The specific targeting of covalent bonds in a local, anisotropic fashion using mechanical methods offers useful opportunities to direct chemical reactivity down otherwise prohibitive pathways. Here, we report that embedding the highly inert 1,2,3-triazole moiety (which is often prepared using the canonical "click" coupling of azides and alkynes) within a poly(methyl acrylate) chain renders it susceptible to ultrasound-induced cycloreversion, as confirmed by comprehensive spectroscopic and chemical analyses. Such reactivity offers the opportunity to develop triazoles as mechanically labile protecting groups or for use in readily accessible materials that respond to mechanical force.
While the field of polymer mechanochemistry has traditionally focused on the use of mechanical forces to accelerate chemical processes, theoretical considerations predict an underexplored alternative: the suppression of reactivity through mechanical perturbation. Here, we use electronic structure calculations to analyze the mechanical reactivity of six mechanophores, or chemical functionalities that respond to mechanical stress in a controlled manner. Our computational results indicate that appropriately directed tensile forces could attenuate (as opposed to facilitate) mechanochemical phenomena. Accompanying experimental studies supported the theoretical predictions and demonstrated that relatively simple computational models may be used to design new classes of mechanically responsive materials. In addition, our computational studies and theoretical considerations revealed the prevalence of the anti-Hammond (as opposed to Hammond) effect (i.e., the increased structural dissimilarity between the reactant and transition state upon lowering of the reaction barrier) in the mechanical activation of polyatomic molecules.
The burgeoning field of polymer mechanochemistry has garnered significant interest in recent years. Mechanochemical transformations are those that are promoted by exogenous forces, and polymer mechanochemistry concentrates on the use of polymer chains to translate mechanical forces to chemical systems. Acoustic fields, particularly ultrasound, have proven to be highly efficient progenitors of tensile stresses within macromolecules and are frequently used to facilitate mechanochemical phenomena. Mechanochemical activation often arises when mechanophores, or functional groups that respond to mechanical perturbation in a controlled manner, are appropriately positioned within a polymer chain to experience tensile forces. A variety of interesting transformations have been realized when well‐designed mechanophores have been properly outfitted with polymer chains, including: thermally inaccessible isomerizations and cycloreversions, symmetry‐forbidden electrocyclic ring openings and activation of latent catalysts. Herein, the chemistry of known mechanophores is summarized and augmented with implications for new opportunities in synthesis and materials science. The focus of this mini‐review is limited to mechanophores that have been specifically adapted for polymer mechanochemistry under acoustic activation. Copyright © 2012 Society of Chemical Industry
Mechanically responsive polymers harness mechanical energy to facilitate unique chemical transformations and bestow materials with force sensing (e.g., mechanochromism) or self-healing capabilities. A variety of solution- and solid-state techniques, covering a spectrum of forces and strain rates, can be used to activate mechanically responsive polymers. Moreover, many of these methods have been combined with optical spectroscopy or chemical labeling techniques to characterize the products formed via mechanical activation of appropriate precursors in situ. In this tutorial review, we discuss the methods and techniques that have been used to supply mechanical force to macromolecular systems, and highlight the advantages and challenges associated with each.
A number of recent studies have shown that mechanical stress can significantly lower or raise the activation barrier of a chemical reaction. Within a common approximation due to Bell [Science 200, 618 (1978)], this barrier is linearly dependent on the applied force. A simple extension of Bell's theory that includes higher order corrections in the force predicts that the force-induced change in the activation energy will be given by -FΔR - ΔχF(2)∕2. Here, ΔR is the change of the distance between the atoms, at which the force F is applied, from the reactant to the transition state, and Δχ is the corresponding change in the mechanical compliance of the molecule. Application of this formula to the electrocyclic ring-opening of cis and trans 1,2-dimethylbenzocyclobutene shows that this extension of Bell's theory essentially recovers the force dependence of the barrier, while the original Bell formula exhibits significant errors. Because the extended Bell theory avoids explicit inclusion of the mechanical stress or strain in electronic structure calculations, it allows a computationally efficient characterization of the effect of mechanical forces on chemical processes. That is, the mechanical susceptibility of any reaction pathway is described in terms of two parameters, ΔR and Δχ, both readily computable at zero force.
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