Styrene polymerizations with a series of nonbridged half-titanocenes of the type Cp'TiCl2(L) [L = O-2,6-(Pr2C6H3)-Pr-i, Cp' = Cp (1), 1,3-Me2C5H3 (2), 1,2,4-Me3C5H2 (3), C5Me5 (Cp*, 4); Cp' = Cp*, L = OPh (5), O-4-MeC6H4 (6), O-2,6-Me2C6H3 (7), O-2,6-(Bu2C6H3)-Bu-t (8); Cp'= 1,3-Me2C5H3, L = NMeCy (9, Cy = cyclohexyl), N(2,6-Me2C6H3)(SiMe3) (10); L = Cl, Cp' = Cp (1'), 1,3-Me2C5H3 (2'), 1,2,4-Me3C5H2 (3'), C5Me5 (Cp*, 4')] have been explored in the presence of MAO cocatalyst under various conditions. The catalytic activities for syndiospecific styrene polymerization with a series of half-titanocenes containing an aryloxo ligand, Cp'TiCl2(O-2,6-(Pr2C6H3)-Pr-i), were higher than those with the trichloro analogues, Cp'TiCl3, and the activity with a series of Cp* analogues (4, 4', 5-8) and 1,3-Me2C5H3 analogues (2, 2', 9, 10) were highly dependent upon the anionic donor ligand used. ((BuC5H4)-Bu-t)TiCl2(O-2,6-(Pr2C6H3)-Pr-i) (11) exhibited a relatively high catalytic activity for syndiospecific styrene polymerization at 25 degreesC when both [PhMe2NH]B(C6F5)(4) and a mixture of (AlBu3)-Bu-i/Al(n-C8H17)(3) were used as the cocatalyst, and the activity with a series of (1,3-Me2C5H3)TiCl2(OAr) depended on the aryloxide ligand used. The role of the anionic donor ligand, like the aryloxide ligand, toward both the catalytic activity and the M-w values of polystyrene has thus been observed irrespective of the kind of cocatalyst employed.