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.

Separation of parent homopolymers from diblock copolymers by liquid chromatography under limiting conditions of desorption 4. Role of eluent and temperature

“…Broad adjustability of barrier efficiency is needed for the LC LCD separation of multicomponent polymer systems. Especially challenging is the control of the barrier action when polymer samples possess distribution in their chemical structure (composition) or physical architecture 37, 38. In any case, the (more) adsorbed polymer is to be decelerated by the barrier while the non‐adsorbed or the less‐adsorbed sample constituent elutes freely in the SEC mode, without any barrier effect.…”

“…It is anticipated that in LC LCD, polymer chains are accumulated on the barrier edge at the composition that corresponds to the critical adsorption point – which is molar mass independent 47–49. However, polymer adsorption was shown to depend on the molecular architecture of polymers 38, 57, 72–74. As a result, LC LCD enabled to separate highly stereoregular polymers according to their tacticity 38.…”

“…However, polymer adsorption was shown to depend on the molecular architecture of polymers 38, 57, 72–74. As a result, LC LCD enabled to separate highly stereoregular polymers according to their tacticity 38. Therefore, identification of appropriate eluent and barriers should be performed with polymers of at least similar stereoregularity as the samples to be separated.…”

Separation of Parent Homopolymers from Nonpolar Block Copolymers by Means of Liquid Chromatography under Limiting Conditions of Enthalpic Interactions

“…The important advantage of LC LCD is its applicability to complex polymer systems, which contain very low, even less than 1% of macromolecular additive . LC LCD proved high efficiency in the practical one‐step baseline separation of two to four component blends of homopolymers, in discrimination of low‐solubility polymers, macromolecules according to their tacticity, and, especially, in division of parent homopolymers from the block copolymers containing at least one polar chain . Enthalpic partition (absorption) and solubility (phase separation) retention mechanisms were so far used only tentatively, and the resulting methods are denoted as LC under limiting conditions of unpartition (LC LCU) and LC under limiting conditions of insolubility (LC LCI), respectively.…”

Comprehensive Molecular Characterization of Complex Polymer Systems by Sequenced Two-Dimensional Liquid Chromatography. Principle of Operation

“…The concentrations or volumes of sample introduced into the 1-d column in the course of particular injections are so adjusted that the polymer detection of the 2-d column effluent is feasible. The group of methods which well fulfill the above conditions for the first-dimension separation is liquid chromatography under limiting conditions of enthalpic interactions (LC LC). ,− LC LC utilizes the significantly different elution rates of small, pore-permeating solvent molecules and large, partially or fully pore-excluded macromolecules. Appropriately chosen slowly eluting low-molecular-mass substances promote interactions of certain macromolecules in the chromatographic system.…”