Liquid Chromatography under Limiting Conditions of Desorption IV. Separation of Macromolecules According to Their Stereoregularity

Abstract

Liquid chromatography under limiting conditions of adsorption (LC LCD) is a novel technique for separation of polymers that exhibit different extent of adsorption on the porous column packing of appropriate polarity. LC LCD is based on the selective ‘‘barrier effect’’ induced by a narrow zone of the adsorption promoting liquid that is introduced into column in front of separated macromolecules of different nature. The small molecules that form the barrier elute slowly because they permeate the packing pores. The partially or fully pore excluded polymer molecules elute rapidly. The barrier hinders fast elution of macromolecules that tend to be adsorbed within column packing; their elution is slowed down. At the same time, the non-adsorbed polymer species elute non-hindered. It is shown that due to the controlled barrier effect even a very small difference in the extent of adsorption between isotactic and syndiotactic macromolecules of poly(methacrylate)s can lead to the significant distinction in their retention volumes. While the direct LC LCD separation of poly(methyl methacrylate)s of different stereoregularity is often precluded by the aggregation of unlike macromolecules, the highly stereoregular poly(ethyl methacrylate)s of distinct kind can be easily discriminated irrespective of their molar mass. Simultaneously, the low molecular and oligomeric impurities can be removed and, if necessary they can be identified and quantified by independent methods. In this way, LC LCD can assist purification of stereoregular polymers and serve as the first step in the twodimensional liquid chromatography of their non-aggregated mixtures.