Catalyst metal-ligand design for rapid, selective, and solventless depolymerization of Nylon-6 plastics

Developing effective catalysis to address end-of-life Nylon pollution is urgent yet remains underdeveloped. Nylon-6 is a resilient synthetic plastic and a major contributor to ocean pollution. Here, we report a metallocene catalytic system based on earth-abundant early transition and lanthanide metals that mediates Nylon-6 depolymerization at unprecedented rates up to 810 (ε-caprolactam)·mol(Cat.)−1·h−1 at 240°C in ≥99% yield. This solventless process operates with catalyst loadings as low as 0.04 mol % at temperatures as low as 220°C—the mildest Nylon-6 depolymerization conditions reported to date. This metallocene catalysis can be carried out in a simulated continuous process, and the resulting ε-caprolactam can be re-polymerized to higher-quality Nylon-6. Experimental and DFT analyses identify effective depolymerization pathways involving catalytic intra-Nylon-chain “unzipping” assisted by π-ligand effects and inter-chain “hopping.” A robust chelating ansa-yttrocene is particularly effective in depolymerizing diverse commodity end-of-life articles, such as fishing nets, carpets, clothing, and plastic mixtures.