Determination of DS and purity of fatty acid esterified celluloses using ³¹P-NMR

Cellulose, as a naturally occurring biopolymer, has received much attention for its possibilities to be used as a starting point for advanced bio-based materials. Modification of cellulose is usually performed on the hydroxyl groups located on the backbone of the polymer. Determination of the degree of substitution of these hydroxyl groups in the product is crucial for understanding the properties arising due to the modifications. In this work derivatives of various celluloses were synthesized and their degrees of substitution were determined using a novel method based on ³¹P-NMR. Hydrophobic fatty acid esters of cellulose were synthesized homogeneously using lithium chloride/N,N-dimethyl-acetamide as a solvent system. Celluloses with varying degrees of polymerization and hemicellulose content were used for reactions with palmitoyl- and decanoyl chloride. IR-, ¹H-NMR- and ³¹P-NMR spectroscopies were used to verify the reactions and the purity of the products. ³¹P-NMR was also utilized to determine the degrees of subtitution of the fatty acid derivatized celluloses. In order to do this the remaining hydroxyl groups were first derivatized to phosphite esters. These fully esterified products were then analyzed alongside of an internal standard to evaluate the degree of substitution and purity of the samples. This was shown to be a precise method regardless of which cellulose was used. Comparison of the results also revealed how the changes in the intensities of the carbonyl region, shown on the IR spectra, were comparable to results obtained using the ³¹P-NMR-method.