Multithiol-Functionalized Lignin as a Bio-Based Macromolecular Thiol for Thiol-ene Click Chemistry: Proof-of-Concept for Surface Property Control in Packaging Applications

This work reports a scalable, two-step, gram-scale synthesis of a macromolecular multithiol-lignin (degree of substitution, DS ≈ 0.70; ∼70 SH per 100 C 9 units), which serves as a thiol component in thiol-ene chemistry with allylcholine chloride as a green “ene” partner. Lignin was first chloroacetylated (92% yield). Subsequently, reaction with thiourea tethered abundant −SH groups along the polymer backbone, affording the multithiol intermediate in quantitative yield. The intermediate was evaluated in thermal-, base-, and UV-initiated thiol-ene reactions; among these, UV-mediated coupling with allylcholine chloride resulted in a cationic, quaternary-ammonium-lignin (DS ≈ 0.80, nitrogen content = 2.72 wt %). Fourier transform infrared (FTIR), 1 H-/ 13 C nuclear magnetic resonance (NMR), heteronuclear single quantum coherence (HSQC), scanning electron microscopy (SEM)–energy-dispersive X-ray spectroscopy (EDS), and EA confirmed successful functionalization, while differential light scattering (DLS), contact angle, and surface free energy (SFE) analyses indicated a marked increase in hydrophilicity after quaternization. The functionalized lignin was formulated into membranes for food-packaging tests, which blocked ≈100% UV light and showed ≈75% antioxidant activity, indicating potential as a sustainable, surface-active, and antimicrobial material. Overall, this mild and efficient strategy introduces multithiol groups onto lignin, which can be exploited for property tuning via the degree of functionalization and the hydrophobic–hydrophilic–ionic balance for diverse applications.