Spectroscopic observation of the propagating species in the cationic polymerization of vinyl monomers

The reaction of several vinyl aromatic monomers with CF3SO3H and H2SO4 was carried out in di‐chloroethane at 30°C. The rapid formation of the cationic species was followed by the stopped‐flow method, and the transient species were identified by rapid‐scan spectra and rapid quenching of the reaction mixture. The monomeric cation was formed predominantly in the reaction of 1,1‐diphenylethylene and excess CF3SO3H; however, an indanic dimer was formed in the presence of excess diphenylethylene. The rate constant for the formation of the cationic species was (3∼4) × 103 M−1.sec−1. On the other hand, the reaction of diphenylethylene and H2SO4 produced two short‐lived species: the monomeric cation and, probably, a monomeric radical cation. The transient species (lifetime ∼50 msec) were similarly observed in the reaction of CF3SO3H with styrene, methylstyrenes, and indene.