A palladium precatalyst for direct arylation polymerization

Abstract

Direct arylation has gained acceptance as an important method for the synthesis of conjugated polymers, given several advantages over traditional cross coupling techniques. However, direct arylation also introduces drawbacks including homocoupling, crosslinking and branching that can lead to poor solubility and electronic properties. These side reactions need to be addressed for direct arylation polymerization (DArP) to be a broadly applicable method for creating high performance conjugated polymers. The problems of the reaction can be circumvented by utilizing monomers with directing groups to prevent unwanted functionalization, blocking any potentially reactive positions or using bulky carboxylic acids. Catalyst design may prove to be important in gaining more control over the side reactions that affect DArP, and even though the tailoring of catalysts has had a profound impact on cross-coupling reactions it often remains overlooked for DArP. Here, we outline how by utilizing a pre-catalyst bound with a phosphine ligand known to promote DArP, a catalytic species that can perform DArP in a controlled fashion when compared to traditional palladium sources was created. The pre-catalyst allowed for the controlled generation of poly(9,9-dioctyl-2,7-fluorene-alt-2,2′bithiophene) (PFBT) with a high M_n and for the facile processing of the material at room temperature in THF while also allowing for lower catalytic-and-phosphine-loadings.