This paper reports a novel mixed ligand catalyst for palladium catalyzed direct arylation polymerization (DArP) of 2,6-diiododithienosilole (DTS-I-2) and thienopyrroledione (TPD-H-2) to give poly(DTS-alt-TPD). It has been documented that this monomer combination has a marked tendency to form homocoupling and branching defects in polymer chains, and the latter defects eventually lead to the formation of insoluble materials. This paper demonstrates that the combined use of P(o-MeOC6H4)(3) and TMEDA ligands effectively prevents the defect formation. The side reactions that afford structural defects constitute a sequential process triggered by the reduction of DTS-I units. TMEDA as a simple diamine effectively inhibits the reduction of DTS-I units, giving poly(DTS-alt-TPD) (M-n(GPC) = 20 000) in high cross -coupling selectivity (99%).