Molecular Catch Bonds and the Anti-Hammond Effect in Polymer Mechanochemistry

While
the field of polymer mechanochemistry has traditionally focused
on the use of mechanical forces to accelerate chemical processes,
theoretical considerations predict an underexplored alternative: the
suppression of reactivity through mechanical perturbation. Here, we
use electronic structure calculations to analyze the mechanical reactivity
of six mechanophores, or chemical functionalities that respond to
mechanical stress in a controlled manner. Our computational results
indicate that appropriately directed tensile forces could attenuate
(as opposed to facilitate) mechanochemical phenomena. Accompanying
experimental studies supported the theoretical predictions and demonstrated
that relatively simple computational models may be used to design
new classes of mechanically responsive materials. In addition, our
computational studies and theoretical considerations revealed the
prevalence of the anti-Hammond (as opposed to Hammond) effect (i.e.,
the increased structural dissimilarity between the reactant and transition
state upon lowering of the reaction barrier) in the mechanical activation
of polyatomic molecules.