(300e) Chain Transfer to Polymer Reactions In Thermal Polymerization of Methyl Acrylate: Computational Study

In the past sixty years, North American regulations on volatile organic content (VOC) of coatings have been driving changes in basic nature of resins in the paint and coatings industries [1, 2], to replace high molecular weight, non-functional polymer solutions with low molecular weight, high functionalized polymer solutions [3]. High-temperature free-radical polymerization involves a large set of complex competing reactions.

Chain transfer to polymer during free-radical polymerization can lead to the formation of mid-chain radicals and highly branched polymers [4]. The formation of mid-chain radicals, and short and long chain branches at different initial monomer concentration was studied using nuclear magnetic resonance spectroscopy in free-radical solution polymerization of n-butyl acrylate [5]. Recently, a macroscopic model with new rate expressions for inter- and intra-molecular transfer to polymer was used to determine the overall polymerization rate, average chain length, and level of branching in thermal polymerization of n-butyl acrylate [6]. However, the mechanism of chain transfer to polymer and the location of the radical center on a polymer chain is not yet well understood.  

Previous studies [6-11] have shown the successful use of computational quantum chemistry to better understand self-initiation and propagation reactions in polymerization systems. Before this study there was no computational study of the mechanism of chain transfer to polymer reactions in self-initiated high-temperature free-radical polymerization of alkyl acrylates.

This paper presents a computational study of chain transfer to polymer reactions in self-initiated high temperature polymerization of methyl acrylate. Energy barriers and rate constants of chain transfer to polymer reactions are predicted using density functional theory calculations. Pathways for the reactions of interest are determined through intrinsic reaction coordinate calculations. The effects of polymer chain length on energy barriers and rate constants of the reactions are also determined.

References:

[1] Superintendent of Documents, Title 1, US Government Printing Office, Washington, DC, 1990, p.1.

[2] Superintendent of Documents, Clean Air Act Amendments of 1990, Title 111, US Government Printing Office, Washington, DC, 1990, p.236.

[3] Adamsons, K.; Blackman, G.; Gregorovich, B.; Lin, L., Matheson, R.; Oligomers in the Evolution of Automotive Clearcoats : Mechanical Performance Testing as a Function of Exposure, Progress in Organic Coatings 1998, 34, 64-74.

[4] Lovell, P.A.; Shah, T.H.; Heatley, F.; Chain Transfer to Polymer in Emulsion Polymerization of n-Butyl Acrylate Studied by Carbon -13 NMR Spectroscopy and Gel Permeation Chromatography, Polym. Commun. 1991, 32, 98-103.

[5] Ahmad, N.M.; Heatley, F.; Lovell, P.A.; Chain Transfer to Polymer in Free-Radical Solution Polymerization of n-Butyl Acrylate Studied by NMR Spectroscopy, Macromolecules, 1998, 31, 2822-2827.

[6] Nikitin, A.N.; Hutchinson, R.A.; Kalfas, G.A.; Richards, J.R.; Bruni, C.; The effect of Intramolecular Transfer to Polymer on Stationary Free-Radical Polymerization of Alkyl Acrylates, Macromolecular, 2009, 18, 247-258.

[7] Heuts, J.P.A.; Gilbert, R.G.; Radom, L.; A Prior Prediction of Propagation Rate Coefficients in Free Radical Polymerizations: Propagation of Ethylene, Macromolecules, 1995, 28, p. 8771-81.

 [8] Khuong, K.S.; H. Jones, W.H.; Pryor, W.A.; Houk, K.N.; The Mechanism of Self-Initiated Thermal Solution of a Classic Problem, J. Am. Chem. Soc. 2005. 127, 1265-77.

[9] Yu, X.; Pfaendtner, J.; Broadbelt, L.J.; Ab Initio Study of Acrylate Polymerization Reactions: Methyl Methacrylate and Methyl Acrylate Propagation, J. Phys. Chem. A. 2008, 112, 6772-82.

[10] 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.

[11] 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.