Molecular Ball Joints: Mechanochemical Perturbation of Bullvalene Hardy-Cope Rearrangements in Polymer Networks.

The solution-state fluxional behavior of bullvalene has fascinated physical organic and supramolecular chemists alike. Little effort, however, has been put into investigating bullvalene applications in bulk, partially due to difficulties in characterizing such dynamic systems. To address this knowledge gap, we herein probe whether bullvalene Hardy-Cope rearrangements can be mechanically perturbed in bulk polymer networks. We use dynamic mechanical analysis to demonstrate that the activation barrier to the glass transition process is significantly elevated for bullvalene-containing materials relative to "static" control networks. Furthermore, bullvalene rearrangements can be mechanically perturbed at low temperatures in the glassy region; such behavior facilitates energy dissipation (i.e., increased hysteresis energy) and polymer chain alignment to stiffen the material (i.e., increased Young's modulus) under load. Computational simulations corroborate our work that showcases bullvalene as a reversible "low-force" covalent mechanophore in the modulation of viscoelastic behavior.

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.

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.

Heat- and Light-Responsive Materials Through Pairing Dynamic Thiol-Michael and Coumarin Chemistry.

Covalent adaptable networks (CANs) based on the thiol-Michael (TM) linkages can be thermal and pH responsive. Here, a new vinyl-sulfone-based thiol-Michael crosslinker is synthesized and incorporated into acrylate-based CANs to achieve stable materials with dynamic properties. Because of the reversible TM linkages, excellent temperature-responsive re-healing and malleability properties are achieved. In addition, for the first time, a photoresponsive coumarin moiety is incorporated with TM-based CANs to introduce light-mediated reconfigureability and postpolymerization crosslinking. Overall, these materials can be on demand dynamic in response to heat and light but can retain mechanical stability at ambient condition.

Dynamic Covalent Bonds in Polymeric Materials.

Dynamic covalent bonds (DCBs) have received significant attention over the past decade. These are covalent bonds that are capable of exchanging or switching between several molecules. Particular focus has recently been on utilizing these DCBs in polymeric materials. Introduction of DCBs into a polymer material provides it with powerful properties including self-healing, shape-memory properties, increased toughness, and ability to relax stresses as well as to change from one macromolecular architecture to another. This Minireview summarizes commonly used powerful DCBs formed by simple, often "click" reactions, and highlights the powerful materials that can result. Challenges and potential future developments are also discussed.

Anilinium Salts in Polymer Networks for Materials with Mechanical Stability and Mild Thermally Induced Dynamic Properties.

Dynamic nucleophilic exchange of quaternary anilinium salts has been incorporated into rehealable and malleable polymeric materials that can be activated under mild (60 °C) thermal stimulus. The mechanism of dynamic exchange between quaternary anilinium salt and free aniline was assessed in small-molecule model experiments. The dynamic exchange was found to be dissociative in nature, due to the indirect SN2 mechanism, where initially the bromide anion attacks the anilinium salt to generate an alkyl bromide which undergoes subsequent attack by a free aniline group. A quaternary anilinium-based cross-linker was synthesized to act as dynamic linkages in the polymer network. Cross-linked polymeric materials showed thermoresponsive rehealing and malleability properties at 60 °C along with being resistant to irreversible creep under ambient conditions. The use of anilinium salts enables dynamic exchange to occur with significantly milder thermal stimulus than other comparable materials, while maintaining mechanical stability.

Probing the mechanism of thermally driven thiol-Michael dynamic covalent chemistry.

The kinetics and mechanism of the thermally activated dynamic covalent exchange of thiol-Michael adducts is investigated. A model system of thiol-Michael adducts between thiophenol and phenylvinylketone derivatives and adducts between 2-mercaptoethanol phenylvinylketone derivatives in N,N-dimethylformamide (DMF) at elevated temperatures is used to probe the underlying exchange mechanism. The kinetic data show negligible free Michael acceptor, which is consistent with the highly efficient thiol-Michael reaction being a "click"-like reaction that significantly favors the adduct form. At elevated temperatures of 90 °C in DMF the thiol-Michael adducts reach equilibrium after 24 h, although equilibration did not occur within 24 h at 60 °C or 75 °C, and negligible exchange occurs under ambient conditions. A kinetic model was developed to describe the dynamic covalent exchange and equilibration. The experimental and simulation kinetic data of dynamic covalent exchange are consistent with the thiol-Michael adducts undergoing a retro-Michael reaction, followed by subsequent addition of a free thiol to the liberated Michael acceptor. Kinetic analysis is consistent with the fragmentation, or retro-Michael reaction, being the rate-determining step in the dynamic covalent exchange. This suggests that the key step in dynamic covalent exchange is not enhanced by addition of free thiol or free Michael acceptor, since the addition reaction is much faster than the retro-Michael reaction. This fundamental study will guide the design of organic compounds, materials, and bioconjugates that utilize the thermally activated dynamic covalent thiol-Michael linkages.

Effect of Polymer Network Architecture, Enhancing Soft Materials Using Orthogonal Dynamic Bonds in an Interpenetrating Network.

Doubly dynamic polymer networks were synthesized with two distinct exchangeable cross-linkers. The first linker is highly dynamic and rapidly exchanging hydrogen bonded 2-ureido-4[1H]-pyrimidinone (UPy) and the second is a thermoresponsive furan-maleimide Diels-Alder adduct (FMI). Two network architectures were considered: an interpenetrating network (IPN) where one network is cross-linked with the UPy linker and the other is cross-linked with the FMI linker, and a single network (SN) where both the UPy and FMI linkers are in the same single network. Remarkably, the IPNs were superior to the SNs with the same composition of the UPy and FMI cross-linkers when comparing peak stress, strain at break, fracture toughness, malleability, and self-healing. Both materials studied were stable and creep resistant under ambient conditions.