Toward Controlled Gilch Synthesis of Poly(p-phenylene vinylenes): Anionic vs Radical Chain Propagation, a Mechanistic Reinvestigation

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Abstract

It is a broad consensus that the first step in the Gilch synthesis of poly(p-phenylene vinylenes) (PPVs) is 1,6-dehydrohalogenation of the 1,4-bis(halomethylene)benzene starting materials. The mechanism of the subsequent chain growth of the resulting α-halo-p-quinodimethane monomers, however, is still a matter of discussion. We re-evaluated the arguments presented for anionic chain propagation and set them against the evidence obtained for a radical mechanism. We conclude that (i) the initial dehydrohalogenation of the starting material represents an E2 type 1,6-elimination without anionic intermediates, (ii) anionic chain propagation does not play a role in standard Gilch syntheses, but instead, (iii) the PPVs grow predominantly via radical chain polymerization. However, since the growing species are α,ω-macro-diradicals, recombination does not cause chain termination as in conventional radical polymerizations. This is one reason for the formation of very high-molecular weight PPVs. The monofunctional benzylhalogenides, sometimes assumed to act as initiators of anionic chain growth and to suppress gelation of the reaction mixtures by lowering the PPVs' molar masses, clearly do not play this role: while we could verify that these additives lower the risk of gelation, they are neither incorporated as end groups into the PPVs nor do they lower the molar masses. Instead, gelation is most probably due to physical crosslinking, induced by the very high entanglement density of the PPV chains immediately after their formation. Additives such as monofunctional benzylhalogenides seem to accelerate de-entanglement, possibly either by retarding the conversion of the still quite flexible poly(p-xylylene) (PPX) precursors into the semirigid PPVs, thereby giving the chains a better chance to de-entangle, or by preferential solvation and successful competition with segment−segment interactions. In agreement with the proposed mechanism is the reproducible observation that additives which antagonize gelation efficiently, simultaneously increase the magnitude of the only relevant side reaction of Gilch reactions, i.e., formation of [2.2]paracyclophanes.

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