POLYOLEFIN PRODUCTION

Abstract: Use of a metallocene compound of the general formula: Ind2R''MQ2 as a component of a catalyst system for the production of a polyethylene copolymer substantially in the absence of comonomer, wherein each Ind is the same or different and is indenyl or substituted indenyl; R'' is a bridge which comprises a C1 to C4 alkylene radical, a dialkyl germanium or silicon or siloxane, alkyl phosphine or amine, which bridge is substituted or unsubstituted, M is Group IV metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen; and the ratio of meso to racemic forms of the metallocene in the catalyst system is at least 1:3.

Description: POLYOLEFIN PRODUCTION FIELD OF THE INVENTION The present invention relates to a process for the preparation of polyethylenes and to the use of metallocene compounds as catalyst components for use in such a process. BACKGROUND TO THE INVENTION Metallocene-catalysed polymerisation of ethylene is well-known and it is common practice to add to the ethylene monomer a comonomer such as butene. This has the advantage of modifying the properties of the polyethylene so as to make a range of copolymer products with various mechanical properties. Racemic (bis indenyl) ethane zirconium dichloride complexes are known to be very active polymerisation catalysts for the production of polyethylene and low molecular weight isotactic polypropylene in combination with a cocatalyst such as methyl aluminoxane. The conventional synthetic procedures for producing the racemic complexes also produce a side product, which is the meso isomer, at a typical level of 3-5%. CA-A-2104036 is directed to the use of various rac/meso mixtures primarily as catalysts in the production of isotactic polypropylenes . This patent application also discloses the use of the meso form of certain metallocenes to produce atactic polypropylene. In one example, this patent application also discloses the use of a 95:5 rac/meso metallocene in the production of a propylene, ethylene block copolymer. EP-A-0743324 is directed to a process for producing polyethylene under gas phase polymerisation conditions. Ethylene is polymerised in the presence of hexene comonomer using a mixture of racemic and meso stereoisomers of a bridged metallocene catalyst. EP-A-0643078 describes preparation of ethylene homopolymers using a meso-ethylene-bis- (4 , 7-dimethyl-l-indenyl) zirconium dichloride catalyst . The same catalyst is also used to produce an ethylene/1-butene copolymer by copolymerising ethylene and 1- butene . CA-A-2017190 and its corresponding EP-A-0399348 disclose the use of a broad range of metallocene catalysts in the preparation of ethylene homopolymers or in the preparation of ethylene copolymers by copolymerisation of ethylene with 1-olefins having 3 to 20 carbon atoms. EP-A-0835886 is directed to a process for preparing a meso/racemic metallocene compound and a process for the polymerisation or copolymerisation of olefins. SUMMARY OF THE INVENTION The present applicants have surprisingly found that the meso form of certain etallocenes can be advantageously used as a catalyst in the production of ethylene copolymers without the need to add exogenous butene to the reaction mix. The present invention provides use of a metallocene compound of general formula Ind 2 R"MQ 2 as a component of a catalyst system for the production of a polyethylene copolymer substantially in the absence of comonomer, wherein each Ind is the same or different and is indenyl or substituted indenyl; R" is a bridge which comprises a C : to C 4 alkylene radical, a dialkyl germanium or silicon or siloxane, alkyl phosphine or amine, which bridge is substituted or unsubstituted, M is a Group IV metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen; and the ratio of meso to racemic forms of the metallocene in the catalyst system is at least 1:3. The present invention further provides a process for the preparation of a polyethylene copolymer, which comprises polymerising ethylene, optionally with hydrogen, in a substantially comonomer-free reaction medium which comprises a catalyst system comprising (a) a metallocene compound of general formula Ind 2 R"MQ 2 as defined in any one of claims 1 to 7; and (b) a cocatalyst which activates the metallocene compound; wherein the ratio of meso to racemic forms of the metallocene in the catalyst system is at least 1:3. In this aspect of the invention no comonomer is added to the ethyl and hydrogen during the polymerisation. This avoids the need to remove unwanted excess butene or other comonomer. In a further aspect, the preset invention provides use of a metallocene compound of general formula Ind 2 R"MQ 2 as a component of a catalyst system for the production of a long chain branched polyethylene copolymer in a solution or slurry process, wherein each Ind is the same or different and is indenyl or substituted indenyl; R" is a bridge which comprises a C x to C 4 alkylene radical, a dialkyl germanium or silicon or siloxane, alkyl phosphine or amine, which bridge is substituted or unsubstituted, M is a Group IV metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen; and the ratio of meso to racemic forms of the metallocene in the catalyst system is at least 1:3. In a further aspect, there is also provided a process for the preparation of a polyethylene copolymer, which comprises polymerising ethylene, optionally with hydrogen, in a reaction medium which comprises a solution or slurry of a catalyst system comprising (a) a metallocene compound of general formula Ind 2 R"MQ 2 as defined in any one of claims 1 to 7; and (b) a cocatalyst which activates the metallocene compound; wherein the ratio of meso to racemic forms of the metallocene in the catalyst system is at least 1:3 so that the polyethylene formed thereby has long chain branching . In these further aspects the presence of comonomer in the reaction is optional. Advantageously, there is substantially no comonomer present , thereby avoiding the need to remove unwanted excess comonomer on completion of the reaction. Each indenyl may bear one or more substituent groups, each of which may be independently chosen from those of formula XR V in which X is chosen from group IVA, oxygen and nitrogen and each R is the same or different and chosen from hydrogen or hydrocarbyl of from 1 to 20 carbon atoms and v+1 is the valence of X. X is preferably C. If the cyclopentadienyl ring is substituted, its substituent groups must not be so bulky as to affect coordination of the olefin monomer to the metal M. Substituents on the cyclopentadienyl ring preferably have R as hydrogen or CH3. More preferably, at least one and most preferably both cyclopentadienyl rings are unsubstituted. Each indenyl may be present in reduced form with up to 4 hydrogen substituents, such as in a , 5, 6, 7 tetrahydroindenyl . In a particularly preferred embodiment, both indenyls are unsubstituted . R" is preferably an ethylene bridge which is substituted or unsubstituted . The metal M is preferably zirconium, hafnium or titanium, most preferably zirconium. Each Q is the same or different and may be a hydrocarbyl or hydrocarboxy radical having 1-20 carbon atoms or a halogen. Suitable hydrocarbyls include aryl, alkyl, alkenyl, alkylaryl or aryl alkyl. Each Q is preferably halogen. Ethylene bis (1- indenyl) zirconium dichloride is a particularly preferred bis indenyl compound of the present invention. The metallocene catalyst component used in the present invention can be prepared by any known method. A preferred preparation method is described in J. Org . Chem. 288. 63-67 (1985) . The cocatalyst which activates the metallocene catalyst component can be any cocatalyst known for this purpose such as an aluminium-containing cocatalyst or a boron-containing cocatalyst. The aluminium-containing cocatalyst may comprise an alumoxane, an alkyl aluminium and/or a Lewis acid. The alumoxanes used in the process of the present invention are well known and preferably comprise oligomeric linear and/or cyclic alkyl alumoxanes represented by the formula: (I) R- (Al-0) n -AlR2 I R for oligomeric, linear alumoxanes and (II) (-Al-0-) m I R for oligomeric, cyclic alumoxane, wherein n is 1-40, preferably 10-20, m is 3-40, preferably 3-20 and R is a C x -C 8 alkyl group and preferably methyl. Generally, in the preparation of alumoxanes from, for example, aluminium trimethyl and water, a mixture of linear and cyclic compounds is obtained. Suitable boron-containing cocatalysts may comprise a triphenylcarbenium boronate such as tetrakis-pentafluorophenyl- borato-triphenylcarbenium as described in EP-A-0427696 , or those of the general formula [L'-H] + [B Ar 1 Ar 2 X 3 X - as described in EP-A-0277004 (page 6, line 30 to page 7, line 7) . Preferably, the same catalyst system is used in both steps (i) and (ii) of the process. The catalyst system is preferably employed in a solution polymerisation process, which is homogeneous, or a slurry process, which is heterogeneous. In a solution process, typical solvents include hydrocarbons with 4 to 7 carbon atoms such as heptane, toluene or cyclohexane. In a slurry process it is advantageous to immobilise the catalyst system on an inert support, particularly a porous solid support such as talc, inorganic oxides and resinous support materials such as polyolefin. Preferably, the support material is an inorganic oxide in its finally divided form. Suitable inorganic oxide materials which are desirably employed in accordance with this invention include Group 2a, 3a, 4a or 4b metal oxides such as silica, alumina and mixtures thereof. Other inorganic oxides that may be employed either alone or in combination with the silica, or alumina are magnesia, titania, zirconia, and the like. Other suitable support materials, however, can be employed, for example, finely divided functionalized polyolefins such as finely divided polyethylene. Preferably, the support is a silica having a surface area comprised between 200 and 900 m2/g and a pore volume comprised between 0.5 and 4 ml/g. The amount of alumoxane and metallocenes usefully employed in the preparation of the solid support catalyst can vary over a wide range. Preferably the aluminium to transition metal mole ratio is in the range between 1:1 and 100:1, preferably in the range 5:1 and 50:1. The order of addition of the metallocenes and alumoxane to the support material can vary. In accordance with a preferred embodiment of the present invention alumoxane dissolved in a suitable inert hydrocarbon solvent is added to the support material slurried in the same or other suitable hydrocarbon liquid and thereafter a mixture of the metallocene catalyst component is added to the slurry. Preferred solvents include mineral oils and the various hydrocarbons which are liquid at reaction temperature and which do not react with the individual ingredients. Illustrative examples of the useful solvents include the alkanes such as pentane, iso-pentane, hexane, heptane, octane and nonane; cycloalkanes such as cyclopentane and cyclohexane; and aromatics such as benzene, toluene, ethylbenzene and diethylbenzene . Preferably the support material is slurried in toluene and the metallocene and alumoxane are dissolved in toluene prior to addition to the support material. The polyethylene formed in accordance with the present invention preferably has long chain branching with a shear ratio of preferably at least 20, more preferably at least 30, and still more preferably at least 40. These properties confer upon the polyethylene good processability characteristics and a smooth glossy surface. Without wishing to be bound by any theory it is postulated that comonomer is not required in the polymerisation because butene is formed in si tu at the active site of the metallocene catalyst, probably by ethylene dimersation. Butene found in si tu would react very quickly with a growing polymer chain because no diffusion barrier would be present. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in further detail, by way of example only, with reference to the following Examples and the accompanying drawings, in which: FIGURE 1 shows a 13C NMR spectrum of a polyethylene made in accordance with the present invention; and FIGURE 2 shows a 13C NMR spectrum of a further polyethylene made in accordance with the present invention. BRIEF DESCRIPTION OF THE INVENTION Example 1 Catalyst Preparation (a) Synthesis of the Stereoisomers High yield rapid synthesis of the rac/meso (bisindenyl) ethanezirconium dichloride: The freshly prepared diaromatized bisindenylethane ligand is suspended in pentane and reacted with an equimolar suspension of ZrCl 4 in pentane. The slurry is stirred for three Hours and filtered. Yellow solid is extracted with methylene chloride to separate the LiCl . According to NMR of the crude product a more or less quantitative yield for the rac/meso mixture is obtained according to this method. (b) Isolation of the Pure Stereoisomers Solubility test showed that the meso isomer has about three times higher solubility in toluene. In this way the meso can be concentrated in toluene and completely separated from the racemic byproduct . The support used in a silica having a total pore volume of 4.217 ml/g and a surface area of 322 m 2 /g- This silica is further prepared by drying in high vacuum on a schlenk line for three hours to remove the physically absorbed water. 5g of this silica are suspended in 50 ml of toluene and placed in a round bottom flask equipped with magnetic stirrer, nitrogen inlet and dropping funnel . An amount of 0.31 g of the metallocene is reacted with 25 ml of methylalumoxane (MAO 30 wt% in toluene) at a temperature of 25°C during 10 minutes to give a solution mixture of the corresponding metallocenium cation and the anionic methylalumoxane oligomer. Then the resulting solution comprising the metallocenium cation and the anionic methylalumoxane oligomer is added to the support under a nitrogen atmosphere via the dropping funnel which is replaced immediately after with a reflux condenser. The mixture is heated to 110°C for 90 minutes. Then the reaction mixture is cooled down to room temperature, filtered under nitrogen and washed with toluene . The catalyst obtained is then washed with pentane and dried under a mild vacuum. Example 2 Polymerisation Procedure and Results Ethylene was polymerised under the conditions described in Table 1 in a 41 batch reactor at a temperature of 80°C for a residence time of 60 mins . Supported catalyst was precontacted with triisobutylaluminium (TBAC) and introduced into the reactor in which 21 of isobutene were used as diluent. The metallocene was present at 100 mg and the cocatalyst at 390ppm. Table 1 Part 1 Runs A to H, Et y ene wt ; uns o , y ene w Table 1 - Part 2 Table 1 represents the polymerisation conditions, results and polymer analysis for the meso stereoisomer . The polymerization activity increases with increasing comonomer content in the feed regardless if hydrogen is present or not. A maximum activity of 4000g PE/g cat for 6% ethylene and lOOOOg PE/g cat for 10% ethylene concentration has been reached with different hexene concentration at this stage. The density behaviour is also interesting with respect to the hexene concentration; it does not decrease gradually with increasing hexene concentration. This behaviour is related to the in si tu butene formation (cf. 13C NMR) . For a monomodal polymer formed with a single site catalyst the SRs of the polymers are very large (35-60) . The most important practical consequence of large SR, related to long chain branching (cf. 13C NMR), is the fact that the specimens that have been obtained from the melt indexer show no signs of melt fracture (good processibility) and the corresponding plaques are of very smooth and glossy surfaces. 13C NMR Analysis of PE Homo- and copolymers Table 2 sets out the results of 13C NMR analysis of the polyethylenes produced under the conditions of some of the runs detailed in Table 1. The averages of three NMR analyses are shown. Representative spectra from runs A and C are shown in Figures 1 and 2 respectively. The spectra indicate a very good comonomer incorporation capability and the formation of long chain branching. In addition to the hexene (Butane branching) substantial amounts of butene (ethyl branching) are observed in the backbone of the polymers. Most important is the signal pattern of the spectrum in Figure 1 of the polymer produced in the absence of any type of external comonomer. It shows up to 1 wt% Butene and confirms the in si tu formation of butene via dimerization of ethylene. Since no other signals related to other comonomers is observed it can be concluded that the dimerization mechanism is very specific. The formation of long chain branching and comomoner is part of the unique properties of this catalytic system. %Cm = molar % compared with ethylene %Cw = weight % compared with ethylene

Claims: CLAIMS : 1. Use of a metallocene compound of general formula Ind 2 R"MQ 2 as a component of a catalyst system for the production of a polyethylene copolymer substantially in the absence of comonomer, wherein each Ind is the same or different and is indenyl or substituted indenyl; R" is a bridge which comprises a Cj to C, alkylene radical, a dialkyl germanium or silicon or siloxane, alkyl phosphine or amine, which bridge is substituted or unsubstituted, M is a Group IV metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen; and the ratio of meso to racemic forms of the metallocene in the catalyst system is at least 1:3. 2. Use according to claim 1, wherein R" is an ethylene bridge which is substituted or unsubstituted. 3. Use according to claim 1 or claim 2, wherein the cyclopentadienyl ring of at least one of the indenyls is unsubstituted . 4. Use according to claim 3 , wherein both indenyls are unsubstituted . 5. Use according to any one of the preceding claims, wherein M is Zr. 6. Use according to any one of the preceding claims, wherein Q is halogen. 7. Use according to claim 1, wherein the bis indenyl compound is ethylenebis 1-indenyl zirconium dichloride. 8. Use according to any one of the preceding claims, wherein the catalyst system further comprises a cocatalyst capable of activating the metallocene compound. 9. Use according to any one of the preceding claims, wherein the cocatalyst which activates the metallocene compound comprises an aluminium-containing cocatalyst or a boron-containing cocatalyst . 10. Use according to claim 9, wherein the cocatalyst comprises an aluminium-containing cocatalyst comprising an alumoxane, an alkyl aluminium and/or a Lewis acid. 11. Use according to any one of the preceding claims, wherein the catalyst system further comprises an inert support . 12. A process for the preparation of a polyethylene copolymer, which comprises polymerising ethylene, optionally with hydrogen, in a substantially comonomer-free reaction medium which comprises a catalyst system comprising (a) a metallocene compound of general formula Ind 2 R"MQ 2 as defined in any one of claims 1 to 7; and (b) a cocatalyst which activates the metallocene compound; wherein the ratio of meso to racemic forms of the metallocene in the catalyst system is at least 1:3. 13. A process according to any one of claim 12, wherein the polyethylene formed thereby has short chain branching arising from C incorporation in the backbone of the polyethylene. 14. A process according to claim 12 or claim 13 , wherein the polyethylene formed thereby has long chain branching. 15. A process according to claim 14, wherein the polyethylene formed thereby has a shear ratio of at least 20. 16. A process according to any one of claims 12 to 15, wherein the catalyst system further comprises an inert support . as a component of a catalyst system for the production of a long chain branched polyethylene copolymer in a solution or slurry process, wherein each Ind is the same or different and is indenyl or substituted indenyl; R" is a bridge which comprises a C λ to C, alkylene radical, a dialkyl germanium or silicon or siloxane, alkyl phosphine or amine, which bridge is substituted or unsubstituted, M is a Group IV metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen; and the ratio of meso to racemic forms of the metallocene in the catalyst system is at least 1:3. 18. Use according to claim 17, wherein the catalyst system is as defined in any one of claims 2 to 11. 19. A process for the preparation of a polyethylene copolymer, which comprises polymerising ethylene, optionally with hydrogen, in a reaction medium which comprises a solution or slurry of a catalyst system comprising (a) a metallocene compound of general formula Ind 2 R"MQ 2 as defined in any one of claims 1 to 7; and (b) a cocatalyst which activates the metallocene compound; wherein the ratio of meso to racemic forms of the metallocene in the catalyst system is at least 1:3 so that the polyethylene formed thereby has long chain branching.

Inventors: PETERS LILIANE, RAZAVI ABBAS, PETERS, LILIANE, RAZAVI, ABBAS
Applicants: FINA RESEARCH, PETERS LILIANE, RAZAVI ABBAS, FINA RESEARCH S.A., PETERS, LILIANE, RAZAVI, ABBAS
Application year: 1999
Main classification: C08F110
IPC codes: C08F4, C08F10, C08F110, C08F210