A supported catalyst composition and process for preparing high molecular weight polymers of one or more addition polymerizable monomers, especially propylene, said composition comprising: 1) a substrate comprising the reaction product of a solid, particulated, high surface area, metal oxide, metalloid oxide, or a mixture thereof and an organoaluminum compound, 2) a Group 4 metal complex of a polyvalent, Lewis base ligand; and optionally, 3) an activating cocatalyst for the metal complex.