Retention mechanism, isocratic and gradient-elution separation and characterization of (co)polymers in normal-phase and reversed-phase high-performance liquid chromatography

Synthetic (co)polymers or (co)oligomers with two (or more) repeating groups show not only molar mass distribution, but also composition and sequence d istribution of the individual repeat units. To characterize such two- (or m ore-) dimensional distribution, liquid chromatography under "critical condi tions" has been suggested, where the separation according to one type of re peating units is suppressed by balancing the adsorption and the size-exclus ion effects. In present work it is shown that by combination of adequately selected separation conditions in normal-phase and in reversed-phase system s, the two-dimensional distribution mode can be adjusted to result in the s eparation following the distribution of any of the two repeat units in ethy lene oxide-propylene oxide block (co)oligomers. Based on the retention mech anism suggested, prediction and optimization of the conditions for isocrati c and gradient-elution separations of (co)oligomers is possible. HPLC-MS wi th atmospheric-pressure chemical ionization is a Valuable tool for unambigu ous identification of the individual (co)oligomers and their tracking in co urse of method development. Gradient elution can be used for the separation and characterization of block (co)oligomers of ethylene oxide (EO) and pro pylene oxide (PO) according to the number of the units in one block, while the separation according to the distribution of the units in the other bloc k is suppressed. The effects of the arrangement of the individual EO and PO blocks in the block (co)oligomers (the sequence distribution) affects sign ificantly the retention behavior and the selection of the optimum separatio n conditions. (C) 2000 Elsevier Science B.V. All rights reserved.