AbstractThe triphenylmethyl hexafluoroarsenate‐initiated cationic polymerization of 1,2‐butylene oxide in dichloroethane between −25 and +25°C and its sodium‐initiated anionic polymerization in bulk at 20°C have been carried out. Gel permeation chromatography (GPC) molecular weight distribution curves of the reaction products are multinodal. Nuclear magnetic resonance (NMR) and infrared (IR) analyses show that the cationically prepared polymers are composed of cyclic oligomers and linear high‐molecular‐weight products, while the anionically prepared polymers contain only linear products some of which include double bonds. NMR analyses further reveal that the cationically prepared polymers consist of monomer repeat units, while the anionically prepared polymers are essentially made of side products originating from the reaction of 1,2‐butylene oxide with the sodium mirror used as the an