Abstract
It is extremely difficult to achieve a rapid and efficient modification of natural polysaccharides, due to the intrinsic strong hydrogen bonding networks and the slow mass transfer process during the reaction process. Herein, we found a fascinating anion-tunnel transmission phenomenon in the imidazolium-based ionic liquids with carboxylate anions. A novel click esterification of natural polysaccharides thus was demonstrated under a catalyst-free condition within a very short reaction time of 15 min at 0-80°C. Such a super-rapid and highly efficient modification strategy is available for various polysaccharides (cellulose, starch, inulin, pullulan, dextran, and xylan), different esterification reactions (acetification, propionation, benzoylation, and cyclohexyl formylation), and high concentrations, claiming a revolutionary potential in polysaccharide chemistry industries.
Highlights
As the natural renewable resource, polysaccharides are polymeric carbohydrate structures, in which monosaccharide or disaccharide units are covalently joined by Oglycosidic bonds in either a linear or a branched configuration [1,2,3,4]
We found an extremely rapid anion-tunnel transmission phenomenon in the imidazolium-based ionic liquids (ILs) with carboxylate anions (R′mimRCOO), based on the hydrogen bond network formed by imidazolium cations and carboxylate anions
Taking the famous cellulose acetate (CA) for example, a homogeneous click acetylation of cellulose is accomplished at 25°C for 15 min in four kinds of imidazolium-based ILs with acetate anions (RmimAc), 1-ethyl-3-methylimidazolium acetate (EmimAc), butyl3-methylimidazolium acetate (BmimAc), 1-allyl-3-methylimidazolium acetate (AmimAc), and 1-ethyl-2,3-dimethylimidazolium acetate (EdmimAc) (Figure 2(a))
Summary
As the natural renewable resource, polysaccharides are polymeric carbohydrate structures, in which monosaccharide (e.g., glucose, fructose, and galactose) or disaccharide (e.g., sucrose and lactose) units are covalently joined by Oglycosidic bonds in either a linear or a branched configuration [1,2,3,4]. The slow mass transmission tends to cause the local inhomogeneity in the reaction system, so the intensified stirring force and/or extended reaction time are always necessary to eliminate the local inhomogeneity phenomenon. These operations are likely to induce the depolymerization of polysaccharides. It is challenging and practical to realize a fast, efficient, and homogeneous modification of polysaccharides, after overcoming their intrinsic strong hydrogen bonding networks and a slow mass transfer process during the reaction process. A general, easy-to-scale, and homogeneous esterification is proposed to achieve a click modification of polysaccharides (Figure 1)
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