Broad molecular weight distribution, and high molecular weight olefin polymers, are prepared by polymerizing at least one olefin in the presence of a vanadium - and a titanium-containing catalyst composition. A catalyst precursor is synthesized by contacting a solid, porous carrier sequentially with a metal or a compound of a metal of Group IIB of the Periodic Chart of the Elements, e.g., a zinc compound, a halogen-containing aluminum compound, a vanadium compound and a titanium compound. The precursor is then combined with a suitable co-catalyst and a halogenating agent to produce the catalyst composition used to polymerize olefins.