Nuclear magnetic resonance spectroscopic characterization of carboxymethylcellulose

Previous assignment of the signals in the spectra of carboxymethylcellulose, based on an incremental calculation and hence on low‐molar‐mass model substances, made determination of the partial degree of substitution fundamentally impossible because overlapping of the relevant signals was predicted. However, the methods of acid and enzymatic hydrolysis employed generate the monomer or a mixture of polymers, oligomers and monomers and hence a large number of end groups, which also influence the nuclear magnetic resonance (NMR) spectra. By using ultrasound, it was possible to degrade the molar mass (lower limit of molar mass approx. 100000 g/mol), without oligomers being generated and cleavage of side groups occuring. The viscosity of 10 wt.‐% polymer solutions was so low that the partial degree of substitution could be determined quantitatively for the first time. An additional ¹³C NMR spectroscopic examination of the acid hydrolysate enabled the composition of the eight monomers of sodium carboxymethylcellulose (NaCMC) to be determined in a degree of substitution (DS) range of 0,8–3,0. Knowledge of the monomer compositions is then used to reassign the signals of the polymer spectra, which are then evaluated. Comparison with titrimetric methods (polyelectrolyte titration and ASTM method) showed some wide discrepancies with the absolute method (NMR) in the case of samples with a DS greater than 1. The results of the ¹³C NMR spectroscopic examination of the acid hydrolysate were also used to determine the relative rate constants of the etherification reaction (k₂:k₃:k₆ = 3,0:1,0:2,1), and these were then compared with published data.