The copolymerization of ethylene and styrene at 120 degrees C was investigated with a series of titanocenes: Cp'TiCl2L [1, 2: Cp' = cyclopentadienyl (Cp) versus 3, 4: Cp' = {2-(dimethylamino)ethyl}cyclopentadienyl (Cp-N); 1, 3: L = Cl versus 2, 4: L = 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)], in combination with methylaluminoxane (MAO). At 120 degrees C, titanocenes bearing a pendant amine ligand (3 and 4) are effective catalysts for the ethylene-styrene (ES) copolymerization, whereas complexes lacking the pendant group (1 and 2) afford only mixtures of polyethylene (PE) and atactic polystyrene (aPS). At these temperatures, copolymerization with complexes 3 and 4 yields mixtures of ES copolymers and atactic polystyrene that are not readily separated by solvent extraction or gel permeation chromatography as both components of the mixture exhibit similar molecular weights and Solubility in THF and acetone. The Source of the atactic polystyrene was the autopolymerization of styrene as deduced by carrying Out the copolymerization in the presence of a catalytic chain transfer agent, cobalt tetraphenylporphyrin (Co(tpp)). The addition of Co(tpp) caused a decrease in the molecular weight of the atactic polystyrene but had no effect on the molecular weight of the ES copolymer.