Successful combination of quasiliving carbocationic (QLCP) and atom transfer radical polymerizations (ATRP) was carried out by initiating ATRP with polyisobutylene (PIB) macroinitiators obtained by QLCP. It has been found that 1-chloro-1-phenylethyl-telechelic PIBs with (M) over bar(n) of 7 800 and 30700 are efficient macroinitiators for ATRP of styrene in bulk and in xylene solution and for p-acetoxystyrene (pAcOSt) in xylene. Size exclusion chromatography (SEC) traces clearly indicated quantitative initiation and the formation of the desired polystyrene-block-polyisobutylene-block-polystyrene (PSt-PIB-PSt) and PpAcOSt-PIB-PpAcOSt triblock copolymers. Experiments also revealed absence of thermal self-initiation of styrene under ATRP conditions.