The functionalization of polyisobutylene-co-isoprene, commonly referred to as butyl rubber, was investigated with the aim of preparing multifunctional materials. First, the acid catalyzed ring-opening of epoxidized butyl rubber was investigated. Ring-opening followed by elimination resulted in the formation of different dienes depending on the reaction conditions. It was possible to cleanly isolate the exo-diene, which readily undergoes Diels-Alder [4 + 2] cycloaddition reactions and it was demonstrated that this chemistry could be used to prepare multifunctional graft copolymers. For example, poly(ethylene oxide) (PEO) or polystyrene (PS) could be grafted while at the same time introducing carboxylic acid moieties along the polymer backbone, and both polymers could be simultaneously grafted to form mixed graft copolymers with tunable tensile properties. Furthermore, it was demonstrated that reaction with a dimaleimide led to efficient cross-linking of the rubber and this cross-linking could be reversed upon heating due to the thermoreversible nature of the Diels-Alder reaction. Thus, this chemistry allows for clean and facile synthesis of new rubber derivatives, opening possibilities for new rubber applications.