The efficient synthesis of film-forming colloidal nanocomposite particles is described. A 50:50 mass ratio of styrene and n-butyl acrylate is statistically copolymerized using a cationic azo initiator at 60 degrees C under aqueous emulsion polymerization conditions in the presence of a commercially available glycerol-functionalized ultrafine silica sol. This new formulation leads to the production of copolymer/silica particles with relatively narrow size distributions and a well-defined "core-shell" morphology. These nanocomposite particles contain up to 43% silica by mass, and the silica aggregation efficiency can exceed 95%, so very few nonaggregated silica nanoparticles remain in solution after the in situ copolymerization is conducted. Upon drying these dispersions, highly transparent free-standing nanocomposite films are obtained. Control experiments confirm the importance of using the cationic azo initiator in combination with the glycerol-functionalized silica sol. Moreover, it is also shown that simple admixtures of a film-forming copolymer latex with the glycerol-functionalized silica sol give much more opaque nanocomposite films, while the deliberate addition of excess silica sol prior to nanocomposite film formation leads to extensive film cracking. Thus this in situ copolymerization route appears to offer a decisive advantage for nanocomposite coatings applications.421137213728University of SheffieldRoyal Society Wolfson Trust Research Merit Award