Carboxymethyl chitosan can also be obtained by the reaction of chloroacetic acid with carboxymethyl chitin in the presence of alkali, just like the preparation of carboxymethyl chitin. However, the carboxymethyl of carboxymethyl chitin is substituted on the C6-OH of sugar residue, and a small amount of carboxymethyl is substituted on the c3-oh to produce O-carboxymethyl chitin. The situation of chitooligosaccharides is more complicated. Carboxymethyl can be substituted not only on - Oh, but also on - NH to form O-carboxymethyl and N-carboxymethyl chitosan. In fact, there are several possible substitutions: c6-o-carboxymethyl and c2-n_ Carboxymethyl, c3-o-carboxymethyl, c6-o, c2-n-carboxymethyl, c6-o, c3-o, c2-n-carboxymethyl, etc. Because of the steric hindrance effect on C3 and the intramolecular hydrogen bond between C2 and C3, the carboxymethylation on C3 is difficult to occur, so the carboxymethyl substitution on hydroxyl group is less, c3-o-carboxymethyl is less, and c6-o-carboxymethyl is the main one. For C6-OH and C2 -- NH, the substitution activity of carboxymethyl on hydroxyl group is higher than that of amino group under alkaline condition. Therefore, when the degree of substitution is less than 1, the substitution of carboxymethyl is mainly on hydroxyl group rather than amino group. Only when the degree of substitution is close to 1 and higher than 1, carboxymethyl substitution occurs on amino group to form o, N-carboxymethyl chitosan. The water solubility of carboxymethyl chitosan is not only because it is a kind of sodium carboxylate and soluble in water, but also because the introduction of carboxymethyl destroys the secondary structure of chitosan molecules and makes its crystallinity greatly reduced and almost amorphous.