Laccase-Catalyzed Copolymerization of Tannin and Technical Lignin: Polymerization Kinetics and Enhanced Properties of Polyphenols

Polymerization of lignin has garnered significant interest due to its potential to unlock innovative and value-added applications for lignin to replace petroleum-based phenolic synthetics in future polymeric materials. However, conventional strategies for lignin polymerization often adopt complex cross-linking reactions with large amounts of unsustainable chemicals, which may impose environmental and ecological concerns. Herein, a green method for polyphenol polymerization is developed, in which tannin-rich extracts serve as building blocks to facilitate lignin polymerization under laccase-catalyzed oxidation. The molar mass and chemical structure of polymerized lignin, tannin, and their mixture and polymerization kinetics are comprehensively studied. Results show that the incorporation of tannin can increase the polymerization degree and rate of lignin and preserve more phenolic-OH groups after laccase incubation compared with self-polymerized lignin or tannin. Moreover, the possible polymerization mechanisms for lignin and tannin samples are proposed based on NMR and FTIR spectrum analysis. The polymerized lignin-tannin samples have superior or similar physicochemical properties compared to self-polymerized lignin or tannin in terms of UV shielding, thermal stability, and antimicrobial properties. Overall, this study offers novel insights into the copolymerization of lignin and tannin catalyzed by laccase and underpins further applications of upgraded polyphenols as polymers.