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

• Published in: Molecules (Basel, Switzerland) Vol. 26; no. 8; p. 2131
• Main Authors: Antonino, Leonardo Dalseno, Gouveia, Júlia Rocha, de Sousa Júnior, Rogério Ramos, Garcia, Guilherme Elias Saltarelli, Gobbo, Luara Carneiro, Tavares, Lara Basílio, Dos Santos, Demetrio Jackson
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI AG 07.04.2021; MDPI
• Subjects: Aliphatic compounds; Biopolymers; Carbon; chemical reactivity; Diphenyl methane diisocyanate; hydroxyl group; Hydroxyl groups; Lignin; lignin-based polyurethanes; NMR; Nuclear magnetic resonance; Phenolic compounds; Phenols; Polyurethane; Polyurethane resins; Prepolymers; Quantitative analysis; Reactivity; Solvents; Spectrum analysis; Brazil; United States > US; Massachusetts; chemical reactivity; hydroxyl group; lignin-based polyurethanes
• ISSN: 1420-3049; 1420-3049
• DOI: 10.3390/molecules26082131

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. In this work, the hydroxyl reactivity of two kraft lignin grades towards 4,4'-diphenylmethane diisocyanate (MDI) was investigated. P NMR allowed the monitoring of the reactivity of each hydroxyl group in the lignin structure. FTIR spectra revealed the evolution of peaks related to hydroxyl consumption and urethane formation. These results might support new PU developments, including the use of unmodified lignin and the synthesis of MDI-functionalized biopolymers or prepolymers.