Versatile synthesis, characterization and properties of β-chitin derivatives from aqueous KOH/urea solution

Chitin is the second most abundant natural polysaccharide with biocompatibility, biodegradability, and bioactivity. Homogeneous modification of chitin is an efficient way to improve or to impart new properties to chitin. Here, amide-modified β-chitin (AMC), hydroxyethyl β-chitin (HEC), and hydroxybutyl β-chitin (HBC) through Michael addition, Williamson reaction, and ring-opening addition, were homogeneously synthesized from aqueous KOH/urea solution, respectively, with controlled structures and uniform properties. The reactions mainly occur at the hydroxyl groups of C-6 positions of β-chitin chains due to mild conditions, and the obtained β-chitin derivatives with high DS values are water-soluble. AMC could transform from sol to gel at acidic condition or upon adding Fe³⁺ due to the presence of partial carboxylate groups. As a specific highlight, HEC and HBC could thermally form smart hydrogels at physiological temperature, with cytocompatibility and blood biocompatibility, which is very useful as drug/ cell carriers for biomedical applications. The remarkably mild and "green" condition of aqueous KOH/urea solution for the synthesis of chitin derivatives has pioneered a better way to exploit its great diversity of the natural, sustainable polysaccharide.