Combined size exclusion chromatography, supercritical fluid chromatography and electrospray ionization mass spectrometry for the analysis of complex aliphatic polyesters

• Comprehensive analysis of complex aliphatic polyesters.
• Multiple fractionations by SEC and SFC.
• Two-dimensional SEC–SFC fractionation to analyze single oligomers.
• Analysis of fractions by ESI–MS.
• Detailed structural information on molar mass, chemical composition, end groups.

Aliphatic polyesters are complex products of polycondensation that are distributed regarding the degree of polycondensation, the end group functionality and the molecular topology. To address the molecular heterogeneity of polyesters based on phthalic acid and propylene glycol, for the first time the combination of SEC, SFC and ESI–MS have been used. In a first set of experiments, samples were fractionated by SEC and the collected fractions analyzed by SFC for a tentative assignment of the degrees of polycondensation. More conclusive results were obtained by semi-preparative SFC fractionation of the bulk samples and the subsequent analysis of the collected fractions by ESI–MS. The ESI–MS spectra of the SFC fractions provided detailed information on the presence of linear and cyclic oligomers, their degrees of polycondensation and their end groups. Information on the presence of propylene oxide oligomers was also obtained and it was shown how they were inserted in the polymer structures. Compared to previous work, the present approach provides significantly more detailed information on the molecular complexity of aliphatic polyesters. This is mainly due to the fact that SFC has been used as the second chromatographic dimension which is known to have superior separation capabilities.