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 R-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 E 2 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 R,ω-macro-diradicals, recombination does not cause chain termination as in conventional radical polymerizations. This is one reason for the formation of very highmolecular 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.

It is general consensus that in Gilch polymerizations the 1,4-bis(halomethylene)benzene starting material first changes into an R-halo-p-quinodimethane intermediate which then acts as the real active monomer in the subsequent chain growth process. Recently, we could verify the formation of R-chloro-p-quinodimethane directly via in-situ NMR spectroscopy at low temperatures. However, quantitative formation of this pquinodimethane was not possible there. Now, we show that even such quantitative conversion into the active monomer is possible if bromomethylene-functionalized starting materials are used instead of their chloromethylene counterparts. Moreover, it is even possible to induce chain growth leading to PPV in a very controlled way by carefully warming the obtained solution of p-quinodimethane. In this manner, the temperature can be determined where chain growth startssand hence thermal energy is sufficient for the initiating process. Finally, we could reconfirm that the chain growth is a radical polymerization here as well, initiated by diradicals formed via spontaneous dimerization of a low number of R-bromo-p-quinodimethane monomers. This proof could be provided by quantitatively analyzing the effect of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO): depending on the molar ratio of monomer and scavenger, the chain growth is either retarded or completely prevented.

A consistent picture is presented of the mechanistic details and intermediates of the Gilch polymerization leading to poly(p-phenylene vinylenes) (PPVs). In-situ generated p-quinodimethanes are shown to be the real monomers, and spontaneous formation of the initiating radicals is effected by dimerization of some of these monomers to dimer diradicals, the latter also being the reason why significant amounts of [2.2]paracyclophanes are formed as side-products. Chain propagation predominantly proceeds by radical chain growth, occasionally interrupted by polyrecombination events between the growing α,ω-macro-diradicals. Based on this knowledge, oxygen is identified as a very efficient molar-mass regulating agent, and the temporary gelation of the reaction mixtures is interpreted to be the consequence of a very high entanglement of the polymers immediately after their formation. Last but not least, it is rationalized why the usually considered constitutional defects in Gilch PPVs might not be the only and most relevant ones with respect to the efficiency and durability of the organic light emitting devices produced thereof, and why cis-configurated halide-bearing vinylene moieties should be perceived as being among the most critical candidates. These considerations result in the recommendation of straightforward measures that should lead to clearly improved PPVs.

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Oxygen is shown to act as an efficient molar‐mass regulating agent in Gilch syntheses of PPV. As a scavenger, it undergoes instantaneous recombination with the initiating diradicals as soon as they appear in the system. Regular polymer formation can only start when all oxygen has been used, proceeding predominantly as chain‐growth polymerization of the p‐quinodimethane monomers. Since all radical species involved in this Gilch process are diradicals, some polyrecombination events occur in parallel. Therefore the initially formed peroxy diradicals are also incorporated into the resulting chains. Later, they break under very mild conditions, thereby causing a systematic decrease of the final molar mass of PPV.magnified image

Gelation of the reaction mixture and insolubility of the poly(p‐phenylene vinylenes) (PPVs) when isolated at this stage, but complete redissolution of the gel and excellent solubility of the resulting PPVs after further stirring for hours or days, is a phenomenon in Gilch polymerizations that has not been explained so far. It is verified that, in agreement with the literature, specific additives prevent gelation. However, it is also shown that chemical crosslinking is certainly not the reason for gel formation. Instead, it seems to be the consequence of a very high entanglement density in the pristine PPVs, which requires time for relaxation. The mentioned additives seem to support this dis‐entanglement process.magnified image

On the basis of new insights into the reaction mechanism of the so-called Gilch route leading to poly(p-phenylene-vinylene)s (PPVs), the importance of vinyl halide defects for the performance of organic light-emitting diodes (OLEDs) is stressed in the present contribution. It is found that the current density, the luminance, and luminance efficiency are superior for PPVs that were subject to a long-term dehydrohalogenation. In particular, the device lifetime improves by a factor of 200 as long as the halide content is reduced from 0.4 to 0.05 wt %. The results imply that rather the mentioned vinyl halide defect than the often discussed tolane-bisbenzyl (TBB) defect has to be considered when investigating lifetime and performance of OLEDs. The device behavior is analyzed in view of a detailed study of the charge-carrier transport properties. We suggest that the penetration of electrons from the cathode in the PPV leads to a separation of halogen and thus to free halogen anions. The anions can move in the electric field to the contacts where they form a salt with the counterion present in the electrode material. The charge-carrier transport across the respective contact is thus impeded as a consequence of the appearance of a salt-containing interlayer. The proposed mechanism explains the observed differences in device performance and lifetime.

Cover: Poly(p-phenylene vinylenes) (PPVs) are very useful materials for optoelectronic devices, and the Gilch reaction offers powerful access to these intensely colored polymers. However, minimizing intrinsic defects and tailoring the molar masses are still difficult tasks. The cover picture underlines that oxygen acts very efficiently as a retarder and regulator in this functional polymer synthesis. Further details can be found in the article by T. Schwalm and M. Rehahn* on page 207.