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

Abstract

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 °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 0.1s at 0° C to 0.5–3s at –80 °C (variable with reagent concentra­tions). At the low monomer concentrations (< 0.2M) accessible to the technique, stage I can be quantitatively discriminated from the subsequent steady stage II only below ca . –60 °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 HCIO 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 NCIO 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 ± p 2000 at –80 °C; 4000–5000 at –60 °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 x 10 –7 mol dm –3 at –60 to –80 °C.