Browsing by Subject "Mechanochemistry"
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Item Open Access A Model Elastomer with Modular Metal-Ligand Crosslinking(2022) Johnson, Patricia NicoleMetallosupramolecular polymers are increasingly of interest for functional and degradable polymeric materials. In these materials, the metal-ligand bonds often bear an external mechanical load, but little is yet understood about the nature of mechanically-triggered reactions of metal-ligand bonds and how that reactivity influences the mechanical limits of the material. This dissertation presents a poly(cyclooctene) polymer bearing 2,6-bis(1′-methyl-benzimidazolyl)pyridine (Mebip) ligands on sidechains, which provides easy incorporation into polymer backbones and sidechains, binding to a large variety of metal species, and facile synthesis with sites for future study substituent effects. This platform is employed in proof-of-concept studies comparing the crosslinking behavior of iron(II) trifluoromethanesulfonate and copper(II) trifluoromethanesulfonate. It was found through small molecule spectroscopic studies that both metal species bind in the desired 2:1 MeBip:metal stoichiometry for crosslinking. When these small molecule complexes are polymerized as crosslinkers in gel and solid networks, though the extent of crosslinking is found to be similar, the copper(II)-crosslinked networks exhibited a faster relaxation than the iron(II)-crosslinked networks. Further, under high strains, the copper(II)-crosslinked networks exhibited significantly higher extensibility. This work lays the foundation for further investigations of the effect of metal-ligand bonding on force-coupled properties of materials.
Item Open Access Characterization and Applications of Force-induced Reactions(2015) Wang, JunpengJust as heat, light and electricity do, mechanical forces can also stimulate reactions. Conventionally, these processes - known as mechanochemistry - were viewed as comprising only destructive events, such as bond scission and material failure. Recently, Moore and coworkers demonstrated that the incorporation of mechanophores, i.e., mechanochemically active moieties, can bring new types of chemistry. This demonstration has inspired a series of fruitful works, at both the molecular and material levels, in both theoretical and experimental aspects, for both fundamental research and applications. This dissertation evaluates mechanochemical behavior in all of these contexts.
At the level of fundamental reactivity, forbidden reactions, such as those that violate orbital symmetry effects as captured in the Woodward-Hoffman rules, remain an ongoing challenge for experimental characterization, because when the competing allowed pathway is available, the reactions are intrinsically difficult to trigger. Recent developments in covalent mechanochemistry have opened the door to activating otherwise inaccessible reactions. This dissertation describes the first real-time observation and quantified measurement of four mechanically activated forbidden reactions. The results provide the experimental benchmarks for mechanically induced forbidden reactions, including those that violate the Woodward-Hoffmann and Woodward-Hoffmann-DePuy rules, and in some cases suggest revisions to prior computational predictions. The single-molecule measurement also captured competing reactions between isomerization and bimolecular reaction, which to the best of our knowledge, is the first time that competing reactions are probed by force spectroscopy.
Most characterization for mechanochemistry has been focused on the reactivity of mechanophores, and investigations of the force coupling efficiency are much less reported. We discovered that the stereochemistry of a non-reactive alkene pendant to a reacting mechanophore has a dramatic effect on the magnitude of the force required to trigger reactivity on a given timescale (here, a 400 pN difference for reactivity on the timescale of 100 ms). The stereochemical perturbation has essentially no measurable effect on the force-free reactivity, providing an almost perfectly orthogonal handle for tuning mechanochemical reactivity independently of intrinsic reactivity.
Mechanochemical coupling is also applied here to the study of reaction dynamics. The dynamics of reactions at or in the immediate vicinity of transition states are critical to reaction rates and product distributions, but direct experimental probes of those dynamics are rare. The s-trans, s-trans 1,3-diradicaloid transition states are trapped by tension along the backbone of purely cis-substituted gem-difluorocyclopropanated polybutadiene using the extensional forces generated by pulsed sonication of dilute polymer solutions. Once released, the branching ratio between symmetry-allowed disrotatory ring closing (of which the trapped diradicaloid structure is the transition state) and symmetry-forbidden conrotatory ring closing (whose transition state is nearby) can be inferred. Net conrotatory ring closing occurred in 5.0 ± 0.5% of the released transition states, as compared to 19 out of 400 such events in molecular dynamics simulations.
On the materials level, the inevitable stress in materials during usage causes bond breakage, materials aging and failure. A strategy for solving this problem is to learn from biological materials, which are capable to remodel and become stronger in response to the otherwise destructive forces. Benzocyclobutene has been demonstrated to mechanically active to ortho-quinodimethide, an intermediate capable for [4+4] dimerization and [4+2] cycloaddition. These features make it an excellent candidate for and synthesis of mechanochemical remodeling. A polymer containing hundreds of benzocyclobutene on the backbone was synthesized. When the polymer was exposed to otherwise destructive shear forces generated by pulsed ultrasound, its molecular weight increased as oppose to other mechanophore-containing polymers. When a solution of the polymer with bismaleimide was subjected to pulsed ultrasonication, crosslink occurred and the modulus increased by two orders of magnitude.
Item Open Access Expanding the Scope of Mechanically Active Polymers(2012) Klukovich, HopeThe addition of mechanically active functional groups (mechanophores) to polymer scaffolds has resulted in new chemical transformations and materials properties. The novel functions in these polymers are achieved in response to a universal input: mechanical force. This dissertation describes studies that expand our ability to elicit and modify chemical reactivity through the application of force (mechanochemistry), both through fundamental studies of mechanochemical coupling and through the synthesis and characterization of new mechanophores.
In order to probe mechanochemical coupling, single molecule force spectroscopy was used to directly quantify and compare the forces associated with the ring opening of gem-dibromo and gem-dichlorocyclopropanes (gDBCs and gDCCs) affixed along the backbone of cis-polynorbornene (PNB) and cis-polybutadiene (PB). At a tip velocity of 0.3 μ sec-1, the isomerization of gDBC-PNB, gDCC-PNB, gDBC-PB, and gDCC-PB to their respective 2,3-dihaloalkenes occurs at 740, 900, 1210 and 1330 pN, respectively. In contrast to their relative importance in determining the rates of the thermal gDHC ring openings, the polymer backbone has much greater impact on gDHC mechanochemistry than does the halogen. The root of the effect lies in more efficient chemomechanical coupling through the PNB backbone, which acts as a phenomenological lever with greater mechanical advantage than the PB backbone. The ability to affect the reactivity of a mechanophore by polymer backbone manipulation provides a previously underappreciated means to tailor mechanochemical response. The experimental results are supported computationally and provide the foundation for a new strategy by which to engineer mechanical reactivity.
The ability to increase the reactivity of mechanophores by changing their polymer scaffold can lead to the realization of mechanically-induced transformations that were otherwise inaccessible. To probe this increased mechanophore reactivity, 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.
Commercially available fluorinated polymers were also investigated as previously unrealized mechanophore-bearing polymers and as candidates for thermally re-mendable materials by examining their response to applied stress. Perfluorocyclobutane (PFCB) polymer solutions were subjected to pulsed ultrasound, leading to mechanically induced chain scission and molecular weight degradation. 19F 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 re-mended simply by subjecting the polymer solution to the original polymerization conditions, i.e., 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.
Item Open Access Mechanisms, Dynamics and Applications of Mechanically-Induced Reactions(2011) Lenhardt, Jeremy MichaelThe mechanical forces typical of daily life have the potential to induce dramatic reactivity at the molecular level. In the past few years, several studies have demonstrated that macroscopic mechanical forces can be harnessed at the molecular level, creating a new tool for the organic and materials chemist alike. These studies have created a new opportunity to develop novel, responsive materials by designing and synthesizing mechanically activated functional groups ("mechanophores") and incorporating them as stress-sensing and/or stress-responsive elements in materials.
The addition of dihalocarbenes to polybutadiene polymers forms polymeric materials that are highly susceptible to mechanochemical transformations. The mechanochemistry of the as formed gem-dihalocyclopropanated (gDHC) polymers is reported herein. The mechanochemical transformations of gDHC polymers are investigated (i) during activation in solution by the application of pulsed ultrasound, (ii) by single molecule force spectroscopy and (iii) in the solid state as a result of compressive stress.
Solution state mechanochemistry first observed during pulsed ultrasonication of gem-dichlorocyclopropanated (gDCC) polybutadiene polymers. The electrocyclic ring opening reactions of up to hundreds of gDCCs are observed on the timescale of molecular weight degradation from C-C bond scission. Mechanistic insights into the shear-induced mechanochemical transformations are obtained by monitoring the mechanochemistry as a function of gDHC halogen (dichloro-, dibromo-, bromochloro- and chlorofluoro-) and stereochemistry. The relative susceptibility of the anti-Woodward Hoffman and anti-Woodard-Hoffman-DePuy ring opening reactions through conventional pathways is explored, as is the relationship between mechanophore activity and initial polymer molecular weight.
The irreversible ring opening reaction of cis-gem-dibromocyclopropane is quantified as a function of mechanical restoring force through single molecule force spectroscopy experiments. The force-induced rearrangement proceeds at forces below covalent scission leads to a dramatic increase in the toughness of single polymer chains. Kinetic data are extracted from the force-induced rearrangment, the analysis of which reveals challenges in deconvoluting the proper reaction coordinate in force-induced reactions in polymers.
In the solid state, compressive stress is observed to induce the ring opening reactions of gDCC, gDBC and gem-bromochloro (gBCC) embedded polybutadiene polymers. Analysis of the 1H-NMR spectra following compressive activation of the materials allows the mechanoactive domains along single polymer chains to be characterized, with domain sizes on the order of only a few (3-5) monomers. The ring opening reactions of isomeric gBCCs are observed to proceed at different rates, providing the first quantitative study of selectivity in competing mechanochemical reactions in the solid state.
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. Additionally, the bimolecular reaction of adjacent, tension trapped 1,3-diradicals was monitored resulting in the first example of a reaction between two formal transition state species.
Item Open Access Mechanochemical Reaction Development for Addition, Elimination, and Isomerization(2022) Wang, LiqiIt is now well appreciated that coupled mechanical forces can influence the rates and outcomes of covalent chemical reactions in isolated polymers and in bulk polymeric materials. Mechanochemical reactions have been widely used in mechanocatalysis, release of small molecules and protons, biasing and probing reaction pathways, stress reporting, stress strengthening and degradable polymers. Mechanochemical reactions involve much more than simply breaking of bonds, and there exist rich opportunities for new reactions to be developed for a wide range of potential applications. In this dissertation, we report new mechanochemical reactions of three types: addition, elimination, and isomerization.Mechanical forces have been used previously to activate latent catalysts by accelerating dissociation of an inhibiting ligand, but tuning catalytic activity by force remains limited to a single demonstration of force-dependent enantioselectivity of Heck and Trost reactions. Chapter 2 describes how force affects the rate of oxidative addition, often the first step within a catalytic cycle. We study the effect of force applied to the biaryl backbone of a bisphosphine ligand on the rate of oxidative addition of bromobenzene to a ligand-coordinated palladium center. Local compressive and tensile forces on the order of 100 pN are generated using a stiff stilbene force probe. We find that a compressive force increases the rate of oxidative addition, whereas a tensile force decreases the rate, relative to that of the parent complex of strain-free ligand. Rates vary by a factor of ~6 across ~340 pN of force applied to the complexes. The applied forces exert an opposite effect on oxidative addition relative to that for reductive elimination, laying the groundwork for mechanically switchable catalysts that can be optimized for individual steps within a closed catalyst cycle. Chapter 3 demonstrates a new mechanochemical elimination reaction, namely the mechanical release of hydrogen fluoride, and its application to triggered polymer degradation. As a versatile reagent for material remodeling, hydrogen fluoride has applications in self-immolative polymers, remodeled siloxanes, and degradable polymers. The responsive, in situ generation of HF in materials therefore holds promise for new classes of adaptive material systems. We achieve the mechanochemically coupled generation of HF from 2-methoxy-gem-difluorocyclopropane (MeO-gDFC) mechanophores in polymers. Pulsed ultrasonication of a MeO-gDFC containing polymer leads to one equivalent of HF release per MeO-gDFC activation. We further quantify the mechanochemical reactivity of MeO-gDFC by single molecule force spectroscopy, and force-coupled rate constants for ring opening reach ~36 s-1 at a force of ~890 pN, 400 pN lower than is required in dialkyl gDFC mechanophores that lack the methoxy substituent. The SMFS and sonication results suggest that MeO-gDFC is a more efficient mechanophore source of HF than its 2-methoxy-gem-dichlorocyclopropane analog is of HCl, in contrast to expectations based on trends in force-free reactivity. We apply the mechanical release of HF to accelerate the degradation of a copolymer containing both MeO-gDFC (3 mol%) and an HF-cleavable silyl ether (25 mol%). The mechanochemical reaction of MeO-gDFC thus provides a mechanically coupled mechanism of releasing HF for polymer remodeling pathways that complements previous thermally driven mechanisms. Finally, in Chapter 4, we report the mechanically driven isomerization of cubane, a compound of longstanding fascination to chemists due to its structure, symmetry, and strain. The mechanical coupling is explored at three regiochemical dispositions: ortho, meta and para. In contrast to the fact that all compounds can be activated thermally, cubane is mechanically activated only when coupled at ortho positions. Through mechanical activation, cubane reacts to form a thermally inaccessible syn-tricyclooctadiene product, in comparison to cyclooctatetraene, which is observed in thermal rearrangements of cubane. Pulsed ultrasonication of such cubane-containing polymer leads to efficient isomerization (57% activation after 4 h sonication). We further quantify the mechanochemical reactivity of cubane by single molecule force spectroscopy, and force-coupled rate constants for ring opening reach ~33 s-1 at a force of ~1.55 nN, lower than required forces of cyclobutanes which are typically 1.8-2.0 nN.
Item Open Access Mechanochemistry for Active Materials and Devices(2016) Gossweiler, Gregory RobertThe coupling of mechanical stress fields in polymers to covalent chemistry (polymer mechanochemistry) has provided access to previously unattainable chemical reactions and polymer transformations. In the bulk, mechanochemical activation has been used as the basis for new classes of stress-responsive polymers that demonstrate stress/strain sensing, shear-induced intermolecular reactivity for molecular level remodeling and self-strengthening, and the release of acids and other small molecules that are potentially capable of triggering further chemical response. The potential utility of polymer mechanochemistry in functional materials is limited, however, by the fact that to date, all reported covalent activation in the bulk occurs in concert with plastic yield and deformation, so that the structure of the activated object is vastly different from its nascent form. Mechanochemically activated materials have thus been limited to “single use” demonstrations, rather than as multi-functional materials for structural and/or device applications. Here, we report that filled polydimethylsiloxane (PDMS) elastomers provide a robust elastic substrate into which mechanophores can be embedded and activated under conditions from which the sample regains its original shape and properties. Fabrication is straightforward and easily accessible, providing access for the first time to objects and devices that either release or reversibly activate chemical functionality over hundreds of loading cycles.
While the mechanically accelerated ring-opening reaction of spiropyran to merocyanine and associated color change provides a useful method by which to image the molecular scale stress/strain distribution within a polymer, the magnitude of the forces necessary for activation had yet to be quantified. Here, we report single molecule force spectroscopy studies of two spiropyran isomers. Ring opening on the timescale of tens of milliseconds is found to require forces of ~240 pN, well below that of previously characterized covalent mechanophores. The lower threshold force is a combination of a low force-free activation energy and the fact that the change in rate with force (activation length) of each isomer is greater than that inferred in other systems. Importantly, quantifying the magnitude of forces required to activate individual spiropyran-based force-probes enables the probe behave as a “scout” of molecular forces in materials; the observed behavior of which can be extrapolated to predict the reactivity of potential mechanophores within a given material and deformation.
We subsequently translated the design platform to existing dynamic soft technologies to fabricate the first mechanochemically responsive devices; first, by remotely inducing dielectric patterning of an elastic substrate to produce assorted fluorescent patterns in concert with topological changes; and second, by adopting a soft robotic platform to produce a color change from the strains inherent to pneumatically actuated robotic motion. Shown herein, covalent polymer mechanochemistry provides a viable mechanism to convert the same mechanical potential energy used for actuation into value-added, constructive covalent chemical responses. 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 actuating device might provide a useful design and optimization tool, revealing spatial and temporal force evolution within the actuator in a way that might also be coupled to feedback loops that allow autonomous, self-regulation of activity.
In the future, both the specific material and the general approach should be useful in enriching the responsive functionality of soft elastomeric materials and devices. We anticipate the development of new mechanophores that, like the materials, are reversibly and repeatedly activated, expanding the capabilities of soft, active devices and further permitting dynamic control over chemical reactivity that is otherwise inaccessible, each in response to a single remote signal.
Item Open Access Metal-Ligand Interactions in Stimuli Responsive Molecules(2018) Hall, KaceyThe ability to control changes in physical and chemical properties has allowed stimuli responsive molecules to be used in a variety of applications. Incorporating metals into stimuli responsive molecules introduces properties unique to organometallics that allow for additional modes of control. Specifically, metals offer increased bonding geometries, lower dissociation energies, reversible chelation, and well-characterized catalytic properties that expand the chemical toolbox for the design of stimuli responsive molecules.
Molecules respond to a range of stimuli, including force and light. Force is an interesting stimulus because it is ubiquitous. It is typically considered destructive, but molecules have been designed to respond to mechanical force to produce constructive chemistries. While much progress has been made towards understanding the role of mechanical forces in accelerating and directing covalent chemical reactivity, the behavior of coordinative complexes subjected to the same is far less understood. This work describes the design and synthesis of novel metallomechanophore constructs intimately connected with polymer chains suitable for testing by single molecule force spectroscopy (SMFS) to probe the effect mechanical forces on the behavior of coordination complexes. Efforts towards characterizing the behavior of these metallopolymers upon application of force by (SMFS) are described. Further, preliminary data on expanding the toolbox of stimuli-responsive molecules by incorporating various metals and higher denticity ligands into these polymer constructs using newly established design principles are discussed. These results create a roadmap for accessing strained metal complexes with the goal of enabling the unique physical and chemical properties of metal ions to be leveraged for creating novel materials.
Alternatively, light can be harnessed as a stimulus. Light allows for precise control over a molecule’s response since these molecules can be design to respond to an exact wavelength of light. The range of applications for light-responsive is diverse, and the ability to design photoswitches with variable photochemical and physical properties is consequently important for realizing their potential. Addition of metals adds an additional level of control that allows for the creation of increasingly sophisticated responsive molecules. Work in our lab has reported on the photochromism of (E)-N'-(1-(2-hydroxyphenyl)ethylidene)isonicotinohydrazide (HAPI), a chelating aroylhydrazone. Building upon this, we report the synthesis of structurally related aroylhydrazone chelators, and conducted a structure-activity study to explore the effect the modifications on their photoreactivity, photostationary state composition, photoisomer thermal stability, and relative iron(III) binding ability.
Organometallics provide opportunities for greater tunability and reactivity compared to organic molecules. This work demonstrates the utility of incorporating metals into stimuli responsive molecules, and provides a framework for the design of future force- and light- responsive metal complexes.
Item Open Access Polymer Remodeling Enabled by Covalent Mechanochemistry(2013) Ramirez, Ashley Lauren BlackMaterial failure is a ubiquitous problem, and it is known that materials fail at much lower stresses than the theoretical maximum calculated from the number and strength of the individual bonds along the material cross-section. The decreased strength is attributed to inhomogeneous stress distributions under load, thus causing the stress to accumulate at localized regions, initiating microcrack formation and subsequent propagation. In many cases, these initiation and propagation steps involve covalent bond scission.
Over the past decade there has been increased interest in channeling the mechanical forces that typically trigger destructive processes (e.g., chain scission) during use into constructive chemical transformations. In an ideal system, these stress-induced chemical transformations would redistribute load prior to material failure, thus extending material lifetime. In this Dissertation, the work of developing constructive transformations through the response of a small molecule "mechanophore" is discussed.
The gem-dihalocyclopropane mechanophore is capable of undergoing a non-scissile electrocyclic ring opening reaction under molecular scale tensile load. The mechanochemistry is demonstrated both in solution via pulsed ultrasound (Chapter 2) and in the bulk via extrusion and uniaxial tension (Chapter 3). In solution, dramatic remodeling at the molecular level occurs under the elongational flow experienced during pulsed ultrasound. Because elongational flow results in regiospecific stress distributions along a polymer main chain, this remodeling converts a gem-dichlorocyclopropane-laden homopolymer into phase separating diblock-copolymers. In the bulk, it is shown that the increased reactivity of an activated gem-dibromocyclopropane mechanophore towards nucleophilic displacement reactions leads to more non-destructive intermolecular bond-forming reactions than chain scissions, indicating the potential of the gem-dibromocyclopropane mechanophore as a self-strengthening platform.
Coupling the idea of mechanophore activation under high forces and covalent bond formation, an autonomous remodeling platform is developed, utilizing the gem-dibromocyclopropane mechanophore and a carboxylate nucleophile (Chapter 4). The system can be either two components, with a mechanophore-based polymer and a small molecule cross-linker, or a one-component system in which the mechanophore and nucleophile are embedded within the same polymer backbone. Both in the bulk and in solution, the autonomous remodeling polymer undergoes mechanophore activation followed by covalent bond formation, creating a cross-linked network in response to high shear forces. This form of remodeling leads to orders of magnitude increases in elastic modulus in response to forces that otherwise degrade polymer molecular weight and material properties. In all cases, the covalent bond formation through nucleophilic displacement of the allylic bromine by a carboxylate is confirmed as the source of polymer remodeling by FTIR as well as numerous control studies.
Together, these studies show that covalent polymer mechanochemistry can be used as a constructive tool for polymer chemistry (the direct conversion of homopolymers into well-ordered diblock copolymers) and materials science (polymers that self-strengthen in response to an applied force). This work paves the way for the future development of new mechanophores that will optimize the proof-of-principle behaviors demonstrated here.
Item Open Access Structure-Activity Relationships in Mechanophores with Latent Conjugation(2017) Brown, Cameron L.Materials often fail as a result of the mechanical loads they experience during use. On the molecular level, forces within polymers are distributed unevenly throughout the material, and some polymer subchains experience greater stress than others. In some cases, the forces experienced by these overstressed subchains can trigger chain scission events. Chain scission in turn might nucleate the formation of a microcrack that subsequently propagates, ultimately leading to material failure. In recent years, force reactive functional groups, or mechanophores, have emerged as the basis of a potential strategy for combatting this destructive cascade. The strategy, known as activated remodeling via mechanochemistry (ARM), comprises embedding mechanophores along the polymer backbone or within cross-‐‑links, so that otherwise destructive force within an overstressed subchain triggers a constructive, rather than a destructive, response. ARM functions in both solution and bulk to form remodeled polymer networks where the number of bonds formed exceeds the number of bonds broken under typically destructive mechanical conditions. It requires no additional external stimulus or energy input beyond the imposed shear and results in orders-‐‑of-‐‑magnitude increases in bulk moduli.
These demonstrations have spurred a range of important and fundamental questions about stress-‐‑responsive remodeling, including how to dissect the complex interplay between material deformation, mechanophore activation, nascent cross-‐‑link rupture, mechanochemically triggered cross-‐‑link formation, and the impact of various stages of each on the mechanical properties and eventual failure of the material. The answers to these questions require new mechanophores that not only activate and then cross-‐‑link efficiently, but that give clear spectroscopic signatures of their state so that the levels of both activation and cross-‐‑linking can be measured in situ and in real time.
In this dissertation, we design and explore two families of mechanophores for use in the context of the ARM concept. The first family is based on a substituted cyclobutene scaffold, which undergoes a force-‐‑induced electrocyclic ring-‐‑opening reaction to unveil butadiene. In Chapter 2, we investigate the intrinsic stability of a variety of substituted cyclobutenes, and then utilize pulsed ultrasound to change electronic distributions and spectroscopic signatures via mechanically unveiling latent conjugation pathways. Furthermore, we show the potential ARM-‐‑type utility of the cyclobutene mechanophore by using click chemistry to react the activated butadiene with 4-‐‑phenyl-‐‑1,2,4-‐‑triazoline-‐‑3,5-‐‑dione (PTAD).
These studies motivated quantification of the mechanical reactivity of the cyclobutene system as a function of substitution. In Chapters 3 and 4, we use single-‐‑ molecule force spectroscopy (SMFS) to pull individual polymers comprised of cyclobutene mechanophore repeating units, and measure the force required to mechanically induce the ring-‐‑opening reaction on the time scale of several hundred
milliseconds. We show that changes in polymer attachment near a reacting benzocyclobutene mechanophore can have dramatic effects when pulling from cis handles, but not when pulling from trans handles. Additionally, we provide evidence that electronic effects further away from the cyclobutene ring can be tuned without significantly altering the force at which CBE mechanically ring-‐‑opens. As demonstrated in Chapter 2, these electronic effects can still have substantial effects for altering conjugation pathways and unveiled reactivity in the mechanically ring-‐‑opened butadiene product.
The second family of mechanophore investigated in this dissertation is based on the ring-‐‑opening of an oxabicyclo[2.1.0’pentane (OBP) to reversibly generate a highly colored carbonyl ylide. In Chapter 5, we synthesize a dibromoaryl substituted OBP and characterize the carbonyl ylide generated from application of UV light or heating above 100 ̊C. The carbonyl ylide is highly reactive with dipolarophiles or in the presence of oxygen. Unfortunately, most derivatives are highly sensitive to trace amounts of acid and we were unable to incorporate the putative mechanophore in a polymer. Through our efforts, however, we were able to identify two stable, sulfur-‐‑based OBPs that we utilize in Chapter 6 in single-‐‑molecule conductance experiments. In these experiments, we observe no evidence of mechanophore activation as a function of break-‐‑junction elongation, which suggests that the guiding principles used to understand force-‐‑induced reactivity may not hold in systems of high confinement.
The final chapter of this dissertation describes an easy-‐‑to-‐‑implement science outreach demonstration featuring a mechanically and photochemically color-‐‑changing polymer. 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 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 to literally ‘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.