Stimuli-responsive double hydrophilic block copolymer micelles with switchable catalytic activity

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, phi(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.