Furan monomers

Within the broad spectrum of systems described, only few can be considered well understood. Unfortunately, many past investigations were superficial and more involved studies have been avoided because of the inherent complex phenomenology. This has been harmful, and misleading conclusions have been reached about the real potential of furan monomers. It is hoped that this review has shown the fallacy of this approach and will therefore stimulate further skilled work in this field. 

Some of the transformations of F into furanic monomers are shown in Scheme 1. In recent years renewed interest in these syntheses has produced much progress in terms of yields, simplicity and economy. Thus, for example, very convenient routes leading to 2-vinylfuran (9) and 2-mryl oxirane (10) have been reported. The structures of the monomers in Scheme 1 simulate those of aliphatic and aromatic counterparts which are the basis of most polyaddition reactions and which are prepared in typical petrochemical operations. The only (but major) difference stems from the presence of the furan ring in each of the structures. 

The purpose of this paper is to highlight the achievements and drawbacks related to the use of furanic monomers in recent investigations, including work in progress. 

The reactions of free radicals with furan and its derivatives can give both addition and substitution products depending on the specific system (11-13). With 2-substituted furans, the attack takes place predominantly at C5 and leads, by additon, to the corresponding furyl radicals which must be viewed as relatively stabilized interemediates because of the dienic-aromatic character of the furan heterocycle. These premises are essential to the understanding of the varied responses of furan monomers to free-radical activation. 

Within the past 10 years, a number of investigators have explored the use of 2,5-disubstituted furan monomers in polymer condensation reactions. In spite of this attention, no significant commercial application has developed thus far. The general unavailability and high price of these monomers are drawbacks as are the lower temperature and oxidative stability and enhanced color development characteristics of some of the polymers developed from them. 

The present choice of systems based on furan monomers is not exhaustive of course, but hopefully sufficient to illustrate the enormous potential of this area of polymer science, which does justice to the plea for more macromolecular materials from renewable resources. 

In the above condensation resist designs, the phenolic resin offers a reaction site as well as base solubility. Self-condensation of polymeric furan derivatives has been utilized as an alternative crosslinking mechanism for aqueous base development (Fig. 126) [375]. The copolymer resist is based on poly[4-hydroxy-styrene-co-4-(3-furyl-3-hydroxypropyl)styrene], which was prepared by radical copolymerization of the acetyl-protected furan monomer with BOCST followed by base hydrolysis. The furan methanol residue, highly reactive toward electrophiles due to a mesomeric electron release from oxygen that facilitates the attack on the ring carbons, readily yields a stable carbocation upon acid treatment. Thus, the pendant furfuryl groups serve as both the latent electrophile and the nucleophile. Model reactions indicated that the furfuryl carbocation reacts more preferentially with the furan nucleus than the phenolic functionality. 

This chapter is devoted to adhesives and resins prepared from totally furanic monomers or formulations in which furanic compounds are added. In this realm, only... 

From the above survey, it appears that the industrial use of furanic monomers such as furfuryl alcohol and furfural, i.e., chemicals based on renewable resources, as binders in foundry molds is highly successful. Similar furan-based resins can also be used as efficient adhesives in wood particle composites and thus are interesting alternatives to petroleum-based counterparts. The fact that the substitution of formaldehyde by furfural has not yet met with a reasonable industrial success probably stems from the higher cost of the furan aldehyde. The increasing pressure on the reduetion of formaldehyde emission and the renewable character of furfural should play in its favor in the near future. 

Whereas 2,5-dihydroxymethylfuran (DHMF) has been proposed frequently as a diol component for polyester synthesis, the high reactivity of the hydroxymethyl groups under acidic conditions primarily leads to resin formation, which is commercially exploited for the production of thermoset furan resins [55]. Although HMFA is more chemically stable than DHMF, it is also prone to resinification at high temperatures. In contrast, FDCA and derivatives have proven to be chemically stable under conditions relevant to polyester synthesis, making them the most versatile and industrially viable furan monomers for step growth polymers. 

Scheme 5 Di-furan monomers derived from 2-substituted furans...

HMF is a suitable precursor for the synthesis of bifunctional furan monomers as summarized in Scheme 4. All these monomers can be used to prepare polycondensates by step-growth reactions with the other corresponding bifunc-tional monomers derived either from petrochemical precursors or from renewable resources. The polycondensates obtained, such as polyesters, polyamides, and polyurethanes, etc. have been characterized [107-113]. Scheme 5 shows another approach for the synthesis of bifunctional monomers through acid-catalyzed condensation of the corresponding mono-functional furan derivatives with an aldehyde... 

This paper describes novel approaches to the exploitation of both furan monomers and a specific facet of furan reactivity in order to synthesize either conjugated oligomers incorporating the heterocycle in their backbone, or polymeric structures which can be crosslinked and returned to linear structures through the reversible chemistry of the Diels-Alder reaction. The first family of compounds showed interesting features in terms of conductivity, luminescence, mesogenic character and photoactivity. The second class of materials owes its interest to the possibility of recycling otherwise intractable polymers, e.g. tires, thanks to a simple thermal process. 

Furan derivatives are among the best examples of a source of chemicals and materials based on renewable resources because furfural is readily produced in good yields from the hemicelluloses contained in a wide variety of agricultural and forestry by-products, through a simple and economically viable process. The use of furan monomers to synthesize polymers and copolymers and of some specific features of fiiran chemistry to develop original macromolecular architectures have been major research topics for many years in our laboratory... 

In order to study elastomeric networks, simulating the type of polymers used for tires, we switched to polymers with low glass transition temperatures and oligoether bis-maleimides. A typical random copolymer structure, built from the radical copolymerization of n-hexyl acrylate and 2-furfuryl methacrylate, is shown below. These reactions were conducted in toluene at 80°C with AIBN as initiator. After 8 h, the copolymers were recovered by precipitation in 70 to 80% yields. The compositions varied fi om 2 to 30% of the furanic monomer (monomer feed and copolymer composition were always very similar, suggesting that ri and ti must have both been close to unity). The corresponding Tgs went from -70 to 30°C for molecular weights of about 20,000. Both homopolymers were also prepared as reference materials. 

The use of furfural as such, as well as its transformation into a variety of furan monomers is discussed in Chapter 6 together with the S5Uithesis and properties of the corresponding macromolecular materials. 

Two striking differences distinguish the essence of this chapter from most other chapters, namely (i) the fact that the furan compounds relevant to polymer synthesis are not found as such in nature but are instead obtained from parent renewable resources and (ii) it is possible in principle to envisage a whole new realm of polymer materials based on furan monomers and furan chemistry, covering a very wide spectrum of macromolecular structures. Concerning the first point, the massive availability of saccharidic precursors and their relatively sirt5)le conversion into furan derivatives, eliminate in fact any apparent problem of absence of such natural structures. As for the second point, its unique relevance has to do with the potential perspective of a viable alternative to the present reality based on polymer chemistry derived from fossil resources. In other words, the biomass refinery concept would be applied here to the synthesis of different furan monomers, simulating the equivalent petroleum counterpart. 

The impressive achievements of the petroleum-based synthesis of monomers, and the huge success of the ensuing polymeric materials, are ineluctable realities which combine pervasive technical performances with remarkable economy (at least for the time being). It follows that the challenges faced by the possibility of a novel branch of polymer chemistry based on furan monomers are daunting. We hope to show in this chapter that daunting does not mean impossible within a realistic framework of research efforts and time span. 

The fact that the typical furyl radicals in Scheme 6.6 are unable to propagate in the classical mode of free radical polymerization because of their resonance stabilization, has profound consequences in terms of the reactivity of furan monomers in this type of polymerization and the role of furan derivatives as possible perturbing agents in free radical chain reactions in general. These specific aspects will be illustrated below. 

These two basic furan derivatives obtained directly from cheap vegetable resources find important uses as chemical precursors in a variety of industrial and fine chemical processes which are well documented [4e, 11,13], Only those leading to furan monomers and to other compounds which can be exploited in polymer synthesis will be examined further hereafter. 

F and MF are typical precursors to furan monomers bearing a moiety which can be polymerized by chain-reaction mechanisms. Scheme 6.11 provides a non-exhaustive array of entries into such structures, which have all been synthesized, characterized and polymerized [4], A major exception to this general postulate is constituted by furfuryl alcohol (2-hydroxymethylfuran, FA), which is in fact still today the most important commercially available furan compound, obtained by the catalytic reduction of F involving more than 80 per cent of its world production. FA is widely used as a polycondensation monomer and does not therefore belong to the class of coirpounds shown in Scheme 6.11. 

HMF on the other hand, is ideally suited as a precursor to bifunctional furan monomers to be used in step growth reactions as shown in Scheme 6.12 which includes FDCA and FCDA. Again, all these compounds wctc synthesized and used to prepare the corresponding polycondensates, like polyesters, polyamides, polyurethanes, etc [4]. 

Copolymerization systems involving the combination of a furan monomer and a conventional counterpart, confirmed this general state of affairs in that only the use of polymerizable furan monomers yielded the expected copolymers. Conversely, comonomers like 2-vinyl furoate either retarded the homopolymerization of well-stabilized standard monomers like methacrylates, or actually inhibited the polymerization of poorly stabilized conventional monomers like vinyl acetate [4d, 7]. 

Other furan monomers which polymerize cationically include 2-furfuryl vinyl ether, 2-vinyl furoate (albeit through a polyalkylation mechanism giving a polyester incorporating the ring into the pol5mer backbone), F and MF as co-monomers in conjunction with substituted styrenes and vinyl ethers, as well as 2-furfurylidene methyl ketone (obtained by the base-catalyzed condensation of F with acetone) and its homologues [4d]. 

The future of FA as a cheap precursor to a variety of useful materials appears therefore to reflect a growing exploitation in traditional as well as state-of-the-art technologies. Additionally, as pointed out in various subsequent sections, FA is finding other novel utilizations as precursor to multifunctional furan monomers. 

This comprehensive study showed first of all that the use of both types of furan monomers did not entail the occurrence of any unwanted side reaction, since all the polymers displayed a perfectly regular structure, as assessed by FTIR and NMR spectroscopy. 

It is worth noting that all these latter examples of materials whose key feature calls upon a specific behaviour of furan moieties, require modest contributions from these heterocycles in terms of its quantitative presence in the macromolecules. This situation is similar to that described in Section 6.5.2, in which furan derivatives were used to modify the end groups of some polymers or to synthesize block copolymers by cationic polymerization. In both instances, therefore, it is not the dominant presence of the heterocycle that determines the specific properties of the final materials (as in the case of polymers and copolymers bearing furan monomer units), but instead the fact that a small or even minute percentage of these structures introduces an original mechanistic feature, associated with the peculiar chemistry of the heterocycle, which transforms the behaviour and properties of the final material. This crucial point will be met again in the context of the application of the DA reaction in Section 6.8. 

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