Breaking the Ring and Closing the Loop: New Bicyclic Monomers for Sustainable Polymers

Designing polymers that combine high performance with efficient chemical recycling is essential for building a circular materials system. To this end, lactone-based polyesters are attractive candidates, yet traditional monocyclic lactones offer limited ring strain and narrow thermodynamic windows. As a result, balancing polymer stability with effective depolymerization poses a challenge in the field. This review focuses on recent progress in the development and use of bicyclic lactone monomers. Specifically, we examine how fused, bridged, and spiro structures introduce higher ring strains, more rigid conformations, and definable stereochemical environments in the designed monomer. These features allow the precise tuning of polymerization behavior, material properties, and monomer recovery. By linking molecular architecture to thermodynamic principles and polymer structure, we establish design rules for next-generation chemically recyclable polyesters. Looking ahead, computational approaches such as DFT and machine-learning-based screening, may support the discovery of new monomers and help accelerate progress in this promising field of polymer science and engineering.