Hydrogen-Bond-Assisted Diels–Alder Kinetics or Self-Healing in Reversible Polymer Networks? A Combined Experimental and Theoretical Study

Abstract

Diels–Alder (DA) cycloadditions in reversible polymer networks are important for designing sustainable materials with self-healing properties. In this study, the DA kinetics of hydroxyl-substituted bis- and tetrafunctional furans with bis- and tris-functional maleimides, both containing ether-functionalized spacers, is investigated by modelling two equilibria representing the endo and exo cycloadduct formation. Concretely, the potential catalysis of the DA reaction through hydrogen bonding between hydroxyl of the furans and carbonyl of the maleimides or ether of the spacers is experimentally and theoretically scrutinized. Initial reaction rates and forward DA rate constants are determined by microcalorimetry at 20 °C for a model series of reversible networks, extended with (i) a hydroxyl-free network and hydroxyl-free linear or branched systems, and (ii) polypropylene glycol additives, increasing the hydroxyl concentration. A computational density-functional theory study is carried out on the endo and exo cycloadditions of furan and maleimide derivatives, representative for the experimental ones, in the absence and presence of ethylene glycol as additive. Additionally, an ester-substituted furan was investigated as a hydroxyl-free system for comparison. Experiment and theory indicate that the catalytic effect of H-bonding is absent or very limited. While increased concentration of H-bonding could in theory catalyze the DA reaction, the experimental results rule out this supposition.

Accelerated Degradation of DiXyl-α,ε-Lys-ARP via Interaction between Extra-Added Xylose and Monosubstituted Lys-ARPs during Maillard Reaction

“…This could have influenced the changes in the electron density of the surrounding atoms and the overall reactivity of the molecule. , On the other hand, intramolecular hydrogen bonds might be formed through intramolecular spatial arrangements and molecular movements near the N α-site under specific conditions, such as a low-temperature environment. These hydrogen bonds could lower the energy barrier of the reaction, − thereby promoting the elimination and hydrolysis reactions at the N α -site. In contrast, the ε-position was farther from the −COOH, was less affected by the electron-withdrawing effect, and remained relatively stable in reactivity.…”

Self‐Healing Elastic Electronics: Materials Design, Mechanisms, and Applications

“…Depending on the type of dynamically reversible bonds, the mechanisms of self-healing can be generally categorized into dynamic covalent bonding reactions and non-covalent bonding interactions (Figure 1). Dynamic covalent bonding reactions includes Diels-Alder reactions, [55][56][57] dynamic exchange reactions, [58][59][60] amine bonds re-formation, [61][62][63] boroxine bonds re-formation, [64][65][66] as well as dynamic non-covalent bonding interactions includes hydrogen bonds (H-bonds), [67][68][69] metalligand bonds, [70][71][72] ionic interactions, [73][74][75] 𝜋-𝜋 interactions [76,77] host-guest interactions, [78,79] Van der Waals interactions. [80][81][82] All of these dynamic bonds can be switched multiple times, resulting in multiple healing processes.…”

Intrinsic and extrinsic self‐healing fiber‐reinforced polymer composites: A review

“…12,13 Extrinsic polymers utilize external implementations to realize self-healing, such as microcapsules, [14][15][16][17][18] microvascular networks [19][20][21][22][23] and thermoplastics. [24][25][26] While, the intrinsic polymers themselves poses the ability to heal based on reversible chemical bonds or intermolecular interactions, 27 such as transesterification, [28][29][30][31][32] Diels-Alder (D-A) reaction, [33][34][35][36][37][38][39] disulfide exchange, [40][41][42] imide exchange, [43][44][45] hydrogen bonding, [46][47][48][49][50] etc. Owing to the excellent capability of selfhealing polymers in extending the service life of materials, the relevant research has been flourishing in recent years.…”