Several titanium(IV) complexes of the type CpЈTi(NMe 2 ) 3 [CpЈ ϭ cyclopentadienyl (1), (dimethylaminoethyl)cyclopentadienyl (2), indenyl (3), and pentamethylcyclopentadienyl (4)] were prepared, and their catalytic properties in the polymerization of ␣-olefins were examined. Complexes 1 and 2 catalyzed the polymerization of ethylene in the presence of methylaluminoxane with a much higher activity than 3 or 4. Complexes 3 and 4 polymerized ethylene with an activity similar to that of CpTiCl 3 (6). The preactivation of 2, 3, or 4 with trimethylaluminum (TMA) resulted in an increase in ethylene polymerization activities. Also, 1 and 2 were successfully used as ethylene/1-hexene copolymerization catalysts, producing polymers with various amounts of 1-hexene incorporation, depending on the amount of 1-hexene in the feed mixture. Complex 1 likewise effectively polymerized styrene with a higher activity and higher syndiospecificity than the other three catalysts. Complexes 3 and 4 polymerized styrene with low syndiospecificity, whereas 2 produced only atactic polystyrene. The preactivation of 3 or 4 with TMA resulted in an increase in styrene polymerization activities and increased the syndiotacticity percentage of the polymers produced.