Combining flow synthesis and heterogeneous catalysis for the preparation of conjugated polymers

Abstract

Current flow systems, used for conjugated polymer syntheses, have been limited to the use of homogeneous catalysts. Conventional homogeneous catalysts are often not ideal for mass production due to their poor air stability and high cost. Here, a heterogeneous palladium catalyst (Pd/C) was applied for the first time in a flow system to synthesize a conjugated copolymer, PTB7, via the Stille polymerization. During our initial trials, catalyst deactivation was observed during the reaction, and this led to a low number average molecular weight (M_n) of 8.2 kDa with a large molecular weight dispersity (Đ) of 3.35 even after 30 min of reaction time. With the aid of deactivation studies, we identified that catalyst deactivation occurred with the highest rate at the early reaction stream mainly due to poisoning of the catalyst by monomers and palladium leaching. By adding triphenylphosphine (TPP), as an antidote for the S poisoning, we demonstrated the successful synthesis of the conjugated polymer, PTB7, with an intermediate M_n of 13.6 kDa and Đ of 2.9 in 30 min. For achieving comparable results with batch reactions, 72 hours of reaction time and additional days of purification were required. These results were highly reproducible (2.15% errors in M_n between runs), and this demonstrates the great potential of combining heterogeneous catalysts and the flow synthesis system for conjugated polymer synthesis.