A series of new copolyimides were synthesized by the reaction of pyrometallic dianhydride, benzophenone tetracarboxylic acid dianhydride, and 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane (6FDA) with two diamines, 4,4"-bis(aminophenoxy)-3,3"-trifluoromethyl terphenyl and 4,4'-diaminodiphenyl ether, at a 50:50 ratio by a thermal imidization route. The complete spectral assignment of the copolymer derived from 6FDA was done with the help of distortionless enhancement of polarization transfer 90degrees and two-dimensional heteronuclear (C, H) shift correlation spectroscopy. The various types of couplings in the copolymers were assigned with the help of two-dimensional correlated spectroscopy (COSY) homonuclear correlation spectroscopy. A long-range connectivity was also observed by a heteronuclear correlation spectroscopy via long-range coupling experiment. The copolyimides derived from 6FDA were soluble in common organic solvents such as N,N-dimethyl formamide, N,N-dimethyl acetamide, dimethyl sulfoxide, N-methyl pyrrolidone, and chloroform. Water absorption values, after the samples were boiled in water for 72 h, were in the range 0.3-0.7%. These polyimides had low dielectric constants of 2.79-3.01 at 1 MHz, low refractive indices of 1.655-1.705 at 589.3 nm, and high optical transparencies (>90%). The onset temperature for 5% weight loss for these polyimides, as assessed by thermogravimetric analysis, was up to 551degreesC in air, indicating a very high thermal stability, and the polyimides showed glass-transition temperatures as high as 316degreesC. Transparent thin films of these polyimides exhibited tensile strengths up to 93 MPa, moduli of elasticity up to 3.1 GPa, and elongations at break up to 24% depending on the exact repeat unit structure. (C) 2004 Wiley Periodicals, Inc.