the invention provides a biodegradable azetidinium - functional polyester . this is useful for treating a substrate such as a cellulosic or keratinaceous textile material , preferably in the presence of a carrier . the polymer can be prepared by : a ) reacting an amine - containing acid or ol with a suitable co - reactant , and , b ) treating the product of step with an epihalohydrin . preferably step occurs in the presence of a catalyst selected from the group comprising sulphuric acid , p - toluenesulphonic acid and a hafnium compound . the preferred diacid are iminodiacarboxylic acids in which each carboxylic acid moiety has a carbon number of 2 - 4 . the preferred diols are polyalkylene glycols in which the repeat unit has a carbon number of 2 - 3 .