Gilch synthesis

A second problem is that the formation of defects that lower the emission efficiency is intrinsic in the polymerization process due to nonregular coupling of the quinodimethane intermediates, as illustrated in Scheme 6.5 for the Gilch synthesis. This has been demonstrated by NMR studies on a 13C-labelled PPV derivative, in which the ethyne and ethane moieties could be detected in 1.5-2.2% amount [38-40], By appropriate choice of substituents the levels of these defects can be minimized to obtain high-efficiency polymers [41]. 

Scheme 6.5 Formation of defects during Gilch synthesis.

PAVs with nitrile substituents on the vinylene moieties can be prepared by Knoevenagel condensation of a dialdehyde 26 with a bisnitrile 27 as illustrated for the synthesis of CN-PPV (4) in Scheme 6.7 [7]. Both monomer components are readily prepared from the bischloromethyl compound 28 otherwise used as a monomer for a Gilch synthesis. Here also, molecular masses are lower than from Gilch synthesis. 

The second approach to PPV, is the Gilch synthesis [147], which consists in the introduction of solubilizing side groups, as represented in the case of the most studied dialkoxy derivative poly[2-methoxy-5-(2 -ethylhexyloxy)-l,4-phenylene vinylene] (MEH-PPV, XII) [29,148-150] (Fig. 9.20). 

Although direct proof was not available until recently, chemical arguments, experimental indications, and the analogy to the aforementioned methods led to the assumption that the Gilch synthesis proceeds according to Scheme 29.3 p-quinodimethane-type species 11 probably are the active monomers, and... 

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.

Scheme 29.5 Chain propagation during Gilch synthesis via regular addition of monomers 11.

The results of these lifetime and efficiency investigations have implied that the hahde-mediated defects must be considered whenever different synthesis conditions or different PPVs are to be screened. Furthermore, such results continue to cast doubt on the attention appHed to TBB defects in Gilch-polymerized PPVs. It must not be assumed, a priori, that any dehydrohalogenation achieved during the Gilch synthesis would be sufficient to exclude the impact of halide defects when, for example, the correlation between the extent of the TBB defects and the device hfetime is examined [49]. 

SCHEME 4.2 Formation of defects in PAVs during Gilch synthesis. 

Yin, C., and C.-Z. Yang. 2001. Mechanism analysis of the PEG-participated Gilch synthesis for soluble poly(p-phenylene vinylene) derivatives. J Appl Polym Sci 82 263. 

In certain cases the fully conjugated polymer may not be soluble. It is then possible to use a precursor variant of the Gilch synthesis, where only one equivalent of base is used. This produces an intermediate soluble halide-substituted polymer that can be dehydrohalogenated thermally (Scheme 13) [104-107] or partially eliminated in an alcoholic solvent [108]. However, the intermediate halide-containing polymers may not be very stable, though this can depend on storage conditions [104]. 

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