- Radical loss in RAFT-mediated emulsion polymerizations. SW Prescott, MJ Ballard, E Rizzardo and RG Gilbert. Macromolecules, 38(11), 4901–4912, 2005.
Radical loss processes in RAFT-controlled emulsion polymerization are investigated using -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 -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 -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.
Last edited: Friday September 10, 2010
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