Mechanical activation of catalysts for C-C bond forming and anionic polymerization reactions from a single macromolecular reagent

Recent advances utilized in artificial switchable catalysis

Developing “green” catalytic systems with desirable performance such as solubility, recyclability, and switchability is a great challenge. However, inspired by nature, the studies on synthesis and activity of artificial switchable metal catalysts and organocatalysts have become an intense, fervid, and challenging field of research. The peculiarity of these catalysts is that they can be generally triggered in the “on” or “off” states by several external stimuli such as light, heat, solvents, pH change, coordination events or ion influxes, redox processes, mechanical forces, or other changes in reaction conditions. A large number of review articles are available in these areas. However, most efforts are currently focused on the invention of new types of switchable catalysts with different forms of stimuli–response units incorporated within their architectures in order to achieve control over the catalytic activity and regio-, chemo- and stereocontrol of various chemical reactions. Thus, in this review, we begin with a brief introduction to switchable catalysts, followed by discussion of types of stimuli and the influence factors on their activities in the field of biomedical engineering, and catalysis as well as related catalytic mechanisms summarized and discussed. The emphasis is on the recent advances utilized in artificial switchable catalysis.

Catalytic systems based on the use of stimuli–responsive materials can be switched from an “on” active state to an “off” inactive state. Consequently, switchable catalysis, both chemical and biological, has played a pivotal role in this ‘greening’ of the pharmaceutical industry.

Supramolecular Regulation of Catalytic Activity for an Amphiphilic Pyrene-Ruthenium Complex in Water

A switchable catalytic system has been designed and constructed with a host-guest interaction between cucurbituril (CB) and an amphiphilic metal complex pyrene-ruthenium (Py-Ru). Py-Ru can self-assemble into positively charged nanoparticles in water, and exhibits an enhanced catalytic efficiency in the transfer hydrogenation of NAD⁺ to NADH. After forming an inclusion complex with CB, Py-Ru aggregates are broken, leading to a decrease in catalytic efficiency, which can be recovered by competitive replacement with amantadine. This supramolecular strategy provides an efficient and flexible method for constructing reversible catalytic system, which also extends the application scope of the host-guest interaction.

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.

Mechanically-Driven Vase-Kite Conformational Switch in Cavitand Cross-Linked Polyurethanes

The eligibility of tetraquinoxaline cavitands (QxCav) as molecular grippers relies on their unique conformational mobility between a closed (vase) and an open (kite) form, triggered in solution by conventional stimuli like pH, temperature and ion concentration. In the present paper, the mechanochemical conformational switching of ad hoc functionalized QxCav covalently embedded in an elastomeric polydimethylsiloxane and in a more rigid polyurethane matrix is investigated. The rigid polymer matrix is more effective in converting mechanical force into a conformational switch at the molecular level, provided that all four quinoxaline wings are covalently connected to the polymer.

Visible light-mediated ring-opening polymerization of lactones based on the excited state acidity of ESPT molecules

A visible light-regulated ring-opening polymerization of lactones has been developed based on the excited state acidity of ESPT molecules.

Photoacid-mediated ring opening polymerization driven by visible light

Visible light regulated ring opening polymerization in the presence of reversible merocyanine-based photoacid is reported in this article.

Manual control of catalytic reactions: Reactions by an apoenzyme gel and a cofactor gel

Enzymes play a vital role in catalysing almost all chemical reactions that occur in biological systems. Some enzymes must form complexes with non-protein molecules called cofactors to express catalytic activities. Although the control of catalytic reactions via apoenzyme–cofactor complexes has attracted significant attention, the reports have been limited to the microscale. Here, we report a system to express catalytic activity by adhesion of an apoenzyme gel and a cofactor gel. The apoenzyme and cofactor gels act as catalysts when they form a gel assembly, but they lose catalytic ability upon manual dissociation. We successfully construct a system with switchable catalytic activity via adhesion and separation of the apoenzyme gel with the cofactor gel. We expect that this methodology can be applied to regulate the functional activities of enzymes that bear cofactors in their active sites, such as the oxygen transport of haemoglobin or myoglobin and the electron transport of cytochromes.

Polymer mechanochemistry: from destructive to productive

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

Visible-light-controlled homo- and copolymerization of styrenes by a bichromophoric Ir–Pd catalyst

Visible-light-controlled polymerization was achieved by a bichromophoric organopalladium catalyst which possesses a naphthyl-substituted cyclometallated Ir(iii) light-absorbing moiety.

Molecular Mechanochemistry: Engineering and Implications of Inherently Strained Architectures

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

Dioxetane scission products unchanged by mechanical force

Dioxetane-based force-induced light emission from polymers, or mechanoluminescence, is a powerful new way of characterizing the behavior of polymeric materials under stress. Here, we reveal that breaking the dioxetane mechanically gives strikingly similar products to those formed on thermal activation, with a singlet/triplet ratio of 1:9.9 and a total quantum yield of 9.8%. A sensitized relay scheme ensured high reproducibility in the detection of the short-lived triplet products. In addition to guiding the design of more sensitive mechanoluminescent probes, the similarity in the scission products indicates that once mechanical force releases the steric lock between the adamantyl groups, the dioxetane undergoes scission in a pathway that resembles the thermal process. Excited states are formed only after the main transition state in a region in which the excited- and ground-state surfaces are nearly degenerate, which, thus, accounts for the remarkable similarity in the scission products.

Mechanochemistry with metallosupramolecular polymers

The transduction of mechanical force into useful chemical reactions is an emerging design approach to impart soft materials with new functions. Here, we report that mechanochemical transductions can be achieved in metallosupramolecular polymers. We show that both reversible and irreversible reactions are possible and useful to create mechanically responsive materials that display new functions. The metallopolymer studied was a cross-linked network assembled from a europium salt and a telechelic poly(ethylene-co-butylene) with 2,6-bis(1'-methylbenzimidazolyl)pyridine (Mebip) ligands at the termini. The Eu(3+) complexes serve both as mechanically responsive binding motifs and as built-in optical probes that can monitor the extent of (dis)assembly due to their characteristic photoluminescent properties. Indeed, dose-dependent and reversible metal-ligand dissociation occurs upon exposure to ultrasound in solution. The absence of ultrasound-induced dissociation of a low-molecular weight model complex and in-depth studies of temperature effects confirm that the dissociation is indeed the result of mechanical activation. The influence of the strength of the metal-ligand interactions on the mechanically induced dissociation was also explored. Metallopolymers in which the Mebip ligands were substituted with more strongly coordinating dipicolinate (dpa) ligands do not dissociate upon exposure to ultrasound. Finally, we show that mechanochemical transduction in metallosupramolecular polymers is also possible in the solid state. We demonstrate mending of damaged objects through ultrasound as well as mechanochromic behavior based on metal-exchange reactions in metallopolymers imbibed with an auxiliary metal salt.

Shockwave loading of mechanochemically active polymer coatings

Thin films of mechanochemically active polymer were subjected to laser-generated, high amplitude acoustic pulses. Stress wave propagation through the film produced large amplitude stresses (>100 MPa) in short time frames (10-20 ns), leading to very high strain rates (ca. 1 × 10(7) to 1 × 10(8) s(-1)). The polymer system, spiropyran (SP)-linked polystyrene (PS), undergoes a force-induced chemical reaction causing fluorescence and color change. Activation of SP was evident via a fluorescence signal in thin films subject to high strain-rates. In contrast, quasi-static loading of bulk SP-linked PS samples failed to result in SP activation. Mechanoresponsive coatings have potential to indicate deformation under shockwave loading conditions.

Mechanical Activation of Mechanophore Enhanced by Strong Hydrogen Bonding Interactions

A mechanically active spiropyran (SP) mechanophore is incorporated into the backbone of prepolymer which is further end-capped with ureidopyrimidinone (UPy) or urethane. Strong mechanochromic reaction of SP arises in the bulk films of UPy containing materials whereas much weaker activation occurs in urethane containing counterparts, coincident with their stress-strain responses. The difference in the magnitudes of supramolecular interactions leads to different degrees of chain orientation and strain induced crystallization (SIC) in the bulk and consequently distinct capabilities to transfer the load to the mechanophores. This study may aid the design of novel mechanoresponsive materials whose mechanoresponsiveness can be tailored by tuning supramolecular interactions.

Joining forces: integrating the mechanical and optical single molecule toolkits

Combining single molecule force measurements with fluorescence detection opens up exciting new possibilities for the characterization of mechanoresponsive molecules in Biology and Materials Science.

Mechanochemistry of Topological Complex Polymer Systems

Although existing since the concept of macromolecules, polymer mechanochemistry is a burgeoning field which attracts great scientific interest in its ability to bias conventional reaction pathways and its potential to fabricate mechanoresponsive materials. We review here the effect of topology on the mechanical degradation of polymer chains and the activation of mechanophores in polymer backbones. The chapter focuses on both experimental and theoretical work carried out in the past 70 years. After a general introduction (Sect. 1), where the concept, the history, and the application of polymer mechanochemistry are briefly described, flow fields to study polymer mechanochemistry are discussed (Sect. 2), results of mechanochemistry study are presented for linear polymers (Sect. 3), cyclic polymers (Sect. 4), graft polymers (Sect. 5), star-shaped polymers (Sect. 6), hyperbranched polymers and dendrimers (Sect. 7), and systems with dynamic topology (Sect. 8). Here we focus on (1) experimental results involving the topological effect on the coil-to-stretch transition and the fracture of the polymer chains, (2) the underlying mechanisms and the key factor that determines the mechanical stability of the macromolecules, (3) theoretical models that relate to the experimental observations, and (4) rational design of mechanophores in complex topology to achieve multiple activations according to the existing results observed in chain degradation.

Alkyne mechanochemistry: putative activation by transoidal bending

Mechanochemical transoidal bending of triple bonds lead to an unexpected reaction between alkynes and azide traps.

Stress-responsive polymers containing cyclobutane core mechanophores: reactivity and mechanistic insights

A primary goal of covalent mechanochemistry is to develop polymer bound mechanophores that undergo constructive transformations in response to otherwise destructive forces. The [2 + 2] cycloreversion of cyclobutane mechanophores has emerged as a versatile framework to develop a wide range of stress-activated functionality. Herein, we report the development of a class of cyclobutane bearing bicyclo[4.2.0]octane mechanophores. Using carbodiimide polyesterification, these stress-responsive units were incorporated into high molecular weight polymers containing up to 700 mechanophores per polymer chain. Under exposure to the otherwise destructive elongational forces of pulsed ultrasound, these mechanophores unravel by ∼7 Å per monomer unit to form α,β-unsaturated esters that react constructively via thiol-ene conjugate addition to form sulfide functionalized copolymers and cross-linked polymer networks. To probe the dynamics of the mechanochemical ring opening, a series of bicyclo[4.2.0]octane derivatives that varied in stereochemistry, substitution, and symmetry were synthesized and activated. Reactivity and product stereochemistry was analyzed by (1)H NMR, which allowed us to interrogate the mechanism of the mechanochemical [2 + 2] cycloreversion. These results support that the ring opening is not concerted but proceeds via a 1,4 diradical intermediate. The bicyclo[4.2.0]octanes hold promise as active functional groups in new classes of stress-responsive polymeric materials.

Mechanoresponsive PS-PnBA-PS Triblock Copolymers via Covalently Embedding Mechanophore

A mechanically active spiropyran (SP) mechanophore is incorporated into the center of poly(n-butyl acrylate) (PnBA) block to construct a series of mechanoresponsive polystyrene (PS)-PnBA-SP-PnBA-PS triblock copolymers. Similar mechanical activations of SP occur in all of the copolymers in solution, whereas a unique PS fraction-dependent mechanochromism is observed in the bulk. Effective mechanical activation occurs in the copolymer with a medium PS block length, whereas a very weak color change is observed in the samples bearing low PS fractions and activation appears only in the vicinity of the fracture point in the copolymer bearing long PS blocks. The difference in chemical compositions of the triblock copolymers leads to different microphase separated structures in the bulk and consequently the unique stress-strain responses and mechanochemistry. This platform promises to open way to the design of a wide range of useful mechanoresponsive triblock copolymers having different hard/soft blocks and various types of mechanoresponsive motifs.

Mechanocatalytic Polymerization and Cross-Linking in a Polymeric Matrix

A latent olefin metathesis catalyst, bearing two polymeric NHC ligands, was embedded in a semicrystalline polymer matrix containing cyclic olefins. The catalyst was activated by straining the solid material under compression, resulting in polymerization and cross-linking reactions of the monomers in situ. Catalyst activation in the solid state may be employed in new self-healing materials.

"Flex-activated" mechanophores: using polymer mechanochemistry to direct bond bending activation

We describe studies in mechanochemical transduction that probe the activation of bonds orthogonal to an elongated polymer main chain. Compression of mechanophore-cross-linked materials resulted in the release of small molecules via cleavage of covalent bonds that were not integral components of the elongated polymer segments. The reactivity is proposed to arise from the distribution of force through the cross-linking units of the polymer network and subsequent bond bending motions that are consistent with the geometric changes in the overall reaction. This departure from contemporary polymer mechanochemistry, in which activation is achieved primarily by force-induced bond elongation, is a first step toward mechanophores capable of releasing side-chain functionalities without inherently compromising the overall macromolecular architecture.

Methods for activating and characterizing mechanically responsive polymers

Mechanically responsive polymers harness mechanical energy to facilitate unique chemical transformations and bestow materials with force sensing (e.g., mechanochromism) or self-healing capabilities. A variety of solution- and solid-state techniques, covering a spectrum of forces and strain rates, can be used to activate mechanically responsive polymers. Moreover, many of these methods have been combined with optical spectroscopy or chemical labeling techniques to characterize the products formed via mechanical activation of appropriate precursors in situ. In this tutorial review, we discuss the methods and techniques that have been used to supply mechanical force to macromolecular systems, and highlight the advantages and challenges associated with each.

Performance of Mechanochemically Activated Catalysts Is Enhanced by Suppression of the Thermal Effects of Ultrasound

In this work, we demonstrate that the performance of mechanochemically activated transesterification and alkene metathesis catalysts is significantly enhanced when the thermal effects of ultrasound are suppressed. Suppression of these effects is realized by performing the reaction under methane instead of argon. Not only do these results provide further confirmation of the true mechanochemical nature of the ultrasonic activation of the catalysts, but it also strongly recommends the use of methane as standard saturation gas when studying the mechanochemical effects of ultrasound.