Synthesis of carboxymethyl chitin in aqueous solution and its thermo- and pH-sensitive behaviors

• Carboxymethyl chitins synthesized homogeneously in the green aqueous solution.
• Less deacetylation and degradation and higher yield than heterogeneous method.
• Simultaneous determination of DA, DS and carboxymethylation fraction at C3 and C6.
• Carboxymethyl chitins with lower DS exhibited thermo- and pH-sensitive behaviors.
• Potential bioapplications as injectable degradable hydrogels due to the nontoxicity.

Homogenous modification of natural chitin offers the advantage of fair structure control. In this work, novel carboxymethyl chitins (CMCHs) with broad range of degree of substitution (DS: 0.035 to 0.74), high degree of acetylation (DA) and little de-polymerization were synthesized homogeneously in aqueous NaOH/urea solution. The simultaneous determination of DA, DS and carboxymethylation fraction at C3 and C6 for these CMCHs was achieved by proton NMR in acidic deuterated aqueous solution for the first time. Due to the good homogeneity, the prepared CMCH-4 with lower DS of carboxymethylation exhibited, for the first time to our knowledge, dual thermo- and pH-sensitive properties. The nontoxic thermo-sensitive polymer systems gel at body temperature (37 °C) in physiological condition, which is very useful as injectable hydrogels for drug delivery and tissue engineering.