Reactions get a new driving force

Optimizing a reaction traditionally involves fine-tuning the temperature, pressure or catalyst. Now, mechanical force may provide another powerful tool, say researchers at the University of Illinois at Urbana-Champaign. They have shown that tensile forces produced by ultrasound can affect mechanically active molecules, called mechanophores. This can profoundly impact some reactions, accelerating them or creating otherwise unattainable results.

“This is a fundamentally new way of doing chemistry,” says Jeffrey Moore, professor of chemistry. “By harnessing mechanical energy, we can go into molecules and pull on specific bonds to drive desired reactions.”

“We have demonstrated that it is now possible to use mechanical force to steer chemical reactions along pathways that are unattainable by conventional means,” Moore notes. “We created a situation where a chemical reaction could go down one of two pathways. By applying force to the mechanophore we could bias which of those pathways the reaction chose to follow.” For instance, tensile force causes the cis and trans isomers of a benzocyclobutene to produce an identical product, rather than different ones as usually would result, he explains.

The researchers use ultrasound to create cavitation in a solution. The collapsing bubbles produce flow fields that have a stretching action on mechanophores, he notes. Ultrasound-induced cavitation already features in some commercial reactors (see "Tiny reactors aim for big role") to enhance localized heating and mixing, but evidence suggests that these effects aren’t responsible for the results the Illinois researchers have achieved, says Moore. Temperature would affect a molecule of any molecular weight, but the mechanical forces impact only high molecular weight materials.

The mechanical-force effect has so far only been seen in polymers with a minimum molecular weight of 20,000, Moore notes. The impact is most pronounced when the mechanophore is near the center of the chain. The approach also might work with viscoelastic solid materials, he believes.

Moore explains that certain types of molecules are likely to be mechanophores. For instance, each benzocyclobutene isomer has a four-member carbon ring, which is an unnatural, strained arrangement. The ring opens when subjected to mechanical force. During experiments, ring opening occurred in more than 50% of the molecules after 15 minutes. Rings can reform once the mechanical force ceases, he says, so the researchers add a “trapping material” to prevent this.

The experiments took place at atmospheric pressure and 10°C. To achieve the same results for the trans isomer would require 12 hours at 100°C, and even at 140°C, most of the cis isomer wouldn’t have undergone transformation, he notes.

The next step, according to Moore, will be to test the approach on a solid polymer that has a mechanophore repeated at various points in its chain. Such tests should begin in the summer.

Besides more control of the direction and improved speed of reactions, the approach promises better polymer life, says Moore. The stress from a crack in a fabricated part containing mechanophores, he explains, could actually spur crosslinking of neighboring chains to reinforce the part. This would particularly suit items in inaccessible or inconvenient locations, like in human implants or in space, he adds.

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