Theoretical Studies of Reaction Mechanisms for Half-Titanocene-Catalyzed Styrene Polymerization, Ethylene Polymerization, and Styrene–Ethylene Copolymerization: Roles of the Neutral Ti(III) and the Cationic Ti(IV) Species

Reaction mechanisms of styrene polymerization, ethylene polymerization, and styrene–ethylene copolymerization catalyzed by the half-titanocene [(Me5C5)TiCl2(OAr)] complex were studied based on density functional theory (DFT) calculations. Neutral Ti(III), cationic Ti(IV), and ion-paired Ti(IV) [Ti(IV)-B(C6F5)4] complexes were examined as candidates of the active species in styrene and ethylene insertion reactions to investigate the influence of oxidation states in catalytic activity and selectivity. Our results clearly indicated that the cationic Ti(IV) species gives the highest catalytic activity for ethylene polymerization, which is consistent with previous studies. This is because of the higher Lewis acidity of the Ti(IV) center and the presence of a C–H agostic interaction to stabilize the transition state for the insertion reaction. On the other hand, only the neutral Ti(III) species is able to explain the high stereoselectivity in syndiospecific styrene polymerization (SSP), though both the Ti(III) and the Ti(IV) species are catalytically active for styrene polymerization. With the results from our previous XANES study, it is concluded that the neutral Ti(III) species produced by reduction of the Ti(IV) catalyst in the presence of styrene is the most plausible active species for SSP. In the Ti(IV) species, the activation energies of styrene and ethylene insertion reactions to polyethylene and ethylene insertion reaction to polystyrene were estimated to be very close to each other, suggesting that the Ti(IV) species is a plausible active species either for ethylene polymerization or styrene–ethylene copolymerization.