Olefin polymerization using supported metallocene catalysts: development of high activity catalysts for use in slurry and gas phase ethylene polymerizations

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Reaction of hydroxylated silica and alumina supports with methyl aluminoxane in toluene suspension provides chemically modified supports suitable for use in slurry and gas-phase polymerizations of ethylene or propylene on treatment with a variety of metallocene dichloride complexes. In particular, aluminas derived from calcination of sol–gel precursors feature high degrees of surface hydroxylation in comparison with commercially available silica (or even alumina) of similar surface area and total porosity. This feature provides a mechanism for increasing the amount of aluminoxane on the former supports, such that commercially acceptable productivities (>10 kg PE/g support×h) are observed at relatively low, total levels of aluminoxane or other alkylaluminum compounds in slurry or gas-phase polymerizations, respectively. A variety of evidence indicates that leaching of active catalyst from these alumina supports occurs to a minor extent under slurry conditions, particularly at higher temperatures in the presence of additional aluminoxane. At lower temperatures, this does not occur to an appreciable extent but the morphology and bulk density of the polymer formed is unsuitable for use in a gas-phase process. This can be attributed to the method for synthesis of the sol–gel alumina precursor which results in irregular particles with a broad particle size distribution. Copolymerization of ethylene with 1-octene or 1-hexene results in formation of linear, low density, PE with a narrow composition distribution as revealed by temperature rising elution fractionation. These studies indicate that less comonomer is incorporated using these supported metallocene catalysts than their soluble analogues under otherwise identical conditions. Finally, some of the resins prepared under slurry conditions (and to a lesser extent in a gas-phase process), exhibit properties consistent with the presence of low levels of long-chain branching; this feature appears to be reasonably general for a variety of simple metallocene complexes.