Synthesis, conformation transition, liquid crystal phase, and self-assembled morphology of thermosensitive homopolypeptide

Thermosensitive diethylene glycol-derived poly( L -glutamate) homopolypeptides (i.e., poly- L -EG 2 -Glu) with different molecular weights (MW) ( M n,GPC = 5380–32520) were synthesized via the ring-opening polymerization (ROP) of EG 2 - L -glutamate N -carboxyanhydride (EG 2 -Glu-NCA) in N , N -dimethylformamide solution at 50 °C. Their molecular structure, conformation transition, liquid crystal (LC) phase behavior, lower critical solution temperature (LCST) transition, and morphology evolution were thoroughly characterized by means of FTIR, 1 H NMR, gel permeation chromatography, differential scanning calorimetry, wide angle X-ray diffraction, polarized optical microscope, transmission electron microscope, and dynamic light scattering. In solid state, the homopolypeptide poly- L -EG 2 -Glu presented a conformation transition from α-helix to β-sheet with increasing their MW at room temperature, while it mainly assumed an α-helix of 80–86% in aqueous solution. Poly- L -EG 2 -Glu showed a thermotropic LC phase with a transition temperature of about 100 °C in solid state, while it gave a reversible LCST transition of 34–36 °C in aqueous solution. The amphiphilic homopolypeptide poly- L -EG 2 -Glu self-assembled into nanostructures in aqueous solution, and their critical aggregation concentrations decreased with increasing MW. Interestingly, their morphology changed from spherical micelles to worm-like micelles, then to fiber micelles with increasing MW. This work provides a simple method for the generation of different nanostructures from a thermosensitive biodegradable homopolypeptide. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012