Poly(isobutylene-b-methyl methacrylate), p(IB-b-MMA), copolymers were synthesized by reacting the two corresponding living homopolymers, obtained by living carbocationic and group transfer polymerization, respectively. IB was polymerized by the 2,4,4-trimethyl-2-chloropentane (TMPCl)/TiCl4/methylcyclohexane:CH3Cl 60:40 v:v/-80 degrees C system. When the polymerization was complete, 1,1-diphenylethylene was added to convert PIB-Cl to the corresponding diphenylcarbenium ions. To this solution living PMMA, obtained by group transfer polymerization, isolated and dissolved in CH3Cl was added, and the reaction mixture was allowed to warm to -40 degrees C. Depending on the livingness of PMMA, up to 80% coupling efficiency could be obtained. It was found that at -40 degrees C after reaching a maximum of similar to 80% the coupling efficiency decreases with time. In a very slow competitive reaction, the formed block copolymers are cleaved by a mechanism suggested to involve the complexation of TiCl4 to the carbonyl oxygen adjacent to the coupling site, followed by anion formation and irreversible termination. This side reaction can be minimized by limiting the reaction time to <4 h at -40 degrees C and by using a low [TiCl4]/[TMPCl] = 4 ratio.