Abstract:
An ethylene polymerization catalyst comprising a supported precursor of vanadium trihalide/electron donor complex and alkylaluminum or boron halides, when combined with alkylaluminum cocatalyst and alkyl halide promoter provides enhanced polymerization and productivity plus superior polyethylene.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an improved catalyst used in ethylene polymerization, and more particularly, to vanadium containing catalyst compositions having high activity and providing an improved process in the gas phase for producing polyethylenes having a desirable balance of properties useful in certain applications. 
     2. Description of the Prior Art 
     Polyethylenes having a relatively broad molecular weight distribution possess wide utility in the areas of wire and cable insulation, blow molding and pipe fabrication. Such broad molecular weight distribution polyethylenes, produced for example by the process disclosed in U.S. Pat. No. 3,324,095 (Carrick et al.), possess instability through the presence of a relatively high level of unsaturation. Relatively narrow molecular weight distribution polyethylenes can be made in the gas phase as described in for example U.S. Pat. No. 4,302,565 (Goeke et al.) and U.S. Pat. No. 4,302,566 (Karol et al.). Such gas phase polymerization procedures use catalysts with high activity to produce polyethylene low in unsaturation and possessing high utility in various film applications. Such polyethylenes, however, have limited utility in other areas requiring the properties of increased melt flow and extrudability as well as lower melt fracture found in broad molecular weight distribution polyethylenes. It would therefore be desirable to develop a high activity catalyst composition and process for producing a highly saturated, polyethylene in the gas phase having a broad molecular weight distribution. 
     Vanadium catalysts have been used to produce polyethylenes. U.S. Pat. No. 3,956,255 (Ort) discloses an ethylene polymerization catalyst composition consisting essentially of a vanadium compound combined with an alkylaluminum alkoxide and a trialkylaluminum. This catalytic composition is deposited on silica which has been reacted with an alkylaluminum alkoxide. Optionally, a halogenated alkane may be used as promoter. 
     U.S. Pat. No. 4,173,698 (Kanoh et al.) describes an ethylene polymerization catalyst which is the unsupported product of a vanadium trichloride-ether complex combined with an aluminum trihalide. An organoaluminum compound is used as cocatalyst. 
     U.S. Pat. No. 4,202,958 (Yamaguchi et al.) pertains to an ethylene polymerization process using an unsupported, solid catalyst obtained by reducing a vanadium halide with an organic aluminum compound in the presence of an ether compound. An organic aluminum cocatalyst is also provided. 
     U.S. Pat. No. 4,263,171 (Shida et al.) is representative of disclosures of olefin polymerization catalysts using magnesium containing compounds. The patent discloses a silica supported catalyst of a magnesium-aluminum alkyl complex and halogen-containing transition metal compound (including vanadium). 
     U.S. Pat. No. 4,294,947 (Doerk et al.) describes an ethylene/butene-1 copolymerization in a liquid butene-1 solvent using an unsupported catalyst comprising the reaction product of a vanadyl chloride and/or alkoxide with alkylaluminum chloride and organoaluminum cocatalysts. 
     Many of these references, representative of the prior art, are directed towards providing catalyst with high activity or towards select improvements in particular polymerization procedures or polyethylene properties. Because of the complexity, it has not been possible, prior to the present invention, to provide a commercially useful, optimized process in the gas phase using a vanadium catalyst to produce polyethylenes having a desirable balance of properties. In order to provide such a process, the catalyst must be one that can, simultaneously; provide a superior hydrogen response in order to achieve very low levels of unsaturation in the polyethylene; provide a high degree of productivity such that the catalyst residues in the polyethylene are so small, preferably less than about 10 ppm of vanadium, so as to allow them to remain therein thereby avoiding catalyst removal steps; provide a polyethylene which can be readily molded into a variety of molding applications, i.e. provide a polyethylene having relatively broad molecular weight distribution and wide melt index range; provide enhanced copolymerization of ethylene with other alpha olefins to enable a wide range in density control of the polyethylene as well as a broader range of comonomers which can be used; provide a polyethylene possessing a high bulk density in order to be commercially acceptable in handling and transportation costs; provide a polyethylene having a particle size which can be readily fluidized in a fluid bed reactor, and which is low in fines content; provide freely-flowing particulate polyethylene free of fused or chunk like agglomerations; preferably using the simplest catalyst composition possible consisting essentially of a minimum number of components. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a catalyst composition and process for producing polyethylene. The process comprises polymerizing ethylene with or without at least one C 3  to C 10  alpha-olefin monomer in the gas phase at a temperature between about 30° to about 115° C. The monomers are contacted with a catalyst composition comprising a specific supported precursor, cocatalyst and promoter. The supported precursor consists essentially of a particular class of vanadium compounds and modifiers which are impregnated on a solid, inert carrier. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention relates to a high activity, catalyst composition and process for producing polyethylene. The polyethylene so produced possesses a superior balance of properties in fluidity, molecular weight distribution, bulk density, and saturation. Such polymers have a density and melt index which can be selected from among a broad range of densities and melt indices. 
     Polyethylene 
     The polyethylene prepared in the process of the present invention is ethylene homopolymer or copolymers of a major mole percent, usually at least about 70%, of ethylene, and a minor mole percent, usually no more than about 30%, of one (copolymer) or more (ter-, tetra-polymers, etc.) C 3  to C 10  alpha-olefins. Preferred alpha-olefins are propylene, butene-1, hexene-1, 4-methylpentene-1 and octene-1. Additional monomers may be provided, including conjugated or nonconjugated dienes such as butadiene, 1,4 hexadiene and ethylidene norbornene. 
     The polyethylenes possess a desirable balance of properties. The polyethylenes can range in density from about 0.88 to about 0.96 g/m 3 . They can possess a melt index of between a no-flow value up to about 300 dg/min, and preferably from about 0.1 up to about 50 dg/min. These polymers possess a broad molecular weight distribution defined by a weight average molecular weight/number average molecules weight ratio (M w  /MW n ) of greater than about 10, and preferably between about 14 to 22. These polymers also possess a high bulk density of at least about 20 lbs/ft 3 , and preferably between about 24 to about 32 lbs/ft 3 . The unsaturation level is very low, being much less than 1 carbon-carbon double bond per thousand carbon atoms (C═C/1000C), preferably less than about 0.2 C═C/1000C and most preferably less than about 0.1 C═C/1000C. The copolymers have a residual catalyst content, in terms of parts per million of vanadium metal, of less than about 10 ppm, and preferably less than about 6 ppm. The preferred polymers produced by the gas phase process of the present invention are freely flowing, particulate solids, substantially free of fused or chunk like agglomerations. 
     The polyethylenes of the present invention are useful in a wide variety of applications including blow molding, film and pipe manufacture, as well as wire and cable coatings. 
     Catalyst 
     The catalyst composition of the present invention comprises a supported precursor, a cocatalyst and a promoter. The supported precursor consists essentially of a vanadium compound and modifier impregnated on a solid, inert carrier. The vanadium compound in the precursor is the reaction product of a vanadium trihalide and an electron donor. The halogen in the vanadium trihalide is chlorine, bromine or iodine, or mixtures thereof. A particularly preferred vanadium trihalide is vanadium trichloride, VCl 3 . 
     The electron donor is a liquid, organic Lewis base in which the vanadium trihalide is soluble. The electron donor is selected from the group consisting of alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic ketones, aliphatic amines, aliphatic alcohols, alkyl and cycloalkyl ethers, and mixtures thereof. Preferred electron donors are alkyl and cycloalkyl ethers, including particularly tetrahydrofuran. Between about 1 to about 20, preferably between about 1 to about 10, and most preferably about 3 moles of the electron donor are complexed with each mole of vanadium used. 
     The modifier used in the precursor has the formula: 
     
         MX.sub.a                                                   ( 1) 
    
     wherein: 
     M is either boron or AlR.sub.(3-a) and wherein each R is independently alkyl provided that the total number of aliphatic carbon atoms in any one R group may not exceed 14; 
     X is chlorine, bromine or iodine; and 
     a is 1 or 2 with the provision that when M is boron a is 3. 
     Preferred modifiers include C 1  to C 6  alkylaluminum mono- and dichlorides and boron trichloride. A particularly preferred modifier is diethyl aluminum chloride. About 0.1 to about 10,and preferably about 0.2 to about 1.5 moles of modifier are used per mole of electron donor. 
     The carrier is a solid, particulate porous material inert to the polymerization. The carrier consists essentially of silica or alumina, i.e., oxides of silicon or aluminum or mixtures thereof. Optionally, the carrier may contain additional materials such as zirconia, thoria or other compounds chemically inert to the polymerization or mixtures thereof. 
     The carrier is used as a dry powder having an average particle size of between about 10 to 250, preferably about 20 to about 200, and most preferably about 30 to about 100 microns. The porous carrier has a surface area of greater than or equal to about 3 and preferably greater than or equal to about 50 m 2  /g. A preferred carrier is silica having pore sizes of greater than or equal to about 80, and preferably greater than or equal to about 100 angstroms. The carrier is predried by heating to remove water, preferably at a temperature of greater than or equal to about 600° C. 
     The amount of carrier used is that which will provide a vanadium content of between about 0.05 to about 0.5 mmoles of vanadium per gram of precursor (mmole V/G), and preferably between about 0.2 to about 0.35 mmole V/g, and most preferably about 0.25 mmole V/g. 
     The carrier is free of preparative chemical treatment by reaction with an alkylaluminum compound prior to the formation of the supported precursor. Such treatment results in the formation of aluminum alkoxides chemically bonded to the carrier molecules. We have discovered that the use of such a treated carrier in the catalyst composition and process of the present invention is not only nonessential but instead results in undesirable agglomeration of polyethylene resulting in a chunk like, non-freely flowing product. 
     In addition, we have also discovered that the presence of a magnesium compound in the supported precursor does not significantly raise catalyst activity, and is therefore a nonessential component. 
     The cocatalyst has the formula: 
     
         AlR.sub.3                                                  (II) 
    
     wherein R is as previously defined in the definition of M in equation I. Preferred cocatalysts include C 2  to C 8  trialkylaluminum compounds. A particularly preferred cocatalyst is triisobutyl aluminum. Between about 5 to about 500, and preferably between about 10 to about 30 moles of cocatalyst are used per mole of vanadium. 
     The promoter has the formula: 
     
         R&#39;.sub.b CX&#39;.sub.(4-b)                                     (III) 
    
     wherein: 
     R&#39; is hydrogen or unsubstituted or halosubstituted lower, i.e., up to about C 6  containing, alkyl; 
     X&#39; is halogen; and 
     b is 0, 1 or 2. 
     Preferred promoters include flouro, chloro or bromo substituted ethane or methane having at least 2 halogens attached to a carbon atom. Preferred promoters include CCl 4 , CHCl 3 , CH 2  Cl 2 , CBr 4 , CFCl 3 , CH 3  CCl 3 , and CF 2  ClCCl 3 . Particularly preferred promoters are methylene dichloride (CH 2  Cl 2 ), 1,1,1,trichloroethane, (CH 3  CCl 3 ), and chloroform (CHCl 3 ). Between about 0.1 to about 10, and preferably between about 0.2 to about 2, moles of promoter are used per mole of cocatalyst. 
     Catalyst Preparation 
     The catalyst composition of the present invention is produced by first preparing the supported precursor. In one embodiment, the vanadium compound is prepared by dissolving the vanadium trihalide in the electron donor at a temperature between about 20° C. up to the boiling point of the electron donor for a few hours. Preferably, mixing occurs at about 65° C. for about 3 hours. The vanadium compound so produced is then impregnated onto the carrier. Impregnation may be effected by adding the carrier as a dry powder or as a slurry in the electron donor or other inert solvent. The liquid is removed by drying at less than about 100° C. for a few hours, preferably between at about 45° to 70° C. for about 3 to 6 hours. The modifier, dissolved in an inert solvent, such as a hydrocarbon, is then mixed with the vanadium impregnated carrier. The liquid is removed by drying at temperatures of less than about 70° C. for a few hours, preferably at about 45° C. for about 3 hours. 
     The cocatalyst and promoter are added to the supported precursor either before and/or during the polymerization reaction. The cocatalyst and promoter are added either together or separately, and either simultaneously or sequentially during polymerization. The cocatalyst and promoter are preferably added separately as solutions in inert solvent, such as isopentane, during polymerization. 
     Polymerization 
     The ethylene polymerization is conducted in the gas phase using these procedures well established in the art. It is preferred that the polymerization occur using a continuous, fluidized bed process. Using such a procedure, portions of the catalyst composition and monomers are fed to a reactor vessel from which polyethylene product is continuously removed. The density of the ethylene copolymer produced may be varied over a wide range depending upon the amount of alpha-olefin comonomer addition and upon the particular comonomer employed. The greater the mole percent of alpha-olefin added the lower the density. 
     The polymerization is conducted at a temperature below the sintering of the polyethylene. The operating temperature will range from between about 30° C. to about 115° C. Preferred operating temperatures will vary depending upon the polyethylene density which is desired. High density polyethylenes of greater than about 0.94 g/cc are produced at operating temperatures of between about 90° to about 110° C., and preferably at about 100° C. Low density polyethylenes ranging in density from about 0.91 to about 0.94 g/cc are preferably produced at an operating temperature of between and 75° to about 90° C. Very low density polyethylenes of less than about 0.91 g/cc are preferably produced at an operating temperature of between about 35° C. to about 75° C., using procedures as described in copending U.S. patent application Ser. No. 480,296 filed concurrently herewith by Karol et al., entitled &#34;Preparation of Low Density, Low Modulus Ethylene Copolymers in a Fluidized Bed&#34;. 
     The fluid bed reactor is operated at pressures of up to about 1000 and preferably between about 50 to about 350 psi. 
     A chain transfer agent, such as hydrogen, is used to terminate the polymerization. The ratio of hydrogen to ethylene will vary between about 0.005 up to about 2.0 moles of hydrogen per mole of monomers. We have discovered that hydrogen response is improved utilizing the catalyst composition of the present invention such that less hydrogen is required to produce polyethylene having extremely low unsaturation content. 
    
    
     EXAMPLES 
     The following examples are designed to illustrate the catalyst composition and process of the invention and are not intended as a limitation of the scope thereof. 
     The properties of the polymers produced in the examples were determined by the test procedures: 
     
         ______________________________________Property     Test Procedure______________________________________Ash Content  Polyethylene is ashed and weighed;        productivity can be calculated in        pounds of polyethylene produced        per pound of total catalyst.Bulk Density ASTM-D-1895 (Method B).(lb/ft.sup.3)Density      ASTM-1505, based on ASTM-D-1928(g/cc)       (Procedure C) plaque preparationFines        Amount of polymer particles which(wt. %)      pass through a 200 mesh screen.Flow Index   ASTM D-1238-Condition F(dg/min)Melt Flow    Flow Index/Melt Index; with meltRatio        index measured using ASTM D-1238        Condition E.Particle     Based on visual observation ofagglomeration        polyethylene product exiting from        the reaction.Unsaturation Based on infrared spectrophoto-(C═C/1000C).sup.a        metric measurements of absorbance        (at 10.35 microns for        transvinylidine, 11.0 microns for        terminal vinyl and 11.25 microns        for pendant vinyl unsaturations)        of 25 mil thickness pressings of        polymer. Unsaturation        concentration is proportional to        absorbance per mil divided by        absorptivity (as taken from        absorptivity values published by        R. J. deKock et al., in J. Polymer        Science, Part B, 2, 339, 1964).______________________________________  .sup.a Number of carboncarbon double bonds per 1000 carbon atoms 
    
     Shorthand designations used in the Examples are defined as follows: 
     
         ______________________________________Designation     Description______________________________________DEAC            Diethylaluminum chlorideED              Electron donor compoundFI              Flow indexMFR             Melt flow ratioSTY             Space time yieldTEAL            TriethylaluminumTHF             TetrahydrofuranTIBA            Triisobutylaluminum______________________________________ 
    
     Catalyst Preparation 
     The catalyst compositions used in the Examples were prepared using the following standard procedures. To a flask containing 4 liters of anhydrous THF is added 34 g VCl 3  (0.216 moles). The mixture is stirred for 5 hrs. at 65° C. under a nitrogen blanket until the VCl 3  has dissolved. To this solution is added 800 g of silica (dehydrated by heating to 600° C.) and stirring is continued for 4 hrs. at 65° C. In Comparative Examples A to F the silica has been chemically treated with 5.5 wt.% TEAL. The flask is vented and the solution is dried to the mud stage at 70° C. The temperature is dropped to 45° C. and a nitrogen purge is used for 6 hrs. or until a 4-10% by weight THF level is reached in the resulting precursor. The vanadium compound so produced is a free flowing solid which has 0.25 mmoles of vanadium per gram of vanadium compound. The solid is removed from the flask and stored under nitrogen. 
     Then, except in Comparative Example C, the modifier was introduced using the following procedure. To a flask containing 4 liters of anhydrous isopentane is added 500 g of the vanadium compound described above. To this mixture is added, with stirring, a 25 weight percent solution of diethylaluminum chloride, as modifier, in anhydrous hexane. The amount of diethylaluminum chloride was employed in an amount sufficient to provide the number of moles of aluminum per mole of THF in the vanadium compound as set forth in the Examples. This mixture is heated to 45° C. and purged with nitrogen for three hours or until the product is a free flowing powder. The vanadium precursor so produced is then removed from the flask and stored under nitrogen. 
     Polymerization 
     The polyethylene was produced using the following standard procedure, under the operating conditions set forth in Table I. Ethylene was copolymerized with the comonomer set forth in the Examples. TIBA cocatalyst was added during polymerization as a 5 weight percent solution in isopentane. Chloroform, CHCl 3 , was added, except in Comparative Example D, as a 5 weight percent solution in isopentane unless otherwise indicated, in proportion to the cocatalyst as set forth in the Examples. 
     Each polymerization was continuously conducted for greater than one hour after equilibrium was reached under a pressure of about 300 psig and at a gas velocity, a nitrogen gas concentration and temperature set forth in Table I. 
     Examples 1-8 and Comparative Examples A-D 
     Using the procedures set forth above, and based on the parameters set forth in Table I, these examples were conducted using a variety of catalyst compositions. Examples 1-6 present embodiments of the present invention using butene-1 copolymerization. Examples 7 and 8 pertain to additional embodiments of the present invention invention using hexene-1 copolymerization. Comparative Examples A and B were conducted using an inert carrier which had been chemically treated with an alkylaluminium prior to impregnation. Comparative Example C was conducted using a catalyst composition without a modifier. Comparative Example D was conducted using a catalyst composition without a promoter. 
     An analysis of the polyethylene, using the above-described testing procedures, is set forth in Table I. 
     
                                           TABLE I__________________________________________________________________________USE OF CLAIMED CATALYST AND COMPARATIVE DATA__________________________________________________________________________Example     1     2     3     4     5     6__________________________________________________________________________Catalyst:Modifier    DEAC  DEAC  DEAC  DEAC  DEAC  DEACPromoter    CHCl.sub.3.sup.a             CHCl.sub.3.sup.a                   CHCl.sub.3                         CHCl.sub.3                               CHCl.sub.3                                     CHCl.sub.3Modifier/ED Ratio       0.9   0.9   0.9   0.9   0.9   0.9Carrier Pretreatment       Untreated             Untreated                   Untreated                         Untreated                               Untreated                                     UntreatedCatalyst/Promoter Ratio       0.45-0.5             0.45-0.5                   0.5   0.5   0.5   0.6Al/V Ratio  20-25 20-25 20-25 20-25 20-25 20-25Polymerization:Temperature (°C.)       100°             100°                   100°                         100°                               90°                                     100°Gas velocity (ft/sec)       2     2     2     2     2     2N.sub.2 pressure (mole %)       10%   10%   10%   10%   10%   10%Comonomer   Butene-1             Butene-1                   Butene-1                         Butene-1                               Butene-1                                     Butene-1Comonomer/C.sub.2 H.sub.4 Ratio       0.006 0.006 0.002 0.002 0.006 0.002H.sub.2 /Monomer Ratio       0.061 0.05  0.051 0.046 0.045 0.028STY (lb/hr-ft.sup.3)       6.6   6.4   5.6   6.4   7.2   8.0Polyethylene:Density (g/cc)       0.957 0.958 0.958 0.958 0.956 0.954FI (dg/min) 33-38 23-27 34-38 23.27 35-38 23-25MFR         70-75 70-75 70-75 70-75 90-95 70-75Bulk Density (lb/ft.sup.3)       26.5  26.5  28    28    27    27.5Fines (wt. %)       0.01  0.01  0.01  0.01  0.01  0.01Ash (wt. %) 0.05  0.05  0.052 0.052 0.050 0.048Particle Agglomeration       Minor to             Minor to                   Minor to                         Minor to                               Minor to                                     Minor to       none  none  none  none  none  noneComments:   All Examples 1-8 produced polyethylene with high bulk       density, low       fines content, low ash content (i.e. high catalyst activity),       good       particle fluidity (no agglomeration problem) and had good       hydrogen       response (shown by low H.sub.2 /Monomer Ratios)__________________________________________________________________________Example     7     8     A     B     C     D__________________________________________________________________________Catalyst:Modifier    DEAC  DEAC  DEAC  DEAC  None  DEACPromoter    CHCl.sub.3             CHCl.sub.3                   CHCl.sub.3                         CHCl.sub.3                               CHCl.sub.3                                     NoneModifier/ED Ratio       0.9   0.9   0.25  0.95  --    0.95Carrier Pretreatment       Untreated             Untreated                   Treated                         Treated                               Treated                                     TreatedCatalyst/Promoter Ratio       0.75  0.75  1     0.05  0.05  --Al/V Ratio  20-25 20-25 20    20    25    12Polymerization:Temperature (°C.)       100°             90°                   100°                         100°                               100°                                     100°Gas velocity (ft/sec)       2     2     1.9   1.9   1.1   1.9N.sub.2  pressure (mole %)       16.5% 20%   15%   15%   15%   15%Comonomer   Hexene-1             Hexene-1                   Butene-1                         Butene-1                               Butene-1                                     Butene-1Comonomer/C.sub.2 H.sub.4 Ratio       0.010 0.011 0.002 0.002 0.002 0.009H.sub.2 /Monomer Ratio       0.022 0.038 0.043 0.03  0.07* 0.14*STY (lb/hr-ft.sup.3)       3.7   3.8   5.5   4     4     4Polyethylene:Density (g/cc)       0.945 0.943 0.956-8                         0.956-8                               0.956-8                                     0.956FI (dg/min) 5-6   5-6   20-22 20-22 20-22 20-22MFR         70-75 90-95 70-75 70-75 70-75 75Bulk Density (lb/ft.sup.3)       26.5  27.5  26-27 28-29 20-21*                                     32Fines (wt. %)       0.01  0.01  0.08  0.05  8*    6*Ash (wt. %) 0.036 0.036 0.036 0.025-                               0.040-                                     0.062*                         0.030 0.045Particle Agglomeration       Minor to             Minor to                   Severe*                         Severe*                               None  None       none  noneComments:               *Examples A &amp; B                               *Inferior                                     *Inferior                   demonstrated                               hydrogen                                     hydrogen                   severe agglo-                               response,                                     response,                   meration problems                               bulk  fines                               density                                     and                               and fines                                     activity__________________________________________________________________________ .sup.a using 1 wt % (instead of 5 wt %) solution in isopentane. 
    
     The results set forth in Table I indicate that the catalyst composition of the present invention provides high activity as demonstrated by the low ash values, as well as a desirable balance of product properties, demonstrated by lower H 2  /monomer ratios (improved hydrogen response), higher bulk densities, improved particle fluidity, and lower fines content. In contrast, the particle agglomeration in Comparative Examples A and B was discovered to be particularly severe involving catalyst compositions similar to those of the present invention, but which utilized carrier which had been pretreated with alkylaluminum compounds. In addition, without the presence of a modifier as in Comparative Example C, hydrogen response decreases (as shown by an increase in H 2  /monomer ratio), bulk density is lowered and the fines content is greater, all at less desirable values. Similarly, in the absence of the promoter as in Comparative Example D, the hydrogen response is less, the fines content is greater and the catalyst activity is lowered, all to less desirable values. 
     Example 9 and Comparative Examples E and F 
     These examples set forth and compare the superior saturation properties of polyethylene produced using the catalyst composition and process of the present invention, with the results set forth in Table II. Using those procedures set forth above, Example 9 was conducted to produce an ethylene/butene copolymer having the designated level of unsaturation. Comparative Example E was conducted under similar procedures using TEAL as cocatalyst, but without DEAC modifier. The polyethylene produced by the present invention contains superior low unsaturation levels even when compared with a highly similar catalyst composition as in Example E. 
     For comparative purposes, Example F sets forth the saturation content of polyethylene produced by an alternative process of the prior art, and is shown to possess a significantly higher level of unsaturation leading to instability of the polymer which degrades over time. In contrast, polyethylene produced by the catalyst composition and the process of the present invention has been shown to exhibit superior stability due to the very low saturation content in the polymer. 
     
                       TABLE II______________________________________POLYETHYLENE UNSATURATION COMPARISION           ComparativeExample   9           E          F______________________________________Catalyst  Modified.sup.a                 Unmodified.sup.b                            Silyl-     V           V          chromate.sup.cUnsaturation:(C═C/1000C)Trans vinyl     0.0108      0.0197     0.038Terminal vinyl     0.0104      0.0508     1.148Pendant   0.0619      0.0812     0.106vinylideneTotal     0.0831      0.1517     1.292______________________________________ .sup.a using VCl.sub.3 /THF/SiO.sub.2 /DEAC/TIBA/CHCl.sub.3 catalyst of the present invention .sup.b using VCl.sub.3 /THF/SiO.sub.2 /TEAL/CHCl.sub.3 catalyst; at 90° C. polymerization temperature .sup.c using polymerization process of U.S. Pat. No. 3,324,095 (Carrick e al.)