Suzuki–Miyaura catalyst-transfer polycondensation with Pd(IPr)(OAc)2 as the catalyst for the controlled synthesis of polyfluorenes and polythiophenes

Abstract

Controlled Suzuki–Miyaura catalyst-transfer polycondensations (SCTPs) of fluorene- and thiophene-based AB-type monomers have been demonstrated with a N-heterocyclic carbene (NHC)-based Pd complex, Pd(IPr)(OAc)₂, as the catalyst. The number average molecular weights (M_ns) of the resulting poly(9,9-dioctyl-9H-fluorene)s (PF8s) were linear to the conversions of the monomer. PF8s with M_ns in the range of 10.5–69.2 kDa and polydispersity indices (PDIs) of ∼1.60 were successfully synthesized by tuning the feed ratios ([monomer]₀/[Pd]). The protocol is also applicable to the controlled synthesis of poly(3-hexylthiophene) (P3HT). Polymers with M_ns of 9.5–63.8 kDa, which were linearly correlated to feed ratios, were obtained when the catalyst loading was tuned to 5–0.5 mol%. The “living” characteristics of the polymerization were also confirmed by monomer-addition and block copolymerization experiments. In addition, PF8 with a moderate molecular weight (MW) was mainly end-capped with Br/H end groups as evidenced by matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass and ¹H NMR spectra, indicating that this polymerization involves a catalyst-transfer mechanism. The mechanism was further confirmed by the fact that the cross-coupling of dibromofluorene (1 equiv.) and biphenylboronic acid ester (1 equiv.) with Pd(IPr)(OAc)₂ as the catalyst preferentially gave a di-substituted product.