Backbiting and β-scission reactions in free-radical polymerization of methyl acrylate

Multiple mechanisms of backbiting and β-scission reactions in free-radical polymerization of methyl acrylate are modeled using different levels of theory, and the rigid-rotor harmonic-oscillator (RRHO) and hindered-rotor (HR) approximations. We identify the most cost-effective computational method(s) for studying the reactions and assess the effects of different factors (e.g., functional type and chain length) on thermodynamic quantities, and then identify the most likely mechanisms with first-principles thermodynamic calculations and simulations of nuclear magnetic resonance (NMR) spectra. To this end, the composite method G4(MP2)-6X is used to calculate the energy barrier of a representative backbiting reaction. This calculated barrier is then compared with values obtained using density functional theory (DFT) (B3LYP, M06-2X, and PBE0) and a wavefunction-based quantum chemistry method (MP2) to establish the benchmark method. Our study reveals that the barriers predicted using B3LYP, M06-2X, and G4(MP2)-6X are comparable. The entropies calculated using the RRHO and HR approximations are also comparable. DFT calculations indicate 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 energetically more favored than 1:3 backbiting and 1:9 backbiting mechanisms. The thermodynamic favorability of 1:5 versus 1:7 backbiting depends on the live polymer chain length. The activation energies and rate constants of the left and right β-scission reactions are nearly equal. The calculated and experimental 13C and 1H NMR chemical shifts of polymer chains affected by backbiting and β-scission reactions agree with each other, which provides further evidence in favor of the proposed mechanisms. © 2013 Wiley Periodicals, Inc.