Combining Metadynamics and Mean Force Integration for studying chemical reactions in solution: an application to backbiting of poly-Butyl Acrylate.

Free radical polymerization (FRP) and pyrolysis processes are fundamental technologies with wide-ranging applications in fields such as material synthesis and recycling. Understanding the underlying reaction mechanisms and considering the effect of solvents is crucial for the design and scale-up of such processes. However, since pyrolysis lacks selectivity and radical intermediates are hard to measure due to their extremely short lifetimes, rate coefficients from experimental campaigns on this subject are scarce. Nevertheless, experiments on free radical polymerization have been a flourishing field of study in past two decades. Knowledge of rate parameters of elementary reactions in solution or in bulk is key for formulating a valid kinetic mechanism. To this day, individual kinetic rate parameters of radical chain propagations, depropagations as well as isomerizations (backbiting) can be accessed experimentally through Pulsed Laser Polymerization technique (PLP) coupled with Size Exclusion Cromatography (SEC) [1] as well as semibatch solution polymerization coupled with Nuclear Magnetic Resonance (NMR) [2]. The resulting rate coefficients are gathered in the IUPAC Database for standard monomers which represents the gold standard for the validation of theoretical calculations. In particular, the experimental studies taken as reference [1-2] report the propagation, backbiting and beta-scission rate coefficients for poly-Butyl Acrylate in bulk and in solvent (mainly a mixture of ortho/meta/para-xylene). In this work, accelerated molecular dynamics in conjunction with Mean Force Integration (MFI) have been employed for exploring the free energy landscape of the backbiting of poly-Butyl Acrylate (PBA) in gas phase as well as in solution with non-polar (o-/m-/p-xylene) and polar (water) solvents