“Mechanochemical activation” of mechanophores often collectively refers to 1) generation of the key intermediates and 2) transformation of the intermediates to the desired products. If the tension could be removed between step 1 and step 2, would the intermediates continue to the expected mechanochemical products or revert to the starting material?

To probe the effect of applied tension on mechanochemical intermediates experimentally, we envisioned a system in which radical intermediates could be generated mechanochemically, but the tension could be released before the radicals exhibited any reactivity. However, the collapse of ultrasonication induced cavitation bubbles (~10^-5 s) is longer than the lifetime of tension-promoted carbon diradicals (10^-13 to 10^-9 s), making it experimentally difficult to apply tension selectively for the generation of the intermediates. We hypothesized that we could artificially extend the lifetime the mechanochemically generated diradicals of interest by designing a system in which the diradicals could be separated from each other via a recursive reaction in which the same radical was regenerated, analogous to the propagation step of a polymerization. Using the polymer main chain as a radical relay to separate the generated radicals would extend the lifetime from 10^-9 s to ~10^-5 s. If this recursive reaction were fast enough, it could selectively propagate many times; 100 reactions da capo al coda would artificially further extend the radical lifetime by 2 orders of magnitude, allowing us to potentially study radical reactivity during bubble collapse (applied tension) and post bubble collapse (no applied tension).

In this talk, I discuss how we came up with the system, our experiences, potential applications of this system, and other designs for reversibly-cascading polymechanophore systems.

Yan Xia

Associate Professor, Stanford University

Appearances