In the last sixty years, the paint and coatings industries have undergone considerable changes because of the stringent environmental regulations for reduction of volatile organic content [1-3]. Conventional, low temperature (ca. 50–80o C) thermal polymerization processes for the production of acrylic resins have been replaced with high temperature (ca. above 100oC) processes [4-7]. Sustained, reproducible, spontaneous initiation in the high-temperature polymerization of ethyl acrylate in the absence of any known added initiator was reported in [8]. It was also reported that secondary reactions such as spontaneous initiation, backbiting, and β-scission are prevalent at these temperature ranges. Quan et al. [9] performed detailed studies to gain a better understanding of the secondary reactions using electron spin ionization - Fourier transform mass spectroscopy and nuclear magnetic resonance spectroscopy. However, no conclusive evidence to point to a meaningful initiation mechanism or initiating species was found [9].

Spontaneous initiation was reported to occur in thermal polymerization of styrene via Mayo's AH mechanism of self initiation [10, 11] and in high-temperature thermal polymerization methyl methacrylate (MMA) through Flory's diradical mechanism [12, 13]. To our best knowledge, there have not been any experimental or theoretical studies yet to determine the initiating species or the mechanism of spontaneous initiation in high-temperature polymerization of ethyl acrylate.

In this paper, we present results from density functional theory calculations using B3LYP/6-31G(d) [14] conducted to identify the initiation mechanism and the initiating species in spontaneous thermal polymerization of ethyl acrylate. The molecular geometries of the reactant, transition state, intermediates and products were calculated on the singlet and triplet potential energy surfaces. Diradical formation was observed on the singlet and triplet energy surfaces. The singlet state diradical was found to be of higher energy and unstable, while the triplet state diradical was found to be highly stable. These are in good agreement with previous postulates on Flory's mechanism. The energy barrier for the formation of triplet state diradical intermediate was found to be comparable to that of MMA [15]. The singlet energy surface was mapped, and the presence of a Diels–Alder (DA) intermediate, a dimer, in general, seen in Mayo's mechanism, was observed. Cyclobutane dimer (CBD), an inactive species, which is incapable of propagation, was also found to form. The lowest energy structure identified was that of CBD. Energy map of the triplet surface demonstrated the presence of the stable diradical intermediate only. The rate constant for the diradical formation was calculated using transition state theory. It is widely known in the literature that monoradicals are preferred initiating species. Monoradical formation from the diradical and the DA dimer via hydrogen abstraction involving a third monomer was studied. The energy barrier and rate constant for the monoradical formation were calculated.

References

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