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For unheard-of success, just add water.

For unheard-of success, just add water

There is a joke that describes a theoretician as one who says, "Yes, it works in practice, but will it work in theory?" Sometimes, even practical chamists find something that contradicts previous experience so strongly that they spend a lot of time trying to make an experiment agree with theory rather than accepting what they see. That happened to two chemists at the California Institute of Technology in Pasadena, who discovered an unexpected and important chemical reaction for making polymers.

Plastics, paints and other polymers with special electronic or chemical properties are at the forefront of the materials revolution. But making polymers to order in an economical fashion can be extremely difficult because the metallic catalysts required in many such reactions can be destroyed or inhibited by many different molecules.

For instance, water and oxygen are almost always the enemy of metal-catalyzed, or "organometallic," reactions and so must be rigorously excluded from the reaction vessel. Nevertheless, Bruce M. Novak and Robert H. Grubbs discovered to their surprise that a reaction that previously would work only in the absence of water or oxygen could be made to work in water in an open container.

While testing a ruthenium-based molecule as a catalyst, Novak and Grubbs started by removing water and oxygen from the reaction mixture. When the reaction took too long, they attributed this to trace amounts of water and oxygen, as is usually the case. But the harder they worked to remove these molecules, the longer the reaction took. Finally, the two chemists thought of adding a little water, and the beneficial effects were "dramatic," Novak says. "It's amazing how long it took to sort it out."

Adding a little water was good, and adding more turned out to be even better, Novak says. This led to the thought -- highly unusual in organometallic chemistry -- of running the reaction in water itself. It worked fine, they report in the Oct. 26 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY.

Their water-based reaction is important for the painting and coating industry, Novak says. Anytime you have to evaporate a solvent to get a coat to harden, "from an economic and environmental point of view you want to use water" as the solvent, he says. Novak and Grubbs say they have had "quite a response" from companies interested in using the technique.

The process also allows researchers to create paints with special properties, such as conductivity of high-temperature stability, by adding extra chemical groups that hang off the polymer chain like trinkets on a charm bracelet.

In the type of reaction they modified, called ring-opening metathesis polymerization or ROMP, it has been very difficult to add side chemical groups for the same reason water and oxygen had to be excluded -- the catalyst reacted too easily with the side rings and became deactivated, Novak says. "The same changes you make to stabilize the catalyst to a side group stabilize it to water," he says.

It's not entirely clear why the modified ROMP reaction works so well, but it seems that water actually works as a catalyst alongside the ruthenium, and that, for some reason, the ruthenium acts specifically on the polymer bonds and not on side groups or oxygen atoms, Novak says. For industry, one additional benefit of this specificity is that the catalyst isn't deactivated by reacting with the trace impurities that are expensive to remove from the original chemical ingredients used in a reaction. All of this adds up to a form of organometallic chemistry of interest both to scientists and to those who want to apply it. "Too often before," Novak says, "we would come up with a very beautiful catalyst and industry would say, 'Nice work, but we can't use this practically.'"
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Title Annotation:ring-opening metathesis polymerization
Publication:Science News
Date:Dec 10, 1988
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