title: Mechanistic Investigation of Catalyst-Transfer Suzuki-Miyaura Condensation Polymerization of Thiophene-Pyridine Biaryl Monomers with the Aid of Model Reactions.
abstract: We have investigated the requirements for efficient Pd-catalyzed Suzuki-Miyaura catalyst-transfer condensation polymerization (Pd-CTCP) reactions of 2-alkoxypropyl-6-(5-bromothiophen-2-yl)-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine (12) as a donor-acceptor (D-A) biaryl monomer. As model reactions, we first carried out the Suzuki-Miyaura coupling reaction of X-Py-Th-X' (Th=thiophene, Py=pyridine, X, X'=Br or I) 1 with phenylboronic acid ester 2 by using tBu3PPd0as the catalyst. Monosubstitution with a phenyl group at Th-I mainly took place in the reaction of Br-Py-Th-I (1 b) with 2, whereas disubstitution selectively occurred in the reaction of I-Py-Th-Br (1 c) with 2, indicating that the Pd catalyst is intramolecularly transferred from acceptor Py to donor Th. Therefore, we synthesized monomer 12 by introduction of a boronate moiety and bromine into Py and Th, respectively. However, examination of the relationship between monomer conversion and the Mnof the obtained polymer, as well as the matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra, indicated that Suzuki-Miyaura coupling polymerization of 12 with (o-tolyl)tBu3PPdBr initiator 13 proceeded in a step-growth polymerization manner through intermolecular transfer of the Pd catalyst. To understand the discrepancy between the model reactions and polymerization reaction, Suzuki-Miyaura coupling reactions of 1 c with thiopheneboronic acid ester instead of 2 were carried out. This resulted in a decrease of the disubstitution product. Therefore, step-growth polymerization appears to be due to intermolecular transfer of the Pd catalyst from Th after reductive elimination of the Th-Pd-Py complex formed by transmetalation of polymer Th-Br with (Pin)B-Py-Th-Br monomer 12 (Pin=pinacol). Catalysts with similar stabilization energies of metal-arene η2-coordination for D and A monomers may be needed for CTCP reactions of biaryl D-A monomers.
authors.affiliation: Department of Material and Life Chemistry, Kanagawa University Rokkakubashi, Kanagawa-ku, Yokohama, 221-8686, Japan.
authors.forename: Yu
authors.initials: Y
authors.lastname: Tokita
authors.affiliation: Department of Material and Life Chemistry, Kanagawa University Rokkakubashi, Kanagawa-ku, Yokohama, 221-8686, Japan.
authors.forename: Masaru
authors.initials: M
authors.lastname: Katoh
authors.affiliation: Department of Material and Life Chemistry, Kanagawa University Rokkakubashi, Kanagawa-ku, Yokohama, 221-8686, Japan.
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Ohta
authors.affiliation: Department of Material and Life Chemistry, Kanagawa University Rokkakubashi, Kanagawa-ku, Yokohama, 221-8686, Japan.
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Yokozawa
keywords: Suzuki-Miyaura coupling reactions; catalyst-transfer condensation polymerization; conjugated polymers; pyridine; thiophene