Functional design of caprolactam for aliphatic polyamide with enhanced flame retardancy, mechanical stability, and chemical recyclability

Polyamide 6 (PA 6) is an important polymer widely used in daily life. However, increasing demands for high performance and recyclability, driven by cutting-edge applications and the ongoing energy crisis, have presented major challenges to the development of PA 6. In this work, we report a simple copolymerization method to design a PA 6 copolymer with high performance and chemical recyclability. A functional monomer named ISA was synthesized via the imidization reaction between isatoic anhydride and the active amino group in α-amino-ε-caprolactam, a mature bio-based platform molecule. The chemical similarity between ISA and caprolactam (CPL) ensured high-quality polymerization, resulting in the corresponding copolymer PA 6- co -ISA with high molecular weight, excellent spinnability, great melt-crystallization performance, and mechanical robustness. As expected, high flame retardancy was obtained thanks to the gaseous dominated flame-retardant effect of the N-containing heterocyclic ring. More importantly, owing to the unchanged polymeric main chain structure, the copolymer could be easily recycled to CPL, and the repolymerized polymer showed great overall properties equal to the original one. This work provides a feasible approach for the preparation of high-performance and closed-loop recyclable polymeric materials.