Understanding Polymer Zip Length through the Lens of Mechanistic Modeling

Abstract Zip length, or the number of monomers released between initiation of depolymerization and loss of the depropagating radical species via termination and/or transfer, is a key metric for assessing the kinetic recyclability of a polymer, with higher values indicating greater potential for monomer recovery. In the context of radical depolymerization, mechanism-dependent analytical zip length expressions have been derived as a function of kinetic rate coefficients, molecular weight, density, and temperature; continuum models based on the method of moments (MoM) reproduce these expressions because they share the same underlying assumptions. However, we demonstrate that these analytical expressions do not accurately reflect the true zip length under typical conditions due to the accumulation of monomer radicals that cannot undergo further depropagation. To investigate this phenomenon, we compared two kinetic modeling approaches, kinetic Monte Carlo (KMC) and population balance modeling with the MoM, across a broad parameter space spanning two initiation mechanisms, multiple kinetic parameter sets, and a range of initial chain lengths and dispersities. The relationship between initial chain length and simulated zip length is not a simple threshold comparison but rather depends on the ratio of the initial degree of polymerization to zip length, the initiation mechanism, and the kinetic parameter set, which we have quantified using a dimensionless value we term “scaled kinetics of zip length”, or the Skoz number. We then apply an expanded KMC model with kinetic parameters derived from DFT calculations to poly(diethyl muconate), a recently developed recyclable polyolefin system with a mechanism akin to those analyzed, demonstrating how a mechanistically grounded understanding of zip length, including its evolution during depolymerization and its dependence on initiation barrier, can guide the rational design of kinetically recyclable polymers.