Free radical cross-linking copolymerization of styrene with large amounts of ethylene glycol dimethacrylate (monomer feed: ST/EGDM = 5/5) has been investigated in toluene as a homogeneous solution. The conversion, the ratio of pendant vinyl groups and the size of the nanoparticles in the pregel state were measured at various reaction times until the onset of macrogelation. It was found that the highest possible polymer yields were 65% at 0.556 mol/dm 3 monomer concentration and nearly 100% at 0.278 mol/dm3 monomer concentration without gelation. The molecular weight distributions were monomodal at low conversions and multimodal at higher conversions. The multimodal molecular weight distributions were caused by branching reactions and crosslinking within the nanoparticles. The copolymers obtained were soluble in standard organic solvents such as toluene and chloroform. Transmission electron micrographs proved the formation of spherical particles (TEM).