Synthesis of Thermoresponsive Shell Cross-Linked Micelles via Living Cationic Polymerization and UV Irradiation

A series of well-defined thermoresponsive poly(2-ethoxyethyl vinyl ether)-block-poly(2-hydroxyethyl vinyl ether) (PEOVE-b-PHOVE) block copolymers have been synthesized via sequential living cationic polymerization, containing methacryloyl groups derived from 2-(vinyloxy)ethyl methacrylate (VEM) in the PHOVE segment, PEOVE-b-P(HOVE/VEM). The ratio of EOVE/HOVE in the block copolymers was fixed at 1:2. The obtained PEOVE-b-P(HOVE/VEM) dissolved molecularly in aqueous solution, when below the lower critical solution temperature (LCST) of PEOVE. Upon heating above LCST of PEOVE, spherical micellization occurred, comprising of PEOVE cores and PHOVE shells. This was confirmed by dynamic light scattering (DLS) and 1H NMR spectroscopy at various temperatures. The methacryloyl groups in the block copolymer are located around the surface of the PHOVE shells as a random copolymer of HOVE and VEM. The micelles were cross-linked by UV (254 nm) irradiation in water to obtain shell cross-linked (SCL) micelles. The SCL micelles were characterized by 1H NMR spectroscopy, DLS, and atomic force microscopy. The micellar core was reversibly hydrated or dehydrated, depending on the solution temperature. The size of SCL micelles was controlled by molecular weight, with longer lengths leading to increased size of the SCL micelles, as expected. These SCL micelles served as nanoreactors for the synthesis of gold or silver nanoparticles prepared by in situ chemical reduction of Au(III) or Ag(I), respectively. The gold- or silver-loaded SCL micelles exhibited colloid stability, and the nanosized SCL micelle image containing the Au(0) was observed by transmittance electron microscopy.