Credit: © 2009 ACS

Supramolecular polymers are of great interest in the development of functional nanomaterials. In this type of polymer, the monomers are held together by intermolecular forces rather than covalent bonds. The usual strategy for preparing such polymers is to make building blocks that contain at least two recognition units that enable a polymer structure to be built up by host–guest interactions. However, the synthesis of such double recognition units can be far from easy, and does not lend itself to simple modification.

Now, Hiroshi Danjo and co-workers from Tokushima Bunri University in Japan have made1 supramolecular polymers using easily prepared tris(spiroborate)s that form an anionic host–host dimer with a structure that resembles two back-to-back bowls . When mixed with a suitable cationic guest — an iridium complex in this case — a supramolecular polymer chain is formed, the structure of which is reminiscent of a vertebrate spine.

Mass spectrometric studies indicate the formation of short oligomers from trimers to heptamers at room temperature. The supramolecular polymer shows temperature-responsive gel formation. When a solution of the two components was heated above 80 °C, a gel formed rapidly and reversibly.