The combined use of P(2-MeOC6H4)(3) (L1) and TMEDA as ligands effectively prevents defect formation in palladium-catalyzed direct arylation polymerization (DArP) to give donor acceptor type alternating copolymers (DA polymers) with diketopyrrolopyrrole (DPP) units. The reaction of 3,6-bis(5-bromo-2-thienyl)diketopyrrolopyrrole (1-Br) and 3,4-dicyanothiophene (2-H) in toluene at 100 degrees C for 6 h in the presence of only L1 afforded a notable amount of insoluble materials via branching and cross-linking; in addition, the soluble part (M-n = 3100) included a large quantity of homocoupling defects (12.5%). In contrast, in the presence of L1 and TMEDA, the formation of insoluble materials was completely suppressed, and homocoupling defects decreased to 1.6%. Furthermore, the molecular weight of poly(1-alt-2) remarkably increased (M-n, = 24 500, 97% yield). The mixed ligand catalyst using L1 and TMEDA was also effective against DArP of 1-Br with thienopyrroledione (3-H) or 3,4-propylenedioxythiophene (4-H) to afford DA polymers with DPP units in high yields (poly(1-alt-3): M-n, = 36 800, 100% yield; poly(1-alt-4): M-n = 19 000, 80% yield). Thermal and optical properties of the resulting polymers are reported.