Substituent effect on structure and physical properties of semicrystalline Diels–Alder network polymers

The Interplay between Kinetics and Thermodynamics in Furan Diels-Alder Chemistry for Sustainable Chemicals Production

Biomass-derived furanic platform molecules have emerged as promising building blocks for renewable chemicals and functional materials. To this aim, the Diels-Alder (DA) cycloaddition stands out as a versatile strategy to convert these renewable resources in highly atom-efficient ways. Despite nearly a century worth of examples of furan DA chemistry, clear structure-reactivity-stability relationships are still to be established. Detailed understanding of the intricate interplay between kinetics and thermodynamics in these very particular [4+2] cycloadditions is essential to push further development and truly expand the scope beyond the ubiquitous addend combinations of electron-rich furans and electron-deficient olefins. Herein, we provide pertinent examples of DA chemistry, taken from various fields, to highlight trends, establish correlations and answer open questions in the field with the aim to support future efforts in the sustainable chemicals and materials production.

Designing End-of-life Recyclable Polymers via Diels-Alder Chemistry: A Review on the Kinetics of Reversible Reactions

The purpose of this review is to critically assess the kinetic behaviour of the furan/maleimide Diels-Alder click reaction. The popularity of this reaction is evident and still continues to grow, which is likely attributed to its reversibility at temperatures above 100°C, and due to its bio-based "roots" in terms of raw materials. This chemistry has been used to form thermo-reversible crosslinks in polymer networks, and thus allows the polymer field to design strong, but also end-of-life recyclable thermosets and rubbers. In this context, the rate at which the forward reaction (Diels-Alder for crosslinking) and its reverse (retro Diels-Alder for de-crosslinking) proceed as function of temperature is of crucial importance in assessing the feasibility of the design in real-life products. Differences in kinetics based from various studies are not well understood, but are potentially caused by chemical side groups, mass transfer limitations, and on the analysis methods being employed. In this work we attempt to place all the relevant studies in perspective with respect to each other, and thereby offer a general guide on how to assess their recycling kinetics. This review sheds light on the kinetics on the furan/maleimide Diels-Alder reaction. This popular reaction opens up a path to develop end-of-life recyclable polymer networks with self-healing properties. The factors affecting reaction kinetics are discussed, and the importance of accurate reaction kinetics in the context of polymer reprocessing is highlighted. This article is protected by copyright. All rights reserved.

Reversible networks of degradable polyesters containing weak covalent bonds

The synthesis of reversible polyester networks based mainly on the Diels–Alder chemistry, alkene [2 + 2] cycloaddition or transesterification reactions and studies of their reversibility and its consequences are reviewed.

The effect of electron density in furan pendant group on thermal-reversible Diels-Alder reaction based self-healing properties of polymethacrylate derivatives

Herein, we discuss the effect of electron density in a furan pendant group on the thermally reversible Diels–Alder (DA) reaction based self-healing efficiency in polymethacrylate derivatives. First, the furan-functionalized polymethacrylates (rPFMA and dPFMA) having different electron density in the furan pendant groups were prepared through free-radical polymerization. The healing efficiency of rPFMA, which was expected to have high healing efficiency due to the high reactivity of DA reaction originating from the electron density in the furan moiety, was shown to be 95.89% in the first and 69.86% in the second healing process, respectively, where it is higher than that of dPFMA having relatively low electron density in the furan moiety. To illustrate these results, kinetic tests of the DA reaction for rPFMA64 and dPFMA64 were performed, where the reactivity of the DA reaction for rPFMA64 was much higher than that for dPFMA64. This could be explained by the electron density in the furan pendant groups which controls the reactivity of DA reaction having a major effect on the efficiency of self-healing performance in furan-functionalized polymethacrylates.

Herein, we discuss the effect of electron density in a furan pendant group on the thermally reversible Diels–Alder (DA) reaction based self-healing efficiency in polymethacrylate derivatives.

Insight into the Mechanism of Reversible Ring-Opening of 1,3-Benzoxazine with Thiols

The reversible ring-opening addition and fragmentation reaction of p-cresol-based N-phenylbenzoxazine with aliphatic and aromatic thiols was investigated in solvent-mediated and solvent-free reactions. Independently of the used thiol, N-phenylbenzoxazine and the thiols reacted to equilibrium with comparable amounts of reactants and products in aprotic solvent, whereas in protic solvent almost full conversions were reached. In contrast, thiol reactivity was a crucial factor in solvent-free reactions yielding fast and complete conversions for a more acidic thiol and balanced equilibrium concentrations in case of thiols with high pK_a values. The strong influence of thiols with low pK_a values emphasizes the relevance of the protonation step in the ring-opening reactions of 1,3-benzoxazines with thiols in absence of solvents where acidity predominates nucleophilicity. The reverse reactions, namely adduct dissociation and benzoxazine recovery, were successfully conducted at elevated temperatures and reduced pressure facilitated by the removal of the formed thiols yielding up to 95% recovered 1,3-benzoxazine. These results provide deeper understanding of the reversible ring-opening reaction mechanism of 1,3-benzoxazine with thiols.

Interplay between crystallization and the Diels–Alder reaction in biobased multiblock copolyesters possessing dynamic covalent bonds

Biobased multiblock copolyesters possessing physical cross-links via crystallization and chemical cross-links via the Diels–Alder reaction at the same time.

A self-healing, re-moldable and biocompatible crosslinked polysiloxane elastomer

The self-healable polysiloxane elastomers cross-linked with DA bonds show high healing efficiency, good mechanical properties and good biocompatibility.

Healable shape memory (thio)urethane thermosets

(Thio)urethane networks combining SM properties and self-healability under mild temperature conditions via the DA chemistry were developed. The effect of various network architectures in SM-aided scratch healing was evaluated.

Polymerization-Depolymerization System Based on Reversible Addition-Dissociation Reaction of 1,3-Benzoxazine with Thiol

The reversible nature of the addition reaction of 1,3-benzoxazine and thiol at ambient temperature was discovered by investigating the reaction with using p-cresol-derived N-phenyl benzoxazine 1 and 1-octadecanethiol 2. The reaction was performed in several deuteriated media involving CDCl₃ and CDCl₃ + CD₃OD, for monitoring their reaction by ¹H NMR spectrometry. CDCl₃ was a favorable solvent for the efficient progress of the reaction, and its combination with CD₃OD allowed further acceleration of the reaction. In both cases, the reaction proceeded until conversion of 1 reached a certain ceiling value, to suggest that the reaction was reversible. This reversible nature was concretely confirmed by finding a dissociation reaction of isolated 3 into 1 and 2 in CDCl₃. Analogously, a bisphenol A-derived bifunctional benzoxazine 4 and 1,6-hexanedithiol 5 underwent the polyaddition in CDCl₃ + CD₃OD at ambient temperature to afford the corresponding polymer 6. Successful depolymerization of 6 into small fragments was achieved by dissolving 6 in CDCl₃.