Masters Degrees (Chemistry and Polymer Science)
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Browsing Masters Degrees (Chemistry and Polymer Science) by browse.metadata.type "Thesis"
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- Item4-Aza-Podophyllotoxins: synthesis and use as anti-cancer agents(Stellenbosch : Stellenbosch University, 2020-03) Gould, Khaya TImothy; Van Otterlo, Willem A. L.; Blackie, Margaret; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Podophyllotoxins, and their derivatives, have shown significant anti-proliferative properties. Etoposide and other semi-synthetic analogues, such as teniposide, have been successfully employed as clinical anti-cancer agents. 4-Aza-podophyllotoxins, which are structurally simplified synthetic analogues of podophyllotoxin, have been successfully synthesised and have shown comparable biological activity to the naturally occurring (-) podophyllotoxin. To further investigate the method of inhibition of this class of compounds, which have been reported to act as either tubulin inhibitors or topoisomerase II poisons, two libraries of 4-aza-podophyllotoxin analogues were synthesised. The biological test results from the first library of analogues was used to inform the synthesis of a second library of compounds. The first library contains a diverse group of twelve 4-aza- podophylltoxin analogues, six with differences in the 4N- position. Half of these compounds were modelled after etoposide, and half modelled after the naturally-occurring (-)-podophyllotoxin. This was achieved through N- functionalization of the starting material, an aniline, prior to its use in a multicomponent reaction with tetronic acid and a functionalized benzaldehyde. The second library of four compounds was generated using an azide alkyne cycloaddition to modify two alkyne-containing compounds from the first library. The first library has shown IC50 values of nano-molar concentrations (125-250 nM) against a WHCO1 oesophageal cell line assay. A small structure activity relationship from these biological results was obtained.
- ItemA study on the reversible photo-induced isomerisation of platinum(II) and palladium(II) complexes of the N,N-dialkyl-N’-acyl(aroyl)thioureas with reversed-phase HPLC separation from related rhodium(III), ruthenium(III) and iridium(III) complexes(Stellenbosch : Stellenbosch University, 2014-04) Nkabyo, Henry Ane; Koch, Klaus R.; Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science.ENGLISH ABSTRACT: See item for full text
- ItemAB diblock copolymers via RAFT-mediated miniemulsion polymerization(Stellenbosch : Stellenbosch University, 2008-12) Bailly, Nathalie; Sanderson, R. D.; Tonge, M. P.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.The Reversible addition fragmentation chain transfer (RAFT) technique is a robust and versatile technique that enables the synthesis of polymers of controlled molecular weight and polydispersity. The application of the RAFT technique in heterogeneous aqueous media has attracted great interest in academics and industry due to it being more environmentally friendly, besides its other advantages. To date, the synthesis of well-defined high molecular weight polymers via the RAFT process under industrially relevant conditions still remains a challenge for polymer chemists. The study addresses the application of the RAFT process in heterogeneous media, namely in miniemulsion polymerization, for the synthesis of AB diblock copolymers of n-butyl methacrylate and styrene. AB diblock copolymers of high molecular weight were successfully prepared via a twostep method. In the first step, a dithiobenzoate monofunctional RAFT agent was used in bulk polymerization with the first monomer, n-butyl methacrylate. After the polymerization, the majority of the polymer chains contained the thiocarbonyl-thio RAFT agent functionality, which makes the chains potentially active for chain extension. The polymeric RAFT agent (also referred to as the starting block) obtained in the first step was chain extended in the second step, in miniemulsion, upon further addition of fresh initiator and the second monomer, styrene. The effects of the initiator/RAFT agent concentration ratio on the miniemulsion systems were investigated. The miniemulsion systems used for the high molecular weight AB diblock copolymers exhibited living features despite the high polydispersity indices. Kinetic results showed an increase in the rate of polymerization throughout the polymerization. Size exclusion chromatography (SEC) results indicated significant broadening in the molecular weight distributions and a steep increase in the polydispersity during the polymerization. It was concluded that the broad molecular weight distributions and steep increase in the polydispersity was not only related to the initiator concentration but possibly due to other factors such as inhomogeneity in the miniemulsion system and a transition in the kinetic behavior during the polymerization. Secondary particle formation emerged from kinetic data and transmission electron microscopy (TEM) results, but this were not supported by the SEC results. The effect of the use of a water-soluble initiator on the miniemulsion system was also investigated. Results indicated a similar behavioral pattern as observed in the AIBNinitiated systems, and not much improvement in terms of the molecular weight distributions and polydispersity was seen. The effect of the molecular weight of the diblock copolymers on the miniemulsion system was investigated. Poly(n-butyl methacrylate)-b-poly(styrene) diblock copolymers of lower molecular weight were synthesized via the two-step process. Kinetic results indicated a similar behavioral trend as to that of the high molecular weight diblock copolymers synthesized, however SEC chromatograms showed narrower molecular weight distributions and low polydispersity indices.
- ItemAn Ab Initio density functional study of the structure and stability of transition metal ozone complexes(Stellenbosch : Stellenbosch University, 2002-12) Venter, Gerhard (Gerhard Abraham); Dillen, J. L. M.; Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science.ENGLISH ABSTRACT: A thorough search through the literature as well as through the Cambridge Crystallographic Structural Database resulted in no examples of a neutral ozone acting as ligand in a complex with any metal. Ionic compounds containing ozonide as anionic species, however, are well known throughout the literature and not surprisingly the only result for 0₃ and a metal in the CCSD was an ionic rubidium ozonide compound. What follows is a systematic study into the result of placing an ozone ligand within complexing distance of a transition metal (the first transition row from titanium to copper). Due to the novelty of the system, as first approximation four different orientations of the ozone ligand relative to the metal (a metal cation in these calculations) were investigated. It was found that coordination through the terminal oxygens resulted in energy minima for all the metal cations, although not necessarily the absolute energy minimum on the potential energy surface for the specific cation. A further structural study was done by adding carbonyl and hydrogen ligands to the system, according to the 18-electron rule. For these calculations coordination through the terminal oxygens was employed. In both series the dissociation energy was also calculated. The dissociation energies for the M(CO)nHm(0₃) complexes were all positive, indicating that they are theoretically stable structures. The resulting wave functions were then analysed with the help of three techniques: Atoms in Molecules (AIM), Charge Decomposition Analysis (CDA) and Natural Bond Orbital Analysis (NBO). AIM showed that bonds were indeed formed between the ozone ligand and the transition metal and hinted that the bonding model can be interpreted with the Dewar-Chatt-Duncanson (DCD) model of σ-donation and л-back donation. CDA confirmed that this was the case. NBO results proved erroneous due to the largely delocalized electronic structure of the complexes.
- ItemAcolein in wine : bacterial origin and analytical detection(Stellenbosch : University of Stellenbosch, 2010-03) Bauer, Rolene; Crouch, A.; Burger, B.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Wine quality is compromised by the presence of 3-hydroxypropionaldehyde (3-HPA) due to spontaneous conversion into acrolein under wine making conditions. Acrolein is highly toxic and is presence has been correlated with the development of bitterness in wine. Lactic acid bacterial strains isolated from South African red wine, Lactobacillus pentosus and Lactobacillus brevis, are implicated in accumulating 3- HPA during anaerobic glycerol fermentation. The environmental conditions leading to its accumulation are elucidated. In aqueous solution 3-HPA undergoes reversible dimerization and hydration, resulting in an equilibrium state between different derivatives. Interconversion between 3-HPA derivatives and acrolein is a complex and highly dynamic process driven by hydration and dehydration reactions. Acrolein is furthermore highly reactive and its steady-state concentration in complex systems very low. As a result analytical detection and quantification in solution is problematic. This study highlights the roles played by natural chemical derivatives and shows that the acrolein dimer can be used as a marker for indicating the presence of acrolein in wines. Solid-phase microextraction (SPME) coupled to gas chromatograph mass spectrometry (GC-MS) was validated as a technique for direct detection of the acrolein dimer in wine. The potential of a recently introduced sorptive extractive technique with a sample enrichment probe (SEP) was also investigated. The SPME technique simplifies the detection process and allows for rapid sampling of the acrolein marker, while SEP is more sensitive.
- ItemAlkaloidal constituents of Crinum variabile and Crinum paludosum, two southern African Amaryllidaceae species(Stellenbosch : Stellenbosch University, 2020-03) Steyn, Kim; Van Otterlo, Willem A. L.; De Villiers, Andre J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: The extraction of dried bulbs of Crinum variabile and subsequent targeted fractionation led to the isolation of lycorine, 1,2-O,O-diacetyllycorine, 2-O-acetyllycorine, 1-O-acetyllycorine, a diastereomeric mixture of haemanthidine and 6-epihaemanthidine, bulbispermine and criwelline. HPLC-MS/MS analysis of the extract was carried out, providing information regarding other alkaloidal constituents of C. variabile. The isolation of lycorine, as well as a brief HPLC-MS/MS dereplication study was carried out on an extract of Crinum paludosum, allowing for the tentative identification of ten alkaloidal constituents therein, with five alkaloids remaining unidentified. A brief synthetic study was carried out on higginsianins A and B, resulting in the generation of some analogues thereof. These included acetylated analogues of both, as well as bromination products of higginsianin A, and reductive hydrogenation and epoxidation products of higginsianin B.
- ItemAlternating copolymerisation of carbon monoxide with a-olefins with a homogeneous Pd-Type catalyst(Stellenbosch : Stellenbosch University, 1997-12) Barkhuysen, L; Van Reenen, A. J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.AFRIKAANSE OPSOMMING: Hierdie studie behels die alternerende kopolimerisasie van koolstofmonoksied en a-olefiene met 'n homogene palladiumkatalisator. 'n Deeglike studie oor die agtergrond van die kopolimerisasie reaksies met oolstofmonoksied en a-olefiene is gedoen. Die homogene katalisatorsisteem, wat suksesvol gesintetiseer en gebruik is in die kopolimerisasie reaksies, is saamgestel uit 'n bidentate fosfienligand, nl. 1,3 bis(difenielfosfinopropaan), 'n palladium(II)spesies, nl. palladium asetaat, en 'n suur komponente, pora-tolueensulfoonsuur. Hierdie homogene palladium-katalisatorsisteem is gebruik om die volgende te produseer: - volkome alternerende etileen/koolstofmonoksied-kopolimere, -volkome alternerende 1-penteen/koolstofmonoksied-kopolimere en -volkome alternerende etileen/koolstofmonoksied/1-penteen terpolimere. - Die ko- enterpolimere is sover moontlik gekarakteriseer om die verwantskap tussen die hoeveelheid l-panteen geinkorpereer en die termiese eienskappe van die etileen/koolstofmonoksied/l-penteen-terpolimere in besonder, aan te toon. 'n Verwantskap tusen die reagens-gasdruk en opbrengs van die polimerisasiereaksies is ook waargeneem.
- ItemAmino acid analysis in wines by liquid chromatography : UV and fluorescence detection without sample enrichment(Stellenbosch : Stellenbosch University, 2003-12) Douglas, C. A. (Claire Anne); Sandra, P. J. F.; Lynen, F.; Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science .ENGLISH ABSTRACT: In this study, the analysis of ammo acids usmg High Performance Liquid Chromatography (HPLC) with pre-column derivatisation was optimised. The derivatisation reagents include o-phthaldialdehyde (OPA), 9- fluorenylmethylchloroformate (FMOC) and iodoacetic acid (IDA). Detection was performed using UV and fluorescence in series. The developed method was utilised for the analysis and quantitation of amino acids in eighteen wines. The application of chemometric data evaluation was initiated.
- ItemAmphiphilic electrospun fibres of poly(methacrylic acid)-graft-poly(dimethylsiloxane) copolymers as a means to controlling electrospun fibre morphology and obtaining nanofibre hydrogels(Stellenbosch : Stellenbosch University, 2014-04) Meltz, Freda-Jean; Mallon, Peter; Klumperman, Bert; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Novel poly(methacrylic acid)-graft-poly(dimethylsiloxane) copolymers were synthesised by conventional free radical reactions using a poly(dimethylsiloxane) macromonomer. The polymers were electrospun to investigate how the fibre morphology can be modified by manipulating the electrospinning solution parameters, and to determine the possibility of using the polymers as new materials for the production of polymer nanofibre hydrogels. The electrospinning solution parameters were varied by electrospinning the highly amphiphilic copolymers in solvents with variable solvent qualities. Scanning Electron Microscopy (SEM) and Field Emission Scanning Electron Microscopy (FE–SEM) was used to investigate the fibre morphology. Internal morphology was studied using a freeze fracture technique prior to FE-SEM imaging. It is revealed that the polymers in this study does not form any fine structure or pores even when self-assembled structures are present in the solution. Attempts were made to visualise any self-assembled structures of films produced from dilute solutions using TEM. Further studies included investigating the fibres properties, primarily with regards to their rate and extent of moisture and water uptake. The fibres showed hydrogel behaviour and the PDMS content were found to have an impact on the hydrogel stability. Post electrospinning crosslinking of the nanofibres was also explored.
- ItemAnalysis of organochloro-pesticides in Eritrean water and sediment samples(Stellenbosch : Stellenbosch University, 2004-12) Weldegergis, Berhane Tekle; Sandra, P. J. F.; Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science.ENGLISH ABSTRACT: The choice of the sample preparation technique used in combination with a chromatographic technique in environmental analysis strongly influences the performance of the method as a whole. The main aim of this work was to evaluate methods like: micro liquid-liquid extraction (цLLE)and solid phase micro extraction (SPME) in combination with gas chromatography-electron capture detector (GCECD) and a recently developed technique named stir bar sorptive extraction (SBSE) combined with thermal desorption system-programmable temperature vaporizationgas chromatography-mass spectrometer (TDS-PTV-GC-MS) for the analysis of organochloro-pesticides (OCPs) in natural water and to make a comparison between them. The most suitable method was then selected for the analysis of water samples collected from Eritrea. An additional goal was to develop a sensitive method for the analysis ofOCPs in soil and sediment samples. цLLE-GC-ECD and SPME-GC-ECD methods were evaluated for the analysis of OCPs in water. The former allowed achieving a sensitivity of less than 100 ppt and the latter was able to detect down to the 1 ppt level. Both methods were found to be repeatable with %RSD of3% to 19% and 5% to 12% as well as linear with a range of 1:103 and 1:5x103 respectivey. SBSE-TDS-GC-MS was also investigated for the analysis of OCPs in water samples. With this technique a good sensitivity down to 1 ppt was also obtained. The less sensitive nature of the MS compared to ECD was hereby compensated for by the large amount of polydimethylsiloxane (PDMS) phase on the stir bar. The method was repeatable and linear with a range of 1:5000, which is similar to the one obtained for the SPME-GC-ECD method. All three methods were also tested with real contaminated water samples and a comparison of the three techniques in terms of sensitivity, linearity, repeatability, availability and cost effectiveness was done. As a method of choice in terms of the above criteria, SPMEGC- ECD was applied to the water samples collected from Eritrea. The presence of some oePs such as a-BBC (benzenehexachloride), heptachlor, heptachlor epoxide, endosulfan I, p,p'-DDE (dichlorodiphenylchloroethylene), endosulfan II, p,p'-DDD (dichlorodiphenyldichloroethane), endosu!fan sulfate and p,p'-DDT (dichlorodiphenyltrichloroethane) in some of the Eritrean water samples was demonstrated The SPME-GC-ECD technique for the analysis OCPs in soil and sediment samples was investigated. It was shown to be able to detect down to 1 pg/g (lppt) with good linearity and repeatability. The method was also evaluated for authentic soil samples. Due to a lack of time the method could not be applied for the analysis of the soil and sediment samples collected from Eritrea.
- ItemAnioniese Fischer-tipe karbeenkomplekse as ligande(Stellenbosch : Stellenbosch University, 2003-12) Du Toit, Aletta; Raubenheimer, H. G.; Cronje, S.; Stellenbosch University. Faculty of Science . Dept. of Chemistry & Polymer Science.ENGLISH ABSTRACT: Please refer to fulltext for abstract.
- ItemApplication of comprehensive 2-dimensional liquid chromatography for the analysis of complex phenolic fractions(Stellenbosch : University of Stellenbosch, 2009-12) Kalili, Kathithileni Martha; De Villiers, A. J.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: The separation of apple, cocoa and green tea phenolic compounds by comprehensive 2-dimensional liquid chromatography (2-D-LC) has been studied. In the first dimension, phenolic compounds were separated according to polarity by hydrophilic interaction chromatography (HILIC) on a diol stationary phase with a mobile phase containing acetonitrile, methanol, acetic acid and water. Gradient reversed-phase (RP) LC using a C18 column with fluorescence detection was employed in the second dimension to separate compounds according to hydrophobicity. Compounds were identified using negative electrospray ionisation mass spectrometry (ESI-MS) coupled to both HILIC and RP separations. The coupling of HILIC and RP separations proved to be especially beneficial since this provided simultaneous information on both the polarity and hydrophobicity of phenolics. The low degree of correlation (r2 < 0.21) between the two LC modes afforded peak capacities in excess of 3000 for the off-line method. An on-line method was also developed utilizing a short, small particle-packed column to provide fast separation in the second dimension. A 1 mm i.d. column was used in the first dimension for the on-line system to reduce injection volumes onto the second dimension column. A significantly lower practical peak capacity was measured for the on-line system, due largely to the reduction in second dimension peak capacity. On the other hand, analysis could be performed in an automated fashion using the online system reducing the risk of sample alteration and guaranteeing better operation reliability and reproducibility. Especially the off-line comprehensive HILIC × RP-LC method developed demonstrated its utility in the analysis of various groups of phenolic compounds including proanthocyanidins, phenolic acids, flavonols and flavonol conjugates in a variety of natural products.
- ItemApplication of comprehensive two dimensional gas chromatography to wine analysis(Stellenbosch : Stellenbosch University, 2011-12) Vestner, Jochen; De Villiers, A. J.; Tredoux, A. G. J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: This study focused on the potential of comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC-TOF-MS) for the improved analysis of volatile wine constituents. Solid phase microextraction (SPME) in combination with GC×GC-TOF-MS was successfully used for the detailed investigation of the impact of three commercial Oenococcus oeni lactic acid bacteria (LAB) strains on the volatile composition of Pinotage wines subjected to malolactic fermentation (MLF). Due to increased separation power and enhanced sensitivity obtained by using two orthogonal separations coupled with the structural information provided by deconvoluted TOF-MS spectra, GC×GC-TOF-MS allowed for the identification and semi-quantitative analysis of much larger numbers of compounds compared to previous studies applying one-dimensional gas chromatography. The combination of univariate and multivariate statistical assessment was used as a powerful tool for data interpretation. The obtained results contribute significantly to the understanding of the impact of MLF on the volatile composition of Pinotage wine Some compounds have been linked to MLF for the first time. Moreover, the impact of these commercial starter cultures on the composition of volatile sulfur and nitrogen compounds in the same wines was studied by one-dimensional gas chromatographic methods with headspace injection and solid supported liquid-liquid extraction together with sulfur selective detection and tandem mass spectrometry. This study demonstrated also for the time, the impact of MLF on the composition of volatile sulfur and nitrogen compounds in Pinotage wine. GC×GC-TOF-MS was further used for the evaluation of the suitability of a new phase for stir bar sorptive extraction (SBSE) analysis of wine volatiles. Despite instrumental complications, beneficial extraction properties of the new stir bar phase for especially more polar compounds could be demonstrated. In addition, the extraction ability of this novel phase was evaluated for the analysis of selected thiazoles in wine using heart-cutting two dimensional gas chromatography in combination with nitrogen selective detection. Advantageous extraction performance of the new stir bar phase compared to a conventional polydimethylsiloxane (PDMS) phase for the determined thiazoles was demonstrated.
- ItemApplication of comprehensive two-dimensional gas chromatography for the characterisation of the volatile composition of Honeybush tea(Stellenbosch : Stellenbosch University, 2016-03) Ntlhokwe, Gaalebalwe Elizabeth; De Villiers, Andre J.; Stellenbosch University. Faculty of Science. Department of Chemistry and Polymer ScienceENGLISH ABSTRACT : Honeybush tea (Cyclopia spp.), like many other natural products, contains a large number of volatile compounds with different physiochemical properties and present at widely varying concentrations. This presents a significant analytical challenge for conventional one-dimensional gas chromatography (GC). The aim of this study was to explore the potential of comprehensive two-dimensional gas chromatography (GC×GC) for the improved analysis of honeybush tea volatile compounds. The first part of the study focused on the evaluation of a GC×GC system equipped with a novel single-stage thermal modulator. A chromatographic method well suited for the analysis of honeybush volatiles was successfully developed by systematic optimisation of instrumental parameters. Solid phase micro-extraction (SPME) was used for extraction of volatile compounds prior to GC×GC separation. Three Cyclopia species were analysed: C. genistoides, C. subternata and C. maculata. Chromatographic separation of honeybush volatiles was significantly improved on this system. 69 compounds were identified based on the retention data obtained by injection of reference standards in combination with flame ionisation detection (FID). Hyphenation to quadrupole mass spectrometry (qMS) was also explored, although the slow acquisition rate of the detector provided limited scope for identification of additional compounds. The highly reproducible performance of this modulator proved beneficial in the use of multivariate data analysis of the GC×GC-FID data. Significant differences in the volatile composition between the three Cyclopia species were observed, in general agreement with sensory studies for the same set of samples. In the second part of the study, a commercial GC×GC instrument equipped with a cryogenic modulator and hyphenated to time-of-flight MS (TOF-MS) was used for the detailed qualitative analysis of the same set of samples. This configuration provided new insight into the volatile composition of honeybush tea: the combination of enhanced GC×GC separation and sensitivity, coupled to the high-speed acquisition of TOF-MS allowed the tentative identification of 158 compounds in these samples for the first time. A total of 272 compounds were identified using either reference standards or mass spectral and retention index (RI) data. Significant differences in the volatile composition of C. genistoides, C. subternata and C. maculata were also observed in the GC×GC-TOF-MS data. (E)-cinnamaldehyde was identified in honeybush tea for the first time, only in C. maculata samples. This compound
- ItemThe application of ultrasonic nebulization to ICP-AES analysis of metallurgical and geological samples(Stellenbosch : Stellenbosch University, 1987) Barnardt, C. A. (Cornelius Andries); Cornell, D. H.; Stellenbosch University. Faculty of Science. Department of Chemistry and Polymer Science.ENGLISH ABSTRACT: The purpose of this project was to investigate the application of ultrasonic nebulization (with and without desolvation) to inductively coupled plasma atomic emission spectroscopy (ICP-AES) for the detection of low concentration elements in metallurgical and geological samples. The investigation included a study on the droplet sizes, the influence of desolvation on the plasma parameters and the determination of the detection limits. Many promising results were obtained in this project. The ultrasonic nebulizer has proved to be a very efficient component in ICP-AES. The results have shown the ultrasonic nebulizer is especially suited for elements at low concentration levels as the superior power of detection produced by the ultrasonic nebulizer cannot be matched by pneumatic nebulizers. It was found that desol vation is not essential for analyses, although the results have shown that even better sensitivity and therefore better powers of detection are obtainable with the use of desolvation. An important and hitherto unknown result of the desolvation investigation is the existence of an optimum desolvation temperature where the analyte emission intensity displays a maximum. A complete Simplex optimization of the whole instrument and procedure was therefore vital for optimum results. A knowledge of the influence of the solvent on the plasma parameters proved to be essential for a better understanding of the applicability of the ultrasonic nebulizer for analyses. However the ultrasonic nebulizer system developed in this project is unfortunately not quite suitable for quick routine analysis due to slow sample changeover, but certain modifications is proposed to improve the speed of analysis.
- ItemApplications of inherently chiral calix[4]arenes(Stellenbosch : Stellenbosch University, 2013-03) Van Laeren, Laura Jane; Arnott, Gareth E.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Calixarenes are large cyclic molecules that have a distinctive bowl-shaped geometry. The nonplanar nature of these molecules means that, by selectively functionalizing the calixarene on one side of the symmetry plane, one introduces inherent chirality. As with other varieties of chirality, this asymmetry can be utilised to impart stereoselectivity to the formation of new bonds through asymmetric catalysis. The evaluation of inherently chiral calixarenes has been hindered thus far by the difficulty in obtaining enantiomerically pure samples. Using a recently developed stereoselective methodology, incorporating chiral oxazoline directed ortholithiation, the synthesis of a series of upper-rim meta-substituted inherently chiral calixarenes is described. With the use of this methodology, the desired calixarene ligands are synthesised in high diastereoselectivity (from 75% de to >99% de). The inherently chiral meta-substituted bidentate thioether-oxazoline calixarenes synthesised were subsequently investigated as asymmetric ligands for palladium catalysed allylic alkylation. The debutylated series of calixarenes showed good catalytic efficiency, achieving high levels of conversion (>90% isolated yield). A rate enhancement relative to a planar model system was observed. Moderate levels of enantioselectivity (31% ee to 89% ee) were achieved. The influence of the central chirality of the chiral oxazoline was determined to be the predominant stereoselective effect. Increasing the steric bulk on this chiral carbon resulted in a significant increase in the stereoselectivity. Inherent chirality was found to have a subtle but significant effect. Increasing the steric bulk on the calixarene bowl, through the use of analogous tert-butylated calixarene, had an adverse effect on the catalytic efficiency. These ligands formed unstable complexes that decomposed before any appreciable yield of the desired product could be formed.
- ItemAn assessment of variation within IAS 41 compliant methodology used in South Africa to estimate the value of pulpwood plantations(Stellenbosch : Stellenbosch University, 2016-03) Baptista, Carl; Ham, Cori; Howard, Michael; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.ENGLISH ABSTRACT: This study sought to assess the possible variations within International Accounting Standard (IAS) 41 compliant valuation methodologies used within South Africa to estimate the value of pulpwood plantations. IAS 41 compliant valuation models were collected from valuation consultants and companies active within the South African forestry sector. Along with the collection of models, model input parameters and methods for the determination of input parameters were retrieved. Models were amended to accept default standardised inputs. These default inputs consisted of case study plantation data sourced from an unnamed plantation in South Africa. Valuations were calculated for this case study plantation using the various models, and used to assess the possible variances between model valuation outputs. In this way the variances derived from the different model mechanisms could be compared to each other. A sensitivity analysis was then performed in order to understand the effect of each parameter upon the valuation output of each model. This study indicated that there are significant differences between valuation outputs as calculated from a range of IAS 41 compliant valuation models. The collected parameters and parameter classification data also highlighted that certain parameters, age class and growth rate in particular, were being calculated or determined in different ways by users resulting in further sources of potential variance in the calculated values produced by the models. The study concluded with an evaluation of each of the six unique models in the study. Two main aspects were identified that need to be addressed namely: (i) The standardisation of a model to be used for all valuation purposes; (ii) The provision of rigid guidelines regarding the standardisation of model input parameters.
- ItemAsymmetric flow field flow fractionation (AF4) of polymers with focus on polybutadienes and polyrotaxanes(Stellenbosch : Stellenbosch University, 2012-03) Makan, Ashwell Craig; Pasch, H.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Over the past two decades, field flow fractionation (FFF), as a polymer characterization technique, has become cutting edge technology. The demand for molar mass and size characterisation of complex polymer systems has increased, especially in cases where classical calibration techniques such as size exclusion chromatography (SEC) has shown several shortcomings. FFF is a technique resembling chromatography. It has several significant advantages over SEC, especially for the characterisation of ultrahigh molar mass (UHMM), branched and gel-containing polymers. In this study, polybutadienes, which often contain the abovementioned species, were analysed by SEC and asymmetric flow field flow fractionation (AF4). Both separation techniques were coupled to refractive index and multi-angle laser light scattering detection. Similarly, polyrotaxanes, which are polymers with complex and unique molecular architectures, were also investigated. Results showed that AF4 can explicitly be used as a superior tool over SEC. In the case of UHMM polybutadienes, much higher molar masses could be detected by AF4, due to the absence of shear degradation which is often encountered in SEC. Gel-containing species could be detected by AF4 as no filtering is required prior to injection. Abnormal retention behaviour, a phenomenon often encountered in UHMM branched polymers, was observed in SEC analysis of the polyrotaxanes materials. AF4 provided sufficient separation from low to high molar masses, without out any irregularities.
- ItemAtom transfer radical polymerisation of unusual monomers(Stellenbosch : Stellenbosch University, 2003-12) Staisch, Ingrid; Sanderson, R. D.; Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science.ENGLISH ABSTRACT: Controlled free radical polymerisation techniques offer several practical and theoretical advantages compared to many other polymerisation techniques. Living polymerisation techniques such as anionic polymerisations require the total exclusion of impurities such as oxygen and moisture. Controlled free radical polymerisations, however, do not require such stringent methods of practice. This is very advantageous for industrial purposes. Atom Transfer Radical Polymerisation (ATRP) is a form of a controlled/living free radical polymerisation technique by which one is able to synthesize controlled architectural structures and predetermine the molecular weights of macromolecules. The monomers that were investigated for this research project include methyl methacrylate (MMA), 4-vinylpyridine (4VP) and lauryl methacrylate (LMA). The latter two monomers (4VP and LMA) are not commonly used in ATRP-mediated reactions. The synthesis of block copolymers ofMMA and LMA were attempted. The homopolymerisation of 4VP did not give the control expected when polymerising by means of ATRP. This prompted an investigation into possible side reactions that could take place with 4VP in this specific ATRP system. This included possible quatemization of 4VP with the alkyl halide initiator species.
- ItemAttempted routes towards the synthesis of fluorinated analogues of ornithine as potential inhibitors of ornithine decarboxylase(Stellenbosch : University of Stellenbosch, 2007-03) De Villiers, Jandre; Strauss, Erick; Jardine, Anwar; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.Human African Trypanosomiasis (HAT) is a disease that threatens more then 60 million men, woman and children in Africa. It is known that the inhibition of the enzyme, ornithine decarboxylase (ODC) leads to cell arrest and subsequent death of Trypanosoma brucei, the parasite that causes the disease. The fluorinated ornithine analogue, DFMO (difluoromethylornithine or eflornithine) is a known inhibitor of ODC. Although various syntheses for DFMO exist they have some practical drawbacks which prevent the cost effective production of this compound as a drug for HAT treatment. This work focuses on the synthetic preparation of the fluorinated ornithine analogue DFMO as well as the fluorinated ornithine analogues 2-MFMO, 3-fluoro-ornithine and 3,3-difluoro-ornithine. Our chosen synthetic methodology focused on the introduction of the fluorine functionality using a simpler, safer and more convenient method than current direct fluorination techniques, or those that rely on the use of CFCs. Instead we decided to develop and optimise a fluorodehydroxylation method based on the transformation of hydroxylated ornithine analogues. The fluorodehydroxylation method substitutes a hydroxyl group to the corresponding fluorine and can also be used to transform an aldehyde or ketone to the corresponding difluoro group. Application of this fluorination method requires the synthesis of appropriate ...