Analysis of the positions of substitution of acetate and propionate groups in cellulose acetate–propionate by the reductive-cleavage method

Abstract

The degree of substitution (ds) and the distribution pattern of the two ester substituents in commercial samples of cellulose acetate–propionate (CAP) were determined by sequential neutral methylation, direct reductive cleavage, and in situ acetylation. When the reductive-cleavage reaction was conducted with 35 equiv (per anhydroglucose unit) of Et₃SiH, 70 equiv of MeSO₃SiMe₃, and 14 equiv of BF₃·OEt₂ at room temperature for seven days, the O-acetyl groups were converted to O-ethyl groups, and the O-propionyl groups were converted to O-propyl groups concurrent with reductive cleavage of the glycosidic linkages. Acetylation of the products gave 27 partially methylated, ethylated, and propylated 4-O-acetyl-1,5-anhydro-d-glucitol derivatives that were identified by GLC–CIMS (NH₃) and GLC–EIMS. Integration of the GLC profile and correction for molar response gave the mole percent of each product. From these data, the fractional degree of substitution for each ester at each position of the anhydroglucose unit was determined. The combined fractional degree of substitution of both esters at each position and the overall ds were also determined by sequential neutral methylation, acyl–ethyl exchange, and reductive cleavage, and the values so obtained were in good agreement with those derived by sequential neutral methylation and direct reductive cleavage.

Introduction

Cellulose acetate–propionate (CAP) has been produced commercially for a wide variety of applications, such as printing inks, hot-melt dip coatings, lacquer coatings, and desalination membranes[1]. The functional properties of these products depend upon their degree of substitution (ds) as well as the distribution pattern of the two ester substituent groups on the ($\text{1}\text{→}$4)-β-d-glucopyranosyl residues of the polysaccharide. The structural characterization of cellulose acetate–propionate samples is therefore of significant importance, both for elucidating structure–property relationships and for achieving quality control in production processes. The distribution patterns of acetyl and propionyl groups in CAP samples have not previously been established. In related work, however, the positions of substitution of O-acetyl and O-butyryl groups in cellulose acetate–butyrates were established by sequential methylation under neutral conditions and direct reductive cleavage under conditions that reduced the O-acetyl and O-butyryl groups to O-ethyl and O-butyl groups, respectively, concurrent with reductive cleavage of glycosidic linkages[2]. From these results it was obvious that it should be possible to establish the distribution pattern of O-acetyl and O-propionyl groups in CAP samples using the same procedure. The 27 possible products so obtained (Table 1), which contain a single O-acetyl group (at the 4-position) and varying numbers of O-methyl, O-ethyl, and O-propyl groups, were separated and characterized, revealing the fractional degree of substitution of each ester group at each position of the 4-linked d-glucopyranosyl residues of the polysaccharide. The same CAP samples were subjected to sequential neutral methylation, acyl–ethyl exchange, and reductive cleavage, and the eight products so obtained were separated by GLC and identified as previously described[3]. The latter experiment was not capable of establishing the fractional degree of substitution of each ester at each position but could, for purposes of comparison, establish the combined fractional degrees of substitution of both esters at each position as well as the overall ds.