Palladium-catalyzed oxidative direct arylation polymerization (Oxi-DArP) of an ester-functionalized thiophene

Poly(hexyl thiophene-3-carboxylate), an alkyl ester side-chain functionalized thiophene polymer herein referred to as poly(3-hexylesterthiophene) (P3HET), is synthesized via a novel palladium-catalyzed oxidative dehydrogenative polycondensation method. The ester promotes the generation of high molecular weight polymers as a directing group, enabling direct hetero C–H/C–H coupling and bypassing the functionalization requirements of traditional direct arylation (halogenation) and transition metal-catalyzed cross-coupling (halogenation and metalation) methods. Despite lacking functional groups in the 2- and 5-position, these unique reaction conditions achieve good regioregularity (around 85%) through the utilization of a phosphine ligand. To evaluate this new polymerization method, poly(hexyl thiophene-3-carboxlates) and poly(3-hexylthiophene) polymers are synthesized via Oxi-DArP, traditional DArP, and Stille polycondensation and subsequently characterized by electrochemical HOMO determination, UV-Vis, GIXRD, and space-charge limited current (SCLC) hole mobilities. High quality polymers via Oxi-DArP were only acquired when the ester functional group was present, whereas molecular weight, regioregularity, and yields suffered with 3-hexylthiophene. Optimized Oxi-DArP P3HET exhibited absorption coefficients, electrochemical HOMO levels, and semi-crystallinity comparable to DArP and Stille P3HET. This work expands on an emerging synthetic method and develops attractively simple and mild conditions toward the generation of high quality polymers promoted by carbonyl directing groups.