| Abstract: | Oligomers of glycerol and succinic acid have been prepared by three different methods and characterised by solution 13C-NMR spectroscopy at 125.8 MHz. The first series of five oligomers was prepared by reacting glycerol with succinic anhydride; only one acid function of each succinic acid residue formed an ester by this means. They were readily distinguished as the shifts of their glycerol residues were dispersed over 16 ppm, and their shifts provided a guide to the assignment of shifts in the more elaborate oligomers. The second set of oligomers was prepared by treating glycerol with a small quantity of succinic acid, ester links being promoted by means of the reagent dicyclohexylcarbodiimide (DCCDI). When we used 2 mol of DCCDI/mol of acid in the presence of an excess of glycerol, no free acid functions remained, and a new set of oligomers was obtained. Furthermore, within this set the proportion of ring molecules was enhanced by repeating the reaction under much more dilute conditions. In this way the shifts of two ring oligomers were recognized in the spectrum. A different set of oligomers again was obtained when the esterification was performed with 1 mol of DCCDI/mol of succininic acid. After the first generation of oligomers had been identified in the mixtures produced by these three experiments, the reactions continued to produce larger oligomers with new fine structure features in the spectra. Glycerol trisuccinate was prepared in a pure form and heated in a vacuum to eliminate succinic acid groups and allow the formation of oligomers with two and three branch points. The structures successively produced by this reaction were readily recognized. In all we were able to recognize the formation of component structures in at least 17 different oligomers. The shifts of the carbons of the glycerol residues are sensitive to the substitution pattern at that residue, to whether the succinic acid residue to which they are linked has reacted a second time, and in some cases the methine shift is sensitive to how the succinic acid residue attached to a neighboring methylene carbon has reacted. © 1994 John Wiley & Sons, Inc. |
