Green Circular Design for Unmodified Lignin: Solvent-Free Valorization into High-performance Covalent Adaptable Networks

Abstract

This study presents a solvent- and purification-free solubilisation-cross-linking strategy to valorize industrial lignin waste into covalent adaptable networks (CANs). Unmodified lignin from pulping wastewater undergoes hydrogen bond-driven solubilisation in bio-based 1,3-propanediol (PDO), forming homogeneous solubilised lignin without chemical modification. Subsequent catalyst-free copolymerization with itaconic anhydride (ITA) yields CANs with exceptional mechanical strength (63 MPa), exceeding reported lignin-based CANs and rivaling bisphenol-A epoxies. Ortho-carboxyl group enables internal catalysis of ester exchange, allowing >74% strength retention after thermal reshaping and selective degradation by PDO for closed-loop oligomer repolymerization. Pre-cross-linked films exhibit strong interfacial bonding to tinplate (7.9 MPa) and wood (4.6 MPa) with heat-triggered debonding. The process avoids volatile organics, metal catalysts, petrochemicals, and purification, achieving 100% atom economy and >98% bio-based content. This work establishes a paradigm for transforming lignin into circular materials that address plastic sustainability and biomass valorization.