Polymerization, of furan

The radical-catalyzed polymerization of furan and maleic anhydride has been reported to yield a 1 1 furan-maleic anhydride copolymer (89,91). The stmcture of the equimolar product, as shown by nmr analyses, is that of an unsaturated alternating copolymer (18) arising through homopolymerization of the intermediate excited donor—acceptor complex (91,92). 

The spontaneous polymerization of furan adsorbed on carbon black with or without SnCl4 vapours35 has been explained by a similar cationic mechanism. Also, the polymerization of gaseous furan on liquid acidic surfaces35 has the same origin, but in these systems the polymers suffer an acid-catalyzed hydrolysis of their tetrahydrofuran rings which produces a considerable proportion of hydroxyl and carbonyl groups. 

A comparison of the cationic polymerization of 2,3-dihydrofurans with that of furan and 2-alkylfurans shows that the complications of the latters two, arising from the dienic character of the monomers, obviously vanish when the monomer is a simple cyclic vinyl ether with just one reactive site, viz. the carbon-carbon double bond. However, it also points out that ring opening in the polymerization of furans by acidic catalysts in the absence of water is unlikely, because otherwise it would also occur to some degree in the polymerization of dihydrofurans. 

Cationic polymerization of furan (53) and alkylated furans is often complicated by the formation of crosslinked, insoluble materials (77MI11102). Early reports postulated 1,2-polyaddition as a primary reaction followed by crosslinking through the resultant dihydrofuran moieties. More recent results (80MI11106) shed considerable doubt on the 1,2-addition sequence, at least beyond the trimer stage. The ultimately formed poly(furans) are believed to be composed mainly of units with structural features similar to those of tetramers (54 Scheme 14). 

Furan is a colorless, water-insoluble liquid with a boiling point of 31 °C and is described as having a pleasant odor. Polymerization of furan occurs readily at room temperature but can be stabilized by the addition of hydroquinone or phenol. Thiophene, the sulphur congener of furan, is a stable, colorless liquid with a boiling point of 84 °C and an odor similar to benzene. 

Polyfurans. Relative to polythiophenes and polypyrroles, little has been reported on polyfurans (PFu) this is likely a result of the high oxidation potential of furan (>1.7 V vs Ag/Ag+), which results in side reactions during polymerization (145). While chemical polymerization of furan has been reported (146), the majority of PFu publications focus on electrochemically prepared PFu. 

Lewis acids for traditional Friedel-Crafts procedures (BF3, AICI3, etc.) cause polymerization of furans and unsatisfactory yields in alkylation and acylation reactions. Exceptions are, for example, (i) the formation of 2,5-di(tert-butyl)furan (9) by (CH3)3CBr/Si02 and (ii) the formation of 2-trifluoroacetylfuran (10) by trifluoroacetic anhydride without catalyst ... 

Concentrated H2SO4 or HCIO4 induce polymerization of furans via cations 42. Dilute acids, hke HCIO4 in aqueous dimethyl sulfoxide (DMSO), cause hydrolysis to 1,4-dicarbonyl compounds 44, probably via nucleophilic attack of H2O at the 2-position of 42 and ring-opening of the thus obtained 2-hydroxy-2,3-dihydrofuran (43). The transformation 41 -> 44 is the reversal of the ring-closing steps in the Paal-Knorr furan synthesis (cf. p. 71). 

The first mention of polythiophenes as potential conducting polymers can be found in the literature in 1967. A. G. Davies and coauthors investigated the acid catalyzed polymerization of furan, pyrrole, and thiophene hetero-cycles. Although this polymerization reaction was known before, several new and interesting facts could be elucidated. Besides the confirmation of the cyclic structure of the polymer units (instead of ring opened), the electric conductivity was investigated. In contrast to the already known types of electronically conducting polymers (polyaniline and polypyrrole, see Chapter 1), the conductivity of all polyheterocycles in Armour s paper, isolated as trichloro- or trifluoroacetates (triflate), is ionic. In the case of the polythiophene, ion pairs formed by the reaction of polythiophene with trifluoro acetic acid dissociated in methylene chloride solution to the protonated polymer and triflate anions. The polythiophene triflate decomposed at relatively low temperatures (60°C-80°C). ... 

Rubber Modifiers. Derivatives of furan and tetrahydrofurfuryl alcohol are used in the polymerization of synthetic mbber to control stereoregularity and otherproperti.es (149,150). 

Alkylation of furan and thiophene has been effected with alkenes and catalysts such as phosphoric acid and boron trifluoride. In general, Friedel-Crafts alkylation of furans or thiophenes is not preparatively useful, partly because of polymerization by the catalyst and partly because of polyalkylation. 

Group of plastics composed of resins in which the furane ring is an integral portion of the polymer chain made from polymerization or polyconden-sation of furfural, furfural alcohol and other compounds containing furane rings also formed by reaction of furane compounds with an equal weight or less of other compounds. 

If on the other hand the polymerization of a furan derivative takes place through a substituent containing an adequate functionality, such as C=C or C=0, the furan ring should in principle conserve its structure and the polymers obtained will bear it as a side group. It has been found, however, that in some of these systems the normal propagation is accompanied by other reactions which involve participation of the ring and which therefore alter the normal structure of the macromolecule. The second section of this chapter deals with monomers, such as 2-vinylfuran and 2-furaldehyde, which exhibit this general behaviour. 

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