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 [(η5-C5Me5)­Sc­(CH2SiMe3)­(THF)n]+ (n = 0 (A), 1 (thfA)), [(η5-C5Me5)­Sc­(CH2C6H4NMe2-o)]+ (B), and [(η5-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.