Chain polymerizations, furans

Recent work on the synthesis, structure and some properties of macromolecules bearing furan rings is discussed. Two basic sources of monomers are considered, viz. furfural for monomers apt to undergo chain polymerization and hydroxymethylfurfural for monomers suitable for step polymerization.Within the first context, free radical, catiomc and anionic systems are reviewed and the peculiarities arising from the presence of furan moieties in the monomer and/or the polymer examined in detail. As for the second context, the polymers considered are polyesters, polyethers, polyamides and polyurethanes. Finally, the chemical modification of aU these oligomers, polymers and copolymers is envisaged on the basis of the unique reactivity of the furan heterocycle. 

Furan, Furfural, Hydroxymethylfurfural, Furan polymers. Chain polymerizations. Step-growth polymerizations, Furan polyesters, Furan polyamides, Furan polyurethanes, Furan-containing conjugated oligomers, Diels-Alder reaction, Dendrimers, Reversible crosslinking... 

Whether by chemical or electro-chemical means, the expected structure has eluded chemists for some 25 years because what is in fact obtained is a material which suggests the existence of extensive conjugation by its blackish colour, but whose macromolecular architecture is much more irregular and complicated than the linear chain 34, notwithstanding some claims related to a thorough structural elucidation. It follows that what all these authors happily call polyfuran is true to its name only because it is obtained by polymerizing furan, albeit through insofar... 

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. 

In conclusion, furan and 2-alkylfurans can be polymerized only by acidic initiators or by y-radiation because the other standard methods of polyaddition fail to induce a chain-propagation reaction. 

Finally, 2-vinyl furan 2a displays an intermediate behaviour in that it polymerizes slowly (because "normal" radicals formed from addition to the vinyl group are relatively stabilized), but gives modest DPs and limiting yields due to the fact that the furan rings pendant to the polymer chains act as radical traps which retard the polymenzation and inhibit it above a certain concentration (equivalent to a given polymer yield). 

The propensity of the C5 site towards electrophilic substitution has been exploited to prepare functionalized oligomers by cationic polymerization. Thus monomers like isobutene, s ene, the vinyl ethers, etc. polymerize in the presence of simple furan derivatives such as 2-methyl furan to give essentially short chains (DP between 2 and 100 depending on the specific experimental conditions) with a terminal furan ring as a result of predominant transfer onto the C5 position of the added furan compound (20). 

Scheme 12. First, quantitative addition of to-furan functionalized PIB (A ), obtained from a simple reaction between living PIB and 2-BusSnFu, to living PIB (A) could be achieved in Hex/CH2Cl2 (40/60 v/v) at -80 °C in conjunction with TiCl4. The resulting living coupled PIB-Fu+-PIB was successfully employed for the subsequent chain ramification polymerization of MeVE. This technique has its unique ability to control A and A block length independently. Pure poly(IB-s-IB -s-MeVE) three-arm star-block copolymer was obtained upon purification of the crude product by column chromatography [87].

Polymeric addition compounds (16) of furan and ethylene, where n is a whole number from 1 to 50, have been prepared (58) by addition of >2 mole furan to 1 mole ethylene at 120 °C to 250 °C and high pressures followed by fractional extraction of the solid product formed. These polymers have interesting physical and chemical properties. They appear to be highly crystalline on analysis by X-ray. As a consequence, the polymers show good thermal stability, which increases directly with molecular weight. The presence of ether bridges makes possible conversion (Scheme 4) to other compounds that retain the double hydrocarbon chain (ladder) structure and may bear various func-... 

In the case of isoprene, low molecular weight model compounds (dimer, trimer, etc.) can be prepared in benzene to produce oligomeric analogues of the polyisoprenyllithium active in polymerization [195]. The NMR spectra of such oligomers show that cis and trans forms of the lithium bearing terminal unit occur, and that one predominates at room temperature [195, 196]. It is probably the cis form, although this is difficult to establish without doubt. Except for the one unit chain, the cis—trans ratio varies reversibly with temperature. Transfer to tetrahydro-furan-rich mixtures at low temperatures shows that isomerization occurs when the solution is warmed to —40°C, probably to the trans form. This is the stable form in such solvents at all temperatures. The NMR spectra are basically the same in both hydrocarbon and ether solvents. Only the resonance due to the proton on the y-carbon is shifted upfield in polar... 

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