Ring-Opening Polymerization of Cyclic Acetals: Strategy for both Recyclable and Degradable Materials

To cope with the severe plastic waste crisis, massive efforts are made to develop sustainable polymer materials whose degradation involves a disposing and decomposing to small molecule (DDM) and/or a chemical recycling to monomer (CRM) process. Polyacetals, a type of pH-responsive polymers, are degradable under acidic conditions, while highly stable under neutral and basic circumstances. As for their synthesis, the cationic ring-opening polymerization (CROP) of cyclic acetals is an elegant and promising approach, though suffering from fatal side reactions and polymerization-depolymerization equilibrium. Recent development in CRM restimulates the interest in the long-forgotten CROP method due to its inherent depolymerization characteristics. In terms of the end-of-life options, polyacetals are recyclable materials with both DDM and CRM potentials. They not only expand the scope of materials for closed-loop recycling but also help to tune the degradation properties of traditional polyesters and polyolefins. This review aims to discuss the synthesis of various polyacetals by CROP and their degradation properties from the perspectives of 1) polymerization of cyclic acetals, dioxepins, and hemiacetal esters, 2) copolymerization of cyclic acetals with heterocyclic or vinyl monomers, and 3) degradation and recycling properties of the related polymers.