Heck reaction defect structures

A drawback of the Heck-type reaction is that it is not strictly regioselective [119]. Depending on the substituents >1% of 1,1-diarylation is observed. Soluble 2,5-dialkoxy-PPVs 63 or 2-phenyl-PPV PPPV 93, without 1,1-diarylated moieties, were synthesized by Heitz et al. in a Suzuki-type cross coupling of substituted 1,4-phenylenediboronic acids and fran5-l,2-dibromoethylene, catalyzed by Pd(0) compounds [120]. However, about 3% of biaryl defect structures are observed in the coupling products (M up to 12,000), resulting from homocoupling of boronic acid functions. 

In the reaction of bromo benzene with ethylene trans-stilbene is formed as the target product. The formation of cis stilbene is not found within the detection limit of gas chromatography. The limitation of the molecular weight in the polyreaction is mainly caused by the dehalogenation of bromo benzene. By optimization of the reaction conditions (low reaction temperature, low catalyst concentration) the dehalogenation is pushed back below the detection limits of gas chromatography. The main defect structures in the polymer are caused by 1,1-disubstitution. This deHciency in the regioselectivity of Ae reaction can not be suppressed even by optimal reaction conditions. Therefore polyphenylene vinylene obtained by Ae Heck reaction has statistically a fold in Ae macromolecule after 30 - 100 monomer... 

The model reaction between bromo benzene and phenylacetylene gives information about the chain limiting reactions and defect structures in the polymer (76). The dehalogenation can be neglected in contrast to die Heck reaction. Diin and enin formation as well as trimerization are demonstrated in the model reaction. The trimerization and the formation of enin can be suppressed quantitatively. The formation of diin occurs in die range < 0,5 %. It is not clear if the diin is formed during the reaction or on working up. 

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