The syntheses of chlorobutyl rubber-g-poly(α-methylstyrene) and poly(chloroprene-g-α-methylstyrene) have been accomplished with 90–97% grafting efficiencies. The synthesis principle involves the initiation of α-methylstyrene polymerization by chlorobutyl rubber or polychloroprene rubber in conjunction with BCI3 or SnCU coinitiators in the presence of 2,6-di-t-butylpyridine proton trap. Selective solvent extraction coupled with molecular weight determination and molecular size distribution indicate satisfactory separation procedures and reliable grafting efficiency data. The close agreement between theoretical and experimental graft copolymer molecular weights suggests that backbone scission or gelation does not occur during grafting. According to graft composition and molecular weight data, 6 to 11 poly(α-methylstyrene) branches have been attached per chlorobutyl or polychloroprene rubber backbones.