Helical Polymers Under Stereocontrol

Polyphenylacetylenes, developed in the 1970s by Virgil Percec of the University of Pennsylvania and coworkers, adopt left- and right-handed forms. The polymers are under active investigation as possible molecular devices, chiral catalysts, and memory elements, among other uses, and their helical sense can be switched by changes in temperature, light, or solvent. But those control techniques are difficult, or they require that the polymer be bound to a surface—and none is reversible. José M. Seco, Ricardo Riguera, and coworkers of the University of Santiago de Compostela, in Spain, now report reversibly controlling polyphenylacetylene helicity by attaching chiral phenylglycine methyl ester pendant groups to the polymer backbone and then changing the conformation of the pendant groups by adding or removing Ba²⁺ or by modifying solvent polarity (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200905222). The researchers propose a detailed mechanism by which the Ba²⁺ induced conformational change is transmitted to the polymer backbone. This is “a premier” study, Percec comments, noting that the reversible nature of the process could make the system useful for sensor applications.