| Abstract | The well-known spirocyclic ladder polymer of intrinsic microporosity (PIM-1) [1] was modified using a “click chemistry” [2 + 3] cycloaddition reaction to yield novel PIMs containing tetrazole units. The complete conversion of the nitrile to tetrazole groups on PIM-1 with sodium azide and zinc chloride was achieved without chain degradation. As far as we are aware, this is the first report of the conversion of a polymeric aromatic nitrile to tetrazole. The resulting tetrazole-substituted PIM (TZ-PIM) was characterized by Fourier transform infrared spectroscopy (FTIR), and proton nuclear magnetic resonance (1H NMR spectra), and a methyl-TZ-PIM derivative allowed further elucidation by gel permeation chromatography (GPC), and other methods. A series of TZ-PIMs containing various amounts of tetrazole substituents had distinctly better solubility in protic solvents (such as DMF, DMSO, DMAc) compared with the nitrile-containing PIM-1 precursor. The TZ-PIM series were evaluated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), which showed that the polymeric tetrazoles had an onset of decomposition at ~190 °C. Compared with PIM-1, the TZ-PIM series showed higher selectivity for pure gas pairs, such as CO2/N2 and for mixed CO2/N2, with a corresponding decrease in permeability. Good selectivity coupled with high permeability combines to exceed the Robeson upper-bound (2008) performance limit for pure gas pairs CO2/N2 . Additionally, the CO2/N2 mixed gas with different feed compositions surprisingly shows even higher selectivity with moderate permeability, exceeding the Robeson upper-bound (2008) by a greater extent. This could be rationalized by preferential selective sorption of CO2 in TZ-PIM, enhanced by strong interaction with tetrazole, thereby hindering the transport of N2 in gas mixtures [2]. This work extends the spectrum of PIMs beyond those reported previously and also demonstrates that significant improvements in gas transport properties may be achieved through post-modification of PIM materials containing nitrile groups. |
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