(279d) Understanding Backbiting and Beta-Scission Reactions in Self-Initiated Polymerization of Methyl Acrylate: A Theoretical Study

The polymer industry uses free-radical polymerization of acrylates widely to manufacture variety of paints, coatings and adhesives[1,2]. High-temperature (> 100^oC) polymerization of alkyl acrylates has been extensively studied and is known to allow for generating low molecular weight, low solvent resins[3,4]. Recent studies have shown that secondary reactions such as self-initiation[4], backbiting and beta-scission occur in high-temperature polymerization of alkyl acrylates[5]. A better quantitative understanding of these reactions can facilitate the rational design of chemically self-regulated polymerization processes.

Quantum chemical calculations can be applied to identify the rate constants, intermediates and mechanisms of individual reactions such as backbiting and beta-scission without a  need for a macroscopic model.  Both wave function-based quantum chemical methods and density-functional theory (DFT) have been successfully applied to study the mechanisms and kinetics of different types of reactions in free-radical polymerization of ethylene[6-8], styrene[9], methyl methacrylate[10,11], and methyl, ethyl and n-butyl acrylate[12,13].

In this work, we apply density functional theory (DFT) methods to investigate the mechanisms of backbiting and beta-scission reactions in thermal polymerization of methyl acrylate. This monomer was chosen to make the large size of the polymer system computationally manageable. We choose three different functionals, B3LYP, M06-2X and PBE0 with the 6-31G* basis set. Backbiting reactions from propagating radicals that have been initiated by two types of radicals (1) monomeric mono-radical (MMR) and (2) dimeric mono-radical (DMR) are investigated. Four backbiting mechanisms and two types of beta-scission reactions are explored. We have found that the 1:5 backbiting mechanism with a six-membered ring transition state and 1:7 backbiting with an eight-membered ring transition state are kinetically more favorable than 1:3 backbiting and 1:9 backbiting. Reactions involving longer polymer chains tend to have higher energy barriers to undergo intramolecular hydrogen transfer. The beta-scission reaction of a mid-chain radical (MCR) is found to be symmetric as the radical can form double bond with its closest left or right secondary carbon atom with nearly equal activation energy. The nuclear magnetic resonance (NMR) chemical shifts (¹H and ¹³C) of species generated from these mechanisms are computed and compared. The chemical shifts of carbon nucleus in non-branched polymer chains and branched polymer chains have been estimated for poly(methyl acrylate).

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