Biasing reaction pathways with mechanical force

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. Carefully designed 'mechanophores' can tame the 'brute force' approach needed for breaking chemical bonds in reactions. If incorporated into polymers and activated by mechanical forces, the mechanophores undergo rearrangement reactions to selectively form new molecules. The effect might result in mechanically responsive polymers that warn of impending structural failures, can slow damage or even self-repair. During the course of chemical reactions, reactant molecules need to surmount an energy barrier to allow their transformation into products. The energy needed for this process is usually provided by heat, light, pressure or electrical potential, which act either by changing the distribution of the reactants on their ground-state potential energy surface or by moving them onto an excited-state potential energy surface and thereby facilitate movement over the energy barrier. A fundamentally different way of initiating or accelerating a reaction is the use of force to deform reacting molecules along a specific direction of the reaction coordinate. Mechanical force has indeed been shown to activate covalent bonds in polymers, but the usual result is chain scission1. Here we show that mechanically sensitive chemical groups make it possible to harness the mechanical forces generated when exposing polymer solutions to ultrasound2, and that this allows us to accelerate rearrangement reactions and bias reaction pathways to yield products not obtainable from purely thermal or light-induced reactions. We find that when placed within long polymer strands, the trans and cis isomers of a 1,2-disubstituted benzocyclobutene undergo an ultrasound-induced electrocyclic ring opening in a formally conrotatory and formally disrotatory process, respectively, that yield identical products. This contrasts with reaction initiation by light or heat alone3, in which case the isomers follow mutually exclusive pathways to different products. Mechanical forces associated with ultrasound can thus clearly alter the shape of potential energy surfaces4 so that otherwise forbidden or slow processes proceed under mild conditions, with the directionally specific nature of mechanical forces providing a reaction control that is fundamentally different from that achieved by adjusting chemical or physical parameters. Because rearrangement in our system occurs before chain scission, the effect we describe might allow the development of materials that are activated by mechanical stress fields.