Toward Controlled Gilch Synthesis of Poly(p-phenylenevinylenes):  Synthesis and Thermally Induced Polymerization of α-Bromo-p-quinodimethanes

Abstract

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.

Gel Formation during the Gilch Synthesis of Poly(p‐phenylene vinylenes): Evidence of an Unexpected Explanation

“…This is certainly a sound argument, independent of the fact that we are not in agreement with the offered explanation of why the PPV chains might be shorter in the presence of the mentioned additives: an anionic propagation mechanism was assumed for the Gilch polymerization, and the additives were believed to act as additional carbanionic initiators after their in-situ deprotonation (option A in Scheme 1). In contrast to this, we [20][21][22][23] and others [14,24] are convinced that this polymerization is based on radical chain growth. Nevertheless, for this latter option there is also a way available to explain the decreasing PPV chain lengths with increasing quantities of monofunctional benzylhalogenides: the mentioned additives are known to act as chain-transfer agents in radical polymerizations through their mobile benzylic hydrogen atoms (option B in Scheme 1).…”

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).…”

“…Instead, all models resulted in an essentially unchanged process of spontaneous dissociation. Thus, it was not possible to identify a single stable molecular geometry of an anionic [64] intermediate, nor any corresponding transition state, for the anionic mode of the Gilch polymerization. It must be concluded that the formation of p-quinodimethanes 3 does not represent a step-wise E 1cb (or E 1 ) elimination, but follows a concerted E 2 -type mechanism for this 1,6-elimination.…”

“…Figure 10 shows a selection of 1 H NMR spectra obtained during such a warmup run. [64] It can be seen that a-bromo-p-quinodimethane 3b is stable in solution at À80 8C, but already starts polymerizing at À70 8C! This is evident from the appearance of broad absorptions especially in the aromatic region (d 6.8-7.8 ppm), which are characteristic of the PPX intermediate 4b as well as the final PPV 2b.…”

“…Since bromomethylene functionalities should offer a higher reactivity, the corresponding starting material 1b was tested analogously in the next step. [64] Already the first NMR spectra proved that the bromomethylenefunctionalized starting material 1b is converted into its p-quinodimethane counterpart 3b as well. However, the spectra additionally showed that temperatures of around À50 8C are still too high for selective conversion: soon after the characteristic absorptions of 3b, further broad absorptions appeared as well.…”