AbstractIn the course of further investigations of ring‐opening polymerizations we have studied the bulk polymerization of β‐methylglycidaldehyde (βMG) by aluminum isopropoxide at 20 and 50°C.The polymerization proceeds in two distinct stages. In the first stage the α.β‐epoxybutyl ester of α.β‐epoxybutyric acid (2.3‐epoxy‐n‐butyl‐1′.2′ ‐epoxybutyrate) (DßMG) is formed by a TISHCHENKO‐CLAISEN reaction.In the second stage this ester gives a polyether (PßMG) by polymerization of the epoxy groups adjacent to the ester carbonyl. The pendent epoxy‐butyl groups are attached to the polyether by an ester linkage. Mechanisms are proposed for