Kinetic Model of Radical Ring-Opening Polymerization of Asymmetric Five-Membered Cyclic Ketene Acetals

Abstract

Radical ring-opening polymerization RROP of cyclic ketene acetals (CKAs) provides a promising route to biodegradable polyesters. However, the mechanistic factors determining polymer structure are still not well understood, especially for CKAs with asymmetricly substituted rings. In this study, we investigate a series of five-membered CKAs bearing electron-donating alkoxymethyl groups at the 4-position, synthesized from bio-based precursors. Through detailed NMR analyses, DFT-calculated rate constants, and a comprehensive kinetic model, we clarify how 4-position substitution influences the balance between propagation, β-scission, and backbiting pathways. The model successfully reproduces the experimentally observed polymer structures across a wide range of temperatures and monomer concentrations, and its applicability extends to CKAs with varying alkoxy groups. The incorporation of ester linkages via ring-opening was confirmed by NMR and correlated with partial biodegradation in OECD 301F tests. These findings establish a predictive framework that links monomer structure for advancing the design of sustainable and biodegradable radical polymers.