NMR characterization of sodium carboxymethyl cellulose: Substituent distribution and mole fraction of monomers in the polymer chains.

A series of sodium carboxymethyl cellulose (CMC) samples with degrees of substitution (DS) from 0.68 to 2.84 were prepared from cellulose by multistep carboxymethylation. Two-dimensional NMR spectroscopic analyses allowed complete assignment of the (1)H and (13)C chemical shifts of the eight anhydroglucose units (AGUs) found in CMC: 2,3,6-tri-, 2,3-di-, 2,6-di-, 3,6-di-, 2-mono-, 3-mono-, 6-mono-, and unsubstituted AGUs. A comparative analysis of the chemical shift data revealed the substituent effects of the carboxymethyl groups at the 2-, 3-, and 6-positions on the (1)H and (13)C chemical shifts of the AGUs and that additivity could be applied to the (1)H and (13)C chemical shifts of CMC. Quantitative (13)C NMR spectra of the CMC samples allowed determination of the AGU composition, as well as the substituent distribution at the 2-, 3-, and 6-positions of cellulose, which allowed estimation of the reactivity of the hydroxyl groups toward the carboxymethylation.