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DETERMINATION OF THE STRUCTURE OF CELLULOSE ETHERS BY 13C NMR SPECTROSCOPY OF PRODUCTS OF ACID-CATALYZED HYDROLYSIS

Cellulose ethers (CE) are widely used in the pharmaceutical, food and construction industries to impart the necessary rheological, thixotropic and water-retention properties to water dispersion systems. In this paper, we compare the substitution parameters of hydroxypropyl methylcellulose (HPMC), hydroxyethyl cellulose (HEC) and hydroxyethyl methylcellulose (HEMC), which are obtained from 13C NMR spectroscopy of products of acid-catalyzed hydrolysis and cellulose ethers. Accurate data on the degree of substitution at the 2, 3 and 6 positions (DSC-2, DSC-3 and DSC-6) were obtained from 13С NMR spectra of products of acid-catalyzed hydrolysis. The total degree of substitution (DStotal) and molar substitution (MS) for the hydroxyethyl and hydroxypropyl substituents were determined. The distribution of substituents at different positions of the glucopyranose link indicates that the most reactive are the C-2 and C-6 positions, as well as the hydroxyl of the hydroxyethyl fragment. Within the proposed method, the degree of substitution and molar substitution are determined: DSHPMC = 1.79 and MSHPMC = 0.38; DSHEC = 1.02 and MSHEC = 2.03; DSHEMC = 1.93 and MSHEMC = 1.82. It is shown that the results are consistent with the data obtained from 13C NMR (СРMAS) spectra of the same cellulose ethers. The proposed method of analysis is characterized by the simplicity of the experiment, the accuracy and informative of the results obtained.