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

Schwalm, Thorsten and Wiesecke, Jens and Immel, Stefan and Rehahn, Matthias (2007):
Toward Controlled Gilch Synthesis of Poly(p-phenylene vinylenes): Anionic vs Radical Chain Propagation, a Mechanistic Reinvestigation.
In: Macromolecules, p. 8842, 40, (25), ISSN 0024-9297,
[Online-Edition: http://dx.doi.org/10.1021/ma071337p],
[Article]

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.

Item Type: Article
Erschienen: 2007
Creators: Schwalm, Thorsten and Wiesecke, Jens and Immel, Stefan and Rehahn, Matthias
Title: Toward Controlled Gilch Synthesis of Poly(p-phenylene vinylenes): Anionic vs Radical Chain Propagation, a Mechanistic Reinvestigation
Language: English
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.

Journal or Publication Title: Macromolecules
Volume: 40
Number: 25
Divisions: DFG-Collaborative Research Centres (incl. Transregio) > Collaborative Research Centres > CRC 595: Electrical fatigue > A - Synthesis > Subproject A5: Synthesis of semiconducting model polymers and their characterization before and after cyclic electrical fatigue
DFG-Collaborative Research Centres (incl. Transregio) > Collaborative Research Centres > CRC 595: Electrical fatigue > A - Synthesis
DFG-Collaborative Research Centres (incl. Transregio) > Collaborative Research Centres > CRC 595: Electrical fatigue
Zentrale Einrichtungen
DFG-Collaborative Research Centres (incl. Transregio) > Collaborative Research Centres
DFG-Collaborative Research Centres (incl. Transregio)
Date Deposited: 20 Sep 2011 12:56
Official URL: http://dx.doi.org/10.1021/ma071337p
Additional Information:

SFB 595 A5

Identification Number: doi:10.1021/ma071337p
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