The effect of different initiators (tert-butyl chloride (t-BuCl) and 2-chloro-2-phenylpropane (CumCl) on the cationic polymerization of isobutylene co-initiated by acidic chloroferrate imidazole-based ionic liquid emimCl-FeCl3 (emimCl: 1-ethyl-3-methylimidazolium chloride, molar fraction of FeCl3 (χ(FeCl3)) ≥ 0.6) in the presence of diisopropyl ether at 0 °C and [IB] = 5.2 M has been investigated. Generally, the use of all above-mentioned initiators results in increase of the monomer conversion from 30% to 60% as well as in decrease of molecular weight from 2500 g mol⁻¹ to 1100 g mol⁻¹ and polydispersity from 2.6 to 1.7, respectively, but does not influence the content of exo-olefin end group. It was demonstrated that CumCl does not initiated directly the polymerization at 0 °C or −20 °C, rather, it decomposed with the formation of the proton, a true initiator of the polymerization. It was also found that addition of small amounts (1% by volume) of benzene or its derivatives into the polymerization system leads to further increase of the monomer conversion (>90%). The rate of isobutylene polymerization depends on basicity of aromatic compounds added and increases in the following order: benzene < toluene ≈ mesitylene.