Gilch reaction

Originally, an ionic mechanism was postulated, which is suggestive, as a strong base used to iniriate the polymerization reaction. However, some experimental results do not support the concept of an ionic mechanism. It is rather believed that the reaction is initiated by a radical process 

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J. Wiesecke and M. Rehahn, [2,2]Paracyclophanes with defined substitution pattern — key compounds for the mechanistic understanding of the Gilch reaction to poly(p-phenylene vinylene), Angew. Chem. Int. Ed., 42 567-570, 2003. 

In the Gilch reaction, his-(halogen methyl)benzene derivates are condensed with an excess of a strong base, such as potassium-terr-butyl alcoholate to yield a PPV. A precursor polymer with pendant chlorine groups is formed, as those from the cyclophane polymerizarion, c.f. Figure 3.4, however, this precursor polymer is converted immediately into PPV, if excess of the base is present. On the other hand, the precursor polymer can he isolated when the base is not given in excess. 

In the Gilch reaction, a strong base is added to the monomer yielding polymers with high molecular weight. MEH-PPV can be obtained in the pres-... 

Figure 29.3 H NMR spectra (sections) of a Gilch reaction carried out at —50 C, using 10 as the starting material, KO Bu as the base, and THF-dg as the solvent. Spectra were recorded at -50°C (a) soon after the addition of KO Bu ...

Water-soluble, fluorescent, poly(phenylenevinylene) derivatives with oligo(ethylene glycol) side-chain repeat units have been synthesized via the Gilch reaction [22]. An overall yield greater than 40% could be achieved. 

The yield for the Gilch reaction decreases with the increase in the length of the side chains, i.e., ethylene glycol repeat units. This phenomenon is suspected to occur due to the steric effects that are inhibiting the linking of the monomeric units. 

Example 6.5 Synthesis of Poly [2-(p-Ethylhexyloxy)-5-Methoxy]-Para-Phenylene Vinylene ("MEH-PPV") Via Gilch Reaction... 

As previously mentioned the COVION group has utilized their understanding of the mechanism of defect formation in the Gilch reaction to make relatively defect-free polymers that show particularly good performance in LEDs (high efficiency and long lifetimes) [26]. 

The molecular weight of the polymers can be controlled (from ca. 50,000 to above 1,000,000) by changing the reaction temperature and time, the solvent, the concentration of the monomer, and the amount of base [39,40]. High molecular-weight polymers and a high content of trans double bonds are the reasons for the wide usage of Gilch polymerization in the synthesis of PPV homo- and copolymers. 

Satyanarayana and Elsenbaumer attempted the Gilch polymerization of monomer 191 to afford poly(bipyridinevinylene), 192, with [Ru(bpy)2]2+ complexed to each bpy in the conjugated backbone (Scheme 4.46).117 Unfortunately, the polymer obtained from this reaction was insoluble, and thus difficult to characterize. A large red shift in the optical spectrum was observed, and thermogravimetric analysis and IR spectroscopy also supported the structure assigned. The conductivity of the blue-black product was 4.5 X 10 6Scm 1. 

The substitution of the aromatic backbone ring by alkylphenyl groups is helpful to get soluble PPX types. The polymer synthesis using these monomers can proceed in a THF, dioxane, or benzene solution. The polymer is formed by a dehydrochlorination reaction using tert-butyl oxide. The synthesis is shown in Figure 2.4. The dehydrohalogenation reaction is also referred to as the Gilch route. 

Figure 29.1 Color development during a standard Gilch synthesis. The final color of the reaction mixture is achieved within less than 5 min following the addition of KO Bu. The PPV is finally precipitated in methanol.

In this chapter, the common synthesis routes leading to technologically significant PPV derivatives used in modern organic electronics have been critically reviewed. In particular, a consistent picture has been presented of the mechanism of the Gilch polymerization leading to such PPVs. The reaction cascade involved includes ... 

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