Why can crown ethers and potassium ions combine tightly?

Crown ethers and potassium ions can bind tightly. The biggest feature of crown ethers is that they can complex with positive ions, especially alkali metal ions. Crown ether is a macrocyclic polyether containing multiple oxygen-methylene structural units in the molecule. Common crown ethers include 15-crown-5 and 18-crown-6. The hole structure of the crown ether has a selective effect on ions and can be used as a catalyst in organic reactions. Crown ethers are somewhat toxic, and inhalation of their vapor or contact with skin must be avoided.