By using maximum bubble pressure method (MBPM)

the dynamic surface tension (DST)of styrene (St)and methyl methacrylate (MMA)surfactant-free emulsion copolymerization in aqueous solution of poly(ethylene glycol)(PEG)or without PEG was studied.It was observed that PEG could slow down copolymerization rate of the system.The conversion of the system was above 98% without PEG after reacting 50 minutes

approaching the end point

while

it was just 49% for the system with PEG.In the presence of PEG

the copolymer of St/MMA initiated by potassium persulphate could become a kind of material that had stronger surface activity.This kind of copolymer with lower surface tension was formed when the conversion was just 5%.Lower surface tension was the force that drove copolymer into particle pile to form small and stable particles

and gave the particles enough surface electric charge to escape from the particle pile eventually.