Model reactions were carried out with 2-isothiocyanato-2-phenylpropane(Cum-NCS)/TiCl4 combinations as the initiating system and using 2,4,4-trimethyl-1-pentene (TMP1) as nonpolymerizable monomer. The reaction products were characterized by GC/MS, and their presence was interpreted in terms of cationic processes. These results were compared to those obtained by the analysis of oligoisobutylenes synthesized under the same conditions. We showed that the combination of chromatographic techniques (SFC, SEC, and GC) and of mass spectrometry allowed the analysis of such oligomers. This strategy was complementary to the approach using model reactions. Model experiments, such as the reaction Cum-NCS/TiCl4, showed the production of compounds demonstrating the ionization of the initiator, Cum-OMe due to the quench with MeOH. Similarly, the reaction Cum-NCS/TMP1/TiCl4 confirmed the existence of a direct initiation mechanism. The reaction Cum-NCS/TMP1/TiCl4/4-methyl-2,6-ditertiobutylpyridine (MDtBP) indicated that the direct initiation mechanism made use of the whole quantity of TiCl4. The polymerization of isobutylene (IB) in the presence of the system Cum-NCS/TiCl4 indicated that the product of indanic cyclization could be alkylated. The same system in the presence of MDtBP demonstrated that the hindered pyridine favors the elimination reaction so that the polymer did not bear any pseudohalide function. In the presence of DMSO instead of MDtBP, an unexpected side reaction was evidenced, namely the alkylation of the terminal double bonds of the polymer by a cumyl cation, thus producing a polymer having an aromatic nucleus at both ends. This finding also demonstrated that in this system, propagation is faster than initiation. However, an important product was also the expected polymer, i.e., a PIE having a cumyl moiety at one end and an NCS group at the other end.