A series of aromatic polyimides (PIS) containing pyridazine or pyrimidine in their main chains has been developed. All of the PIS were prepared from newly synthesized diamines, 3,6-bis(4-aminophenylenesulfanyl)pyridazine (APP), 4,6-bis(4-aminophenylenesulfanyl)pyrimidine (APPM) and aromatic dianhydrides, 4,4'-[p-thiobis(phenylenesulfanyl)]diphthalic anhydride (3SDEA) and 4,4'-oxydiphthalic anhydride (ODPA) via the conventional two-step polycondensation. The PIS showed good thermal stability with 10% weight loss at temperatures above 450 degrees C and glass transition temperatures above 190 degrees C. Films with a 10-mu m thickness exhibited good optical transparency above 80% at 500 nm, high refractive indices ranging from 1.7218 to 1.7499, and low birefringence between 0.0066 and 0.0102. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4886-4894, 2009