A reaction to stress

Chemists usually kick-start reactions with heat, light or electricity, but a far less common option is to use mechanical stress. It now seems that stress not only triggers reactions, but can also direct their course. 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 heat- 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.