Poly aggregates, optically active

A novel approach to aggregate control was described by Terao et al., who confined optically active polysilanes inside poly(ureaurethane) microcapsules and then induced aggregation at 20 and — 78 °C by addition of the polysilane-containing microcapsules to THF or ethanol. 5... 

The present knowledge and understanding of 1 may help us to elucidate chiroptical amplification of other optically active polysilanes in isotropic dilute solution, as aggregate forms dispersed in solution, and in a solid film. Indeed, poly(methyl-(-)-/9-pinanylsilane) ((+)-7 Mw = 10200) prepared by Aoki, Oikawa and coworkers showed a bisignate Cotton CD band at 280 and 303 nm, associated with a broad UV absorption at 300 nm in chloroform at... 

Poly (PDBF) s synthesized by helix-sense-selective polymerization using FlLi-Sp and FlLi-DDB complexes did not show detectable CD or optical activity in solution. However, a thin film made by drop-casting the solution onto a quartz plate indicated intense CD absorptions. The spectra of the polymers synthesized with (+)- and (-)-DDB were almost mirror images. The CD spectra are not due to film anisotropy but to conformational chirality of polymer chains. Because the polymers showing clear CD bands in film did not show chiroptical properties in solution, molecular aggregation may amplify and stabilize the single-handed helical conformation induced... 

It was also observed that conjugated polymers that are also electrical conductors (see Chap. 10) exhibit optical activity that depends critically on their structural organization [78]. Thus, strong chiroptical properties can be obtained firom substituted polythiophene [79] (Chap. 10) with optically active side chains, especially when the monomers are coupled within the polymer in a regioregular head-to-tail fashion. Actually, optical activity of these materials is only found when the polymers are aggregated at low temperature, in poor solvent, or in solution cast films. This contrasts with other optically active polymers, like polypeptides, poly(l-alkynes) and polyisocyanates that show an optically active conformation of the main chain in the absence of supramolecular association. 

We have rationalized the optical activity of these PAn.HCSA (and related PAn.tartrate) salts in terms of the preferential adoption of a one-handed helical conformation by the PAn chains, depending on which enantiomer of the CSA anion is employed in the doping. - - Alternatively, as suggested by Meijer et al. ° (and has been established for chiral poly(alkylthiophenes) — see Chapter 6), the observed optical activity may arise from the formation of chiral supramolecular aggregates. However, in contrast to chiral poly(alkylthiopenes), PAn(+)- HCSA exhibits strong CD spectra in "good"... 

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