The cationic polymerization of phthalimidomethyloxirane (1) with methylaluminum bis-(2,6-di-tert-butyl-4-methylphenoxide) (MAD) afforded polyether (2) or polyacetal (3) depending on temperature. Acetalic linkages in the main chain of 3 were formed via isomerization ring opening involving the intramolecular nucleophilic attack of the imide-carbonyl group at the growing end. The polymerization system in toluene at 25 degreesC became heterogeneous during the polymerization, but the resulting 3 nevertheless had a narrow molecular weight distribution and a degree of polymerization nearly equal to the feed ratio of 1 to MAD. This is the first example of a precipitation polymerization in which the growth to nearly uniform chain lengths was controlled over a very narrow molecular weight range by the physical solubility equilibrium of growing species between solution and precipitates, similar to chemical equilibria between reactive growing ends and dormant covalent species.