Abstract
Two series of novel intrinsically microporous polyimides were synthesized from 9,9'-spirobifluorene-2,2'-diamine (SBF) and its bromine-substituted analogue 3,3'-dibromo-9,9'-spirobifluorene-2,2'-diamine (BSBF) with three different dianhydrides (6FDA, PMDA, and SPDA). All polymers exhibited high molecular weight, good solubility in common organic solvents, and high thermal stability. Bromine-substituted polyimides showed significantly increased gas permeabilities but slightly lower selectivities than the SBF-based polyimides. The CO2 permeability of PMDA BSBF (693 Barrer) was 3.5 times as high as that of PMDA SBF (197 Barrer), while its CO2/CH4 selectivity was similar (19 vs 22). Molecular simulations of PMDA SBF and PMDA BSBF repeat units indicate that the twist angle between the PMDA and fluorene plane changes from 0 degrees in PMDA SBF to 77.8 in PMDA BSBF, which decreases the ability of the polymer to pack efficiently due to severe steric hindrance induced by the bromine side groups.