The influence of initiation efficiency and polydispersity of primary chains on the experimental gel points was studied during atom transfer radical copolymerization (ATRO) of monovinyl monomer and divinyl cross-linker. Three initiators with progressively increased initiation efficiency-ethyl 2-bromopropionate (EBrP) < ethyl 2-bromoisobutyrate (FBiB) < 2-bromopropionitrile (BPN)-were used for the ATRcP of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA). With the least efficient EBrP initiator, experimental gelation Occurred when the molar ratio of EGDMA cross-linker to EBrP was as low as 0.25 due to the low initiation efficiency of EBrP. In contrast, no gelation was observed by using the most efficient BPN initiator, even when the molar ratio of cross-linker to initiator was equal to unity. The use of a poorer solvent for copper catalyst also decreased the initiation efficiency and resulted in a gelation at lower monomer conversion. The dependence of experimental gel points on the polydispersity of primary chains was studied by using activators regenerated by electron transfer (ARGET) ATRP for copolymerization of methyl acrylate (MA) and ethylene glycol diacrylate (EGDA). Decreasing the copper concentration from tens of ppm to a few ppm broadened the molecular weight distribution of primary chains, which resulted in an earlier gelation at lower monomer conversion during the copolymerization of MA and EGDA.