Direct Observation of α‐Chloro‐p‐quinodimethane as the Real Monomer in the Gilch Polymerization Leading to Poly(p‐phenylene vinylene)s

Abstract

It is the general consensus that in Gilch polymerizations the 1,4‐bis(chloromethylene)benzene starting material first changes into p‐quinodimethane intermediates which then act as the real monomers. However, direct observation of these intermediates has not been possible so far. This is because usually the p‐quinodimethane auto‐initiates its rapid radical polymerization instantaneously, keeping its concentration extremely low throughout the whole process. Here it is shown that, when the reaction is carried out at very low temperatures, the formation of p‐quinodimethane still proceeds but chain growth is suppressed. Hence, the concentration of the active monomer reaches a level sufficient for NMR analysis.

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

“…[67] This is in full agreement with the assumption of 3a as the real monomer of the Gilch process, but this is not yet the final proof of this statement: so far, no conditions had been found that resulted in an essentially complete conversion of 1a into 3a, but all conversions 1a ! 3a stopped at 70% conversion when carried out at À50 8C, while a slight increase in the reaction temperature resulted in the instantaneous formation of PPV 2a.…”

“…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).…”

“…Based on that hypothesis, a surprisingly simple method was found to prove the existence of 3: when a starting material such as 5-methoxy-2-(b-ethylhexyloxy)-1,4-bis(chloromethylene)-benzene 1a was converted with KO t Bu at below À50 8C directly in NMR tubes located in a cooled NMR probe head, thereby using THF-d 8 as the solvent, the recorded spectra showed very characteristic changes with time (see Figure 8). [67] Spectrum A represents the reaction mixture soon after addition of the base to the solution of starting material 1a.…”

“…Spectra were recorded: A) directly after equilibrium of the reaction temperature, B) after 39 min, and C) after 48 min at À50 8C. [67] solution of 3b at À80 8C for more than 1 h. Moreover, there was an essentially complete conversion 1b ! 3b already after less than one minute!…”

Synthesis of a Fluorescent Conjugated Polymer in the Undergraduate Organic Teaching Laboratory

“…15 This procedure used Gilch polymerization conditions (mechanism discussed in the Supporting Information), 16 and required moderate precautions to limit the reaction's exposure to air and moisture. Once the polymer was synthesized, it was utilized for two applications: (a) synthesis of fluorescent organic nanoparticles; 17 and (b) fabrication of fluorescent thin films.…”