The many flavours of mechanochemistry and its plausible conceptual underpinnings

Response of a Tethered Zn-Bis-Terpyridine Complex to an External Mechanical Force: A Computational Study of the Roles of the Tether and Solvent

Polymeric materials containing weak sacrificial bonds can be designed to engineer self-healing and higher toughness, improve melt-processing, or facilitate recycling. However, they usually exhibit a lower mechanical strength and are subject to creep and fatigue. For improving their design, it is of interest to investigate their mechanical response on the molecular scale. We report on a computational study of the response to a mechanical external force of a Zinc(II) bis-methyl phenyl-terpyridine ([Zn-bis-Terpy]2+) complex included in a cyclic poly(ethylene glycol) (PEG) tether designed to maintain the two partners of the metal-ligand bonds in close proximity after the rupture of the complex. The mechanical response is studied as a function of the pulling distortion by using the CoGEF isometric protocol, including interactions with a polar solvent (DMSO). We show that tethering favors recombination but destabilizes the complex before bond rupture because of the interactions of the PEG units with Terpy ligands. Similar effects occur between the DMSO molecules and the complex. Our results on the molecular scale are relevant for single-molecule force spectroscopy experiments. Interactions of the complex with solvent molecules and/or with the tether lead to a dispersion of the rupture force values, which could obscure the interpretation of the results.

Switchable Mechanochromic Poly(methyl acrylate) Enabled by Dianthryl Disilane

Mechanochromic polymers have potential in visualization of microscopic damages in materials, and the integration of non-invasive switching into mechanochromic polymers can enable rich on-demand functionality of polymer materials. In this work, we report the development of a mechanoresponsive fluorescent polymer with photogated properties, comprising poly(methyl acrylate) (PMA) and the photoswitchable mechanophore S2A2, which features two anthracene groups linked by a disilane spacer. The mechanochromic polymer PMA-S2A2 exhibits ratiometric fluorescence response upon mechanical deformation, which arises from the monomer and excimer emission ratio of anthracene controlled by the polymer strain. The mechanochromic property of the polymer can be switched off by irradiation with 365 nm UV light which induces [4 + 4] cycloaddition of the anthracene groups between and within the polymer chains. Importantly, the anthracene dimers can be reversibly dissociated in situ within the polymer film by irradiation with 254 nm UV, restoring the mechanochromic function. Furthermore, intermolecular photodimerization of anthracene increases the molecular weight and forms an entanglement network, significantly enhancing the material toughness. This reversible photocontrolled switch design offers a novel strategy for developing mechanochromic materials with modulated responses and mechanical properties.

Rapid self-strengthening in double-network hydrogels triggered by bond scission

The scission of chemical bonds in materials can lead to catastrophic failure, with weak bonds typically undermining the materials' strength. Here we demonstrate how weak bonds can be leveraged to achieve self-strengthening in polymer network materials. These weak sacrificial bonds trigger mechanochemical reactions, forming new networks rapidly enough to reinforce the material during deformation and significantly improve crack resistance. This rapid strengthening exhibits strong rate dependence, dictated by the interplay between bond breaking and the kinetics of force-induced network formation. As the network formation is generally applicable to diverse monomers and crosslinkers with different kinetics, a wide range of mechanical properties can be obtained. These findings may inspire the design of tough polymer materials with on-demand, rate-dependent mechanical behaviours through mechanochemistry, broadening their applications across various fields.

Oxygen, light, and mechanical force mediated radical polymerization toward precision polymer synthesis

The synthesis of polymers with well-defined composition, architecture, and functionality has long been a focal area of research in the field of polymer chemistry. The advancement of controlled radical polymerization (CRP) has facilitated the synthesis of precise polymers, which are endowed with new properties and functionalities, thereby exhibiting a wide range of applications. However, radical polymerization faces several challenges, such as oxygen intolerance, and common thermal initiation methods may lead to side reactions and depolymerization. Therefore, we have developed some oxygen-tolerant systems that directly utilize oxygen for initiating and regulating polymerization. We utilize oxygen/alkylborane as an effective radical initiator system in the polymerization, and also as a reductant for the removal of polymer chain ends. Moreover, we employ the gentler photoinduced CRP to circumvent side reactions caused by high temperatures and achieve temporal and spatial control over the polymerization. To enhance the penetration of the light source for polymerization, we have developed near-infrared light-induced atom transfer radical polymerization. Additionally, we have extended photochemistry to reversible addition-fragmentation chain transfer polymerization involving ion-pair inner-sphere electron transfer mechanism, metal-free radical hydrosilylation polymerization, as well as carbene-mediated polymer modification through C-H activation and insertion mechanisms. Furthermore, we propose a new method for polymerization initiation synergistically triggered by oxygen and mechanical energy. This review not only showcases the current advancements in CRP but also outlines future directions, such as the potential for 3D printing and surface coatings, and the exploration of new heteroatom radical polymerizations. By expanding the boundaries of polymer synthesis, these innovations could lead to the creation of new materials with enhanced functionality and applications.

On the Valorisation of Chitin-Derived Furans by Milling

Chitin-derived furans offer a sustainable alternative feedstock for nitrogen appended aromatic compounds. Herein, we address the challenge of using chitin-derived furans, 3-acetamido-5-acetylfuran (3A5AF) and 3-acetamido-5-furfural aldehyde (3A5F), to favour the formation of exo Diels-Alder adducts and 4-acetylaminophthalimides respectively, using a mechanochemical ball-milling technique. Mechanochemical activation is explored through the synthesis of 7-oxa-norbornene backbones with novel substitution pattern from 3A5AF in yields up to 77 % and improved exo:endo selectivity compared to solution-phase reactions. The synthesis of 4-acetylaminophthalimides from 3A5F in yields up to 79 % is also showcased from hydrazone derivatives.

Autonomic Self-Healing of Polymers: Mechanisms, Applications, and Challenges

Mechanical loads degrade polymers by enabling mechanochemical fragmentation of macromolecular backbones. In most polymers, this fragmentation is irreversible, and its accumulation leads to the appearance and propagation of cracks and, ultimately, fracture of the material. Self-healing describes a diverse and loosely defined collection of approaches that aim at reversing this damage. Most reported synthetic self-healing polymers are non-autonomic, i.e., they require the user to input free energy (in the form of heat, irradiation, or reagents) into the damaged material to initiate its repair. Here, we critically discuss emerging chemical approaches to autonomic self-healing that rely on regenerating the density of load-bearing, dissociatively-inert backbone bonds either after the load on a partially damaged material dissipated or continuously and in competition with the mechanochemically driven loss of backbones in the loaded material. We group the reported chemistries into three broad types whose analysis yields a set of criteria against which the potential of a prospective approach to yield practically relevant self-healing polymers can be assessed quantitatively. Our analysis suggests that the direct chain-to-chain addition in mechanically loaded unsaturated polyolefins is the most promising chemical strategy reported to date to achieve autonomic synchronous self-healing of practical significance.

Supramolecular Modulation for Selective Mechanochemical Iron-Catalyzed Olefin Oxidation

The development of a mechanochemical Fe-catalyzed Wacker oxidation of olefins with a sustainable and benign procedure holds significant promise for industrial applications. However, navigating the intricate interactions inherent in ball-milling conditions to fine-tune reaction selectivity remains a formidable challenge. Herein, leveraging the dispersive and/or trapping properties of cyclodextrins, an innovative mechanochemical approach is developed through the integration of cyclodextrins into a Fe-catalyzed system, enabling a streamlined Wacker oxidation process from simple and/or commercially available alkenes. Our efforts have yielded optimized mechanochemical conditions demonstrating exceptional reactivity and selectivity in generating a diverse array of ketone products, markedly enhancing catalytic efficiency compared to conventional batch methods. Mechanistic investigations have revealed a predominantly Markovnikov-selective catalytic cycle, effectively minimizing undesired alcohol formation, hydrogenation, and the other competing pathways, boosting both reaction yield and selectivity.

Sustainable and solvent-free synthesis of molecules of pharmaceutical importance by ball milling

The solvent-free mechanochemical reactions under ball milling have emerged as a promising alternative to traditional solution-based chemistry. This approach not only eliminates the necessity for large quantities of solvents and minimizes waste production, but it also facilitates a unique reaction environment that enables strategies, reactions, and compound syntheses that are typically unattainable in solution. This solvent-less synthetic strategy under ball-milling has been well employed in synthetic organic chemistry in accessing various potential organic molecules including pharmaceutically important molecules and pharmaceuticals or drug-molecules. This review highlights the potential of ball milling in the synthesis of pharmaceutically important classes of molecules without using any solvent (solvent-free conditions).

Mechanochemical Transformations of Pharmaceutical Cocrystals: Polymorphs and Coformer Exchange

Transformations of solid samples under solvent-free or minimal solvent conditions set the future trend and define a modern strategy for the production of new materials. Of the various technologies tested in recent years, the mechanochemical approach seems to be the most promising for economic and ecological reasons. The aim of this review article is to present the current state of art in solid state research on binary systems, which have found numerous applications in the pharmaceutical and materials science industries. This article is divided into three sections. In the first part, we describe the new equipment improvements, which include the innovative application of thermo-mechanochemistry, sono-mechanochemistry, photo-mechanochemistry, electro-mechanochemistry, as well as resonant acoustic mixing (RAM), and transformation under high-speed sample spinning ("SpeedMixing"). A brief description of techniques dedicated to ex-situ and in-situ studies of progress and the mechanism of solid matter transformation (PXRD, FTIR, Raman and NMR spectroscopy) is presented. In the second section, we discuss the problem of cocrystal polymorphism highlighting the issue related with correlation between mechanochemical parameters (time, temperature, energy, molar ratio, solvent used as a liquid assistant, surface energy, crystal size, crystal shape) and preference for the formation of requested polymorph. The last part is devoted to the description of the processes of coformer exchange in binary systems forced by mechanical and/or thermal stimuli. The influence of the thermodynamic factor on the selection of the best-suited partner for the formation of a two-component stable structure is presented.

Accelerated Mechanochemical Bond Scission and Stabilization against Heat and Light in Carbamoyloxime Mechanophores

Current approaches to the discovery of mechanochemical reactions in polymers are limited by the interconnection of the zero-force and force-modified potential energy surfaces since most mechanochemical reactions are force-biased thermal reactions. Here, carbamoyloximes are developed as a mechanophore class in which the mechanochemical reaction rates counterintuitively increase together with the thermal stability. All carbamoyloxime mechanophores undergo force-induced homolytic bond scission at the N-O bond, and their mechanochemical scission rate increases with the degree of substitution on the α-substituent. Yet, carbamoylaldoximes react to both heat and light with a pericyclic syn elimination, while carbamoylketoximes undergo thermal decomposition at high temperature and photochemical homolytic scission only from the triplet state. Thereby, the mechanochemical and thermal reaction trajectories are separated, and the thermal stability increases alongside the mechanochemical reaction kinetics. This approach may play an important role in the future of systematic mechanochemical reaction discovery.

Nonaqueous Contact-Electro-Chemistry via Triboelectric Charge

Mechanochemistry revolutionizes traditional reactions through mechanical stimulation, but its reaction efficiency is limited. Recent advancements in utilizing triboelectric charge from liquid-solid contact electrification (CE) have demonstrated significant potential in improving the reaction efficiency. However, its efficacy remains constrained by interfacial electrical double-layer screening in aqueous solutions. This study pioneered chemistry in nonaqueous systems via CE for catalysis and luminescence. Density functional theory simulations and experiments revealed varying electron transfer capabilities and chemoselectivity of CE across different solvents. Phenol degradation via CE in dimethyl sulfoxide (DMSO) exhibited a rate over 40 times faster than that of traditional mechano-driven chemistry. A more intuitive comparison revealed that CE degradation of phenol in DMSO exhibits a 30-fold rate improvement compared to deionized water, where the degradation remains incomplete. Luminol oxidation by radicals generated solely via CE in DMSO eliminates the dependence on traditional catalysts and side reactions, establishing a pure and simple system for investigating the reaction mechanisms. A high and stable luminescence characteristic was maintained for 3 months, enhancing the imaging accuracy and stability exponentially. This study underscores the impact of triboelectric charge on reaction efficiency and chemoselectivity, establishing a new paradigm in nonmetal catalysis, mechanoluminescence, and providing profound insights into reaction kinetics.

In-situ monitoring of polymer mechanochemistry: what can be learned from small molecule systems

Using mechanical energy to drive chemical transformations is an exciting prospect to improve the sustainability of chemical reactions and to produce products not achievable by more traditional methods. In-situ monitoring of reaction pathways and chemical transformations is vital to deliver the reproducible results required for scale up to realize the potential of mechanochemistry beyond the chemistry lab. This mini review will discuss the recent advances in in-situ monitoring of ball milling and polymer mechanochemistry, highlighting the potential for shared knowledge for scale up.

Mechanochemical Diversity in Block Copolymers

Covalent polymer chains are known to undergo mechanochemical events when subjected to mechanical forces. Such force-coupled reactions, like C-C bond scission in homopolymers, typically occur in a non-selective manner but with a higher probability at the mid-chain. In contrast, block copolymers (BCPs), composed of two or more chemically distinct chains linked by covalent bonds, have recently been shown to exhibit significantly different mechanochemical reactivities and selectivities. These differences may be attributable to the atypical conformations adopted by their chains, compared to the regular random coil. Beyond individual molecules, when BCPs self-assemble into ordered aggregates in solution, the non-covalent interactions between the chains lead to meaningful acceleration in the activation of embedded force-sensitive motifs. Furthermore, the microphase segregation of BCPs in bulk creates periodically dispersed polydomains, locking the blocks in specific conformations which have also been shown to affect their mechanochemical reactivity, with different morphologies influencing reactivity to varying extents. This review summarizes the studies of mechanochemistry in BCPs over the past two decades, from the molecular level to assemblies, and up to bulk materials.

Green synthesis of a superhydrophobic porous organic polymer for the removal of volatile organic compounds at high humidity

Superhydrophobic porous organic polymers are potential sorbents for volatile organic compounds (VOCs) pollution control by suppressing the competition of water molecules on their surfaces. However, the synthesis of superhydrophobic reagents usually requires large amounts of organic solvents and a long reaction time (≥ 24 h). Herein, a green mechanochemical method was developed to synthesize a superhydrophobic polymer (MSHMP-1) with the advantages of using a small amount of organic solvents (5 mL/g) and a short reaction time (2 h). Meanwhile, MSHMP-1 with a water contact angle (WCA) of 162° exhibited a dramatically rich pore structure as revealed by its specific surface area (SSA) of 1780 m2/g. The decrease in the adsorption of benzene on MSHMP-1 due to the competition of water molecules, even at relative humidity of 90 %, was nonsignificant (<10 %), indicating the great application potential of MSHMP-1 in hydrophobic adsorption. Moreover, the adsorption capacity of MSHMP-1 was maintained after at least five adsorption-desorption cycles. Therefore, MSHMP-1 can be a remarkable adsorbent for the removal of hazardous VOCs, especially at high humidity levels.

Mechanochemistry for Organic and Inorganic Synthesis

In recent years, mechanochemistry has become an innovative and sustainable alternative to traditional solvent-based synthesis. Mechanochemistry rapidly expanded across a wide range of chemistry fields, including diverse organic compounds and active pharmaceutical ingredients, coordination compounds, organometallic complexes, main group frameworks, and technologically relevant materials. This Review aims to highlight recent advancements and accomplishments in mechanochemistry, underscoring its potential as a viable and eco-friendly alternative to conventional solution-based methods in the field of synthetic chemistry.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

Productive chemistry induced by mechanochemically generated macroradicals

Large or repeated mechanical loads degrade polymeric materials by accelerating chain fragmentation. This mechanochemical backbone fracture usually occurs by homolysis of otherwise inert C-C, C-O and C-S bonds, generating highly reactive macroradicals. Because backbone fracture is detrimental on its own and the resulting macroradicals can initiate damaging reaction cascades, a major thrust in contemporary polymer mechanochemistry is to suppress it, usually by mechanochemical release of "hidden length" that dissipates local molecular strain. Here we summarize an emerging complementary strategy of channelling mechanochemically generated macroradicals in reaction cascades to form new load-bearing chemical bonds, which enables local self-healing or self-strengthening, and/or to generate mechanofluorescence, which could yield detailed quantitative molecular understanding of how material-failure-inducing macroscopic mechanical loads distribute across the network. We aim to identify generalizable lessons derivable from the reported implementations of this strategy and outline the key challenges in adapting it to diverse polymeric materials and loading scenarios.

Improving reproducibility through condition-based sensitivity assessments: application, advancement and prospect

The fluctuating reproducibility of scientific reports presents a well-recognised issue, frequently stemming from insufficient standardisation, transparency and a lack of information in scientific publications. Consequently, the incorporation of newly developed synthetic methods into practical applications often occurs at a slow rate. In recent years, various efforts have been made to analyse the sensitivity of chemical methodologies and the variation in quantitative outcome observed across different laboratory environments. For today's chemists, determining the key factors that really matter for a reaction's outcome from all the different aspects of chemical methodology can be a challenging task. In response, we provide a detailed examination and customised recommendations surrounding the sensitivity screen, offering a comprehensive assessment of various strategies and exploring their diverse applications by research groups to improve the practicality of their methodologies.

Mechanochemistry: Fundamental Principles and Applications

Mechanochemistry is an emerging research field at the interface of physics, mechanics, materials science, and chemistry. Complementary to traditional activation methods in chemistry, such as heat, electricity, and light, mechanochemistry focuses on the activation of chemical reactions by directly or indirectly applying mechanical forces. It has evolved as a powerful tool for controlling chemical reactions in solid state systems, sensing and responding to stresses in polymer materials, regulating interfacial adhesions, and stimulating biological processes. By combining theoretical approaches, simulations and experimental techniques, researchers have gained intricate insights into the mechanisms underlying mechanochemistry. In this review, the physical chemistry principles underpinning mechanochemistry are elucidated and a comprehensive overview of recent significant achievements in the discovery of mechanically responsive chemical processes is provided, with a particular emphasis on their applications in materials science. Additionally, The perspectives and insights into potential future directions for this exciting research field are offered.

Force Transduction Through Distant Force-Bearing Regioisomeric Linkages Affects the Mechanochemical Reactivity of Cyclobutane

Fundamental understanding of mechanochemical reactivity is important for designing new mechanophores. Besides the core structure of mechanophores, substituents on a mechanophore can affect its mechanochemical reactivity through electronic stabilization of the intermediate or effectiveness of force transduction from the polymer backbone to the mechanophore. The latter factor represents a unique mechanical effect in considering polymer mechanochemistry. Here, we show that regioisomeric linkage that is not directly adjacent to the first cleaving bond in cyclobutane can still significantly affect the mechanochemical reactivity of the mechanophore. We synthesized three non-scissile 1,2-diphenyl cyclobutanes, varying their linkage to the polymer backbone via the o, m, or p-position of the diphenyl substituents. Even though the regioisomers share the same substituted cyclobutane core structure and similar electronic stabilization of the diradical intermediate from cleaving the first C-C bond, the p isomer exhibited significantly higher mechanochemical reactivity than the o and m isomers. The observed difference in reactivity can be rationalized as the much more effective force transduction to the scissile bond through the p-position than the other two substitution positions. These findings point to the importance of considering force-bearing linkages that are more distant from the bond to be cleaved when incorporating mechanophores into polymer backbones.

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.

Mechanical scission of a knotted polymer

Molecular knots and entanglements form randomly and spontaneously in both biological and synthetic polymer chains. It is known that macroscopic materials, such as ropes, are substantially weakened by the presence of knots, but until now it has been unclear whether similar behaviour occurs on a molecular level. Here we show that the presence of a well-defined overhand knot in a polymer chain substantially increases the rate of scission of the polymer under tension (≥2.6× faster) in solution, because deformation of the polymer backbone induced by the tightening knot activates otherwise unreactive covalent bonds. The fragments formed upon severing of the knotted chain differ from those that arise from cleavage of a similar, but unknotted, polymer. Our solution studies provide experimental evidence that knotting can contribute to higher mechanical scission rates of polymers. It also demonstrates that entanglement design can be used to generate mechanophores that are among the most reactive described to date, providing opportunities to increase the reactivity of otherwise inert functional groups.

Mechanochemistry Drives Alkene Difunctionalization via Radical Ligand Transfer and Electron Catalysis

A general and modular protocol is reported for olefin difunctionalization through mechanochemistry, facilitated by cooperative radical ligand transfer (RLT) and electron catalysis. Utilizing mechanochemical force and catalytic amounts of 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO), ferric nitrate can leverage nitryl radicals, transfer nitrooxy-functional group via RLT, and mediate an electron catalysis cycle under room temperature. A diverse range of activated and unactivated alkenes exhibited chemo- and regioselective 1,2-nitronitrooxylation under solvent-free or solvent-less conditions, showcasing excellent functional group tolerance. Mechanistic studies indicated a significant impact of mechanochemistry and highlighted the radical nature of this nitrative difunctionalization process.

In Vivo Polymer Mechanochemistry with Polynucleotides

Polymer mechanochemistry utilizes mechanical force to activate latent functionalities in macromolecules and widely relies on ultrasonication techniques. Fundamental constraints of frequency and power intensity have prohibited the application of the polymer mechanochemistry principles in a biomedical context up to now, although medical ultrasound is a clinically established modality. Here, a universal polynucleotide framework is presented that allows the binding and release of therapeutic oligonucleotides, both DNA- and RNA-based, as cargo by biocompatible medical imaging ultrasound. It is shown that the high molar mass, colloidal assembly, and a distinct mechanochemical mechanism enable the force-induced release of cargo and subsequent activation of biological function in vitro and in vivo. Thereby, this work introduces a platform for the exploration of biological questions and therapeutics development steered by mechanical force.

Physicochemical reactions in e-waste recycling

Electronic waste (e-waste) recycling is becoming a global concern owing to its immense quantity, hazardous character and the potential loss of valuable metals. The many processes involved in e-waste recycling stem from a mixture of physicochemical reactions, and understanding the principles of these reactions can lead to more efficient recycling methods. In this Review, we discuss the principles behind photochemistry, thermochemistry, mechanochemistry, electrochemistry and sonochemistry for metal recovery, polymer decomposition and pollutant elimination from e-waste. We also discuss how these processes induce or improve reaction rates, selectivity and controllability of e-waste recycling based on thermodynamics and kinetics, free radicals, chemical bond energy, electrical potential regulation and more. Lastly, key factors, limitations and suggestions for improvements of these physicochemical reactions for e-waste recycling are highlighted, wherein we also indicate possible research directions for the future.

Mimicking Ozonolysis via Mechanochemistry: Internal Alkynes to 1,2-Diketones using H5IO6

Utilizing periodic acid as an environmentally benign oxidizing agent, this study introduces a novel mechanochemical method that mimics ozonolysis to convert internal alkynes into 1,2-diketones, showcasing effective emulation of ozone's reactivity. Notably, this oxidation occurs at room temperature in aerobic conditions, eliminating the need for toxic transition metals, hazardous oxidants, or expensive solvents. Through control experiments validating the mechanism, substantial evidence supports a concerted reaction pathway. This progress marks a significant stride toward cleaner and more efficient chemical synthesis, mitigating the environmental impact of conventional processes. Assessing the green chemistry metrics in both solvent-free and previously reported solvent-based methods, our eco-friendly protocol demonstrates an E-factor of 7.40, a 51.7 % atom economy, a 45.5 % atom efficiency, 100 % carbon efficiency, and 11.9 % reaction mass efficiency when solvents are not used.

Charge Redistribution in Mechanochemical Reactions for Solid Interfaces

Mechanochemical strategies are widely used in various fields, ranging from friction and wear to mechanosynthesis, yet how the mechanical stress activates the chemical reactions at the electronic level is still open. We used first-principles density functional theory to study the rule of the stress-modified electronic states in transmitting mechanical energy to trigger chemical responses for different mechanochemical systems. The electron density redistribution among initial, transition, and final configurations is defined to correlate the energy evolution during reactions. We found that stress-induced changes in electron density redistribution are linearly related to activation energy and reaction energy, indicating the transition from mechanical work to chemical reactivity. The correlation coefficient is defined as the term "interface reactivity coefficient" to evaluate the susceptibility of chemical reactivity to mechanical action for material interfaces. The study may shed light on the electronic mechanism of the mechanochemical reactions behind the fundamental model as well as the mechanochemical phenomena.

Mechanochemical Approaches to Fundamental Studies in Soft-Matter Physics

Stretching a segment of a polymer beyond its contour length makes its (primarily backbone) bonds more dissociatively labile, which enables polymer mechanochemistry. Integrating some backbone bonds into suitably designed molecular moieties yields mechanistically and kinetically diverse chemistry, which is becoming increasingly exploitable. Examples include, most prominently, attempts to improve mechanical properties of bulk polymers, as well as prospective applications in drug delivery and synthesis. This review aims to highlight an emerging effort to apply the concepts and experimental tools of mechanochemistry to fundamental physical questions in soft matter. A succinct summary of the state-of-the-knowledge of the field, with emphasis on foundational concepts and generalizable observations, is followed by analysis of 3 recent examples of mechanochemistry yielding molecular-level details of elastomer failure, macromolecular chain dynamics in elongational flows and kinetic allostery. We conclude with reasons to assume that the highlighted approaches are generalizable to a broader range of physical problems than considered to date.

Mechanochemistry of Strecker degradation: Interaction of glyoxal with amino acids

Mechanochemistry is rapidly evolving into a versatile and green method for chemical synthesis. Due to its unique reaction conditions, ball milling of sugars and amino acids mainly leads to the formation of Amadori products with minimum degradation. In this study, we milled glyoxal trimer dihydrate with twenty proteogenic amino acids to demonstrate the formation of Strecker degradation products. HS-GC/MS studies indicated that Strecker degradation proceeded to selectively generate Strecker aldehyde and unsubstituted pyrazine as the major volatiles. Moreover, ESI/qToF/MS studies demonstrated for the first time the formation of the proposed key Strecker degradation intermediates, such as the condensation products and their decarboxylated products, indicating the similarity of the mechanism of Strecker reaction under ball milling to that proposed under hydrothermal reaction conditions. These studies provided supporting evidence that ball milling at ambient temperatures could be used as a novel synthetic approach to prepare precursors of aroma-active volatiles through Strecker degradation.

Mechanochemical Hydrolysis of Polysaccharide Biomass: Scope and Mechanistic Insights

Mechanical forces can affect chemical reactions in a way that thermal reactions cannot do, which may have a variety of applications. In biomass conversion, the selective conversion of cellulose and chitin is a grand challenge because they are the top two most abundant resources and recalcitrant materials that are insoluble in common solvents. However, recent works have clarified that mechanical forces enable the depolymerization of these polysaccharides, leading to the selective production of corresponding monomers and oligomers. This article reviews the mechanochemical hydrolysis of cellulose and chitin, particularly focusing on the scope and mechanisms to show a landscape of this research field and future subjects. We introduce the background of mechanochemistry and biomass conversion, followed by recent progress on the mechanochemical hydrolysis of the polysaccharides. Afterwards, a considerable space is devoted to the mechanistic consideration on the mechanochemical reactions.

Intermediates in Mechanochemical Reactions

Mechanochemical reactions offer methodological and environmental advantages for chemical synthesis, constantly attracting attention within the scientific community. Besides unmistakable sustainability advantages, the conditions under which mechanochemical reactions occur, namely solventless conditions, sometimes facilitate the isolation of otherwise labile or inaccessible products. Despite these advantages, limited knowledge exists regarding the mechanisms of these reactions and the types of intermediates involved. Nevertheless, in an expanding number of cases, ex situ and in situ monitoring techniques have allowed for the observation, characterization, and isolation of reaction intermediates in mechanochemical transformations. In this Minireview, we present a series of examples in which reactive intermediates have been detected in mechanochemical reactions spanning organic, organometallic, inorganic, and materials chemistry. Many of these intermediates were stabilized by non-covalent interactions, which played a pivotal role in guiding the chemical transformations. We believe that by uncovering and understanding such instances, the growing mechanochemistry community could find novel opportunities in catalysis and discover new mechanochemical reactions while achieving simplification in chemical reaction design.

Synthesis of bioactive evodiamine and rutaecarpine analogues under ball milling conditions

Mechanochemical reactions achieved by processes such as milling and grinding are promising alternatives to traditional solution-based chemistry. This approach not only eliminates the need for large amounts of solvents, thereby reducing waste generation, but also finds applications in chemical and materials synthesis. The focus of this study is on the synthesis of quinazolinone derivatives by ball milling, in particular evodiamine and rutaecarpine analogues. These compounds are of interest due to their diverse bioactivities, including potential anticancer properties. The study examines the reactions carried out under ball milling conditions, emphasizing their efficiency in terms of shorter reaction times and reduced environmental impact compared to conventional methods. The ball milling reaction of evodiamine and rutaecarpine analogues resulted in yields of 63-78% and 22-61%, respectively. In addition, these compounds were tested for their cytotoxic activity, and evodiamine exhibited an IC50 of 0.75 ± 0.04 μg mL-1 against the Ca9-22 cell line. At its core, this research represents a new means to synthesise these compounds, providing a more environmentally friendly and sustainable alternative to traditional approaches.

Sonogenetics for Monitoring and Modulating Biomolecular Function by Ultrasound

Ultrasound technology, synergistically harnessed with genetic engineering and chemistry concepts, has started to open the gateway to the remarkable realm of sonogenetics-a pioneering paradigm for remotely orchestrating cellular functions at the molecular level. This fusion not only enables precisely targeted imaging and therapeutic interventions, but also advances our comprehension of mechanobiology to unparalleled depths. Sonogenetic tools harness mechanical force within small tissue volumes while preserving the integrity of the surrounding physiological environment, reaching depths of up to tens of centimeters with high spatiotemporal precision. These capabilities circumvent the inherent physical limitations of alternative in vivo control methods such as optogenetics and magnetogenetics. In this review, we first discuss mechanosensitive ion channels, the most commonly utilized sonogenetic mediators, in both mammalian and non-mammalian systems. Subsequently, we provide a comprehensive overview of state-of-the-art sonogenetic approaches that leverage thermal or mechanical features of ultrasonic waves. Additionally, we explore strategies centered around the design of mechanochemically reactive macromolecular systems. Furthermore, we delve into the realm of ultrasound imaging of biomolecular function, encompassing the utilization of gas vesicles and acoustic reporter genes. Finally, we shed light on limitations and challenges of sonogenetics and present a perspective on the future of this promising technology.

Liquid reagents are not enough for liquid assisted grinding in the synthesis of [(AgBr)(n-pica)]n

This study investigates the mechanochemical reactions between AgBr 3-picolylamine and 4-picolylamine. The use of different stoichiometry ratios of the reagents allows [(AgBr)(n-pica)]n and [(AgBr)2(n-pica)]n to be obtained, and we report the new structures of [(AgBr)2(3-pica)]n and [(AgBr)2(4-pica)]n which are characterized by the presence of the following: (a) infinite inorganic chains, (b) silver atom coordinated only by bromide atoms and (c) argentophilic interactions. Furthermore, we studied the interconversion of [(AgBr)(n-pica)]n/[(AgBr)2(n-pica)]n by mechanochemical and thermal properties. The in situ experiments suggest that [(AgBr)(3-pica)]n is kinetically favoured while [(AgBr)2(3-pica)]n is converted into [(AgBr)(3-pica)]n only with a high excess of the ligand. Finally, the liquid nature of the ligands is not sufficient to assist the grinding process, and the complete reaction is observed with the addition of a small quantity of acetonitrile.

Interactions and reactivity in crystalline intermediates of mechanochemical cyclorhodation reactions

There is experimental evidence that solid mixtures of the rhodium dimer [Cp*RhCl2]2 and benzo[h] quinoline (BHQ) produce two different polymorphic molecular cocrystals called 4α and 4β under ball milling conditions. The addition of NaOAc to the mixture leads to the formation of the rhodacycle [Cp*Rh-(BHQ)Cl], where the central Rh atom retains its tetracoordinate character. Isolate 4β reacts with NaOAc leading to the same rhodacycle while isolate 4α does not under the same conditions. We show that the puzzling difference in reactivity between the two cocrystals can be traced back to fundamental aspects of the intermolecular interactions between the BHQ and [Cp*RhCl2]2 fragments in the crystalline environment. To support this view, we report a number of descriptors of the nature and strength of chemical bonds and intermolecular interactions in the extended solids and in a cluster model. We calculate formal quantum mechanical descriptors based on electronic structure, electron density, and binding and interaction energies including an energy decomposition analysis. Without exception, all descriptors point to 4β being a transient structure higher in energy than 4α with larger local and global electrophilic and nucleophilic powers, a more favorable spatial and energetic distribution of the frontier orbitals, and a more fragile crystal structure.

Kinetic Phenomena in Mechanochemical Depolymerization of Poly(styrene)

Synthetic polyolefinic plastics comprise one of the largest shares of global plastic waste, which is being targeted for chemical recycling by depolymerization to monomers and small molecules. One promising method of chemical recycling is solid-state depolymerization under ambient conditions in a ball-mill reactor. In this paper, we elucidate kinetic phenomena in the mechanochemical depolymerization of poly(styrene). Styrene is produced in this process at a constant rate and selectivity alongside minor products, including oxygenates like benzaldehyde, via mechanisms analogous to those involved in thermal and oxidative pyrolysis. Continuous monomer removal during reactor operation is critical for avoiding repolymerization, and promoting effects are exhibited by iron surfaces and molecular oxygen. Kinetic independence between depolymerization and molecular weight reduction was observed, despite both processes originating from the same driving force of mechanochemical collisions. Phenomena across multiple length scales are shown to be responsible for differences in reactivity due to differences in grinding parameters and reactant composition.

Wandering through quantum-mechanochemistry: from concepts to reactivity and switches

Mechanochemistry has experienced a renaissance in recent years witnessing, at the molecular level, a remarkable interplay between theory and experiment. Molecular mechanochemistry has welcomed a broad spectrum of quantum-chemical methods to evaluate the influence of an external mechanical force on molecular properties. In this contribution, an overview is given on recent work on quantum mechanochemistry in the Brussels Quantum Chemistry group (ALGC). The effect of an external force was scrutinized both in fundamental topics, like reactivity descriptors in Conceptual DFT, and in applied topics, such as designing molecular force probes and tuning the stereoselectivity of certain types of reactions. In the conceptual part, a brief overview of the techniques introducing mechanical forces into a quantum-mechanical description of a molecule is followed by an introduction to conceptual DFT. The evolution of the electronic chemical potential (or electronegativity), chemical hardness and electrophilicity are investigated when a chemical bond in a series of diatomics is put under mechanical stress. Its counterpart, the influence of mechanical stress on bond angles, is analyzed by varying the strain present in alkyne triple bonds by applying a bending force, taking the strain promoted alkyne-azide coupling cycloaddition as an example. The increase of reactivity of the alkyne upon bending is probed by Fukui functions and the local softness. In the applied part, a new molecular force probe is presented based on an intramolecular 6π-electrocyclization in constrained polyenes operating under thermal conditions. A cyclic process is conceived where ring opening and closure are triggered by applying or removing an external pulling force. The efficiency of mechanical activation strongly depends on the magnitude of the applied force and the distance between the pulling points. The idea of pulling point distances as a tool to identify new mechanochemical processes is then tested in [28]hexaphyrins with an intricate equilibrium between Möbius aromatic and Hückel antiaromatic topologies. A mechanical force is shown to trigger the interconversion between the two topologies, using the distance matrix as a guide to select appropriate pulling points. In a final application, the Felkin-Anh model for the addition of nucleophiles to chiral carbonyls under the presence of an external mechanical force is scrutinized. By applying a force for restricting the conformational freedom of the chiral ketone, otherwise inaccessible reaction pathways are promoted on the force-modified potential energy surfaces resulting in a diastereoselectivity different from the force-free reaction.

Effects of surface chemistry on the mechanochemical decomposition of tricresyl phosphate

The growth of protective tribofilms from lubricant antiwear additives on rubbing surfaces is initiated by mechanochemically promoted dissociation reactions. These processes are not well understood at the molecular scale for many important additives, such as tricresyl phosphate (TCP). One aspect that needs further clarification is the extent to which the surface properties affect the mechanochemical decomposition. Here, we use nonequilibrium molecular dynamics (NEMD) simulations with a reactive force field (ReaxFF) to study the decomposition of TCP molecules confined and pressurised between sliding ferrous surfaces at a range of temperatures. We compare the decomposition of TCP on native iron, iron carbide, and iron oxide surfaces. We show that the decomposition rate of TCP molecules on all the surfaces increases exponentially with temperature and shear stress, implying that this is a stress-augmented thermally activated (SATA) process. The presence of base oil molecules in the NEMD simulations decreases the shear stress, which in turn reduces the rate constant for TCP decomposition. The decomposition is much faster on iron surfaces than iron carbide, and particularly iron oxide. The activation energy, activation volume, and pre-exponential factor from the Bell model are similar on iron and iron carbide surfaces, but significantly differ for iron oxide surfaces. These findings provide new insights into the mechanochemical decomposition of TCP and have important implications for the design of novel lubricant additives for use in high-temperature and high-pressure environments.

Direct thermodynamic characterization of solid-state reactions by isothermal calorimetry

Despite the growing importance of solid-state reactions, their thermodynamic characterization has largely remained unexplored. This is in part due to the lack of methodology for measuring the heat effects related to reactions between solid reactants. We address here this gap and report on the first direct thermodynamic study of chemical reactions between solid reactants by isothermal calorimetry. Three reaction classes, cationic host-guest complex formation, molecular co-crystallization, and Baeyer-Villiger oxidation were investigated, showcasing the versatility of the devised methodology to provide detailed insight into the enthalpy changes related to various reactions. The reliability of the method was confirmed by correlation with the values obtained via solution calorimetry using Hess's law. The thermodynamic characterization of solid-state reactions described here will enable a deeper understanding of the factors governing solid-state processes.

Dual Ratiometric Fluorescence Monitoring of Mechanical Polymer Chain Stretching and Subsequent Strain-Induced Crystallization

Tracking the behavior of mechanochromic molecules provides valuable insights into force transmission and associated microstructural changes in soft materials under load. Herein, we report a dual ratiometric fluorescence (FL) analysis for monitoring both mechanical polymer chain stretching and strain-induced crystallization (SIC) of polymers. SIC has recently attracted renewed attention as an effective mechanism for improving the mechanical properties of polymers. A polyurethane (PU) film incorporating a trace of a dual-emissive flapping force probe (N-FLAP, 0.008 wt %) exhibited a blue-to-green FL spectral change in a low-stress region (<20 MPa), resulting from conformational planarization of the probe in mechanically stretched polymer chains. More importantly, at higher probe concentrations (∼0.65 wt %), the PU film showed a second spectral change from green to yellow during the SIC growth (20-65 MPa) due to self-absorption of scattered FL in a short wavelength region. The reversibility of these spectral changes was demonstrated by load-unload cycles. With these results in hand, the degrees of the polymer chain stretching and the SIC were quantitatively mapped and monitored by dual ratiometric imaging based on different FL ratios (I525/I470 and I525/I600). Simultaneous analysis of these two mappings revealed a spatiotemporal gap in the distribution of the polymer chain stretching and the SIC. The combinational use of the dual-emissive force probe and the ratiometric FL imaging is a universal approach for the development of soft matter physics.

Modeling single-molecule stretching experiments using statistical thermodynamics

Single-molecule stretching experiments are widely utilized within the fields of physics and chemistry to characterize the mechanics of individual bonds or molecules, as well as chemical reactions. Analytic relations describing these experiments are valuable, and these relations can be obtained through the statistical thermodynamics of idealized model systems representing the experiments. Since the specific thermodynamic ensembles manifested by the experiments affect the outcome, primarily for small molecules, the stretching device must be included in the idealized model system. Though the model for the stretched molecule might be exactly solvable, including the device in the model often prevents analytic solutions. In the limit of large or small device stiffness, the isometric or isotensional ensembles can provide effective approximations, but the device effects are missing. Here a dual set of asymptotically correct statistical thermodynamic theories are applied to develop accurate approximations for the full model system that includes both the molecule and the device. The asymptotic theories are first demonstrated to be accurate using the freely jointed chain model and then using molecular dynamics calculations of a single polyethylene chain.

Illuminating milling mechanochemistry by tandem real-time fluorescence emission and Raman spectroscopy monitoring

In pursuit of accessible and interpretable methods for direct and real-time observation of mechanochemical reactions, we demonstrate a tandem spectroscopic method for monitoring of ball-milling transformations combining fluorescence emission and Raman spectroscopy, accompanied by high-level molecular and periodic density-functional theory (DFT) calculations, including periodic time-dependent (TD-DFT) modelling of solid-state fluorescence spectra. This proof-of-principle report presents this readily accessible dual-spectroscopy technique as capable of observing changes to the supramolecular structure of the model pharmaceutical system indometacin during mechanochemical polymorph transformation and cocrystallisation. The observed time-resolved in situ spectroscopic and kinetic data are supported by ex situ X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy measurements. The application of first principles (ab initio) calculations enabled the elucidation of how changes in crystalline environment, that result from mechanochemical reactions, affect vibrational and electronic excited states of molecules. The herein explored interpretation of both real-time and ex situ spectroscopic data through ab initio calculations provides an entry into developing a detailed mechanistic understanding of mechanochemical milling processes and highlights the challenges of using real-time spectroscopy.

Polymer Mechanochemistry in Microbubbles

Polymer mechanochemistry is a promising technology to convert mechanical energy into chemical functionality by breaking covalent and supramolecular bonds site-selectively. Yet, the mechanochemical reaction rates of covalent bonds in typically used ultrasonication setups lead to reasonable conversions only after comparably long sonication times. This can be accelerated by either increasing the reactivity of the mechanoresponsive moiety or by modifying the encompassing polymer topology. Here, a microbubble system with a tailored polymer shell consisting of an N2 gas core and a mechanoresponsive disulfide-containing polymer network is presented. It is found that the mechanochemical activation of the disulfides is greatly accelerated using these microbubbles compared to commensurate solid core particles or capsules filled with liquid. Aided by computational simulations, it is found that low shell thickness, low shell stiffness and crosslink density, and a size-dependent eigenfrequency close to the used ultrasound frequency maximize the mechanochemical yield over the course of the sonication process.

Sacrificial Mechanical Bond is as Effective as a Sacrificial Covalent Bond in Increasing Cross-Linked Polymer Toughness

Sacrificial chemical bonds have been used effectively to increase the toughness of elastomers because such bonds dissociate at forces significantly below the fracture limit of the primary load-bearing bonds, thereby dissipating local stress. This approach owes much of its success to the ability to adjust the threshold force at which the sacrificial bonds fail at the desired rate, for example, by selecting either covalent or noncovalent sacrificial bonds. Here, we report experimental and computational evidence that a mechanical bond, responsible for the structural integrity of a rotaxane or a catenane, increases the elastomer's fracture strain, stress, and energy as much as a covalent bond of comparable mechanochemical dissociation kinetics. We synthesized and studied 6 polyacrylates cross-linked by either difluorenylsuccinonitrile (DFSN), which is an established sacrificial mechanochromic moiety; a [2]rotaxane, whose stopper allows its wheel to dethread on the same subsecond time scale as DFSN dissociates when either is under tensile force of 1.5-2 nN; a structurally homologous [2]rotaxane with a much bulkier stopper that is stable at force >5.5 nN; similarly stoppered [3]rotaxanes containing DFSN in their axles; and a control polymer with aliphatic nonsacrificial cross-links. Our data suggest that mechanochemical dethreading of a rotaxane without failure of any covalent bonds may be an important, hitherto unrecognized, contributor to the toughness of some rotaxane-cross-linked polymers and that sacrificial mechanical bonds provide a mechanism to control material fracture behavior independently of the mechanochemical response of the covalent networks, due to their distinct relationships between structure and mechanochemical reactivity.

Coumarin Dimer Is an Effective Photomechanochemical AND Gate for Small-Molecule Release

Stimulus-responsive gating of chemical reactions is of considerable practical and conceptual interest. For example, photocleavable protective groups and gating mechanophores allow the kinetics of purely thermally activated reactions to be controlled optically or by mechanical load by inducing the release of small-molecule reactants. Such release only in response to a sequential application of both stimuli (photomechanochemical gating) has not been demonstrated despite its unique expected benefits. Here, we describe computational and experimental evidence that coumarin dimers are highly promising moieties for realizing photomechanochemical control of small-molecule release. Such dimers are transparent and photochemically inert at wavelengths >300 nm but can be made to dissociate rapidly under tensile force. The resulting coumarins are mechanochemically and thermally stable, but rapidly release their payload upon irradiation. Our DFT calculations reveal that both strain-free and mechanochemical kinetics of dimer dissociation are highly tunable over an unusually broad range of rates by simple substitution. In head-to-head dimers, the phenyl groups act as molecular levers to allow systematic and predictable variation in the force sensitivity of the dissociation barriers by choice of the pulling axis. As a proof-of-concept, we synthesized and characterized the reactivity of one such dimer for photomechanochemically controlled release of aniline and its application for controlling bulk gelation.

Induction-heated ball-milling: a promising asset for mechanochemical reactions

While ball-milling is becoming one of the common tools used by synthetic chemists, an increasing number of studies highlight that it is possible to further expand the nature and number of products which can be synthesized, by heating the reaction media during mechanochemical reactions. Hence, developing set-ups enabling heating and milling to be combined is an important target, which has been looked into in both academic and industrial laboratories. Here, we report a new approach for heating up reaction media during ball-milling reactions, using induction heating (referred to as i-BM). Our set-up is attractive not only because it enables a very fast heating of the milling medium (reaching ≈80 °C in just 15 s), and that it is directly adaptable to commercially-available milling equipment, but also because it enables heating either the walls of the milling jars or the beads themselves, depending on the choice of the materials which compose them. Importantly, the possibility to heat a milling medium "from the inside" (when using for example a PMMA jar and stainless steel beads) is a unique feature compared to previously proposed systems. Through numerical simulations, we then show that it is possible to finely tune the properties of this heating system (e.g. heating rate and maximum temperature reached), by playing with the characteristics of the milling system and/or the induction heating conditions used. Lastly, examples of applications of i-BM are given, showing how it can be used to help elucidate reaction mechanisms in ball-milling, to synthesize new molecules, and to control the physical nature of milling media.

Ester Bond: Chemically Labile Yet Mechanically Stable

Due to the dynamic nature of ester linkages, ester-bond-containing materials are well known for their outstanding degradability and stimuli responsiveness. However, whether ester hydrolysis is affected by mechanical forces remains unclear. Here, we develop a single-molecule assay to quantitatively study the force-dependent ester hydrolysis using an engineered circular permutant protein with a caged ester bond as a model. Our single-molecule force spectroscopy results show that the ester hydrolysis rate is surprisingly insensitive to forces, with a ∼7 s-1 dissociation rate that remains almost unchanged in the force range of 80 to 200 pN. Quantum calculations reveal that the ester hydrolysis involves an intermediate state formed by either H3O+- or OH--bonded tetrahedral intermediates. The measured ester-hydrolysis kinetics at the single-molecule level may primarily correspond to the rupture of these intermediate states. However, the rate-limiting step appears to be the formation of the tetrahedral intermediates, which cannot be quantitatively characterized in our experiments. Nonetheless, based on the quantum calculations, this step is also insensitive to applied forces. Altogether, our study suggests that the ester bond is chemically labile yet mechanically stable, serving as the basis for the design of responsive materials using ester bonds as mechanically inert units.

Editing of polymer backbones

Polymers are at the epicentre of modern technological progress and the associated environmental pollution. Considerations of both polymer functionality and lifecycle are crucial in these contexts, and the polymer backbone - the core of a polymer - is at the root of these considerations. Just as the meaning of a sentence can be altered by editing its words, the function and sustainability of a polymer can also be transformed via the chemical modification of its backbone. Yet, polymer modification has primarily been focused on the polymer periphery. In this Review, we focus on the transformations of the polymer backbone by defining some concepts fundamental to this topic (for example, 'polymer backbone' and 'backbone editing') and by collecting and categorizing examples of backbone editing scattered throughout a century's worth of chemical literature, and outline critical directions for further research. In so doing, we lay the foundation for the field of polymer backbone editing and hope to accelerate its development.

Experimental quantitation of molecular conditions responsible for flow-induced polymer mechanochemistry

Fragmentation of macromolecular solutes in rapid flows is of considerable fundamental and practical importance. The sequence of molecular events preceding chain fracture is poorly understood, because such events cannot be visualized directly but must be inferred from changes in the bulk composition of the flowing solution. Here we describe how analysis of same-chain competition between fracture of a polystyrene chain and isomerization of a chromophore embedded in its backbone yields detailed characterization of the distribution of molecular geometries of mechanochemically reacting chains in sonicated solutions. In our experiments the overstretched (mechanically loaded) chain segment grew and drifted along the backbone on the same timescale as, and in competition with, the mechanochemical reactions. Consequently, only <30% of the backbone of a fragmenting chain is overstretched, with both the maximum force and the maximum reaction probabilities located away from the chain centre. We argue that quantifying intrachain competition is likely to be mechanistically informative for any flow fast enough to fracture polymer chains.

Mechanochemically assisted morphing of shape shifting polymers

Morphing in creatures has inspired various synthetic polymer materials that are capable of shape shifting. The morphing of polymers generally relies on stimuli-active (typically heat and light active) units that fix the shape after a mechanical load-based shape programming. Herein, we report a strategy that uses a mechanochemically active 2,2'-bis(2-phenylindan-1,3-dione) (BPID) mechanophore as a switching unit for mechanochemical morphing. The mechanical load on the polymer triggers the dissociation of the BPID moiety into stable 2-phenylindan-1,3-dione (PID) radicals, whose subsequent spontaneous dimerization regenerates BPID and fixes the temporary shapes that can be effectively recovered to the permanent shapes by heating. A greater extent of BPID activation, through a higher BPID content or mechanical load, leads to higher mechanochemical shape fixity. By contrast, a relatively mechanochemically less active hexaarylbiimidazole (HABI) mechanophore shows a lower fixing efficiency when subjected to the same programing conditions. Another control system without a mechanophore shows a low fixing efficiency comparable to the HABI system. Additionally, the introduction of the BPID moiety also manifests remarkable mechanochromic behavior during the shape programing process, offering a visualizable indicator for the pre-evaluation of morphing efficiency. Unlike conventional mechanical mechanisms that simultaneously induce morphing, such as strain-induced plastic deformation or crystallization, our mechanochemical method allows for shape programming after the mechanical treatment. Our concept has potential for the design of mechanochemically programmable and mechanoresponsive shape shifting polymers.