Cationic polymerization, furans

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. 

Of course, these conclusions do not rule out completely the occurrence of other reactions such as those listed above, but their contribution to the overall mechanism must be very small in the production of the oligomers. The dark colour of these products was attributed to hydride transfer reactions, similar in nature to those encountered in the cationic polymerization of 2-vinyl furan [see Section III-B-l-c)]. The subsequent process which transforms these oligomers into cross-linked resins was not investigated. 

Gcwdini, A. The Behaviour of Furan Derivatives in Polymerization Reactions. Vol. 25, pp. 47-96. Gandini, A. and Cheradamc, H. Cationic Polymerization. Initiation with Alkenyl Monomers. Vol. 34/35, pp. 1-289. 

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). 

Preparation of addition polymers having the oxolene (dihydrofuran) functionality can be envisioned to occur in two possible ways (Scheme 13). Both, in fact, have been observed (77MI11102). Whereas furan (53) or its derivatives do not homopolymerize under free radical conditions, 1 1 alternating copolymers possessing the 1,4-structure are produced with maleic anhydride (50). Intermediate formation of a CT complex between monomers (50) and (53) is believed to be necessary before polymerization can occur. On the other hand, cationic polymerization is quite facile. The outcome is straightforward with benzo[f>]furan derivatives, producing 1,2-polymers. Optically active poly(benzofurans) are formed when the cationic polymerizations are conducted in the presence of a chiral anion. 

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). 

Finally, addition polymerization of suitably substituted furans allows incorporation of the furan nucleus into heterocyclic polymers (77MH1102). 2-Vinylfuran apparently exhibits free radical polymerizability comparable with that of styrene, although rates, yields and degrees of polymerization are low under all conditions except for emulsion polymerization. Cationic polymerization is quite facile and leads not only to the poly(vinylfuran) structure (59), as found in free radically produced polymers, but also to structures such as (60) and (61) in which the furan nucleus has become involved. Furfuryl acrylate and methacrylate undergo free radical polymerization in the manner characteristic of other acrylic esters. 

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