Random copolymers of poly(styrene-co-isopropenyl acetate) (SIPA) with an average number of 9 initiating sites per chain were synthesized by free radical copolymerization of styrene with a small amount of isopropenyl acetate using 2,2'-azo-bis-(isobutyronitrile) as an initiator at 70 degrees C. SIPA copolymer could be further used as macroinitiator for the grafting cationic polymerization of isobutylene (IB) from SIPA chain in CH2Cl2 at -40 degrees C to produce graft copolymers of SIPA-g-PIB. The effect of SIPA concentration ([SIPA]), TiCl4 concentration ([TiCl4]) and IB concentration ([IB]) on initiation efficiency of macroinitiator, grafting efficiency of initiating sites, average length of PIB branches of the resulting graft copolymers were investigated. It can be found that almost all of the initiating sites of IPAc units on SIPA chains were active for the cationic polymerization of IB and both initiation efficiency and grafting efficiency were close to 100% at sufficient molar ratio of TiCl4/IPAc. This synthetic route presents quantitative grafting efficiency and possibility to control length of PIB branches. The graft copolymers of SIPA-g-PIB with average 9-branched PIB chains having terminal functional tert-chlorine groups could be successfully obtained. The average molecular weight of PIB branches in SIPA-g-PIB graft copolymers could be mediated from 3900 to 47,300 g mol(-1) by changing the ratios of macroinitiator to monomer and concentration of TiCl4. (C) 2012 Elsevier Ltd. All rights reserved.