Tandem Kumada–Tamao catalyst-transfer condensation polymerization and Suzuki–Miyaura coupling for end-functionalization and block copolymerization of poly(p-phenylene)

Abstract

Successive Kumada–Tamao catalyst-transfer condensation polymerization (CTCP) of [4-bromo-2,5-bis(hexyloxy)phenyl]magnesium chloride (1) and Suzuki–Miyaura end-functionalization with arylboronic acid (ester) is investigated for the synthesis of poly(p-phenylene) (PPP) with a base-sensitive functional group at both ends as an extension of our previous work on end-functionalized poly(3-hexylthiophene) (P3HT). We first examined Kumada–Tamao CTCP of 1 with the PEPPSI-IPr Pd catalyst to see whether poly1 with Br/H ends would be obtained; the H end is expected to be derived from the poly1-Pd–Br end by hydrolysis with hydrochloric acid used for quenching the polymerization. The MALDI-TOF mass spectrum of the product obtained at 30 min showed peaks due to the desired poly1 with Br/H ends. However, peaks assignable to poly1 with Br/N-heterocarbene (NHC) ends became more intense with increasing reaction time. NHC is likely derived from the ligand of PEPPSI-IPr. Therefore, Kumada–Tamao CTCP of 1 with PEPPSI-IPr was conducted at room temperature for 30 min, followed by reactions with various boronic acid (ester)s 2 in the presence of aqueous K₃PO₄. 4-Trifluoromethylphenyl, 4-methoxyphenyl, 4-(methoxycarbonyl)phenyl, 4-(hydroxymethyl)phenyl, and 4-(tert-butoxycarbonylaminomethyl)phenyl groups were introduced at both ends. However, a 4-vinylphenyl group was introduced at only one end, and the reaction with pinacol 4-(α-bromoisobutyryloxymethyl)phenylboronate (2g) afforded a mixture of mono- and dicapped products. Furthermore, successive Suzuki–Miyaura block copolymerization with bromoarylboronate monomer 3 instead of 2 was conducted. The use of thiophene monomer 3a afforded the diblock copolymer of poly1 and poly(3-hexylthiophene) with a narrow molecular weight distribution, whereas fluorene monomer 3b afforded a copolymer with a broad molecular weight distribution, accompanied by unreacted 3b.