In the polymerization of .alpha.-olefins, a titanium trichloride containing catalyst activated by copulverization with at least one additive selected fromI. an organic compound containing oxygen, sulfur or phosphorus such as:A. Organic oxygen compounds selected from ethers, ketones and esters;B. Organic sulfur compounds selected from thioethers, thiophenols and thioalcohols; andC. Organic phosphorus compounds.Ii. an organic compound of (I) containing oxygen, sulfur or phosphorus in combination with an aluminum halide, andIii. a complex or reaction product of an organic compound of (I) with an aluminum halide.Provides substantially improved catalytic activity and reduces amorphous polymeric by-products while enhancing stereoregularity. The catalytic activity of these catalysts is further enhanced by washing with an organic solvent subsequent to copulverization to remove excess additive. Preferred catalysts are those which contain aluminum chloride with the titanium trichloride. The aluminum trichloride may be provided as an integral part of the titanium trichloride, in admixture therewith, in admixture with the additive as a complex with one or more of the additives, as a reaction product of one or more of the additives or any combination of the above.