Nitroxide partitioning between styrene and water

✍ Scribed by John W. Ma; Michael F. Cunningham; Kim B. McAuley; Barkev Keoshkerian; Michael K. Georges

• Publisher: John Wiley and Sons
• Year: 2001
• Tongue: English
• Weight: 151 KB
• Volume: 39
• Category: Article
• ISSN: 0887-624X
• DOI: 10.1002/1099-0518(20010401)39:7<1081::aid-pola1085>3.0.co;2-4

✦ Synopsis

Research into nitroxide-mediated radical polymerization (NMRP) performed in emulsions and miniemulsions has progressed significantly over the past several years. However, our knowledge of the conditions during polymerization (e.g., the nitroxide concentrations in the aqueous and organic phases) is incomplete, 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-TEMPO between styrene and water from 25 to 135 °C. Experiments were performed in a 250-mL Parr reactor that was equipped for the simultaneous sampling of the aqueous and organic phases. Aqueous-phase and organicphase nitroxide concentrations were measured with ultraviolet-visible spectrophotometry. Experiments were also performed at 135 °C in the presence of hexadecane (costabilizer), polystyrene, and sodium dodecylbenzenesulfonate (surfactant) to determine the effects of the miniemulsion polymerization recipe ingredients on the partitioning of TEMPO and 4-hydroxy-TEMPO. On the basis of the measured partition coefficients (expressed as the ratio of the nitroxide concentration in the organic phase to the nitroxide concentration in the aqueous phase), 4-hydroxy-TEMPO was the most hydrophilic of the nitroxides investigated, followed by 4-amino-TEMPO and TEMPO. Hexadecane, polystyrene, and sodium dodecylbenzenesulfonate did not have a significant influence on the partitioning of these nitroxides at 135 °C. Experiments with ethylbenzene instead of styrene demonstrated that thermally generated radicals were not responsible for the observed temperature effects on the measured partition coefficients.