Polysaccharide sulfates. I. cellulose sulfate with a high degree of substitution

Abstract

Previous methods for the sulfation of cellulose have a number of disadvantages among which excessive degradation and incomplete substitution are the most common. These disadvantages are overcome if a complex of sulfur trioxide with a neutral, highly polar compound, such as N,N-dimethylformamide, is used as the sulfating agent. For the sulfation of cellulose with this complex, any grade or type of cellulose is suitable. The resulting products usually have degrees of substitution greater than 2. The viscosities of their aqueous solutions are relatively high, indicating that degradation is minor. Two of the most interesting properties of this relatively undegraded cellulose sulfate are its reactivity with proteins and the gelation of its aqueous solutions to form thermoreversible gels in the presence of potassium, rubidium, or cesium ions. The properties are surprisingly similar to those of carrageenan, a polysaccharide sulfate occurring naturally in a number of red marine algae. During the past few decades, a number of methods have been suggested for the sulfation of cellulose; common reagents include sulfuric acid 2,3, sulfur trioxide 4,5, chlorosulfonic acid in pyridine 6 or quinoline 7, and sulfur trioxide diluted with an inert solvent such as sulfur dioxide 8. Other agents found useful are complexes of sulfur trioxide with tertiary amines, such as pyridine 5 or triethylamine 9. The method that has come closest to practical utilization is sulfation with sulfuric acid in the presence of an aliphatic alcohol 10. However, all of these methods suffer from one or more disadvantages, among which excessive degradation and incomplete substitution are the most common. Often, the substitution is nonhomogeneous, or only the surface of the fiber is substituted. If the reaction is conducted in the presence of a tertiary amine, the resulting cellulose sulfate is obtained in the form of its trialkylammonium salt, necessitating additional effort to obtain the alkali or other metal salt, which, especially for commercial applications, is the more desirable product. It is now reported that complexes of certain neutral, highly polar compounds, in conjunction with sulfur trioxide, are useful sulfating agents for cellulose. These agents produce high degrees of substitution, and cause little degradation of the cellulose molecule, as indicated by viscosity measurements. The type of neutral