Sustainable functionalization of cellulose and starch with diallyl carbonate in ionic liquids†
A functional and environmentally benign alkoxycarbonylation reagent, diallyl carbonate (DAC), was employed for the sustainable and catalyst-free allyloxycarbonylation of cellulose under homogenous conditions using 1-butyl-3-methylimidazolium chloride (BMIMCI) as a solvent. As a result of optimization studies of the reaction conditions and molar ratio of DAC, cellulose was successfully converted to cellulose allyl carbonate exhibiting degrees of substitution (DS) of 0.8–1.3. The optimized conditions were also applied to maize starch leading to a DS of 1.2, thus demonstrating the reproducibility and versatility of the established procedure. The obtained cellulose and starch allyl carbonates were thoroughly characterized by ATR-IR, 1H, 13C, and 31P NMR spectroscopy as well as by size exclusion chromatography (GPC). Thermal properties were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). A transparent thin film was produced from cellulose allyl carbonate and mechanical properties were examined by tensile strength measurements, showing 21.6 MPa of tensile strength with 40% elongation at break. Additionally, cellulose allyl carbonate was successfully modified via thiol–ene chemistry employing 1-butane thiol, resulting in a material with improved thermal properties, showing a thermal transition (Tg) at 84 °C. Furthermore, the reported methodology represents a step forward in terms of sustainability for carbohydrate and especially cellulose modification, since non-toxic reagents were used and BMIMCI as well as the excess of DAC could be recycled and reused for further reactions.