The mechanism of solution polyeondensation of MDl with TMA was preliminarily studied with model compound．by means of Semi-quantitative analysis of infra-red spectroscopy and viscosity．It is found that the polycondensation is an irreversible reaction and its important charaeteristic is that amido and imido groups are formed at the flame time．The viscosity of the solution increases with the increasing of MDI in excess and eventually gel

ation takes place． All the solvents which contain actire hydrogen or unshared electrons are not suitable for this reaction due to the possibi-1ity of addition reaetion with MDI．When the water content of the solvent used is over 0.16％，the polycondensation reaction can not normally proceed．Because of the absence of labile polyamide-acid and only CO2 evolving during the polyeondensation，the storage stability of the condensation solution shows good．The high temperature treatment of the polycondensate induces the further com-pletion of imidezation and changing of the structure of molecular aggregation，and thus causes the molecular weight to increase continuously．The additiun of a certain amount of epoxide resin of low molecular weight into the polyconsation system not only reduces the fabrication temperature of the PAI about 80℃。but also improves both the property of film formed and its storage stability．