This dissertation describes kinetic and mechanistic studies of high performance polyimide formation, synthesis and characterization of fully cyclized, molecular weight and end group controlled polyimides, and investigations of high performance polymer blends based upon polyimides and polybenzimidazole. Imidization kinetics were successfully followed by the quantitative non-aqueous titration of the amic acid functional groups as a function of reaction conditions. The homogeneous solution imidization processes were described by auto-acid catalyzed second order kinetics. The effects of heteroatom bridging groups in the diamines and dianhydrides on reaction rates have been investigated and a possible reaction mechanism for the solution imidization processes has been proposed. Detailed mechanistic investigations of the thermal solution imidization of polyamic acids were performed. A small amount of hydrolysis and possibly some unimolecular decomposition of amide bonds in the polyamic acid during thermal solution imidization processes were observed via combination of NMR and intrinsic viscosity measurements. However, complete "recombination" of the degraded polymer chains and their further cycloimidization could be achieved under proper imidization conditions. Potential side reactions involving intermolecular imide formation reaction were also investigated using a well characterized polyimide and also a model imide. For polyimide systems containing benzophenone tetracarboxylic acid dianhydride (BTDA), direct evidence for network formation involving imine crosslinking, was observed by high field lH-NMR spectroscopy. The gel formation was a strong function of reaction conditions, occurring under extremely dry reaction conditions and being favored at moderate reaction temperatures. Various polyimide homo- and copolymers with controlled molecular weight and end groups were synthesized by the classic two step method and their thermal properties and solution viscosities were evaluated. Further, miscibility behavior of high performance polymer blends based upon polyimide (PI) and polybenzimidazole (PBI) was investigated. Several miscible PI/PBI blend material systems were identified, some of which showed a lower critical solution temperature (LCST), which was consistent with earlier observations. It was found that miscibility was a strong function of polarity and possible specific interactions with the polyimide components. Thus, miscibility was possible over a wide composition range with polyimides containing polar groups such as ketones, sulfones and ethers. However, immiscible blends were obtained when these polar polyimide components were replaced by non-polar groups such as the hexafluoroisopropylidene linkages.Ph. D.