The synthesis of novel arborescent (arb; randomly branched, "tree-like," and often called "hyperbranched") block copolymers comprised of rubbery polyisobutylene (PIB) and glassy polystyrene (PSt) blocks (arb-PIB-b-PSt) is described. The syntheses were accomplished by the use of arb-PIB macroinitiators (prepared by the use of 4-(2-methoxyisopropyl) styrene inimer) in conjunction with titanium tetrachloride (TiCl4). The effect of reaction conditions on blocking of St from arb-PIB was investigated. Purified block copolymers were characterized by H-1 NMR spectroscopy and Size Exclusion Chromatography (SEC). arb-PIB-b-PSt with 11.7-33.8 wt % PSt and M-n = 468,800-652,900 g/mol displayed thermoplastic elastomeric properties with 3.6-8.7 MPa tensile strength and 950-1830% elongation. Samples with 26.8-33.8 wt % PSt were further characterized by Atomic Force Microscopy (AFM), which showed phase-separated mixed spherical/cylindrical/lamellar PSt phases irregularly distributed within the continuous PIB phase. Dynamic Mechanical Thermal Analysis (DMTA) and solvent swelling of arb-PIB-b-PSt revealed unique characteristics, in comparison with a semicommercial PSt-b-PIB-b-PSt block copolymer. The number of aromatic branching points of the arb-PIB macroinitiator, determined by selective destruction of the linking sites, agreed well with that calculated from equilibrium swelling data of arb-PIB-b-PSt. This method for the quantitative determination of branching sites might be generally applicable for arborescent polymers. (c) 2005 Wiley Periodicals, Inc.