The kinetics of free radical decay in the polymerization of MMA initiated by AIBN was studied by means of ESR spectroscopy. It was found that the curves of radical decay are strongly associated with the reaction temperature

the initiator concentration and the solvent. In the case of the radical polymerization carried out at high temperature or in solution

the radical concentration first reached a maximum

then declined monotonously with reaction time. It was also found that the greater the amount of initiator or the higher the temperature

the more rapidly the radicals decay. When the bulk polymerization was implemented at a relatively low temperature

the curves of radical decay became more complicated

i.e.

the radical concentration rapidly rose to a maximum

then dropped to a minimum

finally increased again with reaction time.Taking into account the diffusion effect

a semi-empirical equation is suggested to describe the kinetics of propagating radical decay.