Bio-polyol chemical design for self-healing boronate ester gels by green oxyalkylation of organosolv lignin

Abstract

Lignin, the most abundant aromatic biopolymer, has a high potential as an alternative to fossil resources in the chemical industry. However, the non-uniformity of lignin is currently a drawback for high-end applications. In this work, glycerol carbonate being a green and safe cyclic carbonate was therefore applied in the oxyalkylation of organosolv lignin (weight average molecular weight of ≈ 8,300 g mol⁻¹; aliphatic OH content of ca. 2.61 mmol g⁻¹) to obtain a lignin-based polyol with solely aliphatic OH functionalities. The catalyst type, reaction temperature and time and additional solvents were evaluated in the oxyalkylation with optimal settings using K₂CO_3, 175 °C, 30 min reaction time without any additional solvent to make a modified lignin with a weight average molecular weight of ca. 15,000 g mol⁻¹ and an aliphatic OH content of ca. 4.59 mmol g⁻¹. To support mechanistic understanding it is shown that the carboxylic acid and phenolic hydroxyl functionalities are converted completely into 1,2-diols, while native aliphatic OH functionalities take at most slightly part in the modification reaction. Furthermore, upon the formation of vicinal diols, the functionalities partially react with glycerol carbonate by an internal transesterification into cyclic carbonate functionalities, this undesirable reaction being more dominant at lower temperatures. Notably, the performance of the oxyalkylation strategy is sufficient to crosslink the modified lignin with benzene-1,4-diboronic acid into a gel-like material with identical shear storage and loss moduli before destruction and immediately after destruction (for the lowest amount of crosslinker added = 1:1.15 diol/boronic acid functionalities molar ratio).