Synthesis of thermoresponsive unimolecular polymeric micelles with a hydrophilic hyperbranched poly(glycidol) core

Abstract

This paper describes the reversible phase transition behavior of a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) shell at the surface of a hydrophilic core. Reversible addition-fragmentation transfer (RAFT) polymerization of N-isopropylacrylamide was conducted using a hydrophilic hyperbranched poly(glycidol) (HPG)-based macroRAFT agent. At lower temperatures (<30 °C), the resultant multiarm star block copolymer (HPG–PNIPAM) exists as unimolecular micelles, with hydrophilic HPG as the core and a densely grafted PNIPAM brush as the shell. In laser light scattering (LLS) studies, the concentration used for HPG–PNIPAM is 5 × 10−6 g ml−1, to avoid any possible aggregation between dendritic unimolecular micelles above the lower critical solution temperature (∼32 °C) of PNIPAM. What we observe for the phase transition of HPG–PNIPAM involves only unimolecular process. A combination of dynamic and static LLS studies of HPG–PNIPAM in aqueous solution reveals a reversible phase transition on heating and cooling. Dendritic multiarm star block copolymer hyperbranched poly(glycidol)–poly(N-isopropylacrylamide) (HPG–PNIPAM) was prepared by the reversible addition-fragmentation transfer polymerization technique. In aqueous solution, the unimolecular polymeric micelles HPG–PNIPAM have a core-shell nanostructure, with hydrophilic HPG as the core and thermoresponsive PNIPAM as the shell. Dynamic and static laser light scattering measurements show a unimolecular process of reversible phase transition behavior for the unimolecular polymeric micelles in dilute solution.