Estimation of Free Radical Polymerization Rate Coefficients using Computational Chemistry
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Acrylic free radical polymerization at high temperature proceeds via a complex set of mechanisms, with many rate coefficients poorly known and difficult to determine experimentally. This problem is compounded by the large number of monomers used in industry to produce coatings and other materials. Thus, there is a strong incentive to develop a methodology to estimate rate coefficients for these systems. This study explores the application of computational chemistry to estimate radical addition rate coefficients for the copolymerization of acrylates, methacrylates and styrene. The software package Gaussian is used to calculate heats of reaction (ΔHr) values for monomer additions to monomeric and dimeric radicals, using minimum energy structures identified and characterized for the reactants and products. The Evans-Polanyi relationship is applied to estimate reactivity ratios from the relative differences in ΔHr. The validity of this methodology is tested through a comparison of calculated monomer and radical reactivity ratios for acrylate, methacrylate, vinyl acetate, ethene and styrene systems to available experimental data for copolymerization systems. The methodology is found to work for some systems while there is computational breakdown in others due to steric crowding and/or breakdown of the Evans-Polanyi relationship.