Computational Study of the Self-Initiation Mechanism in Thermal Polymerization of Methyl Acrylate

This computational study deals with the mechanism of spontaneous initiation in thermal polymerization of alkyl acrylates (e.g., methyl, ethyl, and n-butyl acrylate). The mechanism is presently still unknown. Density-functional theory (DFT) and Moller-Plesset (MP2) calculations are used to explore the Flory and Mayo mechanisms of self-initiation in methyl acrylate. On the singlet surface, a low-barrier, concerted [4 + 2] Diels-Alder mechanism for the formation of a dihydropyran adduct (DA) and a high-barrier nonconcerted [2 + 2] diradical (M-center dot(2S)center dot) mechanism for the formation of dimethyl cyclobutane-1,2-dicarboxylate (DCD) were found using B3LYP/6-31G*. Several levels of theory were used to validate the transition states, and the pathways for the DA and DCD formations on the singlet surface were determined using intrinsic reaction coordinate (IRC) calculations. On the triplet surface, a triplet diradical intermediate (center dot M-2t center dot) Was identified that is structurally similar to M-center dot(2s)center dot but lower in energy. The spin-orbit coupling constant for crossover of the diradical from singlet to triplet surface was calculated. Monoradical generation from the two intermediates, DA and M-center dot(2t)center dot via hydrogen transfer to or from a third methyl acrylate was studied. It was found that generation of two monoradical species was possible from center dot M-2t center dot and is proposed as a likely explanation for experimentally observed spontaneous-initiation.