A novel ester diamine, 9-(3,5-diaminobenzoyloxy) fluorene, as a new monomer for preparation of polyimides was synthesized via two successive reactions. In the first step, reaction of 3,5-dinitrobenzoylchloride with 9-hydroxy fluorene in the presence of sodium hydroxide led to preparation of 9-(3,5-dinitrobenzoyloxy) fluorene. Second reaction was reduction of the nitro groups by tin (II) chloride and fuming hydrochloric acid to produce 9-(3,5-diaminobenzoyloxy) fluorene. The new diamine containing bulky fluorene group was characterized and polycondensed with different dianhydrides via two methods to produce polyimides. The new five-membered and six-membered ring polyimides were characterized and their properties including solubility behavior, inherent viscosity, thermal behavior and stability, and crystallinity were studied. They exhibited favorable balance of physical and thermal properties and their solubility were improved without sacrificing their thermal stability. Six-membered rings polyimides showed higher thermal stability and lower solubility in comparison to related five-membered ring polyimides. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012