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-fre e CpTiCH2Ph+ species, with a coordinated benzene molecule to simulate the s olvent, is characterized by two minimum geometries with different hapticiti es of coordination of the benzyl group. The eta (3) coordination is more st able than the eta (7) coordination by 12 kJ mol(-1). Substitution of the so lvent molecule by styrene leads to coordination intermediates which are als o characterized by different hapticities of the styrene. When the benzyl gr oup 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) coord inated. All these coordination intermediates are of similar energy and are separated by low energy barriers. Insertion can occur with a relatively sma ll energy barrier, 47 kJ mol-1, from a coordination intermediate presenting a eta (3) coordinated growing chain, and a eta (4)-coordinated styrene mol ecule. The products of the insertion reaction are characterized by a backbi ting 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 th e type (CpTiP)-P-II should be not able to promote styrene polymerization, w hereas cationic active species of the type (benzene)(TiP+)-P-II should be a ble to promote styrene polymerization, although the latter species should b e less active than species of the type (CpTiP+)-P-III.