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

• A carboxymethyl cellulose series with substituent degrees of 0.68–2.84 was prepared.
• The chemical shifts of the ring protons and carbons were precisely assigned by NMR.
• The 2-, 3-, or 6-position substituent effects on the chemical shifts were estimated.
• The effects explained the chemical shifts of di- and tricarboxymethylated cellulose.
• The monomers comprising carboxymethyl cellulose could be estimated by 13C NMR.

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 1H and 13C 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 1H and 13C chemical shifts of the AGUs and that additivity could be applied to the 1H and 13C chemical shifts of CMC. Quantitative 13C 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.