Closed-Loop Chemical Recycling of Polyamide Thermosets via Reversible Amidation

The development of closed-loop chemically recyclable thermosets is highly desirable for achieving a circular polymer economy and promoting environmental sustainability. A key challenge to identify these goals is to identify chemical reactions with sufficient reversibility to construct polymer networks. In this study, we developed a new type of reversible amidation reaction based on the reaction between dihydrocoumarin and primary amines. The six-membered phenolic lactone group in dihydrocoumarin can be quantitatively ring-opened by a strong nucleophilic primary amine group under ambient and catalyst-free conditions to form an amide bond. This bond can be efficiently cleaved back into the dihydrocoumarin and ammonium groups in the presence of acid under mild heating conditions, via the neighboring group participation effect. We further employed the reversible amidation reaction to construct polyamide thermosets with tunable mechanical properties ranging from those of strong elastomers to rigid plastics by simply tailoring the monomer structures. Notably, these polyamide thermosets can be depolymerized back to their pristine monomers, with yields exceeding 90%, via reverse amidation. The recovered monomers can then be repolymerized to regenerate pristine polyamide thermosets with identical structures and mechanical properties, demonstrating closed-loop chemical recyclability.