Copolymers of tert-butyl-3-isopropenylcumylperoxide (D-120) with styrene (St) were prepared in bulk by using benzoyl peroxide as initiator at 80degreesC. The compositions of the copolymers were determined by H-1-NMR analysis. The reactivity ratios of D-120 and St were calculated by methods of Xelen-TudOs (K-T), Fineman-Ross (F-R), and Yezrielev-Brokhina-Roskin (YBR), respectively. Graft copolymers were obtained by grafting of methyl methacrylate (MMA) onto the copolymers during decomposition of peroxy bonds suspended in the backbone.