Polymerization of olefins in the presence of a supported catalyst

An olefin polymerization process comprising contacting one or more olefins under olefin polymerization conditions with an olefin polymerization catalyst composition comprising

• a supported catalyst component consisting essentially of (A) a solid particulate support having (i) a specific surface area of from 100 to 1000 m.sup.2 /g, (ii) a surface hydroxyl content of not more than 5 mmol hydroxyl groups per g of solid support, (iii) a pore volume of from 0.3 to 3.0 cc/g, (iv) a median particle size of 1 to 200 .mu.m, and (v) a majority of particles of the solid particulate support in the form of an agglomerate of subparticles, (B) a magnesium halide, said magnesium halide (B) being obtained by the method consisting essentially of impregnating the solid particulate support (A) with a solution of a magnesium compound (B'), followed by halogenating the magnesium compound (B') with hydrogen halide as a halogenating agent (C) to transform the magnesium compound (B') to magnesium halide (B), (D) a Group 4 or 5 transition metal compound, (E) a Group 2 or 13 organometal compound, and, optionally, an electron-donor (F), and
• a cocatalyst selected from the group consisting of alumoxanes and compounds corresponding to the formula R".sub.z GX".sub.3-z, wherein G is aluminum or boron, R" independently each occurrence is hydrocarbyl, X" independently each occurrence is halide or hydrocarbyloxide, and z is a number from 1 to 3.