Synthesis of a Well-Defined Conjugated Polymer Via Nickel-Catalyzed Direct Arylation Polymerization (Ni-DArP)

Direct arylation polymerization (DArP) is a sustainable method of conjugated polymer synthesis; however, expensive and low-abundance palladium catalysts are nearly ubiquitous in this chemistry. Catalysts based on earth-abundant first-row transition metals are little-explored but would represent a significant improvement in the sustainability of DArP and potentially enable new reactivity. Beyond previously explored copper catalysts, nickel catalysts offer access to possibly more effective polymerization chemistry. Previous reports of DArP using a Ni catalyst yielded cross-linked, insoluble materials. Here we report the first instance of a linear, well-defined conjugated polymer synthesized using Ni-catalyzed DArP (Ni-DArP) where 5,5′-(2,5-bis(hexyloxy)-1,4-phenylene)dithiazole was copolymerized with 1,4-dibromo-2,5-bis((2-ethylhexyl)oxy)benzene using Ni(OAc)2. Polymerizations proceeded at catalyst loadings of 5–20 mol % affording polymers with molar masses up to 10.2 kg/mol and yields up to 55%, on-par with Cu-DArP. The stable, commercial species Ni(OAc)2·4H2O can be easily dehydrated to yield the active catalyst. Regioselectivity for the 2-position of thiazole with both Ni and Cu was observed, contrasting selectivity for the 5-position with Pd, demonstrated in both polymerizations and small-molecule couplings, along with the Ni-catalyzed conditions potentially favoring ring walking relative to that of Cu. The complementary nature of the catalysts demonstrates the value of developing earth-abundant catalysts for DArP beyond the relevance to sustainability.