The decomposition rate constant (k(d)) of pinacolone diperoxide (PDP, 3,6-diterbutyl-3,6-dimethyl-1,2,4,5-tetraoxacyclohexane) in methyl methacrylate (MMA) is determined by the kinetic study of its thermal decomposition at temperatures from 110 degrees C to 140 degrees C. The calculated k(d) values for PDP are higher than the corresponding values previously determined and reported for diethyl ketone triperoxide (DEKTP, 3,3,6,6,9,9-hexaethyl-1,2,4,5,7,8-hexaoxacyclononane), for example, at 140 degrees C the k(d) for PDP is 75.4 x 10(-5) s(-1), while for DEKTP, it is 50.6 x 10(-5) s(-1). The difference in the k(d) between 130 degrees C and 140 degrees C indicates that the decomposition mechanism, sequential and/or concerted, is a function of temperature. The conformations of both initiators justify the higher k(d) for PDP in MMA than DEKTP, where one single conformer is found for PDP, whereas 212 conformers are found for DEKTP. Bulk polymerization of MMA using PDP as the initiator reveals also the presence of an induction period, such as in DEKTP case. This work provides mechanistic insights into the interactions among the bifunctional cyclic peroxide PDP and the MMA monomer and their influence on the polymerization kinetics. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 997-1007