Polypeptide diblock copolymers: Syntheses and properties of poly(N-isopropylacrylamide)-b-polylysine

Abstract

A hydrolysis-resistant amide-linkage heterofunctional initiator was synthesized and used successfully for polymerization of well-defined rod-coil block copolymers poly(N-isopropylacrylamide)-b-poly(Z-L-lysine) (PNIPAm-b-PZLys) by combination of atom transfer radical polymerization (ATRP) and amine hydrochloride mediated ring-opening polymerization (ROP). The ATRP of NIPAm was carried out at 0 degrees C using CuBr/Me6TREN complex in 2-propanol and resulted in narrow polydispersity and high monomer conversion. The amine hydrochlorides have replaced the primary amine in the PNIPAm macroinitiator resulting in a well-controlled ROP of N-epsilon-(carbobenzoxy)-L-lysine N-carboxyanhydride in DMF at 20 degrees C. These amphiphilic block copolymers are able to form universal micelle morphologies of spherical micelles, wormlike micelles, and vesicles by varying the polymer compositions and the helicogenic common solvents. From synchrotron SAXS, WAXS, and TEM results, the PNIPAm-b-PZLys microphase self-assembly morphology in solid state is a hierarchical lamellar-in-hexagonal structure. After the protective epsilon-benzyloxycarbonyl group is removed, the dual stimuli-responsive behaviors of the PNIPAm-b-PLys investigated by nuclear magnetic resonance spectroscopy in aqueous solution resulted in either coil-to-helix or coil-globule transition by changing the environmental condition of elevating the temperature or increasing the pH value.