Benoit Briou

Bruno Améduri

Bernard Boutevin

Trends in Diels Alder in Polymer Chemistry

Diels Alder (DA) reaction is regarded as a quite 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 that research activity, that is still undergoing a huge interest for both academic and industrial research. DA reaction is a simple and scalable toolbox. Though it is well-established that the 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 petrosourced (cyclopentadiene, anthracene) or bio-sourced (muconic and sorbic acids, myrcene and farnesene derivatives, eugenol, cardanol). That Perspective 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. Third, petro-sourced and bio-sourced (or products becoming biosourced) involved in DA reactions are also listed and criticized. Finally, the authors' opinion is given on 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 DA reaction and their applications. 5. Conclusion .....

Preamble

Although the Diels Alder reaction has been known for many decades, the literature has become more and more abundant from the 90ies. Deeper studies based on weak bonds describe original strategies to synthesize new molecules or macromolecules. One of the unexpected examples deals with reversible deactivation radical polymerizations (RDRP) Durmaz, Colakoglu, Tunca and Hizal [START_REF] Durmaz | Preparation of block copolymers via Diels Alder reaction of maleimide-and anthracene-end functionalized polymers[END_REF], [START_REF] Gacal | Anthracene-Maleimide-Based Diels-Alder "Click Chemistry" as a Novel Route to Graft Copolymers[END_REF] which offer the design of macromolecules with controlled architectures (as block, graft and star copolymers).

Such reactions arise from various challenges developed in both academia and industry with either environmental or economic concerns. Specific topics involving Health (case of drug delivery), packaging (case of recycling), coatings (case of self-repairing) and electronic packaging where cracking occurs due to differences in thermal expansion index need a better control of the chemical reactions, especially on their reversibility.

However, the domains of applications require various reaction temperatures and eventually these of reversed reactions. For example, in Health, the temperatures can range from room temperature to ca. 50 °C while for paints and varnishes, they can reach up to 120 °C and, for industrial post-crosslinking (oven painting, coatings, composites and 3D materials) [3], they may be raised up to 200 °C.

In the last few years, Health (2.4.10 DA in health) has more attracted the researchers' interest rather than conventional paints, coatings, hybrids [START_REF] Imai | Thermally Reversible IPN Organic-Inorganic Polymer Hybrids Utilizing the Diels-Alder Reaction[END_REF], and adhesives. In addition, and more recently, the topic related to self-healing has been even more attractive in the scientific community and many strategies of synthesis have been reported (2.4.5 DA for self-healing.

As summarized in this preamble, Diels Alder (DA), is one of the most used methodology, well-developed and scaled up in industry thanks to the thermal stimuli which can be suitably applied in many applications.

Mainly, has been the pioneer in that area and has comprehensively reviewed the maleimide-furan couple in 2013.

Due to our specific interest in many topics involving DA reaction: self-curing on demand technology (SCOD) to cure elastomers [START_REF] Froidevaux | Thermosetting material by a thermo responsive cross-linking using retroDiels-Alder and, in situ, Thia-Michael reactions[END_REF], repairing coatings [START_REF] Mazajczyk | Reactive organic product and use thereof as a cross-linking agent in self-healing coatings[END_REF][START_REF] Mazajczyk | Reactive organic product and use thereof as a cross-linking agent in self-healing coatings[END_REF] and adhesives [START_REF] Dolci | Diels-Alder thermoreversible isocyanate-free polyurethanes[END_REF] , The growing importance of the use of bio-based and low-toxic molecules, and a massive bibliographic search, we decided to update the actual trends and knowledge of DA reaction.

This present review does not aim at updating Gandini's reviews [START_REF] Gandini | The furan/maleimide Diels-Alder reaction: A versatile click-unclick tool in macromolecular synthesis[END_REF][START_REF] Gandini | Progress of Polymers from Renewable Resources: Furans, Vegetable Oils, and Polysaccharides[END_REF][START_REF] Gandini | Macromolecular materials based on the application of the Diels-Alder reaction to natural polymers and plant oils[END_REF] or the literature on that topic by reporting many articles and patents, but it plans to revisit the Diels Alder reaction globally in the organic and polymer field. This manuscript supplies an overview on the use of new molecules (such as dienes or dienophiles) involved in such a reaction, with a specific part devoted to bio-sourced molecules. Another objective aims at updating the knowledge of the reaction temperatures whatever the DA or retro DA (rDA) reactions, when reported.

However, the rDA has not been so comprehensively investigated [START_REF] Rickborn | The Retro-Diels-Alder Reaction Part I. C=C Dienophiles[END_REF]. Finally, several applications will be proposed, highlighting their advantages and limitations. Hence, a critical balance appraisal of the overall situation of such a reaction will be given, with the perspectives of new couples able to bring new insight on the temperature range in DA and retro DA reactions.

Generalities

Introduction

The Diels Alder (DA) reaction is a cycloaddition between a diene and a dienophile and has been known for a long time [START_REF] Diels | Synthesen in der hydroaromatischen Reihe[END_REF][START_REF] Diels | Synthesen in der hydroaromatischen Reihe. III. Mitteilung: Synthese von Terpenen, Camphern, hydroaromatischen und heterocyclischen Systemen[END_REF][START_REF] Diels | Synthesen in der hydro-aromatischen Reihe, II. Mitteilung: Über Cantharidin[END_REF][START_REF] Diels | Synthesen in der hydroaromatischen Reihe, IV. Mitteilung: Über die Anlagerung von Maleinsäure-anhydrid an arylierte Diene, Triene und Fulvene (Mitbearbeitet von Paul Pries)[END_REF][START_REF] Diels | Synthesen in der hydroaromatischen Reihe, V. Mitteilung: Über Δ4-Tetrahydro-o-phthalsäure (Stellungnahme zu der Mitteilung von E. H. Farmer und F. L. Warren: Eigenschaften konjugierter Doppelbindungen (VII)[END_REF] .The purpose of this reaction, which was initially used in organic synthesis, i.e. to get new cyclic molecules, has evolved over time. In addition, the Diels-Alder reaction has then been required to prepare thermostable polymers, and polyimides, using the addition of bismaleimides on furans, followed by aromatization reaction to stabilize the molecules [START_REF] Bibiao | Synthesis and properties of novel polybismaleimide oligomers[END_REF][START_REF] Crockatt | Diels-Alder reaction with furanics to obtain aromatics[END_REF]. The application of the DA reaction into polymer synthesis constitutes a recent research topic, and the number of studies on DA reaction is still growing constantly (Figure 1). More recently, these reactions have been looked not for molecules stabilization but for their ability of heat reversion with the aim at preparing smart polymers [START_REF] Liu | Self-healing polymers based on thermally reversible Diels-Alder chemistry[END_REF]. Ideally, the idea is to enable that the first reaction could be achieved at room temperature and then, could be undone by a moderate temperature heating. The temperature, here, acts as a stimulus in the construction/deconstruction process. Lastly, along the same line, many applications appear based on the concept of self-repairing, recycling materials and the control of the cross-linking. Thus, it completely makes sense to note the concept of reversibility being nowadays developed in polymer chemistry. In this topic, the DA reaction is probably the most known and studied pathway. Especially, the furan-maleimide reaction (F-M) has almost been comprehensively investigated. In the specific F-M field, though particularly well-documented, updating Gandini's reviews [START_REF] Gandini | The furan/maleimide Diels-Alder reaction: A versatile click-unclick tool in macromolecular synthesis[END_REF][START_REF] Gandini | Progress of Polymers from Renewable Resources: Furans, Vegetable Oils, and Polysaccharides[END_REF][START_REF] Gandini | Macromolecular materials based on the application of the Diels-Alder reaction to natural polymers and plant oils[END_REF] would be of interest. However, in order to use the thermal reversibility behavior of the DA reaction into the polymer industry, it is worth controlling and adapting the building block and the stimuli with regard to the need of the user.

The purposes of this present review aim at i) showing that the DA reaction is omnipresent in the field of polymer and ii) proposing new diene/dienophile couples other than furan/maleimide, thus being able to enlarge the range of heat reversion and its application in polymer industry. First, after a brief recall of DA background, the F/M couple will be presented and used as an example for the application of DA reaction in the area of Polymer Science. Many different strategies will be highlighted, developed and recent use of FM reaction for high performance polymers will be described. In a second part, to underline some possible and recent alternatives ofF/M couple, other dienes and dienophiles, which can be either petro-or bio-sourced, will be developed and criticized in order to try to complete Gandini's reviews [START_REF] Gandini | The furan/maleimide Diels-Alder reaction: A versatile click-unclick tool in macromolecular synthesis[END_REF][START_REF] Gandini | Progress of Polymers from Renewable Resources: Furans, Vegetable Oils, and Polysaccharides[END_REF][START_REF] Gandini | Macromolecular materials based on the application of the Diels-Alder reaction to natural polymers and plant oils[END_REF] and articles, and to broaden the scope to all systems of cycloaddition.

A special attention will be paid on the availability of these building blocks and examples of their uses in DA polymer chemistry will be added if they exist. Then, a discussion will open toward the future of DA reactions in self-healing, crosslinking-decrosslinking and further applications, especially regarding the process and the industry expectations. In addition, our contribution will not deal with the Huisgen "click" chemistry [START_REF] Delaittre | Cycloadditions in Modern Polymer Chemistry[END_REF], in contrast to many reviews.

Cycloaddition

The cycloadditions represent a powerful strategy in organic chemistry, and it implements many products classified according to two digits [x + y]. x and y identify both the number of π electron involved in the electronic rearrangement and the number of atoms originating the unsaturated six-membered ring. The cycloaddition of two molecules by π orbital can be [2 + 2], [2 + 4], and [4 + 4] (Figure 2). These reactions are broadly irreversible but can sometimes be reversible through thermal or photochemical treatment. By far, the most used method is the [4+2], diene-dienophile, of Diels-Alder reaction. However, the other less-known cycloadditions are more and more frequently used to complete the range of temperature or of the wavelengths applied to reverse the system, such as the case of anthracene [4+4] that occurs at 366 nm and dismantles to 254 nm [START_REF] Diels | Synthesen in der hydroaromatischen Reihe. VIII. Mitteilung: Dien-Synthesen des Anthracens. Anthracen-Formel[END_REF][START_REF] Zhang | Carbocation Lewis Acid Catalyzed Diels-Alder Reactions of Anthracene Derivatives[END_REF]. The cycloaddition reaction can be intramolecular or extramolecular. The most classic ones remain the [4+2], called Diels Alder, that has been the subject of many publications and a comprehensive book written by . However, the Gandini's update [START_REF] Gandini | The furan/maleimide Diels-Alder reaction: A versatile click-unclick tool in macromolecular synthesis[END_REF] on both the selective furan/maleimide couple and the resulting applications on polymers is currently the reference in this area. A first step briefly recalls the basic knowledges, as reported in the Fringuelli and Taticchi's book [24-30]. This reverse reaction has changed the overall thinking of the D-A reaction and leads to reflect on the different possible diene/dienophile couples for the synthesis of the polymers. The diene/dienophile couple is essential to promote the DA reaction and the nature of the couple affects the experimental condition of its success. Some of well-known dienes (Figure 3) and dienophiles (Figure 4) have been already involved in the cycloadditions of DA, welldescribed by . The lists supplied in Figure 3 and Figure 4 are quite relevant. However, they deserve to be updated with, for example, the addition of biomass products which have been increasingly popular nowadays. Indeed, recently, thanks to the gain of growing interest for bio-based building-blocks, new dienes and dienophiles, even if they existed before for other applications, are now started to be involved in the Diels-Alder reaction such as cyclopenta-1,3-diene [33] (Figure 5a) which was already used before [START_REF] Sauer | Asymmetrische induktion bei diels-alder-reaktionen[END_REF][START_REF] Yameen | A facile avenue to conductive polymer brushes via cyclopentadiene-maleimide Diels-Alder ligation[END_REF] or like anthracene [START_REF] Dumitrescu | Diels-Alder polymerization of two anthracene monomers[END_REF][START_REF] Zhang | Enantioselective Diels-Alder reaction of anthracene by chiral tritylium catalysis[END_REF] (Figure 5b). Sorbic acid,[START_REF] Bérard | Sorbic Acid as a Renewable Resource for Atom-Economic and Selective Production of p-Toluic Acid and Alkyl-p-Toluates: Intermediates to Bioterephthalic Acid and Esters[END_REF][START_REF] Malshe | Diels-Alder adduct of sorbic acid and methyl ester of soyabean oil as substitute to conventional dimer acids[END_REF] (Figure 5c Several other furan derivatives produced from biomass have also been quite used. Cho et al.

Electronic effects of the

[57] supplied a comprehensive and interesting overall scheme for the production of various functional furans from biomass (cellulose and hemicellulose) (Scheme 4). Both types of furans, monosubstituted (in 2-position) and disubstituted (in 2-5 positions) have been used.

The former ones are mainly involved in polyadditions between -bis(furan)s and telechelic bismaleimides. The latter ones enable to crosslink polymers bearing furanic groups located in the main chain. That case requires bis(maleimide)s to favour that crosslinking. In addition for further details on this matter, a complete overview on furan and its derivatives was published in 2020 [START_REF] Iroegbu | Sustainable Chemicals: A Brief Survey of the Furans[END_REF]. The term "maleimide" can concern an unsaturated imide and all the associated derivatives, commonly used as building blocks for many organic reactions. Maleimides can be mainly obtained in a two-step synthesis from the reaction of maleic anhydride with amines followed by a dehydration step (Scheme 5). Actually, in contrast to aliphatic ones, aromatic maleimides have already reached the industrial scale obtained by dehydration from acetyl anhydride and sodium acetate It is also possible to find natural maleimides with complex structures produced by fungi, as in the case of antrodia camphorata [START_REF] Stewart | A concise synthesis of maleic anhydride and maleimide natural products found in Antrodia camphorata[END_REF][START_REF] Hattori | Compounds from Antrodia Camphorata having anti-inflammatory and anti-tumor activity[END_REF] able to produce several maleimides and maleic anhydrides or for Paecilomyces farinosus yielding farinomalein [78] (Scheme 8).

Scheme 8 : Examples of natural maleimide compounds

This keen interest of this specific diene/dienophile couple is due to the fact that the pair furan/maleimide (F/M) DA reaction occurs commonly at room temperature and this couple displays a moderately low temperature for its retro-DA reaction (around 110 °C). This smooth thermal reversibility, applied in polymer synthesis, opens an attractive way for recycling, selfhealing, and internal crosslinking processes. It should be noted that the value of the temperature of the retro-DA depends on the diene/dienophile couple used, here F/M, but is also influenced by the structure and the possible substituents born by these molecules. Such a retro-DA happens at an average temperature of around 100-110 °C. However, there are many examples in the literature when the retro-DA of the F/M couple occurs from ca. 80-90 °C [5, 79, 80] to 150-160 °C [START_REF] Ursache | Studies on Diels-Alder thermoresponsive networks based on ether-urethane bismaleimide functionalized poly(vinyl alcohol)[END_REF][START_REF] Liu | Thermally Reversible Cross-Linked Polyamides with High Toughness and Self-Repairing Ability from Maleimide-and Furan-Functionalized Aromatic Polyamides[END_REF]. DA reaction is also subjected to kinetic and thermodynamic controls. Indeed, the reaction between two cyclic compounds leads to two diastereoisomers: one called "endo" dominates (as the kinetic product that is faster formed) while the other one, named "exo", is thermodynamically favourited at higher temperature. The formation and quantification of these both diastereoisomer can be evidenced by NMR spectroscopy. The endo adduct is always unblocked (unprotected) at lower temperature than the exo one [START_REF] Canadell | Stereoisomeric effects in thermo-remendable polymer networks based on Diels-Alder crosslink reactions[END_REF][START_REF] Froidevaux | Study of the Diels-Alder and retro-Diels-Alder reaction between furan derivatives and maleimide for the creation of new materials[END_REF], which is thermodynamically more stable. For example the retro-DA, monitored by DSC, could start from 60 °C for some endo adducts or 100 °C for the exo adducts [8] Therefore, it is coherent to think that these phenomena are commonly applicable at some other diene/dienophile couples and enlarge the range of DA/retro-DA temperatures achievable for a wider field of application [START_REF] Discekici | Endo and Exo Diels-Alder Adducts: Temperature-Tunable Building Blocks for Selective Chemical Functionalization[END_REF].

In the case of maleic anhydride, Eggelte et al. [86] studied the reaction involving furan onto this anhydride and its derivatives. These authors showed that the endo isomer was quite difficult to be produced because of the very low temperature of r DA .

The versatility of the use of furan and maleimide moieties in polymer synthesis is also a major advantage. Indeed, as very well-described in Gandini's review [START_REF] Gandini | The furan/maleimide Diels-Alder reaction: A versatile click-unclick tool in macromolecular synthesis[END_REF], the F/M couple takes action in different ways to the development of polymer materials by polycondensation strategies. More recently, some pertinent reviews [START_REF] Sun | Architecture-transformable polymers: Reshaping the future of stimuli-responsive polymers[END_REF][START_REF] Orellana | Self-Healing Polymer Nanocomposite Materials by Joule Effect[END_REF] have reported the wide variety of polymer architectures obtained from DA and other stimuli-responsive polymers. That pioneered strategy has inspired many authors for the synthesis of well-known polymers [START_REF] Gandini | Reversible click chemistry at the service of macromolecular materials. Part 1: Kinetics of the Diels-Alder reaction applied to furanmaleimide model compounds and linear polymerizations[END_REF][START_REF] Gandini | Reversible click chemistry at the service of macromolecular materials. Part 4: Diels-Alder non-linear polycondensations involving polyfunctional furan and maleimide monomers[END_REF][START_REF] Gandini | Reversible click chemistry at the service of macromolecular materials. 2. Thermoreversible polymers based on the Diels-Alder reaction of an A-B furan/maleimide monomer[END_REF]. This route consists in functionalizing one or more units of a polymer by a member of the F/M couple to obtain an A-A monomer. The polyaddition is then initiated using another B-B monomer (Scheme 11). For example, in order to produce polyurethanes, Gaina et al. synthesized original compounds in which the furan moieties are surrounding a urethane bond Furthermore, it is possible to directly functionalize a DA adduct to obtain the monomer of interest such as a polyol for the synthesis of polyester [START_REF] Banella | A new approach to the synthesis of monomers and polymers incorporating furan/maleimide Diels-Alder adducts[END_REF][START_REF] Banella | A Novel Approach for the Synthesis of Thermo-Responsive Co-Polyesters Incorporating Reversible Diels-Alder Adducts[END_REF] or some epoxy prepolymer

Networks

By increasing the number of furan [103, 104] and maleimide [START_REF] Froidevaux | Thermosetting material by a thermo responsive cross-linking using retroDiels-Alder and, in situ, Thia-Michael reactions[END_REF]105] functions born by the monomers, it is also possible to synthesize non-linear polymers leading to more or less densely crosslinked networks (Scheme 14).

Scheme 14: Example of a crosslinked F/M network pathway

The F/M pair can also be used in the polymer field as a crosslinking agent in order to Thanks to the researcher's imagination, functionalizing polymer chains with diene/dienophile moieties is a powerful tool for molecular design. For example, located at the end-group of a polymer, different polymer blocks can be linked with reversible DA adduct, allowing an easy path to do and undo the structure obtained. [1, 2](Figure 9). All possible uses of the F/M couple in the synthesis of polymers is not only applied to this couple. Indeed, they are all transposable to other combinations of dienes and dienophiles. By tuning the reactivity of the diene/dienophile couples toward the property of the resulting product and on their DA and retro DA temperatures, a wide variety of materials can be offered as well as applications. Therefore, it is important, before describing about the current application trends of such a reaction, not to be limited to the simple F/M couple but to suggest a non-exhaustive list of other potential diene/dienophile candidates involved in the DA reaction.

Other diene/dienophile couples

Even if F/M couple has been widely described for the DA reaction through the years, it would be a mistake to limit ourselves to only this one couple. Indeed, as stated in the introduction, many dienes and dienophiles have already been reported for a long time in the DA community. In addition, because of the nowadays renewed and growing interest for renewable building-blocks, new dienes, and dienophiles, although they were involved in other applications several decades ago, are starting to be more and more used in DA reaction. Thus, these molecules, able to perform the DA reaction, increase the possibilities drastically and may permit to consider the potential limits of F/M couple. It is therefore necessary to present these potential candidates before getting involved with the significant applicative fields of DA features.

Petro-based dienes

At the beginning of the 20 th century, the industrialization of oil led to a massive generation of molecules. These new petro-based products have influenced and improved the chemical industry. At the same time, the DA reaction was also discovered. Among all dienes, cyclodienes, cyclopentadiene and anthracene have been quite used in DA reaction and are described hereafter.

Cyclodienes and cyclopentadienes

Cyclodienes also play an active role in DA reaction. Indeed, furan and its derivatives are commonly employed as dienes. However, other cyclodienes are worthy of interest such as cyclopentadiene (CPD) which is the second most famous cyclodiene in the DA community. Other cyclodienes, such as cyclohexadiene [190,191] or cycloheptadiene [192] have also been involved in DA reactions. They have a lower reactivity than those of furan or CPD.

However, a recent study has shown an improvement in the reactivity of some cyclodienes with hetero-dienophiles, which is a hopeful perspective for further use of these dienes.

[193].

Besides the furan described above as a diene or cyclopentadiene reactive in hetero Diels Alder

[194], some hetero-cyclopentadiene can be involved in DA reactions such as thiophene or pyrrole (Figure 15). Thiophene can be used as a diene or dienophile, but it is not very reactive reported self-healing ladder-like structured polysilsesquioxane copolymers and terpolymers from a DA reaction between pyrrole and cyclohexenyl-containing silicon moieties. First, the functionalization of both diene and dienophile were achieved on the double-stranded siloxane backbone bearing n-alkyl soft groups inducing some chain mobility (Scheme 24). The DA curing time was optimized to be 90 °C. Then, these authors studied the effect of chain mobility and the chemical structure on self-healing and mechanical properties. All such self-repaired hybrid materials exhibited fast healing times (ca. 5 min) and their decrosslinked adducts displayed remarkably high retention of solubility in conventional organic solvents for good reusability. They also exhibit high thermostability and optical transparency.

Inglis and Barner-Kowollik [202] provided a critical review on recent developments in the area of ultra-rapid and mild conjugation chemistries used in polymer chemistry, as an interesting guide to help the formulation of design strategy for novel functional materials. For example, the DA conjugation between cyclopentadiene-functional polymers and those prepared by RAFT polymerization [203] can be simply achieved under ambient conditions of atmosphere and temperature within a few minutes hence yielding block copolymers (Figure 16). In general, acyclic dienes have a lower reactivity than those of cyclodienes. However, some of acyclic dienes with a specific structure, can be more reactive than the others. This is the case of an electron-rich diene so-called "Danishefsky's diene", reported in 1974 [210](Figure 19a).These reactants can be used to synthesize molecules of interest such as Xestocylamine Indeed, from the acrylate, only DA occurred while with the methacrylate, both radical polymerization and DA were concomitantly observed. In another study, the same authors

[231] showed that N- (1-anthryl) maleimide in a thermal reaction led to AD while by anionic initiation, homopolymerization only was noted. From these initial features, Schiraldi's group

[220] revisited these reactions and evaluated the retroDA and evidenced that this occurred at high temperature (250-300 °C). In 2001, Grigoras and Colotin [225] synthesised polyimines from the DA reaction between anthracene-based dienes and four telechelic dienophiles containing bismaleimide functions (Scheme 27). The polycondensation reaction was performed in N,N-dimethylacetamide at 120 °C yielding polymers, soluble in polar solvents, with molar masses ranging between 3500 and 5600 g mol -1 . From TGA analysis, these authors observed a two-step decomposition: i) the first one at 250-300 °C (r DA ) while the second one, starting from 450 °C, corresponds to the decomposition of the imine function.

Scheme 27 : Synthesis of Diels-Alder polymers from the addition of dienes containing anthracene groups and telechelic bismaleimides [225]

A further interesting survey achieved from Schiraldi's team [223] deals with the polytransesterification of bis(2-hydroxyethyl) terephthalate with 2-hydroxyethyl 2anthracenecarboxylate (in a small amount) leading to low molar mass anthracene-terminated macromonomers (Scheme 28). Their anthracene end-group functionality ranged between 1.66 and 1.85. These macromonomers underwent a rapid chain extension with di(4maleimidophenyl)methane via Diels-Alder cycloadditions resulting in consumption of the anthracene and maleimide end groups to generate polymers with Mn >2.0*10 4 g .mol -1 .

Scheme 28 : Polytransesterification of bis(2-hydroxyethyl) terephthalate with 2-hydroxyethyl 2anthracenecarboxylate yielding telechelic macromonomers bearing anthracene end-group followed by chain extension with di(4-maleimidophenyl)methane [223]

Interestingly, these authors also reported a model study from anthracene methyl carboxylate and phenyl maleimide and the reversibility was monitored by 1 H NMR spectroscopy at 260 °C. Thus, they determined a constant of reversibility but with the polymers, even at 350 °C, they could only conclude that partial degradations occurred even if reversion also happened.

Hence, for self-healing, the reversion temperature, is too high, especially for this polymer.

In an interesting article, Haddleton's team studied and compared the efficiency of AD furanmaleimide and anthracene [74] by synthesizing well-defined linear and star PMMA polymers bearing DA adducts in their macromolecular backbone. They also reported a preliminary evaluation of their ability to cleave and reform it under external thermal stimuli (Scheme 29). From the furanic derivative, 100% reversion was obtained in THF reflux for 24 hr and 50% at 60 °C for 24 hr. From the anthracene compound, this is not as easy and the authors, within the reversion at 200 °C in DMSO, used rhodamine to block the reaction. They noted a 20%decrease of the molar mass that suggested some Diels Alder reactions.

Some other articles describe that anthracene was brightly involved in the synthesis of aromatic polyamides containing anthracene units by tuning with its aptitude for retro DA (Scheme 32) The copolymer bearing methacrylate was identified as the most reactive combination. The self-healing efficiency, assessed by 3D microscopy, interestingly showed that these new fullerene-containing copolymers feature a peculiar low healing temperature (40-80 °C and in 15 min only, at that upper temperature) when compared to other DA-based self-healing polymers. Additionally, in other studies, the concept could be particularly useful self-healing studies of DA-based semiconductors achieved for low temperature. These unexpected results are quite interesting since previous authors reported that healing polymers bearing anthracene occurred at rather high temperatures (250 °C).

Bio-sourced dienes

Currently, the use of sustainable raw materials and eco-responsible processes is becoming more and more frequent and increasingly searched in the world of the polymer industry. Therefore, in this context, it is relevant to list and show a growing interest in the biosourced dienes which can be used in the DA reaction. Several candidates have already demonstrated their efficiency, such as sorbic acid and its derivatives, muconic acid, myrcene, farnesene, eugenol and cardanol (Figure 21). Vegetable oils will be not mentioned in this part even if they are currently described when related to bio-based molecule. Indeed, despite the abundance of unsaturation in the alkyl chain of common oil, the lack of conjugated double bond does not allow to use them as dienes but as dienophiles. However, it is worth noting that some conjugated vegetable oil exceptions exist such as conjugated linoleic acid.

Muconic acid

Muconic acid is a telechelic bis(carboxylic acid) with two conjugated double bonds. Muconic acid exists in the form of three isomers (Figure 22) and is of growing interest both its synthesis process and for its numerous applications. As eugenol, cardanol is also a phenol naturally available. Then, the cardanol-DEAD adduct allows a self-condensation with the free cardanol (Scheme 41). In this particular case, cardanol acts as both a diene and a dienophile. Of course, this enumeration of other dienes involved in DA reaction is not exhaustive. For example, as explained in the introduction of this section, also some vegetable oils or levopomaric acid have demonstrated a real potential in DA application. Levopomaric acid is specially used with acrylic acid and is described in section 3.3.2. Acrylic acid and derivatives Citraconic acid could also be synthesized from citric acid as itaconic acid by thermal isomerization of this previous acid or anhydride and vice versa [373, 374]. DA reactions between citraconic acid or anhydride is possible with furan, and with enough reactivity for an intra DA reaction [START_REF] Murali | Intra-molecular Diels-Alder reactions of citraconamic acids from furfurylamines and citraconic anhydride: effects of substitution in the furan ring on regioselectivity[END_REF]. However, most articles about DA reactions with citraconic derivatives (Figure 3) are devoted to citraconimide molecules. In the 90s, the crosslinking reaction of citraconimides with rubber during the reversion process was described and revealed its role as an anti-reversion agent

Discussion and outlooks

Through this review, we aimed to highlight the wide variety of reagents involved in the Diels-Alder reaction. The Furan/Maleimide pair is mostly described in the literature [5-7] and is used here as a reference for the other diene/dienophile pairs presented in the section 3. Other diene/dienophile couples (insertion). The emergence and success of increasingly specialized fields of application such as health drug delivery (p31), smart or self-repairing materials (p24) and 3D printing (p29), and an increasingly growing environment is a fertile ground for DA's reaction and its versatility. These different activities bring with them their share of constraints that the F/M couple, alone, cannot fully meet.

The Self-Healing aspect or the recyclability of materials enabled by the reversibility of the diene/dienophile system via the retro-DA maintains this reaction as a hot topic and a relevant solution to the locks of tomorrow. However, special attention should be paid to the origin of dienes and dienophiles. They must be bio-sourced, as less toxic as possible and the synthetic route in which they are used must be sustainable [383]. As described in the section 3. Other diene/dienophile couples, today, many molecules are studied through the reaction of DA. This is the case of sorbic acid, muconic acid, myrcene, farnesene, eugenol and cardanol as diene and maleic acid, itaconic and citraconic as dienophile to name just some of them. Of course, this is without counting on vegetable oils and other natural raw materials which will, tomorrow, be a source of new compounds of interest for DA reaction.

Despite this increased interest in bio-based compounds, a great deal of information is still lacking on the reactivity and DA and retro-DA temperatures of these compounds. Indeed, even with the couple F/M, certain article omits to specify the temperatures of retro-DA. As discussed in the section 2.4.1 Generalities about furan and maleimide, the temperature of DA and retro-DA depends both on the compounds, on the functionalization of these compounds, on their associations between them and on the kinetic or thermodynamic forms of DA adducts obtained.

The subject is complex and sorely lacking in data. In addition, various studies use DA strategy to allow the coupling without claiming on possible reversible reaction which causes a hole in the data because the reaction of retro-DA does not interest these authors and therefore does not appear in these articles. Especially since these DA and retro-DA temperatures are particularly important to know depending on the intended application.

For example, all studies achieved on F-M couple that displays a reparation temperature close to 110 °C while the exo structure is mainly obtained. However, that temperature leads to controversies for desired applications since it is too high for biomedicine and too low for materials. Thus, revisiting other couples mentioned above should open news trends and opportunities.

In the case of anthracene, a molecule well-known since a lot of time, the results obtained are not always in agreement between different papers, and in various cases, led to controversies because of different values of retro-DA temperature (50 to 200 °C). As mentioned above, it is requested to adapt the temperatures of DA and retro-DA to the desired application, and this is not the only F-M couple which able to achieve this. Hence, deeper research on various dienesdienophiles couples to establish a real temperature scale of retro-DA (when available) between room temperature and ca. 200 °C will be pertinent and useful.

Besides the temperature values of DA and retro-DA, one of the added values of DA is the fact that it is much more than a simple reaction. DA, thanks to its reversibility, is revealed as a real toolbox for the organic and polymeric field.

In the last few years, "reparation" of materials and recycling have been of growing interest. In addition, "self-healing" is a quite fashionable trends especially for coatings, car industries, composites, adhesives, and biomedical applications.

In case of reparation, huge efforts have been made to find out solutions to limitations, by varying the structure of the polymers as well as the DA groups either well-dispersed in the matrix or within the polymer backbone or for both. The chain length and the nature of reactants between DA groups have also been studied to optimize the properties of the resulting materials. Sometimes, other concepts have also been included such as shape memory, intramolecular bonds, etc.. All these strategies have been used to obtain a material that recovers its initial properties after damage. Furthermore, many authors have investigated successive degradations-reparations to get an everlasting healed material. The resulting materials were able to be reprocessed multiple times still retaining similar properties (Tg, thermal stability, mechanical responses) as those of the pristine material. For this, a great work has been accomplished on the technical part by using several methods to tune the properties and to improve the quality of the materials after several reparation-degradation cycles. These contributions have led to huge progress whereas none of these systems is adaptable to all materials and many approaches and studies required to be investigated.

By leaving space for the creativity of researchers, this ability to make and break on demand has enabled, in addition to its use in self-healing and shape memory materials, the development of certain technologies such as DART (section 2. Hence, aspects of reparation and recycling, as well as DART or SCOD to some extent, are real targets for applications from DA reactions in materials. In these fields, many articles for furan/maleimide couple are encompassing the whole area of polymers including elastomers, thermoplastics, and thermosets, that it is not possible to be exhaustive in citing all of them.

We can therefore imagine in the near future that with a more in-depth study of biobased dienes and dienophiles, a scale of known DA and retro-DA temperatures and the growth of applications linked to reversibility behaviours, the interest of researchers and industrialists for DA's reaction will only increases and be the source of new innovative technologies of tomorrow.

Conclusion

Though discovered in 1928, DA reaction has been studied for several decades by many of academic and industrial researchers and is still of growing interest. This review summarizes non-exhaustive strategies in DA ranging from basic and fundamental methodologies to industrial applications. Beside conventional and well-known Furan/Maleimide couple, a wide variety of dienes and dienophiles reactants have been studied whatever they are petro-sourced or bio-sourced. A huge resurgence of articles for F-M involve main variety as elastomers, thermoplastics and thermosets. For the diene family, first petro-sourced were cyclopentadiene and furan but also not so well studied as pyrrole, thiophene or oxidized thiophene, as well as acyclic dienes and anthracene, whereas emerging bio-sourced dienes have recently been involved in DA reactions: sorbic and muconic acids and derivatives, myrcene, eugenol and cardanol to cite a few. Regarding the dienophile series, maleimides, maleic anhydride and derivatives, acrylic acid and derivatives, and vinyl ketones as petro-sourced have led to many surveys while itaconic and citraconic acids and their derivatives, as well as levopimaric acid as bio-sourced reactants, have also been approached a few times. This list of possible diene and dienophile reactants opens up and enlarges the possibilities about DA reaction.

Furthermore, many efforts must be done on bio-based derivatives and sustainable process due to the actual environmental crisis we are facing.

However, from a fundamental aspect, many articles do not report the presence of both exo and endo isomers which are usually produced. In addition, for most new products obtained, temperatures of DA and retroDA (r DA ) reactions are not well-known or even controversies have been reported.

By tuning the reactivities of the diene/dienophile couples toward the properties of the searched product and on the DA and retro DA temperatures, a wide variety of possible uses can be offered as well as applications. However, for most of these new products, temperatures of DA and r Da are not known or not yet reported.

In addition, and more and more noted in the literature, that DA reaction has also played a crucial role to offer self-healing materials via crosslinking/decrosslinking strategies. To use the thermal reversibility behavior of such DA reaction in the polymer industry, it is essential to control and adapt both the building blocks and the stimuli towards the needs of the applications. For example, biomedical applications require low temperature (ca. 40 °C) for r DA , while many efforts still deserve to be made to find out suitable higher temperatures for high performance materials involved in specific applications (car and aerospace industries).

Furthermore, these reactions also enable to design well-architectured polymers (block and graft copolymers, including amphiphilic block copolymers, dendrimers and networks as well as shape memory polymers). Moreover, the smart choice of the diene or dienophile allowed researchers to tune the searched applications (e.g., incorporating PDMS for low Tg elastomers or non-isocyanate polyurethanes for green coatings and adhesives). Finally, SCOD concept as a possibility to obtain one K component formulation appears as a relevant strategy.

It is expected that the design of DA reaction has the potential to be a promising growing pathway to generate new materials with unique properties and will certainly play a significant role in the future of polymer science in the coming years, hence attracting the interest of academic and industrial scientists and engineers, as well as end users.

Figure 1 :

 1 Figure 1: number of publications on Diels Alder per year, taking all subjects into account from 1930 up to now. Source from SCI Finder.

Figure 2 :

 2 Figure 2 : Example of cycloaddition reactions [2+2] and [4+2]

Scheme 1 :Scheme 2 :

 12 Scheme 1 : Normal electron-demand Diels-Alder reaction between a diene and a dienophile Another case exists in which the electron-withdrawing group of the diene enhanced the D-A reaction with an electron-donating dienophile [32] (Scheme 2). This reaction is called "inverse electron-demand DA reaction".

Figure 3 :Figure 4 :

 34 Figure 3 : List of well-known representative diene structures involved in DA reactions

  ), myrcene [40, 41] (Figure 5d) its homologue farnesene [42] (Figure 5e) or 3-hydroxy-2-pyrone [43, 44]( Figure 5f), just to cite some of them, have gained more interest in the DA community.

Figure 5 :

 5 Figure 5 : Examples of diene structures nowadays used

Figure 6 :Scheme 3 :

 63 Figure 6 : Example of structures of dienophiles used nowadays

Scheme 4 :

 4 Scheme 4: Synthetic pathways of furanic compounds prepared from biomass-based polymeric carbohydrates[START_REF] Cho | Synthesis of carbohydrate biomass-based furanic compounds bearing epoxide end group(s) and evaluation of their feasibility as adhesives[END_REF] (reproduced from ref[START_REF] Cho | Synthesis of carbohydrate biomass-based furanic compounds bearing epoxide end group(s) and evaluation of their feasibility as adhesives[END_REF] with permission from ACS, copyright 2013)

6 :

 6 Scheme 5 : Synthetic way for the preparation of maleimides from maleic anhydride[START_REF] Alizadeh | A new method for the synthesis of functionalized maleimides[END_REF][START_REF] Barikani | Preparation and properties of polyimides and polyamide-imides from diisocyanates[END_REF]

Figure 7 :

 7 Figure 7 : Various structures of telechelic and grafted maleimide structures based on PPG and PEG [8]

2. 4 . 2 Scheme 9 :Scheme 10 :

 42910 Scheme 9: Polyaddition of telechelic bis(furan) onto bis(maleimide) It took 25 years before DA polyaddition involved the F/M pair. In his article, Tesoro and Sastri [90] functionalizes a siloxane with two furan functions and then initiates polyaddition using a bis(maleimide) (Scheme 10).

[ 80 ]

 80 while Shibata succeeded in the production of high performance hybrid polyesters [94]. Plenty of different A-A and B-B monomers such as poly-L-lactide [95], non-isocyanate hydroxyurethane [96] or epoxy [97] can be designed from this way and some of them do not necessary involve the functionalization of an existent monomer [68]. Then, it is possible to undo and redo the desired hybrid polymer, facilitating its recycling or increasing its lifespan thanks to a self-healing behavior.

Scheme 11 :Scheme 12 :

 1112 Scheme 11 : DA polyaddition of bismaleimides containing urethane moities and bisfuranAnother technique consists in developing polyfunctional polymers using the DA reaction between a first moieties of monomer comprising a furan and another one containing a maleimide group (Scheme 12). Once the monomer has been synthesized by DA reaction, polymerization can then take place like is the case of telechelic bis(cyclocarbonate)s and bis(amine)s for non-isocyanate polyurethane (NIPU) synthesis[START_REF] Dolci | Remendable thermosetting polymers for isocyanate-free adhesives: a preliminary study[END_REF].

[ 100 ].

 100 Scheme 13 : Synthesis and DA polyaddition of an AB furan/maleimide monomer [102].

  crosslink polymers. The thermally reversible faculty of the DA reaction associated with crosslinking is widely studied and exploited in industry. As a model, Chen et al. obtained crosslinked hard and transparent monoliths from tetrafurane and tris(2-maleimidoethyl) amine [106] , monitoring the DA reaction by UV and solid state NMR spectroscopies at various temperatures. At 24 °C, a DA portion up to 60-70 % required 5 days while at 75 °C, only 3 hrs were enough to reach completion. Solid state reversibility was tested via a series of heating cycles at different temperatures and then quenching in liquid nitrogen.

Figure 8 illustrates

 8 fives cycles and the sample was noted to be irreversible below 120 °C while at 130 and 150 °C, ca. 12% and 25% of the DA adduct disconnected.

Figure 8 :Scheme 15 :

 815 Figure 8 : Thermal reversibility of polymer resulting from DA of a tetrafurane with a tris(2-maleimidoethyl) amine. (A) to (B), 130 °C for 25 min, followed by quenching in liquid nitrogen. (B) to (C), (D) to (E), (F) to (G), and (H) to (I), 80 °C for 1 hour. (C) ) to (D), (E) to (F), (G) to (H), and (I) to (J), 150 °C for 15 min, followed by quenching in liquid nitrogen. (reproduced from [106] with permission from Sciencemag, copyright 2002).

Figure 9 :Scheme 17 :Scheme 19 :

 91719 Figure 9 : Reversible diblock and grafted polymer strategies with anthracene in presence of a dienophile

Figure 10 :

 10 Figure 10 : Wide variety of strategies to prepare Self-Healing Materials

Figure 11 :

 11 Figure 11 : Fold-deploy experiment results for the printed samples with polyurethane acrylate (PUDA)-40: (a) recovery angle of the printed sample with time in a water bath at different (b) the recovery process of the sample in an 80 °C water bath; (c, d, e) visual demonstration of shape recovery processes of the printed petal, lamp-chimney and gripper in a water bath or with a dryer. [145] (reproduced from [145] with permission from Elsevier, copyright 20202).

Figure 12 :

 12 Figure 12 : Stress-strain-temperature diagram from SMC test of the printed samples with the photopolymer resin PUDA-40 [145] (reproduced from [145] with permission from Elsevier, copyright 2020).

Figure 13 :

 13 Figure 13 : Schematic representation of a thermoreversible network based on polydimethylsiloxane [154].

Figure 14 :

 14 Figure 14 : aromatic telechelic bis(maleimide) poly(ether) [161]

2. 4 .

 4 10 DA in health As demonstrated, DA reaction plays a massive role in the future of synthetic organic and polymer chemistry. Accordingly, its importance for pharmaceutical or biomedical applications is steadily increasing. DA is present in the synthesis of biomolecules[162], for drug delivery [163, 164], fluorescent sensor [165] and vaccines [166]. it is easy to quickly find excellent recent reviews describing the use of the DA tool in the field of drug delivery [167] and more generally in chemical biology [168, 169] confirming the interest of this reaction in this specific field.

Scheme 22 :

 22 Scheme 22 : Dimerization of cyclopentadiene [173] Since the beginning of the 20 th century, many DA reactions involving CPD have been reported with a lot of dienophiles [175], such as quinones [176], unsaturated trihalosilanes [177], acrylate derivatives [178], vinyl ketones [179] or fullerenes [180]. DA reaction with CPD is well-documented and many articles focus on the ab initio calculations with plenty of dienophiles and solvents for a better understanding of the reactivity of CPD and to anticipate some possible reactions [181-183]. Other studies have been devoted to the elaboration of original catalysts [184, 185] or on the impact of the possible function borne by CPD on the reactivity of the resulting diene [186]. Very recently, reactions of CPD with hexadienone [187] and benzoquinone have been reported [188, 189] (Scheme 23), enhancing the major role of CPD in the DA field.

[ 195 ].Figure 15 :

 19515 Figure 15 : Structure of cyclopentadiene and hetero-cyclopentadiene Less studies have been developed utilizing pyrrole as the diene. Hwang's group [201]

Scheme 24 :

 24 Scheme 24: DA and rDA reactions involving pyrrole and cyclohexenyl-containing polysilsesquioxane copolymers [201].

Figure 16 :

 16 Figure 16 : Ultra-rapid RAFT-HDA click chemistry [202] (reproduced from [202] with permission from Wiley, copyright 2010)

Figure 17 :

 17 Figure 17 : Inverse electron-demand Diels-Alder reaction between tetrazines and strained alkenes

Figure 18 :

 18 Figure 18 : Model of dienes structure from butadiene. In addition, many articles have reported the use of acyclic dienes in hetero-DA [207] such as Aza [208], or Oxo. [209] (Scheme 25)

[ 211 ]

 211 or involved in hetero-DA as oxo-DA [212, 213](Figure 19b) or aza-DA [214

Figure 19

 19 Figure 19 : a) Structure of a Danishefsky's diene (where TMS stands for trimethylsilyl; [210]. b) Example of an oxo-DA involving a Danishefsky's diene [213]. c) Example of an aza-DA involving a Danishefsky's diene. [214]

Scheme 29 :

 29 Scheme 29 : Synthesis of telechelic bis(acrylate) and original ATRP initiators from furan/maleimide and anthracene/maleimide adducts[START_REF] Syrett | Self-healing polymers prepared via living radical polymerisation[END_REF]

[ 232 ]Scheme 30 :Figure 20 :

 2323020 Scheme 30 : Formation of a foldamer from anthracene adducts [233]

Scheme 32 :

 32 Scheme 32 : Schematic representation of the DA reaction of poly(methacrylate) copolymers bearing anthracene functions with C60-fullerene resulting in CP1F and CP2Fmaterials (only one possible fullerene anthracene adduct is depicted, crosslinks are also possible) [240] (reproduced from [240] with permission from Wiley, copyright 2018)

Figure 21 :Scheme 33 :

 2133 Figure 21 : Structures of different bio-based dienes used for the DA reactions in polymer fields

Figure 22 :

 22 Figure 22 : Structures of the three isomers of muconic acid (MA) Muconic acid can be obtained from a transformation step of catechol using biotechnology. The sourcing can arise from aromatic compounds such as toluene, benzoates, [257-260] or from glucose [261].

Figure 23 :Scheme 34 :Scheme 37 :

 233437 Figure 23 : Structures of lactones and -caprolactam obtained from muconic acidThis acid is also well-known in the field of polymers as a bio-sourced precursor agent ofNylon 6,6 [258]. It can be used as a monomer in the synthesis of polyester by polycondensation with succinic acid [266, 267]( Scheme 34) or for the production of (co)polymers under radical polymerization. [268]

Scheme 41 :

 41 Scheme 41 : An example of a Diels-Alder self-condensation reaction from cardanol-DEAD adduct [321].

3. 3

 3 Scheme 42 : Chemical structures of maleic anhydride and its two diacid isomers Many examples of DA reactions involving maleic anhydride and its derivatives as dienophiles have been described with piperylene [323], furan [324], some vegetable oils [325, 326], cyclopentadiene, acrolein, oxazole [327], and numerous other dienes [328]. Nowadays, finding a sustainable and eco-friendly production has been more and more attractive. Maleic anhydride and its derivatives, as important raw materials in the polymer field, do not escape the rule [329]. With the aim to substitute hazardous and petro-based products from the synthesis of these dienophiles, several studies have been implemented on maleic anhydride and derivatives. An interesting review summarizes the current work on the bio-based synthesis of maleic acid and anhydride from furfural, furan and 5-hydroxymethylfurfural (HMF) [330]. (Scheme 43). Fumaric acid is already present in many plants; for example Fumaria officinalis, as the plant from which it was first isolated, while it could also be produced by sugar fermentation from fungi [331].

Figure 24 :

 24 Figure 24 : Structures of (meth)acrylic acid and its derivatives The presence of the double bond on these molecules allows the opportunity to react with some dienes via the DA reaction. Indeed, acrylic derivatives can react with cyclopentadiene, hexadiene [337, 338] or with conjugated fatty acid [339]. Another example deals with the synthesis of benzoic acid from the DA reaction between acrylic acid and furan [340]. More recently, several studies have described adduct (Figure 25) from a DA reaction between acrylic acid and levopimaric acid (LPAA) [341] for the synthesis of polyketones [342], polyesters [343], a bio-based pressure-sensitive adhesives [344] and as a crosslinking agent for poly(epoxide) [345], showing the renewed interest of acrylic acid and its derivatives in the DA field.

Figure 25 : 44 .Scheme 44 :

 254444 Figure 25 : Structure of Diels-Alder adduct of levopimaric acid with acrylic acid (LPAA) Moreover, these monomers could be synthesized from renewable raw materials as lactic acid [346], glycerol via the 3-hydroxypropionic acid (3-HP) [347], via dehydration into acrolein [348], or from direct fermentation of sugar [349], strengthening its position in the industry during the current conjectures. The different pathways to obtain acrylic acid are detailed in Scheme 44.

Figure 26 :Scheme 45 :

 2645 Figure 26 : Structure of vinyl ketones

Figure 27 :

 27 Figure 27 : Structures of itaconic acid and citraconic acid, and their derivatives Some articles describe the dienophile behaviour of itaconimides present in the formulation of PVC plasticizers [365, 366]. Thanks to these compounds, a DA reaction occurs during the degradation of the plasticized PVC and the resulting product stabilises the polymer from a further degradation. Since this finding, the DA reaction between itaconic acid and cyclopentadiene [367] has been reported for the synthesis of polyols for polyesters [368]. Other itaconic derivatives in DA reactions have been suggested as itaconimides with benzofuran for the synthesis of benzoisoquinoline [369], or with oxazolones [370], itaconic anhydride with furfuryl alcohol [48], or, more recently, with isopropene [371] or furan [372] for bio-thermoset synthesis (Scheme 46).

4 . 8

 48 DA reversible thermoset (DART) for 3D) or SCOD (section 2.4.9. Sealant cured on demand), in particular, one of interest to use DA reactions with SCOD concept deals on the possibility in obtaining one K component formulation. This permits to protect a reactive starting material with another reactive component in one composition. It also allows the development of a whole bunch of new applications described in the section (section 2.4.4 Miscellaneous and different uses of furan-maleimide couple and DA reaction) as Sequence-controlled RAFT quaternary polymerization [129].

Figure 28 :

 28 Figure 28 : Graphic overview of the DA reaction

  Cardanol is obtained from cashew nutshell liquid (CNSL), directly extracted from the shell of the cashew nut, Anacardium or reactive amides [320] with anhydride maleic. Recently,Biswas et al. [321] have shown that cardanol can also have a dienophile role in DA reaction. Indeed, after a first enereaction with diethyl azodicarboxylate (DEAD), the double bonds of cardanol are conjugated.

	occidental growing in China, India, Viet-Nam, Nigeria and Brazil. [304]. Cardanol is
	composed of a mixture of at least four constituents diff ering in the unsaturated side chain,
	namely saturated (5-8%), monoene (48-49%), diene (l6-17%) and triene (29-30%) as
	illustrated in Figure 21. Cardanol and its derivatives can be found in many area of materials
	[305] such as additives (antioxidants [306], surfactants [307, 308], plasticizers [309, 310]) or
	as building blocks for polymers [311], epoxy curing agents [312], coatings [313], composites
	[314] or phenolic thermosets [315, 316]. Thanks to the same mechanism involving eugenol,
	some DA reactions on cardanol as a diene have been reported for the synthesis of novolac
	thermosets with maleimides [317], and the synthesis of coating binders [318], epoxy resins
	[319],

Scheme 47 : Synthetic way to obtain polyfunctional hydrophthalimide monomers from Diels-Alder adduct of citraconimide [379] Of

  [375] and comparative studies about such a crosslinking with squalene as a model system were investigated byDatta [365, 376]. The synthesis of azasteroides was

	reported [377], involving maleimides and citraconimides on dihydronapht	alene while
	Gaina described the preparation and polymerization via DA reaction of monomers containing
	citraconimide or maleimide groups with allyphenyl group [298]. Wang and Welker [378]
	achieved the DA reaction between boron diene and some dienophiles, including
	citraconimides. Recently, a patent has claimed the syntheses of polyfunctional
	hydrophthalimide monomers from DA citraconimide adducts for polyester resins [379],
	underlining the real interest for DA reaction with citraconimides (Scheme 47).

course, this enumeration of other dienophiles involved in DA reaction is not exhaustive. Other studies report quinones [247, 277], cycloalkenones [380] and triazolines [249] as potential dienophiles. Some vegetable oils and saturated fatty acids can also be used as dienophiles [255, 381] or as diene [382]. This abundance of possible dienes and dienophiles and the current growing interest for DA reaction in the polymer area permits to open up a wide range of possibilities for further applications.

To succeed the DA reaction between allylphenol and dienophile, a first ene-reaction between both these reactants occurs allowing the formation of a diene moiety on the molecule (Scheme 39).