Sustainable succinylation of cellulose in a CO2-based switchable solvent and subsequent Passerini 3-CR and Ugi 4-CR modification†
A rapid and effective dissolution and activation of cellulose was demonstrated by a reversible reaction of CO2 with the hydroxyl groups of the cellulose backbone in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The dissolved cellulose was subsequently subjected to in situ derivatization with succinic anhydride without the need of any additional catalyst under very mild conditions. As a result of our optimization studies, cellulose was successfully converted to cellulose succinates with degrees of substitution (DS) ranging from 1.51 to 2.59, depending on the reaction conditions and the molar ratio of succinic anhydride. The optimized reaction conditions were successfully applied to different types of cellulose samples including microcrystalline cellulose (MCC) and organosolv wood pulp (WP), exhibiting similar conversions. Furthermore, the carboxylic acid moiety, introduced by the succinylation, was further converted via Passerini three-component reactions (Passerini-3CR) and Ugi four-component reactions (Ugi-4CR). 31P NMR revealed the quantitative conversion of carboxylic acid moieties on the cellulose backbone under mild conditions. All obtained products were thoroughly characterized by ATR-IR, 1H, 13C, and 31P NMR spectroscopies as well as by size exclusion chromatography (SEC). Thermal properties of the obtained products were investigated by differential scanning calorimetry (DSC) and by thermogravimetric analysis (TGA), revealing glass transitions (Tg) for all the Passerini and Ugi products between at 76–116 °C and high thermal stability between 263–290 °C. The reported methodology represents a very mild, highly efficient and sustainable route for the dissolution of cellulose and the synthesis of cellulose succinates. The subsequent modifications of the obtained cellulose succinates via multicomponent reactions resulted in materials with improved thermal properties and offers a straightforward and versatile modification strategy for cellulose.