Investigation of Pd–Phosphine Ligands Suitable for Suzuki–Miyaura Polymerization through Intramolecular Catalyst Transfer

The suitability of four Pd–phosphine ligands, di( tert -butyl)-4-methoxyphenylphosphine ( t Bu 2 PC 6 H 4 OMe), di( tert -butyl)-3,4,5-trimethoxyphenylphosphine ( t Bu 2 PC 6 H 2 (OMe) 3 ), di(1-adamantyl)-4-methoxyphenylphosphine (Ad 2 PC 6 H 4 OMe), and di(1-adamantyl)-4-dimethylaminophenylphosphine (Ad 2 PC 6 H 4 NMe 2 ), for Suzuki–Miyaura nonstoichiometric polycondensation and catalyst-transfer condensation polymerization (CTCP) was investigated. These four phosphine ligands were synthesized by the reaction of the corresponding phenyl lithium with di( tert -butyl)chlorophosphine or di(1-adamantyl)chlorophosphine. To avoid oxidation, we quickly converted the as-obtained phosphines into the corresponding tetrafluoroboric acid salts and purified them by recrystallization. We first conducted a Suzuki–Miyaura coupling reaction of 2.0 equiv of phenylene dibromide 1 with 1.0 equiv of pinacol phenylboronate in the presence of Pd(OAc) 2 /synthesized ligands and CsF/18-crown-6 as a base at room temperature. The use of Ad 2 PC 6 H 4 OMe and Ad 2 PC 6 H 4 NMe 2 resulted in the exclusive formation of the disubstituted products. Accordingly, nonstoichiometric Suzuki–Miyaura polycondensation of 1.3 equiv of 1 with 1.0 equiv of phenylenediboronate was conducted in the presence of Pd(OAc) 2 /Ad 2 PC 6 H 4 OMe or Ad 2 PC 6 H 4 NMe 2 . High-molecular-weight poly( p -phenylene) was obtained with Ad 2 PC 6 H 4 OMe, whereas the molecular weight was low when Ad 2 PC 6 H 4 NMe 2 was used. We next investigated CTCP of fluorene AB monomer 5 with a Pd–Ad 2 PC 6 H 4 NMe 2 initiator. The polymerization of 5 yielded a high-molecular-weight polymer with a narrow dispersity. The matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra showed peaks due to the polymer containing the initiator unit, indicating that the polymerization proceeded through the CTCP mechanism.