Water-Soluble Copolypeptides with Oligo-Ethylene-Glycol and Benzyl Pendants: Synthesis, Characterization, and Thermoresponsive Properties

A series of random copolypeptides, i.e. poly(γ-propyl-l-glutamate)-graft-(oligo-ethylene-glycol)-random-poly(γ-benzyl-l-glutamate)s (PPLG-g-OEG3-ran-PBLG) with similar main chain lengths and various compositions of OEG and benzyl pendants were synthesized in three steps, including ring-opening polymerization of γ-chloropropyl-l-glutamic acid-based N-carboxyanhydride (CPLG-NCA) and γ-benzyl-l-glutamic acid-based N-carboxyanhydride (BLG-NCA), nucleophilic substitution in the presence of NaN3, and copper-mediated alkyne–azide 1,3-dipolar cycloaddition. 1H NMR, Fourier transform infrared spectroscopy, and gel permeation chromatography results confirmed the molecular structures of the resulting copolypeptides and characterized their molar masses, molar mass distributions, and the OEG/benzyl compositions. Fourier transform infrared spectroscopy and circular dichroism analysis revealed that PPLG-g-OEG3-ran-PBLG samples adopted α-helical conformations both in aqueous solution and solid state. Variable-temperature UV-visible spectroscopy revealed that PPLG-g-OEG3-ran-PBLG samples with high OEG molar content (x ≥ 0.57) possessed lower critical solution temperature-type phase transitions in water. The cloud point temperature, in the range of 22–53°C, can be readily obtained by controlling the OEG/benzyl composition.