Ab Initio Quantitative Prediction of Tacticity in Radical Polymerization of Poly(methyl methacrylate) by a Molecular Simulation Technique with the Conformation Indexing for Multiple Transition States

Free-radical polymerization is a critical route for generating vinyl polymers owing to its versatility and productivity, while the realization of stereospecific propagation has been considered very difficult and demanding. Therefore, a concrete investigation on the mechanism of tacticity obtained in radical polymerization is urgent. Experimentally, tacticity of poly(methyl methacrylate) produced by radical polymerization in bulk was reported to be syndiotactic and temperature-dependent. In this study, we intended to rationalize those behaviors on the microscopic scale via density functional theory calculations and a hybrid Monte Carlo/molecular dynamics simulation [Red Moon methodology]. We devised a unique conformation indexing based on the multidimensional conformational analysis and obtained as many as 576 transition state conformations for the propagation reaction. Boltzmann factors of the 576 conformations implied the overall higher stability of pro-racemo conformations than pro-meso ones and, in addition, the more syndiotactic tendency at low temperature. By the Red Moon simulations utilizing the energy barriers of those conformations, we successfully simulated the propagation reaction process and achieved quantitative agreement of the product tacticity. These results exhibit the validity of our ab initio molecular simulation procedure with the conformation indexing to realize quantitative analysis of the selectivity of reactions.