Sterically hindered aluminum alkyl complexes have been found to be active catalysts and/or initiators compared to sterically non-congested complexes in the polymerization of acrylates and styrene type of monomers to yield polyacrylates and polystyrene with high molecular weights. Electron-withdrawing groups on the ligands enhance the polymerization activity of the catalysts. Mechanistic investigations suggest that both radical and ionic active species are involved in the polymerizations depending on the monomers used.