The effect of LiNTf2 on the propagation rate coefficient of methyl methacrylate

Abstract

In the present work we use accurate Pulsed Laser Polymerization (PLP) to measure the influence of various concentrations of lithium bis(trifluromethane)sulfonamide (LiNTf₂) on the propagation rate coefficient of methyl methacrylate (MMA). We also perform ¹H-NMR analysis to evaluate the effect of LiNTf₂ on poly(MMA) stereochemistry. Additionally, we perform high-level quantum-chemical calculations to model the interactions between Li⁺ and the MMA monomer and propagating radical. Across a broad range of concentrations, LiNTf₂ only slightly increases the isotacticity of the resultant poly(MMA). However, a significant increase in the propagation rate coefficient was noted upon addition of LiNTf₂. The magnitude of this increase was found to be dependent on the LiNTf₂ concentration and temperature. Theoretical calculations reveal the complexities associated with Lewis acid-mediated stereocontrol. On the basis of this theoretical work, we suggest that the potential stereocontrol afforded by Lewis acids is being hindered by their action as propagation catalysts though non-stereoselective binding modes.