Mechanoradical

A mechanoradical is a radical species formed when mechanical energy is converted into chemical reactivity. Unlike photochemical or thermal radicals, mechanoradicals arise from mechanical stress, friction, or agitation of a molecule or solid, often in polymers or condensed-phase materials.

Under mechanical force, covalent bonds may undergo homolytic cleavage, generating radical centers. The effect is more pronounced for bonds that are weak under strain or in bond-labile states; in polymers, chain scission can yield macroradicals at chain ends or along the backbone. Mechanoradical formation can also occur via stress-induced homolysis of peroxides or metal–ligand bonds, depending on the system.

Mechanoradicals can be produced by ultrasonication in solution, ball milling or dry grinding, high-shear mixing, or tensile/fatigue deformation of solids, and friction. In polymers, simple mechanical activation can initiate radical processes that drive cross-linking or polymerization.

Electron paramagnetic resonance (EPR) spectroscopy is commonly used to directly observe radical species. Spin-trapping agents and radical scavengers help identify the types of radicals formed. Changes in mechanical properties, rheology, or chemistry (for example grafting or crosslinking) can indicate mechanoradical activity.

Mechanoradicals enable mechanochemical reactions such as solvent-free polymerizations, surface functionalization, self-healing materials, and stress-responsive chemistries. They also relate to material aging and degradation under mechanical use.