Polycondensation and imidization of benzoguanamine and pyromellitic dianhydride under microwave radiation were studied. The effects of the microwave radiation time (power), the composition of the monomer on the imidization, and the polymer yield were discussed. The polycondensates' third-order optical nonlinearities and time responses were measured by a phase conjugate forward, 3-dimensional degenerate, four-wave mixing technique. At the same time they were compared with relevant conventional thermopolymerization. The results showed that the polycondensates had high third-order nonlinear optical coefficients [polyimide (PI) chi ((3)) = 1.181 x 10(-13) esu] and fast time responses (22 ps). The third-order nonlinear optical coefficients under microwave radiation were higher than that of conventional thermopolymerization (microwave radiation PI chi ((3)) = 1.181 X 10(-13) esu: thermopolymerization PI chi ((3)) = 0.686 X 10(-13) esu). The experimental results also showed that the third-order optical nonlinearities were not only affected by the length of the polycondensate chain, but also the polycondensate pi -electron density distribution, which was explained and confirmed by computation with the semiempirical Austin method.