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

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.

Diels–Alder Cycloadditions of Bio-Derived Furans with Maleimides as a Sustainable «Click» Approach towards Molecular, Macromolecular and Hybrid Systems

This mini-review highlights the recent research trends in designing organic or organic-inorganic hybrid molecular, biomolecular and macromolecular systems employing intermolecular Diels–Alder cycloadditions of biobased, furan-containing substrates and maleimide dienophiles. The furan/maleimide Diels–Alder reaction is a well-known process that may proceed with high efficiency under non-catalytic and solvent-free conditions. Due to the simplicity, 100% atom economy and biobased nature of many furanic substrates, this type of [4+2]-cycloaddition may be recognized as a sustainable “click” approach with high potential for application in many fields, such as fine organic synthesis, bioorganic chemistry, material sciences and smart polymers development.

Trends in the Diels-Alder reaction in polymer chemistry

The Diels-Alder (DA) reaction is regarded as quite a useful strategy in organic and macromolecular syntheses. The reversibility of this reaction and the advent of self-repair technology, as well as other applications in controlled macromolecular architectures and crosslinking, have strongly boosted the research activity, which is still attracting a huge interest in both academic and industrial research. The DA reaction is a simple and scalable toolbox. Though it is well-established that furan/maleimide is the most studied diene/dienophile couple, this perspective article reports strategies using other reversible systems with deeper features on other types of diene/dienophile pairs being either petro-sourced (cyclopentadiene, anthracene) or bio-sourced (muconic and sorbic acids, myrcene and farnesene derivatives, eugenol, cardanol). This review is composed of four sections. The first one briefly recalls the background on the DA reactions involving cyclodimerizations, dienes, and dienophiles, parameters affecting the reaction, while the second part deals with the furan/maleimide reaction. The third one deals with petro-sourced and bio-sourced (or products becoming bio-sourced) reactants involved in DA reactions are also listed and discussed. Finally, the authors' opinion is given on the potential future of the crosslinking-decrosslinking reaction, especially regarding the process (e.g., key temperatures of decrosslinking) or possibly monocomponents. It presents both fundamental and applied research on the DA reaction and its applications.

Structural and solvent control over activation parameters for a pair of retro Diels-Alder reactions

We report the temperature dependent NMR of two Diels-Alder adducts of furan: one formed with maleic anhydride and the other with N-methylmaleimide. These adducts are the products of so-called 'click' reactions, widely valued for providing simple, reliable, and robust reactivity. Under our experimental conditions, these adducts undergo a retro Diels-Alder reaction and we use our temperature dependent NMR to determine the rates of these reactions at multiple temperatures-ultimately providing estimates of the activation parameters for the reversion. We repeat these measurements in three solvents. We find that, in all solvents, the barrier to reversion is larger for the adduct formed with N-methylmaleimide. The barrier to reversion for this adduct is relatively insensitive to changes in solvent while the adduct formed with maleic anhydride responds more strongly to changes in solvent polarity. The differences in reaction barrier and solvent dependence arises because the adduct formed with N-methylmalemide is more stable-leading to a larger barrier to reversion-while the adduct formed with maleic anhydride experiences a larger change in dipole during the reaction-leading to a larger solvent dependence.

Remarkable Reactivity of Boron-Substituted Furans in the Diels-Alder Reactions with Maleic Anhydride

The reactivity of boron-substituted furans as dienes in the Diels-Alder reaction with maleic anhydride has been investigated. Gratifyingly, the furans with boryl substituents at C-3 gave the exo cycloadduct exclusively with excellent yields. In particular, the potassium trifluoroborate exhibited outstanding reactivity at room temperature. Theoretical calculations suggested that the trifluoroborate group is highly activating and also that the thermodynamics is the main factor that determines whether the products can be obtained efficiently or not.

Control of Gelation and Properties of Reversible Diels-Alder Networks: Design of a Self-Healing Network

Reversible Diels-Alder (DA) type networks were prepared from furan and maleimide monomers of different structure and functionality. The factors controlling the dynamic network formation and their properties were discussed. Evolution of structure during both dynamic nonequilibrium and isothermal equilibrium network formation/breaking was followed by monitoring the modulus and conversion of the monomer. The gelation, postgel growth, and properties of the thermoreversible networks from tetrafunctional furan (F4) and different bismaleimides (M2) were controlled by the structure of the maleimide monomer. The substitution of maleimides with alkyl (hexamethylene bismaleimide), aromatic (diphenyl bismaleimide), and polyether substituents affects differently the kinetics and thermodynamics of the thermoreversible DA reaction, and thereby the formation of dynamic networks. The gel-point temperature was tuned in the range T_gel = 97-122 °C in the networks of the same functionality (F4-M2) with different maleimide structure. Theory of branching processes was used to predict the structure development during formation of the dynamic networks and the reasonable agreement with the experiment was achieved. The experimentally inaccessible information on the sol fraction in the reversible network was received by applying the theory. Based on the acquired results, the proper structure of a self-healing network was designed.

Enantioselective Diels-Alder-lactamization organocascades employing a furan-based diene

α,β-Unsaturated acylammonium salts are useful dienophiles enabling highly enantioselective and stereodivergent Diels-Alder-initiated organocascades with furan-based dienes. Complex polycyclic systems can thus be obtained from readily prepared dienes, commodity acid chlorides, and a chiral isothiourea organocatalyst under mild conditions. We describe the use of furan-based dienes bearing pendant sulfonamides leading to the generation of oxa-bridged, trans-fused tricyclic γ-lactams. This process constitutes the first highly enantio- and diastereoselective, organocatalytic Diels-Alder cycloadditions with these typically problematic dienes due to their reversibility. Computational studies suggest that the high diastereoselectivity with these furan dienes may be due to a reversible Diels-Alder cycloaddition for the endo adducts. In addition, the utility of this methodology is demonstrated through a concise approach to a core structure with similarity to the natural product isatisine A and a nonpeptidyl ghrelin-receptor inverse agonist.