Reversible Switching and Recycling of Thermoresponsive 1,2,4-Triazolium-Based Poly(ionic liquid) Catalysts for Porous Organic Cage Synthesis in Organic Media

Homogeneous catalysts of high activity and selectivity often face challenges in the separation from feedstocks and products after reactions. In contrast, heterogeneous catalysts are easier to separate, usually at the cost of compromised catalytic performance. By designing catalysts capable of switching between homogeneous and heterogeneous states for catalysis and separation, the merits of both could be synergistically combined. In this study, a thermoresponsive 1,2,4-triazolium-based poly(ionic liquid) (PIL) was applied as a temperature-switchable organocatalyst for the controlled synthesis of porous organic cages in methanol. Variation of the reaction temperature induced a phase transition of the PIL, causing the polymer chains to dissolve or collapse in methanol, thereby exposing or shielding the catalytically active sites to proceed or retard the reaction, respectively. To note, at a sufficiently low temperature, the PIL as a catalyst precipitated out of its methanol solution and could be separated by centrifugation or filtration for reuse, similar to common heterogeneous catalysts. Such switchable and recyclable properties of polymeric catalysts will inspire the design of efficient and adaptable organic or hybrid nanoreactors in liquid media.