Gold shell indicates that carboxymethyl chitosan is a kind of chemical synthesis in recent years. It has important significance in medicine, chemical industry, environmental protection and health care products. Its stable nature, anti-bacterial, anti-infective, lipid-lowering and anti-atherosclerosis pharmacological effects have doomed it to play a greater role in human daily life in the future. Carboxymethyl chitosan is a water-soluble derivative of chitosan. It has many properties, such as strong antimicrobial activity, fresh-keeping effect and amphoteric polyelectrolyte. It has many applications in cosmetics, preservation, medicine and so on. It is also one of the chitosan derivatives which have been studied more in recent years.

Carboxymethyl chitosan can also be prepared by reaction of chloroacetic acid with carboxymethyl chitin in the presence of alkali, but the carboxymethyl of carboxymethyl chitin is substituted on C6-OH of carbohydrate residue, and a small amount of carboxymethyl is substituted on C3-OH, resulting in O-carboxymethyl chitin. The situation of chitosan is more complicated. Carboxymethyl can be substituted not only on - OH, but also on - NH to produce O - carboxymethyl and N - carboxymethyl chitosan. In fact, there are several possible substitutions: C6 - O - carboxymethyl, C2 - N - carboxymethyl, C3 - O - carboxymethyl, C6 - O, C3 - O, C2 - N - carboxymethyl. Base and so on. Because of the steric hindrance effect on C3 and the intramolecular hydrogen bond between C2 and C3, carboxymethylation on C3 is difficult to occur, so the substitution of carboxymethyl on hydroxyl group is less than that on C3-O-carboxymethyl group, but mainly C6-O-carboxymethyl group. For C6-OH and C2-NH, the gold shell indicates that the substitution activity of Carboxymethyl Chitosan 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 group is mainly on hydroxyl group rather than amino group. Only when the degree of substitution is close to 1 and higher than 1, can the substitution of carboxymethyl group occur simultaneously on amino group. Carboxymethyl is substituted to form O, N-carboxymethyl chitosan. The water solubility of carboxymethyl chitosan is not only because it is a kind of sodium carboxylate but also because the introduction of carboxymethyl destroys the secondary structure of chitosan molecule, which greatly reduces its crystallinity and almost becomes amorphous.