The design of novel macromolecular architectures is a continuous focus in polymer science. Many of these architectures, such as block copolymers, possess unique properties, which make them interesting candidates for special applications in nanotechnology and biomedical materials. Controlled ring-opening polymerization (ROP) of cyclic esters, such as lactide, glycolide, cyclic carbonate, and/or ε-caprolactone (ε-CL), have received significant attention due to the good mechanical properties, degradation behavior and biocompatibility of the resulting polymers [1][2][3]. Polyisobutylene (PIB) has been combined with materials widely used for biomedical applications (polyacrylates and -methacrylates, polysiloxanes, polylactones, polyurethanes, poly(ethylene oxide), and poly (vinyl alcohol)), and some devices that use PIB-based materials are approved by the Food and Drug Administration (FDA) [4][5][6]. One of the most relevant combinations is poly(styrene-b-isobutylene-b-styrene) (SIBS). SIBS is a very soft, transparent material resembling silicone rubber, with superior mechanical properties. It is used as a drug-eluting coating of coronary stents [6][7][8]. PIB is not degradable under biological conditions, however, its copolymers can be. Block copolymers of PIB with L-lactide [9] and pivalolactone [10] have been synthesized from primary hydroxyl functionalized PIBs and metal-containing activators. It was found that the blocks had phase-separated morphologies, and the crystallization behavior of the polylactide and polypivalolactone was influenced by the presence of the PIB blocks. However, Abstract. The synthesis of poly(isobutylene-b-ε-caprolactone) diblock and poly(ε-caprolactone-b-isobutylene-b-ε-caprolactone) triblock copolymers was accomplished using a combination of living carbocationic polymerization of isobutylene (IB) with the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). OH-PIB-allyl was prepared by living carbocationic polymerization of IB initiated with 1,2-propylene oxide/TiCl 4 followed by termination with allyltrimethylsilane. Hydroxyl telechelic HO-PIB-OH was obtained by living IB polymerization initiated by 2,4,4,6-tetramethyl-heptane-2,6-diol/TiCl 4 , termination with allyltrimethylsilane and subsequent thiol-ene click reaction with mercaptoethanol. The structure of the hydroxyl PIBs was confirmed by 1 H NMR (proton Nuclear Magnetic Resonance spectroscopy). OH-PIB-allyl and HO-PIB-OH were then successfully used as macroinitiators for the polymerization of ε-CL catalyzed by Candida antarctica Lipase B (CALB), yielding poly(ε-caprolactone-b-isobutylene) diblock and poly(ε-caprolactone-b-isobutylene-b-ε-caprolactone) triblock copolymers, respectively. Differential Scanning Calorimetry (DSC), Transition Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) demonstrated that the amorphous PIB and the semicrystalline PCL block segments phase separated, creating nanostructured phase morphology.