Analogies and discrepancies between cationic and anionic polymerizations

Abstract

Abstract Results are reviewed on cationic polymerizations initiated by γ‐rays, by carbocation salts, and by strong acids, especially HClO 4 , with the conclusion that in no case are all criteria for true living polymerization satisfied. Detailed information is presented on bimodal molecular weight distributions found in polystyrenes initiated by HClO 4 in CH 2 Cl 2 between 0 and −97°C, showing that at all temperatures, polymerization can take place via both pseudo‐cationic and true ionic species, with the former in numerical preponderance. Transfer reactions prevent these species from being in simple ionization/recombination equilibria with each other, hence precluding a true living polymerization situation. The chief reason for these kinetic complexities is concluded to be a strong monomer‐complexation of the HClO 4 initiator and of propagating species. Indications of similar complexation in styrene/H 2 SO 4 and Ph 3 CClO 4 , and between HClO 4 and five other monomers suggests it may be a fairly widespread feature in cationic polymerizations.