(508e) Kinetics of Mechanisms of Chain Transfer to Polymer Reactions in Alkyl Acrylates: A Theoretical Study

Environmental regulations [1,2] to reduce the volatile organic contents (VOCs) of coatings have been responsible for changes in the basic formulation of resins used in automobile coatings. High-temperature (>100 ^oC) polymerization has been used widely to produce high-molecular-weight high-solid-content resins. High-temperature free-radical polymerization is a complex process, which involves many competing secondary reactions that strongly influence the polymerization rate and polymer end-use properties [3]. Examples are b-scission, monomer self-initiation, and intra- and inter-molecular chain transfer reactions.

Previous studies [4-9] showed the successful use of computational quantum chemistry to better understand self-initiation and propagation reactions in polymerization systems. In our previous work [10], we studied the kinetics of a chain transfer to polymer (CTP) mechanism in polymerization of methyl acrylate using B3LYP/6-31G*. To the best of our knowledge, a comprehensive computational study of CTP reactions in polymerization of alkyl acrylates has not been reported as of yet.

This paper presents a comprehensive computational study of CTP reaction mechanisms in self-initiated high-temperature polymerization of alkyl acrylates. Different levels of theory (functionals and basis sets) have been applied to predict molecular structures, transition state geometries and barriers. The sensitivity of the levels of theory (B3LYP, X3LYP and M06-2X functionals, and 6-31G(d), 6-31G(d,p), 6-311G(d), and 6-311G(d,p) basis sets) to barriers and rate constants was evaluated. The effects of polymer chain-length and type of radical (e.g. tertiary vs. secondary radical) on the rate constant for chain transfer to polymer have been investigated. We have identified that the abstraction of hydrogen from a tertiary carbon atom in a dead polymer by a mono-radical is the most likely mechanism for CTP in ethyl acrylate (EA) and n-butyl acrylate (n-BA). We have found that that the mono-radicals generated via self-initiation have little influence on the capability of EA and n-BA live polymer chains to undergo CTP reactions. The effect of live polymer-chain length on the rate coefficients of chain transfer to polymer reactions was found to be insignificant.

References

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[8] Srinivasan, S.; Lee, M.W.; Grady, M.C.; Soroush, M.; Rappe, A.M.; Computational Study of the Self-Initiation Mechanism in Thermal Polymerization of Methyl Acrylate, J. Phys. Chem. A 2009, 113, 10787-94.

[9] Srinivasan, S.; Lee, M.W.; Grady, M.C.; Soroush, M.; Rappe, A.M.; Self-Initiation Mechanism in Spontaneous Thermal Polymerization of Ethyl and n-Butyl Acrylate: A Theoretical Study, J. Phys. Chem. A 2010, 114, 7975-83. [10] Moghadam, N.; Soroush, M.; Srinivasan, S.; Rappe, A. M.; Grady, M. C.; Chain Transfer to Polymer Reactions in Thermal Polymerization of Methyl Acrylate: Computational Study, Paper 300e, AIChE Annual Meeting 2011.

[10] Moghadam, N.; Soroush, M.; Srinivasan, S.; Rappe, A. M.; Grady, M. C.; Chain Transfer to Polymer Reactions in Thermal Polymerization of Methyl Acrylate: Computational Study, Paper 300e, AIChE Annual Meeting 2011.