Ethylene/styrene copolymerizations using Cp'TiCl2(O-2,6-(Pr2C6H3)-Pr-i) [CP' = Cp* (C5Me5, 1), 1,2,4-Me3C5H2 (2), tert-BuC5H4 (3)]-MAO catalyst systems were explored under various conditions. Complexes 2 and 3 exhibited both high catalytic activities (activity: 504-6810 kg-polymer/mol-Ti h) and efficient styrene incorporations at 25, 40 degrees C (ethylene 6 atm), affording relatively high molecular weight poly (ethylene-co-styrene)s with unimodal molecular weight distributions as well as with uniform styrene distributions (M-w = 6.12-13.6 x 10(4), M-w/M-n = 1.50-1.71, styrene 31.7-51.9 mol %). By-productions of syndiotactic polystyrene (SPS) were observed, when the copolymerizations by 1-3-MAO catalyst systems were performed at 55, 70 degrees C (ethylene 6 atm, SPS 9.0-68.9 wt %); the ratios of the copolymer/SPS were affected by the polymerization temperature, the [styrene]/[ethylene] feed molar ratios in the reaction mixture, and by both the cyclopentadienyl fragment (Cp') and anionic ancillary donor ligand (L) in Cp'TiCl2(L) (L = Cl, O-2,6-(Pr2C6H3)-Pr-i or N=(CBu2)-Bu-t) employed. Co-presence of the catalytically-active species for both the copolymerization and the homopolymerization was thus suggested even in the presence of ethylene; the ratios were influenced by various factors (catalyst precursors, temperature, styrene/ethylene feed molar ratio, etc.). (C) 2008 Wiley Periodicals, Inc.