Chromatographic resolution, polymers

Chromatographic resolution is also dependent on column efficiency (i). Column efficiency is directly dependent on the nature of the support matrix and how well that support is packed in its column. Available chromatographic supports are based on dextran, agarose, polystyrene, acrylic, cellulose, silica gel and a variety of other polymers. Althou cellulosic supports are manufactured in both microcrystalline and leaded forms, most supports are beaded. Newer supports may use hybrid bead construction where the base support is coated with a second materid (e.g., dextran or silica coated with agarose). 

The synthesis of optically active polymers was tackled with the purpose not only of clarifying the mechanism of polymerization and the conformational state of polymers in solution, but also to explore the potential of these products in many fields as chiral catalysts, as stationary phases for chromatographic resolution of optical antipodes, for the preparation of liquid crystals, and so on. 

Among the possible uses of optically active polymers, the preparation of stationary chiral phases for chromatographic resolution of enantiomers is the... 

The polymer of high molecular weight in the solid stage exhibited high crystallinity under a polarized microscope and insoluble in common organic solvents. When the polymer with high optical rotation was used as stationary phase or sorbent for the chromatographic resolution of racemic compounds, it showed the ability of resolution for many kinds of compounds, such as alcohols, amines, esters, and even hydrocarbons (28). 

Isocyanide Polymers Bulky isocyanides give polymers having a 4 1 helical conformation (115) [154]. An optically active polyisocyanide was first obtained by chromatographic resolution of poly(f-butyl isocyanide) (poly-116) using optically active poly((S)-sec-butyl isocyanide) as a stationary phase and the polymer showing positive rotation was found to possess an M-helical conformation on the basis of CD spectral analysis [155,156]. Polymerization of bulky isocyanides with chiral catalysts also leads to optically active polymers. 

Many different mobile phases have been utilized to provide the forward velocity for nonadsorbed molecules. If die mobile phase is a gas, then the technique used is gas chromatography (GC). In GC, die surface to which die molecules adsorb can be a wide variety of materials which are often prepared by coating an inert surface widi a polymer whose properties are related to its structure. In this way die surface properties and hence adsorption of die solid surface can be varied to give die best chromatographic resolution. 

Imprinted polymers have been used as stationary phase in liquid chromatographic separations and allowed the enantiomer separation for a wide range of compounds [445]. Improved chiral chromatographic resolution faetors have been archived by means of gradient-elution [448]. Imprinted polymer thin films have been tested as selective membranes [450,487], including enantiomer separation [451]. 

Fig. 2.10. Enantiomer separation factor (a) and specific surface area versus the level of MAA in the monomer mixture for the chromatographic resolution of D,L-PA on an L-PA imprinted polymer using acetonitrile as inert solvent. Eluent 5% acetic acid in acetonitrile. Column temperature 80°C and flow rate 0.2 ml/min. From Sellergren [12],...

Fig. 2.13. Separation factor (a) versus column temperature in the chromatographic resolution of D,L-PA on L-PA imprinted polymers prepared by thermochemical initiation at 60/ 90/120°C (24 h at each temperature) using acetonitrile as porogen and photochemical initiation at 15°C for 24 h using dichloromethane as porogen. For the thermochemically polymerised material the mobile phase was 5% acetic acid in acetonitrile and for the photochemically polymerised material the mobile phase was acetonitrile/water/acetic acid 92.5/2.5/5 (v/v/v). From Sellergren et al. [27] and Sellergren and Shea [13].

Fig. 4.1. Chromatographic resolution of o,L-la on a polymer imprinted with 1 (elution with a solvent gradient at 90°C) [19].

Molecular imprinting technique was recently used to prepare highly selective tailor-made synthetic affinity media used mainly in chromatographic resolution of racemates or artiftcial antibodies [130-133]. A complex between the template molecule and the functional monomer is first formed in solution by covalent or non-covalent interactions (Figure 3.10). Subsequently, the three-dimensional architecture of these complexes is confined by polymerization with a high concentration of cross-linker. The template molecules are then extracted from the polymer leaving behind complementary sites (both in shape and functionahty) to the imprinted molecules. These sites can further rebind other print molecules. 

As early as 1904, Willstatter attempted to separate optical isomers on the optically active natural polymers wool and silk [10]. About 35 years later, the first partial chromatographic resolution of the enantiomers ofp-phenylene-bis-imino-cam-phor on lactose was achieved by Henderson and Rule [11], and a few years later by Lecoq for the enantiomers of ephedrine [12], and by Prelog and Wieland for the enantiomers ofTroeger s base [13]. 

One of the most studied polymerization systems employs alkyllithium initiators that are modified by chiral amine ligands for the polymerization of sterically bulky methacrylates [8,38,39,40,41], acrylates [42],crotonates [43], and acrylamides [44]. A primary example is the reaction of triphenylmethyl methacrylate with an initiator derived from 9-fluorenyllithium and (-)-sparteine (3) at -78 °C (Scheme 4). The resultant isotactic polymer is optically active, and is postulated to adopt a right-handed helix as it departs from the polymerization site. This polymer has been particularly successful as a chiral stationary phase for the chromatographic resolution of atropisomers [8]. Many modifications of the or-ganolithium initiator/chiral ligand system have been explored. Recently, Okamo-to has applied enantiopure radical initiators for the enantioselective polymerization of bulky methacrylate monomers [45]. 

An example of an enantiomerically pure polymer is also shown [11]. Aoki et al. showed that films of a polyacetylene substituted with a (-)-p-pinene derivative formed an effective membrane for chromatographic resolutions of racemic mixtures. ( )-2-Butanol was resolved to 29.8% eje. and unsaturated polymers for both liquid-phase and gas-phase separation applications (8, 9, 79]. It has been suggested that the rigidity and irregularity of the highly substituted polyacetylene chain, combined with the presence of aliphatic substituents which reduce interchain interactions, are important for the polymers transport properties [10]. 

Sellergren, B. Ekberg, B. Mosbach, K. Molecular imprinting of amino acid derivatives in macroporous polymers. Demonstration of substrate- and enantio-selectivity by chromatographic resolution of racemic mixtures of amino acid derivatives. J. Chromatogr. 1985, 347, 1-10. 

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