Catalyst-Transfer Suzuki-Miyaura Coupling Polymerization for Precision Synthesis of Poly(p-phenylene)

Catalyst-transfer Suzuki-Miyaura coupling polymerization of 2,5-bis(hexyloxy)-4-iodophenylboronic acid (1b) with Bu3PPd(Ph)Br (2) was investigated. When 1b was polymerized with 2 at 0 degrees C in the presence of 4 equiv of CsF, 8 equiv of 18-crown-6, and a small amount of water (to dissolve CsF), poly-(p-phenylene) (poly1b) with a narrow molecular weight distribution was obtained. The conversion-number average molecular weight (M-n) and feed ratio-M-n relationships indicate that this polymerization proceeded in a chain-growth polymerization manner. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra showed that poly1b with moderate molecular weight uniformly had a phenyl group, derived from 2, at one end of each polymer chain and a hydrogen atom at the other, indicating involvement of a catalyst-transfer mechanism. However, the molecular weight distribution of the polymer gradually became broader with increase of the feed ratio of 1b to 2, and polymer chains with other end groups were formed. Successive catalyst-transfer Suzuki-Miyaura coupling polymerization of 2-(7-bromo-9,9-dioctyl-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3) and then 1b yielded a well-defined block copolymer of polyfluorene and poly(p-phenylene).