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 C₉ 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), ¹H-/¹³C 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.