The present invention relates to medical devices which contain isobutylene copolymers. The present invention also relates to biocompatible copolymer materials for therapeutic agent delivery comprising a therapeutic-agent-loaded isobutylene copolymer. According to an aspect of the present invention, a medical device is provided, which includes: (a) a substrate and (b) at least one polymeric layer, which contains a copolymer, disposed over all or a portion of the substrate. The copolymer contains one or more polymer chains, within which isobutylene and elevated Tg monomers (and, optionally, other monomers) are incorporated in a random, periodic, statistical or gradient distribution.