Reactivity of Aliphatic and Phenolic Hydroxyl Groups in Kraft Lignin towards 4,4' MDI.

Several efforts have been dedicated to the development of lignin-based polyurethanes (PU) in recent years. The low and heterogeneous reactivity of lignin hydroxyl groups towards diisocyanates, arising from their highly complex chemical structure, limits the application of this biopolymer in PU synthesis. Besides the well-known differences in the reactivity of aliphatic and aromatic hydroxyl groups, experimental work in which the reactivity of both types of hydroxyl, especially the aromatic ones present in syringyl (S-unit), guaiacyl (G-unit), and -hydroxyphenyl (H-unit) building units are considered and compared, is still lacking in the literature.