100th Anniversary of Macromolecular Science Viewpoint: Piezoelectrically Mediated Mechanochemical Reactions for Adaptive Materials

Mechanical-Force-Induced Non-spontaneous Dehalogenative Deuteration of Aromatic Iodides Enabled by Using Piezoelectric Materials as a Redox Catalyst

The development of green and efficient deuteration methods is of great significance for various fields such as organic synthesis, analytical chemistry, and medicinal chemistry. Herein, we have developed a dehalogenative deuteration strategy using piezoelectric materials as catalysts in a solid-phase system under ball-milling conditions. This non-spontaneous reaction is induced by mechanical force. D2O can serve as both a deuterium source and an electron donor in the transformation, eliminating the need for additional stoichiometric exogenous reductants. A series of (hetero)aryl iodides can be transformed into deuterated products with high deuterium incorporation. This method not only effectively overcomes existing synthetic challenges but can also be used for deuterium labelling of drug molecules and derivatives. Bioactivity experiments with deuterated drug molecule suggest that the D-ipriflavone enhances the inhibitory effects on osteoclast differentiation of BMDMs in vitro.

Mechanochemical Vicinal Dibromination of Unactivated Alkenes and Alkynes Using Piezoelectric Material as a Redox Catalyst

Mechanoredox chemistry is a rapidly evolving field at the intersection of mechanical forces and chemical reactions. Herein, we have reported a vicinal dibromination of unsaturated hydrocarbons using piezoelectric material (Li2TiO3) as a redox catalyst. Furthermore, the reaction can be efficiently scaled up to 10 mmol and performed under an air atmosphere at room temperature without solvents or external reductants, and Li2TiO3 can be reused multiple times without a structural change.

Bioinspired mechanical mineralization of organogels

Mineralization is a long-lasting method commonly used by biological materials to selectively strengthen in response to site specific mechanical stress. Achieving a similar form of toughening in synthetic polymer composites remains challenging. In previous work, we developed methods to promote chemical reactions via the piezoelectrochemical effect with mechanical responses of inorganic, ZnO nanoparticles. Herein, we report a distinct example of a mechanically-mediated reaction in which the spherical ZnO nanoparticles react themselves leading to the formation of microrods composed of a Zn/S mineral inside an organogel. The microrods can be used to selectively create mineral deposits within the material resulting in the strengthening of the overall resulting composite.

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

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

Mechanochemical Synthesis of Aryl Fluorides by Using Ball Milling and a Piezoelectric Material as the Redox Catalyst

Aryl fluorides are important structural motifs in many pharmaceuticals. Although the Balz-Schiemann reaction provides an entry to aryl fluorides from aryldiazonium tetrafluoroborates, it suffers from drawbacks such as long reaction time, high temperature, toxic solvent, toxic gas release, and low functional group tolerance. Here, we describe a general method for the synthesis of aryl fluorides from aryldiazonium tetrafluoroborates using a piezoelectric material as redox catalyst under ball milling conditions in the presence of Selectfluor. This approach effectively addresses the aforementioned limitations. Furthermore, the piezoelectric material can be recycled multiple times. Mechanistic investigations indicate that this fluorination reaction may proceed via a radical pathway, and Selectfluor plays a dual role as both a source of fluorine and a terminal reductant.

Tribochemically Controlled Atom Transfer Radical Polymerization Enabled by Contact Electrification

Traditional mechanochemically controlled reversible-deactivation radical polymerization (RDRP) utilizes ultrasound or ball milling to regenerate activators, which induce side reactions because of the high-energy and high-frequency stimuli. Here, we propose a facile approach for tribochemically controlled atom transfer radical polymerization (tribo-ATRP) that relies on contact-electro-catalysis (CEC) between titanium oxide (TiO2 ) particles and CuBr2 /tris(2-pyridylmethylamine (TPMA), without any high-energy input. Under the friction induced by stirring, the TiO2 particles are electrified, continuously reducing CuBr2 /TPMA into CuBr/TPMA, thereby conversing alkyl halides into active radicals to start ATRP. In addition, the effect of friction on the reaction was elucidated by theoretical simulation. The results indicated that increasing the frequency could reduce the energy barrier for the electron transfer from TiO2 particles to CuBr2 /TPMA. In this study, the design of tribo-ATRP was successfully achieved, enabling CEC (ca. 10 Hz) access to a variety of polymers with predetermined molecular weights, low dispersity, and high chain-end fidelity.

Mechanoredox Catalysis Enables a Sustainable and Versatile Reversible Addition-Fragmentation Chain Transfer Polymerization Process

The sustainable synthesis of macromolecules with control over sequence and molar mass remains a challenge in polymer chemistry. By coupling mechanochemistry and electron-transfer processes (i.e., mechanoredox catalysis), an energy-conscious controlled radical polymerization methodology is realized. This work explores an efficient mechanoredox reversible addition-fragmentation chain transfer (RAFT) polymerization process using mechanical stimuli by implementing piezoelectric barium titanate and a diaryliodonium initiator with minimal solvent usage. This mechanoredox RAFT process demonstrates exquisite control over poly(meth)acrylate dispersity and chain length while also showcasing an alternative to the solution-state synthesis of semifluorinated polymers that typically utilize exotic solvents and/or reagents. This chemistry will find utility in the sustainable development of materials across the energy, biomedical, and engineering communities.

Piezoelectric harvesting of mechanical energy for redox chemistry

Much work has been done in the utilization of mechanical force to enable chemical processes. However, this process is limited to thermal- and deformation-driven reactions. In fact, the transfer of energy in mechanical reactors can be quite inefficient, with energy lost to heat and mechanical deformation. Although these losses diminish at larger scales, small-scale reactions (from a few milligrams to a kilogram) can suffer from unfavorable energy demands. Recent work has sought to harvest unused energy in mechanical reactors by converting it to a flow of electrons through the use of piezoelectric materials, as many economically important reactions rely on the transfer of electrons to enact chemical change. Recent work has shown that the addition of piezoelectric powders to mechanochemical reactions results in enhanced yields for reductive and oxidative chemistry. However, these materials ultimately contaminate the end product and must be removed. Additionally, impacts on a piezoelectric material produce an AC output; limiting this approach's usefulness to irreversible reactions. We have developed a cleaner approach using an external piezoelectric element to either supply or sink electrons during milling. Methylene blue was reduced to leucomethylene blue using our approach. Mechanochemical reaction rates for this reduction were determined with respect to media quantities and sizes with a maximum rate of 7.76 μM s^-1. It was found that the conversion rate is linearly dependent on the number of media and geometrically dependent on the size of the media. Our approach allows selective reduction and eliminates contamination of the products with piezoelectric material. Shuttling electrons in a mechanochemical reaction will enable difficult chemistry, such as the reduction of CO₂ or the production of low oxidation state inorganic compounds, to be achieved more easily.

Sonoelectrochemical oxidation of sulfamethoxazole in simulated and actual wastewater on a piezo-polarizable FTO/BaZr x Ti(1-x)O3 electrode: reaction kinetics, mechanism and reaction pathway studies

The sonoelectrochemical (SEC) oxidation of sulfamethoxazole (SMX) in simulated and actual wastewater on FTO/BaZr_(0.1)Ti_(0.9)O₃, FTO/BaZr_(0.05)Ti_(0.95)O₃ and FTO/BaTiO₃ electrodes is hereby presented. Electrodes from piezo-polarizable BaZr_(0.1)Ti_(0.9)O₃, BaZr_(0.05)Ti_(0.95)O₃, and BaTiO₃ materials were prepared by immobilizing these materials on fluorine-doped tin dioxide (FTO) glass. Electrochemical characterization performed on the electrodes using chronoamperometry and electrochemical impedance spectroscopy techniques revealed that the FTO/BaZr_(0.1)Ti_(0.9)O₃ anode displayed the highest sonocurrent density response of 2.33 mA cm^-2 and the lowest charge transfer resistance of 57 Ω. Compared to other electrodes, these responses signaled a superior mass transfer on the FTO/BaZr_(0.1)Ti_(0.9)O₃ anode occasioned by an acoustic streaming effect. Moreover, a degradation efficiency of 86.16% (in simulated wastewater), and total organic carbon (TOC) removal efficiency of 63.16% (in simulated wastewater) and 41.47% (in actual wastewater) were obtained upon applying the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode for SEC oxidation of SMX. The piezo-polarizable impact of the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode was further established by the higher rate constant obtained for the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode as compared to the other electrodes during SEC oxidation of SMX under optimum operational conditions. The piezo-potential effect displayed by the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode can be said to have impacted the generation of reactive species, with hydroxyl radicals playing a predominant role in the degradation of SMX in the SEC system. Additionally, a positive synergistic index obtained for the electrode revealed that the piezo-polarization effect of the FTO/BaZr_(0.1)Ti_(0.9)O₃ electrode activated during sonocatalysis combined with the electrochemical oxidation process during SEC oxidation can be advantageous for the decomposition of pharmaceuticals and other organic pollutants in water.

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.

Dimensional Optimization for ZnO-Based Mechano-ATRP with Extraordinary Activity

Various piezoelectric nanomaterials were utilized in ultrasound-mediated atom transfer radical polymerization (ATRP), owing to their outstanding piezoelectric effect. However, the relationship between the morphology of those piezocatalysts and polymerization has not been clearly established. Herein, we employed different piezoelectric zinc oxide (ZnO) nanomaterials to achieve novel mechano-induced ATRP (mechano-ATRP). Based on the synergistic effect of piezoelectric properties and specific surface area, the catalytic activity of 1D ZnO nanorods (1D-ZnO NRs) with increased aspect ratio outperformed that of 0D ZnO nanoparticles (0D-ZnO NPs). Compared to the conventional ATRP system, this system exhibited extraordinary activity toward the less activated monomer acrylonitrile (67% conversion after 6 h), with a narrow molecular weight distribution (polydispersity index ∼ 1.19). Furthermore, implications of ZnO loading, copper salt amount, degree of polymerization, monomer, and solvent were also studied for the highly efficient mechano-ATRP.

Diaryliodonium salts facilitate metal-free mechanoredox free radical polymerizations

Mechanically-induced redox processes offer a promising alternative to more conventional thermal and photochemical synthetic methods. For macromolecule synthesis, current methods utilize sensitive transition metal additives and suffer from background reactivity. Alternative methodology will offer exquisite control over these stimuli-induced mechanoredox reactions to couple force with redox-driven chemical transformations. Herein, we present the iodonium-initiated free-radical polymerization of (meth)acrylate monomers under ultrasonic irradiation and ball-milling conditions. We explore the kinetic and structural consequences of these complementary mechanical inputs to access high molecular weight polymers. This methodology will undoubtedly find broad utility across stimuli-controlled polymerization reactions and adaptive material design.

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.

Sonochemistry-assisted photocontrolled atom transfer radical polymerization enabled by manganese carbonyl

Sonochemistry-assisted photocontrolled atom transfer radical polymerization (SAP-ATRP) is developed to circumvent the problem caused by the low penetration depth of light.

Mechanically Promoted Synthesis of Polymer Organogels via Disulfide Bond Cross-Linking

Mechanically adaptive polymers could significantly improve the life-cycle of current materials. Piezo-polymerization is a novel approach that harnesses vibrational mechanical energy through piezoelectric nanoparticles to generate chemical promoters for linear polymerization and cross-linking reactions. However, the available piezo-polymerization systems rely on reactions forming irreversible covalent bonds. Dynamic covalent linkages could impart further adaptability to these polymeric systems. Here we show the first example of the piezoelectrochemical synthesis of disulfide bonds to form organogels from polymers with thiol side groups. We demonstrate that the reaction proceeds via piezo-oxidation of the thiol to disulfide in the presence of ZnO nanoparticles and iodide anions under mechanical agitation. We use mechanical energy in the form of ultrasound (40 kHz) and low frequency vibrations (2 kHz) to synthesize a variety of organogels from common synthetic polymers. Additionally, we show that the polymers in these gels can be chemically recycled with a reducing agent. Finally, we study the thermal and mechanical properties of the composites obtained after drying the gels. We believe this new system adds to the piezo-polymerization repertoire and serves as the basis to fabricate mechanically adaptive polymeric materials via dynamic covalent bonds.