Mechanochemical Reactions of Polymers

An external mechanical force is able to activate covalent bonds. Grinding, milling, or extrusion of polymers and polymeric materials leads to polymer degradation, because covalent bonds are broken. Mechanoradicals are generated by homolytic rupture of covalent bonds, but bond hydrolysis or other bimolecular reactions may as well be mechanically activated. Rubber mastication, thermomechanical treatment of food polymers, and recycling of postconsumer plastic waste are technological applications of mechanochemical reactions of polymers. The design of mechanophores opens new possibilities for mechanoresponsive materials. Weak linkages introduce defined rupture points in polymers. Mechanochromic mechanophores lead to color change of polymer solids when exposed to excessive mechanical stress, even before macroscopic damage becomes visible. Further examples are chemiluminescent mechanophores, cross-linking mechanophores, or mechanophores acting as mechanocatalysts. A quantitative understanding of mechanochemical reactions is reached with a wide variety of experimental and theoretical methods. Defined force can be applied to the bulk material, as well as to individual molecules embedded in macrocycles, or single polymer chains manipulated with an atomic force microscope. The latter method forms the basis for single molecule force spectroscopy, which is performed either in the force-ramp or force-clamp mode. The theoretical framework ranges from one-dimensional description of covalent bonds with a Morse potential via the constrained geometries simulate external force method to full-dimensional quantum chemical methods like external force explicitly included or the force-modified potential energy surface. Ab initio molecular dynamics simulations, transition state optimizations, or intrinsic reaction paths following are routinely performed on molecules exposed to a defined mechanical force. Keywords: mechanical activation; mechanophore; mechanoradical; mechanoresponsive material; single molecule force spectroscopy