A polymer that changes its characteristics depending on whether it’s heated in a polar or nonpolar solvent can retain the programmed properties when it’s cooled and the solvent is removed, reports a group led by Ken D. Shimizu at the University of South Carolina (J. Am. Chem. Soc., DOI: 10.1021/ja904234w). The ability to use stimuli such as heat to control and manipulate how a polymer recognizes guest molecules is important for applications such as chromatography and biosensing. The solvent-programmable polymer contains an arene ring with carboxylic acid and methyl substituents. When the polymer is heated, the ring can freely rotate around a Caryl–Nimide bond that tethers it to the polymer backbone. In a polar solvent, the ring orients so that the carboxylic acid is exposed to solvent. After the solution is cooled and the solvent is removed, the polymer strongly binds to a guest molecule, ethyl adenine-9-acetate, which is known to form H-bonding interactions with carboxylic acids. In contrast, when the polymer is heated in a nonpolar solvent, the carboxylic acid group becomes less accessible and when cooled only weakly binds to the guest molecule. The solvent-induced switching is reversible and repeatable.