Browsing by Subject "Polymer"
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Item Open Access Design Rules for Complex Emulsion Targets in Resonant Infrared, Matrix-Assisted Pulsed Laser Evaporation of Polymer Thin Films(2021) Ferguson, SpencerPolymer thin films used in many modern technologies, such as light emitting diodes, solar cells, flexible electronics, and sensors, are fabricated in numerous ways. Of these methods, solution-based processing techniques are the most common with established commercial manufacturing and high throughput. However, these approaches face multiple disadvantages when applied to device heterostructures that are essential for optoelectronic devices. In contrast, vacuum-based techniques are amenable to multi-layer films with varying composition, regardless of polymer solubility. Resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) allows for the deposition of finely tuned polymer structures, such as uniquely graded heterostructures, which solution processing techniques cannot achieve. The primary contribution of this dissertation is to bring to light general design rules for complex emulsions that comprise the targets used in RIR-MAPLE to deposit polymer thin films containing crystalline domains.
Initially, design rules are discussed that lead to the formation of pinhole-free polymer films that also contain crystalline domains. This work addresses a fundamental challenge to promote the crystalline phase, as opposed to the more prevalent amorphous phase, while also maintaining high-quality film surfaces. These design rules are then expanded to include the effects of overall emulsion composition on the resulting emulsion morphology. In order to maximize the content of the crystalline polymer phase in thin films, the use of various surfactants (not yet applied to RIR-MAPLE emulsion targets) is investigated due to the unique interactions that occur, thereby determining the emulsion morphology. As a result, the impact of surfactant molecular structure on the resultant film properties is described for the first time. Finally, the design rules identified for complex emulsions are used in a fundamentally different emulsion chemistry to verify the extent to which these rules are generally applicable.
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 Intrinsically Disordered Protein Polymer Libraries as Tools to Understand Protein Hydrophobicity(2019) Tang, Nicholas ChenIntrinsically disordered protein polymers (IDPPs) are repetitive biopolymers that, when enriched with prolines, glycines, and aliphatic amino acids, have observable lower critical solution temperature (LCST) phase transition behavior at physiologically relevant temperature and concentration ranges. This behavior is a striking feature of disordered proteins in nature, where chemical or physical stimuli lead to sharp conformational or phase transitions. Accordingly, protein-based polymers have been designed to mimic these behaviors, leading to a broad range of biotechnological applications. This work is driven by two approaches. In our science focused approach, we developed a polymer-physics based framework for understanding IDPP hydrophobicity using the relationship between phase transition temperature and globule surface tension. This physics-based framework has allowed us to better understand the unified contributions of chain length, concentration, temperature, and individual amino acid side chains to IDPP hydrophobicity by studying phase transition data. In our engineering focused approach, we developed novel tools that enable the high throughput discovery of new proteins that exhibit phase transitions, in order to increase the number of known stimuli responsive peptide sequence motifs beyond the limits of bioinspired design. The exhaustive discovery of new proteins that exhibit phase transitions consists of gene synthesis and protein screening. We developed two key technologies that has enabled (1) the scalable synthesis of repetitive gene libraries using a novel graph theoretic gene optimization approach (Codon Scrambling) and (2) the pooled synthesis of large complex gene libraries from libraries of oligonucleotides. Combined with pipelines for the screening of phase transition behavior, these technologies have enabled us to generate a diverse library of protein sequences necessary to validate our theoretical models. Finally, we developed an algorithm for the de novo design of nonrepetitive protein sequences that exhibit phase transition behavior, further broadening the sequence space of stimuli responsive synthetic IDPPs.
Item Open Access Matrix-Assisted Pulsed Laser Evaporation of Conjugated Polymer and Hybrid Nanocomposite Thin Films: A Novel Deposition Technique for Organic Optoelectronic Devices(2011) Pate, Ryan JaredThis dissertation develops a novel application of the resonant-infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) technique toward the end goal of conjugated-polymer-based optoelectronic device fabrication. Conjugated polymers are attractive materials that are being investigated in the development of efficient optoelectronic devices due to their inexpensive material costs. Moreover, they can easily be combined with inorganic nanomaterials, such as colloidal quantum dots (CQDs), so as to realize hybrid nanocomposite-based optoelectronic devices with tunable optoelectronic characteristics and enhanced desirable features. One of the most significant challenges to the realization of optimal conjugated polymer-CQD hybrid nanocomposite-based optoelectronics has been the processes by which these materials are deposited as thin films, that is, conjugated polymer thin film processing techniques lack sufficient control so as to maintain preferred optoelectronic device behavior. More specifically, conjugated-polymer-based optoelectronics device operation and efficiency are a function of several attributes, including surface film morphology, internal polymer chain morphology, and the distribution and type of nanomaterials in the film bulk. Typical conjugated-polymer thin-film fabrication methodologies involve solution-based deposition, and the presence of the solvent has a deleterious impact, resulting in films with poor charge transport properties and subsequently poor device efficiencies. In addition, many next-generation conjugated polymer-based optoelectronics will require multi-layer device architectures, which can be difficult to achieve using traditional solution processing techniques. These issues direct the need for the development of a new polymer thin film processing technique that is less susceptible to solvent-related polymer chain morphology problems and is more capable of achieving better controlled nanocomposite thin films and multi-layer heterostructures comprising a wide range of materials. Therefore, this dissertation describes the development of a new variety of RIR-MAPLE that uses a unique target emulsion technique to address the aforementioned challenges.
The emulsion-based RIR-MAPLE technique was first developed for the controlled deposition of the conjugated polymers poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly[2-methoxy-5-(2'ethylhexyloxy)-1,4-(1-cyanovinylene) phenylene] (MEH-CN-PPV) into homogenous thin films. Therein, it was identified that target composition had the most significant influence on film surface morphology, and by tuning the concentration of hydroxyl bonds in the target bulk, the laser-target absorption depth could be tuned so as to yield more or less evaporative deposition, resulting in films with tunable surface morphologies and optical behaviors.
Next, the internal morphologies of emulsion-based RIR-MAPLE-deposited MEH-PPV thin films were investigated by measuring their hole drift mobilities using the time-of-flight (TOF) photoconductivity method in the context of amorphous materials disorder models (Bässler's Gaussian Disorder model and the Correlated Disorder model) in order to provide a quantitative measure of polymer chain packing. The polymer chain packing of the RIR-MAPLE-deposited films was demonstrated to be superior and more conducive to charge transport in comparison to spin-cast and drop-cast MEH-PPV films, yielding enhanced hole mobilities.
The emulsion-based RIR-MAPLE technique was also developed for the deposition of different classes of inorganic nanoparticles, namely un-encapsulated nanoparticles and ligand-encapsulated nanoparticles. These different classes of nanoparticles were identified to have different film growth regimes, such that either rough or smooth films were obtained, respectively. The ligand-encapsulated nanoparticles were then co-deposited with MEH-PPV as conjugated polymer-CQD hybrid nanocomposites, wherein the distributions of the constituent materials in the film bulk were identified to be tunable, from homogeneous to highly clustered. The RIR-MAPLE deposition regime determined the said distributions, that is, if the polymer and CQDs were sequentially deposited from a sectioned target or simultaneously deposited from a single target, respectively. The homogeneous conjugated polymer-CQD nanocomposites were also investigated in terms of their charge transport properties using the TOF photoconductivity technique, where it was identified that despite the enhanced dispersion of CQDs in the film bulk, the presence of a high concentration of CQDs degraded hole drift mobility, which indicates that special considerations must be taken when incorporating CQDs into conjugated-polymer-based nanocomposite optoelectronics.
Finally, the unique capability of RIR-MAPLE to enable novel conjugated polymer-based optical heterostructures and optoelectronic devices was evaluated by the successful demonstration of a conjugated polymer-based distributed Bragg reflector (DBR), a plasmonic absorption enhancement layer, and a conjugated polymer-based photovoltaic solar cell featuring a novel electron-transporting layer. These optical heterostructures and optoelectronic devices demonstrate that all of the constituent polymer and nanocomposite layers have controllable thicknesses and abrupt interfaces, thereby confirming the capability of RIR-MAPLE to achieve multi-layer, conjugated polymer-based heterostructures and device architectures that are appropriate for enhancing specific desired optical behaviors and optoelectronic device efficiencies.
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 POEGMAlation – A Next-Generation PEGylation Technology(2016) Qi, YizhiThe delivery of therapeutic peptides and proteins is often challenged by a short circulation half-life, necessitating frequent injections that limit efficacy, reduce patient compliance and increase treatment cost. The covalent conjugation of therapeutic peptides and proteins, and more recently oligonucleotide-based drugs, with the “stealth” polymer poly(ethylene glycol) (PEG), termed PEGylation, is one of the most commonly used approaches to increase the in vivo half-life and reduce the immunogenicity of these therapeutic biomolecules. However, after several decades of research and clinical use, the limitations of PEGylation have begun to emerge.
Conventional methods for synthesizing peptide/protein-polymer conjugates have drawbacks including low yield, non-trivial separation of conjugates from reactants, and lack of control over site and stoichiometry of conjugation, which results in heterogeneous products with significantly compromised biological activity. Additionally, anti-PEG antibodies have been induced in patients treated with PEGylated drugs and have been shown to correlate with rapid clearance of these drugs. High levels of pre-existing anti-PEG antibodies have also been found in individuals naïve to PEGylated agents, which are associated with serious first-exposure allergic reactions.
To address the synthetic limitations of PEGylation, a general approach for the high-yield synthesis of site-specific (C-terminal) and stoichiometric (1:1) peptide/protein-polymer conjugates, named sortase-catalyzed polymer conjugation, was developed. Demonstrating proof-of-concept of the approach with green fluorescent protein (GFP) as a model protein, sortase A from Staphylococcus aureus was used to site-specifically attach an initiator solely at the C-terminus of GFP, followed by in situ growth of the PEG-based brush polymer, poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA) from the protein macroinitiator by atom transfer radical polymerization (ATRP). Sortase-catalyzed initiator attachment proceeded with high specificity and near-complete (~ 95%) product conversion. Subsequent in situ ATRP in aqueous buffer produced 1:1 stoichiometric conjugates with > 90% yield, tunable MW, low dispersity, and no denaturation of the protein. The extraordinarily high yield compares favorably to order of magnitude losses typically seen in conventional PEGylation processes.
Next, the therapeutic potential of POEGMAlation, or the conjugation of POEGMA to a peptide or protein, was demonstrated by implementing the developed sortase-catalyzed polymer conjugation strategy with exendin-4 (exendin), a therapeutic peptide for treating type 2 diabetes, to synthesize exendin-C-POEGMA conjugates with a wide and tunable range of molecular weights (MWs) and low dispersity. A single subcutaneous injection of exendin-C-POEGMA conjugates lowered blood glucose for up to 120 h in a diabetic mouse model. Most intriguingly, we showed that appending PEG as oligomeric side-chains on the conjugated POEGMA and tuning the side-chain length completely eliminated the reactivity of exendin-C-POEGMA conjugates toward patient-derived anti-PEG antibodies without compromising in vivo efficacy. Clinically, the lack of anti-PEG antigenicity of POEGMA conjugates is expected to completely eliminate serious first-exposure allergic reactions and the accelerated blood clearance of POEGMA-drug conjugates due to pre-existing anti-PEG antibodies in patients.
Collectively, these results establish POEGMAlation as a next-generation PEGylation technology that is highly useful for improving the pharmacological performance of therapeutic biomolecules while providing a timely solution to the increasing levels of pre-existing anti-PEG antibodies in patients that are seriously hindering the safety and efficacy of traditional PEGylated drugs.
Item Open Access Self-assembly of polymer-grafted anisotropic nanoparticles(2021) Lee, BrianWhile anisotropic nanoparticles provide unique building blocks for self-assembling useful nanodevices and nanomaterials ranging from plasmonic sensors to chiral metamaterials, controlling their self-assembly process to achieve targeted structure remains challenging. Recently, surface functionalization of nanoparticles with polymer grafts was shown to be a powerful strategy for tuning the orientation-dependent interactions of the nanoparticles. This technique allows modulation of the interaction between nanoparticles as grafted polymers can provide both repulsive interactions arising from their steric hindrance as well as attractive interactions due to their adsorption to the particle surfaces. Utilizing this approach, experiments have successfully assembled nanoparticles into large structures with highly uniform interparticle orientations. However, many challenges remain in fabricating desired nanostructures with the polymer-grafted anisotropic nanoparticles. First, much of the underlying physics governing assembly of such nanoparticles is not well understood and is difficult to discern using experimental techniques due to the nanoscopic nature of the self-assembly process. Second, the relevant parameter space that affects the particle assembly is vast and investigation of such large parameter space is costly in terms of both time and expenses. Third, computationally investigating the behavior of anisotropic nanoparticles is difficult as calculation of their interaction energies is computationally expensive due to the lack of analytical expressions for these energies.In this dissertation, I tackle these challenges in self-assembly of anisotropic nanoparticles through computational modeling, focusing specifically on polymer-grafted nanocubes and DNA-grafted nanorods. For both systems, computational methods and analytical models for efficiently calculating the interaction energies between the anisotropic nanoparticles are first developed. Using such methods as well as advanced Monte Carlo simulations and atomistic calculations, free-energy landscapes describing the assembly of these anisotropic nanoparticles are obtained. Analysis of the free-energy landscapes demonstrates that understanding the interplay between the different interaction components of the systems as well as their dependencies on the relative configurations of the assembled particles is crucial. Specifically for the nanocubes, the competition between the attractive interactions between the inorganic particle cores lead to face-face type of configurations while the repulsive interactions due to the polymer corona induce edge-edge configurations. For the DNA-grafted nanorods, the competition between attractive and repulsive interactions interplay with the chirality of the bridging DNA to induce chiral assembly of the nanorods. Based on these results, material design rules for assembling both the nanocubes and the nanorods into desired configurations are suggested. These results were not only in agreement with many previous experimental studies but also provided the underlying mechanism that explain such assembly behaviors. In summary, the results presented in this dissertation should both aid in fabrication of nanodevices with precisely controlled particle assemblies as well as provide efficient computational methods for future investigation of anisotropic nanoparticles.
Item Open Access Structure Activity Relationships in the Fracture of Hybrid Covalent/Metallosupramolecular Organogels(2014) Hawk, Jennifer LeeHybrid polymeric networks constructed using both covalent and reversible cross-links have been shown to be effective in preventing fracture and ultimately failure in polymeric materials. The prevention of failure has been largely attributed to the ability of the reversible cross-links to dissipate energy without breaking the covalent cross-links. The ability to rationally design materials that optimize this strategy would benefit from quantitative and systematic studies of the relationship between the number and strength of reversible interactions and the failure behavior of hybrid networks. This dissertation describe studies of fracture under compression in a family of hybrid networks, in which the timescale of reversible cross-linker dissociation is varied over several orders of magnitude, whereas the covalent components are kept constant.
Polymeric networks were constructed with 4-vinylpyridine. Bimetallic pincer Pd and Pt complexes were inserted into the network, forming reversible metal-ligand bonds that cross-link pyridine residues. The additional reversible cross-links prolong the lifetime of the hybrid networks under compressive strain when compared to their covalent counterparts. The observed failure behavior is dependent on the rate at which the networks are compressed as well as the strength of reversible interaction. Most interestingly, the addition of very dynamic and weak reversible interactions, so weak as to make no measurable contribution to bulk modulus, still leads to enhanced fracture strains. The failure of the covalent component within these hybrid networks was probed directly by incorporating a mechanophore that emits light upon chain scission. It was confirmed that the addition of these dynamic and weak reversible cross-links delays the catastrophic bond scission events associated with failure in the materials.
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.