A series of polymer-silica nanocomposites were prepared by grafting poly(methyl methacrylate), poly(butyl acrylate), polystyrene and poly(styrene-co-acrylonitrile) from both aggregated silica nanoparticles and colloidally dispersed silica using atom-transfer radical polymerisation (ATRP). Cross-linking and macroscopic gelation were minimised by using a miniemulsion system. The grafted polymers silica nanoparticles were characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM), gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), fourier transform infrared spectroscopy (FTIR) and elemental analysis. The thermal and mechanical behaviour of the nanocomposites have been examined by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). Grafting polymers chains from the surface of the nanoparticles gave materials with a 10 oC higher glass transition temperature Tg (according to DSC and DMTA) compared to the pure polymers. DMTA measurements revealed that chain grafted nanocomposites showed an increased modulus and significantly lower high-temperature damping over the neat polymers. In contrast, samples prepared from colloidally dispersed silica nanoparticles exhibited a much less pronounced reinforcement effect than aggregated silica and also showed little change in Tg. Further information on the temperature dependence of the relaxation process was obtained using time temperature superposition. A fast and efficient microwave-assisted method for ring-deuteration of polystyrene and poly(4-hydroxystyrene) using “superheated” C6D6 or D2O in sealed microwave reaction vials has also been developed. The optimised procedure will make future work using neutron scattering possible.