Helical polymers, structural features

The gel continuously proceeds towards the stable equilibrium, but does not reach it within reasonable periods of observation. Second, the solvent mobility is modified by the gel formation. Water is included into the structure and stabilizes the helices. These two features seem to be common to a great number of physical polymer gels. 

Since Robinson [1] discovered cholesteric liquid-crystal phases in concentrated a-helical polypeptide solutions, lyotropic liquid crystallinity has been reported for such polymers as aromatic polyamides, heterocyclic polymers, DNA, cellulose and its derivatives, and some helical polysaccharides. These polymers have a structural feature in common, which is elongated (or asymmetric) shape or chain stiffness characterized by a relatively large persistence length. The minimum persistence length required for lyotropic liquid crystallinity is several nanometers1. 

The main feature identifying a cholesteric mesophase in polymers is the presence of optical texture with selective circularly-polarized light reflection. This indicates the formation of 1-helical cholesteric structure in LC copolymers. The X-ray patterns of actually all cholesteric copolymers described (with the exclusion of polymers 3.1 and 4.1, Table 13) correspond to those of nematic and cholesteric low-molecular liquid crystals, which is manifested in a single diffuse reflex at wide scattering angles. At the same time, for copolymers 3.1 and 4.1 (Table 13) small angle reflexes were observed 123), that are usually missing in low-molecular cholesterics. 

Epitaxial crystallization of helical polymers may involve three different features of the polymer chain or lattice. These are (a) the interchain distance (as for stretched out polymers), (b) the chain axis repeat distance, and (c) the interstrand distance - the distance between the exterior paths of two successive turns of the helix. The two former periodicities are normal and parallel to the chain axis direction, and are therefore not usually sensitive to the chirality of the helix (unless the substrate topography is asymmetric and favors a given helical hand). However, the interstrand distance is oblique to the helix axis (it is normal to the orientation of the outer chain path) and therefore has different, symmetric orientations relative to the helix axis for left-handed and right-handed helices (Fig. 2). In other words, epitaxies that involve the interstrand distances are discriminative with respect to helix chirality. This discrimination becomes visible if the crystal structure is based on whole layers of isochiral helices. Such a situation does indeed exist for isotactic poly(l-butene), Form I, that will be considered soon. 

The life that we know also uses proteins for the majority of structural and catalytic functions. Proteins are particularly suited for these functions because of the structural properties of polymers of amino acids. The polyamide backbone of proteins is neutral, unlike that of nucleic acids. Further, the backbone has a repeating dipole able to make hydrogen bonds. These structural features are exploited as proteins fold into globular structures, as they promote the formation of stable secondary structures such as alpha helices and beta sheets. 

Ma et al. have investigated the solid-state structure and properties of three poly(ethylene glycol)-b-poly(L-alanine) (PEG-PLA) diblock copolymers that were prepared from a PEG macroinitiator with a number-average molecular weight of 2000 [62]. The diblock copolymers contained 39.8, 49.6 and 65.5 mol% PLA. PTIR spectra of the diblock copolymers were indicative for an a-helical secondary structure of the peptide block. DSC traces of the polymers, in particular for the sample containing 49.6 mol % PLA, showed features of both of the respective homopolymers, which led the authors to propose a microphase-separated bulk structure. 

Optical activity of natural products may depend on chemical factors such as asymmetric carbon atoms, restricted rotation, etc. These may be termed primary structural features. There are also secondary structures, e.g., helices or random coils, that may confer chirality to a natural product. Optical rotatory dispersion (ORD, i.e., rotation of plane-polarized radiation over a range of wave-lengths usually from approximately 200 to approximately 500/im) has been used in studies of the conformations of many different molecules, including polymers, proteins, and polypeptides [90]. 

Stereoregular polyacetylenes have been attracting great attention due to the characteristic features based on their hehcal structure and the conjugated main chain [22, 23], The dynamic structure change in the helical polymers is particularly attractive. Stereoregular polyacetylenes with the c/s-transoidal structure have been synthesized by rhodium catalysts from various acetylene derivatives. 

The 3-stacks are packed together, like cylinders, to give approximate close-packing of the 3-stacks and also of the individual polypeptide helices °. There is a slow twisting of the individual chains and of the 3-stacks, reversing in direction when certain stability limits are reached. There are disulfide crosslinks, probably producing an alternation of zones with many crosslinks with zones of few crosslinks. These structural features should be studied by polymer scientists interested in making better textile fibers. ... 

Polyacetylenes (for reviews on polyacetylenes and poly(phenylacetylene)s see [1, 2]) are a versatile family of helical polymers that have attracted the attention of a number of research groups in recent years, mostly due to their capacity to adopt helical structures and, correspondingly, to display axial chirality and due to the properties associated with this structural feature. 

AFM and MM studies showed that these polymers in CHCI3 presented identical handedness for the internal (polyene backbone) and the external (pendants) helices (3/1 helix), whereas in THF the internal and external helices (2/1 helix) presented opposite helical senses. DSC traces supported the cis-cisoidal and cis-transoidal helical structures associated with those structural features. 

The information on the structure of the cholesteric mesophase of polymers is currently limited to data on cholesteric polymers of the comb-shaped type. The comb-shaped structure of macromolecules with mesogenic side groups determines their tendency to form layered structures. In this respect, the question arises of how the helical supermolecular structure is formed in such a system and what its features are in comparison to the cholesteric structure of low-molecular-weight liquid crystals. The answer to this question is given in [81, 82], where the structure of homopolymers and copolymers forming the cholesteric mesophase was studied. 

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