The high reproducibility of the flow synthesis system enabled accurate investigation of the relationship between reaction conditions and molecular weight in Stille polycondensation of conjugated polymers. Here, a conjugated polymer, PTB7, was synthesized in a customized flow reactor using various catalyst and solvent combinations, and their effect on molecular weight was studied. Specifically, common palladium catalysts (Pd(PPh 3 ) 4 and Pd 2 (dba) 3 ) were screened in three different solvents (toluene, chlorobenzene, and o -xylene). A significant difference in molecular weight was observed depending on the type of solvent when using Pd 2 (dba) 3 as a catalyst. It was discovered that the solubility of both the catalyst and PTB7 in the solvent played a critical role in determining the molecular weight of the polymer. By using the Pd 2 (dba) 3 catalyst system, a high number-averaged molecular weight ( M n ) for PTB7, ranging from 41.7 to 51.5 kDa, was achieved. This was 16 times higher than that obtained with the Pd(PPh 3 ) 4 catalyst system in any solvent. However, the Pd(PPh 3 ) 4 catalyst system showed high compatibility with dimethylformamide (DMF) co-solvent, and the M n of PTB7 synthesized with the Pd(PPh 3 ) 4 catalyst showed a significant increase, ranging from 53.9 to 58.7 kDa, while the M n of PTB7 synthesized with the Pd 2 (dba) 3 catalyst decreased to 10.9- 15.0 kDa. The results can provide valuable guidance for optimizing the reaction conditions in the future synthesis of conjugated polymers in a flow reactor to achieve the desired molecular weight. The high reproducibility of the flow synthesis system enabled accurate investigation of the relationship between reaction conditions and molecular weight in Stille polycondensation of conjugated polymers.