Catalyst-transfer system in stoichiometry-independent AA+BB-type Migita–Kosugi–Stille coupling polycondensation using ester-functionalized dibromo monomer

Catalyst-transfer systems have emerged in the condensative chain polymerization of AB-type monomers and stoichiometry-independent polycondensation of AA- and BB-type monomers to synthesize π-conjugated polymers. Although catalyst-transfer polycondensation (CTP) systems have often been explained by a ring-walking process of catalytic metals with a main chain, the functional-group effect of aromatic monomers on such CTP phenomena is unclear. In this study, stoichiometry-independent Migita–Kosugi–Stille coupling polycondensation was performed between 2,5-bis(trimethylstannyl)thiophene and an ester-functionalized dibromo monomer, which is bis(2-butyloctyl) 2,5-dibromoterephthalate, to obtain high-molecular-weight π-conjugated poly(phenylene thienylene) (Mn = 16,800). The method uses a 2-fold excess of the dibromo monomer toward the distannylated monomer. Such successful nonstoichiometric polycondensation may be derived from the intramolecular Pd(0) catalyst transfer on the aromatic dibromo monomer after the first coupling reaction between the dibromo monomer and stannylated compounds during polymerization. The results of model reactions and DFT calculations using ester-, ether- and alkyl-functionalized dibromo compounds suggest that incorporating the ester-carbonyl group into aromatic dibromo monomers is a key molecular design strategy to realize an efficient intramolecular Pd(0) catalyst-transfer system.