A series of amphiphilic poly(2-hydroxyethyl methacrylate)-b-polydimethylsiloxane-b-poly(2-hydroxyethyl methacrylate) (pHEMA-b-PDMS-b-pHEMA) (A-B-A) triblock copolymers were synthesized from three different carbinol-terminated polydimethylsiloxanes with varying molecular weight. A carbinol-terminated polydimethylsiloxane was modified with 2-bromoisobutyryl bromide to obtain a macroinitiator. The block copolymers were characterized by NMR, GPC, and dynamic light scattering (DLS). Reverse micelles of a copolymer were formed in mixture of benzene/methanol solution which served as nanoreactors for the synthesis of magnesium fluoride (MgF2) nanoparticles. The MgF2 was prepared via chemical precipitation using magnesium chloride and potassium fluoride as reactants. The MgF2-triblock copolymer composites were synthesized as a function of MgF2weight ratio (0.5, 5, and 10 wt%) in copolymer. The MgF2 colloids were dissolved in three organic solvents: methanol, isopropanol, and tetrahydrofuran. The polymer nanoparticles were characterized by DLS, transmission electron microscopy, thermogravimetric analysis, and X-ray diffraction (XRD) analysis. The formation of MgF2 crystals was observed by XRD. Particle size and particle size distribution showed significant changes in different solvents. The thermal stability of MgF2 colloids increased as the amount of nanoparticle increased in polymeric matrix. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012