Development of unctional membranes that are capable of selectively recognizing and transporting ions have key importance for the recovery and separation of specific icons (ed K+, Li+, Na+) from multicomponent mixtures. In this thesis, new membrane materials based on crown ether-metal ion host-guest interactions are developed. Crown ethers are widely used as guest molecules for ions and are able to bind selectively specific cationic species. The main focus of this thesis is the direct incorporation of crown ether units into the polymer main chain, as repeating unites, in contrast to the blending of crown ethers with polymers or postfunctionalization of linear polymers. A major disadvantage of the two last options is the lower effectiveness or washing out of the selective crown ether. With the inchain crown ether unites, crown ether polymers show unique membrane functions, ie the selective binding of a specific ion to a very high degree of specificity.