Syndioselective MMA polymerization by group 4 constrained geometry catalysts: A combined experimental and theoretical study

This contribution reports a combined experimental (with kinetics and tangible effects on syndioselectivity) and theoretical (with density functional theory) study of (CGC)M catalysts [M = Ti, Zr; CGC = Me₂Si(η ⁵-Me₄C₅)(^tBuN)], addressing a need for a fundamental understanding of the stereoselectivity observed for such catalysts in polymerization of methyl methacrylate (MMA) and an explanation for the chain-end control nature of the syndioselective MMA polymerization by the chiral (CGC)Ti catalyst. The living/controlled MMA polymerization by (CGC)TiMe⁺MeB(C₆F₅)₃^- (1) follows the zero-order kinetics in [MMA], implying a faster ring-opening process of the cyclic chelate relative to MMA addition within the catalyst-monomer complex in a unimetallic propagation cycle. The syndioselectivity of 1 is insensitive to monomer and catalyst concentrations as well as to ion-pairing strength varied with counterion structure and solvent polarity. Comparative studies using identical (CGC)M bis(isopropyl ester enolate) structures show that the (CGC)Ti system exhibits noticeably higher syndioselectivity than the isostructural Zr system at ambient temperature. Density functional calculations rationalize the higher syndioselectivity observed for the (CGC)Ti catalyst and lend a theoretical support for the mechanism of MMA- or counterion-assisted catalyst site epimerization after a stereomistake, which accounts for the formation of the predominately isolated m stereoerrors.