Novel polyhydantoins (PHYs) were synthesized from original aromatic diisocyanates and bisiminoacetates by a two-step polycondensation procedure, which involved the cyclization of polyurea intermediates promoted by acid catalysis. The physical properties of the novel PHYs were evaluated by comparing them with a classical PHY derived from 4,4-methylenediphenyl diisocyanate. All PHYs were soluble and could be processed into dense films, which showed good mechanical properties (tensile strength up to 110 MPa) and thermal stability of >400 degrees C. High glass transition temperatures (T(g)s), ranging from 260 to 410 degrees C, were observed. Fractional free volume (FFV) was strongly dependent on the chemical structure, and a linear correlation between gas permeability and FFV of PHYs could be found. The gas separation properties were comparable to those of the commercial polyimide Matrimid((R)), with the exception of one of the PHYs which exhibited very promising properties as its gas productivity was comparable to the gas separation performance of well-established experimental polyimides. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4052-4060