Radical Loss in RAFT-Mediated Emulsion Polymerizations

Abstract

Radical loss processes in RAFT-controlled emulsion polymerization are investigated using γ-initiated seeded emulsion polymerization of styrene in “relaxation mode”, i.e., following the rate of polymerization after removal from the radical source. This provides direct measurement of radical loss processes. A water-insoluble RAFT agent, 2-phenylprop-2-yl phenyldithioacetate, was transported to preformed seed particles using acetone, the acetone then removed, the particles swollen with monomer, and RAFT-mediated polymerization initiated by γ-irradiation. The systems show good control over the molecular weight of the products and a retardation dependent on the concentration of RAFT agent. Kinetic parameters are obtained from the γ-relaxation experiments, using pseudo-bulk kinetics as an approximation to full compartmentalization/chain-length-dependent kinetics to describe the systems. The rate of radical loss was strongly affected by the presence of RAFT agent and was found to decrease with increasing length of the dormant chain. The interpretation of this observation is as follows. The termination rate coefficient depends on chain length; the dominant mode of termination is the reaction between chains of similar length in RAFT-mediated systems but between short and long chains in conventional systems. Radical entry into particles is assumed to be by chains of degree of polymerization z formed in the aqueous phase. Dormant z-meric chains in the particles are postulated to lead to an increase in the rate of radical exit from the particles, with the reactivation of these species generating a z-meric radical that is able to desorb from the particle surface in a RAFT-induced exit mechanism, leading to the rapid exchange of radicals between particles and hence radical loss. A simple model for the radical loss rate coefficient, expected to be valid for longer chain lengths, both reproduces much more computationally expensive Monte Carlo calculations and (when used with the scaling suggested by reptation theory) gives quantitative accord with the relaxation data.

Modeling of Diffusion Limitations in Bulk RAFT Polymerization

“…This can easily be observed in Figure 7(b), where the dashed line representing dP n =dt ð Þ t is very closet to the thick full line representing R n . In the previous discussion, we concluded that, as expected, [21][22][23] the termination of shorter chains is favored with respect to that of longer chains by increasing the values of f, the ratio DP R n =DP crit and the polydispersity of the radical chains. Let us now see how this is connected to the deviation between the predictions of the conversion curves obtained by DP and the CLD model.…”

Computational Studies of RAFT Polymerization–Mechanistic Insights and Practical Applications

“…As noted above, there is some debate as to whether the RAFT-adduct radical is capable of undergoing irreversible and/or reversible termination reactions (see Scheme 3) to any significant extent in normal polymerizing RAFT systems. [6,7,[76][77][78][79][80][81][82][83][84] Although computational quantum chemistry is not currently capable of predicting diffusion-controlled rate coefficients, computational calculations could nonetheless be used to calculate the chemically controlled component of the termination rate and establish a lower boundary to the termination rate coefficient. To this end, work is currently underway to Figure 1.…”

On the Origins of Nitroxide Mediated Polymerization (NMP) and Reversible Addition–Fragmentation Chain Transfer (RAFT)

“…We have discussed these challenges in our contributions to this topic [78][79][80] and in our comprehensive reviews of the RAFT process. [81][82][83] RAFT polymerization can be initiated by any source of free radicals including UV irradiation [84] and gamma irradiation [85] and can be performed over a wide range of temperatures, including room temperature.…”