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Kinetic Study of Acrylate Polymerization at High Temperature

Xinrui Yu1, Robin A. Hutchinson2, and Linda J. Broadbelt1. (1) Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60201, (2) Chemical Engineering, Queen’s University, Kingston, ON K7L 3N6, Canada

The automobile coating industry is undergoing reformation driven by environmental regulations which demand low content of volatile organic compounds. Traditional solvent-borne acrylic resins, which consist of high molecular weight polymers that are produced at low temperatures (<80 șC), need a high level of solvent (70%) to meet the requirements of the coating process. Alternatively, novel resin compositions consist of acrylic oligomers with multiple crosslinkable functional groups, which have low viscosity and can undergo crosslinking reactions on metal surfaces with added agents. Polymerization at high temperature (>120 șC) is an economical approach to produce such pre-polymers.

High temperature prompts secondary reactions, which affect the quality of the oligomers. Given the potential complexity of resin recipes and the diversity of the reactions that can occur during polymerization reactions, it is desirable to have a method to predict the characteristics of the products. In particular, methods for predicting rate coefficients for copolymerization and side reactions such as intermolecular and intramolecular hydrogen transfer and scission reactions would be valuable since these quantities are difficult to access experimentally.

In our research, computational quantum chemistry and transition state theory are employed to estimate kinetics parameters, including frequency factors and activation energies of these acrylate polymerization reactions. The stable conformations were located using a conventional optimization method in combination with relaxed potential energy scans for all the dihedrals in the species. Typically, the harmonic oscillator model is the basis for calculating frequencies and thermodynamic data. However, it has been shown to result in deviations in the values of low frequencies (<200 cm-1), and consequently, deviations in the estimation of frequency factors. A methodology for the treatment of low frequencies has been developed in our research, in which low frequencies were treated using a hindered rotor model.

To date, the propagation rate coefficients of methyl methacrylate (MMA) and methyl acrylate (MA) homopolymerization and MMA-MA copolymerization have been investigated. In this investigation, the influence of chain length of the propagating radicals was also studied. The methodology was then applied to calculate rate coefficients for transfer reactions in the MMA, MA and butyl acrylate systems.