Kinetic Analysis of "Living" Polymerization Processes Exhibiting Slow Equilibria. 2. Molecular Weight Distribution for Degenerative Transfer (Direct Activity Exchange between Active and "Dormant" Species) at Constant Monomer Concentration

Abstract

The MWD function and moments are derived for a living polymerization process which proceeds via active and dormant species and where the activity is directly exchanged between these chain ends in a bimolecular reaction (degenerative transfer). Such a mechanism is believed to be applicable to many living polymerizations (e.g., anionic, group transfer, cationic, and radical). For constant monomer concentration (slow addition of monomer or low conversion), the polydispersity index, P w /P n , depends on the ratio of molar concentrations of monomer and initiator, γ = M/I 0 , the degree of polymerization, P n , and the ratio of rate constants of exchange and propagation, β = k ex /k p . In a limiting case (β > 1 and P n >> 1), P w /P n 1 + 2γ/(βP n ). The molecular weight distributions are always narrower than those obtained for a batch process, where monomer concentration decreases during polymerization.