Functional star block copolymers with a cholane core: Thermo-responsiveness and aggregation behavior

Star block copolymers composed of hydrophilic poly(ethylene glycol) (PEG) as an inner block and tuneable poly(allyl glycidyl ether) (PAGE) as an outer block were made by sequential anionic polymerization on a cholane core. The 4-armed CA(EGn-b-AGEm)4 star block copolymers show dual thermo-responsive behavior in aqueous solutions, where the two cloud points (CPs) vary in the range of 10-34 °C for the first CP and 16−39 °C for the second CP, depending on the length of the PEG and PAGE segments. 1H NMR spectroscopy, dynamic light scattering and atomic force microscopy all indicate a two-step aggregation process of flower-like micelles, where the micelles form through the association of the hydrophobic PAGE blocks and the bile acid core. The allyl groups of the star copolymers were further functionalized by radical thiol-ene coupling to introduce NH2 or COOH groups in the outer block. As a result, star-shaped block copolymers with dual pH- and thermo-sensitivity were obtained with different aggregation characteristics corresponding to the pKa of the functionalized blocks. Interestingly, the deprotonated NH2-bearing polymers showed dual thermo-responsive characteristics different from their allyl group-bearing precursor.