Stress-responsive polymers containing cyclobutane core mechanophores: reactivity and mechanistic insights.

A primary goal of covalent mechanochemistry is to develop polymer bound mechanophores that undergo constructive transformations in response to otherwise destructive forces. The [2 + 2] cycloreversion of cyclobutane mechanophores has emerged as a versatile framework to develop a wide range of stress-activated functionality. Herein, we report the development of a class of cyclobutane bearing bicyclo[4.2.0]octane mechanophores. Using carbodiimide polyesterification, these stress-responsive units were incorporated into high molecular weight polymers containing up to 700 mechanophores per polymer chain. Under exposure to the otherwise destructive elongational forces of pulsed ultrasound, these mechanophores unravel by ∼7 Å per monomer unit to form α,β-unsaturated esters that react constructively via thiol-ene conjugate addition to form sulfide functionalized copolymers and cross-linked polymer networks. To probe the dynamics of the mechanochemical ring opening, a series of bicyclo[4.2.0]octane derivatives that varied in stereochemistry, substitution, and symmetry were synthesized and activated. Reactivity and product stereochemistry was analyzed by (1)H NMR, which allowed us to interrogate the mechanism of the mechanochemical [2 + 2] cycloreversion. These results support that the ring opening is not concerted but proceeds via a 1,4 diradical intermediate. The bicyclo[4.2.0]octanes hold promise as active functional groups in new classes of stress-responsive polymeric materials.