Efficient continuous waste PLLA degradation via stearyl alcohol-mediated transesterification catalyzed by tin(II) octadecyl alkoxide

The chemical recycling of waste poly(L-lactic acid) (PLLA) into its monomer, L-lactide, offers a promising strategy for establishing a circular economy for PLLA-based polymers. However, conventional approaches typically require high reaction temperatures and suffer from significant side reactions, thereby hindering the efficient recycling of waste PLLA. In this study, we present a low-temperature, continuous depolymerization process for waste PLLA, catalyzed by stearyl alcohol (StOH) and stannous octanoate (Sn(Oct) 2 ) at a mild temperature of 165 °C. This system achieves an L-lactide yield of 99.31 % and purity of 98.02 % (ee = 99.73 %). Characterization of the reaction mechanism, supported by density functional theory (DFT) calculations, reveals that StOH displaces the octanoate ligands in Sn(Oct)₂ to form tin(II) octadecyl alkoxide. This nucleophilic species interacts with the carbonyl groups of PLLA, promoting chain scission via ligand-assisted attack. The mechanism reduces the energy barrier for backbiting reactions, thereby enhancing the rate of L-lactide formation. This method exhibits strong practical viability, achieving a 97 % yield from real-world PLLA waste and selectively degrading PLLA in mixed plastic streams to enable efficient separation. Furthermore, a simplified continuous processing system was developed, demonstrating stable and sustained PLLA degradation. This work provides an innovative and scalable strategy for sustainable PLLA chemical recycling under mild conditions.