For the first time, chemists have designed catalysts that are activated by mechanical stress. Embedded in self-healing coatings, such catalysts might initiate repair reactions when scratched or stressed. The mechanically triggered catalysts could also find applications in industry to improve yields of plastics, drugs, and other substances.
The mechanically activated catalysts were designed by Rint Sijbesma, a professor of chemistry at Eindhoven University of Technology, in the Netherlands, and are described today in the journal Nature Chemistry. Some existing catalysts can be activated by heat or light pulses, allowing chemists greater control over the progress of chemical reactions. But the new catalysts are the first example of catalysts activated by mechanical stresses.
Sijbesma’s catalysts take advantage of a property of polymers that chemists have known about for many years. When polymers are exposed to great enough force, they are pulled taut, and the stress causes chemical bonds to break. Where the bond breaks, however, is difficult to control. Sijbesma designed carbon-based polymers containing two catalysts bridged by a metal atom. In this state, the catalysts are inactive. The carbon-metal bond is the polymer’s weakest, and under stress, it’s the one that breaks, leaving behind active catalytic sites.
“The coupling between mechanical energy and chemistry remains less well developed than, say, photo, thermal, or electrical energy,” says Jeffrey Moore, a professor of chemistry at the University of Illinois in Urbana-Champaign. In 2007, Moore was the first to demonstrate a reaction designed to be initiated by mechanical stresses. But this reaction didn’t involve a catalyst and can only happen once. Once Sijbesma’s catalyst is activated, it can make reactions happen again and again. “It is rare that new chemical concepts of such a fundamental nature are uncovered and demonstrated,” says Moore.
The Eindhoven researchers demonstrated mechanical activation of catalysts for several well-known reactions, including one used in biofuel synthesis, one used to close carbon rings during the production of pharmaceuticals, and another used to open such rings for making durable plastics.
“This is an interesting application of mechano-chemical techniques,” says Robert Grubbs, a professor of chemistry at Caltech and winner of the 2005 Nobel Prize in chemistry, who was not involved in the Eindhoven work. In principle, it could find applications in catalysis, says Grubbs. The more tools chemists have at their disposal to fine-tune the progress of reactions, the more efficient chemical production can be, says Alshakim Nelson, a chemist at IBM’s Almaden Research Center. “It gives us another knob to tune,” he says.
Sijbesma says that the catalysts’ earliest uses are likely to be in self-healing materials and stress sensors. “The presence of a lot of stress in a material indicates that it is about to fail,” he says. “Our stress-sensitive catalysts may react to this signal by starting a polymerization reaction that reinforces the material precisely at the place and the time it is needed.” Self-healing coatings would prevent cars, ships, and bridges from rusting without the need for frequent reapplications.
While this work is exciting, it’s still in early stages, cautions Moore. The catalysts were demonstrated in liquid solutions, with ultrasound pulses supplying the mechanical stress. “There’s still a significant gap between ultrasound activation in solution and the development of a mechanoresponsive material,” says Moore.