AbstractCopolymers of isobutylene with 1,4‐13C‐isoprene have been prepared in methyl chloride solvent at −90 to −50°C using aluminum chloride and organoaluminum halide initiators. The examination of these copolymers containing 0.5 to 3.0 mol % of the diolefin by13C‐NMR has given new information about the mechanism of the copolymerization. It appears that every macromolecule ends in an isoprene derived unit; that chain transfer to monomer by an isoprene‐derived propagating carbocation is the dominant chain breaking process. The copolymer triad sequences have been unambiguously assigned and analyzed in both batch and continuous stirred tank reactor copolymers. The relative concentrations of the triads agree with those predicted by published react