Some commercial vinyl polymers such as polystyrene, polymethylmethacrylate and polyvinyl chloride have been synthetized through last years using new initiators, for example those with multiple active sites. Multifunctional initiators in radical polymerization of vinyl monomers allow us to obtain different behaviours during the process and changes in the properties of the final products. In this work, the kinetic study of the thermal decomposition of cyclic pinacolone diperoxide (PDP) in ethylbenzene and its use in the initiation step of the bulk free radical polymerization of styrene is reported. The activation parameters for the thermal decomposition reaction of PDP in styrene were worked out from Eyring equation (Delta H-double dagger = 26.0 +/- 1.2 kcal mol(-1); Delta S-double dagger = -9.5 +/- 3.0 cal mol(-1) K-1). In addition, the evolution of molecular weights during polymerization was studied showing an increase through the process.