Unraveling the Mechanism of Catalyzed Melt-Phase Polyester Depolymerization via Studies of Kinetics and Model Reactions

Developing a mechanistic understanding of catalyzed melt-phase depolymerization processes is of utmost importance to the rapidly expanding field of circular polymers with a closed chemical loop. Herein, we present a methodology to probe the mechanism of metal-catalyzed melt-phase depolymerization of polyesters utilizing an approach centered on studies of kinetics by thermogravimetric analysis and model reactions. Kinetic parameters associated with the prototypical Lewis-acid-catalyzed depolymerization of representative polyesters, including poly(δ-valerolactone) (PVL), poly(lactic acid), and poly(γ-butyrolactone), are elucidated. Focusing on PVL for further investigation of the depolymerization mechanism, effects of its molar mass, topology, and end-group chemistry are examined in detail. Overall, a catalyzed ring-closing depolymerization process to monomer from the polyester hydroxyl-chain ends is proposed as the key mechanistic step, although the process has a relatively large zip length (≈ 320) and follows nonimmortal depolymerization kinetics.