| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | January 11, 2005 |
| PATENT TITLE |
Modified supported catalysts for the polymerization of olefins |
| PATENT ABSTRACT | A modified supported olefin polymerization catalyst modified by prepolymerization with an olefin or an olefin mixture different from the olefin or olefin mixture of the subsequent olefin polymerization. The polymerization catalyst is a metallocene, and the melting or softening point of the polyolefin made using the catalyst is at least 20.degree. C. lower than that of the prepolymerized polyolefin |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | July 11, 2002 |
| PATENT CT FILE DATE | October 2, 2000 |
| PATENT CT NUMBER | This data is not available for free |
| PATENT CT PUB NUMBER | This data is not available for free |
| PATENT CT PUB DATE | April 12, 2001 |
| PATENT FOREIGN APPLICATION PRIORITY DATA | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. A process for preparing polyolefins comprising (I) prepolymerising a supported olefin polymerization catalyst with an olefin or olefin mixture wherein the pre-polymerization olefin or olefin mixture is one or more of 1-butene, 3-methyl-1-butene, 3,3-dimethyl-1-butene, 4-methyl-1-pentene, 4,4-di methyl-1-pentene, 4,5-dimethyl-1-hexene, vinyl cyclopentane, vinyl cyclohexane, vinyl cycloheptane, allyl cyclopentane, allyl cyclohexane or alyll cycloheptane; (II) isolating the resulting prepolymerised olefin polymerisation catalyst; and (III) polymerising or copolymerising olefins in the presence of the prepolymerised olefin polymerisation catalyst; wherein the olefin or olefin mixture of the prepolymerisation is different from that of the subsequent olefin polymerisation or copolymerisation, wherein the melting point or softening point of the polyolefin or olefin copolymer resulting from prepolymerisation is at least 20.degree. C. higher than the melting point or softening point of the polyolefin or olefin copolymer of the subsequent olefin polymerisation and wherein said olefin polymerisation catalyst is a metallocene. 2. A process as claimed in claim 1 wherein the melting point or softening point of the polyolefin or olefin copolymer resulting from prepolymerisation is at least 30.degree. C. higher than the melting point or softening point of the polyolefin or olefin copolymer of the subsequent olefin polymerisation. 3. The process as claimed in claim 1 wherein the polymerization catalyst is two or more metallocenes forming a multi-site catalyst. 4. The process as claimed in claim 1, wherein for the liquid-phase polymerisation of polypropylene, in step (I) the supported catalyst is pre-polymerized with 4-methyl-1-pentene. 5. The process as claimed in claim 1 wherein the pre-polymerization olefin or olefin mixture is one or more of 3-methyl-1-butene, 4-methyl-1-pentene, vinyl cyclopentane or vinyl cyclohexane. 6. The process as claimed in claim 1 wherein the support material is silica. 7. The process as claimed in claim 1 wherein said olefin polymerisation catalyst is a supported metallocene catalyst, comprising A) 90.0-99.9 parts by weight of a catalyst support comprising a hydrophilic inorganic oxide of an element of main groups II to IV or transition group IV of the Periodic Table or a mixture or mixed oxide thereof, which catalyst support is obtained by simultaneous reaction with aluminoxanes and with polyfunctional organic crosslinkers reacted with the inorganic oxide to form the catalyst support, B) 10-0.1 parts by weight of a metallocene compound of the formula I ##STR6## where M is a metal selected from the group consisting of Ti, Zr, Hf, V, Nb and Ta or an element selected from the group consisting of the lanthamides, X.sub.1 and X.sub.2 are identical or different and are each a C.sub.1 -C.sub.10 -alkyl group, a C.sub.1 -C.sub.10 -alkoxy group, a C.sub.6 -C.sub.10 -aryl group, a C.sub.6 -C.sub.10 -aryloxy group, a C.sub.2 -C.sub.10 -alkenyl group, a C.sub.7 -C.sub.20 -alkylaryl group, a C.sub.7 -C.sub.20 -arylalkyl group, a C.sub.8 -C.sub.20 -arylalkenyl group, hydrogen or a halogen atom, L.sub.1 and L.sub.2 are identical or different and are each an unsubstituted, monosubstituted or polysubstituted monocyclic or polycyclic hydrocarbon radical containing at least one cyclopentadienyl unit which can form a sandwich structure with M, R is carbon, silicon, germanium or tin, F and G are identical or different and are each a trimethylsilyl radical of the formula --Si(CH.sub.3).sub.3, where G may also be a C.sub.1 -C.sub.10 alkyl radical, C.sub.6 -C.sub.10 -aryl radical, wherein the supported metallocene catalyst, comprising A and B, has an activity of maximum 100 kg, polyolefin/g supported catalyst/h. 8. The process as claimed in claim 1, wherein the pre-polymerization is carried out in gas phase. 9. The process as claimed in claim 1, wherein the pre-polymerization is slurry phase polymerization in an oil or grease slurry medium. 10. The process as claimed in claim 2 wherein the metallocene catalyst comprises two or more metallocenes forming a multi-site catalyst. 11. The process as claimed in claim 7, wherein the pre-polymerization is carried out in gas phase. 12. The process as claimed in claim 7, wherein the pre-polymerization is slurry phase polymerization in an oil or grease slurry medium. 13. The process of claim 7 wherein G is a C.sub.1 -C.sub.4 -alkyl radical. 14. A method of reducing reactor fouling during an olefin polymerisation comprising (I) prepolymerising a supported olefin polymerisation catalyst with an olefin or olefin mixture wherein the pre-polymerisation olefin or olefin mixture is one or more of 1-butene, 3-methyl-1-butene, 3,3-dimethyl-1-butene, 4-methyl-1-pentene, 4,4-dimethyl-1-pentene, 4,5-dimethyl-1-hexene, vinyl cyclopentane, vinyl cyclohexane, vinyl cycloheptane, allyl cyclopentane, allyl cyclohexane or allyl cycloheptane; (II) isolating the resulting prepolymerised olefin polymerisation catalyst; and (III) polymerising or copolymerising olefins in the presence of the prepolymerised olefin polymerisation catalyst; wherein the olefin or olefin mixture of the prepolymerisation is different from that of the subsequent olefin polymerisation or copolymerisation, and wherein the melting point or softening point of the polyolefin or olefin copolymer resulting from prepolymerisation is at least 20.degree. C. higher than the melting point or softening point of the polyolefin or olefin copolymer of the subsequent olefin polymerisation. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
The invention relates to modified supported catalysts for the polymerization of olefins, for example based on single-site and multi-site metallocene catalysts, supported on inorganic or organic supports, or based on the Ziegler-Natta type catalysts. Metallocenes of the metals of e.g. transition group IV of the Periodic Table of the elements are highly active catalysts for the polymerization of olefins. The resulting polyolefins have new advantageous combinations and supplement the product range of the polyolefins prepared hitherto using known conventional Ziegler-Natta catalysts. It is known that catalysts based on unbridged, substituted and unsubstituted biscyclopentadienyl metallocenes in combination with aluminoxanes as cocatalyst can be used for the preparation of polyethylene and ethylene-olefin copolymers (EP 0 128 046). It is also known that stereoregular polyolefins can be prepared using bridged, chiral-metallocenes. For bridging the ligand systems, use is mostly made of dimethylsilanediyl groups (EP 0 316 155), methylphenylsilanediyl groups (EP 0 320 762), ethylene groups (Brintzinger, J. Organomet. Chem. 288 (1985), 63-67) isopropylidene bridges (EP 0 459 264) and silyl-substituted diyl bridges (WO 97 02 276). Depending on the ligand type and the substituents, isotactic, syndiotactic, hemiisotactic, stereoblock-type and atactic homopolymers and copolymers having aliphatic or cyclic structures can be prepared. As ligands, preference is given to using substituted and unsubstituted cyclopentadienyl units (EP 316 155), substituted and unsubstituted indenyl units (EP a 302 424; EP 0 485 823) and also substituted and unsubstituted cyclopentadienyl units in combination with unsubstituted fluorenyl groups (EP 0 412 416). It is further known that bridged metallocenes having a cyclopentadienyl system and a heteroatom ligand (constrained geometry catalyst) can also be used for the Polymerization of olefins (U.S. Pat. No. 5,096,867). A disadvantage of such homogeneous catalysts in the Polymerization of olefins are the resulting powders, having only a low bulk density. The particle morphology of such products can in principle be somewhat improved by a specific pretreatment of the metallocene with the cocatalyst (EP 0 302 424). However, such a process has the disadvantage of, in particular, heavy deposit formation in industrial reactors (EP 0 563 917). Although the use of methylaluminoxane, which is insoluble in aliphatic solvents, as support material gives a certain improvement in the activity and likewise leads to pulverulent products [Polymer 32(1991), 2671-2673] Supporting the metallocene on oxidic materials such as silicon oxide or aluminium oxide with pretreatment of the starting material, which may be partially dehydrated, with the cocatalyst is a known method (WO 91 09 882) used in homopolymerization and copolymerization of ethylene. However, in this method, the particle size of the polymer particle is determined essentially by the particle size of the support material. Oxidic support using methylaluminoxanes and the subsequent application of the metallocene (EP 0 206 794). However, this method restricts the ability to control the particle size by means of the properties of the support materials. EP 0 685 494 describes a further supported catalyst which is prepared by the application of methylaluminoxane to a hydrophilic oxide, subsequent crosslinking of the methylaluminoxane using a polyfunctional organic crosslinker and subsequent application of an activated methylaluminoxane/metallocene complex. The disadvantage of this supported catalyst is that at the relatively high polymerization conversions achieved in industrial plants, the strength of the supported catalysts is not sufficient to ensure a compact, granular morphology of the polymer product. A support material by drying hydrophilic inorganic oxides and reacting with aluminoxanes and polyfunctional crosslinkers gives, after contacting with catalysts, a supported catalyst system for a stable high bulk density in olefin polymerization even at high conversion rates (EP 0 787 746). Methods for modifying homogeneous catalysts and supported catalyst systems by pre-polymerization are also known. In EP 0 354 893 a homogeneous catalyst is prepared by pre-polymerizing a precipitated complex of an aluminium alkyl and a retallocene catalyst with an olefin monomer at a temperature below the polymerization temperature of the monomer, followed by polymerizing the olefin monomer under polymerization conditions. EP 0 426 638 describes the pre-polymerization of propylene in combination with homogeneous metallocene catalysts for reproducible and controllable polymerization. The disadvantage of homogeneous polymerization, resulting in powders having only a low bulk density, is not resolved in these processes. For supported catalyst systems, pre-polymerization of olefinic monomers is known from EP 0 705 281 using bis-indenyl metallocene reaction product catalysts whereby the indenyl rings are 2-substituted, further from WO 97 02 297 wherein the pores of the catalyst system contain a volume of liquid that is equal to or less than the total pore volume of the supported catalyst system. In WO 96 28 479, a supported metallocene catalyst system having an activity greater than 100,000 g/g/h and at least an .alpha.-olefin monomer are combined under pre-polymerization conditions, hydrogen is added, and after recovering the prepolymerized supported catalyst system, ethylene or propylene is fed for polymerization. However, these pre-polymerizations generally use the same olefin or olefin mixture as the olefin or olefin mixture of the subsequent olefin polymerization, or in the case of WO 99 24 478 the pre-polymerized catalyst is used directly in the subsequent polymerization. One of the main disadvantages of these known pre-polymerization processes is the impossibility to use the known metallocene supported catalysts necessary for the production of high molecular weight polyolefins without reactor fouling, formation of polymer fines resulting from soluble catalyst components and disrupted catalyst components in the initial phase of the olefin polymerization under technical liquid olefin polymerization conditions. It is therefore an object of the invention to find a supported catalyst for liquid-phase polymerization of olefins for the production of high molecular weight polyolefins without reactor fouling, formation of polymer fines resulting from soluble catalyst components and disrupted catalyst components in the initial phase of the olefin polymerization. It has now surprisingly been found, that these requirements are achieved by a modified supported olefin polymerization catalyst, characterised in that the catalyst is modified by pre-polymerization with an olefin or an olefin mixture different from the olefin or olefin mixture of the subsequent olefin polymerization. The supported catalyst may be a metallocene catalyst, for example with one, two or three .eta..sup.5 ligands bonding to a transition metal or lanthanide, preferably one or two .eta..sup.5 ligands such as cyclopentadienyl, indenyl or fluorenyl. The .eta..sup.5 ligands may be linked to each other by bridging groups, for example to yield a bridged bis-.eta..sup.5 ligand. Where only one .eta..sup.5 ligand is bonded to the metal, a pendant group on the ligand can also form a .sigma. bond to the metal, a so-called "scorpionate" catalyst. Ziegler-Natta catalysts are well known, and the invention may also be useful with other catalysts such as Philips (CrO.sub.x) catalysts. Metallocene catalysts may be classified as single-site or multi-site. Both may be coactivated by a cocatalyst, for example methyl aluminoxane, and may be supported on an inorganic or organic carrier. Ziegler-Natta catalysts, which are well known as multi-site catalysts, are supported by their nature. The olefin used for the polymerization, different from the olefin of the subsequent polymerization, may be one or more of propylene, 1-butene, 3-methyl-1-butene, 3,3-dimethyl-1-butene, 3-methyl-1-pentene, 3-methyl-1-hexene, 3,4-dimethyl-1-pentene, 3,4-dimethyl-1-hexene, 1-pentene, 4-methyl-1-pentene, 4-methyl-1-hexene, 4,5-dimethyl-1-hexene, 4,4-dimethyl-1-pentene, 4,4-dimethyl-1-hexene, 6,6-dimethyl-1-heptene, vinyl cyclopropane, vinyl cyclobutane, vinyl cyclopentane, vinyl cyclohexane, vinyl cycloheptane, vinyl cyclooctane, allyl cyclopropane, allyl cyclobutane, allyl cyclopentane, allyl cyclohexane, allyl cycloheptane or allyl cyclooctane. Preferred are one or more of propylene, 1-butene, 3-methyl-1-butene, 3,3-dimethyl-1-butene, 4-methyl-1-pentene, 4,4-dimethyl-1-pentene, 4,5-dimethyl-1-hexene, vinyl cyclopentane, vinyl cyclohexane, vinyl cycloheptane, allyl cyclopentane, allyl cyclohexane or allyl cycloheptane, whilst particulary preferred olefins are 3-methyl-1-butene, 4-methyl-1-pentene, vinyl cyclopentane and vinyl cyclohexane. Typically the pre-polymerization will result in 0.001-5.0, e.g. 0.001-3.0 or 0.01-1.0 parts by weight relative to the catalyst and support of the polyolefin or olefin copolymer different from the polyolefin or olefin copolymer of the subsequent olefin polymerization. The melting point or softening point of the polyolefin or olefin copolymer resulting from pre-polymerization is suitably at least 20.degree. C., preferably 30.degree. C. higher than the melting point or softening point of the polyolefin or olefin copolymer of the subsequent olefin polymerization. One particular supported metallocene catalyst for olefin polymerization which may be modified according to the invention, especially for liquid-phase polymerization of olefins, comprises A) 90.0-99.9 parts by weight of a catalyst support based on a hydrophilic inorganic oxide of an element of main groups II to IV or transition group IV of the Periodic Table or a mixture or mixed oxide thereof, which catalyst support is obtainable by simultaneous reaction with aluminoxanes and with polyfunctional organic crosslinkers, B) 10-0.1 parts by weight of a metallocene compound of the formula I ##STR1## where M is a metal selected from the group consisting of Ti, Zr, Hf, V, Nb and Ta or an element selected from the group consisting of the lanthamides, X.sub.1 and X.sub.2 are identical or different and are each a C.sub.1 -C.sub.10 alkyl group, a C.sub.1 -C.sub.10 -alkoxy group, a C.sub.6 -C.sub.10 -aryl group, a C.sub.6 -C.sub.10 -aryloxy group, a C.sub.2 -C.sub.10 -alkenyl group, a C.sub.7 -C.sub.20 -alkylaryl group, a C.sub.7 -C.sub.20 -arylalkyl group, a C.sub.8 -C.sub.20 -arylalkenyl group, hydrogen or a halogen atom, L.sub.1 and L.sub.2 are identical or different and are each an unsubstituted, monosubstituted or polysubstituted monocyclic or polycyclic hydrocarbon radical containing at least one cyclopentadienyl unit which can form a sandwich structure with M, R is carbon, silicon, germanium or tin, F and G are identical or different and are each a trimethylsilyl radical of the formula--Si(CH.sub.3).sub.3, where G may also be a C.sub.1 -C.sub.10 -alkyl radical, preferably a C.sub.1 -C.sub.4 -alkyl radical, or a C.sub.6 -C.sub.10 -aryl radical, whereby according to the invention said modified supported metallocene catalyst comprising the catalyst support A and the metallocene B has an activity of maximum 100 kg, preferably maximum 50 kg, particularly preferred maximum 20 kg polyolefin/g supported catalyst/h. It is also possible that the supported metallocene catalyst has an activity of maximum 5 kg or maximum 10 kg polyolefin/g supported catalyst/h. The hydrophilic oxides used in the catalyst support usually contain hydroxyl groups and/or physically absorbed water. They are preferably porous and finely divided and usually have a mean particle size of from 10 to 300 microns. Preferably the hydrophilic inorganic oxide is an aluminum oxide (alumina), silicon oxide (silica), magnesium oxide, titanium oxide or zirconium oxide or a mixture or mixed oxide thereof. Particular preference is given to using silicon dioxides of the Grace Davison type. However, other suitable starting materials are finely divided oxides, for example those described in EP 0 585 544, which are prepared by high temperature hydrolysis from gaseous metal chlorides or silicon compounds. Magnesium chloride is another suitable support material. According to the present invention, the aluminoxane used in the catalyst support is a linear aluminoxane of the formula II ##STR2## or an aluminoxane of the cyclic type III ##STR3## where, in the formula II and III, the radicals R can be identical or different and are each a C.sub.1 -C.sub.6 -alkyl group and n is an integer in the range 150. Preferably, the radicals R are identical and are methyl, isobutyl, phenyl or benzyl. The aluminoxane can be prepared in various ways by known methods. One possibility is, for example, the reaction of aluminium alkyls with aluminium sulphate containing one water of crystallisation (EP 0 302 424). In the catalyst support, the molar ratio of aluminium (as aluminoxane) to surface hydroxyl groups of the hydrophilic inorganic oxide is between 1 and 50, preferably between 1 and 30. According to the invention, suitable polyfunctional organic crosslinkers in the catalyst support are all organic compounds having more than one functional group which can react with metal-carbon bond. Preference is given to using a bifunctional crosslinker. Such bifunctional organic compounds can be, for example, aliphatic or aromatic diols, aldehydes, dicarboxylic acids, primary or secondary diamines, diepoxy compounds. Most preference is given to using aliphatic and aromatic diols, secondary amines or diepoxy compounds or mixtures thereof. Particular preference is given to using ethylene glycol, butanediol, bisphenol A and 1,4-butanediol diglycidyl ether. Tri- or higher functional crosslinkers which can be used are, for example, triethanolamine, glycerol, phloroglucinol or tetra-ethylenepentamine. The molar ratio'between the aluminium as aluminoxane and the polyfunctional organic compound in the catalyst support can vary within a wide range and is between 1 and 100, preferably between 1 and 40. Higher molar ratios of aluminium to polyfunctional organic compound are used particularly when use is made of tri- or higher-functional crosslinkers which can form a correspondingly higher number of crosslinks. According to the invention, preferred ligands L, and/or L.sub.2 in the metallocene compound are substituted or unsubstituted cyclopentadienyl, indenyl or fluorenyl radicals. Particular preference is given to cyclopentadienyl, tetramethylcyclopentadienyl, indenyl, 2-methylindenyl, 2-methyl-4-phenylindenyl, 2-methyl-4,5-benzoindenyl and fluorenyl units and also ferrocene- and ruthenocene-substituted units as are described, for example, in EP 0 673 946. According to the invention, the following metallocenes are particularly preferred: bis(trimethylsilyl) silanediyldicyclopentadienylzirconium dichloride, bis(trimethylsilyl) silanediyldiindenylzirconium dichloride, bis(trimethylsilyl)silanediylbis(2-methylindenyl) zirconium dichloride, bis(trimethylsilyl)silanediylbis(2-methyl-4,5-benzoindenyl)zirconium dichloride, bis(trimethylsilyl)silanediylbis(2-methyl-4-phenylindenyl)zirconium dichloride, bis(trimethylsilyl)silanediylbis(2-methyl-4-naphthylindenyl)zirconium dichloride, bis(trimethylsilyl)silanediyldifluorenylzirconium dichloride, bis(trimethylsilyl)silanediyl(fluorenyl) (cyclopentadienyl)zirconium dichloride, bis(trimethylsilyl)silanediyl(fluorenyl) (indenyl) zirconium dichloride, bis(trimethylsilyl)silanediyl(tetramethylcyclopentadienyl) (indenyl)zirconium dichloride, methyl(trimethylsilyl)silanediyldicyclopentadienylzirconium dichloride, methyl(trimethylsilyl)silanediyldiindenylzirconium dichloride, methyl(trimethylsilyl)silanediylbis(2-methylindenyl) zirconium dichloride, methyl(trimethylsilyl)silanediylbis(2-methyl-4,5-benzoindenyl)zirconium dichloride, methyl(trimethylsilyl)silanediylbis(2-methyl-4-phenylindenyl)zirconium dichloride, methyl(trimethylsilyl)silanediylbis(2-methyl-4-naphthylindenyl)zirconium dichloride, methyl(trimethylsilyl)silanediyldifluorenylzirconium dichloride, methyl(trimethylsilyl)silanediyl(fluorenyl) (cyclopentadienyl)zirconium dichloride, methyl(trimethylsilyl)silanediyl(fluorenyl)(indenyl) zirconium dichloride and methyl(trimethylsilyl)silanediyl(tetramethylcyclopentadienyl) (indenyl)zirconium dichloride. According to the invention, in the metallocene compound in addition also amido, phosphido and arsenido radicals can be used as ligands L.sub.2, where the substituents of these ligands are as defined for X, and X, or substituted or fused ferrocenyl- or ruthenocenyl-radicals. The invention further provides a process for preparing the modified supported catalyst as hereinbefore described, comprising the step of modifying a supported olefin polymerization catalyst by pre-polymerization with an olefin or an olefin mixture different from the olefin or olefin mixture of the subsequent olefin polymerization, whereby the melting point or softening point of the polyolefin or olefin copolymer resulting from pre-polymerization is at least 20.degree. C., preferably 30.degree. C. higher than the melting point or softening point of the polyolefin or olefin copolymer, of the subsequent olefin polymerization. The modified catalyst may be isolated from the modofication medium prior to use in the subsequent polymerization reaction. In particular, a process for preparing a modified supported metallocene catalyst for polymerization of olefins, especially in liquid phase, comprises the steps: .alpha.) preparing a catalyst support A) by .alpha.1) drying a hydrophilic inorganic oxide of an element of main groups II to IV or transition group IV of the Periodic Table or a mixture or mixed oxide thereof at from 110 to 900.degree. C., e.g. 110 to 800.degree. C., subsequently .alpha.2) if desired, reacting the free hydroxyl groups of the oxide completely or partially with aluminoxanes or aluminium alkyls and subsequently .alpha.3) reacting the oxide simultaneously with aluminoxanes and polyfunctional organic crosslinkers, .beta.) suspending the catalyst support A) in an inert hydrocarbon and bringing it in contact with a solution of a metallocene compound B) of the formula I in an inert hydrocarbon, wherein 90.0-99.9 parts by weight of a catalyst support A) are mixed with 10-0.1 parts by weight of the metallocene compound B) ##STR4## where M is a metal selected from the group consisting of Ti, Zr, Hf, V, Nb and Ta or an element selected from the group consisting of the lanthamides, X.sub.1 and X.sub.2 are identical or different and are each a C.sub.1 -C.sub.10 -alkyl group, a C.sub.1 -C.sub.10 -alkoxy group, a C.sub.6 -C.sub.10 -aryl group, a C.sub.6 -C.sub.10 -aryloxy group, a C.sub.2 -C.sub.10 -alkenyl group, a C.sub.7 -C.sub.20 -alkylaryl group, a C.sub.7 -C.sub.20 -arylalkyl group, a C.sub.8 -C.sub.20 -arylalkenyl group, hydrogen or a halogen atom, L.sub.1 and L.sub.2 are identical or different and are each an unsubstituted, monosubstituted or polysubstituted monocyclic or polycyclic hydrocarbon radical containing at least one cyclopentadienyl unit which can form a sandwich structure with M, R is carbon, silicon, germanium or tin, F and G are identical or different and are each a trimethylsilyl radical of the formula --Si(CH.sub.3).sub.3, where G may also be a C.sub.1 -C.sub.10 -alkyl radical, preferably a alkyl radical, or a C.sub.1 -C.sub.10 -aryl radical, .gamma.) modifying said supported metallocene catalyst as defined above. In the first stage .alpha.1), the oxide is preferably dehydrated in a stream of nitrogen or under reduced pressure at temperatures of from 110 to 800.degree. C. or 900.degree. C. over a period from 1 to 24 hours. The concentration of free hydroxyl groups, established as a function of the drying temperature selected, is then measured. The free hydroxyl groups can be reacted completely or partially with aluminoxanes or aluminium alkyls in stage .alpha.2). In stage .alpha.3), the dried oxide is reacted simultaneously with aluminoxanes and at least one polyfunctional organic crosslinker, which is suspended, for example, in a suitable hydrocarbon solvent such as toluene in such a way that it is covered with the solvent. The solvents for the aluminoxane and for the crosslinker have to be miscible and the same solvents are preferable used. Particular preference is given to using toluene. To prepare the solution needed in stage .alpha.3), the solvent used for the crosslinker can be the same as for the aluminoxane solution. Owing to the temperature dependence of the solubility of these crosslinkers in the solvent used, the desired concentration can be set in an targeted manner by the choice of the temperature of the solution. Particularly advantageous is the selection of a solvent whose boiling point is below the decomposition temperature of the solid prepared in stage .alpha.3). Preference is given to using aromatic solvents such as xylene, benzene or toluene. When using the polyfunctional crosslinkers in preparing the catalyst support in stage .alpha.3), it is also possible, in a further reaction stage, to deactivate unreacted reactive groups using, for example, alkylaluminium compounds, preferably using trimethylaluminium. The molar ratio between the aluminium used in stage .alpha.3) as aluminoxane and the crosslinker can vary within a wide range and is between 1 and 100, preferably between 1 and 40, particularly preferably between 10 and 25. It is dependent, in particular, on the type and pretreatment of the metal oxides, the type of aluminoxanes used, on the respective molar ratio of aluminium (as aluminoxane) to the surface hydroxyl groups on the metal oxide and on the type of crosslinker. Higher molar ratios of aluminium to the crosslinker are used particularly when use is made of tri- or higher functional crosslinkers which can form a correspondingly higher number of crosslinks. The suspended dried oxide from stage .alpha.1) is preferably Treated with a solution of aluminoxane and a solution of one or more polyfunctional organic crosslinkers in the same solvent. If desired, it is also possible in stage .alpha.2) to react the free hydroxyl groups of the oxide with an up to equimolar amount of an aluminoxane or an aluminium alkyl solution, for example trimethylaluminium prior to the crosslinking reaction. Preference is given to using methylaluminoxane for this purpose. It has been found to be particularly advantageous if all hydroxyl groups have been reacted. However, even a partial reaction of these groups gives a positive effect. According to the invention, in stage .alpha.3) for preparing the catalyst support, the metering in of the solutions is carried out simultaneously and continuously and the crosslinker solution may be heated or cooled if desired. The temperature to which the solution is heated or cooled depends on the solubility of the crosslinker in the solvent selected and on the desired crosslinking density on the support surface. The rate at which the two streams are metered in can be set by means of metering pumps and is in a range between 0.1 and 1000 ml per minute, preferably between 0.5 and 250 ml per minute, particularly preferably between 1 and 50 ml per minute. The reaction is preferably carried out in such a way that all the aluminoxane has been reacted after the simultaneous metering in of the two solutions. Under some circumstances, fluctuations in the reaction conditions on the industrial scale can be lead to unreacted aluminoxane remaining in the solution. The usable catalyst supports as described in EP 0 685 494 display a soluble aluminium proportion in the solvent wised of preferably less than 1.4 molt based on the aluminoxane used. In this case, it is possible to carry out one or more washing steps in order to reduce the concentration below the desired limit. It is also possible to add a further amount of alumoxane and to store the resulting suspension to improve the catalyst performance (DE-OS 19821370). After addition of the reactants in stage .alpha.3) for preparing the catalyst support is complete, the reaction mixture is stirred further for about 60 minutes and the solvent is then removed. The residue can be dried under reduced pressure, but it is preferably used further in the moist state. The metallocene compounds, used for the supported metallocene catalyst B are prepared by reacting metallocenes of the formula IV ##STR5## where L.sub.1, L.sub.2, F, G and R are as defined for the formula I and M' is an alkali metal, preferably lithium, with a compound of the formula V M(X').sub.2 X.sub.1 X.sub.2 (V) Where M, X.sub.1 and X.sub.2 are as defined for the formula I and X' is a halogen atom, preferably chlorine. Preparing the metallocene compounds B), the reaction of the dimetallated compound of the formula IV with the metal halide of the formula V can be carried out, for example, as described in EP 0 659 756. However, the reaction of the dimetallated compound of the formula IV with the metal halide of the formula V is advantageously carried out in solvent mixtures of aromatic and/or aliphatic hydrocarbons, which may also be halogenated, with dialkyl ethers, preferably alkane/ether mixtures such as, for example, hexane/ether mixtures. Aliphatic hydrocarbons can be, for example, all C.sub.5 -C.sub.12 -alkanes. Preference is given to n-pentane, n-hexane, n-heptane or cyclohexane. Among the dialkylethers, preference is given to all di-C.sub.2 -C.sub.4 -alkyl ethers, for example diethyl ether, di-n-propyl ether, diisopropyl ether, di-n-butyl ether or tert-butyl-methyl ether. Examples of suitable halogenated hydrocarbons are all C.sub.1 -C.sub.4 -chloroalkanes. Particular preference is given to dichloromethane. The preparation of the supported metallocene catalyst according to .beta. is carried out by suspending the catalyst support A) in an inert hydrocarbon, preferably toluene, and bringing it into contact with the metallocene compound B). In this procedure, the metallocene compound is dissolved, for example, in an inert hydrocarbon. Inert solvents which can be used are, for example, aliphatic or aromatic hydrocarbons, preferably toluene. The metallocene compounds B) are preferably used in an amount of from 0.3 wt % to 5 wt a based on the total mass of the supported catalyst. The mixing time is from 5 minutes to 24 hours, preferably from 1 to 1 2 hours. The mixing is carried out at a temperature of from -10 to +80.degree. C., in particular from 20 to 70.degree. C. The application of the metallocene compound B) is preferably carried out subsequent to the synthesis of the support A) in order to save a drying step. After the reaction is complete, the solvent is decanted and taken off under reduced pressure until a free-flowing solid remains. According to the invention the pre-polymerization of the supported catalyst is carried out in absence of a solvent or in an inert hydrocarbon solvent. The pre-polymerization temperature is from particularly -20 to 25.degree. C., preferably from 0 to 60.degree. C. The treatment of the supported catalyst may be carried out under reduced, atmospheric or elevated pressure batchwise or continuously. In a preferred embodiment the monomer feed during pre-polymerization reaction is at a rate from 0.05 to 20 g monomer/g supported catalyst/h, more preferably from 0.5 to 2 g monomer/g supported catalyst/h. According to the invention, the pre-polymerization of these monomers in presence of the supported catalyst is carried out preferably in gas phase, although pre-polymerization by pore filling of a dry porous catalyst with liquid monomer, by slurry phase polymerization using oil or grease as a slurry medium or by slurry phase polymerization using an evaporatable hydrocarbon as slurry medium are also possible. In a preferred embodiment especially for liquid-phase polymerization of propylene, the modification of the supported metallocene catalyst is carried out by pre-polymerization of 4-methyl-1-pentene. The modified supported catalyst of the invention is polymerization-active without further activating additives. However, it has been found to be particularly advantageous to use aluminium alkyls, preferably trimethylaluminium, triethylaluminium or triisobutylaluminium, as scavenger and as additional activator. The amount is, based on the aluminium, 50-5000 mol, preferably 100-500 mol, per mol transition metal of the metallocene compound in the modified supported catalyst. According to the invention the modified supported catalysts may be used for the polymerization of olefins or olefin mixtures, in particular C.sub.2 -C.sub.16 -.alpha.-olefins, preferably ethylene, propylene, 1-butene, 1-hexene, 1-octene, 4-methyl-1-pentene, cyclopentene, norbornene and/or norbornadiene. The modified supported catalysts make possible the preparation of homopolymers, copolymers and heterophasic copolymers. Most preferred is the use of the modified supported catalysts for the polymerization of propylene. The invention further provides a process for preparing polyolefins by polymerization or copolymerization of olefins, wherein the polymerization catalyst used is a modified supported catalyst according to the invention. In liquid phase polymerization, it is possible to use inert solvents, for example, aliphatic or cycloaliphatic hydrocarbons such as pentane, hexane or cyclohexane; toluene can also be used. Preference is given to carrying out the polymerization in the liquid monomer. If required, hydrogen is added as molecular weight regulator. The total pressure in the polymerization is usually 0.5-150 bar. Polymerization is preferably carried out in the pressure range of 1-40 bar. In the copolymerization of ethylene with propylene, the polymerization is preferably carried out in liquid propylene or in hexane as suspension medium. In the polymerization in liquid propylene, the ethylene is preferably fed in an amount such that a partial pressure ratio P.sub.propylene /P.sub.ethylene >0.5, in particular >1.0, is established over the liquid phase (P.sub.ethylene =partial pressure of the ethylene in the gas phase over the suspension; P.sub.propylene partial pressure of the propylene in the gas phase over the suspension). In the copolymerization in hexane as suspension medium, an ethylene/propylene gas mixture having a propylene content of from 1 to 50 mol %, preferably from 5 to 30 mol %, is fed in. The total pressure is kept constant during the polymerization by metering in a further amount. The total pressure is from 0.5 to 40 bar, preferably from 1 to 20 bar. The polymerization time is from 1 0 minutes to 6 hours, preferably from 30 minutes to 2 hours. The modified supported catalysts make possible the preparation of high molecular weight polyolefins and olefin copolymers without reactor fouling, without formation of polymer fines resulting from soluble catalyst components and disrupted catalyst components in the initial phase of the olefin polymerization, especially under technical liquid polymerization conditions. Particle size and particle size distribution may be controlled in all stages of polymerization process. The polyolefins and olefin copolymers are obtained in granular form even at high polymerization conversions, they have a high bulk density and a low fines content. The following examples illustrate the invention. Abbreviations used are: .sup.1 C-NMR .sup.13 C nuclear magnetic resonance spectroscopy d.sub.50 Mean particle diameter determined by sieve analysis .sup.1 H-NMR .sup.1 H nuclear magnetic resonance spectroscopy weight average molar mass in g/mol determined by GPC M.sub.n Number average molar mass in g/mol determined by GPC M.sub.w /M.sub.n Polydispersity MAO Methylaluminoxane MC Metallocene compound MS mass spectrometry T.sub.m Melting point determined by DSC cat.sub.pure supported catalyst without pre-polymer cat.sub.pre-pol pre-polymerized catalyst m.sub.cat weight of the supported catalyst M.sub.pre-pol weight of the pre-polymer TEAL triethyl aluminium MFR melt flow rate of PP in g/10 min measured at 230.degree. C. using a weight of 2.16 kg MFR.sub.2 melt flow rate of PE in g/10 min measured at 190.degree. C. using a weight of 2.16 kg MFR.sub.21 melt flow rate of PE in g/10 min measured at 190.degree. C. using a weight of 21.6 kg XS amount of polymer still soluble in xylene at 25 C after precipitation from high temperature solution |
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