Poly . helical conformation optical activity

Isocyanide Polymers Bulky isocyanides give polymers having a 4 1 helical conformation (115) [154]. An optically active polyisocyanide was first obtained by chromatographic resolution of poly(f-butyl isocyanide) (poly-116) using optically active poly((S)-sec-butyl isocyanide) as a stationary phase and the polymer showing positive rotation was found to possess an M-helical conformation on the basis of CD spectral analysis [155,156]. Polymerization of bulky isocyanides with chiral catalysts also leads to optically active polymers. 

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... 

This knowledge and understanding may be helpful to characterize local conformations of other optically active polysilanes in solution. For example, poly(methyl-(-)-(3-pinanylsilane) [(+)-7 Mw = 10,200] prepared by Shinohara and co-workers.281 showed a bisignate CD band at 280 and 303 nm, associated with a broad UV absorption at 300 nm in chloroform at 15°C. Since the spectroscopic features are quite similar to those of i,28d-28e it is possible that the main chain in 7 may contain diastereomeric helical motifs with opposite screw senses and different screw pitches. 

Recently, the first example of chiral solvation of a polysilane was demonstrated dissolution of the inherently optically inactive poly(methylphenylsilyene), PMPS, and poly(hexylmethylsilylene), PHMS, in the optically active solvents (V)-2-methyl-l-propoxybutane and (V)-(2-methylbutoxymethyl)benzene induced the polymer chains to adopt PSS helical conformations as evidenced by (positive-signed) Cotton effects almost coincident with the UV a-a transition at 340 and 305 nm, respectively.332... 

Another interesting chiral chain end effect is exhibited by the helical polymer block co-polymer, poly(l,l-dimethyl-2,2-di-/z-hexylsilylene)- -poly(triphenylmethyl methacrylate), reported by Sanji and Sakurai (see Scheme 7) and prepared by the anionic polymerization of a masked disilene.333 The helical poly(triphenylmethyl methacrylate) block (PTrMA) is reported to induce a PSS of the same sign in the poly(di- -propylsilylene) block in THF below — 20 °C, and also in the solid state, by helicity transfer, as evidenced by the positive Cotton effect at 340 nm, coincident with a fairly narrow polysilane backbone UV absorption characteristic of an all-transoid-conformation. This phenomenon was termed helical programming. Above 20°C, the polysilane block loses its optical activity and the UV absorption shifts to 310 nm in a reversible, temperature-dependent effect, due to the disordering of the chain, as shown in Figure 45. 

Okamoto, Y., Suzuki, K., Ohta, K., Hatada, K., and Yuki, H. (1979) Optically active poly(triphenylmethyl methacrylate) with one-handed helical conformation, J. Am. Chem. Soc. 101, 4763-4765. 

Yashima and co-workers reported the memory of macromolecular helicity of poly((4-carboxyphenyl)acetylene) (poly-98). Poly-98 itself possesses a large number of short helical units with many helix-reversal points, and is therefore achiral. However, in the presence of optically active amine 99, which can interact with the polymer s carboxyl groups, one-handed macromolecular helicity is induced in the polymer. When achiral amino alcohol 100 is added to the helical complex, chiral amine 99 bound to poly-98 is replaced by stronger base 100. Nevertheless, the newly formed complex still shows a one-handed helical conformation. Even after the removal of 99 by gel permeation chromatography, the poly-98-100 complex retains a one-handed helical conformation without a loss of helical intensity. Thus the helicity of poly-98 induced by complexation with a chiral amine was memorized after replacement by an achiral one. The half-life of the chiral memory is as long as four years at room temperature.48... 

The helical conformation of polyacetylene derivatives bearing chiral side chains was first pointed out by Ciardelli in 197 4 24 and later extended and more clearly demonstrated by Grubbs in 199 1 25 and by Yashima and Okamoto in 1994.26a For poly(phenyl-acetylene) derivatives bearing no chiral side groups, Yashima and Okamoto showed that a helical conformation can be induced by interaction with added chiral small molecules.260 Apart from optical activity, a helical conformation of cis-cisoidal poly(phenyl-acetylene) in the solid state was pointed out by Simionescu and Percec.27... 

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