Reactive Molecular Simulation with Size Extrapolation to Bridge the Polymerization Mechanism and Kinetics

Reactive molecular dynamics (MD) is performed to simulate polymerization and bridge the microscopic reaction behavior with kinetics. The reaction rate constant computed by MD simulations shows size dependence and can be extrapolated to estimate the corresponding rate constant at the macroscopic scale. Polymerization of Lennard-Jones monomers with a free radical mechanism is simulated to demonstrate that the reactive simulation with size extrapolation is capable of the qualitative modeling reaction kinetics of chain growth and chain transfer reactions. The approach is extended to the polymerization of polystyrene (PS) by a living free radical polymerization mechanism, in which ethylbenzene is represented by all-atom and coarse-grained molecular representations. The kinetics behavior of PS chain growth agrees favorably with experiments, and the yielded PS product displays a narrow distribution with a polydispersity index of 1.04 for >90% conversion of styrene monomers. The comparisons of simulation results between all-atom and coarse-grained models show that the kinetics and reaction dynamics of the polymerization of PS are consistent at both levels. The method demonstrates the possibility of using reactive MD with size extrapolation to model the polymerization kinetics of real systems, which paves the way toward large-scale modeling of polymerization bridging the reaction mechanism and kinetics and further provides new insights into reaction kinetics from a computational perspective.