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Longer and stronger; stiff but not brittle Hydrogels are highly water-swollen, cross-linked polymers. Although they can be highly deformed, they tend to be weak, and methods to strengthen or toughen them tend to reduce stretchability. Two papers now report strategies to create tough but deformable hydrogels (see the Perspective by Bosnjak and Silberstein). Wang et al . introduced a toughening mechanism by storing releasable extra chain length in the stiff part of a double-network hydrogel. A high applied force triggered the opening of cycling strands that were only activated at high chain extension. Kim et al . synthesized acrylamide gels in which dense entanglements could be achieved by using unusually low amounts of water, cross-linker, and initiator during the synthesis. This approach improves the mechanical strength in solid form while also improving the wear resistance once swollen as a hydrogel. —MSL

We have recently reported a series of ladder-type cyclobutane mechanophores, polymers of which can transform from nonconjugated structures to conjugated structures and change many properties at once. These multicyclic mechanophores, namely, exo-ladderane/ene, endo-benzoladderene, and exo-bicyclohexene-peri-naphthalene, have different ring structures fused to the first cyclobutane, significantly different free energy changes for ring-opening, and different stereochemistry. To better understand their mechanochemistry, we used single molecule force spectroscopy (SMFS) to characterize their force-extension behavior and measure the threshold forces. The threshold forces correlate with the activation energy of the first bond, but not with the strain of the fused rings distal to the polymer main chain, suggesting that the activation of these ladder-type mechanophores occurs with similar early transition states, which is supported by force-modified potential energy surface calculations. We further determined the stereochemistry of the mechanically generated dienes and observed significant and variable contour length elongation for these mechanophores both experimentally and computationally. The fundamental understanding of ladder-type mechanophores will facilitate future design of multicyclic mechanophores with amplified force-response and their applications as mechanically responsive materials.

Mechanophores can be used to produce strain-dependent covalent chemical responses in polymeric materials, including stress strengthening, stress sensing and network remodelling. In general, it is desirable for mechanophores to be inert in the absence of force but highly reactive under applied tension. Metallocenes possess potentially useful combinations of force-free stability and force-coupled reactivity, but the mechanistic basis of this reactivity remains largely unexplored. Here, we have used single-molecule force spectroscopy to show that the mechanical reactivities of a series of ferrocenophanes are not correlated with ring strain in the reactants, but with the extent of rotational alignment of their two cyclopentadienyl ligands. Distal attachments can be used to restrict the mechanism of ferrocene dissociation to proceed through ligand ‘peeling’, as opposed to the more conventional ’shearing’ mechanism of the parent ferrocene, leading the dissociation rate constant to increase by several orders of magnitude at forces of ~1 nN. It also leads to improved macroscopic, multi-responsive behaviour, including mechanochromism and force-induced cross-linking in ferrocenophane-containing polymers. Metallocenes are attractive mechanophores because they are stable in the absence of force, yet reactive under tension. Now, covalently bridging the two cyclopentadienyl (Cp) ligands of ferrocenes embedded in a polymer has been shown to alter their mechanochemical reactivity, leading to a faster dissociation of the Fe–Cp bond, which occurs through a peeling mechanism rather than a shearing one.

The mechanical stresses that materials experience during use can lead to aging and failure. Recent developments in covalent mechanochemistry have provided a mechanism by which those stresses can be channeled into constructive, rather than destructive, responses, including strengthening in materials. Here, the synthesis and mechanical response of a polymer containing multiple benzocyclobutene (BCB) mechanophores along its backbone are reported. When solutions of the BCB polymer were exposed to the normally destructive elongational flow forces generated by pulsed ultrasonication, the number of intermolecular bond-forming reactions was greater than the number of bond-breaking reactions, leading to a net increase in polymer molecular weight. The molecular weight increase could be turned into gelation by introducing a bismaleimide cross-linker that reacts with the ortho-quinodimethide intermediate generated by mechanically assisted ring opening of the BCB mechanophores and using polymer concentrations in exces...

Covalent mechanochemistry within bulk polymers typically occurs with irreversible deformation of the parent material. Here we show that embedding mechanophores into an elastomeric poly(dimethylsiloxane) (PDMS) network allows for covalent bond activation under macroscopically reversible deformations. Using the colorimetric mechanophore spiropyran, we show that bond activation can be repeated over multiple cycles of tensile elongation with full shape recovery. Further, localized compression can be used to pattern strain-induced chemistry. The platform enables the reversibility of a secondary strain-induced color change to be characterized. We also observe mechanical acceleration of a flex-activated retro-Diels–Alder reaction, allowing a chemical signal to be released in response to a fully reversible deformation.

The high shear forces generated during the pulsed ultrasound of dilute polymer solutions lead to large tensile forces that are focused near the center of the polymer chain, but quantitative experimental evidence regarding the force distribution is rare. Here, pulsed ultrasound of quantitatively geminal-dihalocyclopropanated (gDHC) polybutadiene provides insights into the distribution. Pulsed ultrasound leads to the mechanochemical ring-opening of the gDHC mechanophore to a 2,3-dihaloalkene. The alkene product is then degraded through ozonolysis to leave behind only those stretches of the polymer that have not experienced large enough forces to be activated. Microstructural and molecular weight analysis reveals that the activated and unactivated regions of the polymer are continuous, indicating a smooth and monotonic force distribution from the midchain peak toward the polymer ends. When coupled to chain scission, the net process constitutes the rapid, specific, and reagentless conversion of a single homop...

The use of mechanical forces to chemically transform polymers dates back decades. In recent years, the use of mechanochemistry to direct constructive transformations in polymers has resulted in a range of engineered molecular responses that span optical, mechanical, electronic and thermal properties. The chemistry that has been developed is now well positioned for use in materials science, polymer physics, mechanics and additive manufacturing. Here, we review the historical backdrop of polymer mechanochemistry, give an overview of the existing toolbox of mechanophores and associated theoretical methods, and speculate as to emerging opportunities in materials science for which current capabilities are seemingly well suited. Non-linear mechanical responses and internal, amplifying stimulus–response feedback loops, including those enabled by, or coupled to, microstructured metamaterial architectures, are seen as particularly promising. Polymer mechanochemistry converts mechanical forces in materials to chemical reactions through the response of functional groups known as mechanophores. This Review discusses the colorimetric, mechanical, chemical and electronic responses of mechanophores that may be useful in materials for strain sensing and strengthening, soft devices and additive manufacturing.

Photoresponsive materials that change in response to light have been studied for a range of applications. These materials are often metastable during irradiation, returning to their pre-irradiated state after removal of the light source. Herein, we report a polymer gel comprising poly(ethylene glycol) star polymers linked by Cu24 L24 metal-organic cages/polyhedra (MOCs) with coumarin ligands. In the presence of UV light, a photosensitizer, and a hydrogen donor, this "polyMOC" material can be reversibly switched between CuII , CuI , and Cu0 . The instability of the MOC junctions in the CuI and Cu0 states leads to network disassembly, forming CuI /Cu0 solutions, respectively, that are stable until re-oxidation to CuII and supramolecular gelation. This reversible disassembly of the polyMOC network can occur in the presence of a fixed covalent second network generated in situ by copper-catalyzed azide-alkyne cycloaddition (CuAAC), providing interpenetrating supramolecular and covalent networks.

Degradable polymers are desirable for the replacement of conventional organic polymers that persist in the environment, but they often suffer from the unintentional scission of the degradable functionalities on the polymer backbone, which diminishes polymer properties during storage and regular use. Herein, we report a strategy that combats unintended degradation in polymers by combining two common degradation stimuli-mechanical and acid triggers-in an "AND gate" fashion. A cyclobutane (CB) mechanophore is used as a mechanical gate to regulate an acid-sensitive ketal that has been widely employed in acid degradable polymers. This gated ketal is further incorporated into the polymer backbone. In the presence of an acid trigger alone, the pristine polymer retains its backbone integrity, and delivering high mechanical forces alone by ultrasonication degrades the polymer to an apparent limiting molecular weight of 28 kDa. The sequential treatment of ultrasonication followed by acid, however, leads to a further 11-fold decrease in molecular weight to 2.5 kDa. Experimental and computational evidence further indicate that the ungated ketal possesses mechanical strength that is commensurate with the conventional polymer backbones. Single molecule force spectroscopy (SMFS) reveals that the force necessary to activate the CB molecular gate on the time scale of 100 ms is approximately 2 nN.

The mechanical strength of individual polymer chains is believed to underlie a number of performance metrics in bulk materials, including adhesion and fracture toughness. Methods by which the intrinsic molecular strength of the constituents of a given polymeric material might be switched are therefore potentially useful both for applications in which triggered property changes are desirable, and as tests of molecular theories for bulk behaviors. Here we report that the sequential oxidation of sulfide containing polyesters (PE-S) to the corresponding sulfoxide (PE-SO) and then sulfone (PE-SO2) first weakens (sulfoxide), and then enhances (sulfone), the effective mechanical integrity of the polymer backbone; PE-S ∼ PE-SO2 > PE-SO. The relative mechanical strength as a function of oxidation state is revealed through the use of gem-dichlorocyclopropane nonscissile mechanophores as an internal standard, and the observed order agrees well with the reported bond dissociation energies of C–S bonds in each species and with the results of CoGEF modeling., The mechanical strength of individual polymer chains is believed to underlie a number of performance metrics in bulk materials, including adhesion and fracture toughness.

Mechanically coupled proton transduction offers potential for stress-responsive polymeric materials whose properties can be switched via acid-triggered coloration, polymerization/cross-linking, or degradation. The utility of currently available mechanoacids, however, is limited by modest force-free stability or a scissile response that caps mechanoacid generation at one proton per strained polymer chain. Here, we report a new mechanoacid based on 2-methoxy-substituted gem-dichlorocyclopropane (MeO-gDCC). Pulsed ultrasonication leads to the mechanochemical ring opening of the MeO-gDCC and the subsequent elimination of either HCl or MeCl, with ∼0.58 equiv of HCl released per mechanophore activation and ∼67 protons per chain scission event. Single-molecule force spectroscopy reveals that the methoxy substituent lowers the force required for rapid (kopen ∼102 s-1) ring opening to ca. 900 pN, vs 1300 pN required for the parent gDCC. The utility of the mechanoacid is demonstrated in silicone elastomers, where its mechanical activation leads to a strain-triggered color change prior to fracture of the elastomer. The postactivation kinetics of coloration are used to demonstrate a new concept in mechanochromism, namely, a spectroscopic indicator of not only whether and where a mechanical event has occurred but when it occurred.

Recent research on the mechanochemistry of metallocene mechanophores has shed light on the force-responsiveness of these thermally and chemically stable organometallic compounds. In this work, we report a combination of experimental and computational studies on the mechanochemistry of main-chain cobaltocenium-containing polymers. Ester derivatives of the cationic cobaltocenium, though isoelectronic to neutral ferrocene, are unstable in the nonmechanical control experimental conditions that were accommodated by their ferrocene analogs. Replacing the electron withdrawing C-ester linkages with electron-donating C-alkyls conferred the necessary stability and enabled the mechanochemistry of the cobaltocenium to be assessed. Despite their high bond dissociation energy, cobaltocenium mechanophores are found to be selective sites of main chain scission under sonomechanical activation. Computational CoGEF calculations suggest that the presence of a counterion to cobaltocenium plays a vital role by promoting a peeling mechanism of dissociation in conjunction with the initial slipping.

Recent successes in covalent polymer mechanochemistry might more easily be translated to stress-responsive bulk materials if easily scalable and minimalist mechanophores were identified. Epoxides r...

Cyclic polymers containing multiple gem-dichlorocyclopropane (gDCC) mechanophores along their backbone were prepared using ring expansion metathesis polymerization. The mechanochemistry of the cycl...

The encapsulation of guests in supramolecular capsules has long been used to trap reactive intermediates and enhance or reduce the kinetic stability of reactants, and alter the products of chemical reactions that occur within the capsule interior. In recent years, multiple studies have shown that variations of normal reactivity patterns can be induced by trapping reactants under tension, for example along a backbone of an overextended polymer chain, in a manner that is fundamentally very different from, but reminiscent of, encapsulation. Here, we describe the formation of a mechanochemically generated isocyante via a mechanical retro [2 + 2] cycloaddition of a 1,2-diazetidinone (DAO) mechanophore. A single DAO mechanophore is incorporated into the chain center of a poly(methyl acrylate) (PMA) backbone via single electron transfer-living radical polymerization (SET-LRP). Mechanical activation of the DAO via ultrasonic sonication leads to the formation of isocyanate and imine products, as supported by trapping experiments using 9-(methylaminomethyl)anthracene labelling and 1H NMR spectroscopy. Further, we examine the relative mechanical susceptibility of chain-centered DAO mechanophores through a variety of methods, and evaluate the advantage and disadvantage of each.

Polymer networks are complex systems consisting of molecular components. Whereas the properties of the individual components are typically well understood by most chemists, translating that chemical insight into polymer networks themselves is limited by the statistical and poorly defined nature of network structures. As a result, it is challenging, if not currently impossible, to extrapolate from the molecular behavior of components to the full range of performance and properties of the entire polymer network. Polymer networks therefore present an unrealized, important, and interdisciplinary opportunity to exert molecular-level, chemical control on material macroscopic properties. A barrier to sophisticated molecular approaches to polymer networks is that the techniques for characterizing the molecular structure of networks are often unfamiliar to many scientists. Here, we present a critical overview of the current characterization techniques available to understand the relation between the molecular properties and the resulting performance and behavior of polymer networks, in the absence of added fillers. We highlight the methods available to characterize the chemistry and molecular-level properties of individual polymer strands and junctions, the gelation process by which strands form networks, the structure of the resulting network, and the dynamics and mechanics of the final material. The purpose is not to serve as a detailed manual for conducting these measurements but rather to unify the underlying principles, point out remaining challenges, and provide a concise overview by which chemists can plan characterization strategies that suit their research objectives. Because polymer networks cannot often be sufficiently characterized with a single method, strategic combinations of multiple techniques are typically required for their molecular characterization.

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

Polymers are stochastic materials that represent distributions of different molecules. In general, to quantify the distribution, polymer researchers rely on a series of chemical characterizations that each reveal partial information on the distribution. However, in practice, the exact set of characterizations that are carried out, as well as how the characterization data are aggregated and reported, is largely nonstandard across the polymer community. This scenario makes polymer characterization data highly disparate, thereby significantly slowing down the development of polymer informatics. In this work, a proposal on how structural characterization data can be organized is presented. To ensure that the system can apply universally across the entire polymer community, the proposed schema, PolyDAT, is designed to embody a minimal congruent set of vocabulary that is common across different domains. Unlike most chemical schemas, where only data pertinent to the species of interest are included, PolyDAT deploys a multi-species reaction network construct, in which every characterization on relevant species is collected to provide the most comprehensive profile on the polymer species of interest. Instead of maintaining a comprehensive list of available characterization techniques, PolyDAT provides a handful of generic templates, which align closely with experimental conventions and cover most types of common characterization techniques. This allows flexibility for the development and inclusion of new measurement methods. By providing a standard format to digitalize data, PolyDAT serves not only as an extension to BigSMILES that provides the necessary quantitative information but also as a standard channel for researchers to share polymer characterization data.

Here, we report covalent polymer gels in which the macroscopic fracture “reaction” is controlled by mechanophores embedded within mechanically active network strands. We synthesized poly(ethylene glycol) (PEG) gels through the end-linking of azide-terminated tetra-arm PEG (Mn = 5 kDa) with bis-alkyne linkers. Networks were formed under identical conditions, except that the bis-alkyne was varied to include either a cis-diaryl (1) or cis-dialkyl (2) linked cyclobutane mechanophore that acts as a mechanochemical “weak link” through a force-coupled cycloreversion. A control network featuring a bis-alkyne without cyclobutane (3) was also synthesized. The networks show the same linear elasticity (G' = 23~24 kPa, 0.1 – 100 Hz) and equilibrium mass swelling ratios (Q = 10~11 in tetrahydrofuran), but they exhibit tearing energies that span a factor of 8 (3.4 J∙m-2, 10.5 J∙m-2, and 27.1 J∙m-2 for networks with 1, 2, and 3, respectively). The difference in fracture energy is well aligned with the force-coupled scission kinetics of the mechanophores observed in single-molecule force spectroscopy experiments, implicating local resonance stabilization of a diradical transition state in the cycloreversion of 1 as a key determinant of the relative ease with which its network is torn. The connection between macroscopic fracture and small molecule reaction mechanism suggests opportunities for molecular understanding and optimization of polymer network behavior.

The utility and lifetime of materials made from polymer networks, including hydrogels, depend on their capacity to stretch and resist tearing. In gels and elastomers, those mechanical properties are often limited by the covalent chemical structure of the polymer strands between cross-links, which is typically fixed during the material synthesis. Here, we report polymer networks in which the constituent strands lengthen through force-coupled reactions that are triggered as the strands reach their nominal breaking point. Reactive strand extensions of up to 40% lead to hydrogels that stretch 40-50% further than, and exhibit tear energies twice that of, networks made from analogous control strands. The enhancements are synergistic with those provided by double network architectures, and complement other existing toughening strategies.

The mechanical degradation of polymers is typically limited to a single chain scission per triggering chain stretching event, and the loss of stress transfer that results from the scission limits the extent of degradation that can be achieved. Here, we report that the mechanically triggered ring-opening of a [4.2.0]bicyclooctene (BCOE) mechanophore sets up a delayed, force-free cascade lactonization that results in chain scission. Delayed chain scission allows many eventual scission events to be initiated within a single polymer chain. Ultrasonication of a 120 kDa BCOE copolymer mechanically remodels the polymer backbone, and subsequent lactonization slowly (~days) degrades the molecular weight to 4.4 kDa, > 10× smaller than control polymers in which lactonization is blocked. The force-coupled kinetics of ring-opening are probed by single molecule force spectroscopy, and mechanical degradation in the bulk is demonstrated. Delayed scission offers a strategy to enhanced mechanical degradation and programmed obsolescence in structural polymeric materials., The mechanical degradation of polymers is typically limited to a single chain scission event and the loss of stress transfer during the scission process limits the extent of degradation achieved. Here, the authors report a mechanically triggered, delayed scission strategy that allows many eventual scission events to be initiated within a single polymer chain.

Molecular force probes that generate optical responses to critical levels of mechanical stress (mechanochromophores) are increasingly attractive tools for identifying molecular sites that are most prone to failure. Here, a coumarin dimer mechanophore whose mechanical strength is comparable to that of the sulfur-sulfur bonds found in vulcanized rubbers is reported. It is further shown that the strain-induced scission of the coumarin dimer within the matrix of a particle-reinforced polybutadiene-based co-polymer can be detected and quantified by fluorescence spectroscopy, when cylinders of the nanocomposite are subjected to unconstrained uniaxial stress. The extent of the scission suggests that the coumarin dimers are molecular "weak links" within the matrix, and, by analogy, sulfur bridges are likely to be the same in vulcanized rubbers. The mechanophore is embedded in polymer main chains, grafting agent, and cross-linker positions in a polymer composite in order to generate experimental data to understand how macroscopic mechanical stress is transferred at the molecular scale especially in highly entangled cross-linked polymer nanocomposite. Finally, the extent of activation is enhanced by approximately an order of magnitude by changing the regiochemistry and stereochemistry of the coumarin dimer and embedding the mechanophore at the heterointerface of the particle-reinforced elastomer.

We report the preparation of cyclic ferrocenyl olefins with various substituents and different ring sizes by ring-closing metathesis (RCM). These ferrocene-containing monomers were subject to ring-opening metathesis polymerization (ROMP), leading to main-chain ferrocene-containing homopolymers, random copolymers, and block copolymers. Depending on the substituents, ferrocenyl homopolymers are semicrystalline or amorphous with good solubility. A semicrystalline polymer was used in the crystallization-driven self-assembly (CDSA) of block copolymers to generate platelet nanostructures.

Ferrocene is classically regarded as being highly inert owing to the large dissociation energy of metal-cyclopentadienyl (Cp) bonds. 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-derived polymers. Quantitative studies reveal that the Fe-Cp bond is similar in strength to the carbon-nitrogen bond of an azobisdialkylnitrile (bond dissociation energy < −0 kcal/mol), despite the significantly higher Fe-Cp bond dissociation energy (approximately 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 formation of permanent structures upon mild red laser illumination in transparent polydiene solutions is examined in the case of gem-dichlorocyclopropanated polybutadiene ( gDCC-PB) polymers bearing 15% functional units of the dichlorocyclopropane groups. The response was found to be distinct from the precursor PB. Whereas fiber-like patterns were clearly observed in both precursor and gDCC-PB solutions in cyclohexane, these were absent in the case of gDCC-PB/chloroform but were present in the precursor PB/chloroform solutions. The involved mechanical stresses were not sufficient for the gDCC activation to be detected by NMR spectroscopy. Remarkably, addition of even 10 wt % gDCC-PB into the latter solution sufficed to suppress the light-induced patterning. The importance of the chemical environment on the response to light irradiation was further checked and confirmed by use of other PB copolymers. Different diameter patterns and kinetics were observed. The strong solvent and comonomer mediated effect was reflected neither in solvency nor in optical polarizability differences of the polymers solvent couples.

The development of multi-mechanophore polymers (MMPs) has empowered new methodologies for observing, quantifying, and exploiting mechanochemical transformations. For example, techniques such as single molecule force spectroscopy and pulsed ultrasound can be used to induce and observe up to hundreds of chemical reactions within a single polymer, enabling mechanistic insights into mechanochemical reactivity. At the same time, MMPs allow for the substantial mechanochemical remodeling of polymers and associated changes in material properties. This minireview presents synthetic approaches that have been used to make MMPs, methods that have been developed to probe and characterize their reactivity, and changes in properties that have been observed through their mechanochemical response.

An easy-to-implement science outreach demonstration featuring a mechanically and photochemically color-changing polymer is described. The active polymeric material is a filled poly(dimethylsiloxane) (PDMS) elastomer that is covalently functionalized with spiropyran (SP), which is both a photochemical and mechanochemical switch. The material can be reversibly changed from colorless to dark purple by exposing it to light from a blue laser pointer or providing a mechanical stimulus such as hitting the polymer with a hammer or dragging a blunt object across the surface. The keynote demonstration is a PDMS chemical-drawing board that allows children literally to “write without ink” using a laser pointer or a blunt stylus. Collectively, these demonstrations are suitable for various student groups and encompass concepts in polymer and materials chemistry, photochemistry, and mechanochemistry. This demonstration has been successfully employed dozens of times in multiple universities across North America.

Biological tissues are multiresponsive and functional, and similar properties might be possible in synthetic systems by merging responsive polymers with hierarchical soft architectures. For example, mechanochromic polymers have applications in force-responsive colorimetric sensors and soft robotics, but their integration into sensitive, multifunctional devices remains challenging. Herein, a hierarchical nanoparticle-in-micropore (NP-MP) architecture in porous mechanochromic polymers, which enhances the mechanosensitivity and stretchability of mechanochromic electronic skins (e-skins), is reported. The hierarchical NP-MP structure results in stress-concentration-induced mechanochemical activation of mechanophores, significantly improving the mechanochromic sensitivity to both tensile strain and normal force (critical tensile strain: 50% and normal force: 1 N). Furthermore, the porous mechanochromic composites exhibit a reversible mechanochromism under a strain of 250%. This architecture enables a dual-mode mechanochromic e-skin for detecting static/dynamic forces via mechanochromism and triboelectricity. The hierarchical NP-MP architecture provides a general platform to develop mechanochromic composites with high sensitivity and stretchability.

Topological molecular connections and structures, including physical entanglements in polymer networks, knots along polymer chains, and rotaxanes in sliding ring gels, have important consequences for the physical properties of polymeric materials. Often these topologies contribute through their ability to bear mechanical stress, but experimental measures of their relative mechanical strength are rare. Here, we use sonochemical polymer mechanochemistry to assess the relative mechanical strength of a multicatenane copolymer relative to copolymers of cyclic and linear analogs. The relative mechanical strengths are obtained by comparing the limiting molecular weights (Mlim) and contour lengths (Llim) of the polymers under pulsed ultrasound of their dilute solutions. The values of Mlim and Llim, and thus the inferred mechanical strengths of the polymers, are effectively identical. The mechanical bonds of the catenanes are therefore as strong, or stronger, mechanically as the covalent bonds along the polymer backbone.

Single-molecule force spectroscopy (SMFS) of multi-mechanophore polymers has been used to provide kinetic and mechanistic insights into mechanochemical reactions. Whereas biological systems have benefitted from force clamp spectroscopy, synthetic polymers have been studied primarily with constant-velocity methods. Here, force clamp SMFS is applied to the mechanically accelerated conrotatory ring opening of benzocyclobutene, and a comparison with constant-velocity SMFS extends the range of available rate-versus-force data and corroborates the use of constant-velocity SMFS to extract force dependencies.

Polymer bottlebrushes may be viewed as hairy flexible cylinders with a long backbone and a thick corona of densely grafted polymer chains. The corona not only controls the bottlebrush diameter, but also provides an additional drag force. We report the study of backbone scission by external forces caused by ultrasonication. A series of bottlebrushes with the same backbone and different side-chain lengths was prepared by ATRP. Bond fracture was induced by pulsed ultrasound in a dilute chloroform solution and ex-situ monitored through molecular imaging of reaction products by atomic force microscopy. The scission rate was found to increase with side chain length, while the limiting length of fractured bottlebrushes displayed a decrease. The experiment showed a good agreement with the Rouse model of polymer dynamics, which suggests that solvent drains through the corona of bottlebrush side-chains.

The functions of soft robotics are intimately tied to their form-channels and voids defined by an elastomeric superstructure that reversibly stores and releases mechanical energy to change shape, grip objects, and achieve complex motions. Here, we demonstrate that covalent polymer mechanochemistry provides a viable mechanism to convert the same mechanical potential energy used for actuation in soft robots into a mechanochromic, covalent chemical response. A bis-alkene functionalized spiropyran (SP) mechanophore is cured into a molded poly(dimethylsiloxane) (PDMS) soft robot walker and gripper. The stresses and strains necessary for SP activation are compatible with soft robot function. The color change associated with actuation suggests opportunities for not only new color changing or camouflaging strategies, but also the possibility for simultaneous activation of latent chemistry (e.g., release of small molecules, change in mechanical properties, activation of catalysts, etc.) in soft robots. In addition, mechanochromic stress mapping in a functional robotic device might provide a useful design and optimization tool, revealing spatial and temporal force evolution within the robot in a way that might be coupled to autonomous feedback loops that allow the robot to regulate its own activity. The demonstration motivates the simultaneous development of new combinations of mechanophores, materials, and soft, active devices for enhanced functionality.

Structure–activity relationships in the mechanochemistry of gem-dichlorocyclopropane (gDCC)-based polymer solutions triggered by pulsed ultrasound are reported. Insights into the flow-induced mechanochemical transformations of gDCC mechanophores into the corresponding 2,3-dichloroalkenes are obtained by monitoring the mechanochemistry as a function of initial polymer molecular weight and sonication conditions. The competition between gDCC activation and polymer chain scission is invariant to sonication power, temperature, polymer concentration, and solvent but is sensitive to initial polymer molecular weight. The results have practical implications for the use of polymer sonochemistry as a tool for quantifying the relative mechanical strength of scissile polymers and conceptual implications for thinking about the nature of the force distributions experienced during sonochemical experiments.

Mechanochemically responsive (MCR) polymers have been synthesized by incorporating mechanophores – molecules whose chemical reactions are triggered by mechanical force – into conventional polymer networks. Deformation of the MCR polymers applies force on the mechanophores and triggers their reactions, which manifest as phenomena such as changing colors, varying fluorescence and releasing molecules. While the activation of most existing MCR polymers requires irreversible plastic deformation or fracture of the polymers, we covalently coupled mechanophores into the backbone chains of elastomer networks, achieving MCR elastomers that can be repeatedly activated over multiple cycles of large and reversible deformations. This paper reports a microphysical model of MCR elastomers, which quantitatively captures the interplay between the macroscopic deformation of the MCR elastomers and the reversible activation of mechanophores on polymer chains with non-uniform lengths. Our model consistently predicts both the stress–strain behaviors and the color or fluorescence variation of the MCR elastomers under large deformations. We quantitatively explain that MCR elastomers with time-independent stress–strain behaviors can give time-dependent variation of color or fluorescence due to the kinetics of mechanophore activation and that MCR elastomers with different chain-length distributions can exhibit similar stress–strain behaviors but very different colors or fluorescence. Implementing the model into ABAQUS subroutine further demonstrates our model's capability in guiding the design of MCR elastomeric devices for applications such as large-strain imaging and color and fluorescence displays.

Purpose– This paper aims to explore and demonstrate the ability to integrate entry-level additive manufacturing (AM) techniques with responsive polymers capable of mechanical to chemical energy transduction. This integration signifies the merger of AM and smart materials.Design/methodology/approach– Custom filaments were synthesized comprising covalently incorporated spiropyran moieties. The mechanical activation and chemical response of the spiropyran-containing filaments were demonstrated in materials that were produced via fused filament fabrication techniques.Findings– Custom filaments were successfully produced and printed with complete preservation of the mechanochemical reactivity of the spiropyran units. These smart materials were demonstrated in two key constructs: a center-cracked test specimen and a mechanochromic force sensor. The mechanochromic nature of the filament enables (semi)quantitative assessment of peak loads based on color change, without requiring any external analytical techniques.Originality/value– This paper describes the first examples of three-dimensional-printed mechanophores, which may be of significant interest to the AM community. The ability to control the chemical response to external mechanical forces, in combination with AM to process the bulk materials, potentiates customizability at the molecular and macroscopic length scales.

Here we present a coumarin dimer (CD) mechanophore that, when embedded near the mid-chain of poly(methyl acrylate) polymers, activates under pulsed ultrasound conditions to yield coumarin chain-end functional polymers. Quantitative photochemical scission of the CD polymers provides a reference against which the activation efficiency of chain-centered mechanophores in polymers synthesized by controlled/living radical polymerization (CRP) can be assessed. Activation efficiency is characterized with respect to the polymer molecular weight (MW), polydispersity index (PDI), and distribution of mechanophores along the backbone.

Molecular mechanisms by which to increase the activity of a mechanophore might provide access to new chemical reactions and enhanced stress-responsive behavior in mechanochemically active polymeric materials. Here, single-molecule force spectroscopy reveals that the force-induced acceleration of the electrocyclic ring opening of gem-dichlorocyclopropanes (gDCC) is sensitive to the stereochemistry of an α-alkene substituent on the gDCC. On the ∼0.1 s time scale of the experiment, the force required to open the E-alkene-substituted gDCC was found to be 0.4 nN lower than that required in the corresponding Z-alkene isomer, despite the effectively identical force-free reactivities of the two isomers and the distance between the stereochemical permutation and the scissile bond of the mechanophore. Fitting the experimental data with a cusp model provides force-free activation lengths of 1.67 ± 0.05 and 1.20 ± 0.05 Å for the E and Z isomers, respectively, as compared to 1.65 and 1.24 Å derived from computational modeling.

Hydrogels and organogels made from polymer networks are widely used in biomedical applications and soft, active devices for which the ability to sustain large deformations is required. The strain at which polymer networks fracture is typically improved through the addition of elements that dissipate energy, but these materials require extra work to achieve a given, desired level of deformation. Here, the addition of mechanically "invisible" supramolecular crosslinks causes substantial increases in the ultimate gel properties without incurring the added energetic costs of dissipation.

Correction for ‘Empowering mechanochemistry with multi-mechanophore polymer architectures’ by Brandon H. Bowser et al., Polym. Chem., 2018, 9, 3583–3593.

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.

Protein-based biomaterials are an important class of materials for applications in biotechnology and medicine. The exquisite control of their composition, stereochemistry, and chain length offers unique opportunities to engineer biofunctionality, biocompatibility, and biodegradability into these materials. Here, we report the synthesis of a thermally responsive peptide polymer-based hydrogel composed of a recombinant elastin-like polypeptide (ELP) that rapidly forms a reversibly cross-linked hydrogel by the formation of intermolecular disulfide cross-links. To do so, we designed and synthesized ELPs that incorporate periodic cysteine residues (cELPs), and show that cELPs are thermally responsive protein polymers that display rapid gelation under physiologically relevant, mild oxidative conditions. Gelation of cELPs, at concentrations as low as 2.5 wt%, occurs in ≈ 2.5 min upon addition a low concentration of hydrogen peroxide (0.3 wt%). We show the utility of these hydrogels for the sustained release of a model protein in vitro, and demonstrate the ability of this injectable biomaterial to pervade tumors to maximize tumor coverage and retention time upon intratumoral injection. cELPs represent a new class of injectable reversibly cross-linked hydrogels with properties intermediate between ELP coacervates and chemically cross-linked ELP hydrogels that will find useful applications in drug delivery and tissue engineering.

Epoxidized polybutadiene and epoxidized polynorbornene were subjected to pulsed ultrasound in the presence of small molecules capable of being trapped by carbonyl ylides. When epoxidized polybutadiene was sonicated, there was no observable small molecule addition to the polymer. Concurrently, no appreciable isomerization (cis to trans epoxide) was observed, indicating that the epoxide rings along the backbone are not mechanically active under the experimental conditions employed. In contrast, when epoxidized polynorbornene was subjected to the same conditions, both addition of ylide trapping reagents and net isomerization of cis to trans epoxide were observed. The results demonstrate the mechanical activity of epoxides, show that mechanophore activity is determined not only by the functional group but also the polymer backbone in which it is embedded, and facilitate a characterization of the reactivity of the ring-opened dialkyl epoxide.

In the past five years, the covalent mechanochemistry of polymeric1 and small molecule2 systems has generated a great deal of interest, in terms of both material science and synthetic chemistry. Most studies to date have focused on systems with a single mechanophore per polymer chain,3–7 an approach that has enabled the discovery of new mechanically induced chemistry and tools for stress monitoring. Single molecule architectures are likely to be limited, however, for applications in stress-responsive mechanical properties or scalable stoichiometric reactivity, due to low mechanophore concentration.8 gem-Dihalocyclopropane mechanophores have demonstrated a rich array of mechanochemical activity,9–12 and their post polymerization addition to polybutadiene based polymers has yielded the highest mechanophore content13 and highest single-chain toughness14 demonstrated in a synthetic polymer to date. While simple to synthesize, olefin containing polymers suffer from inherent instability due to ambient light, heat, and oxygen, often causing uncontrolled crosslinking. Furthermore, it is difficult to create complex architectures based on the post polymerization modification of polybutadiene. In considering more robust and useful synthetic approaches, we start from the desire to engineer non-scissile mechanophores, which typically means that ring systems must be embedded along the polymer backbone.15 Backbone rings cannot easily be introduced by the traditional polymerization of vinyl monomers, and although ring-opening metathesis polymerization (ROMP) has been used,11 the synthesis of complex fused ring monomers can be cumbersome. Furthermore, the ROMP methodology produces olefin-containing backbones whose stability is limited by the factors discussed above. While a rich array of functional groups can easily be incorporated into condensation polymers, mild and controllable polycondensation procedures typically generate polymers of low molecular weight (MW). This limits both bulk material utility and mechanochemical activation by pulsed ultrasound, where high molecular weights (> 40 kDa) are typically required to experience sufficient shear forces along the polymer backbone. Here, we report that the synthesis of an ABA triblock copolymer harvests the advantages of polycondensation and radical polymerization, giving access to stable, mechanophore-rich polymers of desirable molecular weight.

In the past five years, the field of covalent polymer mechanochemistry has experienced a renaissance. Once limited to the simple scission of polymer chains, mechanical force can now be used to produce a wide array of productive chemistry. These outcomes have both challenged and supported classically held views of chemical reactivity. The impact of these findings has relevance in both synthetic chemistry and material science. Here, we review our efforts to exploit mechanochemical coupling to produce constructive and stress-responsive covalent chemistry in polymer materials.

The use of mechanical force to break and build chemical bonds in polymers can enable transformations that cannot be conducted using stimuli such as light and heat. Now, an insulating polymer has been mechanically unzipped to create a semiconducting polymer with extended regions of conjugation.

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.

Main-chain reversible coordination polymers were formed from bis-platinum pincer complexes 2 and bis-pyridine-terminated poly-THF-bis-Py 1. The properties of the polymers, both in solution and in fibres drawn from solution, are characterised. Structural perturbations that influence the ligand exchange (dissociation) rate of the defining metal–ligand bond have no measurable impact on either the viscosity of the polymer solutions or the ultimate yield behaviour of the fibres.