Cocatalyst for transition metal-containing .alpha.-olefin polymerization catalyst

In a catalyst system for (co)polymerizing at least one .alpha.-olefin under olefin polymerization conditions consisting essentially of:

• (i) a supported magnesium/titanium-containing solid catalyst component prepared by the steps of:
• (a) pretreating a silica support to remove surface hydroxyl groups;
• (b) contacting said pretreated silica support with a hydrocarbon soluble magnesium-containing compound selected from the group consisting of hydrocarbyloxy magnesium halides having the formula ROMgX where R is a C.sub.1 to C.sub.12 hydrocarbyl, and X is a halogen;
• dihydrocarbyl magnesium;
• alkylmagnesium halides, magnesium aluminum complexes having the formula (MgRR').sub.m (AlR.sub.3 ").sub.n where R, R', R" are the same or different and are alkyl groups and the ratio of m/n is from about 0.5 to about 10, and mixtures thereof;
• (c) optionally, drying the contacted product of step (b) to form a solid product;
• (d) optionally, contacting said solid product of step (c) with at least one modifying compound selected from the group consisting of silicon halides, boron halides, aluminum halides, alkyl silicon halides, and mixtures thereof;
• and
• (e) contacting said product of step (b) or (d) with at least one titanium-containing compound having the formula TiX.sub.p (OR').sub.q wherein R' is aryl, alkyl, cycloalkyl, aralkyl, alkylsilyl or mixtures thereof;
• X is halogen;
• p and q range from 0 to 4, with the proviso that the sum of p and q is 3 or 4;
• the improvement comprising an alkylaluminum first cocatalyst component, and at least one halosilane second cocatalyst component having the formula R.sub.a.sup.2 SiX.sub.b.sup.2 wherein X.sup.2 is halogen, R.sup.2 is hydrogen, alkoxy, aryloxy, silyloxy, alkyl, aryl or cycloalkyl;
• a is 0 or an integer from 1 to 3;
• and b is an integer from 1 to 4, with the proviso that the sum of a and b is 4.