QM/MM Studies on Scandium-Catalyzed Syndiospecific Copolymerization of Styrene and Ethylene

Abstract

The copolymerization of styrene and ethylene by the cationic half-sandwich scandium alkyl species (η5-C5Me5)Sc(CH2SiMe3) has been computationally investigated by using the quantum mechanics/molecular mechanics (QM/MM) method. It has been found that the initiation of styrene polymerization both kinetically and energetically prefers 2,1-insertion (secondary insertion, free-energy barrier of 12.6 kcal/mol, and exergonic by 19.1 kcal/mol) to 1,2-insertion (primary insertion, free-energy barrier of 19.0 kcal/mol, and exergonic by 8.9 kcal/mol). This is in contrast to a titanocene-based catalyst system, in which the initiation of styrene polymerization was computationally found to prefer 1,2-insertion, while the subsequent styrene insertion (polymerization) proceeds in a 2,1-insertion pattern. In the current Sc-based catalyst system, although the insertion of styrene into the metal–alkyl bond of the active species is kinetically slower than that of ethylene, the formation of a styrene π-complex is more favorable than that of an ethylene complex. Also, the insertion of styrene into an ethylene-preinserted species is more energetically favorable than continuous ethylene insertion into the ethylene-preinserted species. These thermodynamic factors could add to a better understanding of styrene–ethylene copolymerization. The thermodynamic preference for the insertion of styrene rather than that of ethylene into the active species with an ethylene end group was not reported for group 4 catalyst systems. It is also found that the syndiospecific selectivity is inherently determined by the substituent of the ancillary ligand η5-C5Me5.