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 1H 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.