Self-assembled structures from PEGylated polypeptide block copolymers synthesized using a combination of ATRP, ROP, and click chemistry

Abstract

In this study, we used a combination of atom transfer radical polymerization, ring opening polymerization, and click chemistry to synthesize new PEGylated polypeptide block copolymers of polystyrene-b-poly(γ-propargyl-L-glutamate-g-ethylene oxide) [PS-b-(PPLG-g-MEO₂)]. We employed Fourier transform infrared spectroscopy and wide-angle X-ray diffraction (WAXD) to determine the secondary structures of the α-helical conformations of these PEGylated polypeptide block copolymers in the solid state, and circular dichroism spectroscopy to characterize them in solution. Hierarchical self-assembly of the PS-b-(PPLG-g-MEO₂) diblock copolymers in the bulk state (characterized using WAXD, small-angle X-ray scattering, and transmission electron microscopy) led to the formation of lamellar structures as a result of microphase separation of the diblock copolymers; the hexagonal cylinder packing nanostructure featured α-helical conformations of PEGylated polypeptide segments, which were oriented perpendicular to the director of the lamellar structure formed by the diblock copolymers. We obtained a range of micellar structures from the PS-b-(PPLG-g-MEO₂) diblock copolymer in DMF (common solvent) upon the addition of selective solvents (H₂O or MeCN). We observed only spherical micelles from the PS-b-(PPLG-g-MEO₂) diblock copolymer in DMF–MeCN systems at all MeCN compositions. The micelle structures changed, however, from spherical structures to long wormlike, vesicles, and large compound micelles in the DMF–H₂O system upon increasing the content of water. These different morphology transformations arose as a result of very different PS–solvent (H₂O or MeCN) interaction parameters.