GB771138A - Process for the manufacture of chitosan esters - Google Patents

Abstract

The invention comprises N-formyl-chitosan sulphuric acid polyesters having an N-formyl content of from 1.5 to 9 per cent, and a process for the preparation of N-formyl-chitosan sulphuric acid polyesters by converting chitosan into a salt thereof with an organic or inorganic acid, precipitating the salt by the addition of a water-soluble precipitating agent, dissolving the precipitated salt in formamide and sulphonating the same with chlorosulphonic acid. The salt may be derived from formic, acetic, perchloric, nitric, hydrochloric or hydrobromic acid. The water-soluble precipitating-agent may be acetone, ethanol or methanol. The sulphonation step may be carried out in the presence of pyridine while maintaining a temperature of between 20 DEG and 100 DEG C., or by treating the solution of the chitosan salt in formamide with chlorosulphonic acid while cooling and stirring thoroughly. The N-formyl-chitosan sulphuric acid polyesters produced by carrying out the treatment with chlorosulphonic acid in the cold have a relatively high content of free amino groups particularly when the chitosan salt is derived from a strong acid, e.g. from hydrochloric or perchloric acid. If the reaction temperature is maintained at between 20 DEG and 100 DEG C. and pyridine is present in the reaction mixture, the product has a high total nitrogen content but a low content of free amino groups. Thus it is possible to control the degree of formylation by the presence or absence of pyridine and the use of chitosan salts derived from strong or weak acids. The amount of sulphur present in the polyester products may be controlled by the time and temperature of sulphonation and the quantity of chlorosulphonic acid used. The process may be modified in that the chitosan salt used for the sulphonation step or the product of the sulphonation step is degraded by treatment with aqueous hydrogen peroxide or with aqueous acid or alkali. The degree of degradation may be controlled by the time and temperature at which the degradation is carried out and the quantity of reagent used. The degradation step may be modified by introducing a fractionation step either before or after the sulphonation step to isolate a particular polyester. In the case of a fractionation step on the N-formyl-chitosan sulphuric acid polyester, it is preferable to convert the polyester into an alkali metal salt thereof before the fractionation. The N - formyl - chitosan sulphuric acid polyesters produced by the above process may also be dissolved in water and treated with varying quantities of a watermiscible precipitating agent, e.g. methanol, ethanol or acetone, to separate polyesters of a higher or lower molecular weight, a small quantity of precipitating agent being required for the high molecular weight polyesters and a large quantity for the low molecular weight polyesters. The N-formyl-chitosan sulphuric acid polyesters produced by the above process may be purified by reprecipitation, if necessary as alkali metal salts, from an aqueous solution by adding methanol, preferably methanol containing sodium acetate, or any other suitable water-soluble precipitating agent such as ethanol or acetone. The polyesters may be further purified by dissolving them, or the corresponding alkali metal salts, in water and precipitating the polyesters in the form of alkaloid salts, e.g. the narcotine salts, converting the alkaloid salts back to alkali metal salts be the addition of an alkali metal hydroxidyl acidifying and adding a water-soluble precipitating agent. A number of detailed examples are given.