Alternating CO₂ and bicycloalkane copolymerization to circular polyesters

Transforming abundant but inert CO₂ into useful polymers has been pursued since the 1960s (ref. ¹) and has typically been achieved by copolymerization with a reactive comonomer, aided by a catalyst, which can address both thermodynamic constraints and high kinetic barriers associated with CO₂ fixation and incorporation^2-5. However, making polyesters remains a challenge as alternating copolymerization of CO₂ with alkenes is thermodynamically infeasible⁶. Here we introduce a closed-loop CO₂-based polyester platform for producing high-performance yet recyclable polyesters by direct alternating copolymerization of CO₂ with bicycloalkanes, bicyclic butane (BCB) and pentane (BCP) monomers. This copolymerization is initiated by a simple organic catalyst and proceeds in a perfectly alternating fashion to high-molar-mass polyesters with maximum (50 mol%) CO₂ incorporation and architecturally defined backbones, in which the in-chain ring structure enables tailorable thermal and mechanical properties. These polyesters exhibit desired orthogonal performance and end-of-life outcomes. Although the BCB-CO₂ polyesters exhibit exceptional thermal and hydrolytic stability across the full pH range, they can be selectively depolymerized in bulk and base-catalysed conditions to regenerate pure BCB monomers in >90% isolated yield. The BCP-CO₂ polyesters can also be selectively depolymerized but to bicyclolactones. Sequential depolymerization-repolymerization cycles establish circular lifecycles for BCB/BCP-CO₂ high-performance polyesters.