Catalytic precursors Ti(IV) and Zr(IV) complexes bearing cyclopentadienyl and substituted cyclopentadienyl and substituted cyclopentadienyl anionic ligands, bonded to phenyl or substituted phenyl through an isopropylidene bridge have been utilized in the polymerization fo propene and styrene and in ethylene-styrene copolymerization and substituted cyclopentadienyl anionic ligands, bonded to phenyl or substituted phenyl through an isopropylidene bridge have been utilized in the polymerization of propene and styrene and in ethylene-styrene copolymerization. In the presence of trichloro [(1,2,3,4,5-eta)-1-(1-methyl-1-phenylethyl)-2,4-cyclopentadien-1-yl] titanium (LTiCl3) we have obtained either partially isotactic (chain-end type) or atactic poly-(propylene), and either atactic or syndiotactic polystyrene depending on the reaction temperature. [1-Methyl-1-naphthylethyl-2-inden-1-yl]titanium(IV) behaves like (LTiCl3) in styrene polymerization, while it affords metal-controlled partially isotactic poly(propylene), as well as the corresponding zirconium compounds. The experimental data are tentatively explained by the temperature dependence of coordination of the bridged aryl group of the ligand.