Strain-induced strengthening of the weakest link : the importance of intermediate geometry for the outcome of mechanochemical reactions

Mechanochemically facilitated retro [2 + 2] cycloaddition of four-membered ketene dimer rings was investigated using density functional theory and ultrasound induced scission experiments. The results reveal that, in contrast to many other mechanochemical processes, the activation energy for mechanochemical ring-opening of cyclobutane-1,3-diones increases in the presence of an external force. Thus, a strengthening of bonds in the four-membered ring is observed under the influence of an extensional force on cis-substituents. A force on trans-substituents weakens the bonds, but to a smaller extent than initially stronger bonds outside the ring. Mechanochemical scission experiments of ketene dimers incorporated in the main chain of a polymer using ultrasound confirmed that scission of the chains does not occur via retro [2 + 2] cycloaddition, but elsewhere in the chain.