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.

The thiol−Michael adduct is used as a thermoresponsive dynamic cross-linker in polymeric materials. Recently, the thiol−Michael reaction between thiols and conjugated alkenes has been used as a ligation reaction for polymer synthesis and functionalization. Here, the thiol− Michael reaction is demonstrated to be thermally responsive and dynamic. Small molecule model experiments demonstrate the potential for the thiol−Michael adducts to be used in dynamic covalent chemistry. Thiol−acrylate adducts are incorporated into a cross-linker to form a soft polymeric material. These thiol−Michael cross-linked materials display healing after being cut and malleability characteristics at 90 °C. Additionally, the data suggest that there is limited creep and stress relaxation at room temperature with complete recovery of creep once the strain is removed. These thiol−Michael cross-linked polymers show dynamic properties upon thermal stimulus, with long-term stability against mechanical deformation in the absence of this stimulus, opening the way for them to be used in various applications.

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.

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.

Dynamic covalent materials have properties such as self-healing, recyclability, and stress relaxation because of the dynamic exchange in polymer networks. Here, kinetic exchange in anilinium salts and viscoelastic properties of dynamic covalent networks with anilinium linkages are extensively studied. Mechanistic studies show that dynamic exchange in anilinium salts follows a dissociative pathway. Small-molecule kinetics study suggests that despite the dissociative mechanism, there is an essentially constant molar composition or bond density across a wide temperature profile. Rheological studies indicate that covalent adaptable networks (CANs) with anilinium linkages provide viscoelastic properties similar to CANs with associative exchange process networks. Additionally, thermal-and microwave-responsive self-healing and malleability properties can be achieved in this network, along with the materials showing excellent creep resistance and creep recovery.

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.

Thiol-maleimide adducts are incorporated as crosslinkers into polymer networks and act as pH-responsive and thermoresponsive dynamic crosslinkers, imparting malleability and self-healing properties into the material.

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 S N 2 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.