Valorization of lignin for renewable non-isocyanate polyurethanes: a state-of-the-art review

Due to the merits of abundant and unique structural features, the valorization of lignin, the second largest naturally abundant polymer, for the preparation of valuable biomaterials, has become a research hotpot. This review emphasized on the opportunities and challenges of using the functionalized lignin as precursor for the production of eco-friendly and non-hazardous non-isocyanate polyurethanes (NIPUs) as alternates to the conventional toxic diisocyanate-based PUs. The chemical structure, the fractionation processes, and the functionalization strategies of lignin (e.g. alkoxylation, phenolation, etherification, urethanation, hydroxymethylation, amination, nitrification, and sulfomethylation) are summarized, followed by shedding light on the current trends in synthesizing the NIPUs using lignin derivatives as the starting molecules. Potential directions for the application of the lignin-based NIPUs and the challenges in commercialization are also highlighted in this review.