Consistent Experimental and Theoretical Evidence for Long-Lived Intermediate Radicals in Living Free Radical Polymerization

The cumyl dithiobenzoate (CDB)-mediated reversible addition fragmentation chain transfer (RAFT)
polymerization of styrene at 30 °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 × 106 L mol-1. Such a high equilibrium constant (for both equilibria)
is consistent with high-level ab initio quantum chemical calculations (K = 7.3 × 106 L mol-1) and thus
appears to be physically realistic. Given that the addition rate coefficient for macroradicals to (polymeric)
RAFT agent is 4 × 106 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.