(214ai) Hydrogen-Bonding-Suppressed Backbiting Reactions in Radical Polymerization of Acrylates: A Computational Study of Solvent Effects

The intramolecular chain transfer (backbiting) reaction is the key reaction in acrylate polymerization responsible for converting a chain-end secondary propagating radical (SPR) to a less reactive mid-chain radical (MCR) [1]. The propagation and termination reactions of MCRs give rise to branching. At elevated temperature, MCRs can undergo β-scission reaction at considerable rate producing polymers with lower molecular weight [2-5]. Therefore, the occurrence of backbiting reaction directly influences the branching levels and molecular weight of final polymer product. Recent experimental studies demonstrate that the backbiting reaction is hindered significantly in the solvent that is capable of hydrogen-bonding to acrylate [6, 7]. However, the exact nature of the hydrogen-bonding interaction and its effect on the kinetics of backbiting reactions have not been fully understood.

This work presents a first-principles density functional theory study using B3LYP/6-31G*, X3LYP/6-31G*, M06-2X/6-31G* and B97-D3/6-31G* to explore the effects of hydrogen-bonding on backbiting reactions in high-temperature spontaneous polymerization of methyl acrylates (MA). Two types of solvent, dimethyl sulfoxide (DMSO) and methanol (CH₃OH) are studied. The effect of solvent is investigated with both conductor-like polarizable continuum model (PCM) [8] and SMD solvation model [9]. Our results suggest that the application of implicit solvation models has no significant effect of on the kinetics of backbiting reactions. Explicit solvent molecules are introduced to explore the hydrogen-bonding interactions between solvent and acrylates. Our computational results indicate that the presence of DMSO and CH₃OH increases the energy barrier of 1:5 backbiting reactions. This study is foreseen to provide insights into the use of hydrogen-bonding interactions to control the branching levels and molecular weight of poly(acrylates).

References

[1]      Junkers, T.; Barner-Kowollik, C. Journal of Polymer Science Part a-Polymer Chemistry 2008, 46, 7585.

[2]      Quan, C. L.; Soroush, M.; Grady, M. C.; Hansen, J. E.; Simonsick, W. J. Macromolecules 2005, 38, 7619.

[3]      Chiefari, J.; Jeffery, J.; Mayadunne, R. T. A.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1999, 32, 7700.

[4]      Junkers, T.; Bennet, F.; Koo, S. P. S.; Barner-Kowollik, C. Journal of Polymer Science Part a-Polymer Chemistry 2008, 46, 3433.

[5]      Yamada, B.; Oku, F.; Harada, T. Journal of Polymer Science Part a-Polymer Chemistry 2003, 41, 645.

[6]      Liang, K.; Hutchinson, R. A.; Barth, J.; Samrock, S.; Buback, M. Macromolecules 2011, 44, 5843.

[7]      Srinivasan, S.; Kalfas, G.; Petkovska, V. I.; Bruni, C.; Grady, M. C.; Soroush, M. J. Appl. Polym. Sci. 2010, 118, 1898.

[8]      Miertuě, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117

[9]      Marenich, A. V.; Christopher, M.; Cramer, J.; Truhlar, D. G. J. Phys. Chem. B 2009, 113 6378.