Chiral phases helical polymers

A few synthetic helical polymers are known to act as chiral selectors.7a,918d l8k i9d i9h ancj are widely used as chiral stationary phases (CSP) in gas or liquid chromatography.73,53 Recently, it has been reported that the preference of one helical sense in isotropic solution can be induced by some interaction between optically inactive polymers and chiral solvents/additives. Examples of this include poly(n-hexyl isocyanate)18d l8k and poly(phenylacetylene)s bearing functional group.19d 19h The polysilane derivatives also show chiral recognition ability in solution at room temperature. Poly(methyl-ft-pinanylsilane) includes two chiral centers per bulky hydrophobic pinanyl side group28 and... 

The realization of nematic side chain polymers implies the possibility of the existence of cholesteric side chain polymers, presuming the mesogenic molecules, which are linked to the backbone, are chiral. For these polymers it is of interest, whether the polymer fixation influences the helical twist and therefore the optical properties of the cholesteric phase. This will be discussed in 2.3.2.2. 

Another type of synthetic polymer-based chiral stationary phase is formed when chiral catalyst are used to initiate the polymerization. In the case of poly(methyl methacrylate) polymers, introduced by Okamoto, the chirality of the polymer arises from the helicity of the polymer and not from any inherent chirality of the individual monomeric subunits (109). Columns of this type (eg, Chiralpak OT) are available from Chiral Technologies, Inc., or J. T. Baker Inc. 

The role of supramolecular chemistry in materials is perhaps expressed most impressively in liquid crystals, in which slight variations of chiral content can lead to dramatic influences in the properties of the mesophases. The helical sense of these mesophases is determined not only by intrinsically chiral mesogens but also by the use of dopants which more often than not interact with achiral host LCs to generate chiral phases (Fig. 7). These phenomena are important both scientifically and technologically, most notably for the chiral smectic and cholesteric liquid crystal phases [68-71]. These materials—as small molecules and as polymers [72,73]—are useful because their order... 

Another type of chiral phase based on hydrogen-bonding interactions is the polyacrylamide-type phases. Developed by Blaschke, the phase is comprised of a polyacrylamide that incorporates phenylalanine ethyl ester. The phase has a helical structure, and the interactions are based on hydrogen bonding between the polar groups of the enantiomer and the CO—NH groups of the polymer [43,44]. 

The chiral side chain polymers derived from asymmetric esters of terephthalic acid and hydroquinone can form (in a broad temperature range, including ambient temperature) an unusual mesophase (the isotropic smectic phase, IsoSm ) characterized by high transparency and optical isotropy within the visible wavelength range, combined with a hidden layered smectic ordering and some elements of helical superstructure at shorter dimensions of 10 to 250 nm. The short-pitch TGB A model seems to be the most adequate for the mesophase structure. 

Mechanistic considerations (e.g., the extensive work published on brush-type phases) or the practitioner s experience might help to select a chiral stationary phase (CSP) for initial work. Scouting for the best CSP/mobile phase combination can be automated by using automated solvent and column switching. More than 100 different CSPs have been reported in the literature to date. Stationary phases for chiral pSFC have been prepared from the chiral pool by modifying small molecules, like amino acids or alkaloids, by the deriva-tization of polymers such as carbohydrates, or by bonding of macrocycles. Also, synthetic selectors such as the brush-type ( Pirkle ) phases, helical poly(meth) acrylates, polysiloxanes and polysiloxane copolymers, and chiral selectors physically coated onto graphite surfaces have been used as stationary phases. 

The packing materials described above separate chemical entities by exploiting chemical differences, e.g., hydrophobicity. Another class of stationary phases separates molecules based on chirality this is accomplished using a silica particle derivatized with a chiral moiety. There are several classes of chiral stationary phases including helical polymers, brushlike functional groups, protein/peptides, and inclusion complexes. Each of these is described in more detail below. Some manufacturers produce chiral stationary phases that operate either in reversed-phase or normal-phase mode, and some chiral stationary phases can be used in both modes. As with other stationary phases, there are numerous manufacturers of chiral stationary phases. However, contrary to Cl 8 and other achiral packing materials, each manufacturer of chiral stationary phases typically offers unique phases with completely separate selectivities. 

Pirkle or brush type bonded phases Helical chiral polymers (polysaccharides) Cyclodextrins and crown ethers Immobilised enzymes Amino acid metal complexes Three-point interaction Attractive hydrophobic bonding Host guest interaction within chiral cavity Chiral affinity Diastereomeric complexation... 

The preparation of the conventional chiral stationary phases can be realized both by surface immobilization of natural chiral selectors (polysaccharides, proteins) on chromatographic supports, and by direct synthesis of stationary phases composed of polyacrylates with pendant chiral groups, amides, or helical polymers. Even if they are quite expensive and poorly resistant to chemical and biological attack, these materials are largely used to separate racemic mixtures for preparative and... 

Although instances of lyotropic PLCs predate studies of thermotropic PLCs, as they involved solutions of comparatively esoteric species — virus particles and helical polypeptides — studies of these liquid crystals were isolated to a few laboratories. Nevertheless, observations on these lyotropic PLCs did stimulate the first convincing theoretical rationalizations of spontaneously ordered fluid phases (see below). Much of the early experimental work was devoted to characterizing the texture of polypeptide solutions. (23) The chiral polypeptides (helical rods) generate a cholesteric structure in the solution the cholesteric pitch is strongly dependent on polymer concentration, dielectric properties of the solvent, and polymer molecular weight. Variable pitch (<1 - 100 pm) may be stabilized and locked into the solid state by (for example) evaporating the solvent in the presence of a nonvolatile plasticizer.(24)... 

Polymerization of triphenylmethyl methacrylate in the presence of a chiral anion catalyst results in a polymer with a helical structure that can be coated onto macroporous silica [742,804). Enantioselectivity in this case results from insertion and fitting of the analyte into the helical cavity. Aromatic compounds and molecules with a rigid nonplanar structure are often well resolved on this phase. The triphenylmethyl methacrylate polymers are normally used with eluents containing methanol or mixtures of hexane and 2-propanol. The polymers are soluble in aromatic hydrocarbons, chlorinated hydrocarbons and tetrahydrofuran which, therefore, are not suitable eluents. 

Table 3 Data relative to selected crystalline polymers for which only chiral crystalline phases, characterized by chiral helical conformations, are known... 

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