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

Abstract

Methylcellulose samples with different degrees of substitution were prepared by a heterogeneous reaction of cellulose. Two-dimensional NMR spectroscopy provided complete assignment of the 1H and 13C 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 the 1H and 13C chemical shifts of the AGUs; additivity could be applied to the 1H and 13C chemical shifts of methylcellulose. Quantitative 13C 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.