Gradient poly(ionic liquid)s with sequence modulation for catalytic CO2 conversion into cyclic carbonates

The reaction to synthesize cyclic carbonates is an efficient and green technical route to utilize CO2. Poly(ionic liquid)s (PILs) have attracted much attention for many active sites carried for efficient catalysis. However, a uncontrollable distribution and insufficient exposure of active sites in traditional PILs also leads to low catalytic activity. In order to fully utilize the active sites to achieve high catalytic activity, a series of PILs with different monomer sequence have been developed through monomer alkyl chain length regulation. Through shortening the chain length of the alkyl group of the ionic liquid monomer, a gradient structure is formed with ionic liquids enriched on the surface, which can be easily stretched out in the reaction solution and thus fully utilized. Among the prepared PILs, PILs with alkyl chain of 4 carbons (P[C4VIM][Br]) was the best catalyst, exhibiting a higher conversion (96 %) of propylene oxide (PO) under optimal conditions. Its high activity stems from the fact that the active sites are densely distributed on the surface composed of long flexible chain segments, which can be fully extended in the reaction system, facilitating the contact with the reacting molecules. Further XPS analysis reveals that as the chain length decreases, Br− is more readily freed from the catalyst thus enhancing the catalytic activity. This work provides a new strategy to regulate the structure of PILs to enhance catalytic activity.