Environmentally responsive micelles from polystyrene-poly[bis(potassium carboxylatophenoxy)phosphazene] block copolymers

Amphiphilic diblock copolymers that contained hydrophilic poly[bis(potassium carboxylatophenoxy)phosphazene] segments and hydrophobic polystyrene sections were synthesized via the controlled cationic polymerization of Cl ₃P=NSiMe ₃ with a polystyrenyl-phosphoranimine as a macromolecular terminator. These block copolymers self-associated in aqueous media to form micellar structures which were investigated by fluorescence spectroscopy, dynamic light scattering, and transmission electron microscopy. The size and shape of the micelles were not affected by the introduction of different monovalent cations (Li ⁺, K ⁺, Na ⁺, and Cs ⁺) into the stable micellar solutions. However, exposure to divalent cations induced intermicellar crosslinking through carboxylate groups, which caused precipitation of the ionically crosslinked aggregates from solution. This micelle-coupling behavior was reversible: the subsequent addition of monovalent cations caused the redispersion of the polystyrene-block- poly[bis(potassium carboxylatophenoxy)phosphazene] (PS-KPCPP) block copolymers into a stable micellar solution. Aqueous micellar solutions of PS-KPCPP copolymers also showed pH-dependent behavior. These attributes make PS-KPCPP block copolymers suitable for studies of guest retention and release in response to ion charge and pH.