Department of Chemistry and Polymer Science

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Advanced 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.
Controlled free radical polymerization in miniemulsion using Reversible Addition-Fragmentation Chain Transfer (RAFT)
(Stellenbosch : Stellenbosch University, 2001-12) Vosloo, Johannes Jacobus; Sanderson, R. D.; De-Wet Roos, D.; Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science .
ENGLISH ABSTRACT: A novel approach to conducting controlled free radical polymerization in aqueous systems using Reversible Addition-Fragmentation Chain Transfer (RAFT) has been studied. When conducting RAFT in aqueous systems, reaction conditions must be chosen such that monomer transport across the aqueous-phase is either eliminated or facilitated. This is to prevent the formation of the red layer associated with RAFT in emulsions. The formation of the red layer is ascribed to the inability of waterinsoluble, dithiobenzoate-endcapped oligomers to be sufficiently transported across the aqueous phase. The novel approach in this study focussed on eliminating monomer transport and comprises two fundamental steps: the synthesis of dithiobenzoate-encapped oligomers in bulk followed by miniemulsification of these oligomers to yield a polymerizable miniemulsion. Dithioesters that act as chain transfer agents in the RAFT -process were synthesized in situ, thereby eliminating laborious and time-consuming organic purification procedures of dithioesters. In situ formation of the RAFT-agents involved conducting the reaction between di(thiobenzoyl) disulfide and conventional azo-initiators of differing structures in the presence of monomer. The structure of the chosen azo-initiator played a role in the efficiency of the RAFT process when the reaction was conducted in the presence of monomer to control the free radical polymerization process. Synthesis of the oligomers was performed by heating di(thiobenzoyl) disulfide and a selected azo-initiator, in the presence of monomer for a specific reaction duration in bulk. After the reaction was stopped, these oligomers were then miniemulsified by adding water, surfactant and cosurfactant, followed by the application of shear to form the resulting mini emulsion. The free radical polymerization of the dithiobenzoate-endcapped oligomers in the miniemulsion proceeded in a controlled manner with molecular weight increasing in a linear fashion with increasing conversion, while polydispersities remained low. The familiar red layer formation associated with RAFT polymerization in conventional emulsions was not observed under these conditions. The effects of changing the cosurfactant (hydrophobe) as well as changing the degree of polymerization of the emulsified oligomers were also investigated and described.
Hydrophobic core/shell particles via miniemulsion polymerization
(Stellenbosch : University of Stellenbosch, 2006-12) Etmimi, Hussein Mohamed; Sanderson, R. D.; Tonge, M.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
Hydrophobic core/shell latex particles were synthesized for use in barrier coatings using the miniemulsion polymerization process. Particles with liquid or with hard cores were successfully synthesized using miniemulsion as a one-step nanoencapsulation technique. Different materials, including an oil (hexadecane, HD) and two different waxes (paraffin and microcrystalline wax), were used as the core of the particles. The shell of the particles was mainly made from a copolymer containing three relatively hydrophobic monomers, namely methyl methacrylate (MMA), butyl acrylate (BA) and vinyl neodecanoate (Veova-10). Before any further investigations could be carried out, it was important to determine the morphology of the synthesized core/shell particles at the nanometer level. Particle morphology was mainly determined by two different techniques: transmission electron microscopy (TEM) and atomic force microscopy (AFM). TEM was used to directly visualize the morphology of the investigated core/shell particles at the nanometer level, while AFM was used to confirm the formation of these core/shell particles. AFM was a powerful technique with which to study the particle morphology of the core/shell latices during the film formation process. As a second part of the study, the effect of various factors on the hydrophobicity and barrier properties of the resulting films produced from the synthesized core/shell latices to water and water vapour was investigated. This included the effect of: (i) the surfactant concentration, (ii) the wax/polymer ratio for both waxes, (iii) the molecular weight of the polymeric shell, (iv) the amount of the most hydrophobic monomer used (Veova-10), and (v) the degree of crosslinking in the polymeric shell. Results showed that all the above-mentioned factors had a significant impact on the water sensitivity of the resultant films prepared from the synthesized core/shell latices. It was found that the presence of wax materials as the cosurfactant, instead of HD, in the miniemulsion formulation could significantly improve the hydrophobicity and barrier properties of the final films to water and water vapour. In addition, increasing the amount of wax, Veova-10, and the molecular weight of the resultant polymeric shell, led to a significant increase in the hydrophobicity and barrier properties of the resultant latex films. In contrast, hydrophobicity and water barrier properties decreased drastically as the quantity of surfactant and degree of crosslinking increased in the final latex films.
The investigation and analysis of RAFT-mediated block copolymers in aqueous dispersed media
(Stellenbosch : Stellenbosch University, 2007-12) Pretorius, Nadine O.; Sanderson, R. D.; McLeary, J. B.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: Polymers prepared via radical techniques are very common in our every day environment. The technique is however limited by a lack of control over the polymerization and an inability to produce block copolymers. Block copolymers have a significant number of potential applications in advanced materials and as a result are a field in which significant research is being conducted. Reversible Addition-Fragmentation chain transfer (RAFT) is a living free radical process that overcomes the disadvantages inherent in the traditional process. In this study the mediation behaviour of two inherently different RAFT agents was investigated by the “living” free radical polymerization of model monomers via the RAFT process in homogeneous and aqueous dispersed media with the focus on differentiating between the two types of agents. To ensure that the agents were comparable a new RAFT agent had to be prepared which has not previously been documented. The efficiency of the RAFT agents was compared in terms of rate effects, the predictability of the molecular weights of the polymers, the polydispersities of the polymers and their ability to allow block copolymer formation via sequential addition of monomers. Block copolymerizations were conducted by the addition of new monomer to the already existing RAFT end-capped chains. Monomer addition was done via three different approaches; namely shot addition, feed addition and pre-swelling (in the case of emulsions). Chromatographic analysis was conducted on the resulting block copolymers via liquid chromatography at critical conditions (LCCC), and its online coupling with size-exclusion chromatography (SEC) to obtain two-dimensional information on the differences in heterogeneity of their molecular distributions. Other analyses included dynamic light scattering analysis (DLS) and transmission electron microscopy (TEM). The detailed analysis enabled the understanding of the different products that are produced via the two different classes of RAFT agent. Potential causes for the differences are discussed and possible areas for future research are highlighted. The work presented here is the most detailed investigation of this class of polymerization to date and will provide new insight for researchers working in this vibrant and important research field.
An investigation into the mechanistic behaviour of RAFT-mediated miniemulsion polymerizations.
(Stellenbosch : University of Stellenbosch, 2005-12) Hermant, Marie-Claire; Sanderson, R. D.; McLeary, J. B.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
Polymerization using the reversible addition-fragmentation chain transfer (RAFT) process affords a researcher control over the molecular weight and polydispersity of the final polymer. Research is being carried out globally, using heterogeneous RAFT systems, as these systems offer superior industrial possibilities. Many emulsion systems fail when incorporating RAFT agents due to phase separation and colloidal instability. Exchanging conventional emulsion polymerizations with predispersed polymerization systems (i.e. miniemulsions) has shown many improvements. Evidence of uncontrolled aqueous phase polymerization (i.e. not mediated by the RAFT process) has however been found. This behaviour is similar to polymerization in a conventional emulsion polymerization system, but is not expected in miniemulsion polymerization.
Mechanistic aspects of RAFT Mediated (Co) Polymerization by in situ ¹H NMR
(Stellenbosch : Stellenbosch University, 2013-03) Monthunya, Mpho; Klumperman, Bert; Phiri, Mpho; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: In this study the kinetic and mechanistic aspects of the Reversible Addition Fragmentation Chain Transfer (RAFT) process on the copolymerization of acrylonitrile (AN) and vinyl acetate (VAc) are investigated by application of in situ 1H nuclear magnetic resonance (NMR) spectroscopy. The focus is on the early stages of the reaction where the first few monomer (M) additions occur; the change in concentration of the leaving group of RAFT species as a function of time is followed. Cumyl dithiobenzoate (CDB), S-sec propionic acid O-ethyl xanthate (PEX) and O-ethyl cumyl xanthate (ECX) were selected for use in this study. The basis for RAFT agent selection was solely the fact that more activated monomers, e.g. acrylonitrile (AN) are controlled by dithiobenzoates while the less activated monomers, e.g. VAc, are controlled by xanthates. Furthermore, the behaviour of the copolymerization, where the reaction medium is composed of a RAFT agent preferring one monomer in the reaction, is largely unexplored in the literature. First, the homopolymerization of each of these monomers was studied. In accordance with the literature, the AN showed good control when CDB was used as the chain transfer agent, whereas VAc showed good control when using PEX to mediate the polymerization. More emphasis is however placed on the CDB-mediated copolymerization as it still showed some preferential consumption of AN even in the presence of the VAc comonomer, although the reaction was retarded. The copolymerization mixtures comprised the monomer pair, the RAFT agent, and the 2,2’-azobis(isobutyronitrile) (AIBN) in mole ratios as specified for each experiment. When using the total monomer to RAFT to initiator ([M]:[CDB]:[AIBN]) ratio of 5:1:0.2, the AN initialization time was found to be 150 min at 60 °C. Copolymerization of AN with VAc under similar conditions resulted in retardation of the initialization reaction; the initialization period was now about 600 min at fVAc = 0.1. In all the copolymerization reactions undertaken under the conditions described, the VAc monomer conversion was 4–6%. This means that VAc, possibly, retards the copolymerization by binding to the cumyl radicals of the CDB, which it then releases due to weak bonds formed with CDB. The results showed excellent correlation between the experimental and fitted data for the CDB- and PEX-mediated systems, but within a narrow experimental data region for ECX at fAN=0.5, thus for [AN]/[VAc] ratios 0.65–0.93.
Mechanistic studies of reversible addition-fragmentation chain transfer mediated polymerization
(Stellenbosch : Stellenbosch University, 2004-03) Calitz, Francois Malan; Sanderson, R. D.; Tonge, M. P.; Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science .
ENGLISH ABSTRACT: To comply with the ever growing demands for materials with better properties and complex architectures, polymer chemistry has resorted to the use of living free radical polymerization techniques. Despite the structural control some of these techniques offer, major disadvantages do exist. For example, most require ultra-pure reagents, hence only a small fraction of the monomers used in industry can be polymerized in this way. This rendered these new living techniques less advantageous from a commercial point of view. Recently, a revolutionary new living free radical process, namely the reversible addition-fragmentation chain transfer process, or RAFT process, was developed that combines the control over the polymer produced with the robustness and versatility of a free radical process. However, the RAFT process is not without its problems. In some dithioester mediated polymerizations, significant inhibition and rate retardation effects have been observed. Two main opposing opinions have been proposed in recent literature to explain these phenomena observed. The main point of difference between these two groups is the fate of the formed intermediate RAFT radicals, i.e., slow fragmentation of the formed intermediate radicals together with possible reversible intermediate RAFT radical termination, or fast fragmentation of the formed intermediate radicals together with possible irreversible intermediate RAFT radical termination. Between these opposing two groups, there is a difference of six orders of magnitude for the rate of fragmentation of the formed intermediate RAFT radicals. The work presented in this thesis is an attempt to clarify some of the mysteries, i.e., inhibition and rate retardation observed in some RAFT polymerizations. Experimental evidence to support or contradict the theories of the above mentioned two opposing groups was investigated. The concentration-time evolution of the intermediate radical concentration (cy), for styrene and butyl acrylate polymerizations mediated by cumyl dithiobenzoate (COB) at 70°C and 90 °C, was followed via in situ electron spin resonance spectroscopy (ESR). The concentration-time evolution profiles observed were ascribed to the formation of very short chains during the early stages of the reaction. It was also found that the RAFT process is not particularly sensitive to oxygen. The intermediate and propagating radical (cp) concentrations (and their ratio) for the cumyl dithiobenzoate mediated styrene polymerizations were examined by ESR spectroscopy and kinetics. The system showed strong chain length effects in kinetics, assuming all chains were of similar number average molar mass (Mn). However, unusual behavior with respect to existing mechanistic knowledge was observed in other aspects of the system. The central equilibrium "constant" (Keq) was found to be dependent on both temperature and initial reactant concentrations. The observed intermediate radical concentrations were not consistent with predictions based on existing literature models. It was also found that the time dependence of the intermediate radical concentration varies significantly with the type of RAFT agent used. Unexpectedly, intermediate radicals were detected at very long reaction times in the virtual absence of initiator, enhancing the belief of possible reversible termination reactions involving the intermediate radicals. An extra radical (nonpropagating or intermediate) species was observed (via ESR spectroscopy) to form during some reactions. Its concentration increased with time. The combination of data from several analytical techniques provided evidence for the formation of dead chains by the termination of intermediate radicals in the free radical polymerization of styrene, mediated by a cumyl dithiobenzoate RAFT agent, at 84°C. Experiments done focused on the early stages of the reactions, targeting very low final number average molar mass values, with high initiator concentrations. The formation of these terminated chains did not occur to a significant extent until a large fraction of the chains reached a degree of polymerization greater than unity. This corresponded to the occurrence of a maximum in intermediate radical concentration. In situ 1H nuclear magnetic resonance (NMR) and electron spin resonance spectroscopy was used to directly investigate the processes that occur during the early stages (typically the first few monomer addition steps) of an AIBN-initiated reversible addition fragmentation chain transfer polymerization of styrene, in the presence of a cyanoisopropyl dithiobenzoate and cumyl dithiobenzoate RAFT agent, at 70°C and 84 °C respectively. 1H NMR spectroscopy allowed the investigation of the change in concentration of important dithiobenzoate species as a function of time. Identification and concentrations of the radicals present in the system could be inferred from corresponding ESR spectroscopy data. An apparent "inhibition" effect was observed in both the cyanoisopropyl and cumyl dithiobenzoate mediated polymerizations. This effect could be reduced by increasing the reaction temperature to 84 °C. However, the use of cumyl dithiobenzoate as RAFT agent prolonged this effect. This apparent "inhibition" effect was attributed to selective fragmentation of the formed intermediate radicals during the early stages of the reaction, and to different propagation rate coefficients (kp) of the resulting (different) radicals. A change in the equilibrium coefficient for the systems investigated was ascribed to possible progressively decreasing addition and fragmentation rate coefficients of propagating and intermediate radicals formed during the reaction. The increase in intermediate radical concentration, and thus possible intermediate radical termination, was shown to also be a probable cause of the rate retardation observed in the RAFT mediated systems investigated. To conclude, probable causes of the observed inhibition and rate retardation in some dithiobenzoate mediated systems were investigated. It was found that intermediate RAFT radical termination does occurs, albeit reversibly or irreversibly. A maximum in the intermediate radical concentration, and thus possible intermediate radical termination, was seen to occur during the observed rate retardation. An apparent inhibition effect observed was ascribed to a possible change in termination kinetics, the formation of terminated intermediate radical products and a rapidly changing kp of the propagating radicals.
Novel electrospun fibres of amphiphilic organic-inorganic graft copolymers of poly(acrylonitrile)-graftpoly( dimethylsiloxane) for silicone composite reinforcement
(Stellenbosch : Stellenbosch University, 2011-12) Bayley, Gareth Michael; Mallon, Peter; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: Novel silicone nanocomposites were prepared using poly(acrylonitrile) (PAN) based reinforcing fibres as well as multi-walled carbon nanotubes (MWCNTs). Compatibility of the fibre fillers with the silicone matrix required the synthesis of novel amphiphilic, organic–inorganic graft copolymers of PAN and poly(dimethylsiloxane) (PAN-g-PDMS). These fibre precursor materials were synthesised via the “grafting through” technique using conventional free radical copolymerisation. The PDMS macromonomer content in the feed was varied from 5 wt% to 25 wt% and the molecular weights of the macromonomer were 1000 g.mol-1 and 5000 g.mol-1. The solvent medium of the precipitation reaction was optimised at a volume ratio of 98% benzene to 2% dimethylformamide (DMF). Successful incorporation of PDMS yielded graft copolymer blend materials of PAN-g-PDMS, blended with PAN homopolymer and unreacted PDMS macromonomer. A gradient elution profile was developed to track the successful removal of the PDMS macromonomer via hexane extraction. The gradient profile showed that as the PDMS content in the feed increased, the number of graft molecules in the blend increased relative to the number of PAN homopolymer molecules. The crystallisability of the PAN segments was shown to decrease as the PDMS content increased. The synthesised polymer was used as precursor material for the electrospinning of fibre fillers. The electrospinning of the precursor material was successfully achieved using 100% DMF as electrospinning solution medium. The amphiphilic nature of the precursor material in DMF resulted in self-assembled aggregate structures in the electrospinning solution. An increasing PDMS content was shown to affect the aggregation of the precursor material, and resulted in an increase in the solution viscosity. The “gel-like” solutions limited the achievable fibre morphological control when altering conventional electrospinning parameters such as voltage, tip-to-collector distance, and solution concentrations. The rapid evaporation and stretching of the solution during electrospinning, combined with the phase segregated amphiphilic molecules in solution and the crystallisation of the PAN segments resulted in (non-equilibrium morphology) fully porous fibres. The crystallinity was shown to decrease after electrospinning of the fibre precursor materials. Successful incorporation of surface oxidised MWCNTs into the electrospun fibres was achieved. The content of nanotubes was varied from 2 wt% to 32 wt%. The MWCNTs reduced the mean fibre diameters by acting as cross-linkers between the PAN segments and increasing the solution conductivity. The nanotubes dispersed well throughout the porous structure of the fibres and aligned in the direction of the fibre axis. Fabrication of silicone composites containing nonwoven and aligned fibre mats (with 8 wt% MWCNTs in the fibres, and without) was successfully achieved. The compatibilisation of the PDMS surface segregated domains allowed excellent dispersion and interaction of the PAN based fibre fillers with the silicone matrix. Mechanical analysis showed improved properties as the PDMS content in the fibre increased. The highest PDMS content fibres did, however, exhibit decreased properties. This was ascribed to increased PDMS (soft and weak) content, decreased crystallinity and increased fibre diameter (lower interfacial area). Dramatic improvements in strength, stiffness, strain and toughness were achieved. The most significant result was an increase in strain of 470%. The mechanical results correlated with results of SEM analysis of the fracture surfaces. The dramatic improvements in properties were a result of the fibre strength and ductility, as well as the mechanism of composite failure.
Novel synthesis of block copolymers via the RAFT process
(2007-12) Bowes, Angela; Sanderson, R. D.; McLeary, J. B.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
The synthesis of complex architectures, namely block copolymers with tailored enduse properties, is currently an important research area in academia and industry. The challenge is finding a versatile polymerization technique capable of controlling the molecular properties of the formed copolymers, which in turn determines their macroscopic properties. Reversible addition-fragmentation chain transfer (RAFT)- mediated living polymerization is a robust technique capable of producing controlled polymer products. With the great advances in living polymerization techniques and the environmental awareness of society there is an increasing demand to produce these polymer products via the RAFT living technique in heterogeneous media. Conventional emulsion and miniemulsion polymerization present various problems when used to produce polymers mediated by the RAFT process. There is an inherent need to find cost effective and flexible operating conditions to conduct RAFT polymerization in heterogeneous media with the ability to produce well-defined block copolymers. In this study the use of three novel trithiocarbonate RAFT agents to produce welldefined AB-type, ABA-type and star block copolymers via the RAFT process was investigated. Optimal operating conditions for the production of living block copolymers in homogenous and heterogeneous media were determined. The main focus was on the development of the RAFT process in heterogeneous media to efficiently produce block copolymer latex products. The RAFT-mediated miniemulsion polymerization system stabilized with non-ionic surfactants was thoroughly investigated. The ability of the ab initio and in situ RAFT-mediated emulsion polymerization systems to produce controlled latexes was demonstrated. Controlled block copolymer products were successfully synthesized in homogenous and heterogeneous media via the RAFT process when the optimum reaction conditions were chosen.
Organic-inorganic hybrid graft copolymers of polystyrene and polydimethylsiloxane
(Stellenbosch : University of Stellenbosch, 2010-03) Sutherland, Aimee Celeste; Mallon, Peter Edward; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: Hybrid graft copolymers of polystyrene (PSty) and polydimethylsiloxane macromonomers (PDMS) were synthesised. PSty-g-PDMS was synthesised employing the grafting through technique via a conventionally free radical polymerization (FRP) using a polydimethylsiloxane macromonomer. In this series the amount of PDMS incorporated into the copolymer was varied by varying the macromonomer to styrene ratios as well as the length of the PDMS side chain. This allows for the study of the effect that the macromonomer content and the branching length has on the efficiency of the grafting process. A second series of PDMS-g-PSty was also synthesized where the PDMS forms the backbone and the PSty the grafts. Two synthetic techniques were employed for the formation of these polymers. Firstly, the grafting onto approach was used where functional polystyrene prepolymers with either an allyl or vinyl end-groups were synthesised anionically (living anionic polymerization) prior to the coupling of a functional prepolymer using a hydrosilylation reaction with a Karstedt platinum catalyst. This technique was successful and gave insight to the effect of the polystyrene prepolymer graft length has on the grafting efficiency as well as the functional groups needed on the PDMS backbone. Furthermore, the effect of the viscosity (of the PDMS macromonomer) plays on the grafting efficiency was also elucidated. Lastly, the grafting from approach was employed for the formation of PDMS-g-PSty. ATRP, atom transfer radical polymerization, of styrene using a bromoisobutyrate functional PDMS macroinitiator was used for the synthesis of these copolymers. This was accomplished by reacting commercial silane functional PDMS molecules via a hydrosilylation reaction (using a Karstedt catalyst) with allyl-2- bromo-2-methyl-propionate to give a PDMS macroinitiator with bromoisobutyrate functional groups. This will allow for the initiation and growth of polystyrene branches from the PDMS backbone (employing ATRP with a suitable catalyst and ligand). The formation of the endproduct, PDMS-g-PSty, via this route proved to be extremely difficult and largely unsuccessful. Liquid chromatography (LC) at the critical point (LCCC) of polystyrene was used to separate the graft material from homo-polymers which might have formed as well as from the PDMS macromonomer. This technique allows for a very fast chromatographic analysis of the grafting reaction. Under the critical conditions of PSty it was found that the graft copolymer eluted at a lower retention time than the unreacted macromonomer and PSty homopolymer. Two-dimensional chromatography, where LCCC (1st dimension) was coupled to size exclusion chromatography (2nd dimension), was used for the evaluation of the CCD and MMD (molecular mass distribution) of the graft material. LC was furthermore coupled off-line to FTIR and TEM using an LC interface. LCFTIR gave insight to the microstructure of the material, whilst LC-TEM gave insight to the morphological nanostructure of the material.
Organocatalytic anionic polymerization of o-phthaldialdehyde and n-butyraldehyde
(Stellenbosch : Stellenbosch University, 2017-03) Weideman, Inge; Pfukwa, Rueben; Klumperman, Bert; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: Self-immolative polymers (SIPs) can be defined as polymers that are capable of complete head-to-tail depolymerization upon cleavage of an end-cap from the polymer chain-end in response to a trigger/stimulus. The use of SIPs has emerged as an alternative strategy in designing smart materials that are capable of responding to selective signals and provide an amplified response since the depolymerization process converts the entire polymer into its monomeric units and other small molecule products that can play a role in the amplified response. The incorporation of these polymers also offers the opportunity to alter the properties of a material after it has been prepared since the depolymerization of the SIP can cause a change in the shape, internal structure and/or surface properties of the material. Poly(phthaldialdehyde) (PPA), a well-known SIP, has been successfully prepared via non-organometallic catalyst based anionic polymerization in recent years. However, the methods that have been reported require expensive reagents and delicate experimental conditions. Hence, a new facile method for the non-organometallic anionic polymerization of PA was introduced and optimized in this work. The effect of a range of phosphazene and amine base catalysts were investigated. The results showed 1,8-diazabicyclo[5.4.0]undec-7-ene with a catalyst to initiator ratio of four to be the ideal catalytic system. It was further determined that the optimum experimental conditions for the DBU catalyzed system consisted of a 1.0 M monomer concentration, tetrahydrofuran as solvent and a reaction time of ten minutes. It was also shown that carboxylic acids can be used to initiate the polymerization reaction, which has thus far only been achieved using primary alcohols. The optimized method for the preparation of PPA was applied to the preparation of polystyrene-poly(phthaldialdehyde) block copolymers (BCPs). Hydroxyl end-functional polystyrene was prepared via activator regenerated by electron transfer atom transfer radical polymerization to serve as macroinitiator for the BCP reaction. Analysis of these BCPs, revealed that they had been prepared with narrow molecular weight distributions and a good agreement between the theoretical and experimentally obtained molecular weights. A systematic study was carried out to optimize the preparation of poly(butyraldehyde) (PBA), a SIP that has to date only been prepared using organometallic catalysts, by non-organometallic catalyst based anionic polymerization. The results of the study showed the phosphazene base catalyst P2-t-Bu with a catalyst to initiator ratio of 1 to 1 to be the ideal catalytic system for the preparation of PBA. Further investigation revealed a monomer concentration of 1.0 M, a nonpolar solvent such as pentane and a reaction time of ten minutes to be the optimum experimental conditions for the phosphazene base catalytic system.
Polymer-clay nanocomposites prepared by RAFT-supported grafting
(Stellenbosch : Stellenbosch University, 2012-12) Chirowodza, Helen; Pasch, Harald; Hartmann, Patrice C.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: In materials chemistry, surface-initiated reversible deactivation radical polymerisation (SI-RDRP) has emerged as one of the most versatile routes to synthesising inorganic/organic hybrid materials consisting of well-defined polymers. The resultant materials often exhibit a remarkable improvement in bulk material properties even after the addition of very small amounts of inorganic modifiers like clay. A novel cationic reversible addition–fragmentation chain transfer (RAFT) agent with the dual purpose of modifying the surface of Laponite clay and controlling the polymerisation of monomer therefrom, was designed and synthesised. Its efficiency to control the polymerisation of styrene was evaluated and confirmed through investigating the molar mass evolution and chain-end functionality. The surface of Laponite clay was modified with the cationic chain transfer agent (CTA) via ion exchange and polymerisation performed in the presence of a free non-functionalised CTA. The addition of the non-functionalised CTA gave an evenly distributed CTA concentration and allowed the simultaneous growth of surface-attached and free polystyrene (PS). Further analysis of the free and grafted PS using analytical techniques developed and published during the course of this study, indicated that the free and grafted PS chains were undergoing different polymerisation mechanisms. For the second monomer system investigated n-butyl acrylate, it was apparent that the molar mass targeted and the monomer conversions attained had a significant influence on the simultaneous growth of the free and grafted polymer chains. Additional analysis of the grafted polymer chains indicated that secondary reactions dominated in the polymerisation of the surface-attached polymer chains. A new approach to separating the inorganic/organic hybrid materials into their various components using asymmetrical flow field-flow fractionation (AF4) was described. The results obtained not only gave an indication of the success of the in situ polymerisation reaction, but also provided information on the morphology of the material. Thermogravimetric analysis (TGA) was carried out on the polymer-clay nanocomposite samples. The results showed that by adding as little as 3 wt-% of clay to the polymer matrix, there was a remarkable improvement in the thermal stability.
RAFT mediated polysaccharide copolymers
(Stellenbosch : University of Stellenbosch, 2006-12) Fleet, Reda; Sanderson, R. D.; McLeary, J. B.; Grumel, V.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
Cellulose, one of the most abundant organic substances on earth, is a linear polymer of D-glucose units joined through 1,4-β-linkages. Cellulose is however not easily processed without chemical modification. A number of techniques exist for the modification of cellulose, of which the viscose process is one of the most widely applied. Grafting of synthetic polymeric chains onto or from cellulosic materials is an useful technique that can be used to combine the strengths of synthetic and natural polymers dramatically, so changing the properties of cellulosic materials (pulp, regenerated cellulose, cellulose derivatives). In this study five model xanthate (Reversible Addition-Fragmentation chain Transfer (RAFT)/Macromolecular Design through Interchange of Xanthates (MADIX)) agents, namely, monofunctional, difunctional, trifunctional and tetrafunctional species of the form S=C(O-Z)-S-R, with different leaving groups and different activating moieties, were prepared and then studied to determine the feasibility of cellulose modification via addition fragmentation processes. These agents were characterized by Nuclear Magnetic Resonance spectroscopy (NMR), Fourier Transform Infrared spectroscopy (FT-IR) and Ultraviolet spectroscopy (UV). Polyvinyl acetates (PVAc) in the form of linear, three armed and four armed star shaped polymers were then successfully synthesized in reactions mediated by these xanthate RAFT/MADIX agents Xanthates were applied to polysaccharide materials using the viscose process (xanthate esters were formed directly on a cellulosic substrate, with subsequent alkylation) Grafting reactions were then conducted with the polysaccharides; cellulose was modified with vinyl acetate, [this is an example of a surface modification of natural polymers that is of interest in various industries, such as textiles and paper manufacture]. Analysis of the graft copolymers was conducted via Size Exclusion Chromatography (SEC), Liquid Adsorption Chromatography (LAC), Thermogravimetric Analysis (TGA), and FT-IR. Polyvinyl acetate was successfully grafted onto three polysaccharides (cellulosic materials), namely Hydroxyl Propyl Cellulose (HPC), Methyl Cellulose (MC) and cellulose. The study showed that the modification of cellulosic substrates with defined grafts of vinyl acetate can be easily achieved through minor modifications to existing industrial techniques.
RAFT-mediated synthesis of graft copolymers via a thiol-ene addition mechanism
(Stellenbosch : Stellenbosch University, 2007-12) Stegmann, Jacobus Christiaan; Sanderson, R. D.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: The main objective of this project was the controlled synthesis of graft copolymers via a thiol-ene addition mechanism. The Reversible Addition-Fragmentation chain Transfer (RAFT) process was used in all polymerization reactions with the aim to achieve a certain degree of control over the molecular weight. Several synthetic steps were required in order to obtain the final graft copolymer and each step was investigated in detail. Firstly, two RAFT agents (cyanovaleric acid dithiobenzoate and dodecyl isobutyric acid trithiocarbonate) were synthesized to be used in the various polymerization reactions of styrene and butyl acrylate. This was done successfully and the RAFT agents were used to synthesize low molecular weight polystyrene branches of the graft copolymer. Different molecular weights were targeted. It was found that some retardation phenomena were present especially at high RAFT agent concentrations. The polystyrene branches that were synthesized contained RAFT end-groups. Various pathways were explored to modify these RAFT end-groups to form thiol end-groups to be used in the thiol-ene addition reaction during the grafting process. The use of sodium methoxide for this purpose proved most successful and no evidence of the formation of disulfide bridges due to the initially formed thiols was detected. Allyl methacrylate (AMA) was chosen as monomer to be used for the synthesis of the polymer backbone because it has two double bonds with different reactivities. For the first time, RAFT was used to polymerize AMA via the more reactive double bond to obtain linear poly(allyl methacrylate) (PAMA) chains with pendant double bonds. However, at higher conversions, gelation occurred and the molecular weight distributions were uncontrolled. NMR was successfully used to study the tacticity parameters of the final polymer. Finally, the synthesis of the graft copolymer, PAMA-g-polystyrene, was carried out by means of the “grafting onto” approach. The thiol-functionalized polystyrene branches were covalently attached to the pendant double bonds of the PAMA polymer backbone via a thiol-ene addition mechanism in the presence of a free radical initiator. A Multi- Angle Laser Light Scattering (MALLS) detector was utilized in conjunction with Size- Exclusion Chromatography (SEC) to obtain molecular weight data of the graft copolymer. The percentage grafting, as determined by 1H-NMR, was low.
Reversible addition fragmentation chain transfer (RAFT) mediated polymerization of N-vinylpyrrolidone
(Stellenbosch : University of Stellenbosch, 2008-03) Pound, Gwenaelle; Klumperman, Bert; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
Xanthate-mediated polymerization was investigated as a tool for the preparation of well-defined poly(N-vinylpyrrolidone) and copolymers of N-vinylpyrrolidone. Some results regarding the monomer vinyl acetate are included, mostly for comparison purposes. The structure of the leaving/reinitiating group of the xanthate mediating agent was tuned to match the monomer reactivity. This was achieved by studying the initialization behaviour of monomer-xanthate systems via in situ 1H-NMR spectroscopy. Additionally, the latter technique was valuable to identify side reactions affecting the monomer, xanthate and/or polymeric species. Subsequently, experimental conditions were defined, and used to optimize the level of control achieved during polymerization. Block copolymers were prepared from a xanthate end-functional poly(ethylene glycol) with both vinyl acetate and N-vinylpyrrolidone. Finally, the preparation of poly(N-vinylpyrrolidone) with a range of well-defined end groups was achieved via postpolymerization treatment of the xanthate end-functional polymerization product. 3 different routes were investigated, which lead to poly(N-vinylpyrrolidone) with 1) aldehyde or alcohol, 2) thiol or 3) unsaturated ω-chain-end functionality, in high yield, while the α-chain-end functionality is defined by the structure of the xanthate leaving group. The ω-aldehyde end-functional poly(N-vinylpyrrolidone) was successfully conjugated to the lysine residues of the model protein lysozyme via reductive amination. Particular attention was drawn to characterizing the polymerization products. NMR spectroscopy, liquid chromatographic and mass-spectroscopic techniques were used. The major achievements emerging from polymer analysis carried out in this study included the following: - a library of NMR chemical shifts for N-vinylpyrrolidone derivatives; - an estimation of the critical conditions for poly(N-vinylpyrrolidone) relevant for separation according to the polymer chain-ends; - conditions for the separation of block-copolymers comprising a poly(ethylene glycol) segment and a poly(N-vinylpyrrolidone) or poly(vinyl acetate) segment via liquid chromatography; - valuable results on matrix-assisted laser ionization-desorption time-of-flight mass spectroscopy (MALDI-ToF-MS) of poly(N-vinylpyrrolidone).
Selected aspects of RAFT agents : the use of thiophene as an activating group in the RAFT mediated polymerization of styrene and the stereo-controlled polymerization of MMA via RAFT
(Stellenbosch : Stellenbosch University, 2007-12) Bshena, Osama; Sanderson, R. D.; Weber, W.; Tonge, M.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: In this study six Reversible Addition-Fragmentation chain Transfer (RAFT) agents with different leaving groups and different activating moieties were prepared. Three are novel thiophene-based RAFT agents having a thiophene substituent as the activating moiety, namely: (2-thienyl thiocarbonyl)disulfide (BTD), 1-cyano-1-methylethyl 2-thiophene dithiocarboxylate (CPDT), and benzyl thiophene-2-dithiocarboxylate (BDTT). The other three are model RAFT agents bearing a phenyl group as the activating moiety, namely: bis(thiobenzoyl) disulfide (BBD), benzyl dithiobenzoate (BDTB), 2-cyano-2-yl dithiobenzoate (CPDB). These agents were characterized by Nuclear Magnetic Resonance spectroscopy (NMR), Fourier-Transform Infrared spectroscopy (FT-IR) and Ultraviolet - visible spectroscopy (UV/vis). These compounds were studied as RAFT agents, and used as mediators in the bulk polymerization of styrene, self-initiated thermally at 100 °C. The novel thiophene-based mediated systems were compared with the phenyl-based ones in terms of polymerization kinetics, molecular weight and polydispersity index (PDI). The polymerization results showed that the novel CPDT aiid !:!!)TT thiophene-based compounds were effective RAFT agents for the RAFT polymerization of styrene with the characteristics of "living"/controlled free radical polymerization. The BTD mediated system showed the poorest control, as the PDI progressively broadened with monomer to polymer conversion, and it had the lowest reaction rate. In general, the thiophene-based mediated systems had slower reaction rates (higher retardation) when compared to the analogous phenyl-based mediated systems. The RAFT technique was then used to synthesize stereo-controlled poly (methyl methacrylates) in the presence of CPDB (since the thiophene-based RAFT agents showed unfavorable rate retardation) in different solvents, namely: toluene, 2-propanol and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) at different temperatures, namely: 60, 30, 4, and -18 °C. The prepared polymers were characterized by 1 H-NMR spectroscopy in order to monitor the tacticity and SEC for the determination of the controlled/living behavior of the polymerization system. Results showed that the simultaneous control of the molecular weight and stereochemistry of PMMA was accomplished via RAFT-mediated polymerization, especially in HFIP at -18 °C, where the syndiotactic content of the polymer was the· highest and low PDI values (< 1.4) were achieved.
Synthesis and characterization of comb-polymers with controlled structure
(Stellenbosch : University of Stellenbosch, 2006-12) Elhrari, Wael; Mallon, P. E.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
Synthesis of a series of poly (methylmethacrylate)-graft-poly (styrene) polymer was carried out via free radical polymerization of methylmethacrylate and polystyrene macromonomers. The macromonomers were synthesized via living anionic polymerization techniques. Two series of macromonomers where synthesized with different polymerizable end group functionalities, by termination with p-vinyl benzyl chloride and 3-(dimethyl chloro silyl) propyl methacrylate. The branch density was varied by controlling the composition feed ratio of the macromonomers to comonomer. Liquid chromatographic techniques were used to fully characterize the chemical composition and branch distributions of the graft polymer. Liquid chromatography under critical conditions of adsorption of styrene coupling with Fourier Transform Infrared Spectra was used to investigate the chemical composition and distribution of the branches in the graft. Physical properties of the graft copolymers such as Tg and free volume were determined using differential scanning calorimetry and positron lifetime spectrometry respectively. The relationship between the chemical composition and the graft copolymer properties such as Tg and free volume were investigated. The results show that for long chain macromonomers phase segregation occurs in the graft copolymers. In the case of shorter chain macromonomers at low content no phase segregation is observed and the macromonomers have an antiplasticization effect on the graft polymers.
Synthesis and characterization of multiphase copolymers
(Stellenbosch : Stellenbosch University, 2011-12) Elhrari, Wael K. S.; Mallon, P. E.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: Multiphase copolymers generally consist of copolymers where the disparate natures of each of the segments results in complex phase-segregated morphologies in the solid state. The outstanding properties and wide range of applications of multiphase copolymers has led to the need for more sophisticated synthesis methods to produce copolymers with controlled structures. Associated with developments in synthetic methods is the need to develop suitable techniques to characterize these materials in order to obtain a better understanding of their structure–property relationships. The synthesis of multiphase copolymers presents many challenges. These are related to the nature of the molecular requirements, were the monomers of each of the different components may not be polymerized by all available polymerization techniques. This has led to the need to combine different polymerization techniques to overcome such limitations. The focus of this study is the combination of living controlled polymerization techniques, namely anionic polymerization and RAFT polymerization, with hydroboration/autoxidation, to produce non-polyolefin block and graft copolymers. Block copolymers were synthesized by coupling anionic polymerization and hydroboration/autoxidation reactions. The first block segment was prepared via anionic polymerization, and then end-functionalized with a suitable functional group (e.g. an allyl group). A hydroboration/autoxidation reaction was then used to initiate the polymerization of the second block by the slow addition of oxygen at room temperature. Graft copolymers were synthesized using the 'grafting from' technique, by coupling RAFT copolymerization with hydroboration/autoxidation reactions. The backbone polymer was synthesized via RAFT copolymerization of symmetric and asymmetric monomer, after which a hydroboration/autoxidation reaction was carried out to produce graft copolymers. The hydroboration/hydroxylation reaction could also be used to modify an unsaturated polymer chain. The EPDM rubber chain was modified by transforming the double bond into an hydroxyl group, which could undergo an esterification reaction with an acid chloride RAFT agent to produce the multifunctional RAFT polymer. This was used for the controlled living free radical polymerization of the graft chains. Significant amounts of homopolymerization in addition to graft formation were obtained. Solid state NMR (SS NMR) and positron annihilation lifetime spectroscopy were used to determine the compositional phase segregation point in the graft copolymers. The spin diffusion data from the SS NMR provided insight into the seemingly anomalous positron data at the phase segregation point. It is demonstrated how these two techniques can provide complimentary data on the solid state morphology of these multiphase materials.
Synthesis and characterization of styrene – maleic anhydride copolymer derivatives
(Stellenbosch : University of Stellenbosch, 2009-12) Mpitso, Khotso; Klumperman, L.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: In this study, a functional styrene – maleic anhydride copolymer (SMA) was synthesized via reversible addition-fragmentation chain transfer mediated polymerization (RAFT). The obtained copolymer had an alternating structure with well controlled molecular weight. The structure of the copolymer was found to alternating when characterized by NMR and MALDI-Tof-MS. SMA copolymer is functional polymer due to the presence of reactive maleic anhydride moiety in its backbone. The SMA copolymer was used as a starting material for synthesis of other three copolymers with potential anti-viral activity. These three copolymers are referred to as SMA copolymer derivatives because they were synthesized by reacting either maleic anhydride or styrene moieties with certain chemical compounds. The three derived copolymers are; styrene-maleimde copolymer (SMI), styrene sulfonate-maleic anhydride copolymer (SSMA) and styrene sulfonate– maleimide copolymer (SSMI). SMI was synthesized by reacting 4-aminomethylbenzene sulfonamide compound with maleic anhydride moieties on the backbone of SMA copolymer. The reaction proceeded in the presence of co-catalysts triethylamine and dimethylamino pyridine to from amide linkages. The copolymer was characterized by NMR spectroscopy, SEC and FTIR spectroscopy. SSMA copolymer was successfully synthesized by reacting styrene moieties of the SMA copolymers with chlorosulfonic acid. The SSMA copolymer was further reacted with amine compound to synthesize SSMI copolymer. The synthesis of SSMI was achieved by reacting the maleic anhydride moieties in the backbone of the SSMA copolymer with N1,N1- dimethylpropane-1,3-diamine. Both copolymers were successfully characterized by FTIR spectroscopy.
Synthesis and characterization of surfmers for latex stabilization in RAFT-mediated miniemulsion polymerization
(Stellenbosch : University of Stellenbosch, 2005-12) Matahwa, Howard; Sanderson, R. D.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
Synthesis of two surfmers (cationic and anionic) was carried out and the surfmers were used to stabilize particles in miniemulsion polymerization. Surfmers were used to eliminate adverse effects associated with free surfactant in the final product e.g. films and coatings. The Reversible Addition Fragmentation chain Transfer (RAFT) polymerization process was used in miniemulsion polymerization reactions to control the molecular weight distribution. RAFT offers a number of advantages that include its compatibility with a wide range of monomers and solvents. Moreover block copolymer synthesis is possible via chain extension. A comparative study between classical surfactants and surfmers was conducted in regard to reaction rates and molar mass distribution. The rates of reactions of surfmer stabilized RAFT miniemulsion polymerization of Styrene and MMA were similar (in most cases) to classical surfactant stabilized RAFT miniemulsion polymerization reactions. The final particle sizes were also similar for polystyrene latexes stabilized by surfmers and classical surfactants. However PMMA latexes stabilized by surfmers had larger particle sizes compared to latexes stabilized by classical surfactants. The surfmers were also oligomerized in homogeneous media using the RAFT process and their Mn values were estimated using UV-VIS spectroscopy. The oligosurfmers were then used as emulsifiers in RAFT miniemulsion polymerization. The rates of reaction were slower than rates obtain when the surfmers (monomer or oligosurfmers) were used directly as emulsifiers in RAFT miniemulsion polymerization of styrene and MMA. The final latex particle sizes obtained with oligosurfmers were also larger than that of latex stabilized by their parent monomers. The RAFT process was successfully applied in miniemulsion polymerization in both classical surfactant and surfmer stabilized miniemulsions. The molecular weight increased with conversion showing that the molecular weights of the polymers were controlled.

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