Asymmetric biphenyl type polyimides (PI) derived from 2,3,3',4'-biphenyltetracarboxylic dianhydride (a-BPDA) and p-phenylenediamine (PDA) or 4,4'-oxydianiline (ODA) show higher T(g)s, and much better thermoplasticity than the corresponding isomeric PIs from symmetric 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-EPDA). In addition, a-BPDA-derived PIs are completely amorphous owing to their bent chain structures and highly distorted conformations, whereas the PIs from s-BPDA are semicrystalline. a-BPDA-derived PIs possessing these properties or the a-BPDA monomer were used as a flexible blend component or a comonomer to improve the insufficient thermoplasticity of semirigid s-BPDA/PDA homo polymer. The blends composed of s-BPDA/PDA (80%) with a-BPDA-derived PIs (20%), as well as the s-BPDA/PDA-based copolymer containing 20% a-BPDA, showed a certain extent of thermoplasticity above the T(g)s without causing a decrease in T-g. In addition, these blends and copolymer provided comparatively low thermal expansion coefficient (ca. 18 ppm). The improved film properties for the blends are related to good blend miscibility. On the other hand, when s-BPDA/ODX was used as a flexible matrix polymer instead of a-BPDA-derived PIs, the 80/20 blend film annealed at 400 degrees C exhibited nu prominent softening at the T-g. This result arises from annealing-induced crystallization of the flexible s-BPDA/ODA component. Thus, these results revealed that a-BPD-derived PIs are promising candidates ac matrix polymers fur semirigid s-BPDA/PDA for the present purpose.