Determination of the Distribution of the Substituent Group in Cellulose Acetate by Full Assignment of Au Carbonyl Carbon Peaks of 13C{1H} NMR Spectra

Abstract

An attempt was made to assign a number of peaks, observed in carbonyl region (168—171 ppm) of 13C{1H} NMR spectra of cellulose acetate (CA), to twelve magnetically different acetyl groups existing in 8 kinds of glucopyranose units constituting CA. CA samples with total degree of substitution «F» ranging 0.43 to 2.46 were prepared by acid-hydrolysis of CA with «F»=2.92 dissolved in acetic acid. 13C{1H} NMR measurements were made on these ten CA samples in deuterated dimethylsulfoxide and 14 peaks in total were observed definetely over the whole range of «F» from 2.92—0.43. From the spectra of CA samples with highest «F» (2.92) and lowest (0.43) carbonyl carbon peaks at C6, C3, and C2 positions in trisubstituted and monosubstituted glucopyranose units were successfully assigned. Deducing the effect of an acetyl group on the chemical shift of other acetyl groups, and vise versa, based on an empirical rule, 6 carbonyl carbon peaks in 3 kinds of disubstituted glucopyranose units were assigned. Eight average molar fractions «flmn» (l=1 or 0 means that a hydroxyl group attached directly at C2 position is or is not substituted and m=1 or 0 means that C3 position is or is not substituted, and n corresponds to C6 position) [i.e., «f000», «f100», ···, «f111»] were evaluated from integrated intensity ratios. The probability of substitution at Ck position «fk» (k=2, 3, and 6) and «F», both evaluated from the above «flmn» values, fairly agree with those estimated by the conventional NMR method, indicating the validity of the assignment in this paper.