Molecular Tensile Machines: Anti-Arrhenius Cleavage of Disulfide Bonds

Understanding the Mechanochemistry of Mechano-Radicals in Self-Growth Materials by Single-Molecule Force Spectroscopy

Recent research on mechano-radicals has provided valuable insights into self-growth and adaptive responsive materials. Typically, mechanophores must remain inert in the absence of force but respond quickly to external tension before other linkages within the polymer network. Azo compounds exhibit promising combinations of mechanical stability and force-triggered reactivity, making them widely used as mechano-radicals in force-responsive materials. However, the activation conditions and behavior of azo compounds have yet to be quantitatively explored. In this study, we investigated the mechanical strength of three azo compounds using single-molecule force spectroscopy. Our results revealed that these compounds exhibit rupture forces ranging from ~500 to 1000 pN, at a loading rate of 3×104 pN s-1. Importantly, these mechanophores demonstrate distinct kinetic properties. Their unique mechanical attributes enable azo bond scission and free radical generation before causing major polymer backbone damage of entire material during polymer network deformation. This fundamental understanding of mechanophores holds significant promise for the development of self-growth materials and their related applications.

Using oriented external electric fields to manipulate rupture forces of mechanophores

Oriented external electric fields (OEEFs) can facilitate chemical reactions by selectively weakening bonds. This makes them a topic of interest in mechanochemistry, where mechanical force is used to rupture specific bonds in molecules. Using electronic structure calculations based on density functional theory (DFT), we investigate the effect of OEEFs on the mechanical force required to activate mechanophores. We demonstrate that OEEFs can greatly lower the rupture force of mechanophores, and that the degree of this effect highly depends on the angle relative to the mechanical force at which the field is being applied. The greatest lowering of the rupture force does not always occur at the point of perfect alignment between OEEF and the vector of mechanical force. Using natural bond orbital analysis, we show that mechanical force amplifies the effect that an OEEF has on the scissile bond of a mechanophore. By combining methods to simulate molecules in OEEFs with methods applying mechanical force, we present an effective tool for analyzing mechanophores in OEEFs and show that computationally determining optimal OEEFs for mechanophore activation can assist in the development of future experimental studies.

Coupled bond dynamics alters relaxation in polymers with multiple intrinsic dissociation rates

Dynamic networks containing multiple bond types within a continuous network grant engineers another design parameter - relative bond fraction - by which to tune storage and dissipation of mechanical energy. However, the mechanisms governing emergent properties are difficult to deduce experimentally. Therefore, we here employ a network model with prescribed fractions of dynamic and stable bonds to predict relaxation characteristics of hybrid networks. We find that during stress relaxation, predominantly dynamic networks can exhibit long-term moduli through conformationally inhibited relaxation of stable bonds due to exclusion interactions with neighboring chains. Meanwhile, predominantly stable networks exhibit minor relaxation through non-affine reconfiguration of dynamic bonds. Given this, we introduce a single fitting parameter, ξ, to Transient Network Theory via a coupled rule of mixture, that characterizes the extent of stable bond relaxation. Treating ξ as a fitting parameter, the coupled rule of mixture's predicted stress response not only agrees with the network model's, but also unveils likely micromechanical traits of gels hosting multiple bond dissociation timescales.

Sonopharmacology: controlling pharmacotherapy and diagnosis by ultrasound-induced polymer mechanochemistry

Active pharmaceutical ingredients are the most consequential and widely employed treatment in medicine although they suffer from many systematic limitations, particularly off-target activity and toxicity. To mitigate these effects, stimuli-responsive controlled delivery and release strategies for drugs are being developed. Fueled by the field of polymer mechanochemistry, recently new molecular technologies enabled the emergence of force as an unprecedented stimulus for this purpose by using ultrasound. In this research area, termed sonopharmacology, mechanophores bearing drug molecules are incorporated within biocompatible macromolecular scaffolds as preprogrammed, latent moieties. This review presents the novelties in controlling drug activation, monitoring, and release by ultrasound, while discussing the limitations and challenges for future developments.

Effect of functional anisotropy on the local dynamics of polymer grafted nanoparticles

End-functionalised polymer grafted nanoparticles (PGNs) form bonds when their coronas overlap. The bonded interactions between the overlapping PGNs depend on the energy of the bonds (U). In the present study, oscillatory deformation imposed on a simple system with interacting PGNs placed on the vertices of a triangle is employed to examine the local dynamics as a function of energy of the bonds and the frequency of oscillation relative to the characteristic rupture frequency, ω₀ = 2πν exp(-U/k_BT), of the bonds. In particular, the effect of functional anisotropy is studied by introducing bonds of two different energies between adjacent PGNs. A multicomponent model developed by Kadre and Iyer, Macromol. Theory Simul., 2021, 30, 2100005, that combines the features of effective interactions between PGNs, self-consistent field theory and master equation approach to study bond kinetics is employed to obtain the local dynamics. The resulting force-strain curves are found to exhibit a simple broken symmetry where F_x (γ,) ≠ -F_x (-γ,-) and F_y (γ,) ≠ F_y (-γ,-) in systems with functional anisotropy. Fourier analysis of the dynamic response reveals that functional anisotropy leads to finite even harmonic terms and systematic variation of both the elastic and dissipative response from that of the isotropic systems. Furthermore, the intra-cycle variations in the strain stiffening and shear thickening ratios obtained from the analysis indicate that functional anisotropy leads to anisotropic nonlinear response.

Influences of the Periodicity in Molecular Architecture on the Phase Diagrams and Microphase Transitions of the Janus Double-Brush Copolymer with a Loose Graft

The backbone of the Janus double-brush copolymer may break during long-term service, but whether this breakage affects the self-assembled phase state and microphase transitions of the material is still unknown. For the Janus double-brush copolymers with a periodicity in molecular architecture ranging from 1 to 10, the influences of the architectural periodicity on their phase diagrams and order–disorder transitions (ODT) were investigated by the self-consistent mean field theory (SCFT). In total, nine microphases with long-range order were found. By comparing the phase diagrams between copolymers of different periodicity, a decrease in periodicity or breakage along the copolymer backbone had nearly no influence on the phase diagrams unless the periodicity was too short to be smaller than 3. For copolymers with neutral backbones, a decrease in periodicity or breakage along the copolymer backbone reduced the critical segregation strengths of the whole copolymer at ODT. The equations for the critical segregation strengths at ODT, the architectural periodicity, and the volume fraction of the backbone were established for the Janus double-brush copolymers. The theoretical calculations were consistent with the previous theoretical, experimental, and simulation results.

Azo-Crosslinked Double-Network Hydrogels Enabling Highly Efficient Mechanoradical Generation

Double-network (DN) hydrogels have recently been demonstrated to generate numerous radicals by the homolytic bond scission of the brittle first network under the influence of an external force. The mechanoradicals thus generated can be utilized to trigger polymerization inside the gels, resulting in significant mechanical and functional improvements to the material. Although the concentration of mechanoradicals in DN gels is much higher than that in single-network hydrogels, a further increase in the mechanoradical concentration in DN gels will widen their application. In the present work, we incorporate an azoalkane crosslinker into the first network of DN gels. Compared with the traditional crosslinker N,N'-methylenebis(acrylamide), the azoalkane crosslinker causes a decrease in the yield stress but significantly increases the mechanoradical concentration of DN gels after stretching. In the azoalkane-crosslinked DN gels, the concentration of mechanoradicals can reach a maximum of ∼220 μM, which is 5 times that of the traditional crosslinker. In addition, DN gels with the azoalkane crosslinker show a much higher energy efficiency for mechanoradical generation. Interestingly, DN gels crosslinked by a mixture of azoalkane crosslinker and traditional crosslinker also exhibit excellent radical generation performance. The increase in the mechanoradical concentration accelerates polymerization and can broaden the application range of force-responsive DN gels to biomedical devices and soft robots.

Activation of Antibiotic-Grafted Polymer Brushes by Ultrasound

The ultrasound-mediated activation of drugs from macromolecular architectures using the principles of polymer mechanochemistry (sonopharmacology) is a promising strategy to gain spatiotemporal control over drug activity. Yet, conceptual challenges limit the applicability of this method. Especially low drug-loading content and low mechanochemical efficiency require the use of high carrier mass concentrations and prolonged exposure to ultrasound. Moreover, the activated drug is generally shielded by the hydrodynamic coil of the attached polymer fragment leading to a decreased drug potency. Here we present a carrier design for the ultrasound-induced activation of vancomycin, which is deactivated with its H-bond-complementary peptide target sequence. We show that the progression from mechanophore-centered linear chains to mechanophore-decorated polymer brushes increases drug-loading content, mechanochemical efficiency, and drug potency. These results may serve as a design guideline for future endeavors in the field of sonopharmacology.

Ultrasound-controlled drug release and drug activation for cancer therapy

Traditional chemotherapy suffers from severe toxicity and side effects that limit its maximum application in cancer therapy. To overcome this challenge, an ideal treatment strategy would be to selectively control the release or regulate the activity of drugs to minimize the undesirable toxicity. Recently, ultrasound (US)-responsive drug delivery systems (DDSs) have attracted significant attention due to the non-invasiveness, high tissue penetration depth, and spatiotemporal controllability of US. Moreover, the US-induced mechanical force has been proven to be a robust method to site-selectively rearrange or cleave bonds in mechanochemistry. This review describes the US-activated DDSs from the fundamental basics and aims to present a comprehensive summary of the current understanding of US-responsive DDSs for controlled drug release and drug activation. First, we summarize the typical mechanisms for US-responsive drug release and drug activation. Second, the main factors affecting the ultrasonic responsiveness of drug carriers are outlined. Furthermore, representative examples of US-controlled drug release and drug activation are discussed, emphasizing their novelty and design principles. Finally, the challenges and an outlook on this promising therapeutic strategy are discussed.

Self-Strengthening of Cross-Linked Elastomers via the Use of Dynamic Covalent Macrocyclic Mechanophores

The creation of polymeric materials that self-strengthen in response to a mechanical force is an important objective in the field of polymer chemistry. Here, the mechanochemical strengthening of cross-linked elastomers using macrocyclic mechanophores that contain a dynamic covalent disulfide bond is reported. Cross-linked poly(hexyl methacrylate) (CPHMA) polymers with macrocyclic mechanophores inserted at the cross-linking points were synthesized via free radical polymerization. Tensile and swelling tests showed that the addition of the macrocyclic mechanophores to the CPHMA polymers successfully impart them with self-strengthening functionality following compression, without the need for any additives such as monomers and modifiers, or any other stimuli.

Porous Molecular Capsules as Non-Polymeric Transducers of Mechanical Forces to Mechanophores

Mechanical grinding/milling can be regarded as historically the first technology for changing the properties of matter. Mechanically activated molecular units (mechanophores) can be present in various structures: polymers, macromolecules, or small molecules. However, only polymers have been reported to effectively transduce energy to mechanophores, which induces breakage of covalent bonds. In this paper, a second possibility is presented-molecular capsules as stress-sensitive units. Mechanochemical encapsulation of fullerenes in cystine-based covalent capsules indicates that complexation takes place in the solid state, despite the fact that the capsules do not possess large enough entrance portals. By using a set of solvent-free MALDI (sf-MALDI) and solid-state NMR (ss-NMR) experiments, it has been proven that encapsulation proceeds during milling and in this process hydrazones and disulfides get activated for breakage, exchange, and re-forming. The capsules are porous and therefore prone to collapse under solvent-free conditions and their conformational rigidity promotes the collapse by the breaking of covalent bonds.

Advanced Materials by Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) has been successfully employed for the preparation of various advanced materials with controlled architecture. New catalysts with strongly enhanced activity permit more environmentally benign ATRP procedures using ppm levels of catalyst. Precise control over polymer composition, topology, and incorporation of site specific functionality enables synthesis of well-defined gradient, block, comb copolymers, polymers with (hyper)branched structures including stars, densely grafted molecular brushes or networks, as well as inorganic-organic hybrid materials and bioconjugates. Examples of specific applications of functional materials include thermoplastic elastomers, nanostructured carbons, surfactants, dispersants, functionalized surfaces, and biorelated materials.

Stereodynamic control of star-epoxy/anhydride crosslinking actuated by liquid-crystalline phase transitions

The epoxy/anhydride copolymerization kinetics of an original star-epoxy monomer (TriaEP) was explored in dynamic heating mode using a series of isoconversional methods. Negative values of the apparent activation energy (E_α) related to an anti-Arrhenius behavior were observed. The transition from Arrhenius to anti-Arrhenius behavior and vice versa depending on the E_α of polymerization was correlated with the dynamics of mesophasic fall-in/fall-out events, physically induced transition (PIT) and chemically induced transition (CIT). This self-assembly phenomenon induces the generation of an anisotropic crosslinked architecture exhibiting both nematic discotic (N_D) and nematic columnar (N_C) organization. Particular emphasis was placed on evaluating the juxtaposition/contribution of the liquid-crystalline transitions to crosslinking, considering both the reaction dynamics and the macromolecular vision.

Following Homolytic Mechanochemical Kinetics with a Pyrenyl Nitrone Spin Trap

The mechanochemical stability of a polymer is a fundamental parameter when choosing the ideal material for many different uses where mechanical loading may induce molecular weight reduction. The use of mechanophores has significantly improved the detection of mechanochemical reaction, but their incorporation to different polymers can be synthetically challenging. Alternatively, we return to the old strategy of using spin traps to quantify the radicals produced as a consequence of mechanochemical homolytic bond scission events. Several new spin traps have been developed in recent decades, and pyrenyl nitrones have been shown to effectively bind radicals, providing a spectroscopic methodology to follow radical concentration. Here we demonstrate the use of these probes as excellent tools to follow mechanochemical chain scission.

Fundamental biomaterial properties of tough glycosaminoglycan-containing double network hydrogels newly developed using the molecular stent method

The purpose of this study was to clarify fundamental mechanical properties and biological responses of the sodium hyaluronate-containing double network (HA-DN) gel and chondroitin sulfate-containing double network (CS-DN) gel, which were newly developed using the molecular stent method. This study discovered the following facts. First, these hydrogels had high mechanical performance comparable to the native cartilage tissue, and the mechanical properties were not affected by immersion in the saline solution for 12weeks. Secondly, the mechanical properties of the CS-DN gel were not significantly reduced at 12weeks in vivo, while the mechanical properties of the HA-DN gel were significantly deteriorated at 6weeks. Thirdly, the degree of inflammation around the HA-DN gel was the same as that around the negative control. The CS-DN gel showed a mild but significant foreign body reaction, which was significantly greater than the negative control and less than the positive control at 1week, while the inflammation was reduced to the same level as the negative control at 4 and 6weeks. Fourthly, these gels induced differentiation of the ATDC5 cells into chondrocytes in the culture with the insulin-free maintenance medium. These findings suggest that these tough hydrogels are potential biomaterials for future application to therapeutic implants such as artificial cartilage.

STATEMENT OF SIGNIFICANCE

The present study reported fundamental biomaterial properties of the sodium hyaluronate-containing double network (HA-DN) gel and chondroitin sulfate-containing double network (CS-DN) gel, which were newly developed using the molecular stent method. Both the HA- and CS-DN gels had high mechanical properties comparable to the cartilage tissue and showed the ability to induce chondrogenic differentiation of ATDC5 cells in vitro. They are potential biomaterials that may meet the requirements of artificial cartilage concerning the material properties. Further, these DN gels can be also applied to the implantable inducer for cell-free cartilage regeneration therapy.

Force-Induced Reversal of β-Eliminations: Stressed Disulfide Bonds in Alkaline Solution

Understanding the impact of tensile forces on disulfide bond cleavage is not only crucial to the breaking of cross-linkers in vulcanized materials such as strained rubber, but also to the regulation of protein activity by disulfide switches. By using ab initio simulations in the condensed phase, we investigated the response of disulfide cleavage by β-elimination to mechanical stress. We reveal that the rate-determining first step of the thermal reaction, which is the abstraction of the β-proton, is insensitive to external forces. However, forces larger than about 1 nN were found to reshape the free-energy landscape of the reaction so dramatically that a second channel is created, where the order of the reaction steps is reversed, turning β-deprotonation into a barrier-free follow-up process to C-S cleavage. This transforms a slow and force-independent process with second-order kinetics into a unimolecular reaction that is greatly accelerated by mechanical forces.

Polymer mechanochemistry: from destructive to productive

When one brings "polymeric materials" and "mechanical action" into the same conversation, the topic of this discussion might naturally focus on everyday circumstances such as failure of fibers, fatigue of composites, abrasion of coatings, etc. This intuitive viewpoint reflects the historic consensus in both academia and industry that mechanically induced chemical changes are destructive, leading to polymer degradation that limits materials lifetime on both macroscopic and molecular levels. In the 1930s, Staudinger observed mechanical degradation of polymers, and Melville later discovered that polymer chain scission caused the degradation. Inspired by these historical observations, we sought to redirect the destructive mechanical energy to a productive form that enables mechanoresponsive functions. In this Account, we provide a personal perspective on the origin, barriers, developments, and key advancements of polymer mechanochemistry. We revisit the seminal events that offered molecular-level insights into the mechanochemical behavior of polymers and influenced our thinking. We also highlight the milestones achieved by our group along with the contributions from key comrades at the frontier of this field. We present a workflow for the design, evaluation, and development of new "mechanophores", a term that has come to mean a molecular unit that chemically responds in a selective manner to a mechanical perturbation. We discuss the significance of computation in identifying pairs of points on the mechanophore that promote stretch-induced activation. Attaching polymer chains to the mechanophore at the most sensitive pair and locating the mechanophore near the center of a linear polymer are thought to maximize the efficiency of mechanical-to-chemical energy transduction. We also emphasize the importance of control experiments to validate mechanochemical transformations, both in solution and in the solid state, to differentiate "mechanical" from "thermal" activation. This Account offers our first-hand perspective of the change-in-thinking in polymer mechanochemistry from "destructive" to "productive" and looks at future advances that will stimulate this growing field.

Molecular Mechanochemistry: Engineering and Implications of Inherently Strained Architectures

Mechanical activation of chemical bonds is usually achieved by applying external forces. However, nearly all molecules exhibit inherent strain of their chemical bonds and angles as a result of constraints imposed by covalent bonding and interactions with the surrounding environment. Particularly strong deformation of bonds and angles is observed in hyperbranched macromolecules caused by steric repulsion of densely grafted polymer branches. In addition to the tension amplification, macromolecular architecture allows for accurate control of strain distribution, which enables focusing of the internal mechanical tension to specific chemical bonds and angles. As such, chemically identical bonds in self-strained macromolecules become physically distinct because the difference in bond tension leads to the corresponding difference in the electronic structure and chemical reactivity of individual bonds within the same macromolecule. In this review, we outline different approaches to the design of strained macromolecules along with physical principles of tension management, including generation, amplification, and focusing of mechanical tension at specific chemical bonds.

Shifting Electronic Structure by Inherent Tension in Molecular Bottlebrushes with Polythiophene Backbones

Bottlebrush macromolecules can be regarded as molecular tensile machines, where tension is self-generated along the backbone due to steric repulsion between densely grafted side chains. This intrinsic tension is amplified upon adsorption of bottlebrush molecules onto a substrate and increases with grafting density, side chain length, and strength of adhesion to the substrate. To investigate the effects of tension on the electronic structure of polythiophene (PT), bottlebrush macromolecules were prepared by grafting poly(n-butyl acrylate) (PBA) side chains from PT macroinitiators by atom transfer radical polymerization (ATRP). The fluorescence spectra of submonolayers of PT bottlebrushes were measured on a Langmuir-Blodgett (LB) trough with the backbone tension adjusted by controlling the side-chain length, surface pressure, and chemical composition of a substrate. The wavelength of maximum emission has initially red-shifted, followed by a blue-shift as the backbone tension increases from 0 to 2.5 nN, which agrees with DFT calculations. The red-shift is ascribed to an increase in the conjugation length due to the extension of the PT backbone at lower force regime (0-1.0 nN), while the blue-shift is attributed to deformations of bond lengths and angles in the backbone at higher force regime (1.0-2.5 nN).

Modeling polymer grafted nanoparticle networks reinforced by high-strength chains

Using a multi-scale computational approach, we determine the effect of introducing a small fraction of high-strength connections between cross-linked nanoparticles. The nanoparticles' rigid cores are decorated with a corona of grafted polymers, which contain reactive functional groups at the chain ends. With the overlap of neighboring coronas, these reactive groups can form weak labile bonds, which can reform after breakage, or stronger bonds, which rupture irreversibly and thus, the nanoparticles are interconnected by dual cross-links. We show that this network can be reinforced by the addition of high-strength connections, which model polymer arms bound together by bonds with energies on the order of 100 kBT. We demonstrate that in the course of these simulations, these high-strength connections can be treated as unbreakable chains. By subjecting networks with a random distribution of the unbreakable chains to tensile deformation at a constant strain-rate, we determine the distribution of strain at break and toughness. With even a small amount of unbreakable chains, the nanoparticle networks can survive strains far greater than the networks without these connections. Furthermore, networks containing the high-strength connections tend to form long, thin threads, which enable a larger strain at break. The findings provide guidelines for creating polymer grafted nanoparticles networks that could show remarkable strength and ductility.

Synthesis and properties of heterografted toothbrush-like copolymers with alternating PEG and PCL grafts and tunable RAFT-generated segments

Novel (A-g-D)(B-alt-C)_mD-type heterografted toothbrush-like copolymers with great potential in smart drug delivery systems and thermo-responsive surface materials are investigated.