Abstract: Silica supported catalyst composition useful in olefin polymerization, having a SiO2 content not more than 70 weight %, a transition metal (M) content between 2.5 and 9.1 weight %, a magnesium content between 0.5 and 3.3 weight %, an aluminium content between 0.3 and 5 weight % and a chlorine content between 5 and 30 weight %. The silica support has, prior to addition of catalytically active ingredients, a residual surface hydroxyl content between 0.6 and 2 mmole/g of silica, and the molar ratio of transition metal (M) to magnesium is between 0.3 and 2.5.

Abstract: The present invention provides a new process for preparing a catalyst for the gas phase polymerization of olefins wherein the sequence of addition of the catalyst components and the selection of the said components provides a catalyst composition which exhibits a superior response to hydrogen, a surprisingly high productivity and an improved activity profile. The catalyst consists of compounds of a group IV transition metal, Mg, C1 and A1 supported on a silica support.