Dynamic Covalent Bond-Based Polymer Chains Operating Reversibly with Temperature Changes

Abstract

Dynamic bonds can facilitate reversible formation and dissociation of connections in response to external stimuli, endowing materials with shape memory and self-healing capabilities. Temperature is an external stimulus that can be easily controlled through heat. Dynamic covalent bonds in response to temperature can reversibly connect, exchange, and convert chains in the polymer. In this review, we introduce dynamic covalent bonds that operate without catalysts in various temperature ranges. The basic bonding mechanism and the kinetics are examined to understand dynamic covalent chemistry reversibly performed by equilibrium control. Furthermore, a recent synthesis method that implements dynamic covalent coupling based on various polymers is introduced. Dynamic covalent bonds that operate depending on temperature can be applied and expand the use of polymers, providing predictions for the development of future smart materials.

Keywords: application; dynamic covalent bonds; equilibrium; reversibility; self-healing; shape memory; synthesis; temperature control.

Figure 16

(a) Preparation of temperature-mediated Diels-Alder (DA) adducts and the epoxy thermoset crack–healing mechanism. Reprinted/adapted with permission from Ref. [146]. 2023, American Chemical Society. (b) The design concept of DA cycloadditions with lower energy barrier and the reprocess ability of the material. Reprinted/adapted with permission from Ref. [147]. 2023, Wiley. (c) Self-healing mechanism of the urushiol-based coating surface [150]. (d) A schematic illustration of the on/off switch for the drug release mechanism from nanofibers using near-infrared (NIR) light. Reprinted/adapted with permission from Ref. [153]. 2024, Elsevier.

Figure 17

Acylhydrazone bond. (a) Structure and mechanism of the acylhydrazone bond. (b) Synthesis of ketone-based polymers via reversible addition–fragmentation chain-transfer polymerization(RAFT) polymerization and acylhydrazone crosslinking. Reprinted/adapted with permission from Ref. [158]. 2017. Royal Society of Chemistry. (c) Doxorubicin (DOX)-delivery system utilizing acylhydrazone bond, which is capable of releasing drugs in response to acidic conditions. Reprinted/adapted with permission from Ref. [160]. 2011, Royal Society of Chemistry.

Figure 18

Boronic/boronate ester (a) Structure and mechanism of boronic/boronate ester bond. (b) Synthesis of phenolic compounds based on dynamic B–O bonding using pyrogallol with three hydroxyl groups. Reprinted/adapted with permission from Ref. [168]. 2021, American Chemical Society. (c) Application to an ultrafast self-healing piezo-ionic elastomer capable of functioning in both air and underwater environments. Reprinted/adapted with permission from Ref. [173]. 2024, Nature.

Figure 1

Dynamic bonding types according to bonding formation position: (a) Intermolecular and (b) Intramolecular bonds.

Figure 2

Types of dynamic covalent bonds that react reversibly to temperature changes and operating temperature ranges.

Figure 3

Mechanism and kinetics of hindered-urea bond. (a) Association–dissociation reaction of the general urea bond. (b) Association–dissociation reaction of the hindered-urea bond. (c) Heterolytic hindered-urea exchange. (d) Homolytic hindered-urea exchange. (e) Kinetics of hindered-urea bond.

Figure 4

Synthesis process of dynamic hindered-urea bond. (a) Synthesis of conjugated and hindered-urea bond using two isocyanates with different characteristics. (b) Synthesis process of dynamic poly(hindered-urea) by a facile step-growth polymerization.

Figure 5

Application of dynamic hindered-urea bonded materials. (a) The healing process and changes in electrical properties based on the concentration of hindered-urea bonds. Reprinted/adapted with permission from Ref. [61]. 2021, Royal Society of Chemistry. (b) Improvement and recovery of electric properties in a bilayered polymer composite film with reversible hindered-urea bond. Reprinted/adapted with permission from Ref. [59]. 2020, American Chemical Society. (c) A schematic illustration of the 3D printing process and photos of 3D samples with permanent shape reconfiguration due to homolytic bond exchange. Reprinted/adapted with permission from Ref. [52]. 2023, Springer Nature.

Figure 6

Dynamic disulfide bonding mechanism and structures. (a) Thiol-disulfide and disulfide-disulfide exchange reactions. (b) Nucleophilic attack mechanism with acid and base. (c) Radical polymerization induced by light or heat.

Figure 7

Kinetics of dynamic disulfide bond. (a) Disulfide exchange reaction based on S_N2 mechanism. Reprinted/adapted with permission from Ref. [83]. 2007, Elsevier. (b) Schematic diagram illustrating changes in thermodynamic states according to disulfide bond formation. Reprinted/adapted with permission from Ref. [85]. 2021, American Chemical Society. (c) Comparison of reaction pathways and equilibrium constants for thiol-ene and disulfide-disulfide reactions. Reprinted/adapted with permission from Ref. [86]. 2022, American Chemical Society.

Figure 8

Synthesis of dynamic disulfide bonds. (a) Synthesis of a polydisulfide network based on the thiol–Michael reaction between thiol and alkene. Reprinted/adapted with permission from Ref. [87]. 2023, American Chemical Society. (b) Synthesis of degradable epoxy resin via the Williamson ether synthesis method. Reprinted/adapted with permission from Ref. [88]. 2016, Elsevier. (c) Synthesis of copolymers containing disulfide networks using cyclic 1,2-dithiolane. Reprinted/adapted with permission from Ref. [89]. 2017, American Chemical Society.

Figure 9

Application of dynamic disulfide bonded materials. (a) Improvement in wettability in adaptive thermosetting adhesives through the introduction of dynamic disulfide bonds. Reprinted/adapted with permission from Ref. [94]. 2020, American Chemical Society. (b) Polyurethane-based stretchable conductors exhibiting low hysteresis characteristics under strain. Reprinted/adapted with permission from Ref. [95]. 2022, Royal Society of Chemistry. (c) Damping material based on the absorption of energy upon release of bonding in aromatic disulfides. Reprinted/adapted with permission from Ref. [96]. 2021, Wiley.

Figure 10

Dynamic imine bond mechanisms and structures. (a) Three types of imine bond formation and dissociation. (b) Mechanism of imine condensation reaction. (c) Mechanism of imine exchange reaction. (d) Mechanism of imine metathesis reaction.

Figure 11

Kinetics of the dynamic imine bond. (a) Local molecular motions involving the imine exchange reaction. Reprinted/adapted with permission from Ref. [112]. 2012, American Chemical Society. (b) [2 × 2] dynamic combinatorial library of imine bonds composed of amines and aldehydes and its thermodynamic products. Reprinted/adapted with permission from Ref. [113]. 2011, American Chemical Society. (c) Kinetic and thermodynamic products of the dynamic covalent library in the presence of hexameric resorcinarene capsules. Reprinted/adapted with permission from Ref. [19]. 2020, American Chemical Society.

Figure 12

Synthesis of dynamic imine bond. (a) Synthesis of a vanillin-based curing agent via a single imine condensation reaction. Reprinted/adapted with permission from Ref. [15]. 2022, American Chemical Society. (b) Synthesis of a green dynamer from biodegradable oligomer and diol component containing imine bonds. Reprinted/adapted with permission from Ref. [114]. 2012, Royal Society of Chemistry. (c) Synthesis process of thermosetting epoxy incorporating Schiff base chemistry. Reprinted/adapted with permission from Ref. [115]. 2021, Elsevier.

Figure 13

Application of dynamic imine bonded materials. (a) Fabrication of biodegradable polymers utilizing imine bonds and cellulose nanofibers-. Reprinted/adapted with permission from Ref. [116]. 2023, Elsevier. (b) Development of a pH-sensitive amphiphilic bolaform driven by benzoic imine bonds. Reprinted/adapted with permission from Ref. [117]. 2011, American Chemical Society. (c) Implementation of a dual-crosslinked polymer network with multi-shape memory through multiple thermal transition states. Reprinted/adapted with permission from Ref. [118]. 2020, American Chemical Society.

Figure 14

(a) The most basic DA reaction. (b) Normal electron-demand DA reactions, with an electron-poor dienophile and an electron-rich diene. (c) Inverse electron-demand DA reaction, with an electron-poor diene and an electron-rich dienophile. (d) Endo- and exo-diastereomer are obtained and lost according to DA reaction and retro DA reaction, respectively, and the energy profile diagram for its transformation.

Figure 15

Three strategies to introduce DA bonds into the polymer structures: (a) using diene/dienophile-crosslinkers; (b) using DA-contained monomer/oligomer; and (c) using monomers bearing diene/dienophile couples.