Polyisobutylene (PIB) and other PIB-based materials are of significant interest for a vast array of applications, but chemical modification is often required to obtain the desired properties. Described here are two new approaches for the preparation of carboxylic acid-functionalized PIB. The ring opening of cyclic anhydrides from an allylic alcohol derivative of butyl rubber and the atom transfer radical polymerization of tert-butyl methacrylate from a rubber derivative both ultimately afford carboxylated materials. These materials displayed significantly enhanced adhesion to stainless steel, as well as increased ultimate tensile strength and Young's modulus in comparison to unmodified rubbers. Rheological studies suggested that they exhibit a greater degree of cross-linking-type behavior than the parent butyl rubber. Combined, these studies suggest that the properties of PIB can be readily tuned through synthetic modifications of the backbone, even at low mole percent, and that carboxylic acid moieties can impart desirable properties for various applications.