Polymerization by Suzuki-Coupling in Aqueous Systems

Transition metal catalyzed coupling reactions of aromatic halides, such as Heck or Suzuki coupling, are used extensively for the synthesis of low-molecular-weight organic compounds. With bifunchonal substrates, polymers can be obtained. By contrast to all other polymerization reactions considered in this chapter, such reactions represent step-growth-type polymerizations. 

The C-C couphng of aromahc hahdes usually requires the addition of a base in stoichiometric amounts, and the most common Bronsted bases are water soluble and can be employed conveniently in biphasic aqueous/organic systems. 

Suzuki couphng, the palladium-catalyzed cross-coupling of aryl or vinyl hahdes with organoboronates, is employed widely in organic synthesis. The reaction is most often carried out in an organic solvent (benzene) in the presence of a base [228]. Because of the reasonable stabihty of boronic acids and esters towards hydrolysis, Suzuki couphng can also be carried out in aqueous media. An extensive study of the reaction has shown that similar results are obtained in aqueous and non-aqueous systems [229]. 

The use of Suzuki couphng for the synthesis of polyphenylene polymers was introduced by Rehahn, Schlueter and Wegner [Eq. (9)] [230]. Poly(p-2,5-di-n-hexylphe-nylene) was prepared in a biphasic mixture of benzene and water as a reaction medium, using sodium carbonate as a water-soluble base. This AB-type polymerizahon afforded polymers containing, e.g., an average of about 28 phenylene units. 

Hereby, poly(p-phenylene) polymers containing ether and carbonyl linkages in the polymer backbone are accessible. By polymerization of the AB2 monomer 3,5-dibromobenzene boronic acid in a biphasic aqueous/organic medium, Kim and Webster obtained hyperbranched polyphenylenes [233]. Suzuki polycondensation in aqueous systems has proven to be a versatile method, which has been applied to the synthesis of various polymer types ]234]. 

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