Better known as alkylating agents, Grignard reagents are investigated as deprotonating agents of an alcohol to generate magnesium alkoxides for the initiation and propagation of the anionic ring-opening polymerization of propylene oxide. By using an excess of tri- isobutylaluminum, this magnesium–aluminum system enables a full conversion polymer- ization in a few hours yielding controlled poly(propylene oxide) up to 10 000 g/mol with relatively low dispersity. Characterizations by NMR and MALDI-ToF-mass spectrometry allowed the determination of the chain-ends and therefore the associated initiation mech- anisms. This study revealed that propylene oxide can be polymerized in presence of a magnesium-based counter-ion. Concomitant initiations by alkoxide, halide and hydride are discussed.