Single-Event Spectroscopy and Unravelling Kinetics of Covalent Domains Based on Cyclobutane Mechanophores

Abstract

Mechanochemical reactions that lead to an increase in polymer contour length have the potential to serve as covalent synthetic mimics of the mechanical unfolding of noncovalent “stored length” domains in structural proteins. Here we report the force-dependent kinetics of stored length release in a family of covalent domain polymers based on cis-1,2-substituted cyclobutane mechanophores. The stored length is determined by the size (n) of a fused ring in an [n.2.0] bicyclic architecture, and it can be made sufficiently large (>3 nm per event) that individual unravelling events are resolved in both constant-velocity and constant-force single-molecule force spectroscopy (SMFS) experiments. Replacing a methylene in the pulling attachment with a phenyl group drops the force necessary to achieve rate constants of 1 s–1 from ca. 1970 pN (dialkyl handles) to 630 pN (diaryl handles), and the substituent effect is attributed to a combination of electronic stabilization and mechanical leverage effects. In contrast, the kinetics are negligibly perturbed by changes in the amount of stored length. The independent control of unravelling force and extension holds promise as a probe of molecular behavior in polymer networks and for optimizing the behaviors of materials made from covalent domain polymers.

Substituent Control of Tribofilm Growth under Mechanochemical Conditions

“…66 In other words, compared to the oct-1-yl groups, the 2-ethylhexyl and particularly 2-cyclohexylethyl groups act as metaphorical crowbars, allowing the shear (or tensile 13 ) stress to be transmitted across a greater distance as the C−O bond-breaking reaction proceeds. 25 This implies that the lever-arm effect, previously identified in polymer mechanochemistry under applied force, 21 can also affect mechanochemical reactivity of comparatively small organometallic compounds such as ZDDP under applied stress. From a practical perspective, ZDDPs with branched and cycloaliphatic substituents could be particularly useful for wear protection in low-temperature, high-stress applications, such as gear oils.…”

“…24 In similar polymers, replacing a methylene in the pulling attachment with a phenyl group dropped the force necessary to achieve a given rate constant in SMFS experiments by a factor of three, which was attributed to a combination of electronic stabilization and mechanical leverage effects. 25 More conventional substituent effects that can be explained through linear free energy relationships, 26 have also been identified using SMFS. For example, spiropyran-derived polymers containing more electronwithdrawing substituents in the para position to the labile spirocyclic C−O bond required less force to break.…”

Substituent Control of Tribofilm Growth under Mechanochemical Conditions

“…69 In other words, compared to the oct-1-yl groups, the 2-ethylhexyl and particularly 2-cyclohexylethyl groups act as metaphorical crowbars, allowing the shear (or tensile 13 ) stress to be transmitted across a greater distance as the C−O bondbreaking reaction proceeds. 25 This implies that the lever-arm effect, previously identified in polymer mechanochemistry under applied force, 21 can also affect mechanochemical reactivity of comparatively small organometallic compounds such as ZDDP under applied stress. From a practical perspective, ZDDPs with branched and cycloaliphatic substituents could be particularly useful for wear protection in low-temperature, high-stress applications, such as gear oils.…”

“…24 In similar polymers, replacing a methylene in the pulling attachment with a phenyl group dropped the force necessary to achieve a given rate constant in SMFS experiments by a factor of three, which was attributed to a combination of electronic stabilization and mechanical leverage effects. 25 More conventional substituent effects that can be explained through linear free energy relationships, 26 have also been identified using SMFS. For example, spiropyran-derived polymers containing more electron-withdrawing substituents in the para position to the labile spirocyclic C−O bond required less force to break.…”

Substituent Control of Tribofilm Growth under Mechanochemical Conditions

“…48 In other words, compared to the oct-1-yl groups, the 2-ethylhexyl and particularly 2-cyclohexylethyl groups act as metaphorical crowbars, allowing the applied stress to be transmitted across a greater distance as the C−O bond-breaking reaction proceeds. 54 This implies that the lever-arm effect, previously identified in polymer mechanochemistry under applied force, 19 can also affect mechanochemical reactivity of comparatively small organometallic compounds such as ZDDP under applied stress. From a practical perspective, ZDDPs with branched and cycloaliphatic substituents could be particularly useful for wear protection in lowtemperature, high-stress applications, such as gear oils.…”