Bulk copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) was initited by methyl α-bromophenylacetate (MBPA) and mediated by copper bromide (CuBr) ligated with 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA). The atom transfer radical polymerization (ATRP) of pure MMA yielded polymers with well controlled molecular weights and the homopolymerization was taken as a reference for the copolymerization analysis. The copolymerization experienced autoacceleration in rate due to a diffusion-controlled radical deactivation. The onset of autoacceleration came earlier with an increase in the EGDMA fraction. At low EGDMA fractions, the molecular weight versus conversion data deviated from linearity due to branching. The ratio of the copolymer molecular weight over homo-poly(methyl methacrylate) gave an estimate for the branching density, which increased initially and leveled off at high conversion. At high EGDMA fractions, the ATRP system experienced gelation. The pregel branching density increased with conversion, and at the gel point, it agreed with Flory's gelation theory assuming cross-linking free of cyclization. The branching densities at the high gel fractions were very close to the maximum values possibly achieved with the added EGDMA fractions. These results suggested that the ATRP system was very effective in preparing homogeneous polymer networks with a high cross-linking efficiency. © 2005 Society of Plastics Engineers.