Polymer Pulls Ion Pairs from Water

IT CAME TOO LATE to help chemistry professor Jonathan L. Sessler's mother, but a soluble polymer he and colleagues designed that can concurrently extract pairs of strongly hydrophilic cations and anions from an aqueous solution to an organic phase may help others, as well as the environment.

Sessler, of the University of Texas, Austin, says he initially thought about designing such a polymer while watching his mother ingest 40-g daily doses of ion-exchange resins.

Her kidneys were failing. For 10 years, dialysis cleaned her blood of urea and many ions but left behind high levels of phosphate and potassium, Sessler explains. Left untreated, buildup of these ions in the blood can wreak havoc on the body. Consuming piles of different resins not only kept her potassium and phosphate levels low but also inspired her son to look for more selective materials that might reduce dosage size.

Sessler teamed up with UT Austin assistant professor of chemistry Christopher W. Bielawski and Ahmet Akar of Istanbul Technical University, in Turkey, and coworkers. They created a bifunctional polymer composed of a methyl methacrylate backbone appended with two known ion receptors (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200803970).

They used a calix[4]pyrrole moiety previously developed by Sessler's group to bind anions. And to capture cations, the team turned to benzo-15-crown-5-ether, a classic compound that earned Charles J. Pederson a share of the 1987 Nobel Prize in Chemistry.

In laboratory tests, the bifunctional polymer extracted aqueous KF into methylene chloride. The team used 19F NMR to confirm that the fluoride ions were present in the organic phase and used flame emission spectroscopy to demonstrate coextraction of the potassium ions. The researchers found that the polymer was even more effective at extracting KCl than KF.

Additional tests showed the polymer also extracted KCl more efficiently than NaCl, suggesting it could selectively separate potassium halide salts from other inorganic salts. This selectivity could be especially valuable for controlling hyperkalemia, a potentially dangerous medical condition caused by excessive potassium in the blood, Sessler says.

"Although this paper is focused on salts such as potassium chloride, it is important in a broader context of designing reagents that are selective for different ions," says Spiro D. Alexandratos, a chemistry professor at the City University of New York, Hunter College.

Scott M. Grayson, an assistant professor of chemistry at Tulane University, says the team's simple yet elegant fusion of materials science and supramolecular chemistry is likely to yield materials with commercial applications.

Beyond controlling ion levels in vivo, possible environmental applications include methods to purify water through desalination and to remove phosphate from waterways or transuranic cations from radioactive waste.