Computational Kinetics for Sequential Addition of OH Radicals and Molecular Oxygen Relevant to Combustion of Methyl Methacrylate

Methyl methacrylate is the simplest unsaturated branched ester and can thus serve as a surrogate model for oxygenated and biodiesel fuels. It is also the monomer of poly(methyl methacrylate) (PMMA), a widely used polymer that is also used as a hybrid rocket fuel. PMMA rapidly depolymerizes upon heating, almost exclusively forming the monomer. Therefore, chemical kinetic models developed for the combustion and pyrolysis of MMA are typically also used for the polymer. Currently, models are, however, mostly limited to high-temperature conditions and are tuned to reproduce selected characteristics of MMA flames. Hence, many important mid- and low-temperature reaction channels are missing. To fill this gap, in a companion paper, we have identified and analyzed unimolecular concerted reactions that occur during the combustion and pyrolysis of MMA and provided a general classification of retroene-type reactions. Here, we use CBS-QB3 composite level calculations to explore the addition of the OH radical to the double bond of MMA and subsequent transformations. Further addition of molecular oxygen to the β-radical centers of the resulting adducts triggers a set of double chemical activation reactions. Remarkably, a low-energy, combustion-related pathway is revealed via a Waddington mechanism that leads to the formation of CH₂O and methyl pyruvate products, which have been observed as a dominant product set in experiments on the direct oxidation of MMA with OH radicals under atmospheric conditions. Kinetic parameters of the newly identified reactions were calculated and analyzed using Arkane as part of the Reaction Mechanism Generator (RMG) automated tool.