Molecular Dynamics Simulation of In Situ Polymerization: Chain Conformation Transition

In the synthesis of polyethylene using supported Ziegler–Natta catalyst, the nascent polyethylene adopts distinct structures influenced by factors such as active site distribution, temperature, and growth rate. While experiments can achieve atomic-level observations through scanning tunnel microscopy (STM), acquiring statistical data on chain conformation and aggregation structures throughout the growth period remains challenging. To address this, we developed an in situ polymerization molecular dynamics simulation model that concurrently characterizes the growth and mobility of nascent polyethylene chains on supported catalysts. We unveiled the competition between the diffusion of monomers and the growth of nascent polymer chains. The simulation results demonstrated that conformational variations of nascent chains affect the diffusion resistance of the polymer melt to monomers with chain conformations aligned along the Z-axis facilitating monomer diffusion. Furthermore, by introducing a sidewall to simulate the nanoparticles on supported catalysts, we derived the compromise in competition between the attractive energy arising from the sidewalls and the cohesive energy within the nascent chains, resulting in diverse spatial and chain length distributions of the nascent chains. This work provides a new approach to investigate the reaction dynamics and the underlying microscopic mechanisms of in situ ethylene polymerization with the potential of further extension to the study of in situ growth for other nascent polymers.