Relative Mechanical Strengths of Weak Bonds in Sonochemical Polymer Mechanochemistry

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.

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.

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.

Mechanochemical Backbone Editing for Controlled Degradation of Vinyl Polymers

The chemically inert nature of fully saturated hydrocarbon backbones endows vinyl polymers with desirable durability, but it also leads to their significant environmental persistence. Enhancing the sustainability of these materials requires a pivotal yet challenging shift: transforming the inert backbone into one that is degradable. Here, we present a versatile platform for mechanochemically editing the fully saturated backbone of vinyl polymers towards degradable polymer chains by integrating cyclobutene-fused succinimide (CBS) units along backbone through photo-iniferter reversible addition-fragmentation chain-transfer (RAFT) copolymerization. Significantly, the evenly insertion of CBS units does not compromise thermal or chemical stability but rather offers a means to adjust the properties of polymethylacrylate (PMA). Meanwhile, reactive acyclic imide units can be selectively introduced to the backbone through mechanochemical activation (pulse ultrasonication or ball-milling grinding) when required. Subsequent hydrolysis of the acyclic imide groups enables efficient degradation, yielding telechelic oligomers. This approach holds promise for inspiring the design and modification of more environmentally friendly vinyl polymers through backbone editing.

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.

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.

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.

Mechanically Triggered Polymer Deconstruction through Mechanoacid Generation and Catalytic Enol Ether Hydrolysis

Polymers that amplify a transient external stimulus into changes in their morphology, physical state, or properties continue to be desirable targets for a range of applications. Here, we report a polymer comprising an acid-sensitive, hydrolytically unstable enol ether backbone onto which is embedded gem-dichlorocyclopropane (gDCC) mechanophores through a single postsynthetic modification. The gDCC mechanophore releases HCl in response to large forces of tension along the polymer backbone, and the acid subsequently catalyzes polymer deconstruction at the enol ether sites. Pulsed sonication of a 61 kDa PDHF with 77% gDCC on the backbone in THF with 100 mM H2O for 10 min triggers the subsequent degradation of the polymer to a final molecular weight of less than 3 kDa after 24 h of standing, whereas controls lacking either the gDCC or the enol ether reach final molecular weights of 38 and 27 kDa, respectively. The process of sonication, along with the presence of water and the existence of gDCC on the backbone, significantly accelerates the rate of polymer chain deconstruction. Both acid generation and the resulting triggered polymer deconstruction are translated to bulk, cross-linked polymer networks. Networks formed via thiol-ene cross-linking and subjected to unconstrained quasi-static uniaxial compression dissolve on time scales that are at least 3 times faster than controls where the mechanophore is not covalently coupled to the network. We anticipate that this concept can be extended to other acid-sensitive polymer networks for the stress-responsive deconstruction of gels and solvent-free elastomers.

Mechanochemistry of Pterodactylane

Pterodactylane is a [4]-ladderane with substituents on the central rung. Comparing the mechanochemistry of the [4]-ladderane structure when pulled from the central rung versus the end rung revealed a striking difference in the threshold force of mechanoactivation: the threshold force is dramatically lowered from 1.9 nN when pulled on the end rung to 0.7 nN when pulled on the central rung. We investigated the bicyclic products formed from the mechanochemical activation of pterodactylane experimentally and computationally, which are distinct from the mechanochemical products of ladderanes being activated from the end rung. We compared the products of pterodactylane's mechanochemical and thermal activation to reveal differences and similarities in the mechanochemical and thermal pathways of pterodactylane transformation. Interestingly, we also discovered the presence of elementary steps that are accelerated or suppressed by force within the same mechanochemical reaction of pterodactylane, suggesting rich mechanochemical manifolds of multicyclic structures. We rationalized the greatly enhanced mechanochemical reactivity of the central rung of pterodactylane and discovered force-free ground state bond length to be a good low-cost predictor of the threshold force for cyclobutane-based mechanophores. These findings advance our understanding of mechanochemical reactivities and pathways, and they will guide future designs of mechanophores with low threshold forces to facilitate their applications in force-responsive materials.

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.

Mechanochemical Degradation of Biopolymers

Mechanochemical treatment of various organic molecules is an emerging technology of green processes in biofuel, fine chemicals, or food production. Many biopolymers are involved in isolating, derivating, or modifying molecules of natural origin. Mechanochemistry provides a powerful tool to achieve these goals, but the unintentional modification of biopolymers by mechanochemical manipulation is not always obvious or even detectable. Although modeling molecular changes caused by mechanical stresses in cavitation and grinding processes is feasible in small model compounds, simulation of extrusion processes primarily relies on phenomenological approaches that allow only tool- and material-specific conclusions. The development of analytical and computational techniques allows for the inline and real-time control of parameters in various mechanochemical processes. Using artificial intelligence to analyze process parameters and product characteristics can significantly improve production optimization. We aim to review the processes and consequences of possible chemical, physicochemical, and structural changes.

Nonlocal Intrinsic Fracture Energy of Polymerlike Networks

Connecting polymer network fracture to molecular-level chain scission remains a quandary. While the Lake-Thomas model predicts the intrinsic fracture energy of a polymer network is the energy to rupture a layer of chains, it underestimates recent experiments by ∼1-2 orders of magnitude. Here we show that the intrinsic fracture energy of polymerlike networks stems from nonlocal energy dissipation by relaxing chains far from the crack tip using experiments and simulations of 2D and 3D networks with varying defects, dispersity, topologies, and length scales. Our findings not only provide physical insights into polymer network fracture but offer design principles for tough architected materials.

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.

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.

Covalent Mechanochemistry and Contemporary Polymer Network Chemistry: A Marriage in the Making

Over the past 20 years, the field of polymer mechanochemistry has amassed a toolbox of mechanophores that translate mechanical energy into a variety of functional responses ranging from color change to small-molecule release. These productive chemical changes typically occur at the length scale of a few covalent bonds (Å) but require large energy inputs and strains on the micro-to-macro scale in order to achieve even low levels of mechanophore activation. The minimal activation hinders the translation of the available chemical responses into materials and device applications. The mechanophore activation challenge inspires core questions at yet another length scale of chemical control, namely: What are the molecular-scale features of a polymeric material that determine the extent of mechanophore activation? Further, how do we marry advances in the chemistry of polymer networks with the chemistry of mechanophores to create stress-responsive materials that are well suited for an intended application? In this Perspective, we speculate as to the potential match between covalent polymer mechanochemistry and recent advances in polymer network chemistry, specifically, topologically controlled networks and the hierarchical material responses enabled by multi-network architectures and mechanically interlocked polymers. Both fundamental and applied opportunities unique to the union of these two fields are discussed.

Off-center Mechanophore Activation in Block Copolymers

Block copolymers (BCPs) are used in numerous applications in modern materials science. Yet, like homopolymers, BCPs can undergo covalent bond scission when mechanically stressed (mechanochemistry), which could lead to unexpected consequences in such applications. BCPs' heterogeneity may affect force transduction, perhaps changing force distribution and localization. To verify this, a gem-dichlorocyclopropane (gDCC) embedded linear chain is prepared and extended with a poly(methyl methacrylate) block. When stressed in solution, the mechanochemical ring-opening of gDCC is accelerated compared to homopolymers, even though the mechanophores are at the chain ends. Moreover, a higher mechanophore activation selectivity is obtained. These results indicate that mechanochemical response outside, and even far from the chain center is quite prominent in BCPs, and that forces along the polymer chain can efficiently activate multi-mechanophores regions, even when far from the polymer midchain.

Competitive Activation Experiments Reveal Significantly Different Mechanochemical Reactivity of Furan–Maleimide and Anthracene–Maleimide Mechanophores

During the past two decades, our understanding of mechanochemical reactivity has advanced considerably. Nevertheless, an incomplete knowledge of structure-activity relationships and the principles that govern mechanochemical transformations limits molecular design. The experimental development of mechanophores has thus benefited from simple computational tools like CoGEF, from which quantitative metrics like rupture force can be extracted to estimate reactivity. Furan-maleimide (FM) and anthracene-maleimide (AM) Diels-Alder adducts are widely studied mechanophores that undergo retro-Diels-Alder reactions upon mechanical activation in polymers. Despite possessing significantly different thermal stability, similar rupture forces predicted by CoGEF calculations suggest that these compounds exhibit similar mechanochemical reactivity. Here, we directly probe the relative mechanochemical reactivity of FM and AM adducts through competitive activation experiments. Ultrasound-induced mechanochemical activation of bis-adduct mechanophores comprising covalently tethered FM and AM subunits reveals pronounced selectivity-as high as ∼13:1-for reaction of the FM adduct compared to the AM adduct. Computational models provide insight into the greater reactivity of the FM mechanophore, indicating a more efficient mechanochemical coupling for the FM adduct compared to the AM adduct. The methodology employed here to directly interrogate the relative reactivity of two different mechanophores using a tethered bis-adduct configuration may be useful for other systems where more common sonication-based approaches are limited by poor sensitivity.

Ultrasound-Induced Drug Release from Stimuli-Responsive Hydrogels

Stimuli-responsive hydrogel drug delivery systems are designed to release a payload when prompted by an external stimulus. These platforms have become prominent in the field of drug delivery due to their ability to provide spatial and temporal control for drug release. Among the different external triggers that have been used, ultrasound possesses several advantages: it is non-invasive, has deep tissue penetration, and can safely transmit acoustic energy to a localized area. This review summarizes the current state of understanding about ultrasound-responsive hydrogels used for drug delivery. The mechanisms of inducing payload release and activation using ultrasound are examined, along with the latest innovative formulations and hydrogel design strategies. We also report on the most recent applications leveraging ultrasound activation for both cancer treatment and tissue engineering. Finally, the future perspectives offered by ultrasound-sensitive hydrogels are discussed.

Examining the Impact of Relative Mechanophore Activity on the Selectivity of Ultrasound-Induced Mechanochemical Chain Scission

Despite recent advances in polymer mechanochemistry, a more complete understanding of the factors that dictate the ultrasound-induced mechanochemical activation efficiency of mechanophores is necessary. Here, we examine how the identity of a mechanophore, and hence its unique force-coupled reactivity, affects the competition between mechanophore activation and nonspecific polymer backbone scission. Polymers incorporating distinct mechanophores but with putatively similar "chain-centeredness" exhibit widely different mechanochemical activation efficiencies. Furthermore, we employ mechanophores that can be orthogonally cleaved following ultrasonication using heat or light to report on the degree of nonspecific backbone scission that occurs for different mechanophore-containing polymers subjected to ultrasound-induced mechanical force. Our results illustrate that the identity of the mechanophore as well as its position in the polymer chain are inextricably important parameters that together control the selectivity of mechanophore activation during ultrasonication.

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

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

Harnessing the Power of Force: Development of Mechanophores for Molecular Release

Polymers that release small molecules in response to mechanical force are promising materials for a variety of applications ranging from sensing and catalysis to targeted drug delivery. Within the rapidly growing field of polymer mechanochemistry, stress-sensitive molecules known as mechanophores are particularly attractive for enabling the release of covalently bound payloads with excellent selectivity and control. Here, we review recent progress in the development of mechanophore-based molecular release platforms and provide an optimistic, yet critical perspective on the fundamental and technological advancements that are still required for this promising research area to achieve significant impact.

Ultrasound-controlled drug release and drug activation for cancer therapy

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

Heterocyclic Mechanophores in Polymer Mechanochemistry

This Account covers the recent progress made on heterocyclic mechanophores in the field of polymer mechanochemistry. In particular, the types of such mechanophores as well as the mechanisms and applications of their force-induced structural transformations are discussed and related perspectives and future challenges proposed.

1 Introduction

2 Types of Mechanophores

3 Methods to Incorporate Heterocycle Mechanophores into Polymer Systems

4 Mechanochemical Reactions of Heterocyclic Mechanophores

4.1 Three-Membered-Ring Mechanophores

4.2 Four-Membered-Ring Mechanophores

4.3 Six-Membered-Ring Mechanophores

4.4 Bicyclic Mechanophores

5 Applications

5.1 Cross-Linking of Polymer

5.2 Degradable Polymer

5.3 Mechanochromic Polymer

6 Concluding Remarks and Outlook

Sonofragmentation of Organic Molecular Crystals vs Strength of Materials

Mechanochemistry, the interface between the chemical and the mechanical worlds, includes the relationship between the chemical and mechanical properties of solids. In this work, fragmentation of organic molecular crystals during ultrasonic irradiation of slurries has been quantitatively investigated. This has particular relevance to nucleation processes during sonocrystallization, which is increasingly used in the processing and formulation of numerous pharmaceutical agents (PAs). We have discovered that the rates of sonofragmentation are very strongly correlated with the strength of the materials (as measured by Vickers hardness and Young's modulus). This is a mechanochemical extension of the Bell-Evans-Polanyi Principle or Hammond's Postulate: the kinetics (i.e., rates) of solid fracture correlate with thermodynamic properties of solids (e.g., Young's modulus). The mechanism of the particle breakage is consistent with a direct interaction between the shockwaves or localized microjets created by the ultrasound (through acoustic cavitation) and the solid particles in the slurry. Comparisons of the sonofragmentation patterns of ionic and molecular crystals showed that ionic crystals are more sensitive to sonofragmentation than molecular crystals for a given Young's modulus. The rates of sonofragmentation are proposed to correlate with the types and densities of imperfections in the crystals.

Reversible regulation of metallo-base-pair interactions for DNA dehybridization by ultrasound

Mechanical force applied by ultrasound in solution leads to the dissociation of DNA metallo-base-pair interactions when these motifs are functionalized with oligodeoxynucleotide sequences of sufficient length. The annealing and force-induced denaturing process is followed by the attachment of distance-sensitive fluorescent probes and is found to be reversible.

Multiresponsive Tetra-Arylethene-Based Fluorescent Switch with Multicolored Changes: Single-Crystal Photochromism, Mechanochromism, and Acidichromism

In this article, we report a simple tetra-arylethene-based fluorescent switch TPS═C4 modified with a flexible alkyl chain by the Schiff base structure. The incorporation of C═N retains the excellent photochromic property of tetra-arylethene and endows TPS═C4 with new multiresponsiveness of mechanochromism and acidichromism and multicolor changes. TPS═C4 shows remarkable mechanochromism from a deep blue emission at 420 nm to bright blue with a new shoulder band at 450 nm, which arises from the force-induced phase transition from the crystal state to an amorphous form. Both the original crystalline powder and the ground amorphous sample exhibit interesting acidichromism, and their emission colors turn yellow (530 nm), due to the protonation of C═N. More interestingly, TPS═C4 displays fascinating photochromism in multiple states, especially in the single-crystal state. The flexible alkyl chain offers enough free space for molecular motion and facilitates single-crystal photochromism. Due to the multiresponsiveness and multicolor switch, TPS═C4 can be satisfactorily used for the multidimensional anticounterfeiting application. To the best of our knowledge, TPS═C4 is a rare multifunctional molecule with a simple structure but featuring multiresponsiveness and multicolor switch.

Force-modulated reductive elimination from platinum(ii) diaryl complexes

Coupled mechanical forces are known to drive a range of covalent chemical reactions, but the effect of mechanical force applied to a spectator ligand on transition metal reactivity is relatively unexplored. Here we quantify the rate of C(sp2)-C(sp2) reductive elimination from platinum(ii) diaryl complexes containing macrocyclic bis(phosphine) ligands as a function of mechanical force applied to these ligands. DFT computations reveal complex dependence of mechanochemical kinetics on the structure of the force-transducing ligand. We validated experimentally the computational finding for the most sensitive of the ligand designs, based on MeOBiphep, by coupling it to a macrocyclic force probe ligand. Consistent with the computations, compressive forces decreased the rate of reductive elimination whereas extension forces increased the rate relative to the strain-free MeOBiphep complex with a 3.4-fold change in rate over a ∼290 pN range of restoring forces. The calculated natural bite angle of the free macrocyclic ligand changes with force, but 31P NMR analysis and calculations strongly suggest no significant force-induced perturbation of ground state geometry within the first coordination sphere of the (P-P)PtAr2 complexes. Rather, the force/rate behavior observed across this range of forces is attributed to the coupling of force to the elongation of the O⋯O distance in the transition state for reductive elimination. The results suggest opportunities to experimentally map geometry changes associated with reactions in transition metal complexes and potential strategies for force-modulated catalysis.

Mechanochemistry of Cationic Cobaltocenium Mechanophore

Recent research on the mechanochemistry of metallocene mechanophores has shed light on the force-responsiveness of these thermally and chemically stable organometallic compounds. In this work, we report a combination of experimental and computational studies on the mechanochemistry of main-chain cobaltocenium-containing polymers. Ester derivatives of the cationic cobaltocenium, though isoelectronic to neutral ferrocene, are unstable in the nonmechanical control experimental conditions that were accommodated by their ferrocene analogs. Replacing the electron withdrawing C-ester linkages with electron-donating C-alkyls conferred the necessary stability and enabled the mechanochemistry of the cobaltocenium to be assessed. Despite their high bond dissociation energy, cobaltocenium mechanophores are found to be selective sites of main chain scission under sonomechanical activation. Computational CoGEF calculations suggest that the presence of a counterion to cobaltocenium plays a vital role by promoting a peeling mechanism of dissociation in conjunction with the initial slipping.

Clip Chemistry: Diverse (Bio)(macro)molecular and Material Function through Breaking Covalent Bonds

In the two decades since the introduction of the "click chemistry" concept, the toolbox of "click reactions" has continually expanded, enabling chemists, materials scientists, and biologists to rapidly and selectively build complexity for their applications of interest. Similarly, selective and efficient covalent bond breaking reactions have provided and will continue to provide transformative advances. Here, we review key examples and applications of efficient, selective covalent bond cleavage reactions, which we refer to herein as "clip reactions." The strategic application of clip reactions offers opportunities to tailor the compositions and structures of complex (bio)(macro)molecular systems with exquisite control. Working in concert, click chemistry and clip chemistry offer scientists and engineers powerful methods to address next-generation challenges across the chemical sciences.

Mechanism Dictates Mechanics: A Molecular Substituent Effect in the Macroscopic Fracture of a Covalent Polymer Network

The fracture of rubbery polymer networks involves a series of molecular events, beginning with conformational changes along the polymer backbone and culminating with a chain scission reaction. Here, we report covalent polymer gels in which the macroscopic fracture "reaction" is controlled by mechanophores embedded within mechanically active network strands. We synthesized poly(ethylene glycol) (PEG) gels through the end-linking of azide-terminated tetra-arm PEG (M_n = 5 kDa) with bis-alkyne linkers. Networks were formed under identical conditions, except that the bis-alkyne was varied to include either a cis-diaryl (1) or cis-dialkyl (2) linked cyclobutane mechanophore that acts as a mechanochemical "weak link" through a force-coupled cycloreversion. A control network featuring a bis-alkyne without cyclobutane (3) was also synthesized. The networks show the same linear elasticity (G' = 23-24 kPa, 0.1-100 Hz) and equilibrium mass swelling ratios (Q = 10-11 in tetrahydrofuran), but they exhibit tearing energies that span a factor of 8 (3.4 J, 10.6, and 27.1 J·m^-2 for networks with 1, 2, and 3, respectively). The difference in fracture energy is well-aligned with the force-coupled scission kinetics of the mechanophores observed in single-molecule force spectroscopy experiments, implicating local resonance stabilization of a diradical transition state in the cycloreversion of 1 as a key determinant of the relative ease with which its network is torn. The connection between macroscopic fracture and a small-molecule reaction mechanism suggests opportunities for molecular understanding and optimization of polymer network behavior.

Single-Molecule Activation and Quantification of Mechanically Triggered Palladium-Carbene Bond Dissociation

Metal-complexed N-heterocyclic carbene (NHC) mechanophores are latent reactants and catalysts for a range of mechanically driven chemical responses, but mechanochemical scission of the metal-NHC bond has not been experimentally characterized. Here we report the single-molecule force spectroscopy of ligand dissociation from a pincer NHC-pyridine-NHC Pd(II) complex. The force-coupled rate constant for ligand dissociation reaches 50 s^-1 at forces of approximately 930 pN. Experimental and computational observations support a dissociative, rather than associative, mechanism of ligand displacement, with rate-limiting scission of the Pd-NHC bond followed by rapid dissociation of the pyridine moiety from Pd.

Modeling ultrasound-induced molecular weight decrease of polymers with multiple scissile azo-mechanophores

Selective and non-selective chain scission compete upon ultrasonic treatment of polymers with randomly distributed azo units.

Mechanochemical tools for polymer materials

This review aims to provide a field guide for the implementation of mechanochemistry in synthetic polymers by summarizing the molecules, materials, and methods that have been developed in this field.

Stress-responsive properties of metallocenes in metallopolymers

This review article combines the field of metallopolymers and stress-responsiveness on a molecular level, namely, metallocenes, as emerging stress-responsive building blocks for materials.

Mechanochemical activation of disulfide-based multifunctional polymers for theranostic drug release

Drug delivery systems responsive to physicochemical stimuli allow spatiotemporal control over drug activity to overcome limitations of systemic drug administration. Alongside, the non-invasive real-time tracking of drug release and uptake remains challenging as pharmacophore and reporter function are rarely unified within one molecule. Here, we present an ultrasound-responsive release system based on the mechanochemically induced 5-exo-trig cyclization upon scission of disulfides bearing cargo molecules attached via β-carbonate linker within the center of a water soluble polymer. In this bifunctional theranostic approach, we release one reporter molecule per drug molecule to quantitatively track drug release and distribution within the cell in real-time. We use N-butyl-4-hydroxy-1,8-naphthalimide and umbelliferone as fluorescent reporter molecules to accompany the release of camptothecin and gemcitabine as clinically employed anticancer agents. The generality of this approach paves the way for the theranostic release of a variety of probes and drugs by ultrasound.

A theranostic approach for the mechanochemically induced release of drugs is presented to track drug release and uptake in real-time.

Empowering self-reporting polymer blends with orthogonal optical properties responsive in a broader force range

Self-reporting polymers, which can indicate damage with perceptible optical signals in a tailored force range, are useful as stress-sensitive sensors. We demonstrate a simple approach to realize this function by embedding two distinct mechanophores - rhodamine (Rh) and bis(adamantyl)-1,2-dioxetane (Ad), in polyurethane/polylactic acid blends. The deformed blends generate red coloration and red chemiluminescence. Such a unique dual-responsive behavior was evaluated by solid-state UV-vis spectroscopy, macroscopic tensile tests with in situ RGB and light intensity analyses, which supported a stress-correlated occurrence of the ring-opening of Rh, the scission of Ad and the fluorescence resonance energy transfer process between the respective mechanochemical species. Complementarity stemming from the difference in properties and manifestations of the two mechanophores is essential. That is, the more labile Rh allows shifting the appreciable optical changes to a much lower force threshold; the transient nature and high dynamic range of mechanochemiluminescence from Ad map in real time where and when many of the covalently incorporated dioxetane bonds break; besides, the disrupted yet non-scissile structure of Rh acts as a fluorescent acceptor to effectively harvest chemiluminescence from ruptured Ad. The current strategy is thus empowering multi-functional mechano-responsive polymers with greatly improved sensitivity and resolution for multimodal stress reporting.

A Polymer with Mechanochemically Active Hidden Length

Incorporating hidden length into polymer chains can improve their mechanical properties, because release of the hidden length under mechanical loads enables localized strain relief without chain fracture. To date, the design of hidden length has focused primarily on the choice of the sacrificial bonds holding the hidden length together. Here we demonstrate the advantages of adding mechanochemical reactivity to hidden length itself, using a new mechanophore that integrates (Z)-2,3-diphenylcyclobutene-1,4-dicarboxylate, with hitherto unknown mechanochemistry, into macrocyclic cinnamate dimers. Stretching a polymer of this mechanophore more than doubles the chain contour length without fracture. DFT calculations indicate that the sequential dissociation of the dimer, followed by cyclobutene isomerization at higher forces yields a chain fracture energy 11 times that of a simple polyester of the same initial contour length and preserves high energy-dissipating capacity up to ∼3 nN. In sonicated solutions cyclobutene isomerizes to two distinct products by competing reaction paths, validating the computed mechanochemical mechanism and suggesting an experimental approach to quantifying the distribution of single-chain forces under diverse loading scenarios.

Mechanochemical Regulation of Oxidative Addition to a Palladium(0) Bisphosphine Complex

Here, we report the effect of force applied to the biaryl backbone of a bisphosphine ligand on the rate of oxidative addition of bromobenzene to a ligand-coordinated palladium center. Local compressive and tensile forces on the order of 100 pN were generated using a stiff stilbene force probe. A compressive force increases the rate of oxidative addition, whereas a tensile force decreases the rate, relative to that of the parent complex of strain-free ligand. Rates vary by a factor of ∼6 across ∼340 pN of force applied to the complexes. The crystal structures and DFT calculations support that force-induced perturbation of the geometry of the reactant is negligible. The force-rate relationship observed is mainly attributed to the coupling of force to nuclear motion comprising the reaction coordinate. These observations inform the development of catalysts whose activity can be tuned by an external force that is adjusted within a catalytic cycle.

Enhanced polymer mechanical degradation through mechanochemically unveiled lactonization

The mechanical degradation of polymers is typically limited to a single chain scission per triggering chain stretching event, and the loss of stress transfer that results from the scission limits the extent of degradation that can be achieved. Here, we report that the mechanically triggered ring-opening of a [4.2.0]bicyclooctene (BCOE) mechanophore sets up a delayed, force-free cascade lactonization that results in chain scission. Delayed chain scission allows many eventual scission events to be initiated within a single polymer chain. Ultrasonication of a 120 kDa BCOE copolymer mechanically remodels the polymer backbone, and subsequent lactonization slowly (~days) degrades the molecular weight to 4.4 kDa, > 10× smaller than control polymers in which lactonization is blocked. The force-coupled kinetics of ring-opening are probed by single molecule force spectroscopy, and mechanical degradation in the bulk is demonstrated. Delayed scission offers a strategy to enhanced mechanical degradation and programmed obsolescence in structural polymeric materials.

Oxidative regulation of the mechanical strength of a C-S bond

The mechanical strength of individual polymer chains is believed to underlie a number of performance metrics in bulk materials, including adhesion and fracture toughness. Methods by which the intrinsic molecular strength of the constituents of a given polymeric material might be switched are therefore potentially useful both for applications in which triggered property changes are desirable, and as tests of molecular theories for bulk behaviors. Here we report that the sequential oxidation of sulfide containing polyesters (PE-S) to the corresponding sulfoxide (PE-SO) and then sulfone (PE-SO2) first weakens (sulfoxide), and then enhances (sulfone), the effective mechanical integrity of the polymer backbone; PE-S ∼ PE-SO2 > PE-SO. The relative mechanical strength as a function of oxidation state is revealed through the use of gem-dichlorocyclopropane nonscissile mechanophores as an internal standard, and the observed order agrees well with the reported bond dissociation energies of C–S bonds in each species and with the results of CoGEF modeling.

The mechanical strength of individual polymer chains is believed to underlie a number of performance metrics in bulk materials, including adhesion and fracture toughness.

Toward Drug Release Using Polymer Mechanochemical Disulfide Scission

Traditional pharmacotherapy suffers from multiple drawbacks that hamper patient treatment, such as the buildup of antibiotic resistances or low drug selectivity and toxicity during systemic application. To overcome these challenges, drug activity can be controlled by employing delivery, targeting, or release solutions that mostly rely on the response to external physicochemical stimuli. Due to various technical limitations, mechanical force as a stimulus in the context of polymer mechanochemistry has so far not been used for this purpose, yet it has been proven to be a convenient and robust method to site-selectively rearrange or cleave bonds with submolecular precision in the realm of materials chemistry. Here, we present an unprecedented mechanochemically responsive system capable of successively releasing small furan-containing molecules, including the furylated fluorophore dansyl and the drugs furosemide as well as furylated doxorubicin, by ultrasound-induced selective scission of disulfide-centered polymers in solution. We show that mechanochemically generated thiol-terminated polymers undergo a Michael-type addition to Diels-Alder (DA) adducts of furylated drugs and acetylenedicarboxylate derivatives, initiating the downstream release of the small molecule drug by a retro DA reaction. We believe that this method can serve as a blueprint for the activation of many other small molecules.

Molecular Damage Detection in an Elastomer Nanocomposite with a Coumarin Dimer Mechanophore

Molecular force probes that generate optical responses to critical levels of mechanical stress (mechanochromophores) are increasingly attractive tools for identifying molecular sites that are most prone to failure. Here, a coumarin dimer mechanophore whose mechanical strength is comparable to that of the sulfur-sulfur bonds found in vulcanized rubbers is reported. It is further shown that the strain-induced scission of the coumarin dimer within the matrix of a particle-reinforced polybutadiene-based co-polymer can be detected and quantified by fluorescence spectroscopy, when cylinders of the nanocomposite are subjected to unconstrained uniaxial stress. The extent of the scission suggests that the coumarin dimers are molecular "weak links" within the matrix, and, by analogy, sulfur bridges are likely to be the same in vulcanized rubbers. The mechanophore is embedded in polymer main chains, grafting agent, and cross-linker positions in a polymer composite in order to generate experimental data to understand how macroscopic mechanical stress is transferred at the molecular scale especially in highly entangled cross-linked polymer nanocomposite. Finally, the extent of activation is enhanced by approximately an order of magnitude by changing the regiochemistry and stereochemistry of the coumarin dimer and embedding the mechanophore at the heterointerface of the particle-reinforced elastomer.