A series of poly(amide-imide)s were prepared using a new monomer, 1,3-bis(trimellitimido)-2,4,6-trimethyl benzene (BTB), with four different diamines: 1,4-phenylene diamine (PDA), 2,4-diamino mesitylene (DAM), 2,2'-dimethyl-4,4'-diamino biphenyl (DMDB), and 2,2'-bis(trifluoromethyl)-4,4'-diamino biphenyl (TFDB). They were prepared by the condensation method in N-methyl-2-pyrrolidinone (NMP) solvent using triphenyl phosphate and pyridine as condensing agents. The synthesized poly(amide-imide)s were characterized by Fourier transform infrared and H-1 NMR techniques. Films were prepared and characterized using DSC, thermogravimetric analysis (TGA), a prism coupler, and a film dielectric property analyzer. DSC measurement showed that the glass-transition temperatures of the polymers were in the range of 259-327 degreesC. TGA analysis showed 5% weight loss, in the range of 472-514 degreesC. The refractive index varied from 1.6004 to 1.6586 in the following increasing order: BTB-TFBM < BTB-DAM < BTB-DMDB < BTB-PDA. For the poly(amide-imide) films, the birefringence varied in the range of 0.0319-0.0580, in the following increasing order: BTB-DAM < BTB-TFBM < BTB-DMDB < BTB-PDA. The capacitance method showed that the dielectric constant of poly(amide-imide) varied with the diamine structure; no difference was found by the optical method. (C) 2003 Wiley Periodicals, Inc.