A theoretical study of the mechanism of styrene polymerization with models based on the CpTip(+) (P = polymeryl) species is presented. The styrene-free CpTiCH2Ph+ species, with a coordinated benzene molecule to simulate the solvent, is characterized by two minimum geometries with different hapticities of coordination of the benzyl group. The eta (3) coordination is more stable than the eta (7) coordination by 12 kJ mol(-1). Substitution of the solvent molecule by styrene leads to coordination intermediates which are also characterized by different hapticities of the styrene. When the benzyl group is eta (7) coordinated the styrene is eta (2) coordinated, while in the case of eta (3) coordination of the benzyl group, styrene is eta (4) coordinated. All these coordination intermediates are of similar energy and are separated by low energy barriers. Insertion can occur with a relatively small energy barrier, 47 kJ mol-1, from a coordination intermediate presenting a eta (3) coordinated growing chain, and a eta (4)-coordinated styrene molecule. The products of the insertion reaction are characterized by a backbiting of the aromatic ring of the penultimate unit. As for the role of Ti-II active species, our calculations suggest that neutral active species of the type (CpTiP)-P-II should be not able to promote styrene polymerization, whereas cationic active species of the type (benzene)(TiP+)-P-II should be able to promote styrene polymerization, although the latter species should be less active than species of the type (CpTiP+)-P-III.