Ethylene polymerization with silica-supported Cr[N(SiMe3)2]3/alumoxane catalyst

A new silica-supported non-cyclopentadienyl chromium catalyst having high activity for ethylene polymerization is presented. The catalyst was prepared by supporting tris[bis(trimethylsilyl)amido]chromium(III) Cr[N(SiMe3)(2)](3) and alumoxane on silica. The best activity was attained by supporting Cr[N(SiMe3)(2)](3) (0.2 wt%-Cr for silica) on silica calcined at 350degreesC and next supporting a modified-methylalumoxane (Al/Cr molar ratio = 20). Activity during the slurry-phase polymerization was 75 100 g-PE/mmol-Cr(.)hr at 103degreesC and 1.4 MPa ethylene partial pressure in isobutane. The obtained polyethylene had a relatively high molecular weight and broad molecular weight distribution (HLMFR = 15.6g/10 min, M-w = 330000, M-w/M-n = 33.0). The calcination temperature of silica had significant influence on the catalyst performance. As calcination temperature increased, short branches, mainly n-butyl and some ethyl branches, were introduced into the resulting polyethylene. Therefore, poly[ethylene-co-(1-hexene)] was obtained from the ethylene monomer alone. This shows two Cr active sites, a polymerization and trimerization site. The relative amount of each site is considered to be determined by the type of silanol groups obtained after calcination.