Nitroxide partitioning between styrene and water

Research into nitroxide-mediated radical polymerization (NMRP) performed in emulsions and miniemulsions has progressed significantly over the past sev eral years. However, our knowledge of the conditions during polymerization (e.g., the nitroxide concentrations in the aqueous and organic phases) is i ncomplete, and as such we have yet to achieve a clear understanding of the mechanisms involved in these processes. To better understand the conditions present in heterogeneous NMRP, we measured the partition coefficients of 2 ,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO), 4-hydroxy-TEMPO, and 4-amino-T EMPO between styrene and water from 25 to 135 degreesC. Experiments were pe rformed in a 250-mt Parr reactor that was equipped for the simultaneous sam pling of the aqueous and organic phases. Aqueous-phase and organic-phase ni troxide concentrations were measured with ultraviolet-visible spectrophotom etry. Experiments were also performed at 135 degreesC in the presence of he xadecane (costabilizer), polystyrene, and sodium dodecylbenzenesulfonate (s urfactant) to determine the effects of the miniemulsion polymerization reci pe ingredients on the partitioning of TEMPO and 4-hydroxy-TEMPO. On the bas is of the measured partition coefficients (expressed as the ratio of the ni troxide concentration in the organic phase to the nitroxide concentration i n the aqueous phase), 4-hydroxy-TEMPO was the most hydrophilic of the nitro xides investigated, followed by 4-amino-TEMPO and TEMPO. Hexadecane, polyst yrene, and sodium dodecylbenzenesulfonate did not have a significant influe nce on the partitioning of these nitroxides at 135 degreesC. Experiments wi th ethylbenzene instead of styrene demonstrated that thermally generated ra dicals were not responsible for the observed temperature effects on the mea sured partition coefficients. (C) 2001 John Wiley & Sons, Inc.