A new diamine monomer containing rigid planar fluorenone moiety, 2,7-bis(4-aminophenyl)-9H-fluoren-9-one, was synthesized through Suzuki coupling reaction. Then it was reacted with pyromellitic dianhydride to obtain a polyimide (FOPPI) via a conventional two-step polymerization process. The prepared FOPPI exhibits excellent barrier properties, with the oxygen transmission rate and water vapor transmission rate low to 3.2 cm(3).m(-2).day(-1) and 2.9 g.m(-2).day(-1), respectively. The results of wide angle X-ray diffractograms, positron annihilation lifetime spectroscopy, and molecular dynamics simulations reveal that the excellent barrier properties of FOPPI are mainly ascribed to the crystallinity, high chain rigidity, and low free volume, which are resulted from the rigid planar moiety. FOPPI also shows outstanding thermal stability and mechanical properties with a glass transition temperature up to 420 degrees C, 5% loss temperature of 607 degrees C, coefficient of thermal expansion of 1.28 ppm K-1, and tensile strength of 150.8 MPa. The polyimide has an attractive potential application prospect in the flexible electronics encapsulation area. (C) 2017 Wiley Periodicals, Inc.