For the copolymerization of ethylene with propylene or a higher a-olefin, using Et[Ind](2)ZrCl2 metallocene catalyst, modification of silica with silicon tetrachloride prior to MAO adsorption can increase the activity, which is more pronounced for ethylene/1-hexene copolymerization at higher pressure and temperature. The molecular weight of the copolymer produced was lower and the polydispersity tends to be decreased. No significant effect of SiCl4 addition on the microstructure and the