Akio Teramoto

Molecular properties of helical polysilylenes in solution

Abstract This article reviews recent advances in research of helical conformations of optically active polysilylenes in solution. The helical conformation induces strong circular dichroism, chain stiffness, and liquid crystallinity in polysilylene chains, and it sensitively affects electronic state in the σ-conjugated silicon backbone. The helical conformation is determined by the internal rotation potential of the silicon backbone, and it turns out that the internal potential remarkably depends on the chemical structure of side-chains bonded to polysilylenes. Interrelations among these properties, the internal potential, and the side-chain chemical structure are discussed for optically active poly(dialkylsilylene)s.

Cooperative conformational transitions in linear macromolecules undergoing chiral perturbations

Abstract The conformation of a linear polymer chain in the unperturbed state undergoes a cooperative transition between an ordered and disordered states and/or between two different ordered states. Thus this transition is concerned with the conformation of an isolated chain. Typical examples discussed in this review are helix–coil transitions in polypeptides, helix reversals in chiral polyisocyanates, and order–disorder transition in a triple-helical polysaccharide schizophyllan in water, etc. They are linear cooperative systems and well treated theoretically in terms of the linear Ising model. First experimental systems studied are briefly reviewed. Then theories of the transitions based on the Ising model are re-organized with relevant conditions put for each case; both transition curves and global conformations are discussed. Numerical results are given to predict new possibilities. Finally, it is shown that these theories are used to analyze respective experimental data with the polymer molecular weight as the key bridge between experiment and theory, yielding important molecular parameters and thus exploiting the detailed molecular mechanism behind.

Thermo-driven chiroptical switching polysilane featuring 2-cyclopentylethyl side group

A new rod-like helical polysilane, poly{(S)-3,7-dimethyloctyl-(2-cyclopentylethyl)silane}, was found to undergo a thermo-driven, helix-helix transition at –33 ° C in isooctane associated with the discontinuous changes in the Siσ -Siσ *transition energy and intensity in the transition temperature region. This is the first example of a helix-helix transition polysilane with a cycloalkyl group. A similar rod-like polysilane derivative, poly{(S)-3,7-dimethyloctyl-(1-cyclopentylmethyl)silane}, however, did not undergo any helix-helix transition between –61 and 80 ° C.

Cholesteric Structure and Order-Disorder Transition in Aqueous Solutions of Schizophyllan, a Triple-Helical Polysaccharide

Abstract Cholesteric pitch (P=2π/qc ) was measured for aqueous solutions of a triple-helical polysaccharide schizophyllan as a function of temperature (T), concentration (c) and molecular weight (M v). qc was substantially independent of M v but changed with T and c. At higher temperatures, qc changed monotonously with T but showed a sudden decrease around 7–18°C on lowering T, which was due to the order-disorder transition of aqueous schizophyllan. The concentration dependence of qc for D2O solutions in the high temperature disordered state is the same as that for H2O solutions, and interpreted theoretically using the threaded effective freely jointed chain model. On temperature jump experiments. qc did not follow immediately T, but changed with the time elapsed. The relaxation time τch was less than a few minutes. Since the conformation of the triple helix follows T much faster than τch, this relaxation comes from rotational diffusion of the triple helix.

Orientational correlation of liquid–crystalline polymer chains in isotropic solutions. I. Anisotropic light scattering

The orientational fluctuation in isotropic toluene and dichloromethane solutions of a stiff-chain polymer, poly(n-hexyl isocyanate) (PHIC), has been studied by anisotropic light scattering up to the vicinity of the isotropic–nematic phase separation region. The depolarized component ΔRθ,Hv of the Rayleigh ratio divided by the polymer concentration increased with increasing the polymer concentration. The ΔRθ,Hv data for different molecular weights were fitted almost quantitatively to the scaled particle theory for wormlike hard-spherocylinders with the hard-core diameter d, developed by incorporating the higher virial terms in the free energy. However, the fitted d value was appreciably smaller than that chosen to explain the experimental osmotic compressibilities and/or isotropic–nematic phase boundaries of the same systems. Then it was found that the “spinodal concentration” c*, where the isotropic phase becomes thermodynamically unstable, lies within the nematic region, in contrast with the prediction o...