A process is disclosed for the polymerization and interpolymerization of propylene, either alone or in admixture with ethylene, to produce polymers and copolymers of propylene. The catalyst employed is a coordination complex comprising at least some divalent titanium, or other transition metal element in lower valent state. The catalysts are prepared by mixing a higher valent titanium (or other transition metal) halide or titanate ester with a reducing agent, which can be any organometallic compound, such as a metal alkyl or aryl, including Grignard reagents, a metal hydride, or a metal including zinc and metals above zinc in the electromotive series. Hydrocarbon media can be employed advantageously for the preparation of the cataysts and for the polymerizations. The polymeric products (both homopolymers and copolymers) are linear and of high molecular weight and are useful as plastics, fibers and elastomers and for production of blends with polyethylene.