The surfactant-free emulsion polymerization (SFEP) of styrene and methyl methacrylate carried out in the aqueous solution of poly(ethylene glycol)(PEG) was studied.With high power optical microscope

Coulter counter and transmission electron microscopy

particle size

particle size distribution and zeta potential were measured in the process of polymerization.It is interesting to observe that the polymer particle size experiences a process from small to large

which can be more than 1000 m

and then from large to small

which is back to a range of 0.04 ~ 0.18 m.When the particle size returns to nanometer scale

the system has an obvious temperature rising.This phenomenon is different from the traditional nucleation mechanism of SFEP.The particle

which is formed by the homogeneous nucleation or oligomer micelle nucleation mechanism that is popularly received as traditional nucleation mechanism of SFEP

usually has a trend to become bigger and bigger to the end in the process of SFEP.It is obviously found that the freshly formed particles are first coalesced to form a particle pile

which is bigger than 1000 m and even seen with eyes

and then corrupted into tinny ones

which are smaller than 0.04 m and even beyond the ability of high power optical microscope.With transmission electron microscopy (TEM)

it was clearly proven the existence of paricle pile which was composed of a lot of little particles ranging from 40 nm to 80 nm.And by use of Coulter zeta potential measurement

it was found that the zeta potential of particles had a trend from low to high level

and experenced abruptly increase in zeta potential at 60 min which was just the time that the little particle seperated from the paricle pile.This phenomenan was very different from the system that SEFP was carryed out in water without PEG

in which the zeta potential of particle was keeping about -50 mV and just changed a little after 50 min.It was reasonble to think that the potential increase was the driving force to make the little particles to escape from the particle pile.Therefore

the mechanism of coalescence-separate nucleation is proposed.