The polymerization of ethylene was conducted over the Silaned Si02 / Mg(iBu)2 I TiCl I Al (mu)3 and over the Stauffer AA type (1.1)/Al (iBu)3 catalyst systems at 60° C. The activity of the Sb2 — supported catalyst was about one hundred times higher than that of Stauffer AA. The rate — time profiles for both catalysts were found to be of acceleration type • The dependence of the spontaneous rate of polymerization with [Al (mu)3 I has been established for the Si02 — supported catalyst. A maximum rate was achieved at Al: Ti ratio of 512:1. The rate of ethylene production using Si02 — supported catalyst was increased by a factor of two when the prepolymerization time with propylene was 10 minutes.