From Molecular to Macroscopic Mechanochemical Responses

The key concept of polymer mechanochemistry is the rational design of mechanophores. When incorporated into polymeric materials, mechanophores can be used to produce strain-dependent covalent chemical responses, including stress-strengthening, stress-sensing and network remodeling. In general, it is desirable for mechanophores to be highly inert in the absence of force but highly reactive when under applied tension. We show that the Fe−Cp bond in ferrocene is the preferential site of mechanochemical scission in the pulsed ultrasonication of main-chain ferrocene-containing polybutadiene-like polymers. Quantitative studies reveal that the Fe−Cp bond is similar in strength to the carbon−nitrogen bond of an azobisdialkylnitrile (bond dissociation energy < 30 kcal/mol), despite the significantly higher Fe-Cp bond dissociation energy (up to 90 kcal/mol). Mechanistic studies are consistent with a predominately heterolytic mechanism of chain scission. DFT calculations provide insights into the origins of ferrocene’s mechanical lability.

The unexpected combination of force-free stability and mechanochemical activity for ferrocene raises the tantalizing question as to whether similar mechanochemical activity might be present in other metallocenes, and, if so, what features of metallocenes dictate their relative ability to act as mechanophores. We find that ruthenocene, in analogy to ferrocene, acts as a highly selective site of main chain scission despite the fact that it is even more inert. A comparison of ruthenocene and ferrocene reactivity provides insights as to the possible origins of metallocene mechanochemistry, including the relative importance of structural and thermodynamic parameters such as bond length and bond dissociation energy. These results suggest that metallocenes might be privileged mechanophores through which highly inert coordination complexes can be made dynamic in a stimuli-responsive fashion, offering potential opportunities in dynamic metallo-supramolecular materials and in mechanochemical routes to reactive intermediates that are otherwise difficult to obtain.

To further elucidate the mechanistic factors that dictate the mechanochemical activity of metallocenes, we used single molecule force spectroscopy to probe the mechanical reactivity of a series of ferrocenophanes. The force-coupled rate of cyclopentadiene (Cp) dissociation among various ferrocene derivatives varies by several orders of magnitude at ~1 nN, and the differences in reactivity are not correlated with ring strain in the reactants. Instead, a strong correlation with the extent of rotational realignment of the two Cp ligands is observed. Mechanophores with pulling points that are conformationally restricted by distal attachments to an eclipsing orientation are most labile, whereas conformationally unrestricted ligands reorient under force to effectively superpose “catch bond” like contributions onto the overall mechanically assisted dissociation reaction. The ability to program the mechanism of ferrocene dissociation to proceed through ligand “peeling”, as opposed to the more conventional “shearing” mechanism of the parent ferrocene, leads to enhanced macroscopic, multi-responsive behavior including mechanochromism and force-induced crosslinking in ferrocenophane-containing polymers.

Stress sensing at the molecular level in elastomer-based composites is crucial as it could prevent catastrophic material failure from early on and prolong its lifetime. The emerging field of polymer mechanochemistry provides such opportunity by incorporating force probes into the polymer network to generate stress-induced spectroscopic responses. Force probes that could offer irreversible responses when activated is ideal for easy characterization. Here, we designed a coumarin dimer mechanophore that could be covalently embedded into a polymer main chain. Its mechanochemical lability is quantified by competing internally with the ring opening of gem-dichlorocyclopropane (gDCC) and revealed to be comparable to a disulfide bond. The structure-reactivity relationship of coumarin dimer derivatives is investigated to show that the force sensitivity could be tuned by manipulating the force-free activation energy and mechanical coupling, which is further validated by computational modeling. Further, different coumarin dimer derivatives were embedded into the main chain and filler-matrix interface in a particle-filled rubber. The relative activation at these positions under a compressive loading was quantified and revealed that the stress-concentration effect at the heterointerface could result in one order of magnitude higher activation ratio compared to the polymer main chain.