AbstractThe nature and the stability of the various types of propagating species involved in cationic polymerization of ethylenic monomers are examined. After a short discussion of the evidence for and against the presence of covalent species in styrene polymerization, recent data about the identification of carbocationic active centres involved in the polymerization of para‐substituted α‐methyl‐styrenes are given. Carbenium ions were generated in methylene chloride either by reaction of the p‐substituted cumyl chloride with SbF5or by reaction of the monomer with trifluoromethanesulfonic acid (HA). They are stable around −70°C, but some of them give secondary reactions at higher temperature. The various types of species were identified by1H NMR correlated with UV spectroscopy.For the p‐tert‐butyl derivative, monomeric cations remain stable at −70°C (and even at higher temperature) while oligomeric ones are less stable, giving indanic derivatives. With p‐methoxy‐α‐methylstyrene, the monomeric, dimeric and trimeric cations are stable at −63°C but give indanic derivatives at −40°C and above. For both monomers, monomeric cations show a single maximum at 350 or 367 nm with a high absorption coefficient (≁ 30000). In the presence of an excess of monomer, a bathochromic shift of this maximum (≁ 15 nm) is observed, together with the appearance of a new (smaller) maximum around 480 nm. The presence of these two maxima could be correlated with that of “isomerized” species, some of which could be identified by NMR. The possible occurrence of a similar situation with other monomers and particul