Michael K. Georges

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.

Adsorption of sulfur onto a surface of silver nanoparticles stabilized with sago starch biopolymer.

Adsorption of sulfide ions onto a surface of starch capped silver nanoparticles upon addition of thioacetamide was investigated. UV-vis absorption spectroscopy revealed that the adsorption of the sulfide ion on the surface of the silver nanoparticles induced damping as well as blue shift of the silver surface plasmon resonance band. Further increase in thioacetamide concentration led to shift of the resonance band toward higher wavelengths indicating the formation of the continuous Ag2S layer on the silver surface. Thus fabricated nanoparticles were investigated using electron microscopy techniques (TEM, HRTEM, and HAADF-STEM) and X-ray photoelectron spectroscopy (XPS), which confirmed their core-shell structure.

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.

Verdazyl Radicals as Substrates for Organic Synthesis: A Synthesis of 3-Methyl-5-aryl-1,3,4-oxadiazolones

The synthesis of oxadiazolones under hydrolytic conditions is described for a series of 3-methyl-5-aryl-1,3,4-oxadiazolone compounds. The unique starting materials for the hydrolysis reaction are obtained from efficient 1,3-dipolar cycloaddition reactions of styrene and azomethine imine dipoles derived from verdazyl radicals via a disproportionation reaction. A proposed mechanism for the formation of these biologically relevant oxadiazolones includes an opening of the tetrazinone ring followed by a 5-exo-trig ring closure. In support of the mechanism, in one case the ring-opened intermediate was isolated and subsequently treated with acid to give the relevant oxadiazolone.

Biopolymer-protected CdSe nanoparticles.

A synthetic procedure for the encapsulation of cadmium selenide (CdSe) nanoparticles in a sago starch matrix is introduced. The nanocomposite was investigated using structural, spectroscopic, and thermal methods. TEM micrographs of the nanocomposite showed spherical CdSe particles of 4-5 nm in size coated with a biopolymer layer. The absorption edges of both the aqueous solution and the thin film of the CdSe-starch nanocomposite were shifted toward lower wavelengths in comparison to the value of the bulk semiconductor. Infrared measurements revealed that the interaction of CdSe nanoparticles and starch chains takes place via OH groups. Although the onset of the temperature of decomposition of CdSe-starch nanocomposite is lower than that of the pure matrix, thermogravimetric analysis also showed that introduction of CdSe nanoparticles significantly reduced starch degradation rate leading to high residual mass at the end of the degradation process.

Inhibition of Microbial Growth by Silver–Starch Nanocomposite Thin Films

A sago starch biopolymer with embedded silver nanoparticles has been studied as a material for the prevention of microbial growth. Approximately 8 nm in size, silver nanoparticles have been synthesized by reduction of the silver salt in aqueous solution in the presence of sago starch using sodium borohydride as a reducing agent. The obtained solutions were cast on glass plates to obtain thin supported silver–starch nanocomposite films. The morphology of the nanocomposites was investigated by scanning and transmission electron microscopy. UV-Vis absorption spectroscopy showed that during the film formation a part of the silver nanoparticles has been trapped in the water present in the sample, which enabled their partial oxidation into active Ag+ species. The oxidation of the silver nanoparticles was confirmed by X-ray photoelectron spectroscopy. The antimicrobial activity tests have shown that the nanocomposite material can be successfully employed to prevent the viability and growth of the common pathogens Staphylococcus aureus, Escherichia coli and Candida albicans.

A hydrazine- and phosgene-free synthesis of tetrazinanones, precursors to 1,5-dialkyl-6-oxoverdazyl radicals.

A complementary approach to published synthetic methods for tetrazinanones, precursors to verdazyl radicals, is described herein. This approach uses carbohydrazide, a commercially available reagent, as a common starting material. Unlike previous methods described in the literature, this synthetic scheme does not rely on phosgene, phosgene substitutes, or the limited pool of commercially available monosubstituted hydrazines for its execution. A large variety of alkyl substitution patterns at the N-1 and N-5 positions of verdazyl radicals are possible, including both symmetrically and unsymmetrically substituted products. An initial condensation reaction of carbohydrazide with a specific aldehyde introduces the desired C-3 substituent in the final verdazyl radical product and protects the NH(2) groups during the subsequent N-1 and N-5 alkylation reactions. A succeeding methanolysis and concomitant ring-closing reaction gives the tetrazinanone. A number of known oxidation methods can then be employed to form the final verdazyl radical product.

Suspension polymerization processes

Several processes can be employed for manufacturing of polymer materials. Each process presents some intrinsic features that lead to production of resins with peculiar structural and morphological characteristics that define the final polymer properties and the end-use application of the obtained polymer material. Suspension polymerization processes are extensively used because of their many advantages, including easy separation of the polymer particles, easy removal of the heat of reaction, easy temperature control, and low levels of impurities and additives in the final polymer resin. For this reason, suspension polymerization processes are suitable for production of polymer resins intended for many distinct applications, including biotechnological, medical, and dental applications. The main objective of this article is to discuss the fundamental aspects of suspension polymerization processes, focusing upon the effects of the main process variables on the performance of suspension polymerizations. Representative types of suspension polymerization (eg, pearl, precipitation, suspension-emulsion, reverse, microsuspension, and seeded polymerizations), copolymerization, determination of reactivity ratios, and typical operation conditions practiced in commercial processes are also discussed. Keywords: suspension polymerization; polymer; particle; monitoring; copolymerization