Greener Synthesis of the Polymer of Intrinsic Microporosity PIM-1 for Gas Separation

Abstract

Polymers of intrinsic microporosity (PIMs) are studied as membranes for energy-efficient and environmentally friendly separation technologies, but greener polymerization methods are desirable for further scale up. This work aimed to synthesize the prototypical PIM (PIM-1) via a greener synthetic route by changing the solvent system to methyl-5-(dimethylamino)-2,2-dimethyl-5-oxopentanoate (MDDOP), a structural analogue of the green solvent Rhodiasolv PolarClean. Mass-based green metrics analysis was performed on MDDOP, determining atom economy, complete environmental factor, and total carbon intensity, comparing each to synthetic routes to PolarClean. Green metrics analysis found MDDOP synthesis produced less waste than PolarClean. MDDOP solvent capabilities were exemplified via PIM-1 polymerizations using 5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetramethyl-1,1′-spirobisindane (TTSBI) with either tetrafluoroterephthalonitrile (TFTPN) or tetrachloroterephthalonitrile (TCTPN), varying the temperature (120–160 °C) and reaction duration (50 min–6 h). Recovery of methanol and MDDOP post PIM-1 synthesis reduced solvent waste by 22%. Reactions using TCTPN produced polymers with higher molar masses than those produced using TFTPN. All samples showed varied topology, with evidence of branching and colloidal network. The polymer from the most successful reaction conditions (TCTPN, T = 140 °C, 6 h) was fabricated into thick film membranes and tested with pure gases for CO₂/CH₄ and CO₂/N₂ gas pairs, performing comparably with PIM-1 synthesized using conventional solvent systems.