This study synthesized six polythiophenes (PThs) using direct arylation polycondensation (DArP). The C–H monomers employed were β-fluorinated bithiophenes, including 4,4'-difluoro-2,2'-bithiophene (2FBT) and 3,3',4,4'-tetrafluoro-2,2'-bithiophene (4FBT). The C–Br monomers employed were 5,5'-dibromo-2,2'-bithiophenes DCBT-2Br, DABT-2Br, and DTBT-2Br that carry alkoxycarbonyl, alkyl and alkylthienyl side chains at 4,4'-positions, respectively. Both 2FBT and 4FBT are highly reactive for DArP. Successful syntheses of high molecular weight (M_n) PThs without any observable defects were achieved using 2FBT/DCBT-2Br, 2FBT/DABT-2Br, 2FBT/DTBT-2Br and 4FBT/DTBT-2Br monomer pairs. However, DArPs of 4FBT with DCBT-2Br and DABT-2Br resulted in C–H/C–H homo-coupling defects along with lower M_ns. The formation of the defects was found to follow the C–Br reduction provoked C–H/C–H homo-coupling mechanism, which is also responsible for the depressed M_ns. Detailed theoretical calculations revealed that the side reaction was caused by the poor selectivity between the cis- and trans-forms of transition states in concerted metalation-deprotonation (CMD) processes of 4FBT/DCBT-2Br and 4FBT/DABT-2Br monomer pairs. The findings of this research indicate that relying solely on a C–H monomer with high reactivity and regioselectivity is inadequate to achieve well-defined conjugated polymers through DArP. This study highlighted the importance of the C–Br monomer structure in influencing the catalytic cycles of DArP.

Xuwen Zhang

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