(56f) A Theoretical Study of Mechanisms of Alkyl Acrylate Polymer Chain Transfer to Several Solvents

Acrylates are principal monomers in the production of coatings, adhesives, and polymers used in medical and also pharmaceutical applications, due to their transparency and resistance to breakage [1, 2]. The basic design of resins used in automobile coating has been changed because of the environmental limits on allowable volatile organic contents (VOCs) of resins [3, 4]. High-temperature (>100ºC) polymerization of alkyl acrylates allows for reducing the solvent content of the resins, while keeping the resin viscosity within a desired range [5, 6]. However, at the high temperatures secondary reactions such as spontaneous initiation, backbiting, β-scission, and chain transfer to monomer, polymer and solvent reactions have stronger effects on the quality of the resins [7-10].

Previous studies [11-14] showed the successful use of computational quantum chemistry to better understand self-initiation, propagation, and co-initiation reactions in polymerization of alkyl acrylates. In our previous work [15, 16], we used different functionals (B3LYP, X3LYP, M06-2X) to study several mechanisms of chain transfer to monomer (CTM) reaction in thermal polymerization of methyl, ethyl, and n-butyl acrylate (MA, EA, n-BA). The kinetics of the most likely chain transfer to polymer (CTP) mechanisms was also studied for MA, EA, and n-BA [17]. To the best of our knowledge, no computational study of chain transfer to solvent (CTS) reactions in thermal polymerization of alkyl acrylates has been conducted as of yet.

This paper presents a computational study of CTS reaction mechanisms in self-initiated high-temperature polymerization of MA, EA, and n-BA in solvents such as butanol (polar, protic), methyl ethyl ketone (MEK) (polar, aprotic), and p-xylene (nonpolar). Three types of hybrid functionals (B3LYP, X3LYP, and M06-2X) and three different basis sets (6-31G(d,p), 6-311G(d), and 6-311G(d,p)) have been applied to predict energy barriers and molecular geometries of reactants, products, and transition states. The sensitivity of the levels of theory to barriers and rate constants has been evaluated. The effects of live polymer chain length and the type of mono-radical that initiated the live polymer chain on the kinetics of CTS reactions have also been investigated. Solvent effects have been explored for CTS reactions using polarizable continuum model (PCM). Among chain transfer to n-butanol, sec-butanol, and tert-butanol reactions, the tert-butanol reaction has the highest energy barrier and the lowest rate constant. We have identified that the type of mono-radicals generated via self-initiation had little or no effect on the ability of the MA, EA, and n-BA live polymer chains to undergo CTS reactions. The energy barriers do not change significantly with the length of the live polymer chains. Lower activation energy and higher rate constant of chain transfer to n-butanol reaction (than those estimated for chain transfer to MEK and p-xylene reactions) indicate the higher ability of n-butanol to facilitate chain transfer.

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