Abstract: Both alkyl halides promoted atom transfer radical polymerization and peroxide initiated conventional polymerization of methacrylate/dimethacrylate were carried out in an attempt to explore the reaction mechanism of atom transfer radical polymerization in crosslinking systems.The kinetics of the two systems were investigated by DSC.The development of the networks was monitored based on the measurements of gel fraction and swelling ratio.The molecular mass of the primary chains before and after gelation were determined using GPC and an Ubbelohde viscometer.It was found that the polymerization rate of the ATRP system was much lower than that of the BPO system,and the molecular mass of the primary chains increased with vinyl group conversion before and after gelation,but leveled off at a higher conversion.For the three ATRP crosslinking systems with 2.5%,5%,7.5% of PEGDMA as the crosslinking monomer,respectively,the increase in molecular mass leveled off at the conversions of 70%,60%,45% in turn.These results suggest that the ATRP of methacrylate/dimethacrylate crosslinking system had "living"/controlled character,but this character would disappear when the network density arrived at a certain level.