<p>The 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. </p><p>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. </p><p>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.</p><p>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.</p><p>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.</p><p>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.</p> / Dissertation

Mechanical forces can be used to trigger chemical reactions through activating bonds and to direct the course of such reactions in organic materials, particularly in polymers. In inorganic materials, the small molecules present significant challenges in directing the reaction kinetics. This dissertation studied the dynamics and kinetics of oxidation of transitional metals, particularly on tantalum through mechanical forces. This is a new area of research in surface science. Experimentally using a combined electrochemical and mechanical manipulation technique, we compared the equilibrium and non-equilibrium oxidation processes and states of tantalum. An experimental setup was developed with an electrochemical system attached to a sliding mechanical configuration capable of friction force measurement. The surface chemistry of a sliding surface, i.e., tantalum, was controlled through the electrolyte. The mechanical force was fixed and the dynamics of the surface was monitored in situ through a force sensor. The formation of non-equilibrium tantalum oxides was found in fluid environments of hydrogen peroxide, acetic acid and deionized water. We found that the mechanical energy induced the non-stable state reactions leading to metal-stable oxides. Analytically, we compared the energy dispersion, reaction kinetics, and investigate physical chemical reactions. We proposed a modified Arrhenius equation to predict the effect of mechanical energy on non-spontaneous reaction under nonequilibrium conditions. At the end, we also propose a modified Pourbaix diagram known as the Kar-Liang diagram. The Kar-Liang diagram helps to understand the behavior of tantalum under non-equilibrium conditions. A complete understanding of the tribochemical reaction of tantalum is achieved through this dissertation. The dissertation contains six chapters. After the introduction and approach, oxidation of tantalum is discussed in Chapter IV, kinetics in Chapter V. The nonequilibrium Kar-Liang diagram is discussed in Chapter VI, followed by conclusion. This research has important impacts on the field of surface science in understanding the basics of mechanochemical reactions. The resulting theory is beneficial to understand chemical-mechanical planarization (CMP) and to optimize the current industrial processes in microelectronics in making integrated circuits.

Mechanochemistry, or the modulation of chemical reactivity through the application of mechanical forces, has shown to facilitate a number of otherwise prohibitive chemical transformations. Computational approaches employing electronic structure calculations have explained a number of mechanochemically activated processes such as thermally inaccessible isomerizations and cycloreversions, symmetry-forbidden electrocyclic ring openings or activation of latent catalysts and, more recently, have been successfully used to design novel mechanosensitive systems. A significant limitation of such approaches, however, is their high computational cost, as finding force dependent transition states requires multiple saddle searches and consequently, multiple energy evaluations. To circumvent this problem, an approximation has been proposed, extending the well know "Bell formula", which estimates the force-dependent reaction barrier based on zero-force transition state properties. We demonstrate the numerical efficiency of this approximation termed as extended Bell theory (EBT) by comparing to existing theories and experiments. We also apply this method to suggest the unexplored, yet potentially useful possibility of suppressing chemical reactions through mechanical perturbation. Furthermore, in sharp contrast to simple, one-dimensional theories, our analysis reveals that the anti-Hammond effect is dominant in the mechanical activation of polyatomic molecules. Finally, we propose a numerical scheme to address the drawback of the EBT approximation, which is the failure to account for force-induced instabilities. Our approach provides a computationally efficient recipe to track the instabilities and follow the evolution of the reactant or transition states at any explicit force. We provide a classification of the different instability scenarios, and provide an illustrative example for each case. / text

<p>The 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. </p><p>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 <italic>gem<italic>-dibromo and <italic>gem<italic>-dichlorocyclopropanes (<italic>g<italic>DBCs and <italic>g<italic>DCCs) affixed along the backbone of <italic>cis<italic>-polynorbornene (PNB) and <italic>cis<italic>-polybutadiene (PB). At a tip velocity of 0.3 &mu; sec<super>-1<super>, the isomerization of <italic>g<italic>DBC-PNB, <italic>g<italic>DCC-PNB, <italic>g<italic>DBC-PB, and <italic>g<italic>DCC-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 <italic>g<italic>DHC ring openings, the polymer backbone has much greater impact on <italic>g<italic>DHC 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.</p><p>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 (<italic>cis<italic> to <italic>trans<italic> 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. </p><p>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. <super>19<super>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 re-mended simply by subjecting the polymer solution to the original polymerization conditions, i.e., heating to >150 &deg;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.</p> / Dissertation

No description available.

In this research, for the first time, a novel nanofabrication process is developed to produce graphene-based nanoparticles using mechanochemical principles. Utilizing strain energy at the interface of Si and graphite via the use of a tribometer, a reaction between nanometer sized graphite particles with a reducing agent (hydrazine) was initiated. This simple method demonstrated the synthesis of lamellar platelets (lamellae of ~2nm) with diameters greater than 100 micrometers and thicknesses less than 30 nm directly on the surface of a substrate under rubbing conditions. Spectroscopic evaluation of the particles verified them to be graphene-based platelets, with functionalized molecules including C-N and C-Si bonding. Furthermore, the size of the particles was shown to be highly correlated to the applied pressure at the point of contact, such that three-body friction (with intermediate particles) was shown to enhance the size effect, though with greater variation in size among a single test sample. A chemical rate equation model was developed to help explain the formation of the chemically modified graphene platelets, wherein the pressure applied at the surface can be used to explain the net energy supplied in terms of local flash temperature and strain energy. The activation energy calculated as a result of this method (~42kJ/mol) was found to be extraordinarily close to the difference in bond enthalpies for C-O and the C-N, and C-Si bonds, indicating the input energy required to form the platelets is equivalent to the energy required to replace one chemical bond with another, which follows nicely with the laws of thermodynamics. The ability to produce graphene-based materials using a tribochemical approach is a simple, one-step process that does not necessarily require specialized equipment. This development could potentially be translated into a direct-write nanopatterning procedure for graphene-based technologies, which promise to make electronics faster, cheaper and more reliable. The tribochemical model proposed provides insight into nanomanufacturing using a tribochemical approach, and suggests that further progress can be accomplished through the reduction of the activation energy required for graphene formation.

This thesis investigated whether or not it is possible to generate and capture arynes under mechanochemical conditions. The investigation covered mechanochemical parameters, as well as parameters known to influence aryne generation in solution such as crown ether and CsF loading. Experiments were performed in steel vessels, each equipped with a single free moving steel ball in a shaker type mill. Different reactivities were probed, including nucleophilic addition, [4+2] Diels-Alder cycloaddition, [3+2] cycloaddition and Pd-catalyzed cyclotrimerization. Acquired crude product mixtures were analysed with NMR spectroscopy, and spectra were compared to reported spectra of reactants and expected products. The results showed generation and capturing of arynes to be possible under mechanochemical conditions with borylated, as well as non-borylated aryne precursors in moderate to good yields. Regioselectivity did not appear to differ from solvent conditions since only single regioisomers were observed. / Detta kandidatarbete undersökte möjligheten att generera och reagera aryner under mekanokemiska förhållanden. Undersökningen täcker mekanokemiska parametrar likaså parametrar som, sen tidigare, är kända för att påverka generering av aryner i lösning, exempelvis mängden tillsatt kron-eter och CsF. Experimenten utfördes i stålbehållare innehållandes en stålkula vardera som skakades med hjälp av en kvarn av skaktyp. Olika reaktiviteter utforskades, däribland nukleofil addition, [4+2] Diels-Alder cykloaddition, [3+2] cykloaddition och Pd-katalyserad cyklotrimerisering. Erhållna rå-lösningar analyserades med NMR-spektroskopi och erhållna spektran jämfördes därefter med tidigare rapporterade spektran av reaktanterna och de förväntade produkterna. Resultaten visade att generering av och reaktion med aryner är möjligt med både borylerade och icke-borylerade aryn-föregångare under mekanokemiska förhållanden. Regioselektiviteten hos reaktionerna under mekanokemiska förhållanden skilde sig inte från den regioselektivitet som tidigare observerats i lösning.

<p>Material 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. </p><p>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. </p><p>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. </p><p>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. </p><p>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.</p> / Dissertation

Polymer mechanochemistry, wherein exogenous forces are harnessed to drive chemical processes within polymeric matrices, has afforded access to an astounding array of otherwise kinetically prohibitive reactivity. These multifarious mechanochemical transformations include formally symmetry forbidden electrocyclic processes, thermodynamically disfavored isomerizations, and thermally inaccessible cycloreversions of both carbocyclic and heterocyclic functionalities. The fundamental principles that govern mechanochemistry, however, remain elusive. To address this deficiency, we report a series of experimental and computational efforts that probe chemical reactivity under the action of mechanical force. Specifically, we have explored the formal 1,3-dipolar cycloreversion of 1,2,3-triazole moieties in an effort to understand the interplay between kinetic stability and mechanical perturbation. Briefly, 1,4-disubstituted 1,2,3-triazoles were embedded within high molecular weight poly(methyl acrylate) chains and reverted into their azide and terminal alkyne precursors sonochemically. The liberated azide and alkyne moieties were identified by orthogonal chemical ligation to chromophores, and the reactive azido- and alkynyl-polymer fragments could be recoupled through a copper-mediated cycloaddition. Inspired by this result, we developed a computational model to rapidly discover qualitative trends in mechanochemical reactivity. Application of this model to the cycloreversion of 1,2,3-triazoles revealed an intriguing result: the 1,5-disubstitued regioisomer was predicted to exhibit enhanced susceptibility to mechanical cycloreversion in comparison to the 1,4-disubstituted congener. This trend was experimentally verified upon embedding 1,5-disubstituted 1,2,3-triazoles into high molecular weight poly(methyl acrylate) chains and subjecting them to ultrasonication. Specifically, the observed rate constant for chain scission of a poly(methyl acrylate) material containing the 1,5-disubstituted isomer was 20% larger than that of an analogous material containing the 1,4-disubstituted congener. Having established confidence in the predictive capabilities of our model, we undertook an exhaustive evaluation of regiochemical effects on the activation of six previously reported mechanically labile scaffolds. Our theoretical work suggested that all of the evaluated scaffolds could exhibit suppressed reactivity under stress (an underexplored phenomenon), and this result was supported by experimental investigation. Moreover, our theoretical considerations predicted that anti-Hammond effects (i.e., increased structural dissimilarity between reactant and transition state geometries as the two approach energetically) could be predominant in mechanochemical processes. Finally, we endeavored to expand the scope of polymer mechanochemistry beyond traditional chemical systems to biologically relevant species. We found that the photophysical properties of fluorescent protein variants could be modulated by embedding the proteins within poly(methyl methacrylate) matrices and compressing the resulting composites. / text

No description available.