Hydroxyalkylation of xylan using propylene carbonate: comparison of products from homo- and heterogeneous synthesis by HRMAS NMR and rheology

Xylan is a highly available polysaccharide in the plant kingdom and is therefore a valuable resource for novel materials from renewable resources. Hydroxyalkylation is one of the most common reactions to derivatize biopolymers by altering hydroxyl groups. State of the art procedures incorporate epoxides, which are toxic, carcinogenic and highly explosive. In this study, hydroxyalkylation with propylene carbonate (PC) was used as green approach to synthesize xylan derivatives. Reaction pathways under homogeneous conditions, in dimethyl sulfoxide (DMSO), and heterogeneous conditions, without solvent, are compared. Analysis using liquid state and high resolution magic angle spinning (HRMAS) NMR, a novel approach for insoluble but swellable polysaccharide derivatives, as well as FTIR spectroscopy, hydrolysis in combination with borate anion exchange chromatography, and rheology showed significant differences regarding the structure of the products. While the degree of substitution (DS) is similar under both conditions, side chains are significantly longer under heterogeneous conditions, implying a higher rate of homopolymerization. This leads to a non-complete decarboxylation. The higher sterical hindrance imposed by longer side chains therefore leads to a higher tendency of xylose monosaccharides being derivatized at only one native hydroxyl group, while under homogeneous conditions a higher tendency towards double substitution can be observed. Rheology showed a shear thinning behaviour as well as an increase in viscosity with DS for samples from homogeneous synthesis. Products from solvent-free approach were analyzed in swollen state. They showed gel-like behaviour, whose elasticity increases with increasing DS as a result of side chain entanglement.