Our earlier report described that a cyclic hemoglobin (Hb) monomer with two beta subunits of a Hb molecule (alpha(2)beta(2)) bound through a flexible polyethylene glycol (PEG) chain undergoes reversible supramolecular ring-opening polymerization (S-ROP) to produce a supramolecular Hb polymer with a Hb-PEG alternating structure. In this work, we polymerized cyclic Hb monomers with different ring sizes (2, 5, 10, or 20 kDa PEG) to evaluate the thermodynamics of S-ROP equilibrium. Quantification of the produced supramolecular Hb polymers and the remaining cyclic Hb monomers in the equilibrium state revealed a negligibly small enthalpy change in S-ROP (Delta H-p <= 1 kJ.mol(-1)) and a markedly positive entropy change increasing with the ring size (Delta S-p = 26.8-33.2 J.mol(-1).K-1). The results suggest an entropy-driven mechanism in S-ROP: a cyclic Hb monomer with the larger ring size prefers to form a supramolecular Hb polymer. The S-ROP used for this study has the potential to construct submicrometer-sized Hb-PEG alternating polymers having structural regularity.