Pulsed laser polymerization to retrieve kinetic parameters for a propagated mid-chain radical in poly(n-butyl acrylate) synthesis: a combined DFT and kinetic Monte Carlo study

Abstract

Radical polymerization of n-butyl acrylate is characterized by backbiting of end-chain radicals (ECRs), with a propagation rate coefficient kp,e, into mid-chain radicals (MCRs), with a propagation rate coefficient kp,m. In pulsed laser polymerization (PLP) kinetic studies, it is currently assumed that a MCR becomes a (fully developed) ECR after one propagation step. Here we demonstrate that a more gradual transition likely takes place, introducing a propagated mid-chain radical (PMR) with a propagation rate coefficient kp,P or equivalently a transition propagation factor γ that at least theoretically can vary between 0 (kp,P = kp,m) and 1 (kp,P = kp,e). Kinetic Monte Carlo (kMC) simulations under free radical polymerization (FRP) conditions hint at a γ not too close to 0 (e.g. not below 0.01), and Density functional theory (DFT) calculations at a γ not too close to 1 (e.g. not above 0.8). It is further shown that the ratio of the peak heights in a PLP - size exclusion chromatography (PLP-SEC) spectrum is sensitive to a variation in γ. This opens the door to the future experimental determination of kp,P considering different frequencies, provided that the initiator radical generation is well-described. At 325 K and using 500 Hz literature data, we currently put forward a γ of 0.1. In addition, the PMR backbiting potential is evaluated, introducing a backbiting scaling factor δ with respect to the conventional ECR backbiting rate coefficient (kbb). It is showcased that this extra backbiting contributes to a better understanding of the migration mechanism and short branch formation in acrylate radical polymerization, although the experimental determination of δ is less straightforward. Overall the current work highlights how acrylate-specific rate coefficients can be obtained in a roadmap format, considering higher and lower frequencies, and lower and higher temperatures.