Stimuli-Responsive Double Hydrophilic Block Copolymer Micelles with Switchable Catalytic Activity

Abstract

Double hydrophilic block copolymers, poly( N -isopropylacrylamide) -b- poly( N -vinylimidazole) (PNIPAM -b- PVim), were successfully prepared with good control via reversible addition−fragmentation chain transfer (RAFT) using PNIPAM-based macromolecular xanthate agents (i.e., MADIX, macromolecular design via the interchange of xanthates). This represents the first preparation of well-defined block copolymers based on PVim, which has been well-known to be able to catalyze esterolysis reactions. The imidazole-containing diblock copolymers molecularly dissolve at low temperatures in water. Above the phase transition temperatures of PNIPAM or in a proper mixture of methanol/water (cononsolvency), the PNIPAM block becomes hydrophobic and stable micelles form with a dense core consisting of a hydrophobic PNIPAM block and a polar PVim shell. The catalytic activities of PNIPAM 44 -b- PVim 51 and PNIPAM 44 -b- PVim 21 toward the hydrolysis of p -nitrophenyl acetate (NPA) at different temperatures or methanol/water compositions were then determined using a stopped-flow apparatus and compared to that of PVim homopolymer. The Arrhenius plot for the PVim-based diblock copolymers exhibited a pronounced upward curvature above the critical micellization temperature (cmt). Moreover, in the methanol/water mixture, the catalytic activities of PNIPAM -b- PVim diblock copolymers evolved discontinuously as a function of solvent composition and exhibited a maximum in the range of volume fraction of methanol, φ methanol, between 0.3 and 0.5, corresponding to the solvent composition range where cononsolvency-induced micellization took place. We thus observed for the first time that double hydrophilic block copolymer micelles of PNIPAM- b -PVim can serve as self-catalyzing nanoreactors. Most importantly, the catalytic activities can be well-tuned with external temperature or solvent compositions.