Onium salts such as quaternary ammonium and phosphonium halides in conjunction with sterically hindered methylaluminum diphenolates (1) brought about living coordinate anionic polymerization of oxetane, giving a narrow molecular weight distribution (MWD) polyether. In contrast, use of a simple trialkylaluminum such as trimethylaluminum in the place of 1 resulted in no polymerization. X-ray crystallographic studies demonstrated the formation of an ate complex (2b(Cl)) from 1b and Et(4)NCl. 2 in the presence of 1 was capable of initiating the controlled polymerization of oxetane, whereas 2 alone was not. H-1 NMR studies on a mixture of 2b(Cl) and 1b indicated the occurrence of a rapid, reversible migration of the halide group from 2b(Cl) to 1b.