The highly electrophilic borane B(C 6 F 5 ) 3 reacts with n-octadecanol (n-C 18 H 37 OH) and n-octadecanethiol (n-C 18 H 37 SH) to form the 1:1 adducts (n-C 18 H 37 EH)B(C 6 F 5 ) 3 (E ϭ O or S). The latter are acidic and react with Cp*TiMe 3 in methylene chloride and toluene to give methane and the complexes [Cp*TiMe 2 ][(n-C 18 H 37 E)B(C 6 F 5 ) 3 ], which are very good initiators for the carbocationic polymerization of isobutene (IB) from Ϫ40 to Ϫ20°C. High conversions to high molecular weight polyisobutene (PIB) in methylene chloride and moderate conversions to high molecular weight PIB in toluene are observed and are consistent with the anions [(n-C 18 H 37 E)B(C 6 F 5 ) 3 ] Ϫ being very weakly coordinating. Although polymerization in methylene chloride is too rapid for the temperature to be controlled, polymerization in toluene is slower, and the temperatures can be controlled so that Arrhenius-type plots of the logarithm of the number-average molecular weight versus T Ϫ1 ϭ 1/T may be obtained. Activation energies for the degree of polymerization in these polymerization reactions and similar polymerizations carried out with n-C 18 H 37 EH:borane ratios of 1:2 and with the activators [Ph 3 C][B(C 6 F 5 ) 4 ] and Al(C 6 F 5 ) 3 range from Ϫ11 to Ϫ27 kJ mol Ϫ1 , values comparable to those for most conventional IB polymerization initiators. However, the values of the weight-average and number-average molecular weights are unusually high for the temperatures used, and this is consistent with current theories of the role of weakly coordinating anions.