Synthesis of poly(3-hexylthiophene), poly(3-hexylselenophene), and poly(3-hexylselenophene-alt-3-hexylthiophene) by direct C-H arylation polymerization via N-heterocyclic carbene palladium catalysts

Direct C-H arylation polymerization of 2-bromo-3-hexylselenophene and 2-bromo-3-hexylthiophene catalyzed by N-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) (M_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(o-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-alt-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.