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For most chemical reactions to proceed the reactants need to surmount an energy barrier. The energy required is usually provided as heat, light, pressure or electrical potential. Now mechanical force can be added to that list - to the surprise of many a chemist. A reaction can literally be given a shove. In specially designed polymers subjected to ultrasound, rearrangement reactions are accelerated and reaction pathways can be biased to yield products not obtainable from purely thermal or light-induced reactions. The polymers contain a mechanophore positioned at a site where forces from extensional flow are greatest. Besides offering new ways of controlling chemical reactions, this work may also lead to mechanically adaptable materials, polymers that might generate a signal to warn of impending damage, undergo structure modification to slow the rate of damage, or even self-repair. [Letter p. 423; News & Views p. 381]

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Benjamin Grosser, Imaging Technology Group, Beckman Institute, UIUC

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