The precipitate formed form NA capped PMR (polymerization of the monomer react'ants) polyimide solution was isolated and identified. The rate constants of reaction of NA monoethyl ester and 3
3"
4
4'-benzophenone tetracarboxylie acid diethyl ester with 4
4'-diamino diphenyl methane were measured. The stability of resin solution and structure of prepolymer were also discussed.