Crosslinking of Linear Polyimines Into Aminal-Linked Porous Organic Polymers for C₂ Hydrocarbon/Methane Separation

Hypercrosslinked polymers, a class of porous organic polymers (POPs), are constructed by crosslinking linear polymers or knitting small aromatic molecules with a molecular crosslinker, typically via Friedel-Crafts alkylation. In this study, we report a new approach for the synthesis of POPs via the crosslinking of linear polyimines with m‑phenylenediamine through nucleophilic addition of amines to imines, forming aminal linkages. The resulting aminal-linked POPs, with an estimated low cost of 16 USD/kg, exhibited high surface areas up to 650 m²/g and abundant microporosity, in contrast to the ∼100 m²/g observed for the linear polyimine. This method demonstrated good generality: four linear polyimines with different structures were successfully crosslinked into POPs with high porosity. Both experimental results and theoretical calculations indicate that the use of diamines at the meta-position is critical for efficient crosslinking, whereas para-diamines did not initiate crosslinking. With their high surface area and rich microporosity, these POPs displayed high adsorption capacities for C₂ hydrocarbons and CO₂, but significantly lower capacity for CH₄. Dynamic breakthrough experiments confirmed excellent separation performance for C₂ hydrocarbons/CH₄ mixtures, highlighting their potential for hydrocarbon separation. This study provides a new strategy for the synthesis of cost-effective POPs and demonstrates their promising applications in gas separation.