Poly(amirs dialkyl ester) precursors of high performance polyimides were synthesized by employing the direct polymerization method using phenylphosphonic dichloride as an activating agent and pyridine as a Lewis base. Their thermal imidization behavior was investigated by performing both isothermal and nonisothermal TGA(thermogravimetric analysis) experiments. The precursor polymers showed a two-stage imidization behavior, i.e., fast imidization at the early stage and slow imidization at the later stage. In our studies, the kinetic data obtained at the early stage were chosen to determine the rate constants. Kinetic data were interpreted using the first order kinetic plot of the logarithmic wt % loss versus time. The observed imidization behavior was interpreted in view of the length of alkyl leaving group, chemical reactivity and the mobilitl of polymer backbones.