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

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 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.

The Gilch Synthesis of Poly(p‐phenylene vinylenes): Mechanistic Knowledge in the Service of Advanced Materials

“…In order to keep the NMR spectra as simple as possible, we avoided aromatic substituents and focused our investigations on 5-methoxy-2-(b-ethylhexyloxy)-1,4-bis(chloromethylene)benzene 1a and 2,5-bis(bethylhexyloxy)-1,4-bis(bromomethylene)benzene 1b as the standard starting materials (Figure 2). [64,65,67] For comparison purposes, their 2,5-dihexyloxy-substituted (1c) and 2,5-bis(b-ethylhexyl)-substituted counterparts (1d) were also tested in some experiments, which bear linear and oxygen-free solubilizing side chains, respectively. The alkyloxy-substituted starting materials are easily available by the treatment of either hydroquinone 5 or hydroquinone monomethyl ether 6 with the corresponding alkyl bromide and potassium hydroxide, followed by the introduction of either two chloromethylene or two bromomethylene functional groups through twofold halomethylation (Scheme 5).…”

“…[65] The intense, broad and unstructured absorptions represent the characteristic signature of the regularly trans-configurated repeating units of a highmolecular-weight PPV: at d % 7.1 and 7.5 ppm, one observes the signals of the phenylene and vinylene protons, at d % 3.9 ppm those of the OÀCH 2 protons, and above d 2.0 ppm appear all other protons of the lateral b-ethylhexyloxy substituents.…”

“…Reprinted with permission. [65] ether, which might form by nucleophilic attack of Figure 6. Laborious column chromatography allowed fractionation of this mixture into several pure compounds.…”

Anionic PPV polymerization from the sulfinyl precursor route: Block copolymer formation from sequential addition of monomers

“…In the sulfinyl route, a strong base is generally required to eliminate a leaving group from a precursor monomer to form a polymerizable quinodimethane system (see Scheme 1 for structure). Under most reaction conditions these quinodimethanes are unstable and undergo spontaneous radical polymerization (it should be noted that in older literature very often anionic polymerization conditions were erroneously postulated; newer studies indicate that radical polymerization is much more likely to occur) [9]. To switch from radical to anionic polymerization, the base employed in the in-situ-formation of the actively propagating monomer was changed from tert-butoxide to the more sterically hindered base LHMDS (lithium hexamethyldisilazide) while the solvent needs to be replaced by an aprotic solvent such as THF [21].…”

Teaching Organic Electronics: The Synthesis of the Conjugated Polymer MEH-PPV in a Hands-on Experiment for Undergraduate Students

“…The solvent and base used seem to play an important role and depending on the materials used, the mechanism can be radical and/or ionic. In the case of THF as the solvent and potassium tert-butoxide (KO t Bu) as base, it was shown that the polymerization proceeds via a radical chain growth mechanism, as it was verified by using spectroscopic methods (UV-Vis, NMR, EPR spectroscopy) and a radical trapping agent [18,22,23]. In Scheme 1 the reaction mechanism of alkoxy substituted α,α´-dihalogenp-xylene 3 (premonomer) with KO t Bu is shown.…”