Stereoselectivity of an early propagation step in the anionic polymerization of methyl methacrylate – ab initio SCF, MNDO and AM1 study

Abstract

AbstractOn the basis of previous experimental and theoretical studies of low‐molecular models of living poly(methyl methacrylate) and their coordination interactions, molecular states preceding and involving the propagation step of the anionic polymerization of methyl methacrylate (MMA) were investigated. Previously published structures of the intracoordinated form of the model dimer dimethyl 2‐lithio‐2,4,4‐trimethylglutarate (DMMALi) and its free anion (DMMA) optimized using ab initio SCF 3‐21G were reinspected and a new, similar but more stable, geometry was found. AM1 and ab initio 3‐21G calculations of the addition of MMA to the anion derived from methyl isobutyrate were used to explore the applicability of semiempirical methods to the present problem. Intermediate complexes and transition states of the iso‐ and syndiotactic addition of MMA to DMMA and DMMALi were simulated using MNDO and AM1. The activation energy of MMA addition to DMMA is predicted to be substantially lower for the syndiotactic transition state. In contrast, only isotactic MMA addition is possible to the energetically favored intra‐coordinated form of DMMALi (c‐DMMALi). Syndiotactic addition to the uncoordinated form of DMMALi is apparently possible according to MNDO. Considering the earlier finding that c‐DMMALi is favored by dilution and lower temperature in solvating media such as tetrahydrofuran, the differences in stereoregularity of the anionic polymerization of MMA caused by concentration, temperature or solvent polarity can be plausibly explained.