Unlocking Hexahydrotriazine Chemistry for Multifold Thermoset Circularity

Thermosets are made to last. Yet, this very durability creates significant challenges for responsible end-of-life management, rendering conventional recycling nearly impossible. Developing recyclable-by-design thermosets that can be selectively broken down through targeted chemical reactions offers a promising path forward but requires innovative chemistry. While most reported recyclable thermosets are optimized for a single end-of-life route (e.g., chemical depolymerization or reprocessing), achieving multiple distinct circularity pathways in one high-performance thermoset remains challenging. Here, we uncover that hexahydrotriazines─widely used as stable cross-linking units in thermosets─can undergo nucleophilic attack by amines and engage in metathesis reactions, unlocking new avenues for circular material design. Adopting a holistic approach, we first conduct detailed mechanistic studies to elucidate this previously unexplored reactivity, then apply these insights to polyhexahydrotriazine-based thermosets. We demonstrate a single material platform that uniquely combines closed-loop recycling, postsynthetic property reprogramming, repurposing into distinct material architectures, and thermal reprocessing. This multifold circularity introduces a flexible and adaptive strategy for thermoset recycling, addressing the increasing complexity of a circular economy.