Abstract
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 p-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. 31P 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.
Highlights
Polyurethanes (PUs) are polymers conventionally synthesized through reactions between polyols and diisocyanates
The results indicated heterogeneous reactivity of lignin –OH groups, the major limitation for its use in polymers synthesis, such as polyurethanes [27,31]
A significant difference between aliphatic (Figure 4a) and phenolic hydroxyl groups (Figure 4b) reactivity was observed for both lignin samples
Summary
Polyurethanes (PUs) are polymers conventionally synthesized through reactions between polyols and diisocyanates. Hydroxyl groups are abundant in the lignin structure and they have different types randomly allocated, with several levels of reactivity towards diisocyanate. Since each Ph–OH type is located in different chemical environments, differences in their reactivity are expected For this reason, lignin hydroxyl reactivity plays a central role in polyurethane synthesis and might result in phase separation and unsuitable mechanical and thermal properties, if it is neglected during PU formulation and synthesis [5,20]. The reactivity against isocyanate of different types of phenolic hydroxyls (Figure 3) is scarcely explored in the literature. Aiming to contribute to the development of lignin-based PUs, this work investigates the reactivity of different lignin–OH groups toward 4.4 diphenylmethane diisocyanate (MDI). Tetrahydrofuran (THF), which was used as a solvent, was purchased from Labsynth (Diadema, Brazil). 4,4 -diphenylmethane diisocyanate (MDI) with 33.6% NCO groups (value provided by the supplier), N-Hydroxy-5-norbornene-2,3-dicarboximide, N,N-Dimethylformamide, Pyridine, Chloroform-d, Chromium(III) acetylacetonate and 2-Chloro-4,4,5,5-tetramethyl-1,3,2dioxaphospholane were used for NMR analysis were purchased from Sigma Aldrich and used as received
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