Dynamic Covalent Bonds in Polymeric Materials

Abstract

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.

Recent advances in shear‐thinning and self‐healing hydrogels for biomedical applications

“…Dynamic covalent chemistry includes covalent bonds that are able to be formed, broken, and re‐formed, either autonomously or under stimuli . Compared to physical associations, dynamic covalent reactions generally have slower kinetics of bond cleavage and formation, which gives rise to more stable materials . When dynamic covalent chemistry is incorporated into crosslinks in the formation of hydrogels, bond cleavage can occur under mechanical shear and then bonds re‐form through various mechanisms to exhibit self‐healing.…”

“…43 Compared to physical associations, dynamic covalent reactions generally have slower kinetics of bond cleavage and formation, which gives rise to more stable materials. 176 When dynamic covalent chemistry is incorporated into crosslinks in the formation of hydrogels, bond cleavage can occur under mechanical shear and then bonds re-form through various mechanisms to exhibit self-healing. This chemistry has been incorporated into a wide range of materials and exists across various types of reactions and modes of bond reformation (i.e., healing) 176 and here we focus on examples (e.g., Schiff base, oxime chemistry, disulfide bonds, reversible Diels-Alder) ( Figure 2) that have been specifically used in biomedical applications as crosslinkers in hydrogels and even for in vivo applications (Table I).…”

Covalent Adaptable Networks with Tunable Exchange Rates Based on Reversible Thiol–yne Cross‐Linking

“…To illustrate the dissociative bond exchange mechanism operating in dynamic thiol–yne networks, frequency sweep measurements were performed at different temperatures. Whereas associative CANs or vitrimers are characterized by a constant plateau modulus G ′ at different temperatures as a result of their overall constant and temperature‐independent cross‐link density, dissociative CANs should show a decreasing G ′ with increasing temperature as a result of a net de‐cross‐linking . Figure shows the frequency sweep measurements for CAN‐1 and CAN‐3 as representative networks.…”

“…Whereasa ssociative CANs or vitrimers are characterized by ac onstant plateau modulus G' at different temperatures as ar esult of their overallc onstant and temperature-independent cross-linkd ensity,d issociative CANs should show ad ecreasing G' with increasing temperature as ar esult of an et de-crosslinking. [5,52] Figure 5s hows thef requency sweep measurements for CAN-1 and CAN-3 as representative networks. As expected, at emperature-dependent decreasing plateau modulus was observed, confirming wellthe dissociative nature of the dynamic bond exchange.…”

“…An important characteristic of CANs is the underlying bond exchange mechanism as it ultimately determines the mechanical properties of a CAN during the applied stimulus . Reversible polymer networks or so‐called dissociative CANs imply that a bond‐breaking event happens prior to the formation of a new bond (that is, an elimination–addition mechanism), whereas bond exchange in permanent dynamic networks, also called vitrimers or associative CANs, is achieved through a bond‐forming/bond‐breaking sequence (an addition–elimination reaction) . As a result, dissociative CANs can show a temporary decrease in network connectivity during the stimulus application, which may result in a sudden decrease of viscosity, while associative CANs or vitrimers, preserve their network integrity in all conditions and do not undergo a gel‐to‐sol transition .…”