Single-Molecule Force Spectroscopy Studies of Oxy Substituent Effects on the Reactivity of <i>gem</i>-Dihalocyclopropane Mechanophores

Oxy-substituted gem-dihalocyclopropanes (gDHCs) have recently found utility as mechanoacids for the generation of HCl or HF following force-assisted ring opening along polymer backbones. Here, we employ single-molecule force spectroscopy (SMFS) to quantify how oxy substituents influence the force-coupled kinetics of activation in gem-dichlorocyclopropane (gDCC) and gem-difluorocyclopropane (gDFC) mechanophores. Multimechanophore polymers bearing the desired mechanophores are pulled to high forces of extension until their reaction is observed as a plateau in the force–extension curves. At tip retraction velocities of 300 nm s–1, we find that oxy substituents reduce the plateau force associated with the reaction by more than 400 pN relative to unsubstituted analogs. The force reduction is greater for gDFC than gDCC, and main-chain oxy substituents reduce the requisite force by approximately 100 pN relative to pendant oxy substituents. The reactivity patterns are rationalized in terms of the accepted mechanisms of the reactions. The results help guide the design of polymer backbones for efficient mechanoacid activation.