Stephen L. Craig

From molecular mechanochemistry to stress-responsive materials

Current activity in, and future prospects for, the incorporation of mechanochemically active functional groups (“mechanophores”) into polymers is reviewed. This area of research is treated in the context of two categories. The first category is the development of new chemistry in the service of material science, through the design and synthesis of mechanophores to provide stress-sensing and/or stress-responsive elements in materials. The second category is the reverse—the development of new material architectures that efficiently transmit macroscopic forces to targeted molecules in order to generate chemical reactivity that is inaccessible by other means.

Trapping a Diradical Transition State by Mechanochemical Polymer Extension

Forced Open Traditionally, the study of reaction chemistry has relied on random encounters between molecules to initiate the proceedings. Heating and stirring increase the power and frequency of such encounters but provide little finer control. Very recently, chemists have learned how to initiate reactions more directly by embedding precursors in the backbone of a polymer large enough to manipulate with shear forces. Lenhardt et al. (p. 1057) applied this technique to a cyclopropyl ring-opening reaction. When the strained triangular carbon rings were embedded within a polymer, shear force applied by sonication ruptured their bonds as the polymer backbone stretched. The taut polymer then redirected the ring-opened intermediates toward a product differently arranged from that generated by simple heating. Strained carbon rings in the backbone of a polymer can be opened by external application of shear forces. Transition state structures are central to the rates and outcomes of chemical reactions, but their fleeting existence often leaves their properties to be inferred rather than observed. By treating polybutadiene with a difluorocarbene source, we embedded gem-difluorocyclopropanes (gDFCs) along the polymer backbone. We report that mechanochemical activation of the polymer under tension opens the gDFCs and traps a 1,3-diradical that is formally a transition state in their stress-free electrocyclic isomerization. The trapped diradical lives long enough that we can observe its noncanonical participation in bimolecular addition reactions. Furthermore, the application of a transient tensile force induces a net isomerization of the trans-gDFC into its less-stable cis isomer, leading to the counterintuitive result that the gDFC contracts in response to a transient force of extension.

gem-Dichlorocyclopropanes as Abundant and Efficient Mechanophores in Polybutadiene Copolymers under Mechanical Stress

When gem-dichlorocyclopropane (gDCC) copolymers derived from polybutadiene are subjected to ultrasonication, the gDCCs undergo ring opening to form 2,3-dichloroalkenes. The reactivity is not observed in low-molecular-weight (6.5 kDa) copolymers or side-chain gDCCs, consistent with mechanically induced reactivity due to the elongational strain of the polymers in the sonication flow fields. The ring openings occur several hundred times more frequently than polymer chain scission, and cis-coupled gDCCs are slightly more likely to react than their trans isomers. The ability to dramatically and specifically alter the structure of the polymer backbone through a coupled restoring force suggests new routes to postsynthetic polymer modification and motivates the design of easily scalable mechanophores for applications in stress-responsive polymers.

A hybrid polymer gel with controlled rates of cross-link rupture and self-repair

A family of hybrid polymer gels is described, in which covalent cross-links create a permanent, stiff scaffold onto which reversible metal–ligand coordinative cross-links are added. The reversible metal–ligand interactions are shown to bear mechanical stress within the hybrid gel, and relaxations in response to that applied stress are consistent with the stress-free kinetics of ligand exchange in systems that model the reversible cross-links. The stress-induced dissociation of a model metal–ligand complex is examined by a single-molecule force spectroscopy, and its mechanical response is compared with a previously studied complex. The mechanical response of the individual interactions is relevant to those found in the family of hybrid gels, and the modular platform is therefore suitable for the study of stress-induced molecular dissociations, and their subsequent repair, within a macroscopic material of fixed structure.

Mechanochemical strengthening of a synthetic polymer in response to typically destructive shear forces

High shear stresses are known to trigger destructive bond-scission reactions in polymers. Recent work has shown that the same shear forces can be used to accelerate non-destructive reactions in mechanophores along polymer backbones, and it is demonstrated here that such mechanochemical reactions can be used to strengthen a polymer subjected to otherwise destructive shear forces. Polybutadiene was functionalized with dibromocyclopropane mechanophores, whose mechanical activation generates allylic bromides that are crosslinked in situ by nucleophilic substitution reactions with carboxylates. The crosslinking is activated efficiently by shear forces both in solvated systems and in bulk materials, and the resulting covalent polymer networks possess moduli that are orders-of-magnitude greater than those of the unactivated polymers. These molecular-level responses and their impact on polymer properties have implications for the design of materials that, like biological materials, actively remodel locally as a function of their physical environment.

Mechanically induced scission and subsequent thermal remending of perfluorocyclobutane polymers.

Perfluorocyclobutane (PFCB) polymer solutions were subjected to pulsed ultrasound, leading to mechanically induced chain scission and molecular weight degradation. (19)F NMR revealed that the new, mechanically generated end groups are trifluorovinyl ethers formed by cycloreversion of the PFCB groups, a process that differs from thermal degradation pathways. One consequence of the mechanochemical process is that the trifluorovinyl ether end groups can be remended simply by subjecting the polymer solution to the original polymerization conditions, that is, heating to >150 °C. Stereochemical changes in the PFCBs, in combination with radical trapping experiments, indicate that PFCB scission proceeds via a stepwise mechanism with a 1,4-diradical intermediate, offering a potential mechanism for localized functionalization and cross-linking in regions of high stress.

Anion Binding of Short, Flexible Aryl Triazole Oligomers

The flexible, electropositive cavity of linear 1,4-diaryl-1,2,3-triazole oligomers provides a suitable host for complexation of various anions. The binding affinities for various combinations of oligomer and anion were determined by (1)H NMR titrations. Effective ionic radius is found to be a primary determinant of the relative binding interactions of various guests, with small but measurable deviations in the case of nonspherical anions. Solvent effects are significant, and the strength of the binding interaction is found to depend directly on the donor ability of the solvent. A picture emerges in which anion binding can be effectively interpreted in terms of a competition between two solvation spheres: one provided by the solvent and a second dominated by a folded cavity lined with electropositive 1,2,3-triazole CH protons. Implications for rigid macrocycles and other multivalent hosts are discussed.

Cephalopod-inspired design of electro-mechano-chemically responsive elastomers for on-demand fluorescent patterning

Cephalopods can display dazzling patterns of colours by selectively contracting muscles to reversibly activate chromatophores--pigment-containing cells under their skins. Inspired by this novel colouring strategy found in nature, we design an electro-mechano-chemically responsive elastomer system that can exhibit a wide variety of fluorescent patterns under the control of electric fields. We covalently couple a stretchable elastomer with mechanochromic molecules, which emit strong fluorescent signals if sufficiently deformed. We then use electric fields to induce various patterns of large deformation on the elastomer surface, which displays versatile fluorescent patterns including lines, circles and letters on demand. Theoretical models are further constructed to predict the electrically induced fluorescent patterns and to guide the design of this class of elastomers and devices. The material and method open promising avenues for creating flexible devices in soft/wet environments that combine deformation, colorimetric and fluorescent response with topological and chemical changes in response to a single remote signal.

A backbone lever-arm effect enhances polymer mechanochemistry

Mechanical forces along a polymer backbone can be used to bring about remarkable reactivity in embedded mechanically active functional groups, but little attention has been paid to how a given polymer backbone delivers that force to the reactant. Here, single-molecule force spectroscopy was used to directly quantify and compare the forces associated with the ring opening of gem-dibromo and gem-dichlorocyclopropanes affixed along the backbone of cis-polynorbornene and cis-polybutadiene. The critical force for isomerization drops by about one-third in the polynorbornene scaffold relative to polybutadiene. The root of the effect lies in more efficient chemomechanical coupling through the polynorbornene backbone, which acts as a phenomenological lever with greater mechanical advantage than polybutadiene. The experimental results are supported computationally and provide the foundation for a new strategy by which to engineer mechanochemical reactivity.

Molecular Stress Relief through a Force-Induced Irreversible Extension in Polymer Contour Length

Single-molecule force spectroscopy is used to observe the irreversible extension of a gem-dibromocyclopropane (gDBC)-functionalized polybutadiene under tension, a process akin to polymer necking at a single-molecule level. The extension of close to 28% in the contour length of the polymer backbone occurs at roughly 1.2 nN (tip velocity of 3 μm/s) and is attributed to the force-induced isomerization of the gDBCs into 2,3-dibromoalkenes. The rearrangement represents a possible new mechanism for localized stress relief in polymers and polymer networks under load, and the quantification of the force dependency provides a benchmark value for further studies of mechanically triggered chemistry in bulk polymers.