Ethylene copolymerization with 1-hexene, styrene using half-titanocenes containing 1,3-dialkyl- and 1,3-diarylimidazolin-2-iminato ligands,Cp'TiCl2[1,3-R-2(CHN)(2)C=N] [CP' = Cp (1), (BuC5H4)-Bu-i (2): R = Bu-i (a), 2,6-(Pr2C6H3)-Pr-i (b)], have been explored in the presence of MAO cocatalyst. Complex 1a exhibited remarkable catalytic activity for ethylene/1-hexene copolymerization, affording ultrahigh molecular weight copolymers with uniform molecular weight distributions (M-n = 1.07-1.31 x 10(6), M-w/M-n = 1.43-1.82). An opposite trend for effect of N-substituents in 1a,b toward the activity in ethylene and styrene homopolymerization was observed. The ethylene/styrene copolymerizations afforded poly(ethylene-co-styrene)s with uniform compositions and the polymerization by the Bu-i analogues (1a,2a) proceeded in a living manner with moderate activities, whereas the imidazolidin-2-iminato analogues, CpTiCl2[1,3-R-2(CH2N)(2)C=N] (R=Bu-i, Ph, 2,6-Me2C6H3), afforded a mixture of copolymer and homopolymers or exhibited negligible activity under the same conditions. (C) 2012 Elsevier B.V. All rights reserved.