Synthesis of BDT Monomers

However, the aforementioned syntheses not only require thiophene derivatives as key starting materials, also are rather lengthy 

Two main polymerization methods have been employed dispro-portionally towards the synthesis of these BDT based copolymers. 

Stille coupling polycondensations have been used in the synthesis of most BDT based copol)mers [28-32]. Monomers for Stille coupling polymerizations are relatively stable and usually remain as solids at room temperature, a feature that allows easy purifications of the monomers (e.g., via recrystallization). These ultra-pure monomers of solid state are beneficial in achieving an accurate stoichiometry of the two constituting monomers, a general requirement to obtain high molecular weights via polycondensations. The major 

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The report in 1971 by MacDowell and Wisowaty on the S5mthesis of BDT unit started from thiophene derivatives [11,16], which was further improved by Beimling et al. in 1986 (Scheme 3.2) [12, 17]. Instead of using 3-thiophenecarboxaldehyde and 2,3-dibromothio-phene as starting materials in the original report, the improved method started with only 3-bromothiophene. It was first converted to 3-bromothiophene-2-carbaldehyde, which was then treated with 3-thienyllithium to afford compound 1. The newly formed hydroxyl group was reduced off to afford compound 2. The bromine on the 3 position of compound 2 was lithiated and quenched by N,N-dimethylformamide (DMF) to yield compound 3, which imder-went a cyclization in 48% hydrobromic acid solution to complete the synthesis of BDT monomer 4 with high yield. 

Scheme 3.4 Synthesis of BDT monomers with various side chains.

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