Soluble, thermally stable phosphorus containing polyimides have been synthesized to controlled molecular weights and well-defined phthalimide endgroups via solution imidization. They were characterized by intrinsic viscosity, DSC, TGA, and thermomechanical measurements. Molecular weight control was successful and these polyimides were thermally stable via dynamic TGA up to 500°C. Flame resistance was suggested by the 10-20 % of char yield observed in air at 750°C. Good tensile modulus (500-550 ksi) via stress-strain measurements and excellent adhesion to Ti-6AI-4V substrates through single lap shear tests was demonstrated. Investigations of the effects of molecular weight and end group on thermal and mechanical properties of reactive amine terminated and non-reactive phthalic anhydride terminated PMDA-mBAPPO polyimides were pursued. Polyimides with non-reactive end groups do not crosslink and, hence, were essential for controlled processability. Rheological behavior was investigated with a parallel plate type viscometer and the results confirmed the thermoplastic nature of these systems. Phenyl ethynyl endcapped polyimides were synthesized for a broad based investigation of their processibility and mechanical properties. Thermally stable soluble polyimide oligomers were synthesized via the poly(amic acid) precursor route followed by subsequent solution imidization. These polyimides were successfully crosslinked upon heating, without the evolution of volatile products. The phenyl ethynyl endcapped polymers (T_cure ~ 350-400°C) exhibited a wider processing window above Tg, relative to the unsubstituted acetylene endcapped polymers (T_cure ~ 200°C). Studies of the chemorheological behavior of these polyimides as a function of varying temperatures and times were conducted. This research allows the conclusion that high Tg, thermostable, and solvent resistant new networks suitable for structural adhesives and matrix resins have been identified. This was proven by characterization of these novel networks. Ninety-nine percent gel fraction networks were observed after cure at 380°C for 60 minutes. Thermomechanical properties such as tensile properties, adhesion properties, creep behavior, and heat distortion temperatures have been generated which illustrate the excellent dimensional stability and mechanical properties achievable at optimum crosslink density. Polyarylene ether sulfones were also studied which demonstrated the desirable combination of excellent thermal stability and good mechanical properties. Biphenyl-based polysulfones were synthesized and endcapped with 3-phenyl ethyny] functional groups. High Tg, thermostable, ductile and solvent resistant networks were prepared after curing at 380°C. Thermo-mechanical behavior, including tensile properties and dynamic mechanical analyses have been determined and are reported in this thesis. / Ph. D.