13C NMR determination of substituent distribution in carboxymethylcellulose by use of its peresterified derivatives

The distribution of carboxymethyl groups in a series of sodium O-(carboxymethyl)cellulose (CMC) samples was determined by 13C NMR analysis of their peresterified derivatives. Thus sodium carboxymethyl groups were first converted into methyl ester groups by treatment with dimethyl sulfate in Me2SO at 40 °C, to produce methyl-esterified CMC (MCMC). Subsequent propanoation of unsubstituted hydroxyl groups by propanoic anhydride in N,N-dimethyl-acetamide-LiCl at 100 °C in the presence of DMAP produced propanoated MCMC (PMCMC), which was readily soluble in Me2SO-d6 regardless of the degrees of substitution. The propanoyl carbonyl carbon signal in a series of PMCMC samples was found to resolve into three peaks in Me2SO-d6 at 100 °C, corresponding to the position of the substitution position (2, 3, and 6) on the glucose residue, permitting subsequent determination of the distribution pattern of carboxymethyl groups in the parent CMC samples.