Engineering plastics such as polyamides play a crucial role in modern society in areas where the material/product needs to withstand mechanical and environmental conditions such as packaging, infrastructure, and textile industries. Nylon-6 is a thermoplastic polyamide, industrially produced by water-assisted ring-opening polymerization of ε-caprolactam on a scale of 7-8 million tons annually with a market size expected to reach $21.5 billion by 2026. The high demand and good resistance to chemicals and abrasion also prevent it from being biodegradable, causing it to persist and accumulate in the environment resulting in a severe threat to aqueous and terrestrial ecosystems and human health. To avoid such scenarios, a circular economy is urgently needed in which waste plastics are recycled and repurposed. Mechanical recycling, a common method in commodity plastic recycling, is rarely used for Nylon since the elevated temperatures necessary for reprocessing lead to partial degradation, yielding recycled Nylon with inferior properties. The presence of contaminants in post-consumer products also prevents the formation of pristine material. Chemical recycling is an alternative approach to recover the starting building block (monomer), which can create the same high-quality material.<br/><br/>In this work, we report that organolanthanides catalysts can efficiently catalyze the depolymerization of Nylon-6 to recover ε-caprolactam. The reaction is clean (95 % selectivity towards ε-caprolactam), high yielding (&gt;90 %), solvent-free, and to the best of our knowledge, has the lowest reported depolymerization temperature to ε-caprolactam (240 °C). The catalytic activity correlates with the size of the lanthanide ion, with the largest showing the highest catalytic activity. The reaction works for virgin polymer and post-consumer products and is compatible with mixed plastics such as PE, iPP, and PET. Kinetic investigations and DFT calculations suggest a chain-end backbiting depolymerization reaction mechanism.

Selective organolanthanide catalyzed depolymerization of nylon-6 to ε-Caprolactam

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