Morphology-controlled olefin polymerization catalyst

A solid, hydrocarbon-insoluble alpha-olefin polymerization catalyst component comprises a product formed by:

• A. Forming a solution of a magnesium-containing species from a magnesium hydrocarbyl carbonate or a magnesium carboxylate;
• B. Precipitating solid particles from such magnesium-containing solution by treatment with a transition metal halide and an organosilane having a formula:
• R.sub.n SiR'.sub.4-n, wherein n=0 to 4 and wherein R is hydrogen or an alkyl, a haloalkyl or aryl radical containing one to about ten carbon atoms or a halosilyl radical or haloalkylsilyl radical containing one to about eight carbon atoms, and R' is OR or a halogen;
• C. Reprecipitating such solid particles from a solution containing a cyclic ether;
• and
• D. Treating the reprecipitated particles with a transition metal compound and an electron donor;
• in which at least one solution in Steps A, B or C is a toluene-based solution containing about 0.1 to about 2 wt. % C.sub.8 -C.sub.10 aromatic hydrocarbon.