Substituent distribution along the cellulose backbone in O-methylcelluloses using GC and FAB-MS for monomer and oligomeranalysis

The substituent distribution along the cellulose backbone in three heterogeneously prepared methyl celluloses has been investigated by analysis of their monomer and their oligomer compositions. The oligomers were obtained by partial acid hydrolysis of the polymers. The relative hydrolysis rates of the glycosidic bonds at the C-1 positions of the monomers were determined and it was found that these hydrolysis rates are hardly dependent on any substituent combination in the monomers. Thus the partial hydrolysis of the polymers is a random process and the released oligomers are representative for the whole sample. The substituent distributions of the cellotriose units have been measured by fast atom bombardment mass spectrometry of perdeuterio-methylated partial hydrolysates. These distributions were compared with the calculated ones which are expected if the monomeric units would be arranged in a random sequence. The differences between the measured and calculated substituent distributions show that there are areas which are poorly as well as areas which are heavily derivatised in all samples, which prove that the investigated methyl celluloses have indeed an heterogeneous substituent distribution along their cellulose backbones. The degree of heterogeneity is quantified by a newly defined parameter H. Also the substituent distribution in the monomers has been compared with a calculated random distribution. The degree of heterogeneity derived from the monomer composition data was quantitatively in good agreement with the H values derived from the oligomer data, which shows that the degree of heterogeneity is also reflected in the data at the monomeric level. The H parameters correlate with the flocculation points of the methyl celluloses, which illustrates the correlation between the chemical structure and the physical properties of the samples.