title: Consistent experimental and theoretical evidence for long-lived intermediate radicals in living free radical polymerization.
abstract: The cumyl dithiobenzoate (CDB)-mediated reversible addition fragmentation chain transfer (RAFT) polymerization of styrene at 30 degrees C is studied via both kinetic experiments and high-level ab initio molecular orbital calculations. The kinetic data clearly indicate the delayed onset of steady-state behavior. Such an observation is consistent with the slow fragmentation model for the RAFT process, but cannot be reconciled with the cross-termination model. The comprehensive failure of the cross-termination model is quantitatively demonstrated in a detailed kinetic analysis, in which the independent influences of the pre-equilibria and main equilibria and the possible chain length dependence of cross-termination are fully taken into account. In contrast, the slow fragmentation model can describe the data, provided the main equilibrium has a large fragmentation constant of at least 8.9 x 10(6) L mol(-1). Such a high equilibrium constant (for both equilibria) is consistent with high-level ab initio quantum chemical calculations (K = 7.3 x 10(6) L mol(-1)) and thus appears to be physically realistic. Given that the addition rate coefficient for macroradicals to (polymeric) RAFT agent is 4 x 10(6) L mol(-1) s(-1), this implies that the lifetime of the RAFT adduct radicals is close to 2.5 s. Since the radical is also kinetically stable to termination, it can thus function as a radical sink in its own right.
authors.affiliation: Centre for Advanced Macromolecular Design, School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia.
authors.forename: Achim
authors.initials: A
authors.lastname: Feldermann
authors.forename: Michelle L
authors.initials: ML
authors.lastname: Coote
authors.forename: Martina H
authors.initials: MH
authors.lastname: Stenzel
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Davis
authors.forename: Christopher
authors.initials: C
authors.lastname: Barner-Kowollik