Department of Chemistry and Polymer Science
Permanent URI for this community
Browse
Browsing Department of Chemistry and Polymer Science by browse.metadata.type "Thesis"
Now showing 1 - 20 of 585
Results Per Page
Sort Options
- ItemA 195Pt Nuclear magnetic resonance and molecular dynamics computer simulation study of the solvation of simple platinum (IV) chlorido complex anions in water and water-miscible solvent mixtures(2017-03) Engelbrecht, Leon de Villiers; Koch, Klaus R.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: A combined 195Pt NMR spectroscopy and Molecular Dynamics (MD) computer simulation study of the solvation of the octahedral Pt(IV) complex [PtCl6]2‒ in binary mixtures of water and the fully water-miscible organic solvents methanol, 2-methoxyethanol and 1,2-dimethoxyethane has been carried out. A recent 195Pt NMR chemical shift-trends study indicated a preferential solvation of the aforementioned platinum complex by the organic solvent component in such solvent mixtures. The solvent dependence of the intrinsic 1Δ195Pt(37/35Cl) NMR isotope shifts of [PtCl6]2‒ in pure solvents indicate a slight increase in magnitude ~7 ppb in the order water < methanol < 2-methoxyethanol < 1,2-dimethoxyethane. In selected equimolar binary mixtures of water and organic solvents, 1Δ195Pt(37/35Cl) is found to be similar in magnitude to that in the pure organic solvents, supporting the proposed preferential solvation of the platinum complex. 195Pt NMR T1 spin relaxation times and pulsed gradient spin echo (PGSE) translational diffusion measurements were performed for [PtCl6]2‒ in selected solvents and binary mixtures. The results were interpreted in the context of hydrodynamic continuum models of molecular diffusion; while these were found to be not strictly appropriate, the rotational and translational dynamics results appear to be notionally consistent with the preferential solvation phenomenon as indicated. A series of classical MD computer simulations were performed for [PtCl6]2‒ in these equimolar binary solvent mixtures, using a recently revised force field developed by Naidoo et al. The results using the standard force field indicate a strong preference for water in the primary solvation shell region of the complex in all solvent mixtures studied. A similar result is obtained for [PtCl4]2‒ in an equimolar water‒methanol mixture. Simulations were repeated with ionic charges scaled according to the recently developed Molecular Dynamics in Electronic Continuum (MDEC) theory, which is intended to account for the dielectric screening of charges in condensed phases. In these MDEC simulations, a significant reduction in the contribution of water to the primary solvation shells of both complexes is observed; this is particularly evident in the solvation shell of [PtCl6]2‒ in mixtures of water with 2-methoxyethanol and 1,2-dimethoxyethane, for which a strong preferential solvation by the organic components has been indicated. Dynamic properties were also computed from MD trajectories, and are qualitatively consistent with experimental trends, but deviate due to the solvent model combination. Finally, an interpretation of the fascinating 35/37Cl and 16/18O isotope-induced fine structure in the 195Pt NMR spectra of complexes of the type [PtCln(OH)6‒n]2‒, n = 0‒5, is presented, based on the expected trans-influence series of ligands for Pt(IV) OH‒ > Cl‒ > H2O in aqueous solution.
- 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.
- Item59Co NMR, a tool for the study of the structure, reactivity and supramolecular chemistry of Co(III) complexes derived from a series of N-alkyl-N-alkyl(aryl)-N’-acyl(aroyl)thioureas(Stellenbosch : Stellenbosch University, 2020, 2020-03) Barnard, Ilse; Koch, Klaus Robert; Gerber, Wilhelmus Jacobus; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: A large library of Co(III) complexes, derived from selected monopodal and bipodal N,N- dialkyl-N’-acyl(aroyl)thioureas, have been synthesized and characterized. These ligands form tris(chelated) complexes in the thermodynamically more stable fac geometry with the cobalt metal. The monopodal ligands, and their corresponding Co(III) complexes, were separated into two groups based on the two R substituents of the C(S)-NRR’ function. The first group were named the symmetrically substituted acylthiourea derivatives, where R = R’. Due to the increased stability provided by the chelate effect as well as the selective population of the lower-energy d-orbitals, such complexes are generally expected to be kinetically stable in solution. Nonetheless, the ‘symmetrical’ Co(III) complexes were utilized for the study of the unexpected, slow and spontaneous ligand exchange reaction between non-identical homoleptic pairs of low-spin d⁶ octahedral fac-[Co(LA-κS,O)3] and fac-[Co(LB-κS,O)3] complexes in organic solvents. The exchange reaction between these complexes result in mixtures of their corresponding heteroleptic fac-[Co(LA-κS,O)2(LB-κS,O)] and fac-[Co(LA-κS,O)(LB-κS,O)2] complexes in solution. This discovery was followed by a quantitative determination of the ligand exchange equilibria as well as a relative rate study, as a function of temperature, of the reaction using rp-HPLC. It was then established that the high chemical shift sensitivity of ⁵⁹Co NMR is a powerful tool for the easy characterization of the Co(III) complexes derived from symmetrically substituted acylthioureas in solution. The utility of ⁵⁹Co NMR as a spectroscopic tool was exemplified after utilizing this technique for further additional studies of factors shown to dramatically affect the relative rate of ligand exchange in these complexes, specifically light and solvent. The second group of monopodal acylthiourea ligands investigated were named the asymmetrically substituted acylthioureas, where R ≠ R’. The partial double bond character in the C-N bond of the C(S)-NRR’ function results in the E,Z configurational isomerism of these ligands in solution. The isomerism is expressed as duplicate resonances in the ¹H NMR spectra of the uncoordinated ‘asymmetrical’ acylthiourea ligands. The Z to E isomer ratio varies depending on the two R and R’ substituents of the thiourea moiety. Notably, the isomerism in the uncoordinated ligands is passed on to the Co(III) chelates after coordination. As for the symmetrically substituted acylthiourea Co(III) complexes, we find that the ⁵⁹Co NMR chemical shift is very sensitive to the structure of the asymmetrical fac-[Co(Lⁿ-κS,O)3] complexes in solution. Significantly, the presence of the four, theoretically possible, fac- [Co(EEE-Lⁿ-κS,O)3], fac-[Co(EEZ-Lⁿ-κS,O)3], fac-[Co(EZZ-Lⁿ-κS,O)3] and fac-[Co(ZZZ-Lⁿ- κS,O)3] isomers is readily observable by means of ⁵⁹Co NMR, which shows four well-resolved resonances. From the relative peak integrals of the ⁵⁹Co NMR peaks, a semi-quantitative estimate of the relative amounts of the configurational isomers in solution was possible, although assignment of the isomers is not trivial. The assignment of the ⁵⁹Co peaks to each of the EEE, EEZ, EZZ and ZZZ configurational isomers of the asymmetrical fac-[Co(Lⁿ-κS,O)3] complexes were based on the relative E/Z ratio of the uncoordinated ligands, which was established from their ¹H NMR spectra. However, this assignment assumes that the relative E/Z ratio does not vary significantly during coordination to cobalt and remains therefore ambiguous. The distribution of the EEE, EEZ, EZZ and ZZZ configurational isomers was shown to be dependent on the solvent used during the analyses. Moreover, we evaluated the temperature needed to lift the barrier to rotation in the C-N bond of the coordinated ligands. Finally, the ⁵⁹Co NMR trends of several complexes isolated for single crystal X-ray diffraction analysis were complemented by DFT linear transit calculations of their configurational isomers. Finally, we investigate the interesting coordination chemistry of the well-ordered and multinuclear coordination systems of the bipodal acylthiourea analogues, namely aroylbis(N,N-dialkylthioureas). Two ligands were selected for this purpose, including isophthaloylbis(N,N-diethylthiourea) and a bipodal ligand with a modified aromatic spacer derived from catechol. These were utilized as pre-programmed chelating ligands to form metallamacrocyclic octahedral facial Cobalt(III) complexes via self-assembly. The latter catechol-spaced ligand was used for the synthesis of a number of oligometallic Co(III) complexes by one-pot reactions of the ligand, Co³⁺ and a variety of monovalent cations, to form metallacryptates of type {M⁺ ⊂ [Co2(L-κS,O))3]}(PF6), M⁺ = K⁺, Rb⁺, Cs⁺ and NH4+. A significant discovery, owing to the unique sensitivity of the ⁵⁹Co nucleus, was the instrumental role ⁵⁹Co NMR played in our attempt to characterize the metallacryptates and to determine their conditional stabilities in solution. This was achieved by way of several biphasic exchange experiments of cations between an aqueous and non-aqueous phase. This is a novel discovery that has allowed for the study of various factors shown to effect the stability of the metallacryptates in solution.
- 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.
- ItemAdamantane copolymers(Stellenbosch : Stellenbosch University, 2001-12) Coetzee, Liezel; Van Reenen, A. J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science.ENGLISH ABSTRACT: This study concerns the incorporation of adamantane containing monomers 3-(1-adamantyl)-1-propene and 1-(1-adamantyl)-4-vinylbenzene into ethene, propene and higher a-olefins using different catalytic systems. The effect of the incorporation of the adamantane monomer on the physical and thermal properties of the polymers was investigated. A thorough study on the background of adamantane in general, as well as polymerization reactions involving the above-mentioned monomers and a-olefins was done. 3-(1-Adamantyl)-1-propene as well as 1,.(1-adamantyl)-4-vinylbenzene was sucessfully synthesized. The homo polymers of each monomer were made. The above-mentioned monomers were also polymerized: • . 3-( 1-adamantyl)-1-propene with ethene, propene and higher a-olefins, • 1-(1-adamantyl)-4-vinylbenzene with ethene and styrene. The copolymers of 3-(1-adamantyl)-propene as well as 1-(1-adamantyl)- 4-vinylbenzene were characterized as far as possible to show the influence of the incorporation of the adamantane group on the physical and chemical properties of the polymers. A series of 3-phenyl-1-propene copolymers with higher a-olefins were synthesized to compare the influence of the phenyl group to the adamantyl group on the relevant properties of the polymers.
- ItemAdvanced analytical methods for the analysis of complex polymers prepared by RAFT and RITP(Stellenbosch : Stellenbosch University, 2015-04) Wright, Trevor Gavin; Pasch, Harald; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Synthetic polymers are complex compounds that have multiple distributions with regard to molar mass, chemical composition, functionality and molecular architecture. Therefore, the molecular complexity of these compounds can only be analysed using a combination of analytical techniques. Well-defined complex polymers can be prepared by different types of living radical polymerisation, including reversible addition–fragmentation chain transfer polymerisation (RAFT) and reverse iodine transfer polymerisation (RITP). Using these techniques, several different homopolymers and copolymers have been prepared. However, there is still space for some more extended research. Many different types of multifunctional RAFT agents have been reported in literature. A tetrafunctional RAFT agent was prepared in our laboratory and used for the first time in the polymerisation of styrene. The polymerisation reaction was followed using in situ 1H nuclear magnetic resonance (NMR) and the molar masses of the resultant polymers were determined using size exclusion chromatography (SEC). The molar masses of the star-shaped polystyrenes (PS) were found to be less than the theoretical molar masses. This was due to the fact that SEC was calibrated with linear PS standards, while the samples under investigation are branched. Linear and branched polymers have different hydrodynamic volumes at similar molar masses. In order to prove that the star-shaped polymers were in fact four-armed, the samples were cleaved by aminolysis to yield the linear PS arms. The molar masses of the arms were in agreement with the theoretical arm molar masses based in the fourarmed structure. RITP is a relatively new living radical polymerisation technique. Various monomers have been prepared using RITP, including acrylates, methacrylates and styrene. The polymers formed using this technique have been characterised by techniques such as SEC, NMR and mass spectrometry (MS). However, very little advanced characterisation work has been done on polymers synthesised via RITP. Polystyrene-block-poly(n-butyl acrylate) (PS-b-PBA) block copolymers were prepared via RITP and the microstructure analysed by in situ NMR and other advanced analytical techniques. The chromatograms from gradient HPLC of the PS-b-PBA block copolymers showed a separation based on chemical composition. The preparation of deuterated polymers via RITP has not been reported in literature. Hydrogenous-polystyrene-block-deuterated-polystyrene (hPS-b-dPS) was synthesised via RITP and analysed using liquid chromatography at critical conditions. An isotopic separation was achieved when critical conditions were established for hydrogenous PS (h-PS). A separation of the block copolymer from the first block was also achieved under chromatographic conditions where the block copolymer eluted in SEC mode while the first block eluted in LAC mode. The separation according to the block structure was confirmed by two-dimensional liquid chromatography.
- 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.
- ItemAlternating hetero-arm copolymer molecular brushes as scaffolds for inorganic nano-wires(Stellenbosch : Stellenbosch University, 2013-03) Hadasha, Waled Ajili; Klumperman, Bert; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: This study describes the synthesis and self-assembly of hetero-arm molecular brushes (hetero-arm MBs). These MBs consist of two polymeric side chains (SCs) of different natures, alternatingly distributed along the main chain (backbone). Two different types of hetero-arm MBs were prepared: first, alternating amphiphlic hetero-arm MBs (AMBs), and second, alternating hetero-arm MBs (AHMBs). Hetero-arm AMBs were synthesized via two strategies: (I) via a combination of “grafting through” and “grafting onto”, and (II) via “grafting through”. In approach (I), poly[vinyl benzyl (polyethylene glycol)-alt-N-alkyl-maleimide)] (poly[VB-(PEG12)-alt-N-(CnH2n+1)-MI]) was prepared via radical copolymerization of vinyl benzyl-terminated polyethylene glycol (VB-PEG12) with maleic anhydride (MAnh) (grafting through), which produces graft copolymers with PEG SCs and reactive succinic anhydride repeat units alternatingly distributed along the backbone. These graft copolymers were then modified by nucleophilic substitution (imidization) with alkyl amines (CnH2n+1-NH2) on the succinic anhydride residues (grafting onto). Three different primary amines possessing different alkyl chain lengths (n = 4, 12 and 16) were used in the modification process. In this way, hetero-arm AMBs with different hydrophilic to hydrophobic ratios were obtained. In approach (II), similar hetero-arm AMBs were prepared in a one-step grafting through approach. In this case, poly[VB-(PEG17)-alt-N-(CnH2n+1)-MI] (n = 10, 16 and 20) was prepared via radical copolymerization of VB-PEG17 with N-dodecylmaleimide, N-hexadecylmaleimide and N-icosylmaleimide. Following the synthesis step, self-assembly of these hetero-arm AMBs in arm-selective solvents was investigated in relation to the alkyl chain length. The morphology of the obtained assemblies was characterized by Field Emission gun-Scanning Electron Microscopy (FE-SEM), Transmission Electron Microscopy (TEM) and Fluorescence Microscopy (a fluorescent dye was encapsulated). Cylindrical-like aggregates, length 10 μm and diameter ~ 250 nm, were obtained upon hetero-arm AMBs self-assembly. The second type of hetero-arm MBs was hetero-arm AHMBs, in which the SCs consist of PEG and poly(N,N-dimethylamino-ethyl methacrylate) (PDMAEMA). These hetero-arm AHMBs were prepared via a combination of grafting through and grafting from approaches. In this case, poly[vinyl benzyl-(polyethylene glycol)-alt-N-(poly(N,N-dimethylamino-ethyl methacrylate) maleimide] (poly[VB-(PEG17)-alt-N-(PDMAEMA)-MI) was prepared in the following steps: (1) alternating poly[vinyl benzyl-(polyethylene glycol)-alt-N-(4-hydroxyphenyl) maleimide] (poly[VB-(PEG17)-alt-N-(HPh)-MI]) was synthesized via radical copolymerization of VB-PEG17 with N-(4-hydroxyphenyl) maleimide (N-HPhMI). (grafting through), (2) the hydroxyl sites were esterified with 2-bromoisobutyryl bromide to afford poly [vinyl benzyl-(polyethylene glycol)-alt-N-(4-(2-bromobutyryloxy)phenyl) maleimide] (poly[VB-(PEG17)-alt-N-(BrPh)-MI]) (macroinitiator) and (3) an atom transfer radical polymerization (ATRP) reaction of 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) was initiated from the obtained macroinitiator. This approach afforded poly[VB-(PEG17)-alt-N-(PDMAEMA)-MI] hetero-arm AHMBs with two water soluble SCs; however, one is water soluble at all pHs and temperatures (i.e. PEG), while the other is a pH- and temperature-sensitive polymer (i.e. PDMAEMA). Initial attempts were made to fabricate cylindrical organo/silica hybrid materials based on these AMHBs as the organic template and tetra-ethylorthosilicate as the silica precursor. Preliminary results indicate the formation of silica nano-wires, ~ 8 μm in length and ~45 nm in diameter. The self-assembly behavior of these AHMBs in water at a temperature above the lower critical solution temperature of PDMAEMA (> 55 °C) was also investigated. Fibril morphology (~ 30 nm in diameter) was observed. This study addresses initial attempts to fabricate organic/inorganic hybrid materials with controlled size and morphologies via densely grafted hetero-arm molecular brushes.
- ItemAmine end-functional poly(N-vinylpyrrolidone) as amMacroinitiator for L-lysine N-carboxyanhydride polymerization - towards the preparation of pH-responsive micelles for drug delivery(Stellenbosch : Stellenbosch University, 2015-03) Ilchev, Alexander; Klumperman, Bert; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Cancer is a notorious affliction that knows no age, gender, ethnic, racial, or species bounds and is responsible for over 14% of annual worldwide human deaths. There is no universal cure and the treatments that exist have poor probabilities of success. Chemotherapy is often considered the staple for cancer treatment as it can enter areas of the body that are unsafe for surgery and can treat tumors that are too small to be detected, even with modern imaging techniques. However, chemotherapy can induce many harmful and fatal side-effects. It can also lose its therapeutic effect if the cancer mutates to become multi-drug resistant. These shortcomings can be linked to the poor selectivity and pharmacokinetics of conventional chemotherapy drugs. Modern research focusses on improving these aspects of existing chemotherapy regimens through the incorporation of drug delivery principles. This dissertation focusses on the development of a novel, polymeric, pHresponsive drug delivery system for chemotherapy that incorporates the chemotherapeutic drug as well as a cell-penetrating peptide in a prodrug formulation. The system was designed to inhibit the release of its components into healthy tissues while selectively accumulating, through the enhanced permeability and retention effect, and releasing its payload, through reversible hydrolysis of imine bonds, within tumor tissues. Poly(L-lysine) was chosen as the cell-penetrating peptide since it is able to form imine bonds through its ε-amine functional groups on its residues. It was prepared by the primary amine-initiated ring-opening polymerization of Nε-(benzyloxycarbonyl)-L-lysine Ncarboxyanhydride at 0 °C and pressures lower than 1 mbar as these conditions allow for a controlled, living polymerization to occur. The benzyloxycarbonyl end-group was removed by acidolysis with HBr in a mixture of dichloromethane and 1,4-dioxane at 0 °C. The initiator used for the ring-opening polymerization was poly(N-vinylpyrrolidone) with a primary amine end-group, prepared by RAFT-mediated polymerization with O-ethyl-S-(phthalimidylmethyl)xanthate. This RAFT agent was shown to display slow pre-equilibrium kinetics which was linked to the lower relative stability of the phthalimidylmethyl radical compared to the poly(N-vinylpyrrolidone) propagating radical. The pre-equilibrium and main equilibrium kinetics of the RAFT polymerization were optimized by performing the polymerization in semi-batch mode. During the semi-batch polymerization, the monomer to RAFT agent ratio could be controlled by adjusting the monomer feed. This ratio was shown to be inversely proportional to the probability of radical transfer from the propagating radical to the phthalimidylmethyl radical. The phthalimide end-group could be converted to a primary amine both by reacting with hydrazine in methanol as well as reduction with sodium borohydride in water followed by hydrolysis in 1 M HCl at 60 °C. Doxorubicin and benzaldehyde could be conjugated, via imine bonds, to the poly(N-vinylpyrrolidone-block-L-lysine) copolymer spontaneously in methanol. Both types of conjugates would self-assemble into micelles when dispersed in water. However, the Doxorubicin conjugates were unstable, precipitating out of solution within 24 hours. The benzaldehyde conjugates were stable in water for over 24 hours. This suggested that a formulation of Doxorubicin and benzaldehyde conjugated to the block copolymer may be sufficiently stable under the physiological conditions of blood plasma.
- 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 complex tannins by multidimensional techniques(Stellenbosch : Stellenbosch University, 2019-03) Venter, Pieter; De Villiers, Andre J.; Pasch, Harald; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Hydrolysable tannins are plant derived (poly)phenolics which for centuries have been used in tanning, the transformation of animal hides into leather. In addition to their industrial applications, epidemiological evidence suggests that the consumption of hydrolysable tannins can be linked to disease prevention and treatment, although their mode of action at a molecular level is still unknown. In order to investigate their biological and chemical properties, the determination of the molecular composition of hydrolysable tannins in plant material is a critical step. Liquid chromatography hyphenated to mass spectrometry (LC-MS) is the preferred analytical approach to obtain this information, although conventional methods often provide insufficient performance for highly complex mixtures of hydrolysable tannins. The goal of the work reported in this thesis was, therefore, to develop improved multidimensional methods for the analysis of hydrolysable tannins. In the first part of the study, a methodology for the comprehensive analysis of ellagitannins and gallotannins in chestnut and tara using reversed phase liquid chromatography (RPLC) and hydrophilic interaction chromatography (HILIC) combined with ultraviolet (UV), ion mobility (IM) spectrometry and high-resolution mass spectrometry (HR-MS) detection is reported. Good chromatographic performance was achieved using both HILIC and RP-LC for chestnut tannins, with the complementary separation mechanisms proving useful for particular compound classes also pointing to the potential of the combination of HILIC and RP-LC in a comprehensive two-dimensional separation. In contrast, RP-LC provided much better separation of tara gallotannins than HILIC. Incorporation of IM into the LC-MS workflows offered several advantages for such non-targeted screening analyses, including improved mass spectral quality by filtering MS data according to IM arrival time, the availability of an additional identification criterion in the form of collisional cross section (CCS) values obtained from arrival times, and an improved MS duty cycle and, therefore improved sensitivity. Furthermore, additional isomeric species were resolved by IM and, in combination with UV spectral data, IM provided a simple methodology to differentiate between positional isomers. In the second part of the study, an on-line comprehensive two-dimensional HILIC×RPLC method was developed for the analysis of hydrolysable tannins by kinetic optimisation of the relevant experimental parameters. The use of a weak make-up flow to dilute the HILIC effluent and avoid second dimension injection band broadening resulted in excellent performance, with practical peak capacities in excess of 1000 determined for both samples. Despite on-column dilution and the short second dimension analysis times ultimately limiting the number of compounds detected in chestnut and tara, the group-type separations obtained show promise for the fast fingerprint analysis of hydrolysable tannins. Building on previous findings, the final part of the work evaluated the potential of IM spectrometry as a third dimension in a comprehensive three-dimensional HILIC×RPLC×IM configuration. Apart from the inherent advantages of IM alluded to above, the additional separation step offered by IM contributed to an increase in practical peak capacity by a factor of 13 when integrated into the LC×LC workflow. Certain instrumental constraints which limit the ultimate performance of LC×LC×IM-HR-MS are highlighted. Despite these, this approach shows significant promise for the high resolution separation of highly complex mixtures.
- ItemAnalysis of hydroformylation catalysis mechanisms of rhodium catalyst precursors by computational and instrumental means.(Stellenbosch : Stellenbosch University, 2021-05) O'Kennedy, Sean James; Gerber, Wilhelmus J. ; Mapolie, Selwyn Frank ; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: The most industrially prevalent hydroformylation catalysts are based mainly on cobalt or rhodium with various combinations of carbonyl, halide or tertiary phosphine ligands and derivatives thereof. Recently, catalyst designs have branched out to include nitrogenous ligands such as pyridines, imines, triazoles and pyrazoles as well as multidentate combinations of either nitrogen, oxygen and/or tertiary phosphine donor ligands. This dissertation focuses on the mechanistic pathways of hydroformylation catalyst in absence of a detectable hydride intermediate. Due to the transient nature and low abundance of the catalytic intermediates, the study is focused primarily on in situ and in silico modelling of proposed mechanistic pathways. In order to investigate the kinetic trans effect in transition metals an example reaction of the transformation of [RhCl6]3 was investigated. It has been well documented that the transformation of [RhCl6]3 to [Rh(H2O)6]3+ is slow (equilibration takes more than 10 years) in acidic solutions at room temperature. However, on the addition of base to the solution, the [RhCl6]3 complex initially forms the [RhCl5(OH)]3 complex which then rapidly converts to [Rh(OH)6]3. We show using computational methods that the transformation of the [RhCl5(OH)]3 to [Rh(OH)6]3 occurs via labilisation of the trans chloride to form a 5 coordinate [RhCl4(OH)]2 species that can readily react with another hydroxide. The transformation to a hydroxido complex causes a cascade of rapid transformations to form the final [Rh(OH)6]3 complex. The domain based localized pair natural orbital coupled cluster singles doubles and virtual triples (DLPNO-CCSD(T)) was used to calculated accurate transition state energies that agreed well with the experimental data and corroborated our proposed dissociative ligand exchange mechanism. Hydroformylation is a well understood catalytic reaction for the more prevalent cobalt and rhodium phosphine based catalysts that follow a pathway where the catalyst spends most of the catalytic cycle in the Co(I) or Rh(I) oxidation state, respectively. However, we show an alternative pathway where oxidative addition of hydrogen directly to the precursor forms a Rh(III) dihydride which remains in the octahedral geometry throughout most the catalytic cycle. The oxidative addition of hydrogen is shown to form low abundances of transient rhodium dihydride species by computational chemistry, explaining the lack of spectroscopically detectable species in situ. In addition, it is shown via computational fragment analysis and a local energy decomposition scheme that the double electron transfer to form the dihydride occurs on relaxation from the transition state towards the dihydride octahedral complex and not at the transition state itself. The overall catalytic mechanism for a the [[N-(pyridine-_N-methyl)-N0-(3-imine-_N-p-tolyl)]rhodium(I)(_4-1; 5-cyclooctadiene)]+ complex was investigated using in situ 1H NMR in conjunction with UHPLC-MS. The spectroscopic results indicated possible transamination of the imine ligand by reaction with wither the acetone solvent or the formed aldehyde product. Computational chemistry was used to model the proposed hydroformylation mechanism for the [[N-(pyridine-_N-methyl)-N0-(3-imine-_N-p-hydroxyphenyl)]dicarbonylrhodium(I)]+ and [N--(pyridine-_N-methyl)-N0-(3-imine-_N-p-hydroxyphenyl)]chlorocarbonylrhodium(I) complexes, using the [Rh(CO)4]+ and [RhCl(CO)3] complexes as model precursors. The computational results indicated that the overall mechanism is feasible with a dissociative step at each point except that of the alkene addition. Each 5-coordinate intermediate of the proposed dissociative mechanism was stabilised by agostic interaction between the vacant site and the hydrogen of the proximate coordinated alkyl or acyl ligand. Throughout the mechanism, competition is seen between the chloride, hydride, alkyl and acyl ligands. This trans influence was found to destabilise the complex isomers where these ligands are in the trans position. No steric or chiral specificity is expected in these complexes from computationally calculated geometries as there is no hindrance to the coordination of either the linear or branched alkyl ligand explaining the lack of n-iso specificity in experimental results.
- 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.
- ItemAnalysis of vegetable oils, seeds and beans by TGA and NMR spectroscopy(Stellenbosch : University of Stellenbosch, 2011-03) Retief, Liezel; Koch, Klaus R.; McKenzie, Jean; University of Stellenbosch. Faculty of Sciences. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: Due to the commercial, nutritional and health value of vegetable oils, seeds and beans, the analysis of their components is of much interest. In this dissertation oil-containing food products, specifically vegetable oils, seeds and beans, were investigated. Selected minor components of three locally produced vegetable oils, namely apricot kernel, avocado pear and macadamia nut oils were investigated using 31P NMR spectroscopy. These minor components, including 1,2 diacylglycerols, 1,3 diacylglycerols and free fatty acids, were identified in the 31P NMR spectra of each of the three vegetable oils for the first time. Two approaches were used for the quantification of the minor components present in the spectra. A calibration curve approach used known concentrations of standard minor components to establish calibration curves while a direct correlation approach calculated the unknown concentration of minor components in the vegetable oils using a known amount of standard compound within the analysis solution. These approaches aided in determining the concentration of minor components during storage studies in which vegetable oils were stored in five different ways: exposed to light, in a cupboard, in a cupboard wrapped in tin foil, at -8 °C and at 5 °C. It was found that determining the best storage condition for each oil was difficult since individual minor components were affected differently by the various storage conditions. However, in general the best storage conditions appeared to be 5 °C and -8 °C. The oil, protein and carbohydrate contents of sesame, sunflower, poppy, and pumpkin seeds, and soy, mung, black and kidney beans were analysed by thermogravimetric analysis and 13C NMR solid state NMR spectroscopy. It was shown that the first derivative of TGA data for seeds and beans can give valuable information about the carbohydrate, moisture, protein and fat content. This has not been previously demonstrated. For the seeds, the integration of a region between 270–480 ºC was equal to the sum of the oil and protein content and compared well to quantitative results obtained by other conventional methods. For beans the integration of a region between 180-590 ºC, gave a value which represented the sum of the oil, protein and carbohydrate content. 13C solid state NMR spectroscopy, including SPE-MAS, CP-MAS and variable contact time experiments, was carried out on these seeds and beans and gave valuable information on the solid-like and liquid-like components. To our knowledge these seeds and beans have never been previously analysed using this technique. 13C SPE-MAS NMR spectroscopy indicated that the seeds contained more liquid-like components than the beans. In turn the 13C CP-MAS NMR spectra indicated that beans had higher levels of solid-like components than the seeds. These conclusions correlated well with the quantities of liquid-like components and solid-like components that were determined by conventional methods and TGA. Preliminary studies using T1pH experiments on the components present in the seeds and beans led to a few observations. Most interesting is that a model using a two- phase fit in order to determine T1pH values appears to be more accurate than a one-phase model.
- 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.