The non-stationary polymerization of styrene by HCIO ₄ in CH ₂ CI ₂

The non-stationary precursor reaction (stage I) in the polymerization of styrene by perchloric acid in methylene chloride has been examined, by stopped-flow methods over the range 0 to -80 degrees C. At all temperatures there is evidence of a transient, electrically conducting, intermediate species, absorbing at 340 nm, which reaches its peak concentration at times ranging from 0.1 s at 0 degrees C to 0.5-3 s at -80 degrees C (variable with reagent concentrations). At the low monomer concentrations (< 0.2 M) accessible to the technique, stage I can be quantitatively discriminated from the subsequent steady stage II only below $ca$. -60 degrees C. At these low temperatures, contrary to expectations, the conversion during stage I proves to be by a dual mechanism, the ionic reaction producing no more than about half the mass of polymer and a much smaller fraction of the number of polymer chains. The overall time-scale of stage I appears to be determined primarily by that of removal of free CHlO$_{4}$ by the non-ionic mechanism, rather than by the kinetics of the ionic polymerisation. The overall conversions cannot therefore be analysed to yield ionic rate constants. In reactions in presence of the salt $n$-Bu$_{4}$NClO$_{4}$, the instantaneous rates can be separated into their non-ionic and ionic components, and approximate values derived for the paired-ion propagation rate constants. $k_{\text{p}}^{\pm}$ 2000 at -80 degrees C; 4000-5000 at -60 degrees C; (unit: dm$^{3}$ mol$^{-1}$ s$^{-1}$. A more speculative analysis of the rates in salt-free systems permits estimates of the free ion propagation constants some 10-20 times the above values, and of ion-pair dissociation constants in the region of $1-5\times 10^{-7}$ mol dm$^{-3}$ at -60 to -80 degrees C.