Polyisocyanate helical

As compared with rather stiff rod-like polyisocyanates, helical polyacetylenes are considered to be more flexible with relatively short helical persistence lengths. However, polyphenylacetylene derivative having l- or o-alanine-based side group (166) has an exceptionally rigid rod-like helical conformation with a relatively long persistence length (40 nm) in chloroform. This polymer forms a... 

Polyisocyanates have attracted much attention owing to their liquid crystalline properties, stiff-chain solution characteristics, and induced optical activities associated with helical chain conformation (Scheme 8). Pattern and Novak [39]... 

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... 

Several natural and synthetic helical macromolecules such as DNA, polypeptides, and polyisocyanates form, in the appropriate solvent, cholesteric meso-phases. Also self-assembled supramolecular systems formed by guanosine derivatives 2 and 3 (G-wires), which are essentially four-stranded helices (Figure 7.8), behave in a similar way.35... 

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. 

Scheme 24 Chiral photoswitchable polyisocyanates A) schematic representation of the shift in equilibrium between P and M helices upon irradiation. B) illustration ofPto M helix transition in polyisocyanates upon photoisomerization ofthe azobenzene unit (adapted from references 75-78).

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... 

Another significant helical amplification in optically active copolymers with preferential screw-sense helicity is known as the majority rule phenomenon [ 17,18]. In this case, the screw sense of a helical main chain bearing nonracemic chiral side groups is controlled by the ee only and a population of preferential screw-sense helicity and optical activity were nonlinearly amplified by ee of chiral side groups. Since Pino et al. first reported this phenomenon in poly-a-olefins made of vinyl co-monomers bearing nonracemic chiral moieties [21], this majority rule has already been established in stiff polyisocyanates bearing a nonracemic chiral side chain [17,18]. 

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 underlying principle for these phenomena is considered to be the fact that the poly(phenylacetylene)s are a dynamic helical polymer of which right- and left-handed helical conformations are interconvertibly separated by helical reversals like polyisocyanates as mentioned above. Therefore, the remarkable CD induction arises from a drastic change in the population of the right- and left-handed helices of the polymers. 

A more remarkable and dramatic change in the conformation of helical polymers upon photoirradiation has been observed for the azobenzene-modified poly isocyanates. Polyisocyanates are typical dynamic helical polymers [59,60]. Even the optically inactive poly(n-hexyl isocyanate) (20), which is devoid of stereogenic centers, exists as an equal mixture of right- and left-handed helical conformations. Equilibrium exists in solution between both helices separated by... 

The helical sense of polyisocyanates 88 and 89 can be controlled in terms of photoinduced isomerization of the side chain chromophores.165,170 For 88, pho-... 

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