Hydrophobic Collapse of a Poly(ethylene Oxide)–Poly(propylene Oxide) Alternating Multiblock Copolymer in Water

The morphology of single chains and micelles formed by a poly(ethylene oxide) (PEO)–poly(propylene oxide) (PPO) alternating multiblock (AMB) copolymer in water was investigated using scattering techniques. Two series of PEO–PPO AMB copolymers, (EO₂₂₀PO₃₃)_n and (EO₆₈PO₃₃)_n─where EO and PO represent ethylene oxide and propylene oxide units, respectively, and n denotes the number of repeating PEO–PPO units─were synthesized with varying overall chain lengths while maintaining a 1:1 block ratio of PEO to PPO. Small-angle X-ray scattering (SAXS) and dynamic light scattering revealed that both (EO₂₂₀PO₃₃)_n and (EO₆₈PO₃₃)_n adopt a collapsed conformation in water at ambient temperature. Interestingly, the morphology of the AMB copolymers changes with n: in both series, the polymers exhibit increased shrinkage in water as n increases. AMB copolymers possessing longer PEO chains undergo a more pronounced shrinkage. This aligns with the theoretical prediction for the hydrophobic collapse in protein folding [Y. Harano and M. Kinoshita, Biophys. J. 2005, 89, 2701]. As the temperature increases, AMB copolymers associate to form micelles. The SAXS profiles for (EO₂₂₀PO₃₃)_n are independent of n, indicating that the size and morphologies of micelles remain consistent. A similar trend is observed for (EO₆₈PO₃₃)_n. These results suggest that the entropy gain of the surrounding water─resulting from the association of a specific number of PPO blocks─compensates for the entropy loss associated with micelle formation.