Ultrahigh molecular weight copolymers of isobutene (IB) and 2,3-dimethyl-1,3-butadiene (DMBD), analogous to commercial IB-isoprene copolymers, are prepared employing as carbocationic polymerization initiator the highly acidic 1:2 adduct of n-octadecanoic (stearic) acid with the electrophilic borane B(C6F5)(3), [n-C17H35CO2H][B(C6F5)(3)](2). High conversions to gel-free, low-polydispersity materials with weight-average molecular weights well in excess of 3 x 10(6) are readily obtained, demonstrating the effectiveness of {n-C17H35CO2[B(C6F5)(3)](2)}(-) as a weakly coordinating anion. Complete H-1 and (13)Ct{H-1} NMR assignments for the IB-DMBD copolymers, in which the 1,3-diene is incorporated in a 1,4-trans manner, are presented for the first time.