Theoretical study of syndiospecific styrene polymerization with Cp-based and Cp-free titanium catalysts. 1. Mechanism of chain propagation

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 η³ coordination is more stable than the η⁷ 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 η⁷ coordinated the styrene is η² coordinated, while in the case of η³ coordination of the benzyl group, styrene is η⁴ 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 η³ coordinated growing chain, and a η⁴-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 Tin active species, our calculations suggest that neutral active species of the type CpTi^IIP should be not able to promote styrene polymerization, whereas cationic active species of the type (benzene)Ti^IIP⁺ should be able to promote styrene polymerization, although the latter species should be less active than species of the type CpTi^IIIP⁺.