Polyisocyanates helical structure

Similar behavior has been observed in polyisocyanates, which have been shown to possess a helical structure. Unlike polypeptides, polyisocyanates have no stereocenter in their backbone they therefore form a racemic mixture of left-handed and right-handed helices. 1221 Incorporation of chiral azobenzene dyes into the side... 

In the early stage of helical polymer stereochemistry, a few polymers were known to retain a helical main chain with a predominantly single screw sense in solution at room temperature. For example, in cases of poly( f-bulyl isocyanides) [22], poly(triphenylmethyl methacrylate) [23], polyisocyanate [24], and poly-a-olefins [19], helical structures are kept through side group interactions. Since these pioneering works, many synthetic optically active polymers with a chromophoric main chain bearing chiral and/or bulky side... 

The nature of the helical conformation of poly(phenylacetylene) has been studied in detail (74). The stabihty of the helical conformation of poly(phenylacetylenes) was estimated by the chiroptical properties of the copolymers from chiral and achiral phenylacetylenes. When the monomer possesses sterically less bulky ring substitnents, a clear cooperative nature on the copolymerization is not observed. A chiral amplification phenomenon is attainable only when the monomers have bulky ring snbstitnents. This result coincides with the poor chiroptical property of poly(42) (73) and also with the very intense CD effects of poly(44) having bulky chiral silyl groups (256). Computational simulations verified that, unlike polyisocyanates which have a long persistence length of helical structure because of their stiff main chain, the main chain of poly(phenylacetylene) is quite fiexible and that, unless bulky substituents are incorporated, poly(phenylacetylene) exists in essentially randomly coiled conformation or in a helical conformation with very short persistence length. 

It is considered that, if ideal, optically active poly(alkyl(aryl)silane) homopolymer and copolymer systems could be obtained which had stiffer main-chain structures with longer persistence lengths, it should be possible to clarify the relationship between the gabs value and the chiral molar composition. The magnitude of the chirality of the polyisocyanates allowed precise correlations with the cooperativity models.18q In the theory of the cooperative helical order in polyisocyanates, the polymers are characterized by the chiral order parameter M, which is the fraction of the main chain twisting in one helical sense minus the fraction of the main chain twisting in the opposing sense. This order parameter is equal to the optical activity normalized by the value for an entirely one-handed helical polymer. The theory predicts... 

Helix is the most common higher-order structure of synthetic polymers such as peptides, polymethacrylates, polychloral, polyisocyanides, polyisocyanates, and polysilanes. Polyacetylenes bearing appropriate substituents also form a helix. Substituted helical polyacetylenes are promising candidates for enantioselective permeable materials, polarization-sensitive electro-optical materials, asymmetric electrodes, and hence their synthesis is currently under intensive research. This section overviews the synthesis and properties of helical polyacetylenes recently reported. 

In contrast to polypeptides that have many possible conformations, poly(hexyl isocynate) is known to have a stiff rodlike helical conformation in the solid state and in a wide range of solvents, which is responsible for the formation of a nematic liquid crystalline phase.45-47 The inherent chain stiffness of this polymer is primarily determined by chemical structure rather than by intramolecular hydrogen bonding. This results in a greater stability in the stiff rodlike characteristics in the solution as compared to polypeptides. The lyotropic liquid crystalline behavior in a number of different solvents was extensively studied by Aharoni et al.48-50 In contrast to homopolymers, interesting new supramolecular structures can be expected if a flexible block is connected to the rigid polyisocyanate block (rod—coil copolymers) because the molecule imparts both microphase separation characteristics of the blocks and a tendency of rod segments to form anisotropic order. 

Recently polyisocyanates or 1-nylons have received more attention due to their chiral helical [3e] structure and their liquid crystalline properties [3f]. The older methods of polymerization involve anionic polymerization and generally the yields and molecular weights are low. 

Polyisocyanates are comprised of only amide bonding in the main chain and maintain their helical rod structures, so they normally show liquid crystallinity or high crystallinity. For instance, the helix conformation (12/5) of poly(hexylisocyanate) (PHIC) and its high crystallinity in the solid state are well observed by X-ray scattering (Wu et al., 1992). PHIC displays rod structures with a very long persistence length of 50-60 nm in the solution state (Bur and Fetters, 1976). It also shows lyotropic liquid-crystalline behavior at high concentrations (Aharomi, 1979 Aharoni and Walsh, 1979). 

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