Polymer mechanochemistry: from destructive to productive

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.

Mechanically Triggered Bright Chemiluminescence from Polymers by Exploiting a Synergy between Masked 2-Furylcarbinol Mechanophores and 1,2-Dioxetane Chemiluminophores

Mechanoluminescence, or the generation of light from materials under external force, is a powerful tool for biology and materials science. However, direct mechanoluminescence from polymers remains limited. Here, we report a novel design strategy for mechanoluminescent polymers that leverages the synergy between a masked 2-furylcarbinol mechanophore for mechanically triggered release and an adamantylidene-phenoxy-1,2-dioxetane chemiluminophore payload. Ultrasound-induced mechanochemical activation of polymers, in both organic and aqueous solutions, triggers a cascade reaction that ultimately results in bright green light emission. This novel strategy capitalizes on the modularity of the masked 2-furylcarbinol mechanophore system in combination with advances in the design of exceptionally bright and highly tunable adamantylidene-1,2-dioxetane chemiluminophores. We anticipate that this chemistry will enable diverse applications in optoelectronics, sensing, bioimaging, optogenetics, and many other areas.

Foldamer-Based Mechanoresponsive Materials: Molecular Nanoarchitectonics to Advanced Functions

Artificial molecules that respond to external stimuli such as light, heat, chemical signals, and mechanical force have garnered significant interest due to their tunable functions, variable optical properties, and mechanical responses. Particularly, mechanoresponsive materials featuring molecules that respond to mechanical stress or show force-induced optical changes have been intriguing due to their extraordinary functions. Despite the promising potential of many such materials reported in the past, practical applications have remained limited, primarily because their functions often depend on irreversible covalent bond rupture. Foldamers, oligomers that fold into well-defined secondary structures, offer an alternative class of mechanoactive motifs. These molecules can reversibly sustain mechanical stress and efficiently dissipate energy by transitioning between folded and unfolded states. This review focuses on the emerging properties of foldamer-based mechanoresponsive materials. We begin by highlighting the mechanical responses of foldamers in their molecular form, which have been primarily investigated using single-molecule force spectroscopy and other analytical methods. Following this, we provide a detailed survey of the current trends in foldamer-appended polymers, emphasizing their emerging mechanical and mechanochromic properties. Subsequently, we present an overview of the state-of-the-art advancements in foldamer-appended polymers, showcasing significant reports in this field. This review covers some of the most recent advances in this direction and draws a perspective for further development.

Liquid-assisted grinding enables a direct mechanochemical functionalization of polystyrene waste

The plastic waste crisis has grave consequences for our environment, as most single-use commodity polymers remain in landfills and oceans long after their commercial lifetimes. Utilizing modern synthetic techniques to chemically modify the structure of these post-consumer plastics (e.g., upcycling) can impart new properties and added value for commercial applications. To expand beyond the abilities of current solution-state chemical processes, we demonstrate post-polymerization modification of polystyrene via solid-state mechanochemistry enabled by liquid-assisted grinding (LAG). Importantly, this emblematic trifluoromethylation study modifies discarded plastic, including dyed materials, using minimal exogenous solvent and plasticizers for improved sustainability. Ultimately, this work serves as a proof-of-concept for the direct mechanochemical post-polymerization modification of commodity polymers, and we expect future remediation of plastic waste via similar mechanochemical reactions.

Mechanochemical Fabrication of Full-Color Luminescent Materials from Aggregation-Induced Emission Prefluorophores for Information Storage and Encryption

The development of luminescent materials via mechanochemistry embodies a compelling yet intricate frontier within materials science. Herein, we delineate a methodology for the synthesis of brightly luminescent polymers, achieved by the mechanochemical coupling of aggregation-induced emission (AIE) prefluorophores with generic polymers. An array of AIE moieties tethered to the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical are synthesized as prefluorophores, which initially exhibit weak fluorescence due to intramolecular quenching. Remarkably, the mechanical coupling of these prefluorophores with macromolecular radicals, engendered through ball milling of generic polymers, leads to substantial augmentation of fluorescence within the resultant polymers. We meticulously evaluate the tunable emission of the AIE-modified polymers, encompassing an extensive spectrum from the visible to the near-infrared region. This study elucidates the potential of such materials in stimuli-responsive systems with a focus on information storage and encryption displays. By circumventing the complexity inherent to the conventional synthesis of luminescent polymers, this approach contributes a paradigm to the field of AIE-based polymers with implications for advanced technological applications.

Advancing Dynamic Polymer Mechanochemistry through Synergetic Conformational Gearing

Harnessing mechanical force to modulate material properties and enhance biomechanical functions is essential for advancing smart materials and bioengineering. Polymer mechanochemistry provides an emerging toolkit for exploring unconventional chemical transformations and modulating molecular structures through mechanical force. One of the key challenges is developing innovative force-sensing mechanisms for precise and in situ force detection. This study introduces mDPAC, a dynamic and sensitive mechanophore, demonstrating its mechanochromic properties through synergetic conformational gearing. Its unique mechanoresponsive mechanism is based on the simultaneous conformational synergy between its phenazine and phenyl moieties, facilitated by a worm-gear-like structure. We confirm mDPAC's complex mechanochemical response and elucidate its mechanotransduction mechanism through our experimental data and comprehensive simulations. The compatibility of mDPAC with hydrogels is particularly notable, highlighting its potential for applications in aqueous biological environments as a dynamic force sensor. Moreover, mDPAC's multicolored mechanochromic responses facilitate direct force sensing and visual detection, paving the way for precise and real-time mechanical force sensing in bulk materials.

Hydrogel morphogenesis induced by force-controlled growth

Morphogenesis is one of the most marvelous natural phenomena. The morphological characteristics of biological organs develop through growth, which is often triggered by mechanical force. In this study, we propose a bioinspired strategy for hydrogel morphogenesis through force-controlled chemical reaction and growth under isothermal conditions. We adopted a double network (DN) hydrogel with sacrificial bonds. Applying mechanical force to the gel caused deformation and sacrificial bond rupture. By supplying monomers to the gel, the radicals generated by the bond rupture triggered the formation of a new network inside the deformed gel. This new network conferred plasticity to the elastic gel, allowing it to maintain its deformed shape, along with increased volume and strength. We demonstrated that sheet-shaped DN hydrogels rapidly adopted various three-dimensional shapes at ambient temperature when subjected to forces such as drawing and blowing. This mechanism enables morphogenesis of elastic hydrogels and will promote the application of these materials in biomedical fields and soft machines.

Enhancing Polymer Sustainability: Eco-Conscious Strategies

Polymer sustainability is a pressing concern in today's world driven by the increasing demand for environmentally friendly materials. This review paper provides a comprehensive overview of eco-friendly approaches towards enhancing the sustainability of polymers. It synthesized recent research and developments in various areas such as green polymer synthesis methods, biodegradable polymers, recycling technologies, and emerging sustainable alternatives. The environmental impact of traditional polymer production processes and the importance of adopting greener alternatives were critically examined. The review delved into the advancements in polymer recycling technologies like mechanical, chemical, and biological processes aimed at minimizing plastic waste and promoting a circular economy. The innovative approaches such as upcycling, hybrid methods etc., which offer promising solutions for addressing plastic pollution and achieving long-term sustainability goals were also analyzed. Finally, the paper discussed the challenges and future prospects of eco-friendly approaches for polymer sustainability, emphasizing the need for researchers and concerted efforts from scientists across industries and academia to drive meaningful change towards a more sustainable future.

A Strong Supramolecular Mechanophore with Controlled Mechanical Strength

Supramolecular mechanophores typically exhibit much lower mechanical strengths than covalent counterparts, with strengths usually around 100 pN, which is significantly lower than the nN-scale strength of covalent bonds. Inspired by the slow dissociation kinetics of the cucurbit[7]uril (CB[7])-hexanoate-isoquinoline (HIQ) complex, we discovered that charge-dipole repulsion can be utilized to create strong supramolecular mechanophores. When activated at its -COO- state, the CB[7]-HIQ complex exhibits a high mechanical strength of ~700 pN, comparable to weak covalent bonds such as Au-S bonds or thiol-maleimide adducts. The strength of the CB[7]-HIQ complex can also be tuned with pH in a gradual manner, with a minimum value of ~150 pN at its -COOH state, similar to an ordinary supramolecular conjugate. This research may pave the way for the development of supramolecular architectures that combine the advantages of covalent and supramolecular systems.

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.

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.

Effect of the Activation Force of Mechanophore on Its Activation Selectivity and Efficiency in Polymer Networks

In recent decades, more than 100 different mechanophores with a broad range of activation forces have been developed. For various applications of mechanophores in polymer materials, it is crucial to selectively activate the mechanophores with high efficiency, avoiding nonspecific bond scission of the material. In this study, we embedded cyclobutane-based mechanophore cross-linkers (I and II) with varied activation forces (fa) in the first network of the double network hydrogels and quantitively investigated the activation selectivity and efficiency of these mechanophores. Our findings revealed that cross-linker I, with a lower activation force relative to the bonds in the polymer main chain (fa-I/fa-chain = 0.8 nN/3.4 nN), achieved efficient activation with 100% selectivity. Conversely, an increase of the activation force of mechanophore II (fa-II/fa-chain = 2.5 nN/3.4 nN) led to a significant decrease of its activation efficiency, accompanied by a substantial number of nonspecific bond scission events. Furthermore, with the coexistence of two cross-linkers, significantly different activation forces resulted in the almost complete suppression of the higher-force one (i.e., I and III, fa-I/fa-III = 0.8 nN/3.4 nN), while similar activation forces led to simultaneous activations with moderate efficiencies (i.e., I and IV, fa-I/fa-IV = 0.8 nN/1.6 nN). These findings provide insights into the prevention of nonspecific bond rupture during mechanophore activation and enhance our understanding of the damage mechanism within polymer networks when using mechanophores as detectors. Besides, it establishes a principle for combining different mechanophores to design multiple mechanoresponsive functional materials.

Ultrasound-Controlled Prodrug Activation: Emerging Strategies in Polymer Mechanochemistry and Sonodynamic Therapy

Ultrasound has gained prominence in biomedical applications due to its noninvasive nature and ability to penetrate deep tissue with spatial and temporal resolution. The burgeoning field of ultrasound-responsive prodrug systems exploits the mechanical and chemical effects of ultrasonication for the controlled activation of prodrugs. In polymer mechanochemistry, materials scientists exploit the sonomechanical effect of acoustic cavitation to mechanochemically activate force-sensitive prodrugs. On the other hand, researchers in the field of sonodynamic therapy adopt fundamentally distinct methodologies, utilizing the sonochemical effect (e.g., generation of reactive oxygen species) of ultrasound in the presence of sonosensitizers to induce chemical transformations that activate prodrugs. This cross-disciplinary review comprehensively examines these two divergent yet interrelated approaches, both of which originated from acoustic cavitation. It highlights molecular and materials design strategies and potential applications in diverse therapeutic contexts, from chemotherapy to immunotherapy and gene therapy methods, and discusses future directions in this rapidly advancing domain.

Ultrasound-Sensitive Intelligent Nanosystems: A Promising Strategy for the Treatment of Neurological Diseases

Neurological diseases are a major global health challenge, affecting hundreds of millions of people worldwide. Ultrasound therapy plays an irreplaceable role in the treatment of neurological diseases due to its noninvasive, highly focused, and strong tissue penetration capabilities. However, the complexity of brain and nervous system and the safety risks associated with prolonged exposure to ultrasound therapy severely limit the applicability of ultrasound therapy. Ultrasound-sensitive intelligent nanosystems (USINs) are a novel therapeutic strategy for neurological diseases that bring greater spatiotemporal controllability and improve safety to overcome these challenges. This review provides a detailed overview of therapeutic strategies and clinical advances of ultrasound in neurological diseases, focusing on the potential of USINs-based ultrasound in the treatment of neurological diseases. Based on the physical and chemical effects induced by ultrasound, rational design of USINs is a prerequisite for improving the efficacy of ultrasound therapy. Recent developments of ultrasound-sensitive nanocarriers and nanoagents are systemically reviewed. Finally, the challenges and developing prospects of USINs are discussed in depth, with a view to providing useful insights and guidance for efficient ultrasound treatment of neurological diseases.

Reversible strain-promoted DNA polymerization

Molecular strain can be introduced to influence the outcome of chemical reactions. Once a thermodynamic product is formed, however, reversing the course of a strain-promoted reaction is challenging. Here, a reversible, strain-promoted polymerization in cyclic DNA is reported. The use of nonhybridizing, single-stranded spacers as short as a single nucleotide in length can promote DNA cyclization. Molecular strain is generated by duplexing the spacers, leading to ring opening and subsequent polymerization. Then, removal of the strain-generating duplexers triggers depolymerization and cyclic dimer recovery via enthalpy-driven cyclization and entropy-mediated ring contraction. This reversibility is retained even when a protein is conjugated to the DNA strands, and the architecture of the protein assemblies can be modulated between bivalent and polyvalent states. This work underscores the utility of using DNA not only as a programmable ligand for assembly but also as a route to access restorable bonds, thus providing a molecular basis for DNA-based materials with shape-memory, self-healing, and stimuli-responsive properties.

A Thermally Stable SO2-Releasing Mechanophore: Facile Activation, Single-Event Spectroscopy, and Molecular Dynamic Simulations

Polymers that release small molecules in response to mechanical force are promising candidates as next-generation on-demand delivery systems. Despite advancements in the development of mechanophores for releasing diverse payloads through careful molecular design, the availability of scaffolds capable of discharging biomedically significant cargos in substantial quantities remains scarce. In this report, we detail a nonscissile mechanophore built from an 8-thiabicyclo[3.2.1]octane 8,8-dioxide (TBO) motif that releases one equivalent of sulfur dioxide (SO2) from each repeat unit. The TBO mechanophore exhibits high thermal stability but is activated mechanochemically using solution ultrasonication in either organic solvent or aqueous media with up to 63% efficiency, equating to 206 molecules of SO2 released per 143.3 kDa chain. We quantified the mechanochemical reactivity of TBO by single-molecule force spectroscopy and resolved its single-event activation. The force-coupled rate constant for TBO opening reaches ∼9.0 s-1 at ∼1520 pN, and each reaction of a single TBO domain releases a stored length of ∼0.68 nm. We investigated the mechanism of TBO activation using ab initio steered molecular dynamic simulations and rationalized the observed stereoselectivity. These comprehensive studies of the TBO mechanophore provide a mechanically coupled mechanism of multi-SO2 release from one polymer chain, facilitating the translation of polymer mechanochemistry to potential biomedical applications.

Mechanically Controlled Enzymatic Polymerization and Remodeling

In nature, proteins possess the remarkable ability to sense and respond to mechanical forces, thereby triggering various biological events, such as bone remodeling and muscle regeneration. However, in synthetic systems, harnessing the mechanical force to induce material growth still remains a challenge. In this study, we aimed to utilize low-frequency ultrasound (US) to activate horseradish peroxidase (HRP) and catalyze free radical polymerization. Our findings demonstrate the efficacy of this mechano-enzymatic chemistry in rapidly remodeling the properties of materials through cross-linking polymerization and surface coating. The resulting samples exhibited a significant enhancement in tensile strength, elongation, and Young's modulus. Moreover, the hydrophobicity of the surface could be completely switched within just 30 min of US treatment. This work presents a novel approach for incorporating mechanical sensing and rapid remodeling capabilities into materials.

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.

Suppressive Photochromism and Promotive Mechanochromism of Rhodamine Mechanophore by the Strategy of Poly(methyl acrylate)/Polyurethane Interpenetrating Polymer Network

As molecular design and the structure-property relationships of photochemical molecules established in the literature serve as a convenient reference for mechanophore exploration, many typical mechanophores suffer undesired responses to UV light or even sunlight in bulk polymers. We developed a strategy of a poly(methyl acrylate)/polyurethane (PMA/PU) interpenetrating polymer network (IPN) to suppress the photochromic property of the mechanophore and promote its mechanochromic property. A widely used rhodamine mechanophore (Rh-2OH) was first incorporated into polyurethane (P1). Then P1 was swollen in methyl acrylate and photopolymerized to prepare a PMA2.8/PU IPN (P2). Different from photo/force-responsive P1, P2 selectively responded to force because the low free volume in IPN greatly hinders photoisomerization of the rhodamine spirolactam, suggesting that a simple IPN strategy successfully resolves the giant problem of nonselective response to photo/force for photochromic mechanophores. Moreover, PMA/PU IPN enhanced the mechanical property, resulting in a higher mechanochemical activation ratio than PU, and the prestretching effect of PMA/PU IPN promoted the force sensitivity of rhodamine mechanophores significantly. We believe that the strategy can be applied to other mechanophores, promoting their application in more complicated environments.

Bicyclo[2.2.0]hexene: A Multicyclic Mechanophore with Reactivity Diversified by External Forces

Polymer mechanochemistry has been established as an enabling tool in accessing chemical reactivity and reaction pathways that are distinctive from their thermal counterparts. However, eliciting diversified reaction pathways by activating different constituent chemical bonds from the same mechanophore structure remains challenging. Here, we report the design of a bicyclo[2.2.0]hexene (BCH) mechanophore to leverage its structural simplicity and relatively low molecular symmetry to demonstrate this idea of multimodal activation. Upon changing the attachment points of pendant polymer chains, three different C-C bonds in bicyclo[2.2.0]hexene are specifically activated via externally applied force by sonication. Experimental characterization confirms that in different scenarios of polymer attachment, the regioisomers of BCH undergo different activation reactions, entailing retro-[2+2] cycloreversion, 1,3-allylic migration, and retro-4π ring-opening reactions, respectively. Control experiments with small-molecule analogues reveal that the observed diversified reactivity of BCH regioisomers is possible only with mechanical force. Theoretical studies further elucidate that the differences in the positions of substitution between regioisomers have a minimal impact on the potential energy surface of the parent BCH scaffold. The mechanochemical selectivity between different C-C bonds in each constitutional isomer is a result of selective and effective coupling of force to the aligned C-C bond in each case.

Azobenzene as a photoswitchable mechanophore

Azobenzene has been widely explored as a photoresponsive element in materials science. Although some studies have investigated the force-induced isomerization of azobenzene, the effect of force on the rupture of azobenzene has not been explored. Here we show that the light-induced structural change of azobenzene can also alter its rupture forces, making it an ideal light-responsive mechanophore. Using single-molecule force spectroscopy and ultrasonication, we found that cis and trans para-azobenzene isomers possess contrasting mechanical properties. Dynamic force spectroscopy experiments and quantum-chemical calculations in which azobenzene regioisomers were pulled from different directions revealed that the distinct rupture forces of the two isomers are due to the pulling direction rather than the energetic difference between the two isomers. These mechanical features of azobenzene can be used to rationally control the macroscopic fracture behaviours of polymer networks by photoillumination. The use of light-induced conformational changes to alter the mechanical response of mechanophores provides an attractive way to engineer polymer networks of light-regulatable mechanical properties.

Mechanochromic polymer blends made with an excimer-forming telechelic sensor molecule

The ability to monitor mechanical stresses and strains in polymers via an optical signal enables the investigation of deformation processes in such materials and is technologically useful for sensing damage and failure in critical components. We show here that this can be achieved by simply blending polymers of interest with a small amount of a mechanochromic luminescent additive (Py-PEB) that can be accessed in one step by end-functionalizing a telechelic poly(ethylene-co-butylene) (PEB) with excimer-forming pyrenes. Py-PEB is poorly miscible with polar polymers, such as poly(ε-caprolactone) and poly(urethane), so that blends undergo microphase separation even at low additive concentrations (0.1-1 wt%), and the emission is excimer-dominated. Upon deformation, the ratio of excimer-to-monomer emission intensity decreases in response to the applied stress or strain. The approach appears to be generalizable, although experiments with poly(isoprene) show that it is not universal and that the (in)solubility of the additive in the polymer must be carefully tuned.

Solvent Polarity Effects on the Mechanochemistry of Spiropyran Ring Opening

The spiropyran mechanophore (SP) is employed as a reporter of molecular tension in a wide range of polymer matrices, but the influence of surrounding environment on the force-coupled kinetics of its ring opening has not been quantified. Here, we report single-molecule force spectroscopy studies of SP ring opening in five solvents that span normalized Reichardt solvent polarity factors (ETN) of 0.1-0.59. Individual multimechanophore polymers were activated under increasing tension at constant 300 nm s-1 displacement in an atomic force microscope. The extension results in a plateau in the force-extension curve, whose midpoint occurs at a transition force f* that corresponds to the force required to increase the rate constant of SP activation to approximately 30 s-1. More polar solvents lead to mechanochemical reactions that are easier to trigger; f* decreases across the series of solvents, from a high of 415 ± 13 pN in toluene to a low of 234 ± 9 pN in n-butanol. The trend in mechanochemical reactivity is consistent with the developing zwitterionic character on going from SP to the ring-opened merocyanine product. The force dependence of the rate constant (Δx‡) was calculated for all solvent cases and found to increase with ETN, which is interpreted to reflect a shift in the transition state to a later and more productlike position. The inferred shift in the transition state position is consistent with a double-well (two-step) reaction potential energy surface, in which the second step is rate determining, and the intermediate is more polar than the product.

Incorporation of a self-immolative spacer enables mechanically triggered dual payload release

Polymers that release functional small molecules in response to mechanical force are promising materials for a variety of applications including drug delivery, catalysis, and sensing. While many different mechanophores have been developed that enable the triggered release of a variety of small molecule payloads, most mechanophores are limited to one specific payload molecule. Here, we leverage the unique fragmentation of a 5-aryloxy-substituted 2-furylcarbinol derivative to design a novel mechanophore capable of the mechanically triggered release of two distinct cargo molecules. Critical to the mechanophore design is the incorporation of a self-immolative spacer to facilitate the release of a second payload. By varying the relative positions of each cargo molecule conjugated to the mechanophore, dual payload release occurs either concurrently or sequentially, demonstrating the ability to fine-tune the release profiles.

Using Mechanochemistry to Activate Commodity Plastics as Initiators for Radical Chain Reactions of Small Organic Molecules

Radical initiators such as azo compounds and organic peroxides have been widely used to facilitate numerous transformations of free radicals, which enable the efficient synthesis of structurally complex molecules, natural products, polymers, and functional materials. However, these high-energy reagents are potentially explosive and thus often require special precautions or delicate operating conditions. We postulated that a more convenient and safer alternative for radical chain initiation could be developed by mechanical activation of thermodynamically stable covalent bonds. Here, we show that commodity plastics such as polyethylene and poly(vinyl acetate) are capable of acting as efficient initiators for radical chain reactions under solvent-free mechanochemical conditions. In this approach, polymeric mechanoradicals, which are generated by homolytic cleavage of the polymer chains in response to the applied mechanical energy provided by ball milling, react with tris(trimethylsilyl)silane to initiate radical chain dehalogenation of organic halides. Preliminary calculations support our proposed force-induced radical chain mechanism.

Multimechanophore Polymers for Mechanically Triggered Small Molecule Release with Ultrahigh Payload Capacity

Polymers that release small molecules in response to mechanical force are promising for a variety of applications including drug delivery, catalysis, and sensing. While a number of mechanophores have been developed for the release of covalently bound payloads, existing strategies are either limited in cargo scope or, in the case of more general mechanophore designs, are restricted to the release of one or two cargo molecules per polymer chain. Herein, we introduce a nonscissile mechanophore based on a masked 2-furylcarbinol derivative that enables the preparation of multimechanophore polymers with ultrahigh payload capacity. We demonstrate that polymers prepared via ring-opening metathesis polymerization are capable of releasing hundreds of small-molecule payloads per polymer chain upon ultrasound-induced mechanochemical activation. This nonscissile masked 2-furylcarbinol mechanophore overcomes a major challenge in cargo loading capacity associated with previous 2-furylcarbinol mechanophore designs, enabling applications that benefit from much higher concentrations of delivered cargo.

Mechanochemistry in Block Copolymers: New Scission Site due to Dynamic Phase Separation

Mechanochemistry can lead to the degradation of the properties of covalent macromolecules. In recent years, numerous functional materials have been developed based on block copolymers (BCPs), however, like homopolymers, their chains could undergo mechanochemical damage during processing, which could have crucial impact on their performance. To investigate the mechanochemical response of BCPs, multiple polymers comprising different ratios of butyl acrylate and methyl methacrylate were prepared with similar degree of polymerization and stressed in solution via ultrasonication. Interestingly, all BCPs, regardless of the amount of the methacrylate monomer, presented a mechanochemistry rate constant similar to that of the methacrylate homopolymer, while a random copolymer reacted like the acrylate homopolymer. Size-exclusion chromatography showed that, in addition to the typical main peak shift towards higher retention times, a different daughter fragment was produced indicating a secondary selective scission site, situated around the covalent connection between the two blocks. Molecular dynamics modeling using acrylate and methacrylate oligomers were carried out and indicated that dynamic phase separation occurs even in a good solvent. Such non-random conformations can explain the faster polymer mechanochemistry. Moreover, the dynamic model for end-to-end chain overstretching supports bond scission which is not necessarily chain-centered.

Deep-red photoluminescent mechanoresponsive polymers with dynamic CuI-arylamide mechanophores

We demonstrate the use of copper arylamide complexes as efficient photoluminescent mechanophores to design deep-red/near-IR emissive polymers showing reversible changes in photoluminescence intensity in the red/near-IR region in response to mechanical stretching. The mechanoresponse was repeatable over 30 cycles, showing a measurable increase of photoluminescence intensity even at a small applied stress of ca. 0.01 MPa. We demonstrate the potential of using conformationally dynamic copper amide complexes as sensitive and reversible mechanophores for near-IR imaging; systematic control over the emission range was achieved using amide modification.

Ultrasound programmable hydrogen-bonded organic frameworks for sono-chemogenetics

The precise control of mechanochemical activation within deep tissues via non-invasive ultrasound holds profound implications for advancing our understanding of fundamental biomedical sciences and revolutionizing disease treatments. However, a theory-guided mechanoresponsive materials system with well-defined ultrasound activation has yet to be explored. Here we present the concept of using porous hydrogen-bonded organic frameworks (HOFs) as toolkits for focused ultrasound programmably triggered drug activation to control specific cellular events in the deep brain, through on-demand scission of the supramolecular interactions. A theoretical model is developed to visualize the mechanochemical scission and ultrasound mechanics, providing valuable guidelines for the rational design of mechanoresponsive materials at the molecular level to achieve programmable and spatiotemporal activation control. To demonstrate the practicality of this approach, we encapsulate designer drug clozapine N-oxide (CNO) into the optimal HOF nanoparticles for FUS gated release to activate engineered G-protein-coupled receptors in the mice and rat ventral tegmental area (VTA), and hence achieved targeted neural circuits modulation even at depth 9 mm with a latency of seconds. This work demonstrates the capability of ultrasound to precisely control molecular interaction and develops ultrasound programmable HOFs to minimally invasive and spatiotemporally control cellular events, thereby facilitating the establishment of precise molecular therapeutic possibilities. We anticipate that this research could serve as a source of inspiration for precise and non-invasive molecular manipulation techniques, potentially applicable in programming molecular robots to achieve sophisticated control over cellular events in deep tissues.

Shock-Induced Degradation of Guanosine and Uridine Promoted by Nickel and Carbonate: Potential Applications

Experimental studies of the degradation of two ribonucleosides (guanosine and uridine) were carried out by making use of mechanochemistry. Mechanochemical experiments reveal the decomposition of guanosine and uridine, promoted by nickel(II) and carbonate ions, into guanine and uracil, respectively. These nucleobases were identified by HPLC and 1H NMR spectroscopy (this applied only to uracil). Additionally, density-functional theory (DFT) methodologies were used to probe the energetic viability of several degradation pathways, including in the presence of the abovementioned ions. Three mechanisms were analysed via ribose ring-opening: dry, single-molecule water-assisted, and metal-assisted, wherein the last two mechanisms confirmed the mechanochemical degradation of both ribonucleosides into respective nucleobase moieties. These results can contribute to an astrobiological interpretation of the extraterrestrial sample's contents.

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.

Triggered Crosslinking of Main-Chain Enediyne Polyurethanes via Bergman Cyclization

Crosslinking chemistries occupy an important position in polymer modification with a particular importance when triggered in response to external stimuli. Enediyne (EDY) moieties are used as functional entities in this work, known to undergo a pericyclic Bergman cyclization (BC) to induce a triggered crosslinking of polyurethanes (PU) via the intermediately formed diradicals. Diamino-EDYs, where the distance between the enyne-moieties is known to be critical to induce a BC, are placed repetitively as main-chain structural elements in isophorone-based PUs to induce reinforcement upon heating, compression, or stretching. A 7-day compression under room temperature results in a ≈69% activation of the BC, together with the observation of an increase in tensile strength by 62% after 25 stretching cycles. The occurrence of BC is further proven by the decreased exothermic values in differential scanning calorimetry, together with characteristic peaks of the formed benzene moieties via IR spectroscopy. Purely heat-induced crosslinking contributes to 191% of the maximum tensile strength in comparison to the virgin PU. The BC herein forms an excellent crosslinking strategy, triggered by heat or force in PU materials.

Semi-Telechelic Polymers from Mechanochemical C─C Bond Activation

Unstrained C─C bond activation is attained in homopolymers through mechanochemical bond scission followed by functionalization to yield mostly semi-telechelic polymer chains. Ball milling poly(ethylene oxide) (PEO) in the presence of 1-(bromoacetyl)pyrene (BAPy) yields the pyrene terminated PEO. Similarly, milling with 2,4'-dibromoacetophenone followed by Suzuki coupling allows the introduction of various aryl end groups. PEOs with a molecular weight below 20 kDa show no functionalization, supporting a mechanochemical mechanism. The protocol is also tested with doxorubicin, yielding the drug-polymer conjugate. PEO halogenation is also demonstrated by milling PEO with iodine, N-bromosuccinimide, or N-iodosuccinimide, which can then be reacted with an amine substituted anthracene. Grinding additional carbon polymers with BAPy indicates that this functionalization method is general for different polymer chemistries.

Dual reactivity based dynamic covalent chemistry: mechanisms and applications

Dynamic covalent chemistry (DCC) focuses on the reversible formation, breakage, and exchange of covalent bonds and assemblies, setting a bridge between irreversible organic synthesis and supramolecular chemistry and finding wide utility. In order to enhance structural and functional diversity and complexity, different types of dynamic covalent reactions (DCRs) are placed in one vessel, encompassing orthogonal DCC without crosstalk and communicating DCC with a shared reactive functional group. As a means of adding tautomers, widespread in chemistry, to interconnected DCRs and combining the features of orthogonal and communicating DCRs, a concept of dual reactivity based DCC and underlying structural and mechanistic insights are summarized. The manipulation of the distinct reactivity of structurally diverse ring-chain tautomers allows selective activation and switching of reaction pathways and corresponding DCRs (C-N, C-O, and C-S) and assemblies. The coupling with photoswitches further enables light-mediated formation and scission of multiple types of reversible covalent bonds. To showcase the capability of dual reactivity based DCC, the versatile applications in dynamic polymers and luminescent materials are presented, paving the way for future functionalization studies.

Strain-Driven Formal [1,3]-Aryl Shift within Molecular Bows

Delving into the influence of strain on organic reactions in small molecules at the molecular level can unveil valuable insight into developing innovative synthetic strategies and structuring molecules with superior properties. Herein, we present a molecular-strain engineering approach to facilitate the consecutive [1,2]-aryl shift (formal [1,3]-aryl shift) in molecular bows (MBs) that integrate 1,4-dimethoxy-2,5-cyclohexadiene moieties. By introducing ring strain into MBs through tethering the bow limb, we can harness the intrinsic mechanical forces to drive multistep aryl shifts from the para- to the meta- to the ortho-position. Through the use of precise intramolecular strain, the seemingly impractical [1,3]-aryl shift was realized, resulting in the formation of ortho-disubstituted products. The solvent and temperature play a crucial role in the occurrence of the [1,3]-aryl shift. The free energy calculations with inclusion of solvation support a feasible mechanism, which entails multistep carbocation rearrangements, for the formal [1,3]-aryl shift. By exploring the application of molecular strain in synthetic chemistry, this research offers a promising direction for developing new tools and strategies towards precision organic synthesis.

Anomalous photochromism and mechanochromism of a linear naphthopyran enabled by a polarizing dialkylamine substituent

In contrast to common angular naphthopyrans that exhibit strong photochromic and mechanochromic behavior, constitutionally isomeric linear naphthopyrans are typically not photochromic, due to the putative instability of the completely dearomatized merocyanine product. The photochemistry of linear naphthopyrans is thus relatively understudied compared to angular naphthopyrans, while the mechanochromism of linear naphthopyrans remains completely unexplored. Here we demonstrate that the incorporation of a polarizing dialkylamine substituent enables photochromic and mechanochromic behavior from polymers containing a novel linear naphthopyran motif. In solution phase experiments, a Lewis acid trap was necessary to observe accumulation of the merocyanine product upon photochemical and ultrasound-induced mechanochemical activation. However, the same linear naphthopyran molecule incorporated as a crosslinker in polydimethylsiloxane elastomers renders the materials photochromic and mechanochromic without the addition of any trapping agent. This study provides insights into the photochromic and mechanochromic reactivity of linear naphthopyrans that have conventionally been considered functionally inert, adding a new class of naphthopyran molecular switches to the repertoire of stimuli-responsive polymers.

Mechanoluminescent Materials Enable Mechanochemically Controlled Atom Transfer Radical Polymerization and Polymer Mechanotransduction

Organic mechanophores have been widely adopted for polymer mechanotransduction. However, most examples of polymer mechanotransduction inevitably experience macromolecular chain rupture, and few of them mimic mussel's mechanochemical regeneration, a mechanically mediated process from functional units to functional materials in a controlled manner. In this paper, inorganic mechanoluminescent (ML) materials composed of CaZnOS-ZnS-SrZnOS: Mn2+ were used as a mechanotransducer since it features both piezoelectricity and mechanolunimescence. The utilization of ML materials in polymerization enables both mechanochemically controlled radical polymerization and the synthesis of ML polymer composites. This procedure features a mechanochemically controlled manner for the design and synthesis of diverse mechanoresponsive polymer composites.

Naphthopyran molecular switches and their emergent mechanochemical reactivity

Naphthopyran molecular switches undergo a ring-opening reaction upon external stimulation to generate intensely colored merocyanine dyes. Their unique modularity and synthetic accessibility afford exceptional control over their properties and stimuli-responsive behavior. Commercial applications of naphthopyrans as photoswitches in photochromic ophthalmic lenses have spurred an extensive body of work exploring naphthopyran-merocyanine structure-property relationships. The recently discovered mechanochromic behavior of naphthopyrans has led to their emergent application in the field of polymer mechanochemistry, enabling advances in the design of force-responsive materials as well as fundamental insights into mechanochemical reactivity. The structure-property relationships established in the photochemical literature serve as a convenient blueprint for the design of naphthopyran molecular force probes with precisely tuned properties. On the other hand, the mechanochemical reactivity of naphthopyran diverges in many cases from the conventional photochemical pathways, resulting in unexpected properties and opportunities for deeper understanding and innovation in polymer mechanochemistry. Here, we highlight the features of the naphthopyran scaffold that render it a powerful platform for the design of mechanochromic materials and review recent advances in naphthopyran mechanochemistry.

Remote control of mechanochemical reactions under physiological conditions using biocompatible focused ultrasound

External control of chemical reactions in biological settings with spatial and temporal precision is a grand challenge for noninvasive diagnostic and therapeutic applications. While light is a conventional stimulus for remote chemical activation, its penetration is severely attenuated in tissues, which limits biological applicability. On the other hand, ultrasound is a biocompatible remote energy source that is highly penetrant and offers a wide range of functional tunability. Coupling ultrasound to the activation of specific chemical reactions under physiological conditions, however, remains a challenge. Here, we describe a synergistic platform that couples the selective mechanochemical activation of mechanophore-functionalized polymers with biocompatible focused ultrasound (FUS) by leveraging pressure-sensitive gas vesicles (GVs) as acousto-mechanical transducers. The power of this approach is illustrated through the mechanically triggered release of covalently bound fluorogenic and therapeutic cargo molecules from polymers containing a masked 2-furylcarbinol mechanophore. Molecular release occurs selectively in the presence of GVs upon exposure to FUS under physiological conditions. These results showcase the viability of this system for enabling remote control of specific mechanochemical reactions with spatiotemporal precision in biologically relevant settings and demonstrate the translational potential of polymer mechanochemistry.

Preferential Mechanochemical Activation of Short Chains in Bidisperse Triblock Elastomers

Polymer mechanochemistry offers attractive opportunities for using macroscopic forces to drive molecular-scale chemical transformations, but achieving efficient activation in bulk polymeric materials has remained challenging. Understanding how the structure and topology of polymer networks impact molecular-scale force distributions is critical for addressing this problem. Here we show that in block copolymer elastomers the molecular-scale force distributions and mechanochemical activation yields are strongly impacted by the molecular weight distribution of the polymers. We prepare bidisperse triblock copolymer elastomers with spiropyran mechanophores placed in either the short chains, the long chains, or both and show that the overall mechanochemical activation of the materials is dominated by the short chains. Molecular dynamics simulations reveal that this preferential activation occurs because pinning of the ends of the elastically effective midblocks to the glassy/rubbery interface forces early extension of the short chains. These results suggest that microphase segregation and network strand dispersity play a critical role in determining molecular-scale force distributions and suggest that selective placement of mechanophores in microphase-segregated polymers is a promising design strategy for efficient mechanochemical activation in bulk materials.

Acoustodynamic Covalent Materials Engineering for the Remote Control of Physical Properties Inside Materials

Advances in vat photopolymerization (VP) 3D printing (3DP) technology enable the production of highly precise 3D objects. However, it is a major challenge to create dynamic functionalities and to manipulate the physical properties of the inherently insoluble and infusible cross-linked material generated from VP-3DP without reproduction. The fabrication of light- and high-intensity focused ultrasound (HIFU)-responsive cross-linked polymeric materials linked with hexaarylbiimidazole (HABI) in polymer chains based on VP-3DP is reported here. Although the photochemistry of HABI produces triphenylimidazolyl radicals (TPIRs) during the process of VP-3DP, the orthogonality of the photochemistry of HABI and photopolymerization enables the introduction of reversible cross-links derived from HABIs in the resulting 3D-printed objects. While photostimulation cleaves a covalent bond between two imidazoles in HABI to generate TPIRs only near the surface of the 3D-printed objects, HIFU triggers cleavage in the interior of materials. In addition, HIFU travels beyond an obstacle to induce a response of HABI-embedded cross-linked polymers, which cannot be attainable with photostimulation. The present system would be beneficial for tuning the physical properties and recycling of various polymeric materials, but it will also open the door for pinpoint modification, healing, and reshaping of materials when coupled to various dynamic covalent materials.

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.

Microscopic strain mapping in polymers equipped with non-covalent mechanochromic motifs

The mechanical failure of polymers remains challenging to understand and predict, as it often involves highly localised phenomena that cannot be probed with bulk characterisation techniques. Here, we present a generalisable protocol based on optical microscopy, tensile testing, and image processing that permits the spatially resolved interrogation of mechanical deformation at the molecular level around defects in mechanophore-containing polymers. The approach can be applied to a broad range of polymeric materials, mechanophores, and deformation scenarios.

Enhancing Ultrasound-Assisted Iodine-Mediated Reversible-Deactivation Radical Polymerization by Piezoelectric Nanoparticles

The development of mechanochemical tools for regulating the polymerization process has received an increasing amount of attention in recent years. Herein, we report the example of the mechanically controlled iodine-mediated reversible-deactivation radical polymerization (mechano-RDRP) using piezoelectric tetragonal BaTiO3 nanoparticles (T-BTO) as mechanoredox catalyst and alkyl iodide as the initiator. We demonstrated a more efficient mechanochemical initiation and reversible deactivation process than sonochemical activation via a mechanoredox-mediated alkyl iodide cleavage reaction. The mechanochemical activation of the C-I bond was verified by density functional theory (DFT) calculations. Theoretical calculations together with experimental results confirmed the more efficient initiation and polymerization than the traditional sonochemical approach. The influence of BaTiO3, initiator, and solvent was further examined to reveal the mechanism of the mechano-RDRP. The results showed good controllability over molecular weight and capacity for a one-pot chain extension. This work expands the scope of mechanically controlled polymerization and shows good potential in the construction of adaptive materials.

Force-Triggered Atropisomerization of a Parallel Diarylethene to Its Antiparallel Diastereomers

This paper describes a mechanical approach to inducing the atropisomerization of a parallel diarylethene into its antiparallel diastereomers exhibiting distinct chemical reactivity. A congested parallel diarylethene mechanophore in the (R_a,S_a)-configuration with mirror symmetry is atropisomerized to its antiparallel diastereomers with C₂ symmetry under ultrasound-induced force field. The resulting stereochemistry-converted material gains symmetry-allowed reactivity toward conrotatory photocyclization.

A push for mechanochemistry

Chemical reactions can be accelerated by directional local pressure applied to molecules.

Acceleration of Diels-Alder reactions by mechanical distortion

Challenges in quantifying how force affects bond formation have hindered the widespread adoption of mechanochemistry. We used parallel tip-based methods to determine reaction rates, activation energies, and activation volumes of force-accelerated [4+2] Diels-Alder cycloadditions between surface-immobilized anthracene and four dienophiles that differ in electronic and steric demand. The rate dependences on pressure were unexpectedly strong, and substantial differences were observed between the dienophiles. Multiscale modeling demonstrated that in proximity to a surface, mechanochemical trajectories ensued that were distinct from those observed solvothermally or under hydrostatic pressure. These results provide a framework for anticipating how experimental geometry, molecular confinement, and directed force contribute to mechanochemical kinetics.

Designing Site Specificity in the Mechanochemical Cargo Release of Small Molecules

Mechanical force can trigger the predictable and precise release of small molecules from macromolecular carriers. In this article, based on mechanochemical simulations, we show that norborn-2-en-7-one (NEO), I, and its derivatives can selectively release CO, N₂, and SO₂ and produce two distinctly different products, A ((3E,5Z,7E)-dimethyl-5,6-diphenyldeca-3,5,7-triene-1,10-diyl bis(2-bromo-2-methylpropanoate)) and B (4',5'-dimethyl-4',5'-dihydro-[1,1':2',1''-terphenyl]-3',6'-diyl)bis(ethane-2,1-diyl) bis(2-bromo-2-methylpropanoate). Site-specific design in the pulling points (PP) ensures that by changing the regioselectivity, either A or B can be exclusively generated. Controlling the rigidity of the NEO scaffold by replacing a 6-membered ring with an 8-membered ring and concomitantly tuning the pulling groups makes it mechanolabile toward the selective formation of B. The diradical intermediate formed during I → A is predicted to be persistent for ∼150 fs. The structural design holds the key to the trade-off between mechanochemical rigidity and lability.

CO2-responsive gels

CO₂-responsive materials undergo a change in chemical or physical properties in response to the introduction or removal of CO₂. The use of CO₂ as a stimulus is advantageous as it is abundant, benign, inexpensive, and it does not accumulate in a system. Many CO₂-responsive materials have already been explored including polymers, latexes, surfactants, and catalysts. As a sub-set of CO₂-responsive polymers, the study of CO₂-responsive gels (insoluble, cross-linked polymers) is a unique discipline due to the unique set of changes in the gels brought about by CO₂ such as swelling or a transformed morphology. In the past 15 years, CO₂-responsive gels and self-assembled gels have been investigated for a variety of emerging potential applications, reported in 90 peer-reviewed publications. The two most widely exploited properties include the control of flow (fluids) via CO₂-triggered aggregation and their capacity for reversible CO₂ absorption-desorption, leading to applications in Enhanced Oil Recovery (EOR) and CO₂ sequestration, respectively. In this paper, we review the preparation, properties, and applications of these CO₂-responsive gels, broadly classified by particle size as nanogels, microgels, aerogels, and macrogels. We have included a section on CO₂-induced self-assembled gels (including poly(ionic liquid) gels).

Hierarchically Structured Deformation-Sensing Mechanochromic Pigments

Mechanochromic materials alter their color in response to mechanical force and are useful for both fundamental studies and practical applications. Several approaches are used to render polymers mechanochromic, but they generally suffer from limitations in sensing range, capacity to provide quantitative information, and their capability to enable broad and simple implementation. Here, is it reported that these problems can be overcome by combining photonic structures, which alter their reflection upon deformation, with covalent mechanophores, whose spectral properties change upon mechanically induced bond scission, in hierarchically structured mechanochromic pigments. This is achieved by synthesizing microspheres consisting of an elastic polymer with spiropyran-based cross-links and non-close-packed silica nanoparticles. A strain of less than 1% can be detected in a shift of the reflection band from the photonic structure, while the onset strain for the conversion of the spiropyran into fluorescent merocyanine ranges from 30% to 70%, creating a broad strain detection range. The two responses are tailorable and synergistic, permitting the activation strain for the mechanophore response to be tuned. The mechano-sensing photonic pigments are demonstrated to be readily incorporated into different polymeric materials of interest and quantitatively probe spatially heterogeneous deformations over a large strain range.