Polyimides are generally formed by combining a dianhydride and a diamine monomer in a polar aprotic solvent to form a poly(amic acid). The poly(amic acid) is then cyclodehydrated by either thermal or solution imidization at high temperatures to give the fully cyclized polyimides. This research focuses on the development of a low temperature solution polymerization route utilizing a transimidization method to make fully cyclized polyimide homopolymers and polyimide siloxane copolymers. Polyimide oligomers endcapped with 2-aminopyrimidine were reacted with aminopropyl terminated poly(dimethyl siloxane) to give perfectly alternating segmented polyimide siloxane copolymers. The polymerization was conducted under very mild conditions. At reaction temperatures of only 100 - 110°C in chlorobenzene, high molecular weight, fully imidized polyimide siloxane copolymers were obtained. The polyimide siloxane copolymers were cast into tough transparent films. Properties of the polyimide siloxane copolymers were found to be dependent on the molecular weight of the starting polyimide and poly(dimethyl siloxane) oligomers. The transimidization method was also applied to the synthesis of soluble, fully cyclized polyimide homopolymers at reaction temperatures as low as 60°C. Utilizing the transimidization route, fully cyclized polyimides were made at lower temperatures than can be made by conventional polyimide synthetic methods. The polymerization of the bis(N-pyrimidine phthalimide) derivative of 6F dianhydride and 4, 4' oxydianiline at ~65 °C in N-methyl pyrrolidone with acetic acid as a catalyst resulted in a high molecular weight, cyclized polyimide. Fully cyclized, processable, fluorine containing polyimides were developed with very high glass transition temperatures and good thermal stability. The 1, 1-bis(4- aminophenyl)-1-phenyl-2, 2, 2-trifluoroethane (3F diamine) based polyimides were soluble and amorphous, probably as a result of the non-coplanar structure. DSC and dynamic mechanical analysis showed a glass transition temperature exceeding 420°C for the PMDA-3F diamine based polyimide. These polyimides showed good thermooxidative stability at 600°F in air after 500 hours. / Ph. D.