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 incorporation2-5. 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.