Theoretical Study of Syndiospecific Styrene Polymerization with Cp-Based and Cp-Free Titanium Catalysts. 1. Mechanism of Chain Propagation

Abstract

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 TiII active species, our calculations suggest that neutral active species of the type CpTiIIP should be not able to promote styrene polymerization, whereas cationic active species of the type (benzene)TiIIP+ should be able to promote styrene polymerization, although the latter species should be less active than species of the type CpTiIIIP+.