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We studied thermal self-initiation of methyl methacrylate using first-principles density functional theory (DFT) calculations.  Two model mechanisms, the Mayo and Flory mechanisms, were considered and explored on the singlet and triplet energy surfaces using B3LYP/6-31G*. Singlet and triplet potential energy surface maps were 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 was identified. Transition states were calculated using B3LYP/6-31G* and MP2/6-31G*. The presence of a diradical intermediate on the triplet surface was identified. Using MCSCF/6-31G*, the spin orbit coupling constant for the singlet to triplet crossover was calculated to be 2.5 cm⁻¹. The mechanism of monoradical generation via hydrogen abstraction by both triplet and singlet diradicals from a third monomer was identified to be the most likely mechanism of self-initiation in spontaneous polymerization of methyl methacrylate.

We also studied the mechanism of self-initiation in spontaneous thermal polymerization of ethyl and n-butyl acrylate using first-principles calculations.  Density functional theory (with B3LYP functional and /6-31G*basis set) was used to study [4+2] and [2+2] cycloaddition reactions of two monomers on the singlet and triplet potential energy surfaces. Diels-Alder dimers of ethyl acrylate (6-ethoxy-2-(1-ethoxyethenyl)-3,4-dihydro-2H-pyran) and of n-butyl acrylate (6-butoxy-2-(1-butoxyethenyl)-3,4-dihydro-2H-pyran) were found to form on the singlet surface via the concerted pathway proposed by Mayo.  The formation of diethyl cyclobutane-1,2-dicarboxylate (DECD) and dibutyl cyclobutane-1,2-dicarboxylate (DBCD) via a non-concerted pathway was identified on the singlet surface of ethyl and n-butyl acrylate, respectively.  The presence of a diradical transition state for the formation of the DECD and DBCD was predicted.  Triplet potential energy surfaces for the formation of diradical dimer of ethyl and n-butyl acrylate were computed, and the presence of a triplet diradical intermediate was identified for each of the monomers.  The lower energy monoradical generation mechanism was identified to be via hydrogen abstraction by a third acrylate monomer from the triplet diradical species.  The molecular structure of the computed monoradical species was found to correlate with chain initiating species of the dominant series of peaks in our previously-reported electronspray ionization-Fourier transform mass spectra of spontaneously polymerized samples of ethyl and n-butyl acrylate.  In view of these observations, we concluded that the diradical self-initiation mechanism is the most likely initiation mechanism in spontaneous thermal polymerization of alkyl acrylates.

This project was our first serious attempt to use of quantum chemical calculations to better understand monomer self-initiation reactions in free-radical polymerization of acrylates.  The computational skills that we gained in this project have been invaluable, as they have facilitated our successful computational/theoretical studies of other free-radical polymerization reactions.  So far, one postdoctoral research associate and one doctoral-candidate research assistant have participated and been trained in the project. This project provided the research assistant and associate with an invaluable opportunity to gain skills in quantum chemical calculations, parallel computing, polymerization laboratory experimentation and spectroscopic methods.