First principles assessment of solvent induced cage effects on intramolecular hydrogen transfer in the free radical polymerization of acrylates

We investigate the rate constant of poly-butyl acrylate backbiting between 310 and 510 K using semi-empirical metadynamics in the gas phase, bulk and solution. The simulations in the condensed phase are performed through a hybrid quantum mechanics/molecular mechanics approach. The free energy landscape associated with the reactive events under vacuum and in the condensed phase is used to correct harmonic transition state theory (TST) rate constants. The Arrhenius parameters so determined are introduced in a semi-detailed mechanistic kinetic mechanism of butyl acrylate polymerization in bulk and in solution, allowing it to test how the butyl acrylate polymerization rate is affected by solvent-induced cage effects on backbiting. The results show that the backbiting rate constant is higher in the condensed phase than in the gas phase. In addition, a twofold increase is observed in xylene compared to the bulk. These results differ significantly from previous theoretical calculations, especially at high temperatures, aligning better with experimental rate measurements. The semi-detailed model, incorporating our calculated rate coefficients, is validated against monomer concentration profiles from bulk and solution polymerizations in various reactor configurations, demonstrating good agreement with experimental data. This study paves the way for developing detailed kinetic models in the condensed phase using a priori kinetic parameters derived from molecular simulations, thus widening their range of applicability beyond the one experimentally accessible.