Polymer Mechanochemistry and the Emergence of the Mechanophore Concept

Revealing Mechanochemical Force Distributions with Polymechanophore Block Copolymers

Polymer mechanochemistry is most studied under ultrasonication conditions, where the force distribution along a polymer chain is typically modeled as a parabola centered at the chain midpoint. How far the forces required for mechanoactivation extend toward the chain ends remains a question of interest. Investigating the mechanochemical reactivity of mechanophores at defined locations off chain center and toward termini would provide valuable information regarding this, but preparing such polymers to study the effects of mechanophore location and chain length on mechanoactivation has been synthetically cumbersome. Using an operationally simple procedure, we synthesized a series of block copolymers containing a block of mechanophores by living ring opening metathesis polymerization of a ladderene-type mechanophore and a norbornene, with good control over block positions and lengths. We found that for polymers of initial degrees of polymerization (DP) ≈ 1000, terminal mechanophore blocks activated less than more centrally located ones. However, simply by extending the length of the mechanochemically inert block, terminal mechanophore blocks of DP ≈ 200 achieved surprisingly high degrees of mechanoactivation comparable to those of centrally located ones or mechanophore homopolymers, after an induction period. These findings revealed the broad range of high force coverage along a polymer chain under sonication conditions and the possibility to still achieve high degrees of productive mechanochemistry far away from chain centers.

A focus on substituents effect in the force-promoted disrotatory ring-opening of cis-cyclobutenes

Symmetry-forbidden reactions are notoriously difficult to investigate as they are typically overshadowed by the corresponding symmetry-allowed pathway. Mechanical activation allows access to reaction pathways disfavoured using other methods of activation, such as the symmetry-forbidden disrotatory ring-opening of substituted cis-cyclobutenes. In a recent publication, Bowser, et al. have studied the effects of various substituents on this reaction using atomic force microscopy and computational analysis (B. H. Bowser, C. L. Brown, J. Meisner, T. B. Kouznetsova, T. J. Martínez and S. L. Craig, Chem. Sci., 2025, https://doi.org/10.1039/D5SC00253B). The largest effect is observed with substituents close to the scissile bond having the ability to stabilise the diradical character of the disrotatory ring-opening reaction pathway.

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

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

Dual-Mode Emission and Solvent-Desorption Dependent Kinetic Properties of Crystalline-State Chemiluminescence Reaction of 9-Phenyl-10-(2-phenylethynyl)anthracene Endoperoxide

The chemiluminescence (CL) feature and reactivity of the aromatic endoperoxide 9-phenyl-10-(2-phenylethynyl)anthracene endoperoxide (PPEA-O2) were investigated in the crystalline state. For this, PPEA-O2 crystals were prepared using dichloromethane and n-hexane. These crystals exhibited an α-phase structure containing n-hexane as a crystal solvent. The crystal structure of nonperoxidic anthracene (i.e., PPEA) was also confirmed. After optimizing heating conditions to 120 °C for the thermolytic reaction of PPEA-O2 in crystals while maintaining the solid state, its CL characteristic and reactivity were investigated. Two key findings were derived: (1) dual-mode emission with maxima at 510 and 1275 nm and (2) distinct observation of CL emission at the first 2-3 min after the start of heating owing to the rapid thermolytic reaction coupled with n-hexane desorption. The 510 and 1275 nm emissions were attributed to the PPEA excimer and 1O2 (1Δg), respectively. We proposed a mechanism involving the triplet-triplet annihilation of the excited triplet states of PPEA to explain excimer production with postulated pathways for generating these triplet states from PPEA-O2. The rapid thermolytic reaction of PPEA-O2 in α-phase crystals with simultaneous n-hexane desorption was attributed to the formation of transient vacant spaces, which increased the molecular freedom necessary for the reaction ("transient vacant space effect"). Thus, the CL of PPEA-O2 proved useful for identifying characteristic reactivity and analyzing the luminescence mechanism of aromatic endoperoxides in the crystalline state.

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

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

The Restoring Force Triangle: A Mnemonic Device for Polymer Mechanochemistry

In polymer mechanochemistry, mechanophores are specific molecular units within the macromolecular backbone that are particularly sensitive to tension. To facilitate understanding of this selective responsiveness, we introduce the restoring force triangle (RFT). The RFT is a mnemonic device intended to provide intuitive insight into how external tensile forces (i.e., stretching) can selectively activate scissile bonds, thereby initiating mechanically driven chemical reactions. The RFT utilizes two easily computable parameters: the effective bond stiffness constant, which measures a bond's resistance to elongation, and the bond dissociation energy, which is the energy required to break a bond. These parameters help categorize reactivity into thermal and mechanical domains, providing a useful framework for developing new mechanophores that are responsive to force but thermally stable. The RFT helps chemists intuitively understand how tensile force contributes to the activation of a putative mechanophore, facilitating the development of mechanochemical reactions and mechano-responsive materials.

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.

Caged AIEgens: Multicolor and White Emission Triggered by Mechanical Activation

Aggregation-induced emission luminogens (AIEgens) that respond to mechanical force are increasingly used as force probes, memory devices, and advanced security systems. Most of the known mechanisms to modulate mechanoresponsive AIEgens have been based on changes in aggregation states, involving only physical alterations. Instances that employ covalent bond cleavage are still rare. We have developed a novel mechanochemical uncaging strategy to unveil AIEgens with diverse emission characteristics using engineered norborn-2-en-7-one (NEO) mechanophores. These NEO mechanophores were covalently integrated into polymer molecules and activated in both the solution and solid states. This activation resulted in highly tunable fluorescence upon immobilization through solidification or aggregation, producing blue, green, yellow, and orange-red emissions. By designing the caged and uncaged forms as donor-acceptor pairs for Förster resonance energy transfer (FRET), we achieved multicolor mechanofluorescence, effectively broadening the color spectrum to include white emission. Additionally, we computationally explored the electronic structures of activated NEOs, providing insights into the observed regiochemical effects of the substituents. This understanding, together with the novel luminogenic characteristics of the caged and activated species, provides a highly tunable reporter that traces progress with continuous color evolution. This advancement paves the way for future applications of mechanoresponsive materials in areas like damage detection and bioimaging.

In Vivo Polymer Mechanochemistry with Polynucleotides

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

JEDI: A versatile code for strain analysis of molecular and periodic systems under deformation

Stretching or compression can induce significant energetic, geometric, and spectroscopic changes in materials. To fully exploit these effects in the design of mechano- or piezo-chromic materials, self-healing polymers, and other mechanoresponsive devices, a detailed knowledge about the distribution of mechanical strain in the material is essential. Within the past decade, Judgement of Energy DIstribution (JEDI) analysis has emerged as a useful tool for this purpose. Based on the harmonic approximation, the strain energy in each bond length, bond angle, and dihedral angle of the deformed system is calculated using quantum chemical methods. This allows the identification of the force-bearing scaffold of the system, leading to an understanding of mechanochemical processes at the most fundamental level. Here, we present a publicly available code that generalizes the JEDI analysis, which has previously only been available for isolated molecules. Now, the code has been extended to two- and three-dimensional periodic systems, supramolecular clusters, and substructures of chemical systems under various types of deformation. Due to the implementation of JEDI into the Atomic Simulation Environment, the JEDI analysis can be interfaced with a plethora of program packages that allow the calculation of electronic energies for molecular systems and systems with periodic boundary conditions. The automated generation of a color-coded three-dimensional structure via the Visual Molecular Dynamics program allows insightful visual analyses of the force-bearing scaffold of the strained system.

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.

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.

Mechanochemical Activation of DNAzyme by Ultrasound

Controlling the activity of DNAzymes by external triggers is an important task. Here a temporal control over DNAzyme activity through a mechanochemical pathway with the help of ultrasound (US) is demonstrated. The deactivation of the DNAzyme is achieved by hybridization to a complementary strand generated through rolling circle amplification (RCA), an enzymatic polymerization process. Due to the high molar mass of the resulting polynucleic acids, shear force can be applied on the RCA strand through inertial cavitation induced by US. This exerts mechanical force and leads to the cleavage of the base pairing between RCA strand and DNAzyme, resulting in the recovery of DNAzyme activity. This is the first time that this release mechanism is applied for the activation of catalytic nucleic acids, and it has multiple advantages over other stimuli. US has higher penetration depth into tissues compared to light, and it offers a more specific stimulus than heat, which has also limited use in biological systems due to cell damage caused by hyperthermia. This approach is envisioned to improve the control over DNAzyme activity for the development of reliable and specific sensing applications.

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.

Bioorthogonal chemistry for prodrug activation in vivo

Prodrugs have emerged as a major strategy for addressing clinical challenges by improving drug pharmacokinetics, reducing toxicity, and enhancing treatment efficacy. The emergence of new bioorthogonal chemistry has greatly facilitated the development of prodrug strategies, enabling their activation through chemical and physical stimuli. This "on-demand" activation using bioorthogonal chemistry has revolutionized the research and development of prodrugs. Consequently, prodrug activation has garnered significant attention and emerged as an exciting field of translational research. This review summarizes the latest advancements in prodrug activation by utilizing bioorthogonal chemistry and mainly focuses on the activation of small-molecule prodrugs and antibody-drug conjugates. In addition, this review also discusses the opportunities and challenges of translating these advancements into clinical practice.

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.

Polymer mechanochemistry in drug delivery: From controlled release to precise activation

Controlled drug delivery systems that can respond to mechanical force offer a unique solution for on-demand activation and release under physiological conditions. Compression, tension, and shear forces encompass the most commonly utilized mechanical stimuli for controlled drug activation and release. While compression and tension forces have been extensively explored for designing mechanoresponsive drug release systems through object deformation, ultrasound (US) holds advantages in achieving spatiotemporally controlled drug release from micro-/nanocarriers such as microbubbles, liposomes, and micelles. Unlike light-based methods, the US bypasses drawbacks such as phototoxicity and limited tissue penetration. Conventional US-triggered drug release primarily relies on heat-induced phase transitions or chemical transformations in the nano-/micro-scale range. In contrast, the cutting-edge approach of "Sonopharmacology" leverages polymer mechanochemistry, where US-induced shear force activates latent sites containing active pharmaceutical ingredients incorporated into polymer chains more readily than other bonds within the polymeric structure. This article provides a brief overview of controlled drug release systems based on compression and tension, followed by recent significant studies on drug activation using the synergistic effects of US and polymer mechanochemistry. The remaining challenges and potential future directions in this subfield are also discussed. PROGRESS AND POTENTIAL: The precise spatiotemporal control of drug activity using exogenous signals holds great promise for achieving precise disease treatment with minimal side effects. Ultrasound, known for its safety, has found widespread application in clinical settings and offers adjustable tissue penetration depth and drug release control. However, challenges persist in achieving precise control over drug activity using ultrasound. In recent years, ultrasound-induced drug release utilizing the principle of polymer mechanochemistry (Sonopharmacology) has made significant progress and demonstrated its potential in achieving precise drug activation and release. These systems enable drug release at the sub-molecular level, allowing for selective control over drug activation. Sonopharmacology offers a unique advantage by integrating both chemical and biomedical perspectives, positioning it as a promising field with broad implications in polymer chemistry, nanoscience and technology, and pharmaceutics. This review article aims to examine recent advancements in ultrasound-triggered drug activation systems based on polymeric materials and with an focus on polymer mechanochemistry, identify remaining challenges, and propose potential perspectives in this rapidly evolving field. By providing a comprehensive understanding of the progress and potential of sonopharmacology, this article aims to guide future research and inspire the development of innovative drug delivery systems that offer enhanced selectivity and improved therapeutic outcomes.

Swelling-induced Mechanochromism in Multinetwork Polymers

We report a novel and versatile approach to achieving swelling-induced mechanochemistry using a multinetwork (MN) strategy that enables polymer networks to repeatedly swell with monomers and solvents. The isotropic expansion of the first network (FN) provides sufficient force to drive the mechanochemical scission of a radical-based mechanophore, difluorenylsuccinonitrile (DFSN). Although prompt recombination generally occurs in such highly mobile environments, the resulting pink radicals are kinetically stabilized in the gels, probably due to limited diffusion in the extended polymer chains. Moreover, the DFSN embedded in the isotropically strained chain exhibits increased thermal reactivity, which can be reasonably explained by an entropic contribution of the FN to the dissociation. The utility of the MN polymers is demonstrated not only in terms of swelling-force-induced network modification, but also in the context of tunable reactivity of the dissociative unit through proper design of the hierarchical network architecture.

Force-Induced Shuttling of Rotaxanes Controls Fluorescence Resonance Energy Transfer in Polymer Hydrogels

The molecular shuttling function of rotaxanes can be exploited to design mechanoresponsive reporter molecules. Here, we report a new approach to such rotaxane-based mechanophores, in which the fluorescence resonance energy transfer (FRET) between a donor-acceptor pair is mechanically controlled. A cyclic molecule containing a green-light-emitting FRET donor connected to a red-light-emitting FRET acceptor was threaded onto an axle equipped with a quencher at its center and two stoppers in the peripheral positions. In the force-free state, the green emitter is located near the quencher so that charge transfer interactions or photo-induced electron transfer between the two moieties suppress green emission and prevent the FRET from the green to the red emitter. The mechanophore was covalently incorporated into a linear polyurethane-urea (PUU), and stretchable hydrogels were prepared by swelling this polymer with water. Upon deformation of the PUU hydrogels and under an excitation light that selectively excites the donor, the intensity of the red fluorescence increases, as a result of a force-induced separation of the green emitter from the quencher, which enables the FRET. The switching contrast is much more pronounced in the gels than in dry films, which is due to increased molecular mobility and hydrophobic effects in the hydrogel, which both promote the formation of inclusion complexes between the ring containing the green emitter and the quencher.

Thermochemistry of Sulfur-Based Vulcanization and of Devulcanized and Recycled Natural Rubber Compounds

The vulcanization of rubber compounds is an exothermal process. A carbon black-filled and natural rubber-based (NR) formulation was mixed with different levels of sulfur (0.5, 1.0, 2.0, 4.0 and 6.0 phr) and studied with differential scanning calorimetry (DSC) for the determination of the vulcanization enthalpy. It was found that the vulcanization enthalpy is dependent on the amount of sulfur present in the compound and the vulcanization heat released was -18.4 kJ/mol S if referred to the entire rubber compound formulation or -46.0 kJ/mol S if the heat released is referred only to the NR present in the compound. The activation energy for the vulcanization of the rubber compounds was also determined by a DSC study at 49 kJ/mol and found to be quite independent from the sulfur content of the compounds under study. A simplified thermochemical model is proposed to explain the main reactions occurring during the vulcanization. The model correctly predicts that the vulcanization is an exothermal process although it gives an overestimation of the vulcanization enthalpy (which is larger for the EV vulcanization package and smaller for the conventional vulcanization system). If the devulcanization is conducted mechanochemically in order to break selectively the sulfur-based crosslinks, then the natural rubber compounds recovered from used tires can be re-vulcanized again and the exothermicity of such process can be measured satisfactorily with DSC analysis. This paper not only proposes a simplified mechanism of vulcanization and devulcanization but also proposes an analytical method to check the devulcanization status of the recycled rubber compound in order to distinguish truly devulcanized rubber from reclaimed rubber.

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.

Mechanochemical solid-state vinyl polymerization with anionic initiator

Mechanochemistry has been extended to various polymer syntheses to achieve efficiency, greenness, and new products. However, many fundamental polymerization reactions have not been explored, although anionic polymerization of vinyl compounds has been pursued under mechanochemical conditions. Two solid monomers, 4-biphenyl methacrylate and 4-vinyl biphenyl, representing methacrylate and styrenic classes, respectively, were reacted with secondary butyl lithium under high-speed ball-milling. The alkyl-anion-promoted polymerization process was established by excluding radical initiation and producing the expected polymers with good efficiency. However, the generally expected features of anionic polymerization, such as molecular weight control and narrow dispersity, were not observed. Analysis of the milling parameters, reaction monitoring, and microstructural analysis revealed that the mechanism of the mechanochemical process differs from that of conventional anionic polymerizations. The mechanical force fractured the newly formed polymer chains via anionic initiation and generated macroradicals, which participated in the polymerization process. The anionic process governs the initiation step and the radical process becomes dominant during the propagation step.

Excited State Charge-Transfer Complexes Enable Fluorescence Color Changes in a Supramolecular Cyclophane Mechanophore

Mechanochromic mechanophores are reporter molecules that indicate mechanical events through changes of their photophysical properties. Supramolecular mechanophores in which the activation is based on the rearrangement of luminophores and/or quenchers without any covalent bond scission, remain less well investigated. Here, we report a cyclophane-based supramolecular mechanophore that contains a 1,6-bis(phenylethynyl)pyrene luminophore and a pyromellitic diimide quencher. In solution, the blue monomer emission of the luminophore is largely quenched and a faint reddish-orange emission originating from a charge-transfer (CT) complex is observed. A polyurethane elastomer containing the mechanophore displays orange emission in the absence of force, which is dominated by the CT-emission. Mechanical deformation causes a decrease of the CT-emission and an increase of blue monomer emission, due to the spatial separation between the luminophore and quencher. The ratio of the two emission intensities correlates with the applied stress.

Polymer and small molecule mechanochemistry: closer than ever

The formation and scission of chemical bonds facilitated by mechanical force (mechanochemistry) can be accomplished through various experimental strategies. Among them, ultrasonication of polymeric matrices and ball milling of reaction partners have become the two leading approaches to carry out polymer and small molecule mechanochemistry, respectively. Often, the methodological differences between these practical strategies seem to have created two seemingly distinct lines of thought within the field of mechanochemistry. However, in this Perspective article, the reader will encounter a series of studies in which some aspects believed to be inherently related to either polymer or small molecule mechanochemistry sometimes overlap, evidencing the connection between both approaches.

Recycling of plastic wastes generated from COVID-19: A comprehensive illustration of type and properties of plastics with remedial options

Plastic has contributed enormously to the healthcare sector and towards public health safety during the COVID-19 pandemic. With the frequent usage of plastic-based personal protective equipment (PPEs) (including face masks, gloves, protective body suits, aprons, gowns, face shields, surgical masks, and goggles), by frontline health workers, there has been a tremendous increase in their manufacture and distribution. Different types of plastic polymers are used in the manufacture of this equipment, depending upon their usage. However, since a majority of these plastics are still single-use plastics (SUP), they are not at all eco-friendly and end up generating large quantities of plastic waste. The overview presents the various available and practiced methods in vogue for disposal cum treatment of these highly contaminated plastic wastes. Among the current methods of plastic waste disposal, incineration and land filling are the most common ones, but both these methods have their negative impacts on the environment. Alongside, numerous methods that can be used to sterilize them before any treatment have been discussed. There are several new sorting technologies, to help produce purer polymers that can be made to undergo thermal or chemical treatments. Microbial degradation is one such novel method that is under the spotlight currently and being studied extensively, because of its ecological advantages, cost-effectiveness, ease of use, and maintenance. In addition to the deliberations on the methods, strategies have been enumerated for combination of different methods, vis-à-vis studying the life cycle assessment towards a more circular economy in handling this menace to protect mankind.

The Mechanochemical Synthesis and Activation of Carbon-Rich π-Conjugated Materials

Mechanochemistry uses mechanical force to break, form, and manipulate chemical bonds to achieve functional transformations and syntheses. Over the last years, many innovative applications of mechanochemistry have been developed. Specifically for the synthesis and activation of carbon-rich π-conjugated materials, mechanochemistry offers reaction pathways that either are inaccessible with other stimuli, such as light and heat, or improve reaction yields, energy consumption, and substrate scope. Therefore, this review summarizes the recent advances in this research field combining the viewpoints of polymer and trituration mechanochemistry. The highlighted mechanochemical transformations include π-conjugated materials as optical force probes, the force-induced release of small dye molecules, and the mechanochemical synthesis of polyacetylene, carbon allotropes, and other π-conjugated materials.

Advancing the Logic of Polymer Synthesis via Skeletal Rearrangements

Polymers are ubiquitous materials that have driven technological innovation since the middle of the 20th century. As such, the logic that guides polymer synthesis merit considerable attention. Thus far, this logic has often been ‘forward-synthetic’, which constrains the accessible structures of polymer materials. In this article, we emphasize the benefits of ‘retrosynthetic’ logic and posit that the development of skeletal rearrangements of polymer backbones is central to the realization of this logic. To illustrate this point, we discuss two recent examples from our laboratory – Brook and Ireland–Claisen rearrangements of polymer backbones – and contextualize them in prior reports of sigmatropic rearrangements and skeletal rearrangements of polymers. We envision that further development of skeletal rearrangements of polymers will enable advances in not only the chemistry of such rearrangements and the logic of polymer synthesis, but also polymer re- and upcycling.

Polymer mechanochemistry for the release of small cargoes

The field of force-induced release of small cargoes within polymeric materials has experienced rapid growth over the past decade, not only including achieving diversified functional materials that report force, trigger degradation, activate drugs and release catalysts, but also involving investigations on the interesting force-coupled reactivity of mechanophores, such as ferrocenes. In this highlight article, we review the recent progress on polymer mechanochemistry that releases small cargoes, including small molecules and metal ions. Since mechanophores play a key role in force-responsive materials, we introduce the progress by discussing different types of mechanophores and their mechanochemical reactions for the release of acids, gases, fluorophores, drugs, iron ions, and so on. At the end, we provide our perspectives on the remaining challenges and future targets in this growing field.

The mechanochemical synthesis of polymers

Mechanochemistry - the utilization of mechanical forces to induce chemical reactions - is a rarely considered tool for polymer synthesis. It offers numerous advantages such as reduced solvent consumption, accessibility of novel structures, and the avoidance of problems posed by low monomer solubility and fast precipitation. Consequently, the development of new high-performance materials based on mechanochemically synthesised polymers has drawn much interest, particularly from the perspective of green chemistry. This review covers the constructive mechanochemical synthesis of polymers, starting from early examples and progressing to the current state of the art while emphasising linear and porous polymers as well as post-polymerisation modifications.

Accelerated Mechanochemistry in Helical Polymers

Polymer chains, if long enough, are known to undergo bond scission when mechanically stressed. While the mechanochemical response of random coils is well understood, biopolymers and some key synthetic chains adopt well-defined secondary structures such as helices. To understand covalent mechanochemistry in such structures, poly(γ-benzyl glutamates) are prepared while regulating the feed-monomer chirality, producing chains with similar molecular weights and backbone chemistry but different helicities. Such chains are stressed in solution and their mechanochemistry rates compared by following molecular weight change and using a rhodamine mechanochromophore. Results reveal that while helicity itself is not affected by the covalent bond scissions, chains with higher helicity undergo faster mechanochemistry. Considering that the polymers tested differ only in conformation, these results indicate that helix-induced chain rigidity improves the efficiency of mechanical energy transduction.

Activation of Antibiotic-Grafted Polymer Brushes by Ultrasound

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

Light- and mechanic field controlled dynamic soft matter materials

A photochemical reaction system that fuses photo- and mechanochemistry into one macromolecular design for light- and mechano-reversible modification of polymer endgroups is introduced.

Sigmatropic Rearrangements of Polymer Backbones: Vinyl Polymers from Polyesters in One Step

Polymer modification is a fundamental scientific challenge, as a means of both upcycling plastics and extracting a stimulus response from them. To date, the overwhelming majority of polymer modifications has focused on the polymer periphery. Herein, we demonstrate nearly quantitative, scission-free modification of polymer backbones, namely, a metamorphosis of polyesters into vinyl polymers resembling commodity materials via the Ireland-Claisen sigmatropic rearrangement. The glass transition temperature (T_g) and thermal stability of the polyesters undergo dramatic changes post-transformation. Beyond polymer modification, our work advances the application of retrosynthetic analysis in polymer synthesis; the nontraditional production of vinyl polymers from lactones opens the door to a slew of previously inaccessible materials.

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

Tailoring the Properties of Optical Force Probes for Polymer Mechanochemistry

The correlation of mechanical properties of polymer materials with those of their molecular constituents is the foundation for their holistic comprehension and eventually for improved material designs and syntheses. Over the last decade, optical force probes (OFPs) were developed, shedding light on various unique mechanical behaviors of materials. The properties of polymers are diverse, ranging from soft hydrogels to ultra-tough composites, from purely elastic rubbers to viscous colloidal solutions, and from transparent glasses to super black dyed coatings. Only very recently, researchers started to develop tailored OFP solutions that account for such material requirements in energy (both light and force), in time, and in their spatially detectable resolution. We here highlight notable recent examples and identify future challenges in this emergent field.

Heterolytic Bond Cleavage in a Scissile Triarylmethane Mechanophore

Covalent mechanophores display the cleavage of a weak covalent bond when a sufficiently high mechanical force is applied. Three different covalent bond breaking mechanisms have been documented thus far, including concerted, homolytic, and heterolytic scission. Motifs that display heterolytic cleavage typically separate according to non-scissile reaction pathways that afford zwitterions. Here, we report a new mechanochromic triarylmethane mechanophore, which dissociates according to a scissile heterolytic pathway and displays a pronounced mechanochromic response. The mechanophore was equipped with two styrenylic handles that allowed its incorporation as a cross-linker into poly(N,N-dimethylacrylamide) and poly(methyl acrylate-co-2-hydroxyethyl acrylate) networks. These materials are originally colorless, but compression or tensile deformation renders the materials colored. By combining tensile testing and in situ transmittance measurements, we show that this effect is related to scissile cleavage leading to colored triarylmethane carbocations.

Polymer Skeletal Editing via Anionic Brook Rearrangements

This report communicates the first example of polymer backbone metamorphosis affected by anionic 1,2-Brook rearrangement of acyl silane moieties. Introduction of the acyl silane functionality into a polymer backbone was achieved via acyclic diene metathesis copolymerization (ADMET) of diene 1 and two dienes. We demonstrate that, using organolithium species and cyanide as nucleophiles, the backbones of resulting copolymers can be triggered to undergo highly efficient 1,2-Brook rearrangement, which transforms the poly(acyl silane)s into poly(silyl ether)s. Furthermore, the carbanion intermediate of the 1,2-Brook rearrangement can be intercepted by ketone electrophiles to give rise to polymers with quaternary stereogenic centers in the backbone and pendant functionality. Such structural editing of polymer backbones enables a new retrosynthetic paradigm for silicon-containing polymers that could not be accessed by traditional means.

Organic Sonochemistry: A Chemist's Timely Perspective on Mechanisms and Reactivity

Sonochemistry, the use of sound waves, usually within the ultrasonic range (>20 kHz), to boost or alter chemical properties and reactivity constitutes a long-standing and sustainable technique that has, however, received less attention than other activation protocols despite affordable setups. Even if unnecessary to underline the impact of ultrasound-based strategies in a broad range of chemical and biological applications, there is considerable misunderstanding and pitfalls regarding the interpretation of cavitational effects and the actual role played by the acoustic field. In this Perspective, with an eye on mechanisms in particular, we discuss the potentiality of sonochemistry in synthetic organic chemistry through selected examples of past and recent developments. Such examples illustrate specific controlling effects and working rules. Looking back at the past while looking forward to advancing the field, some essentials of sonochemical activation will be distilled.

A Two-Step Baromechanical Cycle for Repeated Activation and Deactivation of Mechanophores

Mechanophores that are embedded in a polymer backbone respond to the application of mechanical stretching forces by geometric changes such as bond rupture. Typically, these structural changes are irreversible, which limits the applicability of functional materials incorporating mechanophores. Using computational methods, we, here, present a general method of restoring a force-activated mechanophore to its deactivated form by using hydrostatic pressure. We use the spiropyran-merocyanine (SP-MC) interconversion to show that repeated activation of the SP mechanophore and deactivation of MC can be achieved by alternating mechanical stretching and hydrostatic compression, respectively. In the baromechanical cycle, MC acts as a "barophore" that responds to hydrostatic pressure by bond formation. The activation and deactivation of SP/MC are understood in terms of strain and electronic effects. Beneficially, this two-step baromechanical cycle can be observed in real time by using UV/vis spectroscopy. Our calculations pave the way for improving the applicability and reusability of force-responsive materials.

Visually Monitoring the Compactness of Polymer Matrixes Coded by Disparate Luminescence

The intrinsic property disclosure of polymer systems by visual monitoring of photoluminescence behaviors is of great value in fundamental interest and promising applications. Three novel polymer films were obtained by simply doping methyl 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine-11-carboxylate (DPC) with three polymer materials. The photoluminescence behaviors of these films represented diverse fluorescence emissions from light orange to blue, especially room-temperature phosphorescence (RTP) emissions with ultralong lifetime, attributing to various configurations of DPC molecules provided by distinct microscopic environments in three polymer systems. The rigidity and regularity of polymer systems would be visually reflexed by luminescence regulation and temperature responses. In addition, irregular distribution of distinct polymer systems could be specifically monitored by both fluorescence and phosphorescence behaviors when doping different polymer materials into one blend film.

Mechanochemical Reactivity of Bottlebrush and Dendronized Polymers: Solid vs. Solution States

We explored the mechanochemical degradation of bottlebrush and dendronized polymers in solution (with ultrasonication, US) and solid states (with ball-mill grinding, BMG). Over 50 polymers were prepared with varying backbone length and arm architecture, composition, and size. With US, we found that bottlebrush and dendronized polymers exhibited consistent backbone scission behavior, which was related to their elongated conformations in solution. Considerably different behavior was observed with BMG, as arm architecture and composition had a significant impact on backbone scission rates. Arm scission was also observed for bottlebrush polymers in both solution and solid states, but only in the solid state for dendronized polymers. Motivated by these results, multi-mechanophore polymers with bottlebrush and dendronized polymer architectures were prepared and their reactivity was compared. Although dendronized polymers showed slower arm-scission, the selectivity for mechanophore activation was much higher. Overall, these results have important implications to the development of new mechanoresponsive materials.

Introduction of a Luminophore into Generic Polymers via Mechanoradical Coupling with a Prefluorescent Reagent

Herein, we report a novel strategy for introducing a luminophore into generic polymers facilitated by mechanical stimulation. In this study, polymeric mechanoradicals were formed in situ under ball-milling conditions to undergo radical-radical coupling with a prefluorescent nitroxide-based reagent in order to incorporate a luminophore into the polymer main chains via a covalent bond. This method allowed the direct and conceptually simple preparation of luminescent polymeric materials from a wide range of generic polymers such as polystyrene, polymethyl methacrylate, and polyethylene. These results indicate that the present mechanoradical coupling strategy may help to transform existing commodity polymers into more valuable functional materials.