Polyisobutylene-graft-polystyrene (PIB-g-PSt) was synthesized by quasiliving atom transfer radical polymerization (ATRP) of styrene. The ATRP process was induced by an industrially produced poly(isobutylene-co-p-methylstyrene-co-p-bromomethylstyrene) macroinitiator (PIB) in the presence of CuBr and 2,2'-bipyridine. Successful graft polymerizations were performed both in bulk and in xylene solution. Preliminary investigations indicate phase-separated morphology and thermoplastic elastomer (TPE) behavior of this new graft copolymer with certain PIB/PSt ratios. These materials are able to elongate to ca. 500% of their original dimension without any irreversible deformation.