Reproducible and Rapid Synthesis of a Conjugated Polymer by Stille Polycondensation in Flow: Effects of Reaction Parameters on Molecular Weight

Abstract The batch-to-batch variations in molecular weight and molecular weight dispersity (Đ) values are large for conjugated polymers synthesized by step-growth polymerization, which can lead to irreproducible device performance. We developed a rapid flow synthesis system that can perform step-growth polymerizations of conjugated polymers in a reproducible manner by utilizing nitrogen gas as a carrier. The use of gas carrier, instead of organic solvents or fluorinated oils, eliminates the common diffusion problems at the beginning and the end of the reaction stream to the carrier fluid. Without the diffusion problem, our system provides high reproducibility and uniform reaction conditions in all regions of reaction stream. To highlight the performance of our flow system, we conducted Stille polycondensations to synthesize a representative conjugated polymer, PTB7. We found that PTB7 with a number average molecular weight (Mn) over 30 kDa was synthesized in only 3 minutes, with very low deviations of 2.6 % and 3.1 % for Mn and Đ values, respectively, compared to deviations of 66 % for Mn and 43 % for Đ in control reactions using pure solvent as a carrier. The effect of catalyst loading and reaction temperature was also studied, which enabled tuning the Mn of PTB7 within the range of 30.6 – 57.9 kDa. The quality of PTB7 synthesized in our system was also confirmed by fabricating photovoltaic devices which gave maximum power conversion efficiency of 7.02% with PC71BM as an acceptor.