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

The degree of substitution (ds) and the distribution pattern of the two ester substituents in commercial samples of cellulose acetate–butyrate (CAB) 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-butyryl groups were converted to O-butyl groups concurrent with reductive cleavage of the glycosidic linkages. Acetylation of the products gave 27 partially methylated, ethylated, and butylated 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.