A series of new aromatic polyimides (PIs) and co-PIs containing bulky tert-butyl phenoxy group was synthesized by one-step high-temperature polycondensation of 1,3-diamino-4-(4′- tert-butylphenoxy)benzene ( tBuPDAB) with different commercially available aromatic dianhydrides. The polymers were obtained in quantitative yields with inherent viscosities of 0.40–0.70 dL g−1. They exhibited high thermal stability with 10% weight loss above 480°C and were cast in films with good mechanical properties capable to be tested as gas separation membranes. These PIs were compared with analogs bearing phenoxy group (PDAB). The incorporation of tBu improved the solubility of the PIs, their free volume fraction, d-spacing, and gas permeability coefficients in comparison with their analogs obtained from PDAB. The permeability enhancement was from 2.5 to 8 times depended on the gas tested. The PI, based on tBuPDAB and 4,4’-(hexafluoroisopropylidene)diphtalic anhydride and, thus, containing two different bulky pendant groups, showed the highest gas permeability coefficient for CO2 (58.3 Barrer) and moderate ideal selectivity to the gas pair CO2/CH4 ( α = 18.0).
The influence of trifluoromethanesulfonic (TFSA) superacid on conditions of the synthesis of polybenzimidazoles, such as OPBI and CF3PBI, was studied. It was shown that the polycondensations proceeded smoother and at lower temperatures in the presence of the TFSA in Eaton’s Reagent and that polymers of high molecular weights, and readily soluble in organic solvents, were obtained. The effect was more pronounced for CF3PBI, where the low reactivity monomer, 4,4′ (hexafluoroisoproylidene)bis (benzoic acid), was used. CF3PBI was obtained at a moderate temperature of 140 °C with no gel fraction and exhibited an inherent viscosity twice higher than the one obtained by the traditional method. In fact, the addition of TFSA allows for the obtention of soluble N-phenyl substituted CF3PBI by direct synthesis, which had not been obtained otherwise. Thus, the use of TFSA is a good media for the synthesis of N-substituted PBIs under relatively mild conditions.
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