Solid-State ¹³C NMR and Raman Studies of Cellulose Triacetate:  Oligomers, Polymorphism, and Inferences about Chain Polarity

13 C CPMAS NMR and Raman spectra have been taken for oligomers (DP = 2−9) and several preparations of cellulose triacetate (CTA). CTA exhibits polymorphism and the NMR spectra of the CTA(I) and CTA(II) allomorphs along with the spectrum of noncrystalline CTA have been isolated. Magnetic inequivalence within the unit cell of CTA(II) is greater than for CTA(I), indicating a lower symmetry in the CTA(II) unit cell. An attempt is made to correlate some of the NMR resonances with proposed X-ray crystal structures for each of these allomorphs. From both the Raman and the NMR spectra of the oligomers, in comparison with the different CTA preparations, it is determined that the pentamer and higher oligomers crystallize into the CTA(I) lattice. On the basis of the Raman spectra, it is argued that CTA(I) and CTA(II) are distinguished by backbone conformational differences. In a single attempt to explore the role of the solvent in determining the crystal habit of the oligomers, the pentamer persisted in the CTA(I) lattice even when crystallization took place in dibenzyl ether, the solvent out of which the CTA polymer forms CTA(II) crystallites. Based on these findings, an approach to obtaining a definitive crystal structure of CTA(I) would be to grow and perform a structural determination on a single crystal of an oligomer of DP > 4. Spectra of these oligomers indicated partial crystallinity; hence, care must be taken in growing single crystals. Since it is known that there is a correlation between the allomorphs of CTA formed by heterogeneous acetylation and the allomorphs of the precursor, cellulose, and since there are still points of ambiguity regarding the crystal structures of the allomorphs of cellulose and CTA, the topic of chain polarity (directionality) is addressed herein. As a contribution to the dialogue on the structure of these materials, we present plausibility arguments, i.e., not definitive arguments, that CTA(I) has parallel and CTA(II) antiparallel chain character.