Modular Access to Aliphatic Polycarbonates with Tunable Properties and Dual Closed-Loop Recyclability by Polycondensation-Depolymerization-Repolymerization Strategy.

Great achievements have been made in CO/epoxide copolymerization and dialkyl carbonate/diol polycondensation; however, efficient preparation of high-molecular-weight (>100 kDa) aliphatic polycarbonates with tunable properties and recyclability under mild conditions still remains as a great challenge. Herein, we presented a "polycondensation-depolymerization-repolymerization" strategy for structurally diverse aliphatic polycarbonates. This involved a step growth polycondensation of dialkyl carbonate and diol to low-molecular-weight (0.5-1.9 kDa) polycarbonates under atmosphere pressure, which are then utilized to produce cyclic carbonate monomers through catalytic depolymerization. The ring-opening polymerization of cyclic carbonate led to high molecular weight (>100 kDa) polymers, which can be converted back to cyclic monomer via ring-closing depolymerization or diol/dialkyl carbonate via alcoholysis, enabling chemical recycling of polycarbonates via dual closed loops. The thermal and mechanical properties of the polycarbonates can be widely adjusted by varying the substituent, and polycarbonate with four-membered spiro-cyclic substituent shows a recorded high melting temperature (217 °C) and mechanical strength within the reported polycarbonate family. A(hard)-B(soft)-A(hard) triblock thermoplastic elastomers with good mechanical performance and elastic recovery were also created by sequential polymerization. The "polycondensation-depolymerization-repolymerization" strategy provided a powerful toolbox for developing high-performance aliphatic polycarbonates.