Issue Date: September 3, 2007
Boston's Chemistry Smorgasbord
AMERICAN CHEMICAL SOCIETY national meetings are like sumptuous buffets for the mind, offering literally thousands of opportunities to find out about the discoveries that chemists and their colleagues around the world have been making recently. At the ACS meeting in Boston late last month, more than 9,500 papers were presented. Here's a small sampling of the research discussed.
Polymer Synthesis, One Carbon At A Time
KENNETH J. SHEA makes his polyethylene the hard way—one carbon atom at a time. While most routes to such carbon backbone polymers incorporate two carbon atoms at a time, Shea, a chemistry professor at the University of California, Irvine, has developed a non-olefinic route to these polymers.
The reaction he has been developing involves the repetitive formation of sp3-sp3 carbon-carbon bonds in a highly controlled manner. Each successive reaction extends the polymer chain by one carbon atom.
"The reason we were looking for new methods of polymerization was that we wanted to address some of the problems that still exist in traditional olefin polymerization-controlling topology, making unusual structures, and controlling molecular weights and composition," Shea said.
His polymerization reaction builds upon a well-known homologation reaction first reported by Nobel Laureate Herbert C. Brown. The transformation involves addition of a nucleophilic carbon to an organoborane, followed by a 1,2-migration from boron to the carbon attached to the organoborane, Shea explained.
"We took this very fundamental reaction in organoborane chemistry—inserting a carbon into a carbon-boron bond—and developed reagents, monomers, and reaction conditions that allowed us to do this repetitively. And it works beautifully for making polymers," Shea told C&EN.
Specifically, the reaction employs ylides or diazoalkanes with alkylboranes (shown). The polymerization process includes no reactions that terminate the polymer chain, so the reaction chugs along until all the monomer is consumed. This so-called living polymerization process remains "alive"— the polymerization continues as long as you add monomer.
Shea's polyhomologation reaction provides excellent control over the polymer's molecular weight, giving a very narrow distribution of polymer chain lengths. It also has been used to make unusual structures, such as 50,000-membered rings and polymer chains that contain bulky substituents on every carbon.
"Our motivation is to make materials that can't be made by traditional olefin polymerization and to make architectures and topologies that are not readily available by existing methods," Shea said. He presented his research in the Division of Organic Chemistry.
Tracking Wildlife With Chromatography
IN THE EASTERN U.S., the population of mountain lions, which are also known as cougars, is thought to have fallen in recent years. To test whether the animals could be on the rebound, biologist Gary Heidt, chemist Ali U. Shaikh, and graduate student Jennifer Shirley at the University of Arkansas, Little Rock, are trying to develop chromatography and mass spectrometry methods to identify mountain lions by the bile acid profiles in their scat. Shaikh described their progress at a poster session organized by the Division of Analytical Chemistry.
Conservation biologists typically estimate mountain lion populations from the amount of mountain lion scat found in a particular region. People originally used the scat's size and shape, at best an extremely qualitative method, to determine whether it came from a mountain lion. Profiling the DNA present in scat is another option, but it's difficult to distinguish the predator's DNA from that of its prey.
Heidt and Shaikh turned to bile acid profiles instead. The liver produces a variety of bile acids, which end up in the intestines, where they aid digestion by emulsifying food. The bile acid profile in an animal's waste depends on the animal itself and the microbes in its gut. These complicated bile acid profiles might help distinguish mountain lions from other wildlife, Shaikh and his colleagues reasoned.
The team started with gas chromatography methods coupled with mass spectrometry. To make the bile acids amenable to GC analysis, the researchers derivatized the acids with trimethylsilyl groups or diazomethane. Since these derivatization methods are time-consuming, and diazomethane is potentially explosive, they turned instead to liquid chromatography paired with MS. The LC method allows the researchers to separate bile acids that are not separated by the GC method. With this method, "we can really extract information about these bile acids even though they're not very well-resolved in the GC," Shaikh said. He and his colleagues are still developing the LC method to distinguish between some isomeric bile acids.
The scientists acknowledge they have their work cut out for them; they have yet to identify a bile acid unique to mountain lions that can serve as a marker. From the GC analysis, they had thought that glycocholic acid could be such a marker, but the LC analysis—with its better resolution and lower detection limits—revealed glycocholic acid in the bile acid profiles of other species. The researchers are continuing to search for an appropriate marker, Shaikh said.
Plastic Plumbing Can Make Water Nasty
DOES YOUR drinking water have an odd bouquet? Does it taste as though it's been sitting outside in a rubber tire? If so, said Andrea M. Dietrich, a civil and environmental engineering professor at Virginia Polytechnic Institute & State University, your plumbing may be to blame.
As more and more plastic takes the place of metal for making water pipes, complaints about drinking-water quality have been increasing. "Utilities sometimes get calls from customers that their water tastes or smells different," said Gary A. Burlingame, a water-quality scientist at the Philadelphia Water Department. "When the utility investigates, the source of the problem is not found to be the utility's water, but the customer's plumbing." He added that "the nation's long-term experience with plastic pipe is lacking, and there is a need for better guidance on the use and application of different plastic materials."
That's where Dietrich's research comes in. To determine which pipes are the worst culprits, she has been studying how different plumbing materials affect the odor and taste of drinking water. She presented her findings in the Division of Polymer Chemistry.
Dietrich and her research team took commercially available plastic pipes and filled them with purified water that they had treated with the natural minerals and disinfectants usually found in drinking water. They let the water sit in the pipes for three days and then subjected it to both instrumental analysis and a panel of sensory experts who evaluated the water's smell and taste.
The pipes Dietrich tested included ones made of chlorinated polyvinyl chloride (cPVC), high-density polyethylene (HDPE), and the cross-linked polyethylene polymers PEX-a and PEX-b. Panelists used phrases such as "waxy plastic citrus" and "burning plastic" to describe the taste and smell of the water exposed to the plastic plumbing.
Dietrich's group found that cPVC has a low odor potential and doesn't seem to release many organic chemicals. HDPE, on the other hand, showed the highest odor production, despite the fact that it didn't release many organic materials. The PEX-b pipe released a moderate amount of odors and organic chemicals into the air. Compared with PEX-b, PEX-a had a less powerful odor and released fewer organic compounds.
The plasticlike odors and flavors weren't long lasting, Dietrich reported. Most disappeared after the pipes had been used regularly for about two months.
"Some materials were very smelly, even though they released only a small amount of organic carbon into the water; others released more organic carbon but did not smell much," Dietrich pointed out. "There's not necessarily a direct correlation between chemical content and odor intensity. Some of these odorants are at parts-per-trillion concentrations. That's why the human nose is so important. It can often pick up these trace amounts of really powerful odorants quicker and better than any instrument."
"Oftentimes, the water industry and pipe manufacturers overlook the impact that materials will have on the taste and odor of the water," noted Mel Suffet, an environmental health sciences professor at the University of California, Los Angeles. "Research like Dietrich's helps the drinking-water industry identify sources of odors and should also direct manufacturers toward improving their processes so that odor-free pipes are produced," he told C&EN.
Nanoporous Alumina Eyed As Dialysis Membrane
A NEW TYPE of dialysis membrane made from nanoporous alumina may better emulate human kidneys' ability to remove waste products from the bloodstream than the technology currently in use. Chemists Mark Schneider and Loyd Bastin of Widener University, in Chester, Pa., described the new membrane in the Division of Biological Chemistry.
Dialysis membranes clear toxins such as urea from the blood and preserve water balance and serum protein levels in blood, while leaving red and white blood cells intact. Aluminum oxide is a well-known nanostructured material, but it represents a complete departure from the flexible organic materials that traditionally make up membranes, such as cellulose and polysulfone. Although alumina makes for stiffer membranes, the material offers the advantage of easily controllable pore sizes and has a more regular pore pattern than polysulfone. Its pore properties can better maintain consistent blood flow through the dialyzer, Schneider and Bastin noted.
In Boston, the researchers reported that an alumina dialysis membrane developed by Widener mechanical engineer Zhongping Huang has passed all of their initial biocompatibility tests.
Huang designed the thin-walled alumina membrane and previously demonstrated that it mechanically outperforms its predecessors (J. Med. Devices 2007, 1, 79). For example, the alumina membrane can handle double the flow rate of a polysulfone membrane, due in part to its regular pore structure. And alumina's higher melting point renders the membrane more resistant to the heat used in sterilization procedures.
To test whether the alumina membrane could function in dialysis procedures, Schneider, Bastin, and Huang flowed bovine blood through the membrane for three hours-roughly the duration of a typical dialysis session. Atomic absorption spectroscopy detected no aluminum leaching from the membrane into the blood and dialysate; leaching could pose a hazard to a patient. In addition, the total free hemoglobin protein content in the blood was constant during dialysis, indicating that red blood cells remained intact upon passage through the membrane. Serum protein levels also were consistent during dialysis.
The Widener team plans to follow up the current work by assaying specific proteins that are abundant in blood, such as lactate dehydrogenase, to confirm that protein structure and function remain intact during alumina-membrane-mediated dialysis.
Calcium Fluoride Goes Soluble
THE MINERAL FLUORSPAR, CaF2, plays a unique role in the chemical industry as the only raw material source for fluorine. That's a significant distinction, given the importance of fluoropolymers and the growing use of fluorine as a substituent to stabilize active pharmaceutical ingredients.
Nonetheless, there are no direct fluorination processes that use CaF2 as a reagent. Instead, CaF2 is treated with sulfuric acid to generate hydrogen fluoride, which is used as a fluorinating reagent and as a key precursor leading to myriad other fluorinating reagents.
Herbert W. Roesky, a chemistry professor at the University of Göttingen, in Germany, decided a dozen years ago to try to remedy this situation. In Boston, Roesky described his group's progress in making CaF2 derivatives that are soluble in organic solvents and, as a result, are potentially more useful for some applications than CaF2 itself. He spoke before the Division of Fluorine Chemistry.
"To our surprise, there is hardly any chemistry known of CaF2, and no derivatives of CaF2 have ever been reported," Roesky said. One reason is that the compound is not very soluble in water or organic solvents, has a high melting point (1,418 °C), and in its mineral form is very stable, he explained. But when heated, CaF2 gives rise to fluorescence, a term that historically derives its name from fluorspar.
Aside from preparing HF, these properties make CaF2 suitable as a coating material for optical applications, Roesky noted. And when applied to the surface of glass, silicon, or metal, CaF2 makes the material resistant to chemical attack. But because of the low solubility, particularly in organic solvents, coating applications must be carried out by vapor deposition at high temperatures, Roesky said.
Seeking to address this issue, Roesky's group came up with the first organic-soluble CaF2 complexes a few years ago. But in these complexes, CaF2 is trapped in a soluble titanocene matrix, which is not particularly useful, he noted.
Earlier this year, Roesky and coworkers finally reported the first well-defined organic-soluble calcium fluoride complex, [LCaF(thf)]2, where L is a β-diketiminate ligand bearing bulky diisopropylphenyl groups and thf is tetrahydrofuran (Angew. Chem. Int. Ed. 2007, 46, 2512). They prepare the complex by reacting LCaN[Si(CH3)3]2(thf) with the tin fluorinating reagent (CH3)3SnF, another Roesky invention.
The isolated calcium monofluoride—a white solid that's soluble in benzene, n-hexane, and tetrahydrofuran—exists as a dimer. The ligands form a cage around the calcium atoms, which are bridged by a pair of fluorine atoms and further stabilized by coordinated tetrahydrofuran molecules.
The team has tested solutions of the new complex by using a dip-coating procedure to deposit thin CaF2 layers on a silicon substrate at room temperature. "With this compound, or any organic-soluble calcium derivative and a fluoride source, it is now possible to generate room-temperature coatings of CaF2 by dip or spraying techniques," Roesky said.
[LCaF(thf)]2 is "the first example of a calcium monofluoride that shows exceptional solubility in organic solvents," according to Roesky. On the basis of these results, he anticipates broad applications for CaF2 coatings.
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