Aromatic polyimides and copolyimides containing bulky t-butyltriphenylmethane units

Triphenylmethane-based polyimides and copolyimides containing bulky t-butyl group (tBu) were obtained by one-step high temperature polycondensation of 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride with diamines of triphenylmethane (TPM) family. The polymers were obtained in quantitative yields with inherent viscosities of 0.45–0.80 dL/g. They exhibited high thermal stability with 5% weight loss above 500 °C and were cast in films with good mechanical properties capable of testing as gas separation membranes. All polyimides were readily soluble in polar aprotic solvents, and the solubility enhanced with the increase in tBu-group content. The amorphous, free-standing membranes were prepared from these polymers, and their permeabilities and selectivities to several gases were measured and discussed with respect to the structural differences in the polymers. It was shown that the presence of bulky tBu-units made the chain packing less efficient; free volume and d-spacing in the polyimides grew accordingly. As a consequence, the membranes with higher content of tBu-groups demonstrated improved permeabilities, showing 1.5–3.0 times higher permeability coefficients depending on the gas tested. The membranes’ separation performance was improved for CO/CH gas pair in comparison with that of structurally similar polyimides, while it did not change for O/N pair. Additionally, the mechanism of formation of triphenylmethane diamines in the reaction between aniline and benzaldehydes was investigated in order to optimize the monomer synthesis and to minimize possible side reactions. It was established that the secondary diamines, so-called aminals, were inevitable side products, particularly important in the condensation between aniline and tBu-benzaldehyde.
This work was supported by grants from DGAPA # IN107117 and by Spain’s MINECO (Projects MAT2016-76413-C2-R2, and MAT2016-76413-C2-R1). The authors thank G. Cedillo Valverde, M.A. Canseco Martinez, E. Hernandez Mecinas, E. R-Morales and A. Tejeda Cruz (all from IIM-UNAM) for different kinds of analyses and also would like to acknowledge Sara Rodriguez for the gas separation measurements. R.A.C.B. is grateful to CONACyT for the financial support.