Synthetic polymers chiral recognition

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... 

Different classifications for the chiral CSPs have been described. They are based on the chemical structure of the chiral selectors and on the chiral recognition mechanism involved. In this chapter we will use a classification based mainly on the chemical structure of the selectors. The selectors are classified in three groups (i) CSPs with low-molecular-weight selectors, such as Pirkle type CSPs, ionic and ligand exchange CSPs, (ii) CSPs with macrocyclic selectors, such as CDs, crown-ethers and macrocyclic antibiotics, and (iii) CSPs with macromolecular selectors, such as polysaccharides, synthetic polymers, molecular imprinted polymers and proteins. These different types of CSPs, frequently used for the analysis of chiral pharmaceuticals, are discussed in more detail later. 

In this chapter, we mainly discuss and deal with photoresponsive, chiral biorelated and synthetic polymers bearing a configurational and/or conformational chirality in the polymer main chains. The photocontrol of the chiral recognition ability of chiral polymers and chirality induction on achiral polymer films by circular polarized light (CPL) are also briefly reviewed. 

Although enantioselective inclusion into the chiral clefts formed by the polymer network may play a minor role in the chiral recognition mechanism, enantiodiscrimina-tion is primarily driven by hydrogen bonding and/or Ti-TT-interactions. A representative selection of successfully resolved racemates employing synthetic polymeric CSPs is listed in Table 9.7. 

A wide spectrum of synthetic polymers, polymeric complexes, and aggregates that have or may have a helical conformation were reviewed. The synthetic method varies from the addition polymerization methods for the vinyl and related polymers to the simple mixing methods for the aggregates. Some of the polymers exhibited functions based on the helical structure such as chiral recognition and asymmetric catalyses. 

Andersson, L.I. O Shannessy, D.J. Mosbach, K. Molecular recognition in synthetic polymers preparation of chiral stationary phases by molecular imprinting of amino acid amides. J. Chromatogr. 1990, 513, 167-179. 

The helical polytriphenylmethyl methacrylate was the first synthetic chiral polymer able to separate a very limited number of enantiomers [28]. Recently a fully synthetic chiral stationary phase based on polymerized diacryloyl derivative of fra s-l,2-diaminocyclohexane [either (R, R) or (S, S)] bonded to silica gel in the form of a very thin layer was proposed as a new LC CSP [29]. This CSP could not resolve many enantiomeric pairs. However, when it could resolve a racemate, it was shown that the amount that could be loaded was much larger than that on most other CSPs. It means that the number of active sites is large. Hydrogen bonds were found to be pivotal in the chiral recognition mechanism of this CSP. The enan-tioselectivity was adjusted by the methanol content in the organic mobile phase. Polysodium A-undecanoyl-L-leucyl-leucinate (poly-SULL) and —L-leucyl-valinate... 

MIP assays can also be utilized in synthetic organic applications. For example, MIP-based assays have been used to measure the chiral purity of samples in organic solvents. An L-phenylalanine anilide (l-PAA) imprinted polymer was utilized as a recognition element to measure the enantiomeric excess (ee) of PAA samples (Chen and Shimizu 2002). The MIP displays greater capacity for l-PAA versus d-PAA samples of similar concentration, and this difference was used to estimate enantiomeric excess. The enantiomeric excess of an unknown solution was determined by comparing the UV absorbance of the PAA remaining in solution after equilibration against a calibration curve. This MIP assay was demonstrated to be rapid and accurate with a standard error of +5% ee. 

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