Methanolysis of carboxymethyl cellulose: a comprehensive study

Two commercial carboxymethyl celluloses (CMC) with degree of substitution (DS) 0.75 and 0.77, respectively, have been submitted to exhaustive methanolysis until the amount of the insoluble residue did not further decrease. Dissolved and insoluble material were determined gravimetrically and further analyzed with respect to their substituent distribution in the glucosyl units by capillary electrophoresis, and by size exclusion chromatography. While the substitution of the soluble fraction was slightly above the average DS and in accordance with a random distribution (Spurlin), unsubstituted glucose was steadily enriched in the retained solid material (10 and 20 mol%), resulting in a final DS of 0.46 and 0.42, respectively. An “excess” of unsubstituted glucose is mainly responsible for the heterogeneity of this portion, since deviation from the Spurlin model could be minimized by reducing the molar fraction of glucosyl units. Estimation of the DS/DP-distribution profile of the insoluble residue from molar mass and substituent distribution data, and comparison with the profile of the original CMC (DS 0.75) showed that roughly only about half of the residue could be covered by sequences of the original CMC if random substitution is assumed. This indicates that there is a certain portion of unsubstituted domains in a low substituted heterogeneous part of the CMC. By this procedure, low or unsubstituted areas as a minor part of the CMC material can be concentrated and heterogeneity detected with higher sensitivity.