Light‐Controlled Conformational Switch of an Aromatic Oligoamide Foldamer

An aromatic oligoamide sequence composed of a light‐responsive diazaanthracene‐based aromatic β‐sheet flanked by two variable diameter helical segments was prepared. Structural investigations revealed that such oligomers adopt two distinct conformations: a canonical symmetrical conformation with the two helices stacked above and below the sheet, and an unanticipated unsymmetrical conformation in which one helix has flipped to directly stack with the first helix. Photoirradiation of the foldamer led to the quantitative, and thermally reversible, formation of a single photoproduct resulting from the [4+4] cycloaddition of two diazaanthracenes within the aromatic β‐sheet. NMR and crystallographic studies revealed a parallel arrangement of the diazaanthracene photoproduct and a complete conversion into a symmetrical conformation requiring a rearrangement of all unsymmetrical conformers. These results highlight the potential of foldamers, with structures more complex than isolated helices, for the design of photoswitches showing nontrivial nanometer scale shape changes.     

Einfluß wasserlöslicher Polyelektrolyte aus 1,6-Hexamethylendiisocyanat und N-(2-Aminoethyl)-2-aminoethansulfonsaurem Natrium auf die Dispersionsbildung bei anionischen Polyurethanen

Zusammenfassung Polyelektrolyte wurden durch Umsetzung von 1,6-Hexamethylendiisocyanat und N-(2-Aminoethyl)-2-amino-ethansulfonsaurem Natrium hergestellt und als wäßrige Lösungen der acetonischen Lösung eines anionischen Polyurethans vor der Dispersionsbildung zugesetzt. Kontinuierliche Leitfähigkeitsmessungen während der Dispersionsbildung ergaben, daß oberhalb eines bestimmten Wassergehaltes die Leitfähigkeit mit steigender Polyelektrolytkonzentration abnimmt. Dies wird auf eine zum Teil reversible Partikelvereinigung zurückgeführt, bei der ein Einschluß von wäßriger Phase zwischen den geflockten Teilchen erfolgt. Der mittlere Partikeldurchmesser nimmt mit steigendem Polyelektrolytgehalt zu. Oberhalb einer kritischen Polyelektrolytkonzentration ist die Partikelvereinigung weitgehend irreversibel, wodurch die Bildung einer Dispersion verhindert wird.

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Summary Polyelektrolytes were prepared by reaction of 1,6-hexamethylene diisocyanate and the Na salt of N-(2-aminoethyl)-2-aminoethane sulfonic acid. Aqueous solutions of these polyelectrolytes were added to acetonic solutions of an anionic polyurethane before formation of the aqueous dispersion. Continuous conductivity measurements showed that above a certain water concentration the conductivity decreased with increasing amount of polyelectrolyte. This was attributed to an in part reversible association of particles with inclusion of water between the flocked particles leading to an increase of the mean particle diameter. Above a critical concentration of polyelectrolyte the association of particles becomes largely irreversible preventing the formation of stable dispersions.

     

Combining Orthogonal Chain‐End Deprotections and Thiol–Maleimide Michael Coupling: Engineering Discrete Oligomers by an Iterative Growth Strategy

Orthogonal maleimide and thiol deprotections were combined with thiol–maleimide coupling to synthesize discrete oligomers/macromolecules on a gram scale with molecular weights up to 27.4 kDa (128mer, 7.9 g) using an iterative exponential growth strategy with a degree of polymerization (DP) of 2ⁿ −1. Using the same chemistry, a “readable” sequence‐defined oligomer and a discrete cyclic topology were also created. Furthermore, uniform dendrons were fabricated using sequential growth (DP=2ⁿ −1) or double exponential dendrimer growth approaches (DP=2 −1) with significantly accelerated growth rates. A versatile, efficient, and metal‐free method for construction of discrete oligomers with tailored structures and a high growth rate would greatly facilitate research into the structure–property relationships of sophisticated polymeric materials.