CPMAS ¹³C NMR and X-ray studies of cellooligosaccharide acetates as a model for cellulose triacetate

A series of crystalline oligomers from α-D-cellobiose octaacetate through α-D-cellohexaose eicosaacetate were prepared by homogeneous acetylation of the corresponding cellooligosaccharides and characterized by cross-polarization and magic angle sample spinning (CPMAS) carbon-13 nuclear magnetic resonance (¹³C NMR) spectroscopy and X-ray analysis to obtain the structural models of cellulose triacetate (CTA) in the solid state. Progressing toward the hexamer, the NMR spectral features of the oligomers, in comparison with two allomorphs of CTA I and CTA II, gradually approached those of CTA I. Specifically, chemical shifts of both the hexamer and pentamer were in agreement with those of CTA I. In addition, X-ray diffraction patterns of the oligomers established that the crystalline pentamer and hexamer had a CTA I lattice despite recrystallization from ethylacetate-n-hexane. Therefore, we conclude that the pentamer and hexamer are useful models for the CTA I structure.