NMR characterization of methylcellulose: Chemical shift assignment and mole fraction of monomers in the polymer chains.

Methylcellulose samples with different degrees of substitution were prepared by a heterogeneous reaction of cellulose. Two-dimensional NMR spectroscopy provided complete assignment of the1 H and13 C chemical shifts of the un-, 2-mono-, 3-mono-, 6-mono-, 2,3-di-, 2,6-di-, 3,6-di-, and 2,3,6-tri-substituted anhydroglucose units (AGUs). Comparative analysis of the chemical shift data revealed the relationship between the methyl substituents at the 2-, 3-, and 6-positions and the1 H and13 C chemical shifts of the AGUs; additivity could be applied to the1 H and13 C chemical shifts of methylcellulose. Quantitative13 C NMR spectra of the samples determined the composition of the eight AGUs and the substituent distribution at the 2-, 3-, and 6-positions of cellulose. This provided estimations of the hydroxyl group reactivity toward methylation and the interactions between the substituent groups within the same AGU.