KINETIC-ANALYSIS OF LIVING POLYMERIZATION SYSTEMS EXHIBITING SLOW EQUILIBRIA .4. DISSOCIATIVE MECHANISM OF GROUP-TRANSFER POLYMERIZATION AND GENERATION OF FREE IONS IN CATIONIC POLYMERIZATION

The MWD moments are derived for a ''living'' polymerization process wh ich proceeds via active and ''dormant'' species and where addition of a catalyst, C, to a ''dormant'' species, P', leads to the formation of an active species, P, and another product, E. Such a mechanism is ap plicable to the ''dissociative'' mechanism of group transfer polymeriz ation (GTP), where the active species is an enolate and E is a silyl e ster, to ''living'' cationic polymerization, where P is a free cation and E is the counterion, and to atom transfer radical polymerization (ATRP), where P' and P are a covalent species and a free radical, res pectively, and C and E are transition metal salts of fewer and higher oxidation states, respectively. Both equilibrium and nonequilibrium in itial conditions are used for the calculation. The results are very si milar to those obtained for the ''associative'' mechanism of GTP (corr esponding to the generation of ion pairs in cationic polymerization) a nd for degenerative transfer (i.e., direct exchange of activity betwee n active and ''dormant'' species). In the absence of added E, the domi nating parameter, beta, is defined as beta = alpha k(2)/(p) where k(2) and K-p are the rate constants of reversible deactivation and propaga tion, respectively, and alpha is the fraction of active chain ends. Th e value of a in turn depends on the equilibrium constant K and the rat io of initial concentrations of catalyst and initiator, C-0/I-0. In co ntrast, for the ''associative'' mechanism of GTP (or ion pair generati on in cationic polymerization) the parameter was defined as beta = k(2 )/(k(p)I(0)), depending on initiator concentration alone, whereas for degenerative transfer it was beta = K-ex/k(p), irrespective of reagent concentrations. Again, for beta > 1 the polydispersity index decrease s with monomer conversion (after a marked increase at low conversions) , coinciding with a common observation in group transfer and cationic polymerizations. In a limiting case, at full conversion, M(w)/M(n) app roximate to 1 + 1/beta. Differences between equilibrium and nonequilib rium initial conditions can only be seen for beta < 1. Added E (e.g., ''livingness enhancer'' in GTP) always leads to narrower MWD's. The re sults are discussed with respect to GTP using nucleophilic catalysts a nd to the cationic polymerization of various monomers. The accessible results indicate that the predominant mechanism for activity exchange in GTP and perhaps also in cationic polymerization is degenerative tra nsfer whereas the mechanism for generation of active species from inac tive ones has to be determined from analysis of the kinetic reaction o rders.