This paper discusses four closely related themes. (a) First, we present the synthesis of new butyl rubber graft copolymers that have a polyisobutylene backbone and several free-radical polymerized polymers, such as poly(methyl methacrylate), in the side chains. The chemistry involves commercial butyl rubber, poly(isobutylene-co-isoprene) and free-radical graft-from reaction using a borane reagent. (b) The selective autoxidation reaction of secondary alkyl-9-borabicyclononane offers the conditions for producing desirable polymeric radicals in graft-from polymerization with high grafting efficiency and controllable copolymer compositions. (c) The structure of graft copolymers is characterized. Several techniques, especially gel permeation chromatography measurement with on-line refractive index, photodiode-array and light scattering detectors, were used to provide detailed microstructural information. (d) Finally, compatibility is studied using graft copolymers as the compatibilizers in polyisobutylene blends.