Formation of Sulfobetaine-Containing Entirely Ionic PIC (Polyion Complex) Micelles and Their Temperature Responsivity

Sulfobetaine, a type of zwitterionic polymer, is highly biocompatible with temperature responsiveness of the upper critical solution temperature (UCST) type. The objective of this research was to construct polyion complex (PIC) micelles in the shell of sulfobetaine that had these properties. We used poly(sulfopropyl dimethylammonium propylacrylamide) (PSPP) as sulfobetaine, poly(sodium styrenesulfonate) (PSSNa) as the anionic polymer, and poly[3-(methacrylamido)propyl trimethylammonium chloride] (PMAPTAC) as the cationic polymer. The fundamental properties of the sulfobetaine-containing polymer and the complex were investigated to construct micelles in which the corona expands and contracts in response to temperature changes. Changes in the cloud point were observed from the transmittance for sulfobetaine homopolymers with different degrees of polymerization and concentration and aqueous solution of temperature-responsive diblock copolymers with different concentrations. The concentration and degree of polymerization dependencies on temperature responsivity were determined. Then we mixed two diblock copolymer aqueous solutions that did not have temperature responsivity so that the charge number of anions and cations became equal, and the temperature responsivity and the formation of micelles were confirmed from ¹H NMR, DLS, and transmittance. This confirmed the formation of PIC micelles with temperature responsivity. The diblock copolymer did not have temperature responsivity due to the influence of the block ratio by introduction of the ionic chain. However, it is considered to have temperature responsivity because the ionic chain becomes the core when PIC micelles are formed. Furthermore, the PIC micelles with temperature responsivity also had a degree of polymerization and concentration dependencies.