Separating ultra-high molecular weight polymers on monolithic capillary columns

The chromatographic behavior of a polystyrene sample with a molecular weight of 20 × 10 6 Da and narrow molecular weight distribution is studied on a monolithic capillary column based on polydivinylbenzene. It is found that either one symmetrical peak or two peaks that cannot be completely separated over the range of flow rates for the mobile phase can be observed in the chromatogram, depending on the flow rate of the mobile phase. The observed changes in the elution profile of polystyrene standards are attributed to the presence of dynamic equilibrium in the system (reaction chromatography), for which the apparent constants of forward and reverse reactions and the apparent equilibrium constant are obtained. It is noted that the apparent constant of direct reaction increases linearly with a rising eluent flow rate, while saturation and even a drop in the apparent rate constant of the reverse reaction is observed when the flow rate of the eluent rises. It is suggested that the observed changes are related to conformational transitions in the macromolecules of polystyrene, but a detailed explanation of the observed effect requires further investigation.