Fig. 11.7 CD and absorption spectra of 21 in hexane. (Reprinted with permission from Ref. 42. Copyright 1988 American Chemical Society.)

units on average. This long helical persistence length (I) arising from the relatively large AGr value directly connects to the observed chiral amplification in polyisocyanates, because the chiral bias 2AGh is multiplied by the number of monomer units between the helical reversals (I), so that many units likely take the same helical sense within the polymer chain. In this way, the small chiral bias of each unit of the polymer chain is significantly amplified [35, 41].

Based on the variable temperature NMR experiments of 20, the activation energy (AG0) for the interconversion process between the right- and left-handed helical conformations (2AGh = 0 in this case) was determined to be 19 kcal mol-1, which is significantly greater than the thermodynamic excess energy of the helical reversal (AGr = 5 kcal mol-1) [43]. This result supports the fact that inversion of the helix readily occurs at ambient temperature, but raises a question about the boundary between the static and dynamic helical conformations. Chiral solvation, while its chiral bias (2AGh) seems to be very weak (0.04 cal mol-1), can also be used to induce a helical conformation with a preferential screw-sense in the dynamically racemic poly(n-hexyl isocyanate) (19) in nonracemic solvents [44]. Green et al. further demonstrated that the copolymers of the isocyanates composed of a mixture of (R)- and (S)-enantiomers with a small enantiomeric excess (ee) also form an excess single-handed helical conformation (Fig. 11.6C) [45]. The minority units obey the helical sense of the majority units in order to avoid introducing energetic helical reversals. They termed this phenomenon the ''majority rule''. Polysilanes

Polysilanes (22-26) are also dynamic helical polymers like polyisocyanates with essentially a 7/3 helix, but different from polyisocyanates with respect to their unique chromophoric and fluorophoric Si s-conjugated backbones. Wide varieties of optically active polysilanes bearing chiral alkyl or aromatic pendants and co-polymers with achiral monomers have been synthesized by Fujiki and coworkers [11, 46]. The structures of the pendant groups, in particular the chain length and position of the branching methyl group at the chiral center, significantly influence the rigidity of the polymer backbones (q = 6 (23), 70 (25), 85 (24) and

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