Stress-Responsive Polymers Containing Cyclobutane Core Mechanophores: Reactivity and Mechanistic Insights

Abstract

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.

Mechanical gating of a mechanochemical reaction cascade

“…The synthesis (Fig. 3) was similar to that of previously reported bicyclo(4.2.0)octane derivatives34. Photochemical (2+2) cycloaddition of maleic anhydride and Z -9,9-dichlorobicyclo(6.1.0)non-4-ene ( 1 ) in the presence of benzophenone yielded DCTCU derivative ( 2 ), which was esterified with 4-pentenol to diene 3 followed by ring closing metathesis54 to form macrocycle 4 .…”

“…and of the Z -2,3-dichloroalkene (δ∼4.5 and 5.8 p.p.m. ), expected from ring opening of cyclobutane34 and cis-g DCC (ref. 59), respectively (Fig.…”

“…Both experimental672427303132333435363738 and theoretical2939404142434445464748 studies have been conducted to understand force-coupled reactivity and mechanisms, and these investigations provide valuable insights into the design of mechanophore response. One concept for a useful reactivity response is mechanochemical gating, in which one mechanophore (a molecular gate) initially prevents another mechanophore (substrate) from experiencing force delivered along a polymer backbone.…”

Molecular Mechanochemistry: Engineering and Implications of Inherently Strained Architectures

“…Similarly, primarily because of the high inherent ring strain, the selective cleavage of a cyclobutane bond is facile, which makes cyclobutane and its derivatives promising mechanophores as well [98][99][100][101]. …”

Mechanochemical Reactions Reporting and Repairing Bond Scission in Polymers

“…9). Finally, two detailed studies on bicyclo[3.2.0]-heptanes emerged from the Craig group in 2013 and 2014 [10,54] (Fig. 10).…”