Abstract
Nitroxide-mediated polymerization (NMP) represents a viable way to produce polymers with predictable molecular weights (MWs) and well-defined architectures. Such polymers have potential application in the fabrication of commercially valuable block copolymers, drug delivery systems, nanotechnology, adhesives, surfactants, and polymer compatibilizers. The distinguishing feature of NMP is the use of a nitroxide, such as 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), to reversibly deactivate active polymer radicals to give a dormant alkoxyamine molecule (see Scheme 1). In Scheme 1, ka is the alkoxyamine dissociation rate coefficient, and kd is the radical deactivation rate coefficient. At 125 8C, the equilibrium coefficient for the reversible deactivation of polystyrene radicals by TEMPO has been measured to be Keq1⁄4 ka/kd1⁄4 2.1 10 11 M. Thus, the chemical equilibrium of the radical deactivation reaction favors the formation of the dormant alkoxyamine. As a result, the concentration of active polymer radicals in NMP systems is lower than in conventional free-radical polymerization systems, which reduces the likelihood Full Paper: A mathematical model has been developed to describe the interfacial mass transfer of TEMPO in a nitroxide-mediated miniemulsion polymerization (NMMP) system in the absence of chemical reactions. The model is used to examine how the diffusivity of TEMPO in the aqueous and organic droplet phases, the average droplet diameter and the nitroxide partition coefficient influences the time required for the nitroxide to reach phase equilibrium under non-steady state conditions. Our model predicts that phase equilibrium is achieved quickly (< 1 10 4 s) in NMMP systems under typical polymerization conditions and even at high monomer conversions when there is significant resistance to molecular diffusion. The characteristic time for reversible radical deactivation by TEMPO was found to be more than ten times greater than the predicted equilibration times, indicating that phase equilibrium will be achieved before TEMPO has an opportunity to react with active polymer radicals. However, significantly longer equilibration times are predicted, when average droplet diameters are as large as those typically found in emulsion and suspension polymerization systems, indicating that the aqueous and organic phase concentrations of nitroxide may not always be at phase equilibrium during polymerization in these systems. Influence of droplet phase TEMPO diffusivity, DTEMPO,drop, on the predicted organic phase concentration of TEMPO. Macromol. Theory Simul. 2002, 11, 953–960 953
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