Barkev Keoshkerian

New water-dispersible silica-based pigments: synthesis and characterization

Abstract Water-dispersible pigments were prepared by grafting reactive dyes to the surface of derivatized silicas. The silica core consisted of either fumed hydrophilic silica with surface areas of 200 or 380 m2g−1 or monodisperse spherical silica ranging in diameter from 40 to 500 nm. The silica surface was derivatized with silane coupling agents, such as aminopropyltriethoxysilane (APS) and N,N-bis(2-hydroxyethyl)-3-aminopropyltriethoxysilane (BHAPS). The coloration proceeded on suspensions of the derivatized silica in water in the presence of a soluble reactive dye. The resulting brightly colored pigments were characterized by particle size analysis and visible absorption spectroscopy.

Nitroxide-mediated radical polymerization of styrene in miniemulsion: model studies of alkoxyamine-initiated systems

Abstract A mathematical model has been developed to describe the behavior of the nitroxide-mediated miniemulsion polymerization (NMMP) of styrene initiated by alkoxyamine initiators. The model includes mechanisms describing reactions in the aqueous and organic phases, particle nucleation, the entry and exit of oligomeric radicals, and the partitioning of nitroxide and styrene between the aqueous and organic phases. The influence of nitroxide partitioning on the polymerization kinetics was examined by modeling systems initiated by the alkoxyamines BST and hydroxyl-BST; BST and hydroxyl-BST are benzoylstyryl radicals terminated by the nitroxides TEMPO and 4-hydroxyl-TEMPO, respectively. Predicted monomer conversions, number average molecular weights and polydispersities were in agreement with experimentally measured values. Simulations and mathematical analysis showed that the rate of styrene NMMP is not strongly influenced by the partitioning properties of TEMPO and 4-hydroxyl-TEMPO because of the complex interaction between reaction equilibrium, phase equilibrium, termination and thermal initiation. However, in the absence of styrene thermal initiation, nitroxide partitioning was found to have a significant influence on the polymerization kinetics. The model was also used to make quantitative estimates of: the population of active and dormant polymer radicals derived from both alkoxyamine initiators and thermal initiation; the population of dead polymer chains; and the number molecular weight distributions of living and dead polymer chains.

Nitroxide partitioning between styrene and water

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 organic-phase 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.

Interfacial Mass Transfer in Nitroxide-Mediated Miniemulsion Polymerization

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

Nitroxide mediated living radical polymerization of styrene in miniemulsion: modelling persulfate-initiated systems

Recently we have constructed a mechanistic model describing the nitroxide mediated miniemulsion polymerization (NMMP) of styrene at 135°C, using alkoxyamine initiators to control polymer growth (Nitroxide-Mediated Polymerization of Styrene in Miniemulsion. Modeling Studies of Alkoxyamine-Initiated Systems, 2001b). The model has since been expanded to describe styrene NMMP at 135°C using TEMPO and the free radical initiator, potassium persulfate (KPS). The model includes mechanisms describing reactions in the aqueous and organic phases, particle nucleation, the entry and exit of oligomeric radicals, and the partitioning of nitroxide and styrene between the aqueous and organic phases. Predicted monomer conversions, number average molecular weights and polydispersities were in agreement with experimentally measured values. Model simulations revealed that for systems employing high ratios of TEMPO:KPS, the consumption of TEMPO by polymer radicals derived from KPS decomposition and styrene thermal initiation (using the accepted literature kinetic rates) is not sufficient to lower TEMPO concentrations to levels where polymer growth can occur. By accounting for the consumption of TEMPO by acid-catalyzed disproportionation, TEMPO concentrations are significantly reduced, allowing for accurate model predictions of monomer conversion, number average molecular weight and polydispersity at every experimental condition considered.