Activated esters in the cationic polymerization of styrenes

Cationic polymerization of styrenes initiated by protonic acids can proceed via covalent intermediates. Calculations of the charge distribution in these species show low probability of transfer reactions. Macromolecular and model covalent species in the perchloric acid initiated polymerization of styrene have been directly observed by NMR. Esters formed with triflic acid are very reactive and participate in Friedel‐Crafts alkylation at −78 °C. Acetates become active only in the presence of protonic and Lewis acids. 1‐Phenylethyl trifluoroacetate activated with the parent acid reacts with styrene faster than the acid does. The racemization of the optically active ester is faster than its incorporation indicating an ionic mechanism of propagation in this system. Activation by Lewis acids proceeds by the reversible formation of complexes. Some systems yield polymers with low polydispersity.