Computational Analyses of the Effect of Lewis Bases on Styrene Polymerization Catalyzed by Cationic Scandium Half-Sandwich Complexes

The styrene polymerizations catalyzed by cationic half-sandwich rare-earth metal complexes [(eta(5)-C5Me5)Sc-(CH2SiMe3)(THF)(n)](+) (n = 0 (A), 1 ((thf)A)), [(eta(5)-C5Me5)Sc-(CH2C6H4NMe2-o)](+) (B), and [(eta(5)-C5Me5)Sc(C6H4OMe-o)](+) (C) have been computationally studied. It has been found that THF as an external Lewis base has no effect on the regioselectivity in the chain initiation step. However, it can make activity lower toward styrene insertion. THF is computationally proposed to move away from the Sc center during chain propagation and thus has no effects on stereoselectivity. Aminobenzyl as an internal Lewis base in B results in no regioselectivity at the chain initiation stage and has no effect on syndioselectivity during chain propagation. The internal Lewis base anisyl induces high-isotactic chain-end microstructure. The discrepancy in chain-end microstructures induced by aminobenzyl and anisyl groups could be ascribed to the different coordination capability of oxygen and nitrogen atoms to Sc metal. The size of the metal-involved ring in the bare cationic species plays an important role in the control of chain-end microstructure of the resulting polystyrene.