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      Figure 1.9 (I) Diels Alder reversible reaction. (II) (a) Synthesis of the tetrafunctional monomer (1); (b) formation of the DA-based network (3); (c)–(e) healing/recycling process. Reprinted with permission from Ref. [72]. Copyright (2014) John Wiley & Sons, Inc.

      Dynamic covalent bonds (DCBs), in contrast to reversible bonds, can be used to produce self-healing intrinsic polymers that maintain their cross-linked structure even at high temperatures during the healing process [19, 76, 77]. The exchange reactions between dynamic bonds need to be activated through an external stimulus. The most usual one is heat, but some dynamic bonds respond to light, and in a lesser extent to other stimuli such as mechanical stress [78–80] or solvents [81, 82]. Light responsive DCBs based on the sulfur chemistry were extensively studied by C. Bowman’s group [83–89]. The healing mechanism of these materials relies on bond exchanges through addition-fragmentation chain transfer. These polymers are capable not only of self-healing, but also of undergoing selective plastic deformation through the use of appropriate masks. This can be used to create 2D patterns on the material, as shown in Figure 1.10-I. Matyjaszewski and co-workers also used light-activated reactions to produce self-healing polymers. Their polymeric networks have either trithiocarbonate (TTC) [90, 91] or thiuram disulfide (TDS) [92] moieties acting as the dynamic crosslinks. Mechanical tests performed on the latter showed that around 90% of the mechanical strength could be recovered by visible light irradiation (Figure 11.0-II). Some of the advantages of the use of light to trigger the self-healing are highlighted by the authors: the healing can be performed at room temperature, preserving the substrate (especially useful when it cannot withstand higher temperatures); no solvents are needed; the air atmosphere does not interfere with the healing process nor it produces any degradation by oxidation, since the temperature remains low.

Thermally activated DCBs can also be harnessed to obtain self-healing polymeric networks. Hydroxyesters, obtained from the reaction of an epoxide with a carboxylic acid and capable of undergo transesterification reactions, were first studied by Leibler and colleagues, who coined the term “vitrimers” to name these self-healing networks [21, 93, 94]. Though many different chemical groups have also been proposed as alternatives [18, 19], vitrimers based on the transesterification reaction remain the most widely studied ones. A variety of epoxy and poly-carboxylic acid precursors have been used in combination with diverse catalysts. The first systems developed by Leibler et al. consisted in a commercial epoxy resin (DGEBA) crosslinked with a mixture of di- and tri-functional fatty acids, with Zn⁺² to catalyze the transesterification [21, 93]. They showed that a thermal treatment at moderate temperature (100–150 °C) produces a very good welding between two pieces of the vitrimer. Other catalysts tested by the same authors include triphenylphosphine (PPh₃) and triazobicyclodecene (TBD) [94]. TBD showed an excellent catalytic activity, similar to Zn⁺². TBD and Zn⁺² are the most frequent catalysts used for epoxy-carboxylic acid vitrimers [21, 81, 93, 95–103]. These vitrimers were used to study other relevant aspects of DCBs. For instance, Legrand and Soulié-Ziakovic studied how the dynamic bonds can be harnessed to obtain a better adhesion between the matrix and glass fibers [99]. We addressed the generation of a crosslinked polymer from difunctional precursors, at the expense of leaving a fraction of soluble smaller chains [104], which several authors have done [81, 95–97]. Other catalysts that proved to work for accelerating the transesterification reactions in vitrimers include tertiary amines [105], imidazoles [106] and Sn⁺² [107]. Also several uncatalyzed systems demonstrated self-healing ability, though higher temperatures or longer times are needed [108–110]. Figure 1.11 shows some examples of self-healing processes of epoxy-carboxylic acid vitrimers. Epoxy-anhydride networks with Zn⁺² as transesterification catalyst have also been studied as vitrimer systems though to a lesser extent [21, 93]. Epoxy-anhydride networks have higher glass transition temperatures, and higher modulus (at room temperature) when compared to epoxy-carboxylic acid ones. However, the – OH concentration, which was proved to be a critical parameter for the transesterification reactions rate [21], is much lower in the former (the epoxy-anhydride reaction yields ester groups, hence another source of hydroxyls is needed).

      Figure 1.10 (I) Mechanopatterning of two different samples of thiol-ene based elastomers. (a) Using a mask with concentric rings. (b) Using a mask with horizontal lines in a two step process, rotating the mask 30° between the irradiation steps. Reprinted with permission from Ref. [84]. Copyright (2011) John Wiley & Sons, Inc. (II). (a)–(c) As-prepared, cut and self-healed sample of a TDS crosslinked polymer. (d) Stress–strain curves of samples with different healing times. (e) Dependence of the elongation at break with the healing time. Reprinted with permission from Ref. [92]. Copyright (2012) John Wiley & Sons, Inc.

      A wide variety of functional groups such as disulfide [111–114], hydroxyurethanes [78], vinylogous urethanes [115–117], anhydrides [118], dioxaborolanes [119], triazolium salts [120], Schiff bases [121, 122], acylhydrazones [123], among others [124] were successfully used as dynamic crosslinks following the groundbreaking work of Leibler and colleagues, with varying degrees of healing efficiencies. Figure 1.12 shows some examples of vitrimers based on different chemistries.

An important aspect to be considered is the temperature needed to activate the exchange reactions that allows the network to flow. A characteristic temperature Tv can be defined, below which the exchange reactions are frozen, or take place only at rates low enough to be dismissed. Above Tv, the exchange reactions proceed at an appreciable rate, and the material flows, and therefore its usage should be always limited to temperatures below Tv. If Tv is low enough to enable self-healing at room temperature, then it would also be expected that the polymeric network can be easily deformed permanently at the same temperature [125]. A balance between good mechanical properties and dimensional stability at the service temperature on one side, and quick self-healing at reasonable temperatures on the other is crucial for a coating whose purpose is to protect the substrate. This compromise can be well illustrated by some disulfide based vitrimers with Tv values below room temperature [111] that also show acceptable mechanical properties. The authors claimed that the formation of quadruple H bonding between urea groups is the key to achieve such behavior, providing an additional healing mechanism, as well as crosslinks that increase the elastic modulus. Most vitrimers, however, show much higher Tv values, up to above 100 °C [18]. The activation energy of the exchange reactions (Ea) is another important parameter, which offers a measure of the sensitivity of exchange reactions rate to temperature. Typical values range from a few tens of kJ/mol to more than 120 kJ/mol. Both Tv and Ea values strongly depend on the type and amount of catalyst used in the material [94, 117].

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