Synthesis of Poly(3-hexylthiophene), Poly(3-hexylselenophene), and Poly(3-hexylselenophene-<i>alt</i>-3-hexylthiophene) by Direct C–H Arylation Polymerization via <i>N</i>‑Heterocyclic Carbene Palladium Catalysts

Direct C–H arylation polymerization of 2-bromo-3-hexylselenophene and 2-bromo-3-hexylthiophene catalyzed by <i>N</i>-heterocyclic carbene (NHC) palladium-based and Pd­(OAc)₂-based systems has been carried out and investigated. Under the optimized conditions, high molecular weight poly­(3-hexylthiohphene) (P3HT) (<i>M</i>_n = 26.9K g/mol) with high head-to-tail regioregularity (94%) can be obtained by using [1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene]­chloro­[3-phenylallyl]­palladium­(II) (Pd-IPr) as the catalyst. Pd-IPr exhibits a wide range of working temperatures from 70 to 140 °C and good catalytic reproducibility as a result of its high thermal stability. It was found that the presence of additional phosphine ligand, such as tris­(<i>o</i>-methoxyphenyl)­phosphine, can increase the polymerization efficiency in the Pd­(OAc)₂ system. This improvement is linked to the stability enhancement for the active species during the course of catalysis. For the first time, poly­(3-hexylselenophene) (P3HS) was also obtained by direct-arylation polymerization in this research. The modest molecular weight is attributed to the poor solubility of P3HS in the used solvents. Density-functional-theory (DFT) calculations were performed to investigate the postulated reaction mechanisms for our catalytic systems. The experimental observations can thus be elucidated by the help of computation. Most significantly, a main-chain alternating and side-chain regioregular (RR = 94%) poly­(3-hexylselenophene-<i>alt</i>-3-hexylthiophene) (Alt-P3HST) with high molecular weight (20.0K g/mol) was successfully synthesized via the Pd-IPr-catalyzed direct-arylation polymerization of a well-designed 2-bromo-3-hexyl-5-(3-hexylselenophen-2-yl)­thiophene monomer. The molecular properties of the Alt-P3HST were characterized to compare with those of P3HT and P3HS. This research demonstrates in-depth investigation on the NHC-based palladium catalysts for the synthesis of conjugated polymers via direct C–H bond polymerization