283104 A Theoretical Study of Mechanisms for Chain Transfer to Monomer Reactions in Alkyl Acrylates
Alkyl acrylates are widely used as primary binders in coatings formulations for the automobile industry [1-3]. The basic nature of acrylic resins and the plants producing the resins have changed considerably over the past decades, as a result of environmental limits on allowable volatile organic contents (VOCs) of resins [4-6]. High temperature (>100ºC) polymerization of alkyl acrylates allows for the production of high-solids low-molecular-weight resins, but it also permits secondary reactions to occur at higher rates, including spontaneous initiation (in the absence of known added initiators), chain transfer to monomer (CTM) and polymer, back-biting, and β-scission [1, 2, 7-11]. The polydispersity index, which is a measure of the breadth of the polymer chain length distribution, was found to be 1.5 to 2.2 in high-temperature homo-polymerization of alkyl acrylates [10, 12]. This indicates that various types of chain transfer reactions occur abundantly at high temperatures.
Propagation rate constants of alkyl acrylates and methacrylates were determined using different density functional theory (DFT)-based hybrid functionals . In our previous work, we used B3LYP/6-31G* to study several mechanisms of CTM reaction in methyl acrylate (MA) . Hydrogen abstraction from methyl substituent group in the monomer by the live chain radical was identified as the most probable mechanism for CTM in MA. Before this work, there was no computational/theoretical comparison of CTM mechanisms in high-temperature free-radical polymerization of alkyl acrylates.
This paper presents newer results from a detailed computational investigation of four possible mechanisms of CTM in self-initiated homo-polymerization of methyl, ethyl and n-butyl acrylate. We have used B3LYP, X3LYP and M06-2X, and WB97X-D functionals, and 6-31G(d), 6-31G(d,p), 6-311G(d), and 6-311G(d,p) basis sets. Energy barriers and rate constants of the reactions involved in the four mechanisms have been estimated. The effects of live polymer chain length, the type of mono-radical that initiated the live polymer chain, and the type of live polymer chain radical (tertiary vs. secondary) on the kinetics of the reactions have been studied. The rigid rotor harmonic oscillator (RRHO) approximation has been applied to investigate the thermodynamics and kinetics of the reactions. Hydrogen abstraction from methylene substituent group in the monomer by the live chain radical has been found to be the most probable mechanism for CTM in ethyl acrylate and n-butyl acrylate. EA and n-BA live chains, initiated by two different types of monoradicals obtained via self initiation, have showed nearly similar rate constants of hydrogen abstraction from monomer. We have determined that increasing the chain length of the live polymer has negligible effect on the barriers and rate constants of chain transfer to monomer. The transition state geometries and activation barriers have been validated using different levels of theory.
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