Synthesis and self-assembly of isotactic polystyrene-block-poly(ethylene glycol)

Isotactic polystyrene-block-poly(ethylene glycol) (iPS-b-PEG) was synthesized via a thiol–ene click coupling reaction of vinyl-terminated isotactic polystyrene (iPS–) with thiol-terminated poly(ethylene glycol) (PEG-SH). iPS– was prepared by the extremely highly isospecific polymerization of styrene with 1,4-dithiabutandiyl-2,2′-bis(6-tertbutyl-4-methylphenoxy) titanium dichloride and methylaluminoxane (MAO) in the presence of 1,7-octadiene as a chain transfer agent. PEG-SH was synthesized by the direct esterification of hydroxyl-terminated PEG (PEG-OH) with 3-mercaptopropionic acid using HfCl4·2THF as a catalyst. The behavior and micelle morphology of the crystallization-driven self-assembly of iPS-b-PEG in N,N-dimethylformamide (DMF) were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic light scattering (DLS), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The results revealed that the crystallinity of the iPS blocks in the micelle cores was increased with increasing the storage time of the micelle solution at room temperature, and the increase of crystallinity led to the transition from spherical to petal-like micelles. With time these petal-like micelles could self-associate into flower-like aggregates. Moreover, when the newly prepared micelle DMF solution of iPS-b-PEG was dialyzed against deionized water, bowl-like micelles were formed due to the diffusion of DMF from the iPS cores into the aqueous phase.