V. A. Davankov

Hypercrosslinked polymers: basic principle of preparing the new class of polymeric materials

Abstract Hypercrosslinked polystyrene networks have been recognised since the 1970s. They were prepared by extensive crosslinking of linear polystyrene chains in a strongly solvating media. Generally, the basic principle of obtaining hypercrosslinked polymers consists in the formation of a rigid highly solvated three-dimensional network. Owing to the high rigidity and reduced degree of chain entanglement, such “expanded” networks are characterized by loose chain packing, i.e., high free volume, and the unique ability to swell in both good solvents and non-solvents. In addition to the above polymer-analogues transformation of polystyrene, the article expands the synthesis techniques to polymerization and polycondensation procedures and also involves other types of polymers and monomers. Typical examples of preparing hypercrosslinked networks include the crosslinking of linear chains of polysulfone and polyarylates, copolymerization of styrene with a large amount of divinylbenzene, self-condensation of p -xylylene dichloride. All resulting materials differ substantially from classical polymers of the same chemical nature and comprise a new class of hypercrosslinked polymeric materials. Some polymers described in the literature were shown to also belong to the hypercrosslinked family.

Separation of unmodified α-amino acid enantiomers by reverse phase HPLC

SummaryA novel chiral phase system is presented for the resolution of unmodified α-amino acid racemates which is composed of a reverse phase packing coated with N-alkyl-L-hydroxyproline (where alkyl is n-C7H15-, n-C10H21- and n-C16H33-) and a hydro-organic eluent containing copper(II)acetate. The factors controlling retention and enantioselectivity such as concentration of Cu(II)ion and pH of the eluent, addition of NH4Ac to the eluent, type and content of organic solvent of hydro-organic eluent and column temperature were examined. The extremely high enantioselectivity observed (α up to 16) is assumed to be caused by a three site sorbate-sorbent interaction involving bidentate coordination of two amino acids to a Cu(II)ion and hydrophobic attractions between hydrocarbon side chains of amino acids and the n-octadecyl groups of the support. The efficiency and selectivity of the system permits resolution of up to 7 racemic amino acids into enantiomers within 35 minutes on a conventional HPLC apparatus.

The nature of chiral recognition: Is it a three-point interaction?

Evolution of the initial “three-point attachment model” resulted in the understanding that an interaction in at least three configuration-dependent points is needed for a chiral selector to recognize entantiomers. Thermodynamic enantioselectivity of this interaction can result in chiral discrimination of the enantiomers, with the exception of a temperature range where enthalpic and entropic contributions to the free energy of discrimination balance each other. Similarly, a three-point interaction is needed for a chiral inductor to modify enantiospecifically a prochiral molecule. The difference between a theoretical interaction point and real interaction sites in chemical molecules is emphasized. The role of conformational rigidity of chiral species is discussed in relation to the dependence of spatial arrangement of three active points on the configuration of the species. Chirality 9:99–102, 1997.

Chiral selectors with chelating properties in liquid chromatography: Fundamental reflections and selective review of recent developments

Abstract Taking ligand-exchange chromatography as a relatively well studied, example, theoretically expected general correlations between retention, enantioselectivity and efficiency of chiral chromatographic systems are discussed. More recent practical achievements and the expansion of the application area of chiral ligand-exchange chromatography are also briefly reviewed.

Enantioselective ligand exchange in modern separation techniques.

As a follow-up to a series of review articles on enantioselective ligand exchange chromatography, the present contribution critically evaluates achievements in this area of active and successful research which have been reported in the scientific since 1992. Also discussed is enantioselective ligand exchange in electromigration techniques which have developed especially fruitfully during the last decade.

Sorption of organic compounds from aqueous media by hypercrosslinked polystyrene sorbents ‘Styrosorbrs

Abstract Sorption experiments with a variety of organic compounds dissolved or suspended in water and a new hypercrosslinked polystyrene-type sorption material ‘Styrosorb’ (‘Macronet-Hypersol’) showed superiority of the latter to other chemically related adsorbents

Investigation of the properties of hypercrosslinked polystyrene as a stationary phase for high-performance liquid chromatography

SummaryThe mechanical rigidity, swelling properties, adsorption selectivity, and chromatographic performance of hypercrosslinked polystyrene (mainly MN-200 or Purosep-200; Purolite, UK) have been studied to evaluate the use of the material as a stationary phase for reversed-phase high-performance liquid chromatography (RPHPLC). By use of inverse size-exclusion chromatography (SEC) this adsorbent, with a high specific surface area of 1500 m2 g−1 was found to have a biporous structure with micropores of ca 1–2 nm and macropores ca 100 nm in diameter. The polymer does not change its volume significantly on changing water for organic solvents. The retention increments for methylene and phenyl groups were calculated and indicated that the mechanism of retention on the hypercrosslinked polystyrene involves π-π interactions and strong hydrophobic interactions. The column performance of the hypercrosslinked polystyrene was found to be acceptable, with reduced plate height increasing very slowly as the linear velocity of the mobile phase increased to high values (up to 20–45 cm min−1). Columns containing hypercrosslinked polystyrene were evaluated for the separation of phenols, dialkyl phthalates, and polyaromatic compounds. On-column preconcentration of trace organic compounds from aqueous media is possible. With smaller particles of hypercrosslinked polystyrene becoming available, this material can be regarded as an alternative to alkylsilica as a hydrolytically stable column-packing material for RPHPLC.

Hypercross-linked polystyrene and its potentials for liquid chromatography: a mini-review

Hypercross-linked polymeric adsorbing materials are obtained under conditions that (i) their polymeric network is formed in the presence of large amounts of a thermodynamically good solvent (porogen) and (ii) the network is rigid. Hypercross-linked polystyrene is a transparent microporous low-density material with an apparent inner surface area of over 1,000 m2/g and an unprecedented adsorption capacity. To enhance the mass transfer, the adsorbent beads may be provided with large transport pores, in addition to the inherent micropores; these beads are opaque. Hypercross-linked polystyrene sorbents are widely used for large scale adsorption of organic compounds from aqueous and gaseous media and for solid-phase extraction of trace components. Novel perspective application areas of the materials are high-performance liquid chromatography column packings and blood purification. Present mini-review summarises basic principles of obtaining hypercross-linked materials, their structural peculiarities and distinguishing properties, as well as major application areas. Important new unpublished data are also included.

High-performance ligand-exchange chromatography of α-amino acid enantiomers

Abstract Three packings with the following surface composition were synthesized on LiChrosorb Si 100, d p  10 μm: [Cu(II) 3-( L -prolyl)propyl] + (packing I), [Cu(II) 3-( L -hydroxyprolyl)propyl] + (packing II) and [Cu(II) 3-( L -hydroxyprolyl)propyl] + with remaining excess 3-(iodopropyl) groups (packing III). The separation of α-amino acid enantiomers was studied in eluents containing a constant concentration of copper(II) acetate (10 −4 M ). The pH of the buffered eluent was varied in the range 4–6 and the ammonium acetate concentration was varied from 0.001 to 0.1 M , and these were established as dominating parameters in controlling the retention and enantioselectivity. In contrast, the addition of an organic solvent such as methanol, acetonitrile or tetrahydrofuran gave only minor changes. The same was observed for the changes in k′ and α on increasing the methanol content up to 30% (v/v) and by varying the column temperature between 298 and 323°K. The order of elution of α-amino acid enantiomers was generally found to be L - D -; there were, however, a series of exceptions, depending on the type of packing and the eluent composition. Enantioselectivity can be understood in terms of complexation of the enantiomer to a trans -bis(amino acidato)—copper complex and additional interaction through complexation of free ligand sites, hydrophobic interaction between the radical R and the n -propyl spacer of the bonded ligand, hydrogen bonding, etc. The columns permitted the separation of six racemic α-amino acids into the corresponding enantiomers, using common high-performance liquid chromatographic equipment with a UV photometer (254 nm).

Polymeric adsorbent for removing toxic proteins from blood of patients with kidney failure

A hypercrosslinked styrenic polymer with an enhanced proportion of mesopores in the range 2-20 nm has been developed. The principle of the synthesis consists of the suspension polymerization of divinylbenzene (or copolymerization of styrene with divinylbenzene) in the presence of a porogen that is a theta-solvent for polystyrene. On the scale of thermodynamic affinity, theta-solvents occupy a border position between good solvents and precipitating media for the growing polymer chains. In this case, microphase separation takes place during the final stages of the polymerization process. The polymer was shown to adsorb 93-98% of beta2-microglobulin from the blood or plasma of patients with chronic kidney failure. At the same time, large essential proteins, like albumin, are not removed to a significant extent, obviously, due to the size-exclusion effect and the difference in the hydrophobicity of the proteins. By replacing surface exposed pendant vinyl groups of the polymer with hydrophilic functional groups, the material was made hemocompatible, according to the standard battery of biocompatibility tests required by ISO 10993 guidelines. No adverse effects such as fever or hypotension were noted in dogs in direct hemoperfusion experiments with the polymer.