Solvolytic Depolymerization of PA6 from Marine Waste Using Structurally Diverse Deep Eutectic Solvents

The accumulation of discarded fishing nets in coastal zones significantly contributes to marine pollution, with polyamide 6 (PA6) being the predominant polymer in these residues ¹. Chemical recycling through depolymerization offers a promising strategy to transform this waste into high-value building blocks, particularly ε-caprolactam, supporting circular economy initiatives while reducing ocean contamination.

Neoteric solvents have the potential to act as catalytic cosolvents for other purposes ². In this study, deep eutectic solvents (DESs) were evaluated as green media for depolymerizing PA6 recovered from fishing nets. The tested DESs included choline chloride (ChCl)/urea, ChCl/glycerol, ChCl/monoethanolamine (MEA), and ChCl/diethanolamine (DEA) under mild conditions (160–180°C). The catalytic effects of 4-(dimethylamino)pyridine (DMAP), solvolysis time, and the reaction system were investigated to determine their effectiveness in promoting depolymerization. After solvolysis, the crude reaction mixture was filtered to assess conversion, and attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy was used to monitor the depolymerization process by identifying the functional groups associated with PA6 chain cleavage.

Although several DESs facilitated polymer dissolution, those containing MEA and DEA exhibited significantly higher depolymerization efficiencies. This enhanced performance was attributed to their dual function, acting not only as solvents but also as nucleophilic agents capable of attacking amide bonds, thereby promoting solvolysis reactions under the tested conditions. The findings of this screening enable the selection of MEA- and DEA-ChCl-based DESs as the most promising candidates for subsequent optimization through experimental design. This approach will contribute to advancing chemical recycling pathways for fishing net waste by enabling the recovery of high-value building blocks for future production of new polyamides and advanced materials, thus supporting environmental sustainability while generating added value within the polymer recycling chain.