Helical pendant chirality

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... 

The chiral groups substituted p-conjugated polyacelylenes form helical structure with a predominantly one-handed screw sense exhibits simultaneous changes in both optical and chiroptical properties [125, 126]. For example, the pendant chiral oxazoline moiety in poly(phenylacetylene) [127] induces an excess one handedness helical conformation in the main chain of the polymer. 

An strategy to obtain new helical polymers based on the incorporation, as pendants of PPAs, of known CDAs with successful past records in other research areas such as configurational assignment by NMR, i.e. PGME (1), MPA (2), MTPA (3), has been tested. The corresponding polymers [poly-(/ )-l, poly-(5)-l poly-(/ )-2, poly-(5)-2 and poly-(/ )-3, poly-(5)-3] behave as sensors of metal cation valence and/or the polar and donor character of solvents. Phenomena such as helical inversion, chiral amplification and axial chirality selection that are displayed by these polymers make them versatile materials. 

In principle, lanthanide complexes of alkyl- (phosphinates) or alkoxy- (phosphonate esters) DOTP derivatives may give rise to 32 stereoisomers, existing as 16 enantiomeric pairs, which are indistinguishable by NMR spectroscopy. The isomers originate from chiral elements inherent in these complexes, including the R or S configuration at each phosphorus and the helicity defined by the pendant arm orientations (AIA). Various Ln3+ complexes of phosphinate and phosphonate ester ligands derived from 1,4,7,10-tetraazacyclododecane (cyclen) have been described in the literature [104-107]. 

Upon complexation with a lanthanide ion, these complexes may form square antiprism or twisted square antiprism (TS APR) structures with a vacant coordination site in the cap position, which is assumed to be occupied by a solvent molecule. Just as in the chelated complexes described previously, two distinct types of chiral stereochemistry are present. In analogy with OC-6 species, the sense of rotation of the pendant arms is denoted as A or A depending upon if the arms rotate clockwise (A) or counterclockwise (A) as one proceeds down the direction of the C4 axis. There is also chirality (or helicity) associated with the nonplanar 12-membered ring. If one looks along the skew-line connecting the coordinated nitrogens, the carbon atoms... 

Figure 13 shows several optically active vinyl polymers and copolymers with azobenzene or stilbene residues in the side groups. All these polymers were prepared by the conventional radical polymerization, so that the main chains should be atactic or syndiotactic and may not be able to take a helical conformation even in the presence of chiral pendants. The homopolymers (28-31) [83-85]... 

A polyacetylene-type helical polymer having chiral amino alcohol pendant groups has also been prepared by the polymerization of chiral (S )-threonine-based... 

X-ray measurements on LC elastomers have shown [6-8] that the reversible transition between a chiral smectic C phase with and without a helical superstructure can be induced mechanically. The helix untwisted state corresponds in this case to a polar ferroelectric monodomain. The piezoelectricity arising from this deformation of the helical superstructure (which does not require a complete untwisting) has been demonstrated [9] for polymers cross-linked by polymerization of pendant acrylate groups (Figure 15). 

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