Computational Study of Chain Transfer Reactions in Self-Initiated High-Temperature Polymerization of Alkyl Acrylates

Abstract

A better understanding of the kinetics of intermolecular chain transfer reactions, such as chain transfer to solvent, monomer and polymer, allows one to produce higher quality polymer resins. In this research project, kinetics of chain transfer to monomer (CTM), chain transfer to polymer (CTP), and chain transfer to solvent (CTS) reactions in self-initiated high-temperature homo-polymerization of alkyl acrylates (methyl, ethyl and n-butyl acrylate) were studied. Possible mechanisms of the chain transfer reactions were studied using density functional theory (DFT) calculations. Transition state theory was used to estimate rate constants of the reactions. Effects of live polymer chain length, type of mono-radical that initiated the live polymer chain, and type of live polymer chain radical (tertiary vs. secondary) on the energy barriers and rate constants of the involved reaction steps were investigated theoretically. The results indicated that abstractions of a hydrogen atom (by a live polymer chain) from the methyl group in methyl acrylate (MA), the methylene group in ethyl acrylate (EA), and methylene groups in n-butyl acrylate (n-BA) are the most likely mechanisms of CTM. Among all possible CTP reaction mechanisms, hydrogen atom abstraction from a tertiary carbon atom, which leads to the formation of a stable tertiary radical, was identified as the most favorable mechanism for CTP reaction in alkyl acrylates. The CTP reactivity of MA, EA, and n-BA was also compared; the difference in the end-substituent group (MA, EA, or n-BA) does not affect the kinetics of the CTP reactions remarkably. Tertiary hydrogens of dead polymers formed by disproportionation reactions are most likely to be transferred to live polymer chains in CTP reactions. Chain transfer reactions to several solvents such as butanol (polar, protic), methyl ethyl ketone (MEK) (polar, aprotic), and p-xylene (nonpolar) were studied theoretically. The estimated lower activation energies and higher rate constants of chain transfer to n-butanol compared to those of MEK and p-xylene are indicative of the higher CTS reactivity of n-butanol. Among n-butanol, sec-butanol, and tert-butanol, tert-butanol has the highest CTS energy barrier and the lowest rate constant. Polarizable continuum model (PCM) and conductor-like screening model (COSMO) solvation models were applied to explore chain transfer to n-butanol, MEK and p-xylene from polymer chains of MA, EA and n-BA. The type of mono-radical generated via self-initiation has little or no effect on the capability of MA, EA and n-BA live polymer chains to undergo chain transfer reactions. Energy barriers of the chain transfer to solvent reactions do not change significantly with the length of the live polymer chain. Before this work, although a general mechanism for chain transfer reactions had been presented, no specific mechanisms for different types of chain transfer reactions were proposed. Moreover, before this study it was inconclusive which dead polymer structure was most likely to provide the hydrogen atom…