WO2009063084A2

WO2009063084A2 - Process for the preparation of ultra high molecular weight polyethylene

Info

Publication number: WO2009063084A2
Application number: PCT/EP2008/065622
Authority: WO
Inventors: Timothy James Kidd; Bernardus Johanna Muskens; Van Gerardus Henricus Josephus Doremaele; Martin Alexander Zuideveld; Edwin Gerard Ijpeij; Van Wouter Meerendonk
Original assignee: Dsm Ip Assets Bv
Priority date: 2007-11-15
Filing date: 2008-11-14
Publication date: 2009-05-22
Also published as: WO2009063084A3

Abstract

The invention relates to a process for the preparation of ultra high molecular weight polyethylene comprising the steps of: (a) supporting a catalyst onto a support at a temperature of between -8O C and 100 C, said catalyst comprising an organometallic compound and an activator, characterized in that the organometallic compound is a compound according to formula 1 : wherein: M is a metal of group 3 -13 or the lanthanide series, and p is the valency of the metal M; A represents a neutral or anionic spectator ligand whose valency v is 0, 1 or 2, and q is an integer denoting the number of spectator ligands A; Z is an optional bridging moiety, and n is the integer number of parallel bridging moieties Z; Y is a ligand represented by formula 2: wherein the ligand is covalently bonded to the metal M via the imine nitrogen atom, Subi is a substituent, which comprises a group 14 atom through which Subi is bonded to the imine carbon atom, Sub2 is a substituent, which comprises a heteroatom of group 15-16, through which SUb2 is bonded to the imine carbon atom,; r is 1, 2, 3, 4 or 5; L is an optional neutral Lewis basic ligand, and j is an integer denoting the number of neutral ligands L, and X is an anionic ligand that may be independently selected from the group consisting of hydride, halide, alkyl, silyl, germyl, aryl, amide, aryloxy, alkoxy, phosphide, sulfide, acyl, pseudo halides such as cyanide, azide, and acetylacetonate, or a combination thereof; and (b) polymerizing ethylene in the presence of the supported catalyst of step (a), thereby producing ultra high molecular weight polyethylene.

Description

PROCESS FOR THE PREPARATION OF ULTRA HIGH MOLECULAR WEIGHT POLYETHYLENE

The invention relates to the process for the preparation of ultra high molecular weight polyethylene (UHMWPE). The invention further relates to a UHMWPE obtainable by said process, to the use of a supported catalyst for the preparation of UHMWPE, to a UHMWPE and to the use of said UHMWPE in a biomedical application.

UHMWPE has a molecular weight that is 10 to 20 times greater than the molecular weight of high-density polyethylene (HDPE). In addition to the chemical resistance, lubricity, and excellent electrical properties of conventional HDPE, UHMWPE offers major advantages in toughness, abrasion resistance, and stress-crack resistance. UHMWPE is herein defined as a substantially linear ethylene homopolymer or copolymer with a weight average molecular weight (Mw) of 400,000 g/mol or more, preferably with an Mw of 1.10⁶ g/mol or more.

UHMWPE can be produced by Ziegler polymerization, as illustrated in for example U.S. Pat. No. 5,756,600. The process requires exceptionally pure ethylene and other raw materials. An α-olefin comonomer, such as 1-butene, may be incorporated into UHMWPE according to U.S. Pat. No. 5,756,600. Like conventional HDPE, UHMWPE made by Ziegler polymerization has a broad molecular weight distribution, and usually its polydispersity Mw/Mn (Mw: weight average molecular weight, Mn: number average molecular weight) is within the range of 5 to 20.

WO2005/090418 discloses a process for the preparation of a polymer, including for example ultra high molecular weight polyethylene (UHMWPE), comprising at least one aliphatic or aromatic hydrocarbyl C₂-₂0 olefin in the presence of an ionic catalyst, comprising an organometallic compound and an activator. Batch polymerization of ethylene using a homogeneous catalyst resulted in UHMWPE with a molecular weight of at least 4 having a weight average molecular weight of at least 4.1O⁶ g/mol and a molecular weight distribution of less than 2.6. UHMWPE with a narrow molecular weight distribution (MJM_n < 2.6) has excellent mechanical properties compared to UHMWPE prepared by Ziegler polymerization, which has a broader molecular weight distribution (i.e M_w/M_n > 5.0). However, the processability of UHMWPE with a narrow molecular weight distribution is relatively poor. Therefore, there is a need for a process for preparing UHMWPE having a good balance between processability and mechanical properties

An object of the present invention is therefore to provide a process for the preparation of UHMWPE having a good balance between mechanical strength and processability.

Surprisingly the object is reached by the process according to the invention, which will be described in detail below.

In one embodiment of the present invention, there is provided a process for the preparation of UHMWPE comprising the steps of: a) supporting a catalyst onto a support at a temperature of between -8O⁰C and

100⁰C, said catalyst comprising an organometallic compound and an activator, characterized in that the organometallic compound is a compound according to formula 1 :

(formula 1 ) wherein:

M is a metal of group 3 -13 or the lanthanide series, and p is the valency of the metal

M;

A represents a neutral or anionic spectator ligand whose valency v is 0,1 or 2, and q is an integer denoting the number of spectator ligands A; Z is an optional bridging moiety, and n is the integer number of parallel bridging moieties Z;

Y is a ligand represented by formula 2:

N

Sub₁^^Sub₂ (formula 2), wherein the ligand is covalently bonded to the metal M via the imine nitrogen atom, Subi is a substituent, which comprises a group 14 atom through which Subi is bonded to the imine carbon atom,

Sub₂ is a substituent, which comprises a heteroatom of group 15-16, through which Sub₂ is bonded to the imine carbon atom,; r is 1 , 2, 3, 4 or 5; L is an optional neutral Lewis basic ligand, and j is an integer denoting the number of neutral ligands L, and

X is an anionic ligand that may be independently selected from the group consisting of hydride, halide, alkyl, silyl, germyl, aryl, amide, aryloxy, alkoxy, phosphide, sulfide, acyl, pseudo halides such as cyanide, azide, and acetylacetonate, or a combination thereof; and

(b) polymerizing ethylene in the presence of the supported catalyst of step (a), thereby producing ultra high molecular weight polyethylene.

It has been surprisingly found that the UHMWPE obtained when applying the supported catalyst system in the above process has an increased molecular weight distribution (M_w/M_n), i.e. typically between 2.8 and 5.0, compared to the unsupported catalyst systems described in the prior art. This result is particularly surprising, given that the molecular weight distribution of UHMWPE has often been considered to increase slightly, but in general not significantly. For example, in a recent review (Severn et al, Chem. Rev. 2005, 105 (11 ), 4073-4147) it is described that generally the molecular weight distribution broadens somewhat (page 4083), but that this is often a small increase. On page 4094 the use of an organometallic catalyst supported on a borate solid activator complex even resulted in a narrower molecular weight distribution compared to the homogeneous catalyst (2.2. for the supported catalyst and 3.1 for the homogeneous catalyst). Moreover, WO 03/059968 discloses the preparation of UHMWPE using a supported transition metal catalyst having at least one quinolinoxy ligand. The examples and comparatives experiments using an unsupported catalyst show only a slight increase of M_w/M_n (at most 9%). The highest recorded MJM_n was 2.72. In contrast, the use of supported catalyst as in the present invention has been found to increase the M_w/M_n from below 2.6, as described in

WO2005/090418, to at least 2.8, preferably at least 2.9 and more preferably at least 3.0. The upper limit of the M_w/M_n is preferably less than 5, less than 4.5, less than 4.0 or less than 3.5 though particular embodiments may use a higher M_w/M_n ratio (eg. less than 10). Not only does the process of the present invention produce

UHMWPE with a M_w/M_n, such that a good balance of processability and mechanical characteristics is achieved (eg. 2. < M_w/M_n < 5.0), it has been surprisingly found that the increase in M_w/M_n may be controlled through controlling the temperature at which the catalyst is supported onto the support. This temperature is chosen between -8O⁰C and 100⁰C, preferable between O⁰C and 100⁰C and more preferable between 2O⁰C and - A -

8O⁰C. The temperature at which the catalyst is supported onto the support can be adjusted to achieve the desired M_w/M_n value.

Properties that determine the ease of handling UHMWPE are for example the dry flow and the bulk density. Parameters expressing the mechanical properties are for example abrasion and wear resistance, stiffness and tensile strength. A narrower molecular weight distribution has been shown to be particularly beneficial for abrasive wear as shown in T.A. Tervoort et al., Macromolecules, 2002, 35, p. 8467-8471. For medical implants, for example hip or knee implants, wear resistance is a very important property. Furthermore, for fiber applications, a narrower molecular weight distribution improves stiffness and fiber strength.

The supported catalyst can be prepared by contacting an organometallic compound with a supported activator, for example in a dispersant such as toluene, pentane, hexane, heptane, chloroform, dichloromethane or other common solvents, wherein the organometallic compound is soluble. The supported activator is prepared by contacting an activator with a solid support for example in a dispersant as mentioned above. In a preferred method the supported activator is prepared by reacting an aluminum (oxide) activator with a solid oxide, for example in a dispersant as mentioned above In one preferred embodiment, the temperature at which the activator is supported onto the support is at least 2O⁰C, more preferably 3O⁰C and even more preferably at least 4O⁰C. A higher temperature results in a higher loading of the activator, and, consequently, also a higher loading of the organometallic compound with the same AhTi ratio. The upper limit of the temperature at which the activator is supported onto the support is preferably at most the boiling temperature of the solvent, for example 7O⁰C, more preferably at most 8O⁰C and even more preferably at most 100⁰C.

The solid support is defined as an inorganic or organic compound that does not dissolve in the inert hydrocarbon solvent in which the process of the invention is carried out. Suitable inorganic supports include silica, magnesium halides, such as MgF₂, MgCI₂, MgBr₂, MgI₂, zeolites, alumina, and silica aluminates. Suitable organic supports include polymers. Some non-limiting examples of polymeric supports are polyolefins such as polystyrene, polypropylene and polyethylene, polycondensates such as polyamides and polyesters, which can be modified to allow the supporting of the organometallic compound or the activator, and combinations thereof. Preferably, the support is selected the group consisting of silica, alumina, silica-alumina, magnesia, titania, zirconia, magnesium chloride, and crosslinked polystyrene. More preferably, the support is selected from a group consisting of silica, alumina, silica-alumina, and magnesium chloride, and even more preferably, the support comprises or consists of silica or magnesium chloride.

The particle size of the support can for example be expressed by D50, D10 and D90, representing the median or the 50th percentile, the 10^th percentile and the 90th percentile of the particle size distribution, respectively, as measured by volume according to ISO 13320-2. That is, the D50 (D10 or D90) is a value on the distribution such that 50% (10% or 90%) of the particles have a volume of this value or less.

The D50 of the support according to the invention is preferably between 2 and 30 μm, more preferably between 4 and 20 μm. Examples of suitable supports are given in the Examples. In a special embodiment, the support is halogen free, preferably in combination with a halogen free catalyst and halogen free activator. The use of a catalyst system free or substantially free of halogen results in UHMWPE which is substantially free of halogen (i.e. < 1 ppm). The production of UHMWPE which is substantially free of halogen may be advantageously used in for example biomedical applications, wherein an inert UHMWPE is preferred. Indeed, inertness combined with good processability and good mechanical properties are particularly desired traits of UHMWPE used in biomedical applications and fibers.

In the process of the invention the catalyst system comprises an activator. Suitable activators include alumoxanes, alkyl aluminums, alkyl aluminum halides, anionic compounds of boron or aluminum, trialkylboron compounds, triarylboron compounds, and mixtures thereof.

Examples are methyl alumoxane (MAO) and polymeric MAO (PMAO), triethylaluminum, trimethylaluminum, diethylaluminum chloride, lithium tetrakis(pentafluorophenyl)borate, triphenylcarbenium tetrakis(pentafluorophenyl)borate, N,N-dimethyl anilinium tetrakis(pentafluorophenyl)borate, lithium tetrakis(pentafluorophenyl)aluminate, tris(pentafluorophenyl)boron, tris(pentabromophenyl)boron, and the like. Preferably, the activator is a MAO.

In one embodiment of the invention ethylene is substantially homopolymerized. In another embodiment of the invention ethylene is copolymerized with up to about 10 % by weight of a comonomer based on the total weight of monomer and comonomer, more preferably from about 0 to about 8 % by weight, most preferably from about 0 to about 5 % by weight and in particular from about 0 to about 1 % by weight. In general, in the presence of a comonomer content, e.g., creep, cross- linkability, impact resistance, flexibility and transparency are improved.

A comonomer is understood to be a molecule containing at least one polymerizable double bond, which differs from ethylene. Suitable comonomers are C₂-₂0 olefins. Preferred monomers include C_3--_I2 α-olefins which are unsubstituted or substituted by up to two Ci_6 alkyl radicals, Ce--_I2 vinyl aromatic monomers which are unsubstituted or substituted by up to two substituents selected from the group consisting of Ci_-4 alkyl radicals, and C_4-I2 straight chained or cyclic hydrocarbyl radicals which are unsubstituted or substituted by a C_1-4 alkyl radical. Illustrative non-limiting examples of such α-olefins are propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-undecene, 1-dodecene, 1-tridecene, 1-tetradecene, 1-pentadecene, 1-hexadecene, 1-heptadecene, 1-octadecene, 1-nonadecene, 1- eicosene, 3-methyl-1-butene, 3-methyl-1-pentene, 3-ethyl-1-pentene, 4-methyl-1- pentene, 4-methyl-1-hexene, 4,4-dimethyl-1-hexene, 4,4-dimethyl-1-pentene, 4-ethyl-1- hexene, 3-ethyl-1-hexene, 9-methyl-1-decene, 1 1-methyl-1-dodecene and 12-ethyl-1- tetradecene. These α-olefins may be used in combination.

The comonomer may also be a polyene comprising at least two double bonds. The double bonds may be conjugated or non-conjugated in chains, ring systems or combinations thereof, and they may be endocyclic and/or exocyclic and may have different amounts and types of substituents. This means that the polyene may comprise at least one aliphatic, alicyclic or aromatic group, or combinations thereof.

Suitable polyenes include aliphatic polyenes and alicyclic polyenes. More specifically, aliphatic polyenes can be mentioned, such as 1 ,4-hexadiene, 3- methyl-1 ,4-hexadiene, 4-methyl-1 ,4-hexadiene, 5-methyl-1 ,4-hexadiene, 4-ethyl-1 ,4- hexadiene, 1 ,5-hexadiene, 3-methyl-1 ,5-hexadiene, 3,3-dimethyl-1 ,4-hexadiene, 5- methyl-1 ,4-heptadiene, 5-ethyl-1 ,4-heptadiene, 5-methyl-1 ,5-heptadiene, 6-methyl-1 ,5- heptadiene, 5-ethyl-1 ,5-heptadiene, 1 ,6-heptadiene, 1 ,6-octadiene, 4-methyl-1 ,4- octadiene, 5-methyl-1 ,4-octadiene, 4-ethyl-1 ,4-octadiene, 5-ethyl-1 ,4-octadiene, 5- methyl-1 ,5-octadiene, 6-methyl-1 ,5-octadiene, 5-ethyl-1 ,5-octadiene, 6-ethyl-1 ,5- octadiene, 1 ,6-octadiene, 6-methyl-1 ,6-octadiene, 7-methyl-1 ,6-octadiene, 6-ethyl-1 ,6- octadiene, 6-propyl-1 ,6-octadiene, 6-butyl-1 ,6-octadiene, 1 ,7-octadiene, 4-methyl-1 ,4- nonadiene, 5-methyl-1 ,4-nonadiene, 4-ethyl-1 ,4-nonadiene, 5-ethyl-1 ,4-nonadiene, 5- methyl-1 ,5-nonadiene, 6-methyl-1 ,5-nonadiene, 5-ethyl-1 ,5-nonadiene, 6-ethyl-1 ,5- nonadiene, 6-methyl-1 ,6-nonadiene, 7-methyl-1 ,6-nonadiene, 6-ethyl-1 ,6-nonadiene, 7- ethyl-1 ,6-nonadiene, 7-methyl-1 ,7-nonadiene, 8-methyl-1 ,7-nonadiene, 7-ethyl-1 ,7- nonadiene, 1 ,8-nonadiene, 5-methyl-1 ,4-decadiene, 5-ethyl-1 ,4-decadiene, 5-methyl- 1 ,5-decadiene, 6-methyl-1 ,5-decadiene, 5-ethyl-1 ,5-decadiene, 6-ethyl-1 ,5-decadiene, 6-methyl-1 ,6-decadiene, 6-ethyl-1 ,6-decadiene, 7-methyl-1 ,6-decadiene, 7-ethyl-1 ,6- decadiene, 7-methyl-1 ,7-decadiene, 8-methyl-1 ,7-decadiene, 7-ethyl-1 ,7-decadiene, 8- ethyl-1 ,7-decadiene, 8-methyl-1 ,8-decadiene, 9-methyl-1 ,8-decadiene, 8-ethyl-1 ,8- decadiene, 1 ,9-decadiene, 1 ,5,9-decatriene, 6-methyl-1 ,6-undecadiene, 9-methyl-1 ,8- undecadiene and 1 ,13-tetradecadiene, 1 ,3-butadiene, isoprene.

Alicyclic polyenes may consist of at least one cyclic fragment. Examples of these alicyclic polyenes are vinylcyclohexene, vinylnorbornene, ethylidene norbornene, dicyclopentadiene, cyclooctadiene, 2,5-norbornadiene, 1 ,4- divinylcyclohexane, 1 ,3-divinylcyclohexane, 1 ,3-divinylcyclopentane, 1 ,5- divinylcyclooctane, i-allyl-4-vinylcyclohexane, 1 ,4-diallylcyclohexane, 1 -allyl-5- vinylcycloocatane, 1 ,5-diallylcyclooctane, 1-allyl-4-isopropenylcyclohexane, 1- isopropenyl-4-vinylcyclohexane and 1-isopropenyl-3-vinylcyclopentane, and 1 ,4- cyclohexadiene. Preferred polyenes are polyenes having at least one endocyclic double bond and optionally at least one exocyclic double bond, such as 5-methylene-2- norbornene and 5-ethylidene-2-norbornene, 5-vinylnorbornene, and 2,5-norbornadiene, dicyclopentadiene, vinylcyclohexene and the like.

Examples of aromatic polyenes are divinylbenzene (including its isomers), trivinylbenzene (including its isomers) and vinylisopropenylbenzene (including its isomers).

In some embodiments, the comonomer may include a sulphur containing compound.

All of the above-mentioned comonomers may be further substituted with at least one group comprising a heteroatom of group 13-17, or combinations thereof.

Homopolymers, copolymers and terpolymers of ethylene and the above-mentioned comonomers and blends thereof can be prepared with the process of the present invention. The process according to the invention involves a polymerization which can be carried out in any manner known in the art. Processes that can polymerize monomers into polymers include, but are not limited to, slurry polymerizations, gas phase polymerizations, and multi-reactor combinations thereof. Thus, any polymerization zone known in the art to produce olefin-containing polymers can be utilized. For example, a stirred reactor can be utilized for a batch process, or the reaction can be carried out continuously in a loop reactor or in a continuous stirred reactor. Suitable diluents used in slurry polymerization are well known in the art and include hydrocarbons which are liquid under reaction conditions. The term "diluent" as used in this disclosure does not necessarily mean an inert material, as this term is meant to include compounds and compositions that may contribute to polymerization process. Examples of hydrocarbons that can be used as diluents include, but are not limited to, cyclohexane, isobutane, toluene, n-butane, propane, n-pentane, isopentane, neopentane, n-hexane and n-heptane. Typically, isobutene or n-heptane is used as the diluent in a slurry polymerization. Examples of this technology are found in U.S. Pat. Nos. 4,424,341 ; 4,501 ,885; 4,613,484; 4,737,280; and 5,597,892; each of which is incorporated by reference herein, in its entirety.

The catalysts used in the process of the present invention are described in WO2005/090418 which is incorporated herein by reference. The catalyst used in the process of the invention comprises an organometallic compound and an activator. The metal (M) in the organometallic compound of formula 1 represents an atom of group 3 - 13 or the lanthanide series. Preferably, the metal is chosen from group 3, 4, 5, 6 or 7, or the lanthanide series, more preferably from group 4-7. Even more preferably, the metal is chosen from Group 4. Most preferably, the metal is Ti.

In the organometallic compound used in the process of the invention, A is a neutral or anionic spectator ligand, and q is an integer denoting the number of spectator ligands A. The valency v of A is 0, 1 , or 2. Examples of monoanions are carbanions, silylanions, germylanions, amides, phosphides, imines, and chalconides. Examples of dianionic ligands are biphenoxides, cyclooctatetraenides, boroles and the like.

The spectator ligand A is preferably an imine ligand, a chalconide, or a cyclopentadienyl-containing ligand.

An imine ligand is defined as a group containing a double bonded nitrogen atom. Examples of imine ligands are ketimine, guanidine, phosphinimine, iminoimidazolidine, (hetero)aryloxyimines, pyrroleimines, indoleimines, imidazoleimines or (hetero)aryloxides, (substituted) pyridin-2-yl-methoxy, (substituted) quinolin-2-yl- methoxy, 8-hydroxyquinoline, 8-aminoquinoline, 8-phosphinoquinoline, 8-thioquinoline, 8-hydroxyquinaldine, 8-aminoquinaldine, 8-phosphinoquinaldine, 8-thioquinaldine and 7-azaindole or indazole and the like.

A cyclopentadienyl-containing ligand comprises at least one cyclopentadienyl (Cp) ring. This ring may be substituted with at least one R' group. When the Cp ring is substituted with at least two R' groups, these R' groups may form at least one ring system. As result, the Cp-containing ligand may be an indenyl or fluorenyl group.

The R' groups may each independently be hydrogen or a hydrocarbon radical with 1-20 carbon atoms (e.g alkyl, aryl, biaryl, aralkyl, alkaryl and the like) or a heteroatom comprising a moiety from group 13-17. Examples of such hydrocarbon radicals are methyl, ethyl, n-propyl, i-propyl, butyl (including isomers), hexyl (including isomers), decyl (including isomers), phenyl, biphenyl (including isomers) and the like. Examples of heteroatom-containing moieties of group 13-17 are borane radicals, silyl radicals, germyl radicals, stannyl radicals, amide radicals, phosphide radicals, oxide radicals, sulphide radicals, halide radicals, halide substituted hydrocarbyl radicals and the like. Also, two adjacent hydrocarbon radicals may be connected with each other resulting in a ring system. Such a group may also contain one or more R' groups as substituents. R' may also be a substituent which instead of or in addition to carbon and/or hydrogen may comprise one or more heteroatoms of groups 13-17.

Suitable ligands A are (substituted) cyclopentadienyl groups, (substituted) indenyl groups, (substituted) fluorenyl groups, (substituted) tetrahydroindenyl groups, (substituted) tetrahydrofluorenyl groups, (substituted) octahydrofluorenyl groups, (substituted) benzoindenyl groups, (substituted) heterocyclopentadienyl groups, (substituted) heteroindenyl groups, (substituted) heterofluorenyl groups, or their isomers. A heterocyclopentadienyl group (hereinafter referred to as 'hetero ligand') is understood to be a group that has been derived from a cyclopentadienyl group, but in which at least one of the C atoms in the 5-ring of the cyclopentadienyl has been replaced by a hetero atom, which heteroatom may be chosen from group 14, 15 or 16. If there is more than one heteroatom present in the 5-ring of the hetero ligand, these heteroatoms may be the same or different. More preferably, the heteroatom is chosen from group 15, while yet more preferably the heteroatom is phosphorus.

If ligand A is a neutral ligand, this ligand may be as defined under L. In the organometallic compound used in the process of the invention Z is an optional bridging moiety, and n is the integer number of parallel bridging moieties Z. In case of n = 0, there is no bridge between A and Y. The optional bridging group Z may contain sp³, sp² or sp hybridized atoms of group 13 to 16 or combinations thereof. The bridging group Z may consist of linear, cyclic fragments, spiro ring systems, or combinations thereof. Examples of a carbon containing Z group may be a hydrocarbon group with 1-20 carbon atoms, e.g. alkylidene, arylidene, biarylene, aryl alkylidene, etc. Examples of such groups are methylene, ethylene, propylene, butylene, phenylene, naphthylene, biphenylene, binaphthylene. Examples of silicon-containing groups are dimethylsilyl, diethylsilyl, dipropylsilyl, including its isomers, (substituted) diphenylsilyl, dimethoxysilyl, diethoxysilyl, dipropoxysilyl, and diphenoxysilyl. In the organometallic compound used in the process of the invention

Y is a ligand, and r is an integer with r > 0. A spectator ligand is a ligand that is represented by formula 2. The amidine-containing ligand is covalently bonded to the metal via the imine nitrogen atom. This means that the imine nitrogen atom of the imine does not have any substituents but the imine carbon atom. Subi comprises a group 14 atom through which Subi is bonded to the imine carbon atom. Sub₂ comprises a heteroatom of group 15-16, through which Sub₂ is bonded to the imine carbon atom. Preferably this atom is selected from the group of nitrogen, phosphorus, oxygen or sulfur. More preferably, the heteroatom is a nitrogen, such that Y is a imidine containing spectator ligand. Subi preferably represents a hydrocarbyl radical, optionally substituted with heteroatoms of group 13 - 17, or a silyl radical, optionally substituted with group 13-17 atoms.

Sub₂ preferably is an amide, imide, phosphide, phospinimide, oxide, sulphide radical, optionally substituted with hydrocarbyl radicals or silyl radicals as described for Subi. Subi or Sub₂ may be bonded to the bridging moiety Z or may be part of a ring system, which ring system may be bonded to the bridging moiety Z.

In the organometallic compound used in the process of the invention L is optionally a neutral Lewis basic ligand, and j is an integer denoting the number of neutral ligands L. The ligand L may be present in the organometallic compound for reasons of stability. If the ligand L is present, L is an ether, a thioether, a tertiary amine, a tertiary phosphane, an imine, or a bi-, or oligodentate, comprising an ether, a thioether, a tertiary amine, or a tertiary phosphane functional group, or combinations thereof. Suitable ethers are tetrahydrofuran and diethylether. Suitable thioethers are thiophene, diethylsulfide, and dimethylsulfide. Suitable tertiary amines are trialkylamines, pyridine, bipyridine, TMEDA, and (-)-sparteine). Suitable tertiary phosphanes are triphenylphoshine, trialkylphosphanes. Suitable of imines are ketimines, guanidines, iminoimidazolidines, phosphinimines, amidines and the like. Suitable bidentate ligands are diimines, alkyl or aryldiphoshanes, dimethoxyethane. Suitable oligodentate ligands are triimines (such as tris(pyrazolyl)alkanes), cyclic multidentate ligands comprising heteroatoms of group 13-17, including crown ethers optionally having heteroatoms of group 13-17, azo-crown ethers optionally having heteroatoms of group 13-17, phospha-crown ethers optionally having heteroatoms of group 13-17, crown ethers having combinations of heteroatoms of group 15-16 optionally having heteroatoms of group 13-17 and crown ethers containing heteroatoms of group 14-17 or combinations thereof. In the catalyst used in the process of the invention, X is an anionic ligand. Each anionic ligand, X, bonded to M, may be independently selected from the group consisting of hydride, halide, alkyl, silyl, germyl, aryl, amide, aryloxy, alkoxy, phosphide, sulfide, acyl, pseudo halides such as cyanide, azide, and acetylacetonate, or a combination thereof. Preferably, X is a hydride or a moiety selected from the group consisting of monoanionic spectator ligands, halide, alkyl, aryl, silyl, germyl, aryloxy, alkoxy, amide, siloxy and combinations thereof (e.g. alkaryl, aralkyl, silyl substituted alkyl, silyl substituted aryl, aryloxyalkyl, aryloxyaryl, alkoxyalkyl, alkoxyaryl, amidoalkyl, amidoaryl, siloxyalkyl, siloxyaryl, amidosiloxyalkyl, haloalkyl, haloaryl, etc.) having up to 20 non-hydrogen atoms. Preferred anionic ligands X include halides and hydrocarbyl anions. A preferred halide is chloride, a preferred hydrocarbyl anion is methyl. In one embodiment of the invention hydrocarbyl groups are anionically charged hydrocarbyl groups. In addition to the usual definition of a hydrocarbyl group, in this application a hydrocarbyl group also comprises a hydride group. The hydrocarbyl groups optionally contain heteroatoms of group 13-17. Preferred hydrocarbyl groups include hydride, alkyl-, aryl-, aralkyl-, alkaryl-, substituted vinyl- and substituted allylgroups. More preferred hydrocarbyl groups include hydride, alkyl-, aryl-, aralkyl- and alkaryl groups. Most preferred hydrocarbyl groups include alkyl-, aryl-, aralkyl- and alkaryl groups. Examples of such most preferred hydrocarbyl groups are methyl, benzyl, methyltrimethylsilyl, phenyl, methoxyphenyl, dimethoxyphenyl, N,N-dimethylaminophenyl, bis (N,N-dimethylamino)phenyl, fluorophenyl, difluorophenyl, trifluorophenyl, tetrafluoropheny, perfluorophenyl, trialkylsilylphenyl, bis(trialkylsilyl)phenyl, tris(trialkylsilyl)phenyl and the like.

The number of ligands (X and L) depends on the valency of the metal and the stability of the organometallic compound. The organometallic compound may be monomeric, oligomeric or a cluster. The number of anionic ligands equals the valency of the metal used. The number of neutral ligands on the organometallic reagent may range from 0 to the amount that satisfies the 18-electron rule, as known in the art. In the process of the invention the catalyst optionally comprises a scavenger. A scavenger is a compound that reacts with impurities present in the process of the invention, which are poisonous to the catalyst. A scavenger in an embodiment of the invention can be a hydrocarbyl of a metal or metalloid of group 1-13 or its reaction products with at least one sterically hindered compound containing a group 15 or 16 atom. Preferably, the group 15 or 16 atom of the sterically hindered compound bears a proton. Examples of these sterically hindered compounds are tert-butanol, iso-propanol, triphenylcarbinol, 2,6-di-tert-butylphenol, 4-methyl-2,6-di-tert- butylphenol, 4-ethyl-2,6-di-tert-butylphenol, 2,6-di-tert-butylanilin, 4-methyl-2,6-di-tert- butylanilin, 4-ethyl-2,6-di-tert-butylanilin, HMDS (hexamethyldisilazane), di- isopropylamine, di-tert-butylamine, diphenylamine and the like. Some non-limiting examples of scavengers are butyllithium including its isomers, dihydrocarbylmagnesium, trihydrocarbylaluminium, such as trimethylaluminium, triethylaluminium, tripropylaluminium (including its isomers), tributylaluminium (including its isomers) tripentylaluminium (including its isomers), trihexyl aluminium (including its isomers), triheptyl aluminium (including its isomers), trioctyl aluminium (including its isomers), hydrocarbylaluminoxanes and hydrocarbylzinc and the like, and their reaction products with a sterically hindered compound or an acid, such as HF, HCI, HBr, HI.

The UHMWPE according to the invention has an M_w/M_n, as measured by high-temperature SEC-MALS, of 2.8 or more, 2.9 or more, or 3.0 or more. The UHMWPE according to the invention has an M_w/M_n, of 5.0 or less, 4.5 or less, 4.0 or less, or 3.5 or less.

The UHMWPE produced by the process according to the invention preferably has a D50 (determined according to ISO 13320-2) of between 50 and 400 μm, preferably between 80 and 300 μm, more preferably between 100 and 200 μm. The D50 is important for the processability of the UHMWPE powder. Generally a UHMWPE with a D50 < 80 causes dust issues and can increase the risk of dust explosion. Moreover too small particles make the powder difficult to handle, while a UHMWPE with a D50>400 may be a problem as they tend to stand out after consolidation as a white spot in the consolidated sheet or rod. For fiber applicatuions too large particles may be difficult to dissolve.

The bulk density of the UHMWPE according to the invention is preferably higher than 250 kg/m³, more preferably higher than 300 kg/m³, in particular higher than 350 kg/m³. The dry flow of the UHMWPE according to the invention is preferably less than 60 seconds, more preferably less than 40 seconds, in particular less than 30 seconds.

The UHMWPE according to the invention preferably has a residual halogen content of less than 1 ppm, a residual metal content of less than 30 ppm, more preferably less than 20 ppm, in particular less than 10 ppm.

The UHMWPE according to the invention can suitably be used in biomedical applications, for example in orthopedics as bearing material in artificial joints. In particular, the UHMWPE according to the invention can be used in for example hip arthroplasty, knee replacements, shoulder replacements and spinal applications such as total disc replacement. These applications are described in detail in for example Steven M. Kurtz in "The UHMWPE Handbook", Elsevier Academic Press, 2004, p. 22-31 in Chapters 4-6 (hip), 7-8 (knee), 9 (shoulder) and 10 (spinal applications), which is herein incorporated by reference. The UHMWPE according can also be used in for example in industrial applications such as pickers for textile machinery, lining for coal chutes and dump trucks, runners for bottling production lines, as well as bumpers and siding for ships and harbors, and in fibers.

The invention is further illustrated by the following examples.

EXAMPLES

Characterization methods

CP-AES

Determination of the Ti and Al contents was performed via ICP-AES according to the following method. The sample was destructed with the use of sulfuric and nitric acid at elevated temperatures. The insoluble support remains were removed via filtration and the contents of dissolved titanium and aluminium were measured on a Perkin Elmer Optima 3000.

Conditions: Plasma gas flow: 15L/min Auxiliary gas flow: 0.5 L/min Nebulizer gas flow: 0.8 L/min RF Power: 1300 Watt Pump: 1 ml/min Sample tube: black - black, internal diameter 0.030mm

XRF

Determination of the Si, Al and Ti contents in the polymer samples was performed on a WDXRF (wave-length dispersive X-ray fluorescence) spectrometer, type PANalytical PW2404 with SuperQ software.

SEC-MALS

The molecular mass distributions were measured using a PL-210

Size Exclusion Chromatograph coupled to a refractive index detector (PL) and a multi- angle light scattering (MALS) detector from Wyatt (type DAWN EOS). Two PL-Mixed A columns were used. 1 ,2,4 trichlorobenzene was used as the solvent, the flow rate was

0.5 ml/min, and the measuring temperature was 160⁰C. Data acquisition and calculations were carried out via Wyatt (Astra) software.

Other measurement details are: Injection volume: 0.3 ml

Software: Wyatt Astra 4.90.08

Solvent/eluent: distilled 1 ,2,4-trichlorobenzene with about 1 g/l of lonol stabilizer

Flow: 0.5 ml/min.

Temperature: 16O ⁰C Refractive Index increment: 0.097 ml/g

Equipment: PL210 (Polymer Laboratories) SEC with build-in PL210 Refractive Index detector. Wyatt DAWN EOS Light scattering detector (laser wavelength 690 nm), sequentially coupled to the refractive index detector

Solvent degasser: Polymer Laboratories PL-DG802 Columns: 2x PL-Mixed A Sample concentration: about 0.01 weight %

The UHMWPE should be completely dissolved under such conditions that polymer degradation is prevented by methods known to a person skilled in the art.

Dry flow

The dry flow (s) was measured according to the method described in ASTM D 1895-69, Method A; 23°C / 50% relative humidity (RH).

Bulk density (poured) The bulk density (kg/m³) was measured according to the method described in DIN 53466; ISO 60; 23°C / 50% RH.

Average particle size

The average particle size was measured on a Malvern LLD according to ISO 13320-2. The particle size distribution was calculated via the formula (D90-D10)/D50.

Materials

Solid support materials (Table 1 ) were obtained from W.R. Grace & Co. and Fuji Silysia Chemical Ltd.

Table 1. Silica solid support materials used.

Toluene was dried by distillation from sodium using benzophenone as indicator. MAO (30 wt% in toluene) was obtained from Albemarle (MAO-1 ),

MAO 10T (10 wt% in toluene) was obtained from Crompton (MAO-2).

All manipulations were performed under an inert nitrogen atmosphere using standard Schlenk techniques.

Preparation of Compound 1 : MesCpTiMe_?(NC(2, 6-F₇PH)CPr₇N)

Compound 1 was prepared as described in patent WO 2005/090418 (designated compound 10M in the patent).

Preparation of SiO^/MAO (SM-A) 5.6 g of silica D was dried for 4h at 220⁰C in a vacuum.

Subsequently, the powder was slurried in 100 ml. of toluene before slowly adding 15 ml. MAO-1.The suspension was heated to boiling point and refluxed for 2 hours. Subsequently the slurry was allowed to cool to room temperature and filtered. The powder was dried in a vacuum for 2h at 175°C. Subsequently, the powder was washed with 3 x 100 mL toluene. Finally the powder was dried under vacuum at 120⁰C for 1 h.

Preparation of SiCb/MAO (S/M-B) Similar to SiO₂/MAO-A, but with silica C as solid support

Preparation of SiCb/MAO (S/M-C)

Similar to SiO₂ZMAO-A, but with silica B as solid support

Preparation Of SiO₇ZMAO (S/M-D)

Similar to SiO₂/MAO-B, but with 8 mL of MAO-1.

Preparation of SiCVMAO (S/M-E)

5.0 g of silica C was dried for 4h at 220⁰C under vacuum. Subsequently, the powder was slurried in 100 mL of toluene before slow addition of 8 mL MAO-1. The suspension was heated to boiling point and refluxed for 1 hour. Subsequently the slurry was allowed to cool to room temperature and filtered. The powder was dried under vacuum for 1 h at 160°C. Subsequently, the powder was washed with 3 x 100 mL toluene. Finally the powder was dried under vacuum at 120⁰C for 1 h.

Preparation of SiO^/MAO (S/M-F)

9.0 g of silica A was dried for 3h at 150⁰C under vacuum. Next,

200 mL MAO-2 was added slowly. The suspension was stirred at room temperature for 16 h before the solvent was removed under vacuum. The powder was dried under vacuum for 2h at 175°C. Next, the powder was washed with 2 x 100 mL toluene at 90⁰C. Finally the powder was dried under vacuum at 120⁰C for 1 h.

Preparation of SiO^MAO (S/M-G) 9.0 g of silica A was dried for 3h at 150°C under vacuum. Next,

200 mL MA0-2was added slowly. The suspension was stirred at room temperature for 16 h before the solvent was removed under vacuum. The powder was dried under vacuum for 2h at 175°C. Next, the powder was washed with 2 x 100 mL toluene at 90°C. Finally, the powder was dried under vacuum at 120°C for 1 h. Preparation of SiCb/MAO (S/M-H)

3.8 g of silica A was dried for 4h at 220⁰C under vacuum. Next, the powder was slurried in 50 ml. of toluene prior to slow addition of 30 ml. MAO-2. The suspension was heated to boiling point and refluxed for 2 hours. Next the slurry was allowed to cool to room temperature and then filtered. The powder was dried under vacuum for 2h at 175°C and then the powder was washed with 2 x 100 ml. toluene. Finally the powder was dried under vacuum at 120⁰C for 1 h.

Preparation Of SiO₇ZMAO (S/M-l) 5.0 g of silica A was dried for 3h at 175°C in a vacuum. Next slowly

100 ml. MAO-2 was added. The suspension was stirred at room temperature for 16 h before the solvent was removed under vacuum. The powder was dried in a vacuum for

2h at 175°C. Next the powder was washed with 2 x 100 ml. toluene at 90⁰C. Finally the powder was dried in vacuum at 120⁰C for 1 h.

Preparation of SiCVMAO/compound 1 (S/M/C1-I)

1.15g of SiO₂/MAO (S/M-H) was slurried in 100 ml. heptane. Next,

2.742 ml. of a Compound 1 solution in toluene (15 mg/ml_, 41.13 mg) was added. The suspension was slowly stirred overnight and used as such.

Preparation of SiO?/MAO/compound 1 (S/M/C1-II)

2.Og of SiO₂/MAO (S/M-A) was slurried in 100 ml. toluene. Next,

4.0 ml. of a Compound 1 solution in toluene (15 mg/mL, 60 mg) was added. The suspension was slowly stirred overnight and used as such.

Preparation of SiO^/MAO/compound 1 (S/M/C1-III)

2.Og of SiO₂/MAO (S/M-B) was slurried in 100 ml. toluene. Next,

Preparation of SiO?/MAO/Compound1 (S/M/C1-IV)

2.Og of SiO₂/MAO (S/M-C) was slurried in 100 ml. toluene. Next,

5.8 ml. of a Compound 1 solution in toluene (15 mg/mL, 87 mg) was added. The suspension was slowly stirred overnight and used as such. Preparation of SiCb/MAO/Compound 1 (S/M/C1-V)

2.9 g of SiO₂/MAO (S/M-B) was slurried in 100 mL toluene. Next, 5.8 mL of a Compound 1 solution in toluene (15 mg/ml_, 87 mg) was added. The suspension was slowly stirred overnight and used as such.

Preparation of SiO?/MAO/Compound 1 (S/M/C1-VI)

5.6g of SiO₂/MAO (S/M-A) was slurried in 100 mL toluene. Next, 5.8 mL of a Compound 1 solution in toluene (15 mg/mL, 87 mg) was added. The suspension was slowly stirred overnight and used as such.

Preparation of SiCb/MAO/Compound 1 (S/M/C1-VII)

2.9 g of SiO₂/MAO (S/M-D) was slurried in 100 mL toluene. Next, 5.8 mL of a Compound 1 solution in toluene (15 mg/mL, 87 mg) was added. The suspension was slowly stirred overnight and used as such.

Preparation of SiO?/MAO/Compound 1 (S/M/C1-VIII)

2.9 g of SiO₂/MAO (S/M-E) was slurried in 100 mL toluene. Next, 5.8 mL of a Compound 1 solution in toluene (15 mg/mL, 87 mg) was added. The suspension was slowly stirred overnight and used as such.

Preparation of SiO?/MAO/Compound 1 (S/M/C1-IX)

2.97 g of SiO₂/MAO (S/M-G) was slurried in 100 mL toluene. Next, 15 mL of a Compound 1 solution in toluene (45 mg/mL) was added. The suspension was stirred under refluxing conditions overnight and washed with 4 x 50 mL of toluene.

Preparation of SiCb/MAO/Compound 1 (S/M/C1-X)

3 g of SiO₂/MAO (S/M-F) was slurried in 100 mL toluene. Next, 15 mL of a Compound 1 solution in toluene (45 mg/mL) was added. The suspension was stirred at 60⁰C overnight and washed with 4 x 50 mL of toluene.

Preparation of SiO?/MAO/Compound 1 (S/M/C1-XI)

2.Og of SiO₂/MAO (S/M-l) was slurried in 100 mL toluene. Next 3.78 mL of a Compound 1 solution in toluene (15 mg/mL) was added. The suspension was slowly stirred overnight and used as such. Svnthesis of MqCI?

The MgCI₂ support was prepared as described in U.S. Patent No. 5,696,044 (Example 1 ) with the exception that CCI₄ was replaced by PhSiCI₃.

Synthesis of MgCI₂ZMAO (MlM-A)

About 5.Og of MgCI₂ slurried in 15 ml. hexane was transferred to a round bottom flask. The majority of the heptane was removed via a canula and replaced with 10 ml. toluene. Subsequently, 2 ml MAO (30wt%, Albemarle) was added and the mixture was slowly stirred at 60⁰C for 1 hour. Subsequently, the MgCI₂/MAO was filtered and washed with 2 x 30 ml. toluene and the white powder was dried under vacuum.

Synthesis of MqCWMAO/compound 1 (M/M/C1-A)

1.1 g of MgCI₂/MAO was slurried in 50 ml. toluene and 2 ml. of a Compound 1 solution in toluene (15 mg/ml_) was added. After being stirred at room temperature for 16 hours, the powder was filtered and washed with 2 x 50 ml. toluene. The catalyst was finally dried under vacuum. The catalyst was reslurried in toluene before use.

Polymerization procedure A

The batch polymerization was performed in a 1.5 L batch autoclave equipped with a double intermig stirrer. The reaction temperature was set to 60⁰C and controlled by a Lauda thermostat. The feed streams (solvent and ethylene) were purified by various absorption media to remove catalyst killing impurities such as water, oxygen and polar compounds as is known to someone skilled in the art. During polymerization ethylene was continuously fed to the gas cap of the reactor. The pressure of the reactor was kept constant at 6.8 barg by a back-pressure valve. In an inert atmosphere the previously dried reactor is filled with 750 ml. Pentamethylheptane (PMH). After the solvent has reached the desired temperature, the catalyst components are added. Ethylene is added at a flow of

150 nL/h to a maximum pressure of 6.8 barg. After the desired polymerization time, the contents of the reactor are collected, filtered and washed with some petroleum ether. After drying under vacuum at 50⁰C overnight, the polymer is weighed and samples are prepared for analysis. Polymerization procedure B

Larger scale batch polymerizations were carried out in a stirred 1OL reactor. The reaction temperature was set to 60⁰C and controlled by a Lauda thermostat. The feed streams (solvent and ethylene) were purified by various absorption media to remove catalyst killing impurities such as water, oxygen and polar compounds as is known to someone skilled in the art.

In an inert atmosphere the previously dried reactor is filled with 4.5 L heptane. After the solvent has reached the desired temperature, the optional scavenger components are added and after 5 minutes the catalyst components are added. Next the ethylene stream is fed into the reactor. During polymerization, ethylene is either dosed to maintain a constant pressure, or at a constant rate (of 320 nL/h).

After the desired polymerization time, the contents of the reactor are collected, filtered and dried under vacuum at 50⁰C overnight, the polymer is weighed and samples are prepared for analysis. Different SiO₂/MAO compounds were prepared according to the experimental details mentioned above. An overview is given in Table 2.

Table 2. SiO?/MAO prepared with solid support materials listed in Table 1.

With the different SiO₂/MAO compounds of Table 2, a number of SiO₂/MAO/Compound 1 catalyst systems were prepared. An overview is given in Table 3. Table 3. SiCb/MAO/Compound 1 Catalyst details

The influence of the supporting temperature of compound 1 on a SiO₂/MAO compound was investigated and the results are displayed in Table 4. A higher temperature also leads to a higher loading of Ti.

Table 4. SiCb/MAO/Compound 1 Catalyst details, influence of Compound 1 supporting temperature.

To check the possibility of supporting compound 1 on a MgCI₂ type of solid support, MgCI₂ was treated with MAO in a manner comparable to SiO₂ excluding the heat treatment step. Details can be found in Table 5.

Table 5. MqCb/MAO/Compound 1 catalyst details, MqCI? supported catalyst

Two batch polymerizations were performed (Table 6) to check the influence of the supporting temperature of compound 1 on a SiO₂/MAO support. The polymerization results show an increase in molecular weight distribution and a decrease in Mw when a higher supporting temperature is used. The experiment at 60⁰C shows that a somewhat milder condition leads to a smaller increase in MWD. When the compound 1 supporting step is carried out a room temperature, MWD are typically between 3.1 and 4 as is shown in Table 7. Table 6. Polymerization details, influence of Compound 1 supporting temperature

Polymerization conditions: Polymerization procedure A, 100 mg catalyst, 60⁰C, 2 hours, no scavenger, 6.8 Bar ethylene, 30 minute run

K) Ul

Table 7. Polymerization details

K)

Polymerization conditions:

K) a) Polymerization procedure A, 60⁰C. b) Polymerization procedure B, 60⁰C. Polymerization was performed with a constant feed rate of ethylene (320 nL/hr).

1.2 mmol (Entry 7-1 1 ) or 0.5 mmol (Entry 2) of 4-methyl-2,6-di-tert-butylphenol was used. The scavengers used were triisobutylaluminium (TIBA) of Chemtura and methyl alumoxane, 30 wt% in toluene of Albemarle (MAO). c) For a dry flow measurement at least 90 g of material is needed

Claims

Process for the preparation of ultra high molecular weight polyethylene

(UHMWPE) comprising the steps of:

(a) supporting a catalyst onto a support at a temperature of between -8O⁰C and 100⁰C, said catalyst comprising an organometallic compound and an activator, characterized in that the organometallic compound is a compound according to formula 1 :

(formula 1 ) wherein:

M is a metal of group 3 -13 or the lanthanide series, and p is the valency of the metal M;

A represents a neutral or anionic spectator ligand whose valency v is 0,1 or 2, and q is an integer denoting the number of spectator ligands A; Z is an optional bridging moiety, and n is the integer number of parallel bridging moieties Z; Y is a ligand represented by formula 2:

N SUb₁ JL Sub₂ (formula 2), wherein the ligand is covalently bonded to the metal M via the imine nitrogen atom, Subi is a substituent, which comprises a group 14 atom through which Subi is bonded to the imine carbon atom, Sub₂ is a substituent, which comprises a heteroatom of group 15-16, through which Sub₂ is bonded to the imine carbon atom,; r is 1 , 2, 3, 4 or 5;

L is an optional neutral Lewis basic ligand, and j is an integer denoting the number of neutral ligands L, and X is an anionic ligand that may be independently selected from the group consisting of hydride, halide, alkyl, silyl, germyl, aryl, amide, aryloxy, alkoxy, phosphide, sulfide, acyl, pseudo halides such as cyanide, azide, and acetylacetonate, or a combination thereof; and (b) polymerizing ethylene in the presence of the supported catalyst of step (a), thereby producing ultra high molecular weight polyethylene.

2. The process according to claim 1 , wherein the activator is selected from a groups consisting of alumoxanes, alkyl aluminums, alkyl aluminum halides, anionic compounds of boron or aluminum, trialkylboron compounds, triarylboron compounds, and mixtures thereof.

3. The process according to any one of the preceding claims, wherein the support is selected from the group consisting of silica, alumina, silica-alumina, magnesia, titania, zirconia, magnesium chloride, and crosslinked polystyrene.

4. The process according to any one of the preceding claims, wherein the support is silica.

5. The process according to any one of the preceding claims, wherein the D50 of the support is between 2 and 30 μm,

6. The process according to anyone of the preceding claims, wherein the supported catalyst is substantially halogen free.

7. The process according to anyone of the preceding claims, wherein the heteroatom of Sub₂ of the ligand Y is nitrogen.

8. UHMWPE having a weight average molecular weight of at least 400,000 g/mol, characterized in that the UHMWPE has a molecular weight distribution Mw/Mn of greater than 2.8 and less than 5.0, preferably greater than 3.0 and less than 4.0.

9. UHMWPE according to claim 8, wherein the UHMWPE has a particle size of between 80 and 400 μm.

10. UHMWPE according to claims 8 or 9, wherein the UHMWPE has a dry flow of 60 s or less.

11. UHMWPE according to any one of claims 8-10, wherein the UHMWPE has a bulk density of 200 kg/m³ or more.

12. Use of UHMWPE according to any one of claims 8-11 in a biomedical application or in a fiber application.

13. Use of the UHMWPE obtained by the process according to any one of claims

1-7 in a biomedical application.

14. Use according to claim 12 or claim 13, wherein the UHMWPE is used in an artificial implant.

15. Use according to claim 12 or claim 13, wherein the UHMWPE is used in hip arthroplasty, knee replacement, shoulder replacement or a spinal application such as total disc replacement.