Poly silanes sense

We also reported an intriguing chiral termini effect in rodlike poly(n-hexyl-(2-methylpropyl)silane) with (5)-2-me(hylbu(yl termini (15).28g Although the value of gabs is only +1 x 10-5, which indicates a very weak preferential screw sense in 15, it is noted that the preferential screw sense of 15 was opposite to that of 4, bearing the same (5)-2-me(hylbutyl side groups.28d,28f,28k... 

In this section, we comprehensively focused on the controlled synthesis, chiroptical characterization, and manipulation of optically active poly(dialkyl-silane)s. Although many artificial polymers adopting preferential screw sense... 

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... 

Figure 11. Screw-sense-selective photolytic cleavage of the M-helical segments in poly[methyl(2-methylbutyl)-silane] and subsequent Wurtz coupling of P-helical segments. (Reprinted with permission from ref 127. Copyright 1994 American Chemical Society.)...

Reversible control over the helix sense in polysilanes was achieved in the case of poly(diaryl-silanes)128 as well as in (co)polymers of ((S)-3,7-dimethyloctyl)(3-methylbutyl)silane,129 which both showed helix reversal upon heating. For the latter polymer (Figure 12a), it was calculated that the potential curve has a double-well ( W j shape (Figure 12b) with a slight preference for the M-helix over the P-helix. CD spectroscopy indeed revealed that above the transition temperature the ordered (low-entropy) M-helical conformation becomes less stable than the entropically more favored P-helical state (Figure 12c,d). 

Figure 12. (a) Structure of poly[((S)-3,7-dimethyloctyl)(3-methylbutyl)silane]. (b) Energy diagram showing the doublewell shape, (c) CD and UV spectra of both screw senses, (d) Schematic representation of the two helical forms. (Reprinted with permission from ref 129. Copyright 2000 American Chemical Society.)... 

Isochains and heterochains may be substituted or unsubstituted. Here, the different significances of the word substitution in inorganic and organic chemistry must be noted. In the strictest sense, unsubstituted isochains occur in cflr/ na-poly(sulfur). On the other hand, silanes, H(SiH2)nH, should be considered as substituted in the sense of inorganic nomenclature. By contrast, the poly(alkanes), H(CH2)nH, are considered to be unsubstituted chains in organic chemistry, since the basic structure is here considered to be the alkanes, and not diamond. 

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