A new method to produce highly filled and well dispersed polymer solid composites using controlled free radical polymerization has been developed. Grafting of polymers onto ultrafine silica was done in bulk polymerization at 120 degreesC in presence of N-tert-butyl-1-diethylphosphono2,2-dimethyl propyl nitroxide (DEPN) as a nitroxide stable free radical. Optimum conditions for tert-butyl hydroperoxide grafting onto fumed silica were first determined. The percentage of grafting, the architecture of grafted polymers, the length of chains, and the polydispersity index can be controlled at will using this approach. The effect of the number of grafted polymer chains combined with its molecular weight on the processing of these materials was investigated. The syntheses performed in this work gave grafting percentages of polymers and copolymers ranging from 12 to 88 wt%. All 'synthesized' composites gave stable suspensions in toluene and tetrahydrofuran. (C) 2003 Elsevier Science Ltd. All rights reserved.