More particularly, this invention relates to a catalyst system comprising a catalytic chromium compound supported on a halide- containing carrier.

In polymerisations of ethylene and optionally other α- olefins at low pressures and temperatures the use of Ziegler-Natta catalysts, often in combination with an organoaluminium compound, is well known. The transition metal compound of such catalysts may be fixed to a solid carrier, such as a magnesium halide. Further, the use of organo-chromium catalysts in combination with an alkyl aluminium halide, such as diethyl-aluminium-chloride (Et₂AlCl or DEAC) , is also known.

DE 26 06 243 teaches catalysts comprising an acetate, acetylacetoneate or chloride of a transition metal . An aluminium alkyl halide, in particular DEAC is used as a cocatalyst. The catalyst components are suspended in a hydrocarbon, e.g. benzene. Polymerisations performed at temperatures from 60 °C to 80 °C are exemplified. In general, the catalysts have low activities, the highest being obtained with Cr-acetate (CrAc₃) and DEAC suspended in benzene. The molar ratio of Cl/Cr employed is 342.

EP 193 146 discloses a homogeneous organo-chromium catalyst comprising a transition metal compound and DEAC. The object is to provide ethylene-based polymers intended for use in injection moulding, i.e. polymers having a narrow molecular weight distribution. The catalyst comprises (i) an organochromium compound, such as Cr-acetylacetonates, Cr (RCOO) ₃ or Cr(OR)₄, where R is an alkyl, aryl or aralkyl group having 1 to 20 carbon atoms, (ii) a Mg, Mn or Ca compound, such as carboxylates, phosphorous-containing compounds or halides, preferably chlorides, and (iii) alkyl-Al- halide, e.g. diethyl -Al-chloride . The molar ratios between magnesium and chromium are preferably from 20:1 to 50:1.

US 5189000 discloses a catalyst system for the

(co) polymerisation of olefins. In the working examples the catalyst is prepared by contacting a Ti compound, at least one transition metal compound and a Mg compound with a carrier. The carrier is preferably microbeads of Si02 having pores between 100 a and 150 a and a high surface area and a narrow pore diameter distribution. As a cocatalyst is used an organo-Al compound.

US 5198400 discloses a catalyst system consisting of a first and second catalyst component comprising silica carriers differing in pore volume by at least 0.3 cm³/g. Both components contain 0.1 to 2.0% by weight of Cr which is thermally activated by calcining at 230 to 925°C. The first catalyst component may contain 2 to 4.5 wt% of Al, P, Ti or Sr. The mixed chromium catalyst composition may also contain a reducing agent, such as alkyl-Al-halides or preferably triethyl-boron. In the examples polymerisations are performed at about 100°C.

In catalysed polymerisations the catalyst will function as a starting site for the growth of polymer chains. Consequently, the catalyst will become intermingled in the polymer particle and cannot easily be removed. Therefore, it is important that the catalyst does not contain substances that are harmful or not desired in the finished article made from the final polymer resin. Halides are particularly objectionable. Halides, in particular chlorine, may be found in the inorganic carrier or in the cocatalyst, such as diethyl-Al- chloride . Hydrolyzable chlorine-containing compounds, typically cocatalysts, may upon heating easily form hydrochloric acid in the presence of moisture, which will corrode processing equipment and also have other drawbacks .

Thus, there remains a need for new catalyst systems containing less chlorine which nevertheless maintain the catalytic activity of the catalyst system.

It has now been surprisingly found that by employing chromium catalysis a particularly low ratio of halogen to chromium catalyst may be tolerated whilst still giving rise to catalyst systems that yield excellent polymer products. Moreover, it has surprisingly been found that by maintaining a low ratio of aluminium to chromium and/or a particular ratio of magnesium to chromium, an improved catalyst system may be prepared.

Thus, viewed from one aspect the present invention provides a catalyst system for olefin polymerisations comprising (i) a particulate solid support impregnated with a magnesium halogenide and a chromium compound having at least one organic ligand; and

(ii) a cocatalyst selected from an alkyl aluminium or alkyl aluminium halogenide or mixtures thereof; wherein the support is not calcined after impregnation with said chromium catalyst; and wherein the molar ratio between the total amount of aluminium and the total amount of chromium is from 1:1 to 50:1, preferably 10:1 to 30:1.

Viewed from another aspect, the invention provides a process for the preparation of an olefin polymer comprising contacting at least one olefin monomer with a catalyst system comprising

(i) a particulate solid support impregnated with a magnesium halogenide and a chromium compound having at least one organic ligand; and

(ii) a cocatalyst selected from an alkyl aluminium or alkyl aluminium halogenide or mixtures thereof wherein the support is not calcined after impregnation with said chromium catalyst; and wherein the molar ratio between the total amount of aluminium and the total amount of chromium is from 1:1 to 50:1.

Viewed from yet another aspect the invention provides the use of a catalyst system as hereinbefore defined in olefin polymerisation.

Viewed from a yet further aspect the invention provides an olefin polymer produced by a polymerisation catalysed by a catalyst system as hereinbefore defined and the use of said polymer in the manufacture of articles.

The particulate support material used is preferably an inorganic material. The support is especially preferably a metal or pseudo metal oxide such as silica, alumina or zirconia or a mixed oxide such as silica- alumina, in particular silica, alumina or silica- alumina. Particularly preferably, the support material is acidic, e.g. having an acidity greater than or equal to silica, more preferably greater than or equal to silica-alumina and even more preferably greater than or equal to alumina. The acidity of the support material can be studied and compared using the TPD (temperature programmed desorption of gas) method. Generally the gas used will be ammonia. The more acidic the support, the higher will be its capacity to adsorb ammonia gas. After being saturated with ammonia, the sample of support material is heated in a controlled fashion and the quantity of ammonia desorbed is measured as a function of temperature.

Especially preferably the support is a porous material so that the chromium catalyst may be loaded into the pores of the support, e.g. using a process analogous to those described in W094/14856 (Mobil) , 095/12622 (Borealis) and WO96/00243 (Exxon) . The particle size is not critical but is preferably in the range 4 to 200 μm, more preferably 8 to 80 μm, e.g. 20 to 80 μm. Useful catalyst supports will have weight-determined median particle sizes and preferably have relatively narrow particle size distribution.

Before loading of the active agent, the particulate support material is preferably calcined, i.e. heat treated, preferably under a non-reactive gas such as nitrogen. This treatment is preferably at a temperature in excess of 100°C, more preferably 200°C or higher, e.g. 200-800°C, particularly about 300°C. The calcination treatment is preferably effected for several hours, e.g. 2 to 30 hours, more preferably about 10 hours.

It should of course be noted that once the chromium compound has been impregnated onto the solid support the support must not be calcined.

The magnesium halogenide is preferably impregnated into the particulate solid support optionally before or after calcination and is preferably magnesium chloride. Such a support structure may be prepared in a number of ways readily determined by the person skilled in the art. For example, the support may be impregnated either with anhydrous magnesium chloride (MgCl₂) dissolved in an alcohol and then dried, or with a magnesium dialkyl in solution, then dried and subsequently treated with a chlorinating agent such as hydrogen chloride. The result of either process is a carrier impregnated with magnesium chloride. The impregnated support, preferably silica support, may contain from 0.1 to 0.2 g, preferably about 0.15 g of magnesium halogenide per g of Si0₂ in the surface layer of the carrier. The overall weight ratio between MgCl₂ and support may be different. The carrier should be of a small particle size, typically having a diameter of about 50 μm.

The chromium compound used in the preparation of the present catalyst system should contain at least one organic ligand. Suitable ligands are preferably bidentate and may be selected from the group comprising various carboxylic acid salts, e.g. carboxylic acid salts of chromium, chromium alkoxides, chromium chelate compounds, chromium π-complexes and chromium aryl compounds. Particular chromium carboxylic acid salts may be represented by the general formula Cr(RCOO)₃ or Cr (RCOO) ₂. (RCO) ₂0, wherein R is an aryl group, an aralkyl or preferably a branched or straight chain C^_o alkyl group. Particularly preferred chromium compounds include Cr (2-ethyl-l-hexanoate) ₃, Cr(CH₃COO)₃, Cr(EtCOO)₃, Cr (C₃H₇COO)₃, Cr(PhCOO)₃, Cr (BzCOO) ₃ especially Cr(2- ethyl-1-hexanoate) ₃. Other chromium compounds of interest may be found in EP-A-193146 which is herein incorporated by reference. The amount of chromium carboxylate should be sufficient to obtain an amount of 0.5 to 2.5% by weight of Cr, calculated as metal, based on the weight of the MgCl₂-Si0₂.

To be useful in polymerisations, the thus prepared procatalyst has to be combined with a cocatalyst, in particular an alkyl aluminium or alkyl aluminium chloride compound. A preferred cocatalyst is an alkyl aluminium halide, e.g. C^o alkyl aluminium chloride preferably a dialkylaluminium chloride, such as the commonly used diethylaluminium chloride (DEAC) .

The procatalyst and the cocatalyst are used in amounts such that the molar ratio between chlorine from the added cocatalyst and chromium joined to the carrier is within the range from 1:1 to 100:1, preferably from 5:1 to 50:1, more preferably from 10:1 to 30:1, in particular about 18:1.

The procatalyst and the cocatalyst are also used in amounts such that the molar ratio between aluminium from the added cocatalyst and chromium joined to the carrier is within the range from 1:1 to 50:1, preferably from 10:1 to 30:1., especially 15:1 to 24:1, e.g. 18:1.

It is also preferred if the ratio of magnesium to chromium is from 3:1 to 15:1.

The final catalyst particles should have a surface area in the range from 100 to 600 m²/g, preferably from 200 to 500 m²/g, and a pore volume in the range between 0.7 and 2 cm³/g-

The catalyst of the present invention will maintain a high activity with a substantially lower level of cocatalyst than conventionally used in catalysts. Thus, it may have its highest activity when the molar ratio between the total amount of chlorine in the catalyst system and the amount of chromium in the catalyst, (Cl_tot/Cr) , is in the range from 5:1 to 210:1, and when the molar ratio between the chloride in the cocatalyst and chromium in the catalyst, (Cl_cc/Cr) , is in the range from 5:1 to 100:1. Preferably, the ratio Cl_cc/Cr is in the range from 5:1 to 50:1, more preferably from 12:1 to 24:1, most preferably about 18:1, i.e. a ratio Cl_cc/Cr of 18. The molar ratio between the alkyl of the alkyl aluminium cocatalyst and chromium in the catalyst will thus be in the range from 10:1 to 100:1. The activity may decrease when said ratio increases, viz. when the level of chlorine increases. Prior art catalysts have their maximum activities at much higher Cl/Cr ratios. The lower level of cocatalyst used in the process of the present invention may result in a reduced concentration of chlorine in the final polymer resin.

The catalyst of the invention may be made by conventional means. The carrier, e.g. silica carrier, impregnated with magnesium chloride, MgCl₂-Si0₂, is readily available in the market place from suppliers such as Grace or may be prepared as hereinbefore described. This may be contacted with a chromium compound by combining a slurry of the MgCl₂-Si0₂ and a solution of the chromium compound under conditions of stirring at room or elevated temperatures for a sufficient period of time. The resulting material is recovered as solid granules.

The cocatalyst is generally added to the reactor in which polymerisation occurs separately from the support impregnated with the magnesium halogenide and chromium catalyst. However, it is possible to impregnate the support with the cocatalyst as long as the resulting catalyst system is employed in a polymerisation reaction rapidly, e.g. within 1 hour, preferably within 0.5 h, especially within 10 mins .

The olefin polymerized by the catalyst system of the invention is preferably ethylene or an alpha-olefin or a mixture of ethylene and an α-olefin or a mixture of alpha olefins, for example C₂_₂₀ olefins, e.g. ethylene, propene, n-but-1-ene, n-hex-1-ene, 4-methyl-pent-l-ene, n-oct-1-ene etc. The olefins polymerized may include any compound which includes unsaturated polymerizable groups. Thus for example unsaturated compounds, such as C₆_₂₀ olefins (including cyclic and polycyclic olefins (e.g. norbornene) ) , and polyenes, especially C₆_₂₀ dienes, may be included in a comonomer mixture with lower olefins, e.g. C₂_₅ α-olefins, especially ethylene. Diolefins (ie. dienes) are suitably used for introducing long chain branching into the resultant polymer. Examples of such dienes include α,ω linear dienes such as 1 , 5-hexadiene, 1, 6-heptadiene, 1 , 7-octadiene, 1,8- nonadiene, 1 , 9-decadiene, etc.

In general, where the polymer being produced is a homopolymer it will preferably be polyethylene. Where the polymer being produced is a copolymer it will likewise preferably be an ethylene copolymer with ethylene making up the major proportion (by number and more preferably by weight) of the monomer residues. Comonomers, such as C₄_₆ alkenes, will generally be incorporated to contribute to the mechanical strength of the polymer product .

The method of polymerisation and the utilized polymerisation conditions are not critical. Polymerisations performed in solution, suspension or gas phase may be employed. Moreover, the polymerisation may be carried out either continuously or in a batchwise process, a preferred polymerisation method is suspension polymerisation. In this case a suitable polymerisation diluent is an alkane, such as isobutane.

Polymerization in the method of the invention may be effected in one or more, e.g. 1, 2 or 3 , polymerization reactors, using conventional polymerization techniques, e.g. gas phase, solution phase, slurry or bulk polymerization . In general, a combination of slurry (or bulk) and at least one gas phase reactor is often preferred, particularly with the reactor order being slurry (or bulk) then one or more gas phase.

For slurry reactors, the reaction temperature will generally be in the range 60 to 110°C (e.g. 70-105°C) , the reactor pressure will generally be in the range 5 to 80 bar (e.g. 10-50 bar), and the residence time will generally be in the range 0.3 to 5 hours (e.g. 0.5 to 2 hours) . The diluent used will generally be an aliphatic hydrocarbon having a boiling point in the range -70 to +100°C. Alternatively aromatic solvents such as toluene could be employed. In such reactors, polymerization may if desired be effected under supercritical conditions.

For gas phase reactors, the reaction temperature used will generally be in the range 60 to 115°C (e.g. 70 to 105°C) , the reactor pressure will generally be in the range 10 to 25 bar, and the residence time will generally be 1 to 8 hours. The gas used will commonly be a non-reactive gas such as nitrogen together with monomer (e.g. ethylene).

For solution phase reactors, the reaction temperature used will generally be in the range 130 to 270°C, the reactor pressure will generally be in the range 20 to 400 bar and the residence time will generally be in the range 0.1 to 1 hour. The solvent used will commonly be a hydrocarbon with a boiling point in the range 80- 200°C.

Generally the quantity of catalyst used will depend upon the nature of the catalyst, the reactor types and conditions and the properties desired for the polymer product. Conventional catalyst quantities, such as described in the publications referred to herein, may be used .

The molecular weight can be controlled by hydrogen. Typically, a hydrogen partial pressure of about 1 bar will be suitable, a higher partial pressure will only have a marginal effect on the catalytic activity of the catalyst system.

An advantage of the catalyst system of the present invention is that the polymerisation rate will increase when the polymerisation is performed at a relatively high temperature. However, the polymerisation temperatures have to be lower than the melting temperature of the produced polymers. On the other hand, a too low temperature will reduce the reaction rate to an unacceptable low level . The present catalyst system has a high thermal stability and therefore a preferred polymerisation temperature is in the range from 70 °C to

105^υC.

The ethylene-based polymers optionally obtained by utilizing the present catalyst in suspension polymerisations will have an improved morphology over a non-supported catalytic system. The main reason for that is the controlled particle size distribution of the carrier, which will result in a substantially reduced amount of produced fines. The obtained ethylene-based polymers will have a high average molecular weight (MW) and a narrow molecular weight distribution (MWD) . The MWD may be expressed as the ratio between the melt flow rates measured with load 21.6 kg (HLMI) and load 2.16 kg (MI) according to ASTM 1238. Preferably HLMI/MI < 60.

The present invention is described in more detail with reference to the following examples.

Examples Preparation of a solid catalyst a combined MgCl₂- Si0₂ carrier available from Grace- Davison under the trade name "Sylopol 5550" was used. This carrier has a surface area of 260 m²/g, a pore volume of 1.3 ml/g, and contains 4.8 wt% of Mg and 10.9 wt% of Cl . The weight average median particle size is 50 μm. This carrier was slurried in pentane and the slurry added to a stirred mineral oil solution of Cr (2-ethyl-l- hexanoate)₃ in an amount sufficient to obtain 1.3 % by weight of Cr (calculated as metal) based on the weight of the MgCl₂-Si0₂. Under continuous stirring the reagents were allowed to react at room temperature. The solid was recovered by filtration and washed with pentane, then dried until constant weight under a reduced pressure at room temperature.

Example 1

Polymerisation of ethylene

A l l reactor provided with a stirrer was charged with 0.5 1 isobutane and heated to 90 °C, and then 200 mg of the solid catalyst (0.05 mmole Cr) prepared above was introduced under a nitrogen atmosphere. It was added 2% by weight of the cocatalyst Et₂AlCl (0.6 mmole Cl) in a pentane solution, giving a molar ratio of Cl : Cr = 12:1 between the chlorine from the cocatalyst and the chromium of the catalyst. Ethylene was then added until a total pressure of 38 bar was reached. The polymerisation was allowed to proceed for 45 minutes. Remaining ethylene was then vented off and the obtained polymer collected and dried under a reduced pressure.

Example 2

Example 1 was repeated, except that also hydrogen was introduced at a partial pressure of 1 bar.

Example 3

Example 2 was repeated, except that the hydrogen partial pressure was 3 bars.

Example 4

Example 2 was repeated, except that the cocatalyst was added to a molar ratio Cl/Cr of 18:1.

Example 5

Example 4 was repeated, except that the cocatalyst was added to a molar ratio Cl/Cr of 24.

Example 6

Example 1 was repeated, except that the catalyst carrier was a Si0₂ free of any MgCl₂.

The polymerisation parameters and the obtained results of examples 1 to 6 are presented in Table 1 below.

Example 7

In this example the catalyst component Cr (2-ethyl-l- hexanoate)₃ on MgCl₂-Si0₂ was prereacted with the cocatalyst Et₂AlCl prior to the polymerisation of ethylene. The catalyst prepared as described above was slurried in a mixture of mineral oil and toluene in a closed bottle. It was then added a 2 % solution of Et₂AlCl in toluene, which was allowed to react for about 15 minutes. The Cl/Cr ratio in the slurry was 15.7:1. The resulting slurry, corresponding to 300 -mg solid catalyst, was introduced into the reactor with nitrogen pressure. Subsequently, 0.5 1 of isobutane was added, the temperature adjusted to 70 °C and ethylene fed continuously to the reactor to keep a constant pressure of 35 bar. After a polymerisation period of 20 minutes at 72.5 °C it was obtained 84.3 g of polyethylene, which corresponds to a catalytic activity of 66.6 kg PE/gCr,h. This activity is comparable with the activities obtained in examples 1 to 5. Example 8

Copolymerisation of ethylene with 1-hexene The polymerisation reactor used in example 1 was charged with 0.5 1 isobutane and 1.0 ml of 1-hexene and heated to 90 °C, and 200 mg of the solid catalyst (0.05 mmole Cr) used in example 1 was introduced under a nitrogen atmosphere. An isobutane solution of the cocatalyst Et₂AlCl was then added (0.6 mmole Cl) , giving a molar ratio Cl : Cr = 12:1. A minor amount of hydrogen was added, and ethylene fed until a total pressure of 38 bars. The hydrogen partial pressure was 1 bar. The polymerisation was allowed to proceed for 45 minutes. Remaining ethylene was then vented off and the obtained polymer collected and dried under a reduced pressure.

Example 9

Example 8 was repeated, except that 2.0 ml of 1-hexene was introduced into the reactor.

Example 10

Example 8 was repeated, except that 10.0 ml of 1-hexene was introduced into the reactor.

Polymerisation parameters and the obtained results of examples 8 to 10 are presented in Table 2 below.

Table 1

d = not detectable

Table 2

The results in tables 1 and 2 show that the catalyst of the present invention maintains a high activity at low levels of chlorine from the cocatalyst. According to examples 2, 4 and 5 the catalyst will have its maximum activity at a Cl/Cr ratio of about 18, and the activity will decrease with an increasing level of chlorine from the cocatalyst. Moreover, example 1 and comparative example 6 in table 1 show that a catalyst comprising a carrier of Si0₂ combined with MgCl₂ will have a substantially higher activity than when similar carrier without MgCl₂ is used. Examples 1 to 5 show that the present catalyst is not much influenced by hydrogen. Examples 8 to 10 show that the catalyst maintains its activity at moderate levels of comonomer, but will be lower at higher comonomer levels.

Example 11

1.0 g MgCl₂-Si0₂ (Sylopol 5550) was slurried in 9g mineral oil. 0.120g of an 8wt% solution of Cr(2-ethyl- hexanoate)₃ was added by syringe under stirring. The resulting slurry was stirred overnight. 0.240 ml of a solution of 10% Et₂AlCl in toluene was added by syringe under stirring and stirred for a further hour. The catalyst contains 1.0 wt% Cr and an Al : Cr ratio of 1.0. Polymerisation was carried out by introducing 4.0 ml of the mineral oil slurry to a 2-1 autoclave using isobutane as diluent. Polymerisation at 92 °C at 39 bar total pressure yielded negligible amounts of polyethylene after 75 minutes.

Claims

1. A catalyst system for olefin polymerisations comprising (i) a particulate solid support impregnated with a magnesium halogenide and a chromium compound having at least one organic ligand; and

(ii) a cocatalyst selected from an alkyl aluminium or alkyl aluminium halogenide or mixtures thereof; wherein the support is not calcined after impregnation with said chromium catalyst; and wherein the molar ratio between the total amount of aluminium and the total amount of chromium is from 1:1 to 50:1.

2. The catalyst system of claim 1 wherein said magnesium halogenide is magnesium chloride.

3. The catalyst system of claim 1 or 2 wherein the cocatalyst is an aluminium alkyl chloride or mixtures thereof .

4. The catalyst system of claim 3 wherein the alkyl aluminium chloride is diethyl aluminium chloride (DEAC) .

5. The catalyst system of any one of claims 1 to 4 , wherein said organic ligand is bidentate .

6. The catalyst system of any one of claims 1 to 5, wherein said organic ligand is selected from the group comprising carboxylates represented by the general formula Cr(RCOO)₃, wherein R is an branched or straight chain C^_o alkyl group.

7. The catalyst system of claim 5 wherein the chromium compound having at least one organic ligand is Cr(2- ethyl -1-hexanoate) ₃.

8. The catalyst system of any one of claims 1 to 7 wherein said carrier is alumina or silica.

9. The catalyst system of claim 8 wherein said carrier is silica.

10. The catalyst system of any one of claims 1 to 9 wherein the molar ratio between the total amount of halogen and the total amount of chromium in the catalyst system is from 5:1 to 210:1.

11. The catalyst system of any one of claims 1 to 10 wherein the molar ratio between the total amount of aluminium and the total amount of chromium is from 10:1 to 30:1.

12. The catalyst system of claim 11 wherein the molar ratio between the total amount of aluminium and the total amount of chromium is from 15:1 to 24:1

13. The catalyst system of claim 12 wherein the molar ratio between the total amount of aluminium and the total amount of chromium is 18:1.

14. The catalyst system of any one of claims 1 to 13 wherein the molar ratio of magnesium to chromium is from 3:1 to 15:1.

15. The catalyst system of any one of claims 1 to 14 wherein the molar ratio of chlorine in the cocatalyst to chromium (Cl_cc:Cr) is in the range from 5:1 to 100:1.

16. The catalyst system of any one of claims 1 to 15, wherein the cocatalyst is added in an amount such that the molar ratio between chlorine from the cocatalyst and the chromium is about 18:1.

17. A catalyst system as claimed in any one of claims 1 to 16 wherein said cocatalyst is impregnated on the support .

18. A process for the preparation of an olefin polymer comprising contacting at least one olefin monomer with a catalyst system comprising

(ii) a cocatalyst selected from an alkyl aluminium or alkyl aluminium halogenide or mixtures thereof wherein the support is not calcined after impregnation with said chromium catalyst; and wherein the molar ratio between the total amount of aluminium and the total amount of chromium is from 1 : 1 to 50:1.

19. A process as claimed in claim 18 wherein said cocatalyst is impregnated on the support and is employed in a polymerisation reaction within 1 hour.

20. A process for the polymerisation of ethylene, optionally with other α-olefins in the presence of a catalyst system as claimed in any one of claims 1 to 17, wherein said process is performed in a slurry or gas phase at a temperature in the range from 70°C to 105°C.

21. The process of claim 20, wherein the polymerisation temperature is about 90°C.

22. Use of a catalyst system as claimed in any one of claims 1 to 17 in olefin polymerisation.

23. An olefin polymer produced by a process as claimed in any one of claims 18 to 21.