KINETIC DISCRIMINATION BETWEEN VARIOUS MECHANISMS IN GROUP-TRANSFER POLYMERIZATION

For various mechanisms of group-transfer polymerization (GTP), the con centration of the active chain ends is calculated. The calculated reac tion orders with respect to catalyst and initiator concentrations, res pectively, are compared to experimental data for the GTP of methyl met hacrylate in THF catalyzed by both bifluoride and benzoate and of n-bu tyl acrylate in toluene catalyzed by mercury iodide. The comparison re veals that for catalysis by benzoate and mercury iodide, the experimen tal reaction orders are only consistent with the associative mechanism of GTP, i.e., with an activated silyl ketene acetal as the active spe cies. For the catalysis with the less nucleophilic benzoate anion, the existence of a dissociative mechanism (with enolate ions or ion pairs as the active species) can be neither excluded nor proven. The effect of silyl esters which have been used in ''livingness enhancers'' is d iscussed and quantified in terms of all mechanisms discussed.