General Synthetic Routes to Polythiophenes

As already mentioned, unsubstituted PT is an insoluble and infusible material. Once the polymer is prepared, it is difficult (if not impossible) to further process it as a material for electronic applications. The solubility can be greatly enhanced by the introduction of side chains at position 3 (or at both, 3 and 4). The most widely studied side chains are n-alkyl substituents that can be easily introduced into the thiophene core by reaction of 3-bromothio-phene with alkyl-Grignard reagents [464]. 

While 3-substitution efficiently improves the solubility and the processability of the PTs, polymerization of 3-i -thiophenes can result in three different types of coupling of the thiophene rings along the polymer main chain, i.e., HT, HH, and TT (Chart 2.96). 

SCHEME 2.60 Synthesis of polythiophene via chemical oxidation polymerization. 

Head-to-tail (HT) Head-to-head (HH) Tail-to-tail (TT) 

SCHEME 2.61 McCullough and Rieke methods of synthesis of regioregular HT-poly(3-alkylthio-phenes) HT-388. (From McCullough, R.D. and Lowe, R.D., J. Chem. Soc., Chem. Commun., 70, 1992 McCullough, R.D., Lowe, R.D., Jayaraman, and Anderson, D.L., J. Org. Chem., 58, 904, 1993.) 

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