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 η 3 coordination is more stable than the η 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 η 7 coordinated the styrene is η 2 coordinated, while in the case of η 3 coordination of the benzyl group, styrene is η 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 η 3 coordinated growing chain, and a η 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 CpTi II P should be not able to promote styrene polymerization, whereas cationic active species of the type (benzene)Ti II P + should be able to promote styrene polymerization, although the latter species should be less active than species of the type CpTi III P + .

Syndioselective Polymerization of Vinylnaphthalene

“…This mainly attributes to the increasing steric hindrance and the reduced Lewis acidity of active species, that helps to inhibit the side‐reactions between the active species and the naphthalene ring. According to the polymerization behaviors of 2VN and MVN, we can deduce that these monomers are polymerized into syndiotactic products via the same pathway to styrene and its derivatives …”

Chirality of Catalysts for Stereospecific Polymerizations

“…169 -171 Despite the many experimental studies which have contributed to clarify even details of the polymerization mechanism, only recently has theoretical study (a combined quantum mechanics/molecular mechanics, QM/MM) of the mechanisms of polymerization and of stereocontrol in the syndiospecific polymerization of styrene been afforded. 172,173 In particular, insertion of styrene on the CpTi III CH 2 Ph + active species, in which the achiral -CH 2 Ph group has been used to simulate the growing chain, was investigated. This choice allowed to explore details of the insertion reaction delaying the complications due to the presence of a chiral growing chain.…”

“…This choice allowed to explore details of the insertion reaction delaying the complications due to the presence of a chiral growing chain. 172 After the basic features of the insertion step have been clarified, the issue of stereoselectivity was afforded. In order to contribute to establish a possible mechanism of growing chain stereocontrol, the insertion of styrene on the CpTi III CH(CH 3 )Ph + active species, which contains a chiral growing chain, has been investigated.…”

Preparation of syndiotactic polystyrene using novel monotitanocene with ortho-substituted phenoxy ligands

“…Since the first preparation of sPS by Ishihara et al [3], the use of titanium/MAO catalyst system has spurred an active search for new types of sPS catalysts. Of the compounds identified as effective catalyst precursors for the synthesis of sPS, the most active are based upon pentamethylcyclopentadienyl titanium (Cp*Ti) complexes activated with methylaluminoxane (MAO) [4][5][6]. For monotitanocene catalysts, steric and electronic effects of ligands play important roles in determining the catalytic activity and the molecular weight of sPS [7][8][9][10][11].…”