This compound is prepared by a modified Williamson ether synthesis in the presence of a templating cation:(CHOCHCHCl)  +   (CHOCHCHOH)  +  2 KOH   →   (CHCHO)  +  2 KCl  +  2 HO
It can be also prepared by the oligomerization of ethylene oxide. It can be purified by distillation, where its tendency to supercool becomes evident. 18-Crown-6 can also be purified by recrystallisation from hot acetonitrile.  It initially forms an insoluble solvate. Rigorously dry material can be made by dissolving the compound in THF followed by the addition of NaK to give [K(18-crown-6)]Na, an alkalide salt.  
Crystallographic analysis reveals a relatively flat molecule but one where the oxygen centres are not oriented in the idealized 6-fold symmetric geometry usually shown.  The molecule undergoes significant conformational change upon complexation.