The synthesis of poly(isobutylene-b-methyl methacrylate) has been accomplished by a novel coupling approach combining living cationic and anionic polymerization techniques. By capping living polyisobutylene (PIB) with 1,3-butadiene, a chloroallyl chain-end-functionalized PIB (PIB-AllylCl) was prepared. Coupling of the purified and isolated product and living poly( methyl methacrylate) (PMMALi) was then attempted in tetrahydrofuran at -78 degrees C. However, coupling was absent, and the addition of cesium iodide as a catalyst to increase the coupling rate was also unsuccessful. Halogen exchange reaction of PIB-AllylCl using an excess of lithium bromide proceeded quantitatively to yield the bromoallyl counterpart (PIB-AllylBr). The PIB-AllylBr was coupled with PMMALi in THF at -78 degrees C with high efficiency (> 95%) and yielded the expected diblock copolymer.