Yamamoto Coupling Polymerization

In particular, H-H PIBs with molecular weights in the range of 3-10 k Dalton were prepared by the Grignard coupling polymerization of 2,2,3,3-tetramethyl-l,4-dibromobutane with copper(I) tris-(triphenylphosphino)bromide as catalyst (20). This catalyst is also referred to as the Yamamoto catalyst (21). The reaction is sketched out in Figure 6.2. 

Polythiophene, as obtained from Grignard-coupling polymerization from dibromothiophene, was vacuum deposited on various substrates and studied by electron diffraction by Yamamoto el al. [51]. They report diffraction patterns obtained from a single crystalline region, showing 44 distinct reflections of hkO-type. The indexing is in accordance with the a and 6-axes values obtained by Mo el al. Curiously the crystal is oriented with its c-axis perpendicular to the substrate, and the c-axis parameter could not be determined. The crystal is reported to deteriorate under the electron beam. 

This type of polymerization is referred to as Yamamoto coupling. Polymers prepared by Yamamoto coupling exhibit a higher degree of polymerization than those prepared by the Heck reaction. ... 

The synthetic route represents a classical ladder polymer synthesis a suitably substituted, open-chain precursor polymer is cyclized to a band structure in a polymer-analogous fashion. The first step here, formation of the polymeric, open-chain precursor structure, is AA-type coupling of a 2,5-dibromo-1,4-dibenzoyl-benzene derivative, by a Yamamoto-type aryl-aryl coupling. The reagent employed for dehalogenation, the nickel(0)/l,5-cyclooctadiene complex (Ni(COD)2), was used in stoichiometric amounts with co-reagents (2,2 -bipyridine and 1,5-cyclooctadiene), in dimethylacetamide or dimethylformamide as solvent. 

An important advancement for the synthesis of well-defined PPPs was the site-controlled nickel-catalyzed coupling of Grignard reagents, derived from dihalobenzenes, to yield linear PPP as shown in Scheme 19 [63,64]. The PPPs obtained by Yamamoto chemistry range from yellow to brown infusible solids of low molecular weights typically with degrees of polymerization ranging from 10-12. 

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