Direct (hetero)arylation: a new tool for polymer chemists

The coupling of aryl halides with catalytically activated aryl C-H bonds provides a desirable and atom-economical alternative to standard cross-coupling reactions for the construction of new C-C bonds. The reaction, termed direct (hetero)arylation, is believed to follow a base-assisted, concerted metalation-deprotonation (CMD) pathway. During this process, carboxylate or carbonate anions coordinate to the metal center, typically palladium, in situ and assist in the deprotonation transition state. Researchers have employed this methodology with numerous arenes and heteroarenes, including substituted benzenes, perfluorinated benzenes, and thiophenes. Thiophene substrates have demonstrated high reactivity toward C-H bond activation when appropriately substituted with electron-rich and/or electron-deficient groups. Because of the pervasive use of thiophenes in materials for organic electronics, researchers have used this chemistry to modularly prepare conjugated small molecules and, more recently, conjugated polymers. Although optimization of reaction conditions such as solvent system, phosphine ligand, carboxylate additives, temperature, and time is necessary for efficient C-H bond reactivity of each monomer, direct (hetero)arylation polymerization (DHAP) can afford high yielding polymeric materials with elevated molecular weights. The properties of these materials often rival those of polymers prepared by traditional methods. Moreover, DHAP provides a facile means for the synthesis of polymers that were previously inaccessible or difficult to prepare due to the instability of organometallic monomers. The major downfall of direct (hetero)arylation, however, is the lack of C-H bond selectivity, particularly for thiophene substrates, which can result in cross-linked material during polymerization reactions. Further fine-tuning of reaction conditions such as temperature and reaction time may suppress these unwanted side reactions. Alternatively, new monomers can be designed where other reactive bonds are blocked, either sterically or by substitution with unreactive alkyl or halogen groups. In this Account, we illustrate these methods and present examples of DHAP reactions that involve the preparation of common homopolymers used in organic electronics (P3HT, PEDOT, PProDOT), copolymers formed by activation of electron-rich (bithiophene, fused bithiophenes) and electron-deficient monomers (TPD, 1,2,4,5-tetrafluorobenzene, 2,2'-bithiazole). Our group is optimizing these reactions and developing ways to make DHAP a common atom-economical synthetic tool for polymer chemists.