1. Chitosan is alkalized with a mixed alkali solution at low temperature.
2. The alkalized chitosan is dispersed in isopropanol and stirred at constant temperature.
3. An isopropanol solution of bromoethanol/ethylene oxide/chloroethanol is added dropwise for hydroxyethylation.
4. Work-up: centrifugation/filtration, dissolution in water, concentration, dialysis to remove salts, and re-concentration.
5. The concentrate is added to ethanol/acetone for precipitation. The solid is filtered and freeze-dried.
