Cation binding properties of crown ether‐modified polyethylenimines

Crown ether‐modified polyethylenimines (PEI‐CR) were prepared to elucidate their cation binding efficiency and selectivity by liquid‐liquid and solid‐liquid extraction methods. With alkali metal cations it was found that polyethylenimines containing benzo‐15‐crown‐5 moieties (Ia and Ib) extracted all cations examined more efficiently than their monomeric analogs, especially in the cases of K+ and Rb+ cations. The solid‐liquid extraction revealed clearly that the polymers complexed with K+ and Rb+ cations according to a 1:2 cation‐to‐crown stoichiometry. The high selectivities of the polymers for K+ and Rb+ were interpreted in terms of the favourable conformation of the polymer chain for the formation of intramolecular 1:2 complexes. On the contrary, these polymers bound alkali earth cations less effectively with low selectivities.