Formation of the intermediate cyclic six‐membered oxonium ion in the cationic polymerization of β‐propiolactone initiated with CH3CO⊕SbF ⊖6

Analysis of the 1H NMR spectra of /β-propiolactone and of the model systems /(CH3)2O and /CH3COOCH3 in liquid SO2 in the temperature region −70 to −20°C revealed that the reaction of the acetylium cation with β-propiolactone leads to the cyclic six-membered oxonium ion 4, participating as an intermediate in the initial stage of polymerization.