Helical structures optical activity

Keywords Transition metal complexes Living polymerization Rigid rod helical structure Optically active polymer Asymmetric polymerization... 

The optically active poly(TrMA) shows a large optical activity and intense circular dichroism (CD) due both to the triphenylmethyl group, indicating that this group has a chiral propeller structure, and to the helicity. Poly(TrMA) of degree of polymerization (DP) over 80 is insoluble in common organic solvents. 

It is considered that, if ideal, optically active poly(alkyl(aryl)silane) homopolymer and copolymer systems could be obtained which had stiffer main-chain structures with longer persistence lengths, it should be possible to clarify the relationship between the gabs value and the chiral molar composition. The magnitude of the chirality of the polyisocyanates allowed precise correlations with the cooperativity models.18q In the theory of the cooperative helical order in polyisocyanates, the polymers are characterized by the chiral order parameter M, which is the fraction of the main chain twisting in one helical sense minus the fraction of the main chain twisting in the opposing sense. This order parameter is equal to the optical activity normalized by the value for an entirely one-handed helical polymer. The theory predicts... 

Koe, J. R. Fujiki, M. Nakashima, H. Motonaga, M. Helical Poly(diarylsilylene)s Effects of Higher Order Structure on Optical Activity. In Synthetic Macromolecules with Higher Order Structure, Khan, I. M., Ed. ACS Symposium Series 812 Washington DC, 2002 pp 67-86. 

Poly(19-< (9-15) and poly(19 -co-2 )) undergo helix-helix transition upon temperature change. All the co-polymers exhibit no optical activity at certain temperatures, which depends on the co-polymer composition. The helical structure of poly(19-/ o-15) carrying long alkyl chains is much affected compared to poly(19-/ o-20). The thermodynamic parameters of helix transition also depend on solvent. 

The co-polymerization of D-alanine-derived A-propargylamide 22, L-valine-derived 23, and pyrene-based monomer 24 gives helical poly(22 -< o-23-c -24) carrying pyrene. The secondary structure of the co-polymer is tunable by the composition of the optically active amino acid units and solvent, which makes it possible to control the direction of the pyrene groups in the side chain. The interaction between the pyrene groups is small when the co-polymer takes a helical structure. The pyrene groups are regularly positioned in the polymer side chain. The co-polymer emits weak... 

Alanine-derived optically active A-propargylamide 22 and azobenzene-containing monomer 25 afford a co-polymer forming a helix. The azobenzene moiety isomerizes from trans-ioxm to cis-ioxm upon UV light irradiation, accompanying transition from helix to random coil. Then upon irradiation of visible light, the m-azobenezene moiety re-isomerizes into trans, while the polymer main chain keeps a random structure. This is presumably due to large steric repulsion around the azobenzene moiety to disturb recovery of a helical structure. 

Our template synthesis of knots implies that the target molecules are obtained as cationic dicopper(I) complexes. Therefore we considered the possibility of interconverting both enantiomers into a pair of diastereomeric salts [137, 138] by combining them with an optically active anion. Binaphthyl phosphate (BNP") [139] drew our attention because its chirality arises from the binaphthyl core, which is twisted. This helical structure is of the same type as that of die copper double helix, precursor of the knot. Besides, both compounds are aromatic and, thus, we could expect some potentially helpful stacking interactions [87],... 

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