Novel Spirobifluorene- and Dibromospirobifluorene-Based Polyimides of Intrinsic Microporosity for Gas Separation Applications

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 CO 2 permeability of PMDA–BSBF (693 Barrer) was 3.5 times as high as that of PMDA–SBF (197 Barrer), while its CO 2 /CH 4 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° 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.