Ethylene and styrene polymerizations and ethylene/styrene copolymerization have been examined with a catalytic system obtained by a combination of zirconium or hafnium complexes bearing a secondary phosphine-pendant cyclopentadienyl ligand {eta(5)-C5H4(CH2)(2)P(H)R} (R = 2,4,6-trimethylphenyl (Mes), 2,4,6-tri(i-propyl)phenyl (Tip)) or a phosphide-pendant cyclopentadienyl ligand {eta(5)-C5H4(CH2)(2)PR-kappaP} with modified methylaluminoxane (MMAO) or a mixture of methylaluminoxane (MAO) and tri-i-butylaluminium (TIBA) as a cocatalyst. These systems show moderate activity in ethylene polymerization (8-316 g mmol(-1) h(-1)) but show inactivity in styrene polymerization. In ethylene/styrene copolymerization, these systems afford ethylene/styrene copolymers with a relatively high styrene content (35-87%). (C) 2004 Elsevier B.V. All rights reserved.