Browsing by Subject "Polymer chemistry"

Metallosupramolecular 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.

A variety of intriguing surface patterns have been observed on developing natural systems, ranging from corrugated surface of white blood cells at nanometer scales to wrinkled dog skins at millimeter scales. To mimetically harness functionalities of natural morphologies, artificial transformative skin systems by using soft active materials have been rationally designed to generate versatile patterns for a variety of engineering applications. The study of the mechanics and design of these dynamic surface patterns on soft active materials are both physically interesting and technologically important.

This dissertation starts with studying abundant surface patterns in Nature by constructing a unified phase diagram of surface instabilities on soft materials with minimum numbers of physical parameters. Guided by this integrated phase diagram, an electroactive system is designed to investigate a variety of electrically-induced surface instabilities of elastomers, including electro-creasing, electro-cratering, electro-wrinkling and electro-cavitation. Combing experimental, theoretical and computational methods, the initiation, evolution and transition of these instabilities are analyzed. To apply these dynamic surface instabilities to serving engineering and biology, new techniques of Dynamic Electrostatic Lithography and electroactive anti-biofouling are demonstrated.

Virtually every consumer product available on the market today contains some form of fossil fuel-based polymer. However, these materials pose environmental, human health, and economic concerns due to their enduring presence in the global ecosystem and their degradation products. Addressing this crisis necessitates scalable production of biodegradable alternatives, such as polyhydroxyalkanoates (PHAs). PHAs are presented as promising substitutes due to their biodegradability, biocompatibility, and the potential for complete renewable utilization post-degradation, but a current challenge to widespread use of these materials lies in understanding the quantitative relationship between the structural characteristics of PHAs, their environmental interactions, and their degradation rates to enhance their industrial production and distribution. To bridge this knowledge gap, the dissertation outlines a comprehensive approach involving the development of a specialized dataset, the application of machine learning (ML) models to predict degradation rates based on structural and environmental factors, and the experimental validation of these predictions. The first part of this research focuses on assembling a manually curated dataset from the extensive, available open-access literature, aimed at understanding the effects of structural and environmental features on PHA degradation. The second part leverages this dataset through ML modeling, employing techniques like random forest regression to predict degradation profiles with over 80% accuracy. This methodology enables a deeper understanding of the complex interplay between chemical structures and degradation properties, surpassing traditional trial-and-error approaches. The final part of this research aims to complete an iterative workflow for dataset development by validating ML model predictions through physical experiments, enriching the original dataset with comprehensive experimental data on PHA degradation in hydrolytic environments with contact angle, molecular weight, and thermal property characterizations. The incorporation of experimental findings into the ML dataset, particularly through expanded ML techniques that emphasize pairwise feature importance such as explainable boosting machines (EBM), helps in pinpointing critical factors influencing PHA degradation, such as environmental temperature and material properties. The model performances indicate a strong performance of manually assembled literature-based datasets when predicting degradation rate for PHAs. In conclusion, a data science-based framework has been developed for exploring PHA biopolyester degradation and explores the combination of features of the material and its environment that integrates the structure, properties, and experimentally verified degradation profiles of the material. This workflow will be a useful and generalizable pipeline for PHAs and other polymers to expand the biopolymer design space with degradation in mind.

The fracture of polymer networks is usually perceived macroscopically and is considered as a mechanical engineering problem. However, to advance a crack in a polymer network, lots of polymer strands that bridge the crack need to be broken, thus network fracture is molecular as well. In the past 80 years, scientists have been trying to build up quantitative connections between network fracture mechanics and the molecular details of the networks, but to date, there is still no well-accepted quantitative molecular model for network fracture. This is due to the lack of understanding of the molecular details (i.e., strand scission reaction, network topology, etc.) in polymer networks. Developments in polymer mechanochemistry and polymer physics open the door to understanding network fracture from the molecular level. With the concepts of polymer mechanochemistry and polymer physic, this dissertation investigates the correlations between bond/strand scission reaction and the network fracture mechanics theoretically and experimentally.The Lake-Thomas theory is the most well-known molecular theory of network fracture which connects the network critical tearing energy to the scission of polymer strands. Although it has been widely used to explain experimental data, the energy parameter in this theory does not capture the correct chemistry of strand scission and the physics of polymer networks. We provided a conceptual framework to modify the molecular energy parameter in the Lake-Thomas theory by considering the force-coupled reactivity of polymer strand scission reaction (SSR) and network connectivity. First, we consider the strand scission during crack propagation as a mechanochemical reaction of polymer strands, the kinetics of which is dictated by the force on the strands instead of the bond dissociation energy of the repeating monomers. By incorporating the data reported from the single-molecule force spectroscopy experiments, we found the elastic energy stored per bond when typical hydrocarbon polymers break is ca. 60 kJ ∙ mol-1, which is well below the typical carbon-carbon bond dissociation energy (ca. 350 kJ ∙ mol-1). This modification introduced the concept of strand scission reaction into the molecular fracture model of polymer networks and explained the underlying criteria of chain scission. Next, we consider the energy contribution of unbroken strands in the polymer networks during crack propagation. This modification includes not only the energy stored in the breaking network strands (bridging strands) but also the energy stored in the tree-like structure of the strands connecting the bridging strands to the network continuum, which remain intact as the crack propagates. We show that the tearing energy stored in each of the generations of this tree depends non-monotonically on the generation index due to the nonlinear elasticity of the stretched network strands. We further show that the energy required to break a single bridging strand is not necessarily dominated by the energy stored in the bridging strand itself but in the higher generations of the tree. To verify our theoretical modification of the Lake-Thomas theory, we designed and synthesized covalent polymer gels in which the macroscopic fracture “reaction” is controlled by mechanophores embedded within mechanically active network strands. The gels were prepared through the end-linking of azide-terminated tetra-arm PEG (Mn = 5 kDa) with different bis-alkyne linkers under identical conditions, except that the bis-alkyne was varied to include either a cis-diaryl or cis-dialkyl linked cyclobutane mechanophore that acts as a mechanochemical “weak link” through a force-coupled cycloreversion. A control network featuring a bis-alkyne without cyclobutane (non-mechanophore) was also synthesized. The networks show the same small strain elasticity and swelling, but they exhibit tearing energies that span a factor of 8 (3.4, 10.6, and 27.1 J ∙ m-2 for networks with cis-diaryl, cis-dialkyl cyclobutane mechanophores, and non-mechanophore control, respectively). The difference in fracture energy is well-aligned with the force-coupled scission kinetics of the mechanophores observed in single-molecule force spectroscopy (SMFS) experiments, implicating local resonance stabilization of a diradical transition state in the cycloreversion of cis-diaryl cyclobutane mechanophore as a key determinant of the relative ease with which its network is torn. The connection between macroscopic fracture and a small-molecule reaction mechanism suggests that the fracture of polymer networks is a chemical reaction. Further characterizations of the force-coupled kinetics of cis-diaryl and cis-dialkyl cyclobutane mechanophores with SMFS suggest opportunities for constructing quantitative correlations between strand scission reaction and network fracture mechanics. Although the tearing energy of polymer networks is highly dependent on the strand scission reaction of network strands, how the networks with mixed strand scission reactions behave remains unclear. Hence, the impact of mixed strand scission reaction is studied through the synthesis of networks with varied ratios of cis-diaryl cyclobutane mechanophore (“weak”) and non-mechanophore (“strong”) control linkers. The strands with mechanophore linkers are about 4 ~ 5 times weaker than the strands with non-mechanophore linkers according to SMFS experiments. Tearing energy versus strong linkers percentage demonstrate the existence of plateau regions for < 40% strong linker at 2 ~ 3 J∙m-2 and >75% strong linker at ~20 J∙m-2. These regions correspond somewhat closely to the expected Flory-Stockmayer percolation thresholds for a tetra-functional network, pc (strong) = 0.67 and pc (weak) = 0.33. From the classical point of view (e.g., Lake-Thomas theory), the crack propagation is usually assumed to be path-determined instead of reactivity-determined. These data suggest the path-determined mechanism, which predicts that the tearing energy should vary linearly with the average strength of the bonds, does not correctly capture the trend in tearing energy, and the reactivity-determined mechanism is likely to be correct. Ongoing experiments on the actual percolation threshold of either weak or strong chains in the networks would provide more understanding of the reactivity-determined mechanism. Our current work on end-linked networks suggests that the network is weak with incorporated weak mechanophores. However, incorporating weak mechanophores as crosslinkers in radically polymerized networks yields an opposite result: the network is tougher with incorporated weak mechanophores. A cis-diaryl cyclobutane-mechanophore is developed as a mechanochemically weak covalent crosslinker and incorporated into controlled radical-polymerized networks. The networks consist of crosslinkers that are mechanochemical weak and long primary chains that are mechanochemically strong. The activation force of the covalent crosslinker is estimated to be ca. 5 times weaker than the corresponding control crosslinkers and the polymer backbone bonds. However, the networks made from the weak crosslinkers are 2 ~ 9 times tougher than the networks made from the corresponding control crosslinkers (non-mechanophore) while the former exhibit the same small strain elasticity as the latter. By altering the degree of polymerization (DP) of primary chains while keeping the crosslinking density similar, we found that, with primary chains that have DP ≈ 1300 and larger, the networks made from the weak crosslinkers are 6 ~ 9 times tougher than that made from control crosslinkers. With primary chains that have DP ≈ 300, the tearing energies of these two types of networks have no significant difference. This suggests that having long enough primary chains is critical for the toughening effect. The underlying principles of this toughening effect are the weak crosslinkers can be activated before the primary chain breaks, and the inherent topological loopy structures of the network can be released as “stored-length”. Such change in topological structures of the networks after weak crosslinker activation can redistribute the load and toughen the networks.

When mechanical force is applied to synthetic materials, polymer chains become

highly strained, leading to bond scission and ultimately material failure. Over the last

decade or so, work in the field of polymer mechanochemistry has coupled this tension to

desired covalent chemical reactions. These functionalities, known as mechanophores,

react to unveil a new molecular structure and triggering a constructive response. This

strategy has been explored for a variety of purposes, including stress sensing, stress

strengthening, small molecule release, catalysis, and development of soft devices.

Additionally, the effect of force on a reaction coordinate, through biasing and probing

reaction pathways and trapping of transition states and intermediates, has been well–

studied experimentally and in theory. This work reports on understanding structure property

relationships for the spiropryan mechanophore and expanding our control of mechanochromism

from the single-molecule to device scale.

First, we report the effect of substituents on spiropyran derivatives substituted

with H, Br, or NO2 para to the breaking spirocyclic C− O bond using single molecule

force spectroscopy. The force required to achieve the rate constants of ~ 10 s−1 necessary

to observe transitions in the force spectroscopy experiments depends on the substituent,

with the more electron withdrawing substituent requiring less force. Rate constants at

375 pN were determined for all three derivatives, and the force coupled rate dependenc

eon substituent identity is well explained by a Hammett linear free energy relationship

with a value of ρ = 2.9, consistent with a highly polar transition state with heterolytic,

dissociative character. The methodology paves the way for further application of linear

free energy relationships and physical organic methodologies to mechanochemical

reactions.

The development and characterization of new force probes has enabled

additional, quantitative studies of force-coupled molecular behavior in polymeric

materials. The relationship between strain and color change has been measured for

these three spiropyran derivatives. The color appears at around the same strain and the

ratio of color intensities remains constant for all three derivatives. This result was not predicted by

previously reported computational work and motivates future studies of

force distribution within filled silicones.

On the material and device scale, we have utilized mechanochromism for soft

and stretchable electronics, which are promising for a variety of applications such as

wearable electronics, human− machine interfaces, and soft robotics. These devices,

which are often encased in elastomeric materials, maintain or adjust their functionality

during deformation, but can fail catastrophically if extended too far. Here, we report

new functional composites in which stretchable electronic properties are coupled to

molecular mechanochromic function, enabling at-a-glance visual cues that inform user

control. These properties are realized by covalently incorporating a spiropyran

mechanophore within poly(dimethylsiloxane) to indicate with a visible color change that

a strain threshold has been reached. The resulting colorimetric elastomers can be molded

and patterned so that, for example, the word “STOP” appears when a critical strain is

reached, indicating to the user that further strain risks device failure. We also show that

the strain at color onset can be programmed through the layering of silicones with

different moduli into a composite. As a demonstration, we show how color onset can be

tailored to indicate a when a specified frequency of a stretchable liquid metal antenna

has been reached. The multi-scale combination of mechanochromism and soft

electronics offers a new avenue to empower user control of strain-dependent properties

for future stretchable devices.

Through the study of the reaction that converts spiropyran into merocyanine, we

are able to teach and connect a number of standard general chemistry course topics

while also introducing students to polymer concepts. By framing a number of different

concepts including molecular orbital theory, quantum mechanics, equilibrium,

hydrogen bonding, mechanical work, and polymer chemistry with the same reaction,

our goal is to allow students to see connections in seemingly disparate sections of

general chemistry.

The reactivity of a mechanically active functional group is determined by the

activation energy of the reaction (ΔG‡) and the force-coupled change in length as the

reaction proceeds from the ground to transition state (Δx‡). Finally, we report a combination

of both principles enhances the mechanochemical reactivity of epoxides:

placing alkenes adjacent to cis-epoxide mechanophores along a polymer backbone

results in ring-opening to carbonyl ylides during sonication, whereas epoxides lacking

an adjacent alkene do not. Upon release, tension-trapped ylides preferentially close to

their trans-epoxides in accordance with the Woodward-Hoffman rules. The reactivity of

carbonyl ylides is exploited to tag the activated species with spectroscopic labels for

force-induced cross-linking through a reaction with pendant alcohols. Even with alkene

assistance, mechanochemical reactivity remains low; single molecule force spectroscopy

establishes a lower limit for ring-opening ca. 1 sec-1 at forces of ~2600 pN.

The development of multi-mechanophore polymers (MMPs) has empowered new methodologies for observing and quantifying mechanochemical transformations. Previously developed techniques such as single-molecule force spectroscopy (SMFS) and pulsed ultrasound can be used to induce and observe up to hundreds of chemical reactions within a single polymer, enabling mechanistic insights into mechanochemical reactivity. The bulk of the work presented herein (Chapters 2-5) involves the use of these techniques to elucidate substituent effects on the covalent mechanochemistry of four-membered carbocycles, namely the force-triggered ring-opening reactions of fused-cyclobutane (CB) and cyclobutene (CBE) mechanophores.

MMPs that contain multiple CB and CBE repeats are typically synthesized via entropy-driven ring-opening metathesis polymerization (ED-ROMP, Chapters 3-5), a technique used widely in the field of polymer mechanochemistry. While useful for generating polymers for fundamental mechanistic investigations, there are several challenges inherent to the ED-ROMP approach that limit its impact and applicability (covered in Chapter 1). We therefore sought to overcome some of these challenges by introducing a new class of mechanophore monomers that are amenable to free-radical addition polymerization. In Chapter 2 we report that cyclobutene carboxylates that are fused to larger rings are amenable to controlled radical polymerization techniques, specifically reversible addition-fragmentation chain transfer (RAFT) copolymerization with butyl acrylate. The fused ring repeats act as stored-length mechanophores along the polymer backbone, and so these polymers are a rare example of high mechanophore content polymers formed by addition polymerization. Analysis of mechanophore activation as a function of polymer scission cycle reveals that these CB-based mechanophores operate in high force regimes. In addition, the kinetics and “controllability” of the RAFT polymerizations are investigated as a function of (co)monomer composition, providing a foundation for the design of bulk polymer networks that contain a tunable amount of these stored-length mechanophores.

In Chapter 3 we report the force-dependent kinetics of stored length release in a family of covalent domain polymers based on cis-1,2-substituted CB mechanophores, which have the potential to serve as covalent synthetic mimics of the mechanical unfolding of noncovalent “stored length” domains in structural proteins. The stored length is determined by the size (n) of a fused ring in an [n.2.0] bicyclic architecture, and it can be made sufficiently large (> 3 nm per event) that individual unravelling events are resolved in both constant-velocity and constant-force SMFS experiments. Replacing a methylene in the pulling attachment with a phenyl group drops the force necessary to achieve rate constants of 1 s-1 from ca. 1970 pN (dialkyl handles) to 630 pN (diaryl handles), and the substituent effect is attributed to a combination of electronic stabilization and mechanical leverage effects. In contrast, the kinetics are negligibly perturbed by changes in the amount of stored length. The independent control over unravelling force and extension holds promise as a probe of molecular behavior in polymer networks and for optimizing the behaviors of materials made from covalent domain polymers.

In Chapter 4 we use a combination of SMFS and computation to gain a fundamental understanding of substituent effects on a different class of force-triggered reactions: the forbidden disrotatory ring-opening reaction of CBE mechanophores. We synthesized a series of cis-ester-substituted CBE mechanophores that probed the effects of substituents attached to the breaking sigma bond and the nascent π bond. In general, we found that substituents connected to the breaking sigma bond had a larger influence on reaction kinetics. Computations reveal that the extent of mechanical coupling is similar between all derivatives studied, so we explain the observed differences (or lack thereof) in reactivity by examining the degree to which the substituents provide electronic stabilization to the TS, which has substantial diradical character. We find that an alkyne substituent connected to the breaking sigma bond provides the most stabilization. In addition, the alkyne is initially insulated from the nascent π bond of CBE, but extends the conjugation of the unveiled butadiene product enough to make it fluorescent, providing a foundation for CBEs to become a class of “turn-on” mechanofluorophores.

We combine our insights from Chapters 3-4 to better understand a new monomer presented in Chapter 5 that uses the photoswitching chemistry of a diarylethene (DAE) substituent to photochemically switch the mechanophore between cyclobutene-like and cyclobutane-like structures. Quantitative results derived from SMFS experiments reveal that the photochemical trigger can alter the reaction rate of the monomer by > 105 at a force of 810 pN (the CB-like monomer being easier to activate). We therefore have designed a polymer system whereby the mechanochemical reactivity of the polymer backbone can be regulated (sped up or slowed down) by a separate photochemical reaction. Interestingly, results from sonication studies reveal that despite their disparate reactivates, the two DAE isomers yield the same mechanochemical product. The mechanistic insights gleaned from these results will serve as a foundation for the future molecular-level design of similar, photoswitchable mechanophores.

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There are about 86 billion neurons in the human brain, connected by trillions of synapses. Deciphering the electrical signaling between neurons is key to understanding the brain in both health and disease. As understanding begins with observation, the past decade has brought significant investment in scalable, stable neural recording technologies. An ideal recording platform would have the ability to record from thousands of neurons simultaneously with millisecond temporal precision and knowledge of the genetic identity of each cell—all while being low-cost, scalable, and amenable to simple data storage. Moreover, deciphering how disease progression remodels neural ensembles requires recordings with months-long stability. To date, no recording technology offers these features in combination. Here, we present an approach that aims to address these limitations. We conceived genetically encoded electrophysiology, in which we establish a covalent link between genetically tagged neurons and surface modified electrodes via novel engineered conductive polymers. The approach retains millisecond temporal resolution native to electrophysiology while combining genetic specificity of tagged neurons. This method utilizes a covalent reaction between the HaloTag protein and its chemical ligand, which was successfully used by the Tadross lab for cell-type specific delivery of drugs. We detail the development of each component of genetically encoded electrophysiology, beginning with engineered conductive polymers and incorporation of HaloTag binding ligand. Through different assays, we verify both successful polymerization onto electrodes and HaloTag ligand availability for HaloTag protein binding. We further demonstrate the concept in cultured neurons using custom microelectrode arrays and development platform. We provide proof-of-concept data to support our approach and demonstrate its feasibility. We discuss the implications of future work which could build on the proof-of-concept technology to refine the approach, optimize it for use in culture and adapt it to in vivo—animal—recordings.

Adequate pain management within the first 3-5 days following a procedure is vital to the enhancement of patient healing and recovery. The current gold standard for relieving post operative pain consists of oral medication which, though effective, only offers temporary relief through frequent doses as pain persists, delivers drug systemically and inefficiently, and introduces an inherent potential of misuse and abuse of pain killers. A probable solution is the fabrication of a drug-load matrix to impart local, sustained release of analgesic compounds. Specifically, a novel class of polymers, poly(ester urea)s (PEUs) have been developed and analyzed for their ability to achieve controlled drug delivery. In recent literature, PEUs have displayed a limited inflammatory response, tunable mechanical properties, and degradation. Due to their versatility, PEUs have been applied to a variety of applications, ranging from sturdy bone implants to grafts for soft tissue repair. Moreover, PEUs have high flexibility in processability and in turn different matrix fabrications for drug delivery are feasible, such as drug-loaded implantable devices or injectable dosage forms (microparticles). Thus far, non-opioid analgesic compounds (i.e. bupivacaine, lidocaine, and etoricoxib) have successfully been incorporated into PEU films. The release profiles of the active pharmaceutical ingredients (APIs) from PEU films reveal sustained release over time that varies with chemical composition, film thickness, and drug-load. Additionally, drug diffusion from the polymer matrices follow Fickian diffusion as suggested through fitting to a Higuchi model. Similar release results have also been shown in vivo with etoricoxib and varying polymer composition. Preliminary results reveal integration of the film with local tissue while limiting exposure of drug systemically. Overall, our work highlights the adaptability of PEUs as an emerging biomaterial that’s potential has yet to be exhausted as well as providing a promising alternative to achieving sufficient post-operative pain management.

The key concept of polymer mechanochemistry is the rational design of mechanophores. When incorporated into polymeric materials, mechanophores can be used to produce strain-dependent covalent chemical responses, including stress-strengthening, stress-sensing and network remodeling. In general, it is desirable for mechanophores to be highly inert in the absence of force but highly reactive when under applied tension. We show that the Fe−Cp bond in ferrocene is the preferential site of mechanochemical scission in the pulsed ultrasonication of main-chain ferrocene-containing polybutadiene-like polymers. Quantitative studies reveal that the Fe−Cp bond is similar in strength to the carbon−nitrogen bond of an azobisdialkylnitrile (bond dissociation energy < 30 kcal/mol), despite the significantly higher Fe-Cp bond dissociation energy (up to 90 kcal/mol). Mechanistic studies are consistent with a predominately heterolytic mechanism of chain scission. DFT calculations provide insights into the origins of ferrocene’s mechanical lability.

The unexpected combination of force-free stability and mechanochemical activity for ferrocene raises the tantalizing question as to whether similar mechanochemical activity might be present in other metallocenes, and, if so, what features of metallocenes dictate their relative ability to act as mechanophores. We find that ruthenocene, in analogy to ferrocene, acts as a highly selective site of main chain scission despite the fact that it is even more inert. A comparison of ruthenocene and ferrocene reactivity provides insights as to the possible origins of metallocene mechanochemistry, including the relative importance of structural and thermodynamic parameters such as bond length and bond dissociation energy. These results suggest that metallocenes might be privileged mechanophores through which highly inert coordination complexes can be made dynamic in a stimuli-responsive fashion, offering potential opportunities in dynamic metallo-supramolecular materials and in mechanochemical routes to reactive intermediates that are otherwise difficult to obtain.

To further elucidate the mechanistic factors that dictate the mechanochemical activity of metallocenes, we used single molecule force spectroscopy to probe the mechanical reactivity of a series of ferrocenophanes. The force-coupled rate of cyclopentadiene (Cp) dissociation among various ferrocene derivatives varies by several orders of magnitude at ~1 nN, and the differences in reactivity are not correlated with ring strain in the reactants. Instead, a strong correlation with the extent of rotational realignment of the two Cp ligands is observed. Mechanophores with pulling points that are conformationally restricted by distal attachments to an eclipsing orientation are most labile, whereas conformationally unrestricted ligands reorient under force to effectively superpose “catch bond” like contributions onto the overall mechanically assisted dissociation reaction. The ability to program the mechanism of ferrocene dissociation to proceed through ligand “peeling”, as opposed to the more conventional “shearing” mechanism of the parent ferrocene, leads to enhanced macroscopic, multi-responsive behavior including mechanochromism and force-induced crosslinking in ferrocenophane-containing polymers.

Stress sensing at the molecular level in elastomer-based composites is crucial as it could prevent catastrophic material failure from early on and prolong its lifetime. The emerging field of polymer mechanochemistry provides such opportunity by incorporating force probes into the polymer network to generate stress-induced spectroscopic responses. Force probes that could offer irreversible responses when activated is ideal for easy characterization. Here, we designed a coumarin dimer mechanophore that could be covalently embedded into a polymer main chain. Its mechanochemical lability is quantified by competing internally with the ring opening of gem-dichlorocyclopropane (gDCC) and revealed to be comparable to a disulfide bond. The structure-reactivity relationship of coumarin dimer derivatives is investigated to show that the force sensitivity could be tuned by manipulating the force-free activation energy and mechanical coupling, which is further validated by computational modeling. Further, different coumarin dimer derivatives were embedded into the main chain and filler-matrix interface in a particle-filled rubber. The relative activation at these positions under a compressive loading was quantified and revealed that the stress-concentration effect at the heterointerface could result in one order of magnitude higher activation ratio compared to the polymer main chain.

Intrinsically 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.

The 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.

The 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.

The insertion of force-sensitive motifs (mechanophores) into polymer backbones provides a mechanism to induce forbidden reactions, stabilize transition state, and build intrinsic stress-responsive materials. Although polymer mechanochemistry has provided the basis for a variety of stress-responsive materials (e.g., those that are mechanochromic, mechanoluminescent, and mechanocatalytic, or that release small molecules or generate novel chemical reactions), many desirable stress-responsive behaviors have yet to be realized. We applied molecular-level design and synthesis to engineer stress-responsive materials that address several gaps in the prior polymer mechanochemistry toolbox:

Chapter 2 presents four studies of structure-reactivity relationships. First, regiochemical effects on mechanophore reactivity is quantified in the context of three spiropyran (SP) derivatives that are incorporated into polydimethylsiloxane (PDMS) elastomers. Under thermodynamic control, we find that the relative activation of the regioisomers is well correlated with the extent of mechanochemical coupling between the equilibrium reaction coordinate and the applied force, as quantified by computational modeling. Second, subtle differences in stereochemistry between two gem-monochlorocyclopropane (gMCC) stereoisomers (i.e., syn and anti, relative to the polymer attachment points through which force is delivered) lead to dramatic differences in reactivity and reactivity outcomes. The two gMCCs were embedded along a polymer backbone and their mechanochemical reactivities were quantified using single molecule force spectroscopy (SMFS). The mechanical ring-opening of syn-gMCC proceeds along an anti-Woodward-Hoffmann-Depuy pathway and exhibits significantly lower reactivity than anti-gMCC. Further, under tension applied through ultrasonication, the syn-gMCC isomer generates about 0.25 equivalents of HCl per ring-opening event, whereas ring opening of the anti-gMCC led to no detectable HCl under identical conditions. Third, we report the dependence of the mechanical strength of a C-S bond on the oxidation state of the S atom (i.e., the relative mechanical strength of sulfide, sulfoxide and sulfone). Ultrasonication of gem-dichlorocyclopropane (gDCC) copolymers of each sulfur-containing group reveals that their relative mechanical strengths follow the order: polysulfide ~ polysulfone > polysulfoxide. Finally, we demonstrate the effect of cyclic polymer structure/architecture on mechanophore activation along a polymer backbone. A multi-mechanophore, cyclic gDCC copolymer was prepared via ring expansion metathesis polymerization (REMP), and its mechanochemical response to ultrasonication was compared to a linear analog prepared using ring opening metathesis polymerization (ROMP). The cyclic polymer experiences less gDCC activation per fragmentation along its backbone than does the linear analog. This observation suggests conformational memory effects in the nascent cyclic polymer during elongation and fragmentation.

Chapter 3 introduces two new mechanophores that convert mechanical input into potentially useful chemical signals, enriching the available toolkit of stress-responsive behaviors. The first mechanophore is a 1,2-diaitidinone (DAO) based four-member ring that generates reactive isocyanate upon mechanical activation via pulsed ultrasonication; evidence for the generation of isocyanate is acquired by 1H NMR analysis and trapping experiments. We anticipate that this latent reactive isocyanate might lead to materials that heal or strengthen in response to a mechanical load. A second mechanophore is a new thermally stable and nonscissile mechanoacid that is based on a methoxy-substituted gDCC and that overcomes drawbacks present in previously reported mechanoacids. The introduction of the methoxy substituent not only facilitates the release of HCl as a result of gDCC ring opening (0.58 equivalents per activation), it significantly lowers the force necessary to trigger rapid ring opening, as evidenced by SMFS studies. The utility of this new mechanoacid is demonstrated in PDMS elastomers, where its mechanical activation leads to a strain-triggered color change in a pH-sensitive dye prior to fracture of the elastomer. The post-activation kinetics of coloration are used to demonstrate a new concept in mechanochromism, namely not only a spectroscopic indicator of whether and where a mechanical event has occurred, but when it occurred.

Chapter 4 describes how the well-known photoswitch azobenzene, when embedded into PDMS elastomers, can be used as a mechanochromic probe of the molecular forces present in strained bulk materials. Specifically, the cis-to-trans isomerization of azobenzene is accelerated under uniaxial tension. The kinetics are cleanly described by a single exponential first-order process (k = 2.7 × 10-5 s-1) in the absence of tension, but they become multi-exponential under constant strains of 40-90%. The complex kinetics can be reasonably modeled as a two-component process. The majority (~92%) process is slower and occurs with a rate constant that is similar to that of the unstrained system (k = 2.3–2.7 × 10-5 s-1), whereas the rate constant of the minority (~8%) process increases from k = 10.1 × 10-5 s-1 at 40% strain to k = 21.3 × 10-5 s-1 at 90% strain. Simple models of expected force-rate relationships suggest that the average force of tension per strand in the minority component ranges from 28 pN to 44 pN across strains of 40-90%.

Finally, in Chapter 5, polymer mechanochemistry is integrated into two degradable polymers to demonstrate new concepts in mechanically coupled degradable polymers. The unintentional scission of chemical degradable functionalities on the polymer backbone can diminish polymer properties, and we report a strategy that combats unintended degradation in polymers by combining two common degradation stimuli—mechanical and acid triggers—in an “AND gate” fashion. A cyclobutane (CB) mechanophore is used as a mechanical gate to regulate an acid-sensitive ketal that has been widely employed in acid degradable polymers. This gated ketal is further incorporated into the polymer backbone. In the presence of acid trigger alone, the pristine polymer retains its backbone integrity, and delivering high mechanical forces alone by ultrasonication degrades the polymer to an apparent limiting molecular weight of 28 kDa. The sequential treatment of ultrasonication followed by acid, however, leads to a further 11-fold decrease in molecular weight to 2.5 kDa. Experimental and computational evidence further indicate that the ungated ketal possesses mechanical strength that is commensurate with the conventional polymer backbones. Single molecule force spectroscopy (SMFS) reveals that the force necessary to activate the CB molecular gate on the timescale of 100 ms is approximately 2 nN. With this success in hand, we noted that mechanical-only polymer degradation is intrinsically limited to one chain scission per stretching event. To overcome this drawback, we integrate multiple copies of a [4.2.0]bicyclooctene (BCOE) based mechanophore into the polymer backbone. Mechanochemical remodeling of the polymer backbone occurs through the force-promoted forbidden ring-opening of BCOE, the product of which undergoes a subsequent, slower cascade lactonization that leads to a spontaneous, force-free decrease in average molecular weight to 4.4 kDa (from an initial molecular weight of over 120 kDa) over the course of 9 days.

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.

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.

Peptide-based vaccines have received growing interest due to their specificity and ability to limit off-target effects, and they are currently being explored toward a variety of infectious diseases and therapeutic targets. However, the efficacy and applicability of such epitope-based vaccines are currently limited by difficulties in predicting immunogenic epitopes in outbred populations and a reliance on carrier proteins and adjuvants that can cause pain and swelling. Current vaccine platforms are further limited in their ability to combine multiple different epitopes, making it difficult to adjust humoral and cellular responses systematically. A vaccine platform containing broadly reactive T-cell epitopes that boosts responses to co-delivered antigens with minimal inflammation could address these limitations. To that end, the focus of this dissertation was to create peptide epitopes that can be incorporated within a supramolecular nanomaterial platform, together acting as a nano-adjuvant, a term that we will use here to describe materials whose adjuvanting properties depend on their nanoscale structure. To achieve this, we took inspiration from a class of materials termed glatiramoids, which promote anti-inflammatory and TH2 immune responses. We created an immunomodulatory supramolecular nanomaterial system inspired by the randomized nature of glatiramoids termed KEYA-Q11. By creating a glatiramoid-like peptide library integrated within self-assembling Q11 nanofibers, numerous epitopes can be presented simultaneously along the nanofibers for maximum antigen presenting cell uptake and activation. The first half of this document (Chapters 3 and 4) describes how this nanomaterial increased immunogenicity of co-assembled epitopes while also creating a KEYA-specific non-inflammatory response to the randomized component. Additionally, capitalizing on the potential for KEYA-Q11 to amplify immune responses to co-assembled epitopes, this technology is applied in the second half of this document (Chapters 5 and 6) to an epitope-based influenza vaccine. Initially we designed and synthesized a self-assembling nanomaterial inspired by glatiramoids and evaluated its TH2 T-cell polarizing properties (Chapter 3). Glatiramoids raise strong, protective immune responses in patients and have been examined in a variety of contexts from Multiple Sclerosis to HIV. However, due to their randomized polysequence structure, it remains challenging to incorporate glatiramoids into other materials and strategies to optimize them for specific therapeutics. Therefore, we designed a polysequence peptide sequence and synthesized it onto the chemically defined, supramolecular Q11 nanofiber platform to straightforwardly titrate it into other nanomaterial formulations. This polysequence nanomaterial was termed KEYA-Q11 for the four amino acids, lysine, glutamic acid, tyrosine, and alanine, that comprise its structure. Due to the extensive number of possible KEYA sequences, multiple batches of KEYA-Q11 were first examined with an array of biophysical characterization techniques to confirm reproducible synthesis and assembly. The optimal number of polysequence amino acid additions was determined to be 20 amino acids as (KEYA)20Q11 could reliably be synthesized and raise strong Type 2/TH2/IL-4 immune responses. Moreover, by modulating the concentration of KEYA-Q11 in a Q11 immunization, the strength of KEYA-specific B-cell responses were similarly altered. KEYA modifications dramatically improved uptake of peptide nanofibers in vitro by antigen presenting cells and served as strong B-cell and T-cell epitopes in vivo, inducing a KEYA-specific Type 2/TH2/IL-4 phenotype. KEYA modifications also increased IL-4 production by T cells, extended the residence time of nanofibers, and decreased overall T cell expansion compared to unmodified nanofibers, further suggesting a TH2 T-cell response with minimal inflammation. Subsequently, we exploited the modularity of the self-assembling system to maximize application of KEYA-Q11 as a nanoscale adjuvant without inflammation (Chapter 4). Adjuvants are commonly required to raise strong immune responses to peptide therapeutics, but often induce swelling and pain at the injection site and typically drive immune phenotype. Relative to common adjuvants, KEYA-Q11 had no detectable injection site swelling and was more effective at raising humoral responses despite a genetically diverse in vivo population. Furthermore, when combined with peptide epitopes KEYA-Q11 augmented antibody production against co-assembled B-cell epitopes for cytokine TNF, D-chiral MMP cross linker, and a conserved segment of the M2 influenza protein, and increased T-cell stimulation specific to co-assembled T-cell epitopes PADRE and a conserved segment of the nucleoprotein of influenza. Likewise, when combined with the influenza surface protein hemagglutinin, KEYA modifications strengthened the resulting influenza-specific cellular immune responses. Augmented immune responses typically followed native epitope polarization, as in a co-assembly of KEYA-Q11 and the nucleoprotein epitope raised Type 2/TH2/IL4 producing KEYA-specific responses and magnified the Type 1/IFN producing nucleoprotein-specific responses that epitope would produce without an adjuvant, and thus using KEYA-Q11 as the adjuvant allowed for finer control over immune phenotype. Building on the success of KEYA-Q11 as a nano-scale adjuvant without inflammation, we utilized these properties to decrease the severity of influenza infection and provide broad protection via immunization with peptide epitopes (Chapters 5 and 6). Much of the current focus on influenza vaccines revolves around partial or whole proteins to induce broadly protective antibodies, while other have demonstrated cross-reactive T-cell responses are vital for heterologous protection. Conserved peptide epitopes have been discovered but typically are included with larger proteins and adjuvants to increase immunogenicity. Supramolecular assemblies based on the Q11 peptide system containing KEYA, a B-cell epitope from a conserved surface protein on influenza, and CD4+ and CD8+ T-cell epitopes from influenza nucleoprotein and polymerase acidic protein, respectively, raised strong immune responses against all three epitopes. Inclusion of the KEYA component in prophylactic immunizations with these materials significantly improved protection following a lethal influenza challenge. It has been established that while peptide-based immunotherapies can have finely directed specificity for chosen epitopes, they generally lack sufficient immunogenicity to provoke suitable immune responses. This new strategy for augmenting immune responses to peptide-based therapeutics, especially those employing nanomaterials, and especially for applications where non-inflammatory responses are prioritized, can be employed for a variety of potential applications in vaccine development, towards infectious diseases and towards non-infectious applications such as inflammatory autoimmune diseases, wound healing, or graft rejection. KEYA-Q11 is a unique fusion of two materials, a highly ordered system with a highly disordered system, and examination of this nanomaterial has provided valuable insight into both randomly polymerized structures and non-inflammatory nano-scale adjuvants.

Toughness, the measure of how much energy a material can absorb before rupture, is an important property of materials. It has been demonstrated that the toughness of a single polymer chain of gem-dihalocyclopropane (gDHC) functionalized polybutadiene (PB) is increased dramatically over PB alone, due to the mechanically triggered electrocyclic ring opening reaction of gDHC into 2,3-dibromoalkenes. This thesis explores whether this molecular mechanical property can also be manifested in bulk material properties. Crosslinked gem-dichlorocyclopropane (gDCC) embedded PB polymers were swollen in various solvents, and the resulting gels were mechanically deformed under tensile stress. Young's modulus and fracture toughness were compared among PBs with gDCC incorporated in the backbones and/or crosslinking positions. The results showed that the incorporation of gDCC does not measurably increase the fracture toughness of the crosslinked polymer gels. Neither NMR nor FT-IR characterization of the post-test samples revealed detectable activation of the gDCC in the crosslinked PB. Further experiments will be focused on optimizing the polymer structure and testing methods to more effectively transfer the macroscopic force to the mechanophore in the material and continuing exploring the correlation between molecular responses and changes in macroscopic properties.

Hybrid 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.