A theoretical study of syndiospecific styrene polymerization with Cp-based and Cp-free titanium catalysts: 2. Mechanism of chain-end stereocontrol

A theoretical study of the mechanism of chain-end stereocontrol in the syndiospecific polymerization of styrene with models based on the CpTi^IIICH(Ph)CH₃⁺ species is presented. Independent of the chirality of styrene coordination, low-energy transition states are always characterized by interactions of the aromatic group of the last enchained unit with the metal atom. However, the most stable transition state leads to formation of a syndiotactic diad, and it is favored by 6 kJ mol^-1 with respect to the most stable transition state which leads to an isotactic diad. According to our calculations, the transition state which leads to a syndiotactic diad is favored because the smallest substituent on the C_α atom of the chain, the H atom, can be pointed toward the Cp ligand. On the other hand, the transition state which leads to an isotactic diad is of higher energy because one group between the aromatic ring on the C_α atom of the growing chain, or the C_β and following groups of the growing chain, must be oriented toward the Cp ring. In agreement with experiments, models of the type Cp*Ti^IIICH(Ph)CH₃⁺ and (benzene)Ti^IICH(Ph)CH₃⁺ are calculated to be more stereoselective than CpTi^IIICH(Ph)CH₃⁺, since the transition states leading to a syndiotactic diad are favored by 16 and 13 kJ mol^-1 with respect to the transition states leading to an isotactic diad.