Microstructural characterization of aromatic copolyesters made by step reactions, by gradient polymer elution chromatography

The potentials of Gradient Polymer Elution Chromatography (GPEC) in both the Reversed-Phase (RP) and Normal-Phase (NP) mode, for the characterization of aromatic copolyesters made by step reactions, according to their chemical microstructure, were studied. Hereto, a number of copolyesters, varying in molar mass and chemical composition (CC) were synthesized, which allowed a systematic study on the effects of those parameters in GPEC. By RP-GPEC, highly detailed separations were obtained. Information on chemical composition differences could, however, only be obtained for the lower molar masses. From these results, qualitative evidence for differences in the chemical microstructure of two strongly resembling copolyesters was found that could not be obtained by other methods such as SEC and NMR. Nevertheless, it was found difficult to unambiguously assign observed differences in the high molar mass parts of RP-GPEC chromatograms. Therefore, RP-GPEC must mainly be considered as a versatile, qualitative fingerprinting tool. In contrast, NP-GPEC provides more and quantitative information on microstructural differences. By a combination of SEC and NP-GPEC the Molar-Mass-Functionality-Type-Distribution (MMFTD) of the (co)polyesters, and the Molar-Mass-Chemical-Composition-Distribution (MMCCD) of the fraction containing two alcoholic end groups of the copolyesters could be studied. Significant differences between strongly resembling copolyesters were found which, for the MMCCDs, can only be the cause of the relative importance of reaction kinetics in step reaction copolymers. This makes the assumption that a predictable, theoretical statistically determined CCD is formed in all cases, questionable