Helix inversion

Figure 15. Models of helix inversion in isotactic polypropylene (a) TT conformation (h) G G conformation. From ref. (178) Copyright Accad. Naz. Lincei.

By addition of each of several diesters of isosorbide, isomannide, and isoidide to a nematic phase, cholesteric phases can be produced. All compounds exhibit a large twisting power. In the cholesteric phase, helix inversion, large or small temperature-dependencies of the pitch, and broad blue phases were achieved.183... 

In these molecular switches, there is a delicate balance between ground state distortion and ease of helix inversion, leading to racemization. It should be emphasized that the possibility of tuning the barriers for thermal and photochemical isomerization processes is important for the construction of, for example, stable molecular... 

Irradiation (k>280nm) of (P,P)-trans-35 at room temperature unexpectedly yielded (P,P)-cts-35, with the same helicity as the starting material and both methyl groups in axial orientations (Scheme 23). Low temperature irradiation (-55 °C), however, gave the expected helix inversion to provide (M,M)-ris-35 (trans-cis ratio 95 5). Upon heating to 20 °C, the less stable (M,M)-cis-35 converted into (P,P)-ris-35 in a irreversible helix inversion step. Continued irradiation at k>280 nm resulted in photoisomerization of (P,P)-ds-35 into (M,M)-trans-35 (cis-trans ratio 10 90), with simultaneous helix inversion. Subsequent heating at 60 °C produced a thermal isomerization of (M,M)-trans-35 into the starting compound (P,P)-trans-35. The combination of the... 

Several biopolymers and synthetic optically active polymers are known to exhibit an inversion of helicity (helix-helix transition) between right- and left-handed helical conformations when changing the external conditions, such as solvent, temperature, or by light irradiation. However, switching of the macromolecular helicity by chiral stimuli is rare, and can be used to sense the chirality of specific chiral guests. The helicity of optically active helical poly(phenylacetylene)s 67-69 can be switched by external chiral and achiral stimuli [123-126]. The first example of such a helix inversion induced by... 

Fig. 26 Schematic illustration of an induced one-handed helicity in optically inactive 73, helix inversion with temperature, and subsequent memory of the diastereomeric macromolecular helicity at different temperatures. The left-handed helical conformation of 73 induced by (R)-39 at low temperature reversibly changes into the opposite right-handed helix at high temperature (a), and these diastereomeric helices of 73 are memorized at different temperatures by replacement of the (R)-39 with achiral 74, resulting in the formation of the enantiomeric mirror image helices of 73 (b). (Reproduced with permission from [129], Copyright 2005 American Chemical Society)...

We published a review paper in this journal entitled Asymmetric Polymerization in 1994 which encompassed this aspect of helical polymer synthesis in addition to the other types of polymerization in which chirality is introduced during the polymerization process.34 There have been several other review papers on asymmetric polymerization and chiral polymers.35-40 On the other hand, if the energy barrier is low enough to allow rapid helix inversion at room temperature, one cannot expect to obtain a stable one-handed helical polymer but may expect to induce a prevailing helical sense with a small amount of chiral residue or stimulant. The existence of this type of polymer was most clearly demonstrated with poly(alkyl isocyanate)s.23,41... 

A. Helical Polymers Having Low Helix Inversion Barriers 4046... 

B. Helical Polymers Having High Helix Inversion Barriers 4053... 

B. Helical Polymers Havinq Hiqh Helix Inversion Barriers... 

Several procedures have been established in the past for the helix-sense-selective polymerization of isocyanides.169 For the introduction of a chiral bias, required for the discrimination of the otherwise enantiomeric helical senses, three distinct approaches can be applied. One is the use of a chiral monomer, which makes the left- and right-handed helices diastereotopic, and as a result one of the two will be energetically favorable (Figure 5). A second approach involves the use of a chiral catalyst or a chiral initiator, in which case during the initiation of the polymerization there will be a preference for the formation of one particular helical sense that will be retained upon propagation due to the helix inversion barrier. The former approach has been employed in numerous cases (vide infra) the accounts of the use of optically active Ni catalysts are limited.189-192 The... 

Fig. 5.10 Examples of chiral azobenzene dopants capable of promoting helix inversion in CLCs...

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