Homogeneous synthesis and characterization of chitosan ethers prepared in aqueous alkali/urea solutions

The etherification of chitosan dissolved in aqueous LiOH/KOH/urea solutions was conducted for the first time. The structure and solution properties of the chitosan ethers were characterized using FT-IR, NMR, SEC-LLS, elemental analysis, and ζ-potential measurement. Chitosan ethers with different degrees of substitution (DSs) were obtained by adjusting the molar ratio of the etherifying agent to N-acetylglucosamine unit (AGU) of chitosan. The total DS for methyl chitosan (MCh) increased from 1.17 to 2.25, and trimethyl chitosan (TMCh) was obtained with DSN-trimethyl as high as 0.48. The total DS for carboxymethyl chitosan (CMCh) was in the range 0.75-1.45. Both N-substitution and O-substitution were observed, and the OH groups at C-6 displayed relative higher reactivity than the OH groups at C-3. In comparison with the traditional methods and solvents, aqueous alkali/urea solutions proved to be a stable and more homogeneous medium for preparing chitosan ethers with higher DS through a one-step reaction.