Thermolysis of cyclic diperoxides gave radicals. When solution polymerization of MMA was initiated by acetone cyclic diperoxide transfer to solvent was the dominant feature of the initiation process. However, transfer in the bulk polymerization of MMA was not a significant feature. Initiation of polymerization of MMA in solution by dibenzyl ketone cyclic diperoxide occurred at lower temperatures,and also aromatic end groups, derived from initiation by benzyl radicals, could be detected by H-1 NMR.