Synthesis of a double-hydrophilic star-block copolymer by aqueous SET-LRP and its dual-stimuli responses

Abstract

The synthesis of a well-defined dual-responsive double-hydrophilic star-block copolymer by single-electron-transfer living radical polymerization (SET-LRP) in aqueous solution is reported. Conditions for iterative block formation have been optimized for an N-isopropylacrylamide three-armed star-shaped polymer, which was prepared with DP = 120–360 with full conversion in < 30 min with polydispersity index (Ð) < 1.18. To demonstrate this approach for true block stimuli-responsive copolymer materials, a well-defined block polymer containing N-isopropylacrylamide, dimethylaminoethyl methacrylate, and N-vinylpyrrolidone has been prepared in high purity, with purification only required in the final step. The Ð after each chain extension was below 1.20. The polymer has been characterized by FTIR spectroscopy, 1H NMR spectrometry, and gel permeation chromatography. The responsive behaviors of the block-star copolymer have been studied by turbidity measurements, dynamic light scattering, and transmission electron microscopy. The phase transition temperature of the star-block copolymer increased with decreasing pH. At pH 3.0, the copolymer displayed weak thermo-responsive behavior. At higher pH (8.6 and 10.0), the star-block copolymer showed a strong thermal response and formed spherical micelles and vesicles.