Synthesis and self-assembly in bulk of star-shaped block copolymers based on helical polypeptides

A series of star-shaped copolymers containing poly (styrene) (PS) and poly(γ-benzyl-l-glutamate) (PBLG) were synthesized by click reaction from alkyne- and azide-functionalized homopolymers. The α-azide PS star-shaped homopolymer was synthesized by copper-mediated atom transfer radical polymerization from a bromine-containing star-shaped initiator, and α-alkyne PBLG homopolymers were synthesized by ring-opening polymerization of γ-benzyl-l-glutamate N-carboxyanhydride with an amino-containing α-alkyne initiator. The molecular structures of the homopolymers and star-shaped block copolymers were confirmed by 1H nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy (FTIR), and gel permeation chromatography analysis. The self-assembling behavior of the star-shaped block copolymers in bulk was investigated using wide-angle X-ray diffraction, and small-angle X-ray scattering. For star-shaped block copolymer at lower fPS of ~0.27, PBLG segment was assigned to a hexagonally packed-cylinder structure (ΦH) based on the rod-like α-helix conformation as shown in FTIR spectra. By increasing fPS to ~0.42, a proposed microphase-separated double-hexagonal morphology was observed, in which PBLG rods formed the core of the columns by interdigitated packing in ΦH phase with PS domain as the matrix in a larger hexagonal columnar structure. The proposed structure was based on calculation from the simulated molecular length of each block of the copolymer and experimental analyses.