High activity polyethylene catalysts

A catalyst composition for copolymerizing ethylene and an alpha-olefin of 3 to 10 carbon atoms, comprising a solid supported catalyst precursor and dimethylaluminum chloride as a cocatalyst to activate said catalyst precursor, wherein the precursor comprises

• (i) silica, wherein the silica has 0.4 to 0.9 mmol of OH groups per gram of silica;
• (ii) a dialkylmagnesium compound, R.sub.m MgR'.sub.n wherein each of R and R' is an alkyl group of 4 to 10 carbon atoms, and wherein m plus n equal the valence of magnesium, wherein the dialkylmagnesium compound is present in an amount to provide a Mg:OH molar ratio of 1.0 to 1.8,
• (iii) tetraethyl orthosilicate, in an amount to provide a tetraethyl orthosilicate:Mg molar ratio of 0.50 to 0.80;
• (iv) titanium tetrachloride in an amount to provide a Ti:Mg molar ratio of 0.7 to 1.4.

A catalyst composition for copolymerizing ethylene and an alpha-olefin of 3 to 10 carbon atoms, comprising a solid supported catalyst precursor and dimethylaluminum chloride as a cocatalyst to activate said catalyst precursor, wherein the improvement comprises a precursor comprising

• (i) silica, wherein the silica has 0.4 to 0.9 mmol of OH groups per gram of silica;
• (ii) dibutylmagnesium, in an amount to provide a Mg:OH molar ratio of 1.0 to 1.8
• (iii) tetraethyl orthosilicate, in an amount to provide a tetraethyl orthosilicate:Mg molar ratio of 0.50 to 0.80;
• (iv) titanium tetrachloride, in an amount to provide a Ti:Mg molar ratio of 0.7 to 1.4 wherein the catalyst precursor is characterized by the K value which is defined as ##EQU4## wherein Ti! is the molar titanium content provided by TiCl.sub.4, Mg! is the molar magnesium provided by said dibutylmagnesium;
• Si! is the molar silicon content provided by said tetraethyl orthosilicate, and K is 0.23 to 0.31.