Molecular dynamics (MD) simulations are undertaken on a series of five copolyimides based on two different dianhydrides: the flexible 4,4'-oxydiphthalic dianhydride (ODPA) and the rigid bulky bicyclo(2.2.2)-oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA). The diamines are respectively 4,4'oxydi(phenylamine) (ODA), 2-trifluoride-4,4'-oxydi(phenylamine) (CF(3)ODA) and 2-methoxy-4,4'-oxydi(phenylamine) (MeOODA). These are potential candidates for gas separation membranes, and the effects of increasing BCDA content in ODPA/BCDA copolyimides as well as adding trifluoromethyl or methoxy substituents on the ODA are studied at the molecular level. Amorphous long-chain models are built using a hybrid pivot Monte Carlo/MD sampling preparation procedure. The reproducibility of this approach is tested on a series of eight independently prepared systems. Densities, cohesive energies, Hildebrand parameters, conformational. characteristics, intermolecular structures, and the available void spaces are analyzed for each system under study. Both the BCDA moiety and the trifluoromethyl substituent on the diamine are found to have similar consequences on the properties of the copolyimide by decreasing chain cohesion and increasing the available void space. This is related to the steric effect of the BCDA dianhydride, while the trifluoromethyl combines both steric and electronic repulsion. The steric effect of the methoxy substituent on the diamine is not strong enough to significantly differ from the unsubstituted system.