Coarse-graining (CG) approaches reduce the total number of particles in molecular dynamics (MD) simulations and often enable the use of longer integration time steps. Both of these effects serve to reduce computational expense and therefore provide access to larger length and time scales for a given system. The Martini model is a widely used CG force field, flexible and transferrable enough to find applications ranging from biological to polymer-based systems, including reactive CG MD simulations. However, CG models like Martini typically combine several atoms into a representative 'bead,' meaning that individual reaction sites effectively disappear as atomic identities are absorbed into the Martini beads. In this work we use this loss of specificity as a tool that enables the simulation of different chemical reaction mechanisms that may arise from the same CG model. This approach therefore isolates reaction mechanism as the variable in these simulations.<br/><br/>As proof of concept, we consider a few trifunctional monomers that share a similar molecular mechanical description but polymerize by different mechanisms: radical chain-growth polymerization and radical step-growth polymerization. We represent these physically similar monomers with the same CG model, a 'Universal Monomer,' and use reactive CG MD simulations to prepare highly crosslinked thermosets of each monomer. Since the monomers are represented by precisely the same model, differences observed in the polymerization reactions or in the mechanical properties of the resulting thermosets are a function of the reaction mechanism. We compare on-the-fly simulated polymerization, network topology, and simulated mechanical testing of the two reactive CG MD systems and discuss related experimental results. Further, we suggest that this Universal Monomer approach is a reasonable survey of 'mechanism-space' with application to the rational design of chemical feedstocks for photopolymerization-based additive manufacturing techniques.

Isolating the Reaction mechanism as a variable: Reactive coarse-grained molecular dynamics simulation of chain-growth versus step-growth polymerization

ACS Spring Meeting 2021

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