Dinuclear Silylium-enolate Bifunctional Active Species: Remarkable Activity and Stereoselectivity toward Polymerization of Methacrylate and Renewable Methylene Butyrolactone Monomers

Novel dinuclear silylium-enolate active species, consisting of an electrophilic silylium catalyst site and a nucleophilic silicon enolate initiating site that are covalently linked as single molecules, and their unique polymerization characteristics and kinetics are reported. Such unimolecular, bifunctional propagating species are conveniently generated from activation of ethyl- and oxo-bridged disilicon enolate (i.e., disilyl ketene acetal, di-SKA) compounds with [Ph3C][B(C6F5)4]. Both the ethyl- and oxo-bridged dinuclear species are much more active for the polymerization of methyl methacrylate (MMA) than the mononuclear SKA-based active species, exhibiting an approximate rate enhancement by a factor of 12 and 44, respectively. The oxo-bridged silylium-enolate species is considerably more active and controlled than the ethyl-bridged one, with their differences being even more pronounced in polymerizing a renewable monomer, γ-methyl-α-methylene-γ-butyrolactone. The polymerization by the oxo-bridged silylium-enolate active species follows first-order kinetics in both monomer and silylium catalyst concentrations, indicating a unimolecular propagation mechanism which involves an intramolecular delivery of the polymeric enolate nucleophile to the monomer activated by the silylium ion electrophile being placed in proximity in the same catalyst molecule. Highly stereoregular poly(methyl methacrylate) (PMMA), with a syndiotacticity up to 92% rr, can be produced in quantitative yield using the oxo-bridged propagator at low temperature.