published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Block copolymers containing pendant pyrene, terpyridine and poly(3- hexylthiophene) moieties with different block ratios and chain lengths were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The block copolymers obtained had narrow molecular weight distribution. The applications of these polymers for non-covalent functionalization of carbon nanotubes and in photovoltaic devices were studied. The molecular weight distribution and block sizes of the block copolymers could be controlled quite well. The polydispersities measured were below 1.25. The block copolymers could be functionalized on the surface of CNTs. The functionalized CNTs had an improved dispersing ability and a maximum dispersing ability of 0.30 mgmL-1 in DMF was achieved. The photosensitizing properties of an individual functionalized CNT were studied by conductive atomic force microscopy. In the presence of the photosensitizing unit, the photocurrent was measured to be 6.4 nAμW-1 at 580 nm. This suggests the role of metal complexes in the photosensitizing process in the block copolymer. Poly(3-hexylthiophene)-block-pendant pyrene copolymers were synthesized by Grignard metathesis and RAFT polymerization. Different loadings of the block copolymers functionalized CNT were employed as the electron accepting materials in bulk heterojunction photovoltaic devices. A maximum power conversion efficiency of 0.77 × 10-3 % was achieved for the poly(3- hexylthiophene): 0.5% polymer functionalized CNT devices. The poor efficiency was attributed to the low CNT loadings that limited the electron transport in the devices. The poly(3-hexylthiophene)-block-pendant pyrene copolymer were employed as compatibilizer for poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction photovoltaic devices. With the addition of 20 % of the block copolymer, a maximum power conversion efficiency of 1.62 % could be achieved. The long term stability of the encapsulated photovoltaic devices was studied. There was more than 30 % reduction in the degradation of performance after 30 days when the block copolymer was added as compatibilizer. These results suggested the role of the block copolymer compatibilizers in improving both the photovoltaic performances and stability of the devices. Differential scanning calorimetry results suggested that the improved photovoltaic performances may be attributed to the enhanced compatibility between poly(3- hexylthiophene) and PCBM. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Well-defined functional block copolymers containing either a fluorinated or a hydrophilic segment can be synthesized via a controlled free-radical technique, known as atom transfer radical polymerization (ATRP). Their self assembly characteristics in the solid state and in solution were examined in this work with the aim of developing ultralow dielectric constant materials and templates for conductive polymer synthesis, respectively. We demonstrated the controlled synthesis via ATRP of block copolymers containing poly(pentafluorostyrene) (PPFS) and a degradable polymer, such as poly(methyl methacrylate) (PMMA), poly([epsilon]-caprolactone) (PCL), or poly(D,L-lactide) (PLA). These block copolymers microphase separate in the solid state to form periodic nanostructures, such as alternating lamellae, a bicontinuous gyroid on a cubic lattice, cylinders on a hexagonal lattice, or spheres on a body-centered-cubic lattice, depending on the volume fraction of each block and the interblock segregation strength. Additionally, we quantified the interblock segregation strength of PPFS/PMMA, demonstrating that this block copolymer is only approximately twice as segregated as its nonfluorinated counterpart poly(styrene-[beta]-methyl methacrylate) due to the symmetric placement of the polar C-F bonds on the benzene ring in PPFS. We also showed that the self-assembly characteristics of PPFS-containing block copolymers can be used to create nanoporous fluorinated films with ultra-low dielectric constants in the range of 1.7 - 1.9. The dielectric constants are tunable through manipulation of the volume fraction of the degradable block in the parent block copolymers. We also demonstrated the controlled synthesis via ATRP of block copolymers containing poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAAMPSA) with either poly(oligo(ethylene glycol) methyl ether methacrylate) (PEGMA) or poly(methyl acrylate) (PMA). We showed that PEGMA/PAAMPSA formed well-ordered nanostructures in the solid state when cast from strong hydrogen bond accepting solvents, such as DMSO and DMF. PEGMA/PAAMPSA can also be used as the acid dopant in the synthesis of conductive polyaniline (PANI). Additionally, we studied the micelle formation of PMA/PAAMPSA and subsequently used these micelles as templates to create spherical conductive PANI nanoparticles. The size and size distribution of these PANI nanoparticles were dictated by the corresponding characteristics of the micellar template. / text

Controlled free radical polymerization facilitates the production of polymers with highly defined microstructures like traditional ionic polymerization; but in contrast allows for previously unattainable monomer combinations such as acrylic acid in its non-protected form. Incorporation of acrylic acid into styrene was done by random copolymerization of acrylic acid (directly and in its protected form as tert-butyl acrylate) with styrene. Styrene/tert-butyl acrylate (S/t-BuA) as well as styrene/acrylic acid (S/AA) mixtures were copolymerized to form tapered or gradient copolymers. Using an alkoxyamine unimolecular initiator, 2-[N- tert-butyl-2,2-(dimethylpropyl)aminooxy] propionic acid (BlocBuilder RTM), along with additional free nitroxide (SG1), the effect of acid protection on polymerization kinetics and copolymer composition was determined. Adding 4.5 mol% SG1/BlocBuilderRTM greatly improved the control of S/t-BuA copolymerization with low polydispersities (1.14-1.22) whereas the S/AA required higher levels of SG1 to produce polymers with low polydispersities that were comparatively still broader compared to the S/t-BuA system (polydispersities ∼ 1.3-1.4 at 9 mol% SG1/BlocBuilderRTM). S/AA copolymerization required higher SG1 concentrations to compensate for degradation of SG1 by attack from the acrylic acid monomer.

No description available.

No description available.

Novel, high molecular weight poly(dimethylsiloxane) / cycloaliphatic polyester segmented copolymers were prepared and characterized. Specifically, polyesters based on dimethyl 1,4-cyclohexane dicarboxylate and 1,4-butanediol were employed. The copolymers were synthesized via a melt process using a high trans content isomer which afforded semi-crystalline morphologies. Aminopropyl terminated poly(dimethylsiloxane) oligonlers of controlled molecular weight were synthesized and then end capped with excess diester to form a diester terminated amide linked oligomer. The latter was then incorporated into the copolymer via melt transesterification step reaction segmented copolymerization. The molecular weight of the polysiloxane and chemical composition of the copolymer were systematically varied to prepare a series of segmented polyester / poly(dimethylsiloxane) copolymers. / Ph. D.

Thesis (MSc)--University of Stellenbosch, 2007. / 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. / AFRIKAANSE OPSOMMING: Die hoofdoel van hierdie projek is die beheerde sintese van ‘n entkopolimeer via ‘n merkaptaan-een addisiereaksie. Die sogenaamde “Reversible Addition-Fragmentation chain Transfer” (RAFT) proses is in al die polimerisasiereaksies gebruik met die doel om ‘n mate van beheer oor die molekulêre massa van die polimere te verkry. Verskeie stappe (waarvan elkeen ten volle ondersoek is) was nodig om die finale entkopolimeer te verkry. Eerstens is twee RAFT-agente (sianovaleriaansuur ditiobensoaat en dodekielisobottersuur tritiokarbonaat) gesintetiseer vir gebruik in verskeie polimerisasiereaksies van stireen en butielakrilaat. Hierdie stap was suksesvol en die RAFT-agente is toe gebruik vir die sintese van lae molekulêre massa polistireensytakke vir die entkopolimeer. Die molekulêre massas van die sytakke is gevarieer en daar is gevind dat vertragings in die polimerisasiereaksies voorgekom het, veral by hoë konsentrasies van die RAFT-agente. Die polistireensytakke wat gemaak is, besit almal ‘n RAFT-eindgroep. Verskeie roetes is bestudeer ten einde die RAFT-eindgroepe tot merkaptaan-eindgroepe te modifiseer om sodoende tydens ‘n merkaptaan-een addisiereaksie gebruik te word. Die gebruik van natriummetoksied was hier die suksesvolste en daar was geen teken van die vorming van disulfiedbrûe as gevolg van die oorspronklik gevormde merkaptane nie. Allielmetakrilaat (AMA) is gekies as die monomeer wat gebruik sou word vir die sintese van die polimeerruggraat omdat die monomeer twee dubbelbindings met verskillende reaktiwiteite besit het. Vir die eerste keer is RAFT gebruik vir die polimerisasie van AMA via die meer reaktiewe dubbelbinding om lineêre poli(allielmetakrilaat) (PAMA) kettings met dubbelbindings in die sygroepe te verkry. Gelvorming en onbeheerde molekulêre massaverspreiding het egter by hoër monomeeromsettings voorgekom. KMR is susksekvol gebruik om die taktisiteitsparameters van die finale polimeer te bestudeer. Ten slotte is die sintese van die entkopolimeer, PAMA-g-polistireen, uitgevoer deur die aanhegting van voorafgevormde sytakke. Die polistireensytakke met die merkaptaaneindgroepe is kovalent geheg aan die dubbelbindings in die sygroepe van die PAMA-polimeerruggraat via ‘n merkaptaan-een addisiemeganisme in die teenwoordigheid van ‘n vrye radikaalinisieerder. ‘n Kombinasie van gelpermeasiechromatografie en multi-hoeklaserligverstrooiing is gebruik om die molekulêre massa van die entkopolimeer te bepaal. Die persentasie sytakke soos bepaal deur 1H-KMR was laag.

Thesis (PhD (Chemistry and Polymer Science)--University of Stellenbosch, 2005. / This study involves the synthesis and characterisation of PEG-based amphiphilic block copolymers for the hydrophilization of polysulphone ultrafiltration membranes. PEG based macro RAFT agents were synthesized and characterised. PEG-b-PS block copolymers were synthesized via the RAFT assisted controlled free radical polymerisation utilizing the synthesized PEG macro RAFT agents. The resulting polymerisation products were then analyzed by two-dimensional chromatography at the “critical conditions” for PS. In the second phase of this study PEG-b-PSU block copolymers were synthesized via the polycondensation of bis (4-chlorophenyl) sulphone, Bisphenol A, and PEG. The resulting products were characterised by NMR spectrometry. PEG-b-PS films and modified PSU membranes (modified by the addition of PEG-b-PSU block copolymer to the membrane casting solution) were prepared and analyzed. Surface analyses included static contact angle, AFM force-distance analysis, and FTIR-PAS analysis. Results showed the successful synthesis of both PEG-b-PS and PEG-b-PSU amphiphilic block copolymers. Surface analysis proved the successful hydrophilization of the surface of the modified PSU membranes.

In 1980’s a new class of material named as dendrimer became popular both in the field of polymer science and engineering. Dendrimer is an example of symmetric, highly branched three dimensional globular nano-object. It possess several interesting physical and chemical properties like low solution and melt-viscosity, lower intermolecular chain entanglement, large number of end groups placed at the molecular periphery, relatively high solubility with respect to their linear counterpart. In order to get this perfectly branched structure, one has to go through the tedious multistep synthetic approach, repetitive chromatographic purification and protection-deprotection strategies in every step; all of which limits the large scale production and thus commercialization. On the other hand, hyperbranched polymer, a highly branched analogue of dendritic polymer with few defects in their branching architecture, which can be prepared in a single step, show similar physical and chemical properties as that of dendrimer. Polymerization of AB2 monomer is one of the well established method to generate hyperbranched polymer which upon polymerization, generates plenty of ‘B ’groups at the periphery along with a single ‘A’ group as a focal point in the resulting hyperbranched polymer as shown in Figure 1. From the structural point of view, hyperbranched polymers consist of three distinctly different compartments such as periphery, interior and a (single) focal point. During the past decade our lab have developed a novel melt trans-etherification process to generate polyethers and have utilized to access to a wide variety of hyperbranched structures. One of the challenges we addressed is to selectively functionalize the periphery of the hyperbranched polymer during the polymerization process. Polycondensation of ‘AB2’ monomer is not sufficient enough to generate a wide variety of hyperbranched polymer as the periphery of hyperbranched polymer is limited to the ‘B’ functional group unless it could be modified via ‘post-polymerization modifications’. Copolymerization of ‘AB2’ monomer with stoichiometric amount of ‘A-R’ monomer should result in hyperbranched polymer decorated with ‘R’ groups in the periphery that can be prepared in a single step. One of the prerequisite in the ‘AB2+A-R’ approach is that the comonomer ‘A-R’ should have silent ‘R’ group which does not interfere during the polymerization. During the copolymerization process with stoichiometric amount of ‘A-R’ monomer, ‘AB2’ monomer having one equivalent excess of ‘B’ can react with the ‘A’ group from ‘A-R’ monomer eventually generating the hyperbranched structure with peripheral ‘R’ groups. By appropriately choosing the ‘R’ group, one can access a wide class of hyperbranched polymer with the required functionality. Further by having a reactive ‘R’ group that is not participating in polymerization can act as a handle for post-polymerization modifications. For instance, copolymerization of 1-(6-Hydroxyhexyloxy)-3,5-bis(methoxymethyl)-2,4,6-trimethylbenzene (Hydroxy as ‘A’ and methoxy as ‘B’) and 6-bromo-1-hexanol where ‘OH’ and ‘-(CH2)6Br’ is ‘A’ and ‘R’ functional groups respectively, generates hyperbranched polymer with peripheral alkyl bromide functional groups as shown in Figure 2. The peripheral alkylbromides has been quantitatively transformed to quaternary ammonium or pyridinium salts using trimethyl amine or pyridine respectively. Thus by the post polymerization modification, we have transformed a hydrophobic hyperbranched polymer to a water soluble cationic hyperbranched polymer by simple and efficient post-polymerization modification. In a slightly different objective we Another problem that I have addressed is the difficulty associated with the aforementioned copolymerization approach. In spite of the fact that stoichiometric amounts of ‘A-R’ type monomer was taken in ‘AB2 + A-R’ approach, the extent of peripheral functionalization i.e. the incorporation of ‘R’ group is relatively lower. Further the molecular weight of the hyperbranched polymer obtained is also not high. One of the reasons we adopted ‘AB2 + A-R’ approach is to provide a functional handle for the subsequent post-polymerization modification. We modified the ‘AB2’ type monomer with a functionalizable handle to circumvent the lower amount of incorporation of the ‘A-R’ type monomer in ‘AB2 + A-R’ approach. Of all the readily functionalizable handles, click chemistry found to be a very useful tool for the post-polymerization modifications as the reactions conditions are mild, no side product, high selectivity, easy purification, etc. Another advantage of this reaction is that, we can incorporate any type of functional group starting from a single clickable parent hyperbranched polymer. In this particular project, I have Earlier design of the ‘AB2’ type monomer in our group, to prepare hyperbranched polymer via melt transetherification process, involved benzylic methoxy groups as ‘B’ in ‘AB2’ monomer leading to a hyperbranched polymer with peripheral methoxy groups. Transetherification under melt-conditions is an equilibrium reaction which was driven towards the hyperbranched polymer by continuous removal of methanol from the system as a volatile alcohol. In the new design of ‘AB2’ monomer; we have used benzylic allyloxy groups as ‘B’ in ‘AB2’ monomer, where in polymerization is driven by the continuous removal of allyl alcohol (instead of methanol as in the previous case), generates hyperbranched polymer with peripheral allyloxy group containing hyperbranched polymer. The allyloxy groups can be subsequently functionalized with a variety of thiol, we prepared a hydrocarbon-soluble octadecyl-derivative, amphiphilic systems using 2-mercaptoethanol and chiral amino acid (N-benzoyl cystine) hyperbranched structures by using thiol-ene click reactions (Figure 3). Polymers prepared from the parent hyperbranched polymer have significantly different physical properties like glass transition temperature (Tg), melting point (Tm) etc; thus considering the versatility of functionalization, parent polymer could be envisioned as a clickable hyperscaffold. More interestingly by functionalizing cystine derivative, we have demonstrated the possibility of biconjugation of the hyperbranched polymer. In summary, the limitations of ‘AB2+A-R’ copolymerization approach (low molecular weight Molecular weight and molecular weight distribution are very important parameters that influence the physical property and thus the application of the polymeric materials. As predicted by Flory, hyperbranched polymers are inherently polydisperse in nature and it tends to infinity when the percent of conversion is very high. Experimentally observed value of polydispersity is also significantly higher compared to their linear analogues. Control of the molecular weight and polydispersity of hyperbranched polymer by using a suitable amount of reactive multifunctional core has been demonstrated in this project. We have substantiated by using very little amount of ‘B3’ core along with ‘AB2’ monomer; wherein ‘B’ in ‘B3’ are more reactive than ‘B’ in ‘AB2’ monomer, regulate the molecular weight and polydispersity of the resulting hyperbranched polymer. As the ratio of core to monomer increases the molecular weight and polydispersity reduces in nearly linear fashion. In a slightly different objective, the core and periphery are functionalized with two different fluorophore by using orthogonal click reactions and demonstrated the possibility of energy transfer from periphery to the core of the hyperbranched polymer. In this section of my thesis, the self-assembly behavior of a periodically grafted amphiphilic copolymer has been studied. Polymer was synthesized via melt transesterification approach where hexaethylene glycol monomethyl ether (HEG) containing diester monomers are reacted with alkylyne diol monomers with varying carbon spacer (C12 and Another interesting problem, I approached is to functionalize the interior part of the hyperbranched polymer. In the case of dendrimer, as it is a step-wise synthesis, internal functionalization could be accomplished with the order of monomer addition i.e. by putting the internal functional group containing monomer first followed by other monomer not having those functional groups, whereas it is a bit challenging task for hyperbranched polymers especially when dealing with polycondensation of AB2 monomers, as it is a single step polymerization process. For a hyperbranched polymer in the polycondensation of ‘AB2’ monomer, the internal functional group should reside in between of the ‘A’ and ‘B’ functional group wherein the internal functional groups are silent during the process of polymerization. In order to do so, we have designed and synthesized a new AB2 monomer (a in Figure: 4). Here decanol is the volatile condensate that was removed during the transetherification reactions leading to a hyperbranched polymer having allyl group as the internal functional group and decyloxy as the peripheral functional group (b in Figure: 4). As a post-polymerization modification, the interior allyl groups were modified by thiol-ene click reaction with variety of thiol derivatives. In one example, the inherent hydrophobic nature of the parent hyperbranched polymer which is enhanced by the decyl chain at the molecular periphery, is converted to a alkaline water soluble hyperbranched polymer by the click reaction with mercapto succinic acid (d in Figure: 4) or mercapto propionic acid (c in Figure: 4) to the internal allyl groups, generating a novel amphiphilic hypersystem. This kind of amphiphilic systems are very interesting to study for their self-assembly behavior, in this particular case, the modified hyperbranched polymer adopts as a large spherical aggregates in alkaline water evidenced by FESEM (Figure: 4) and AFM images. Further investigation is being carried out to understand the exact nature of these aggregates. As the hyperbranched polymer contained ‘-S-‘ group in the interior, we utilized this as the scaffold for scavenging heavy metal ions like Hg2+ from aqueous solutions to the chloroform solution containing polymer. This hyperbranched polymer could trap Hg2+ ions even when present in ppm level of contamination.