Reproducible and rapid synthesis of a conjugated polymer by Stille polycondensation in flow: Effects of reaction parameters on molecular weight

초록

Conjugated polymers are promising material which can be applied in various devices such as organic photovoltaics(OPVs), organic field-effect transistors(OFET) and light emitting diodes(LEDs). However, in the batch synthesis system which is used widely for conjugated polymer synthesis, batch-to-batch variation occurs leading uncontrolled molecular weight and irreproducible device performance. Flow synthesis system can be an excellent alternative to batch synthesis system by providing rapid and reproducible reactions. Flow system effectively transfers heat and mass, so it enables uniform reaction and high reproducibility. In our study, we developed a flow system using nitrogen gas as a carrier instead of pure solvent or fluorinated oils to eliminate 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. Using our system, PTB7, one of the most representative conjugated polymers, was synthesized to highlight performance of our flow system. We found that PTB7 with a number average molecular weight (Mn) over 30 kDa was synthesized in only 3 min, with very low deviations of 2.6% and 3.1% for Mn and Ð values, respectively, compared to deviations of 66% for Mn and 43% for Ð 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.