Weak link.
When tugged, a mechanically sensitive molecule undergoes a reaction, changing from the topmost molecule to the one at the bottom.

Benjamin Grosser, University of Illinois

Forcing New Chemistry

Chemists normally turn up the heat or send in a beam of light when they want to trigger chemical reactions. Now researchers report in tomorrow's Nature that they have managed to initiate a chemical reaction with mechanical force, using sound waves to essentially pull on long polymer chains. Down the road, the approach could lead to new ways to create self-healing plastics and to the development of materials that give off warnings before they fail.

The research adds a new twist to a well known property of polymers: they are often easy to break. That likely comes as no surprise to anyone who has snapped the handle off of a plastic coffee cup or shattered their child’s favorite toy. But in such cases, applying force doesn't trigger chemical reactions. Rather, it normally just breaks molecules apart in random locations. Jeffrey Moore, a chemist at the University of Illinois, Urbana-Champaign, wanted to see if he could offer a little more direction.

So Moore and his colleagues designed polymers with weak links at their core. Most polymers are long molecules made up of one or more building blocks that are linked together over and over, like box cars in a train. That was largely the case for the Illinois group's polymer as well, although in the middle of each polymer chain they placed a double ringed chemical group. One of these rings was made up of only four carbon atoms, a relatively unstable configuration.

The researchers then dissolved their polymers in an organic solvent and blasted it with ultrasound. The high frequency sound waves created zones of localized compression and expansion within the solution that tugged harder on one end of the polymer molecules than the other. This generated a mechanical force that stretched the polymers apart. That extra stress caused the weak link's four-member rings to pop open, rearranging the chemical bonds at the core of each polymer unit. Extensive experiments showed that mechanical force steered the reaction down a unique, predesigned path, exerting a level of control that chemists long for.

"This is a very elegant and spectacular piece of work," says Virgil Percec, a chemist at the University of Pennsylvania, Philadelphia. "This opens up a lot of possibilities in organic and polymer chemistry." One such possibility, Percec says, is to design polymers that generate groups that repair the polymer when they are liberated by damage. Another, Moore adds, would be to add chemical groups that generate a color change if they are broken down by wear and tear, thereby giving off a signal that they need to be replaced. Such materials might just start a bit of a reaction themselves.

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