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 unsubstituted 2,2'-bithiophene units. When only L1 is used as a ligand, the reaction of 5,6-difluoro-4,7-bis[5-bromo-4-(2-octyldodecyl)thiophene-2-yl]benzo[c][1,2,5]thiadiazole (1a-Br) with 2,2'-bithiophene (2-H) in toluene at 100 degrees C forms a notable amount of insoluble materials via branching and cross-linking. In contrast, in the presence of L1 and TMEDA, the formation of insoluble materials is completely suppressed, and PffBT4T [poly(1a-alt-2)] with a well-controlled structure and high molecular weight is obtained (M-n = 88100). Similarly, the reaction of 5,10-bis[5-bromo-4-(2-decyltetradecyl)thiophene-2-yl]naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole (1b-Br) with 2-H in toluene at 110 degrees C forms PNTz4T[poly(1b-alt-2)] with M-n = 43800. The resulting polymers function as good components of bulk-heterojunction type organic solar cells (OSCs) using PC71BM. In particular, PNTz4T performs the best of all the DArP polymers reported so far (PCE = 9.0(1)%).