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.

Living Radical Polymerization by the RAFT Process—A First Update

“…Values of K have also been estimated on the basis of simulation of the polymerization kinetics. [44][45][46] However, most simulations have assumed a simplified RAFT mechanism in which K is assumed independent of the radical structure. Values of K estimated on the basis of the measured concentrations of the radical intermediates are much lower than those predicted by theoretical calculations.…”

“…[135] [61] S/VBSC [62] 8* AN [63] (DBI) [64] DMAEMA [65] 9* (DBI) [64] DMAEMA [68] GMA [68] HPMAM/NMS [66] MMA [67] HEMA [68] (MA) [69] MMA [46,70,71] NIPAM [72] PFMA [73] (S) [44,74] S [75] S [76] S/AN [68] S/MAH [68] S S CN S S CN…”

Bimolecular radical termination: New perspectives and insights

“…The only caveat is that side reactions must be absent. [31][32][33] Such side reactions include intermediate radical termination (IRT), [34][35][36] slow fragmentation (SF), 37,38 or impurities in the RAFT agent. 31,32 By combining RAFT with a simple nonsteady-state method, the effects of i and x on k t can be accurately and reliably evaluated, avoiding the problem of broad MWDs.…”

Use of Spin Trap Technique for Kinetic Investigation of Elementary Steps of RAFT-Polymerization

“…10 4 L mol -1 s -1 ). In this case, model fitting to experimental kinetic data indicates that the intermediate radicals are much more stable (K= 1.06 × 10 7 L mol -1 ) and that there is no significant amount of termination products (Feldermann et al, 2004). However, this so-called slow fragmentation model also predicts intermediate radical concentrations that are incompatible with the available ESR data, measured for polymerizing systems.…”