Direct observation and stability of active species in cationic polymerization: A reexamination of the polymerization of styrene initiated by triflic acid

Polymerization of styrene initiated by triflic acid in CH₂Cl₂ solution was reexamined, using a new stopped-flow device working in high purity conditions over a wide temperature range. Monomer and styryl cation were followed simultaneously through their respective absorbances at 290 and 340 nm. Initiation is very rapid, and cations concentration reaches a plateau the duration of which is depending on temperature. In our conditions (I₀ = 0.5 ? 9.10^?3M, M₀/I₀ = 1 to 20), cations concentration is so low at room temperature that it is almost unmeasurable. At ?65°C, it is 100 times higher, remains constant for several seconds and complete termination takes place within a minute or more. Such a profile of cation evolution agrees with an equilibrium situation between initiation and a much more temperature-dependent backward deprotonation. Apparent initial rate of initiation is first order with respect to monomer, but the order with respect to initiator was found very high and variable with temperature (from 4.5 at ?65°C to 3 at ?20°C). This supports the presence, even if they are in low concentration, of acid high agregates, the reactivity of which increases with size. A first order monomer consumption is observed during the plateau, which leads to k_p values ranging from 10³ at ?65°C to 9.10⁴ M^?1.s^?1 at ?10°C (Ep^# = 43 kJ.mol^?1). The disappearance of cations, which follows the plateau, slows down and becomes unimolecular when monomer consumption is complete, and k_t values range from 6.10^?2s^?1 at ?65°C to 1.2s^?1 at ?23°C (E_t^# = 33 kJ.mol^?1).