Computational and experimental study of spontaneous thermal polymerization of alkyl acrylates

This project is aimed at answering two unanswered fundamental and industrially-important questions: What are the species that initiate the spontaneous thermal polymerization of alkyl acrylates? What are the initiation mechanisms? Density functional calculations (B3LYP/6-31G*) level of theory was used to investigate mechanism of spontaneous initiation in thermal polymerization of methyl, ethyl and n-butyl acrylate. Flory and Mayo mechanisms of self initiation were investigated. Potential energy surface maps for [4+2] and [2+2] cycloaddition reactions were constructed. It was found that diradical mechanism of self-initiation occurs in spontaneous thermal polymerization of alkyl acrylates, with the diradical species in a triplet spin state Hydrogen transfer from the triplet diradical intermediate was found to be the lower energy pathway in comparison to hydrogen abstraction from triplet diradical intermediate and is proposed to generate monoradicals for initiating polymerization. DFT calculations was used to identify the occurrence of diradical mechanism of self-initiation in spontaneous polymerization of MMA and that the diradical exists in a triplet state. This provided theoretical evidence to existing postulate and experimental observations. Experiments of spontaneous thermal homo-polymerization of methyl, ethyl and n-butyl acrylates in the absence of any known added initiators were conducted at 120 and 140°C in xylene, dimethyl sulfoxide (DMSO) and cyclohexanone and in the presence of nitrogen and air in a 1 liter reaction calorimeter (RC1 Mettler Toledo). Highest initial monomer conversion was found to occur in cyclohexanone and lowest in DMSO. Highest average molecular weight polymers were found to form in DMSO. Higher initial monomer conversion and lower average molecular weight polymers was found in air than nitrogen. Mass-spectrometric studies were used to identify that the two dominant mechanisms of chain initiation and transfer in spontaneous thermal polymerization of methyl and n-butyl acrylate are monomer self-initiation and beta-scission respectively. The initiation rate constants estimated from macroscopic process modeling and experimental monomer conversion, number-average and weight-average molecular weight measurements was in good agreement with rate constant for triplet diradical formation predicted via quantum chemical studies.