SUPERCRITICAL-FLUID FRACTIONATION OF POLY(ETHYLENE GLYCOLS)

The partitioning of poly(ethylene glycols), PEGs, in supercritical CO2 is presented. The individual n-mer partition coefficients are determined from the overall polymer compositions, measured by UV-absorption spectroscopy and the polymer molecular-weight distributions measured by FAB mass spectroscopy in two coexisting phases. The results of two parent PEGs, one with an average molecular weight of 400 and n-mer range of 4 to 16, and a second with an average molecular weight of 600 and n-mer range of 4 to 21, are reported at various temperatures and pressures. The molecular-weight dependence of n-mer partition coefficients follows Flory's expression for n-mers with molecular weights above approximately 400 and deviates from the expected behavior for n-mers in the lower molecular-weight range. The observed deviation is attributed to the hydroxyl end groups effects and the resulting intramolecular associations. A critical molecular weight of approximately 400 corresponds to the maximum range of these intramolecular associations. The effects of the operating conditions on the yield and the selectivity of supercritical-fluid fractionation of PEGs are also discussed.