Novel approach towards hydrolytically stable starch acetates for physiological applications

An efficient synthesis path for pure, water-soluble, hydrolytically stable and storable starch acetates with a defined pattern of functionalization and adjustable degree of substitution (DS) and molecular mass is described. The starch acetate samples with DS values in the range from 0.4 to 1.2 can be synthesized homogeneously applying dimethyl sulfoxide as solvent either by conversion of starch with acetic acid/N,N′-carbonyldiimidazole or with acetic anhydride/imidazole. In both cases the reactive intermediate formed is the acetic acid imidazolide that guarantees an efficient and selective acetylation with limited side reactions. The starch acetate samples prepared show an almost selective substitution at position 2 (up to a DS of 1) as can be revealed by 13C NMR- and 1H NMR spectroscopy after perpropionylation. The specific pattern of functionalization and a defined molecular mass in the range from 50,000 to 500,000 g/mol yields polysaccharide-based products that are suitable for biomedical and pharmaceutical applications, in particular as volume expanders. They can be stored under sterile conditions in buffer over a period of more than 200 days without change in viscosity with a slight decrease of the DS only, as determined by 1H NMR spectroscopy and by evaluation of the acetic acid/acetate liberation using an enzymatic method.