(743f) Theoretical Evidence to Diradical Self-Initiation in Spontaneous Thermal Polymerization of Methyl Methacrylate

Extensive studies have shown that spontaneous high-temperature (> 100 oC) polymerization of styrene, methyl methacrylate (MMA) and alkyl acrylates can occur via monomer self-initiation. Experimental studies of spontaneous thermal polymerization of MMA generated higher molecular weight polymers and showed lower monomer conversion than in alkyl acrylates. Also, these studies provided qualitative evidence to the occurrence of diradical mechanism of self-initiation, and measured formation of various dimer species in thermal polymerization of MMA than in alkyl acrylates . Quantum chemical calculations, using B3LYP/6-31G*, identified the occurrence of diradical (Flory) mechanism of self-initiation in alkyl acrylates and verified the Diels-Alder adduct-based (Mayo) self-initiation mechanism in spontaneous thermal polymerization of alkyl acrylates and styrene respectively . Before this study, there was no reported computational study, which could conclusively determine the mechanism of initiation in spontaneous polymerization of MMA. This study explores two self-initiation (Mayo and Flory) mechanisms for MMA on the singlet and triplet energy surfaces using B3LYP/6-31G*. Singlet and triplet potential energy surface maps are constructed. The formation of Diels-Alder adducts, cis and trans- dimethyl 1,2-dimethylcyclobutane-1,2-dicarboxylate and dimethyl 2-methyl-5-methylidene- hexanedioate, on the singlet surface is shown. The presence of a diradical intermediate (.M2t.) on the triplet surface is identified. Transition states are calculated using B3LYP/6-31G* and MP2/6-31G*. The calculated energy barriers and rate constants with different levels of theory are in good agreement with corresponding data obtained from laboratory experiments.