The action of various external stimuli on telechelic poly(ethylene oxide) bearing iron(III) tetraphenylporphyrin chloride or iron(II) tetraphenylporphyrin derivatives at their ends has been investigated in organic and water media. Fully reversible chain end coupling processes were shown to take place yielding bis-iron(III) mu-oxo-tetraphenylporphyrin poly(ethylene oxide) under changes in acido-basic conditions, solvent nature, or the action of redox additives. At low concentration macrocyclic poly(ethylene oxide)s of same molar mass as the linear poly(ethylene oxide) precursors are almost exclusively formed via intramolecular coupling whereas increasing the poly(ethylene oxide) concentration yields both intra-and intermolecular chain coupling and formation of large macrocycles with [bis-iron(III) mu-oxo-tetraphenylporphyrin poly(ethylene oxide)] repeating units. The mechanisms involved are discussed.