KINETIC-ANALYSIS OF LIVING POLYMERIZATION SYSTEMS EXHIBITING SLOW EQUILIBRIA .3. ASSOCIATIVE MECHANISM OF GROUP-TRANSFER POLYMERIZATION ANDION-PAIR GENERATION IN CATIONIC POLYMERIZATION

The averages of the molecular weight distribution are derived for a '' living'' polymerization process which proceeds via active and ''dorman t'' species and where the active species are formed by addition of a c atalyst to a ''dormant'' species. Such a mechanism is applicable to gr oup transfer and ''living'' cationic polymerizations (assuming that io n pairs are formed exclusively in the latter ease). Both equilibrium a nd nonequilibrium initial conditions are used for the calculation. The results are very similar to those obtained for degenerative transfer (i.e., direct exchange of activity between active and ''dormant'' spec ies), The dominating parameter is beta = k(2)/(k(p)I(0)), where K-2 an d k(p) are the rate constants of reversible deactivation and propagati on, respectively, and lo is the initial initiator concentration (for d egenerative transfer this parameter was given as beta = k(ex)/k(p), in dependent of initiator concentration). As an example, for beta > 1 the polydispersity index decreases with monomer conversion (after a marke d increase at low conversions), coinciding with a common observation i n group transfer and cationic polymerizations. In a limiting case, at full conversion, M(w)M(n) approximate to 1 + 1/beta. Differences betwe en equilibrium and nonequilibrium initial conditions can only be seen for beta < 1. The results are compared to experimental data of the GTP of methyl methacrylate using nucleophilic catalysts and to the cation ic polymerization of isobutylene. Typically, exchange is moderately fa st (10 less than or equal to beta less than or equal to 100), thus the equilibrium initial conditions are sufficient for the calculations.