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

Abstract

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) (Mn = 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.