Suzuki–Miyaura Catalyst-Transfer Condensation Polymerization for the Synthesis of Polyphenylene with Ester Side Chain by Using Stable, Reactive 1,1,2,2-Tetraethylethylene Glycol Boronate (B(Epin)) Monomer

Suzuki–Miyaura catalyst-transfer condensation polymerization (CTCP) of 4-bromo-2,5-dihexyloxycarbonylphenylboronic acid ester (2) as an acceptor monomer was investigated. The polymerization of the boronic acid pinacol ester (Bpin) monomer (2a) was first conducted in the presence of ^tBu₃P-ligated tolyl–Pd-Br initiator 1 and CsF/18-crown-6 as a base, affording mainly polymers with H/H ends. We thought that the unsuccessful CTCP of 2a might be due to (1) poor catalyst transfer from the donor initiator unit to the acceptor monomer unit, (2) strong coordination of the Pd catalyst to the alkoxycarbonyl groups on successive repeat units, or (3) nonplanarity of the π-face of the backbone owing to the steric effect of the alkoxycarbonyl groups in the repeat unit. However, model reactions showed that the Pd catalyst successfully underwent intramolecular catalyst transfer, ruling out these possibilities. We then replaced 2a with B(Epin) monomer 2b, which is more stable than 2a. In the polymerization of 2b, the M_n value increased in proportion to the monomer conversion and feed ratio of [2b]₀/[1]₀ and the molecular weight distribution remained narrow. The matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrum showed a series of peaks due to the polymer with Tolyl/H ends. After end-capping of the polymer with 4-methoxyphenylboronic acid, the ¹H NMR spectrum exhibited signals of both end groups. These results indicate that the polymerization of 2b proceeded according to the CTCP mechanism.