The rapid expansion of plastic pollution and environmental concern motivate scientific research to use renewable resources and recyclability of the utilised polymeric materials that can be effective for packaging, construction, furniture and automotive industries [1,2,3]. There are two types of conventional synthetic polymer materials: thermoplastic and thermosetting polymers. While a certain temperature is applied, the thermoplastic polymer is broken its intermolecular connections, such as hydrogen bonds, Van der Waals forces and molecular chain entanglement, resulting in fluidity, and it can be easily reprocessed [4]. Among thermoset polymers, epoxy thermosets are considered the most adaptable and commonly used material due to their resilience to chemicals and heat, high mechanical strength and magnificent insulation [5,6]. Based on its chemical structure, the molecule has at least two epoxy groups and can be found in solid, viscous and liquid forms [7]. Therefore, it is often used in construction, electronic packaging, adhesives, coatings and composites [8]. However, diglycidyl ether of bisphenol A (DGEBA), which is not renewable, makes up about 90% of the epoxy thermosets used today [9]. However, after the completion of their service life, this thermoset decreases the composite material’s feasibility and creates massive environmental and waste resource difficulties. Concerning environmental and human health issues, the utilization of bisphenol A BPA in materials that come into touch with food is also prohibited by several nations, and they are concentrating on BPA replacement in research [10,11,12]. Keep focusing on petroleum availability and price uncertainty; the biobased materials and chemicals have to be synthesised from renewable resources. Therefore, epoxy made by BPA can be replaced by using renewable chemicals and derivatives [10]. Biomass renewable resources are cellulose [13,14,15], lignin [16,17,18], vanillin [19,20,21], vegetable oil [22,23,24], furan [25,26,27], cardanol [28,29,30], rosin [31,32,33], itaconic acid [34,35,36], quercetin [37,38] and so on.

Moreover, conventional and biobased thermosetting polymeric materials cannot be recycled or mended due to irreversible crosslinking networks, so they are treated as less environmentally friendly than thermoplastics [39,40,41]. The problems of the recycling and degradation, reprocessability, and self-healing of epoxy thermosets can be solved by incorporating reversible dynamic covalent bonds in the polymer network. Wuld and coworkers first proposed the concept of adaptive covalent chemistry (ACC) in 2002 [42], while Bowman and coworkers initially used the term covalent adaptive networks, also known as CANs, regarding crosslinked polymer networks with reversible bonds [43]. If certain stimuli and equilibrium controls allow for the reversible formation and breaking of covalent bonds, these bonds can be categorised as dynamic. In addition, these covalent bonds will be constructed as sustainable thermosets, eliminating the use of petroleum-based components and the depletion of fossil resources [44,45,46,47]. Several numbers of reversible covalent bonds are disulfide linkages [48,49,50], boronic ester bonds [31,51], ester bonds [37,52,53], acetal linkages [54,55,56], Schiff bases [2,57], carbamate linkages [58,59], Diels–Alder additions [60,61] and silicon oxygen bonds [62,63]. The dynamic covalent bond containing crosslinked network demonstrates repairability, malleability, degradability, reprocessability and self-healing properties under several extrinsic stimuli, such as solvent, pressure, light and heat [1,64,65,66]. CANs may be divided into two general categories according to how their dynamic structures are created: either bond exchange done kinetically (associative), expressed in Figure 1, or through equilibrium changes that result in reversible depolymerization (dissociative), expressed in Figure 2 [67,68]. In the case of associative dynamic bonds, a new bond is formed as the previous one breaks at the same time, keeping a constant crosslink density (Figure 1) [68,69]. In the case of dissociative dynamic bonds, the covalent networks are initially disrupted and then rebuilt at a different location within the network (Figure 2). During crosslinked network breakdown, the molecular density declines, resulting in network connectivity to be lost or completely degraded. This breakdown of the network might result in either the recovery of molecules or the formation of free chains, according to the composition of the dissociative network. Photo-triggerable reactions and Diels–Alder adducts are two of the most well-known examples of dissociative reversible networks [70,71]. In both situations, the polymeric thermosets are malleable and reshapable, and the crosslinks can be reformed to restore their original mechanical characteristics.

Recently, it has been established that various chemical linkages have adaptive abilities, making the resulting polymer network characteristics typically of CANs [72]. Moreover, the imine bond is a viable choice for CANs and epoxy thermosets among the many dynamic bonds. An imine (C=N) bond is formed in the reaction between the activated carbonyl and primary nucleophilic amine groups [2]. These linkages give polymers exceptional characteristics, including stimuli adaptability, reprocessability, degradability and self-healability.

2. Imine-Based Adaptive Covalent Chemistry