Facile, high-efficiency, and low-cost depolymerization of PA6 to ε-caprolactam enables closed-loop chemical recycling

Chemical recycling of non-biodegradable polyamide-6 (PA6) to monomer epsilon-caprolactam (epsilon-CPL) via a green, simple, and low-cost approach is highly desired and still a great challenge. In this study, the efficiently thermal depolymerization of PA6 and various its post-consumer products to epsilon-CPL was achieved by cheap and commercially available mixed alkali catalysts. The depolymerization was solvent-free, high-yield (similar to 88%), high-purity (similar to 97%), and carried out under simple and facile conditions. Kinetic and theoretical (DFT) mechanistic investigations reveal that this alkali-catalyzed depolymerization was based on a deprotonation of amide groups, and followed by an intramolecular cyclization to form lactam units and sequentially drop from the chain end. Significantly, the repolymerization of recycled epsilon-CPL to the PA6 was realized for the first time, and the chemical structure and mechanical properties of the regenerated PA6 were almost completely consistent with those of the initial PA6, demonstrating a proof-of-concept for closed-loop chemical recycling from PA6 to epsilon-CPL and back to PA6.