Browsing by Subject "Conjugated polymers"

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Applications of N-heterocycles in electrically and ionically conductive polymers
(2010-08) Norris, Brent Carl; Bielawski, Christopher W.; Anslyn, Eric V.; Manthiram, Arumugam; Colwey, Alan H.; Sessler, Jonathan L.
The covalent bond formed between a N-heterocyclic carbene and an aryl-isothiocyanate was discovered to be thermally-reversible. This bond was incorporated into the backbone of an aromatic polymer which, when subjected to heat and excess monomer, would depolymerize to smaller oligomers. In addition these small molecules contain active chain ends and could be repolymerized to reform the original polymer. The high molecular weight material was made into freestanding sheets with desirable mechanical properties and could be made conductive by treatment with iodine. A new poly(triazene) was formed from the reaction of a facially opposed, annulated, bis-N-heterocyclic carbene (NHC) and an organic bis-azide. The NHC as well as the azide were varied and combined to produce a series of polymers which were characterized by GPC, TGA, and NMR. These thermally robust polymers were also coated onto glass slides and rendered electrically conductive by exposure to iodine vapor. A new reagent for Reversible Addition Fragmentation Chain Transfer Polymerization (RAFT) is described. This imidazolium based reagent shows unusually fast kinetics which allows it to control polymerizations at significantly reduced loadings compared to the more traditional neutral dithiocarbamates or dithioesters. The fast kinetics is explained by the rapid rotation of the dithioester about the plane of the cationic N-heterocycle. Sulfonated poly(ether ether ketone) (sPEEK) membranes were blended with imidazoles with varying pKas. The proton conductivity of the membranes was evaluated as a function of pKa and temperature. Interestingly, the conductivity of the dry membranes showed a non-monotonous profile over a temperature range of 25 – 150 C. We use a theoretical model to better understand the mechanistic origins of the observed temperature–conductivity profiles. This model is based on the reaction equilibria between sPEEK’s sulfonic acid groups and the basic sites of the added heterocycles. Using the copper-catalyzed 1,3-dipolar “click” cycloaddition reaction, poly(sulfone)s containing pendant azide moieties were functionalized with various amounts of sodium 3-(prop-2-ynyloxy)propane-1-sulfonate and crosslinked with 1,7-octadiyne. The degree of sulfonation as well as the degree of cross-linking was systematically varied by changing the ratios of the aforementioned reagents. The polymers were cast into membranes, acidified, and then tested for proton conductivity, methanol permeability, and membrane-electrode assembly (MEA) performance.
Catalytic and polymer composite applications of graphite oxide and the controlled synthesis of donor-acceptor conjugated polymers
(2015-05) Todd, Alexander Dean; Willson, C. G. (C. Grant), 1939-; Bielawski, Christopher W; Dong, Guangbin; Humphrey, Simon M; Anslyn, Eric V; Makarov, Dmitrii E; Ellison, Christopher J
While much of the recent work regarding carbon materials has centered on monolayer graphene, we have focused on the relatively inexpensive, and readily accessible graphite oxide (GO) and thermally-reduced graphite oxide (TRG) to develop new catalysts and polymer composites. Indeed, the reactivity and physical properties inherent to GO has led to improved GO-based catalysis and composite materials with more robust mechanical properties. GO in combination with zeolite NaY dehydrated a variety of alcohols to their corresponding olefinic products under mild conditions. Although GO at 15 wt% loading was an active dehydration catalyst, it was found that the GO loading could be reduced by 50% when used cooperatively with NaY (7.5 wt%). Ultimately, it was determined that the acidity of GO was able to convert the NaY into an acidic form without the added step of ammonium exchange or high-temperature calcination. For the former application, polyethylene/thermally reduced graphite oxide (TRG) composites have been synthesized under mild conditions (1 atm ethylene and 40 °C) using (n-BuCp)₂ZrCl₂ activated with methylaluminoxane (MAO) as the polymerization catalyst. Although the thermal properties of the aforementioned composites were relatively unchanged when compared to polyethylene, significant enhancements in the mechanical properties were observed (e.g., up to 57% increase in the tensile strength and 170% increase the Young’s modulus for composites containing 5.2 wt% TRG). Donor-acceptor conjugated polymers, where the backbone consists of an alternating electron rich and electron poor π-conjugated repeat unit, have some of the highest reported power conversion efficiencies to date. Conventional synthetic methods for synthesizing donor-acceptor conjugated polymers have relied upon Pd-catalyzed Stille- or Suzuki-type polycondensations and typically afford ill-defined polymers with variable molecular weights and broad polydispersities (Đs). We were interested in synthesizing well-defined donor-acceptor conjugated polymers and have developed a method to polymerize a donor-acceptor monomer based on thiophene and 5,6-difluorobenzotriazole in a controlled fashion using Ni-catalyzed Kumada catalyst-transfer polycondensation (KCTP). By altering the initial catalyst-to-monomer ratio, the M [subscript n] of the resulting polymers (PFBTzHT) ranged from 6.2 kDa – 22.9 kDa, while maintaining narrow Đs (≤ 1.4). Moreover, under the optimized polymerization conditions, the M [subscript n] was found to increase linearly with monomer conversion. Ultimately, the quasi-living nature of the polymerization allowed for the synthesis of well-defined diblock copolymers of PFBTzHT and poly(3-hexylthiophene) (PFTBTz-b-P3HT).
Electrochemical and electrogenerated chemiluminescence studies of the BODIPY dyes
(2011-08) Nepomnyashchii, Alexander Borisovich; Bard, Allen J.; Crooks, Richard M.; Stevenson, Keith J.; Mullins, Buddie C.; Korgel, Brian A.
Electrochemical and electrogenerated chemiluminescence properties of the BODIPY (boron dipyrromethene) dyes are presented. Some specific features of the BODIPY dyes are obtained and described in the current dissertation. Separation of around 1.0-1.2 V is noticed between two oxidation and reduction waves for one center which is very different from 0.5 V seen for the polycyclic hydrocarbons. Cathodic and anodic stability is shown to depend upon absence or presence of certain degree of substitution. Different ways of electrochemical dimerization are explored and compared with the chemical dimerization. Photophysical and electrochemical properties of monomer, chemically synthesized dimer, trimer and polymer are described and the characteristic features determined.
Ensemble and single molecules fluorescence studies of polymers
(2007-12) Kim, Yeon Ho, 1973-; Vanden Bout, David A.
The effects of chain conformation on the photo-oxidation and green emission of poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) are investigated at both single molecule and ensemble levels. Single molecule studies reveal the conformation of PFO chains to be more globular when cast from THF than from toluene. Intensity transients of single molecules show that the elongated molecules cast from toluene have more fluctuations due to a fewer number of emitting centers on the polymer. Photochemical degradation leads to intensity fluctuations for the elongated molecules, while the globular chains show monotonic decays. Emission spectra of the single molecules show that photochemical oxidation leads to reduction in the emission of the molecule with no change in the emission spectra. No green emission is detected for single molecules indicating that formation of emissive ketone defects occurs rarely. Ensemble studies show that molecule cast from THF develop some green emission upon photodegredation while those cast from toluene don't. The increase in green in the globular molecules suggests that interchain contacts are necessary for the photochemical formation of emissive ketone defects in the PFO. All emission spectra of the aggregated and nonaggregated PFO during photooxidation are also analyzed by using a modified FranckCondon progression model with an additional independent Gaussian component and fitting results from single PFO spectrum. While emission spectrum of single PFO molecule shows a good fitting result to the model, the other two bulk PFO films display needs to introduce an additional term for better fit. This additional independent Gaussian component implies that green emission comes from non-Franck-Condon process. Rotational dynamics of poly(methyl acrylate) is investigated by single molecule spectroscopy. Polarized fluorescence transients from single rhodamine 6G dye embedded in polymer matrix above glass transition are analyzed and the correlation function of reduced linear dichroism is fit by a stretched exponential function. The fitting results suggest that non-exponential decay of correlation function. However, more rigorous study is needed because of the intrinsic statistical error of limited experimental data and the effect of high numerical objective.
Fluorescence microscopy of materials with energy applications
(2020-05-13) Geberth, Geoffrey Thomas; Vanden Bout, David A.; Roberts, Sean T; Baiz, Carlos; Mullins, Charles; Dodabalapur, Ananth
One of the largest drivers of modern materials research is the advancement of renewable energy, particularly solar power. This objective has seen the rise of several different materials systems, each with their own advantages and disadvantages. Before any of these alternatives to traditional inorganic semiconductors can be effectively utilized at a commercial scale, they must first be understood at the fundamental level such that they can be tuned through utilization of the structure-property relationship. This dissertation describes the use of fluorescence-based microscopy techniques to explore material systems relevant to energy production at the smallest possible levels, ranging from single molecules and aggregates to small scale surface structures in order to unravel the microscopic heterogeneities that influence photophysical performance. First, two different conjugated polymers were studied. Poly(3-(2'-methoxy-5'-octylphenyl)thiophene) (POMeOPT) aggregates were studied in bulk solution in order to probe charge transfer character in the excited state. Nonpolar solution environments led to more than hundredfold increases in the fluorescence intensity of this material, demonstrating the importance of the environment in manipulating the photophysics of conjugated polymers and illustrating the role the charge transfer state plays in the excited state. Next, a polyphenylenevinylene (PPV) based block copolymer designed for controlled folding was examined at the single molecule level with excitation polarization spectroscopy, revealing not only the robustness of the folding functionality, but also that the resulting folds spaced the chromophores far enough apart to severely limit interactions between them. Shifting to a different class of energy materials, single perylene diimide (PDI) aggregates were then formed with solvent vapor annealing (SVA) and studied with fluorescence microscopy. These experiments revealed a vast heterogeneity amongst small aggregates as well as provided strong evidence for emissive excited states with triplet character occurring even in small aggregates. The final class of material studied was a Ruddlesden-Popper phase quasi-2D organolead halide perovskite. Confocal fluorescence microscopy was utilized to image film degradation in the presence of moisture, and provided insights into the mechanism behind moisture-driven surface crystallite growth. Taken together these experiments demonstrate the power of fluorescence microscopy to advance the understanding of energy materials systems by examining small scale heterogeneity.
Investigation of the photo-induced charge transfer in organic semiconductors via single molecule spectroscopy techniques
(2009-12) Lee, Kwang Jik; Barbara, P. F. (Paul F.), 1953-
Photo-induced charge transfer which occurs between molecules or different parts of a large molecule is the pivotal process related to performances of organic electronics. In particular, injection of charge carriers into conjugated polymers and dissociation of photo-generated excitons at the heterojunction between a donor and acceptor system are of great importance in determining the luminescence efficiency of organic light emitting diodes (OLEDs) and solar energy conversion efficiency of organic solar cells, respectively. However, the complex nature of organic semiconductors as well as complicated primary processes involved in the functioning of these devices have prevented us from understanding unique characteristics of these processes and thereby engineering better materials for higher performances. In this dissertation, two different types of photo-induced (or -related) charge transfer processes occurring in organic semiconductors were investigated by using single molecule spectroscopy (SMS) techniques to unravel the complexities of these processes. The carefully designed functioning capacitor-like model devices similar to OLEDs and photovoltaic cells were fabricated where isolated single nanoparticles were introduced as an active medium to mitigate the complexities of these materials. We observed that injection of positively charged carriers (holes) into poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) single nanoparticles from the carbazole hole transport layer does not occur in the absence of light. We denoted the observed hole injection in aid of light as the light-induced hole transfer mechanism (LIHT). It was revealed that the charging dynamics are highly consistent with a cooperative charging effect. In addition, the LIHT was proposed as the possible source for the formation of deep trapped hole in organic devices. Local exciton dissociation yields across a nanostructured domain between poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) single nanoparticles and either poly(9,9- dioctylfluorene - co - bis-N,N- (4 -butylphenyl)-bis-N,N-phenyl-1,4-phenylene diamine) (PFB) or poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) film in model photovoltaic devices was also investigated. A wide distribution of exciton dissociation yields was observed from each nanodomain due to the device geometry. The observed hysteresis in fluorescence voltage curve was ascribed to accumulated charges following charge separations. The dynamics of charge separation under the applied electric field was described in more detail.
Single molecule studies of structure-property relationships in well-defined conjugated oligomers and push-pull conjugated polymers
(2017-05) Shao, Beiyue; Vanden Bout, David A.; Roberts, Sean T.; Brodbelt, Jennifer; Anslyn, Eric V.; Shih, Chih-Kang
The electronic properties of conjugated polymers are strongly dependent on their morphologies. Despite continuous research efforts in conjugated polymers (CPs) as a promising alternative to traditional inorganic semiconductors for flexible and wearable optoelectronic devices, the morphological and associated electronic properties in CPs still remain ambiguous due to their inherently heterogeneous structures both at the bulk film level and single molecule level. In the studies presented in this dissertation, single molecule spectroscopy techniques were employed to understand the single chain morphology and how it affected photophysics. Structurally well-defined model materials based on bis(2-ethylhexyl)-p-phenylene vinylene (BEH-PPV) oligomers, as a simplified model of CPs, were strategically designed to control folding at the single molecule level. Significant morphological variations were found by incorporating both rigid bent and linear linkers into the backbone, as well as by manipulating the chromophore size. The longest chromophore with the linear linker displayed best single molecule folding order and the film packing was further reinforced by the stronger π-π interactions between the longer conjugated segments. In addition, single chain folding via hydrogen-bonding side chain inclusion was demonstrated on the otherwise isotropic model material composed of a series of BEH-PPV trimers linked with flexible linkers. In comparison with a control oligomer system with non-hydrogen-bonding side chains, the oligomers with hydrogen-bonding motifs showed highly ordered structures and enhanced monomer interactions. These studies were further extended to a new class of emerging push-pull conjugated polymers (also known as donor-acceptor copolymers), specifically poly[N-(1- octylnonyl)-2,7-carbazole]-alt-5,5-[4’,7’-di(thien-2-yl)-2’,1’,3’-benzothiadiazole] (PCDTBT). By probing the polymer chain folding as a function of molecular weight using single molecule techniques, it was found that there are two emissive species in this material. Above a critical molecular weight, the polymer transitions from a non-interacting, more ordered conformation to a self-interacting, more isotropic conformation.
Single molecule study on the conformation, orientation and diffusion anisotropy of conjugated polymer chains in a liquid crystal matrix
(2007) Chang, Wei-Shun, 1973-; Barbara, P. F. (Paul F.), 1953-
The nature of the solvent plays an important role in the conformation and orientation of polymers in solution. A particularly interesting case is when the solvent itself possesses order, such as when dissolving the polymer in a LC. In this dissertation, the morphology and diffusion behavior of the conjugated, stiff polymer MEH-PPV, (poly[2-methoxy-5((2-ethylhexyl)oxy)-1,4-phenylenevinylene]), in liquid crystal (LC) solvents have been investigated. Using polarization sensitive fluorescence correlation spectroscopy, it was found that in a nematic LC the polymer molecules are extended and highly aligned parallel with the nematic director. The conformation and orientational order of MEH-PPV increase with chain stiffness as a result of an interplay among the conformational entropy, solvation anisotropy, and bending energy of the polymer chains. In the smectic phase, about 10% of the MEH-PPV molecules are aligned perpendicular to the director in between the smectice layers, an effect not previously observed for a polymer solute. When applying an external electric field across the LC cell, the LC director changes orientation from a planar to a homeotropic alignment. The MEH-PPV chains remain aligned parallel with the LC director with applied field in the bulk of the LC device. However, the local structure near the LC-substrate interface is more complex. Single molecule polarization distributions measured as a function of distance from the LC device interface allow us to use MEH-PPV as sensitive local probe to explore complex structures in anisotropic media. Furthermore, diffusion anisotropy of the polymer solute in a LC solvent was studied by a novel two-beam cross-correlation technique. The diffusion anisotropy was observed to be about 2. This value is comparable to the diffusion anisotropy of the solvent and suggests that, despite the high degree of alignment, the solute diffusion is governed by the solvent and not the solute.
Spectroscopic and calorimetric studies of aggregated macromolecules
(2007) Kitts, Catherine Carter, 1979-; Vanden Bout, David A.
Different optical and calorimetric techniques were utilized to gain a better understanding of aggregated macromolecules. This research looked at two different macromolecules: poly(9,9'-dioctylfluorene), a conjugated polymer that forms aggregates in organic solvents; and bovine insulin, which forms amyloid fibrils. Conjugated polymers are of increasing interest due to their thermal stability and ease of solution processing for use in devices. A member of the polyfluorene family, poly(9,9'-dioctylfluorene) (PFO), has been studied due to its blue-emitting spectral properties. However, PFO has been found to form aggregates in solution, which is detected by the presence of a red-shifted absorption peak. This peak is caused when a section of the backbone planarizes forming the [beta]-phase. The [beta]-phase can be removed from the solution upon heating and will not return until the solution is cooled, making it a non-equilibrium process. The dissolution and reformation of the -phase were monitored using absorption spectroscopy and differential scanning calorimetry. Atomic force microscopy (AFM) and near-field scanning optical microscopy (NSOM) were able to probe the aggregates in films. It is important to understand polymer properties in solution in order to understand film morphology. Amyloid fibrils contribute to over 20 different neurodegenerative diseases, in which cures have yet to be found. The fibrils form when a soluble protein misfolds and self-assembles to form insoluble protein aggregates, and the cause of the fibril formation in vivo has still yet to be determined. Spectroscopy studies have been made possible with the use of fluorescent dyes: thioflavin T (ThT), BTA-2, and Congo red (CR). These dyes bind to amyloid fibrils and exhibit changes in their spectral properties. However, the exact mechanism for the binding of these dyes has only recently been studied. Through the use of calorimetry, the forces involved with binding of ThT and CR to amyloid fibrils can be determined. Absorption and fluorescence spectroscopy techniques were employed to study the spectral properties of these dyes. Polarized NSOM was used to determine the ThT or BTA-2's orientation with an individual fibril. Understanding how these dyes bind to fibrils will enable researchers to use spectroscopy to study the early stages of fibril formation.
Studies of conjugated polymer thin film morphology : effect on emission and charge transport
(2007-05) Rozanski, Lynn June, 1980-; Vanden Bout, David A.
Since their discovery, semiconducting conjugated polymers have shown great promise as active materials for a range of electronic devices. Initially desired for their high quantum yield, conjugated polymers have become popular due to their low cost and potential to be transferred to existing technology. Conjugated polymers are liquid crystalline, packing into well ordered domains upon thermal annealing of the films, which often leads to complex polymer interactions that can affect their semiconducting properties such as charge transport, emission color and ultimately device efficiency. Film morphology is difficult to characterize, with the order often varying on the nanoscale within a film. Near field scanning optical microscopy (NSOM) combined with Atomic Force Microscopy (AFM) can probe the degree of order of a film on the nanoscale and correlate it to topography; this can then be related to changes in luminescence emission and device characteristics to infer how charges are moving within a film. The effect of morphology on device function can vary between polymer systems; for example, di-alkyl polyfluorenes (PFs), a popular blue emitter for LEDs, undergo fluorescence degradation from ketone-based defects. Ordering of PF films containing some chemical defects increased the energy transfer from pristine chains to defects, increasing the defects’ degrading effect on the film emission. In comparison, the air-stable di-alkyl polyphenylene ethynylenes (PPEs) have numerous chain interactions in the amorphous pristine film, but show evidence of fewer interactions between these chains after ordering the film rather than more interactions. PPE polymers with varied lengths of sidechains produce dissimilar electroluminescence intensities, due to differences in their morphologies that affected how charges moved and recombined within the films. Understanding the effect of changes in polymer film morphology on luminescence and charge movement will help future efforts in understanding more complex polymer interactions, such as seen in blended polymer films.
Studies of conjugated polymer thin film morphology: effect on emission and charge transport
(2007) Rozanski, Lynn June; Vanden Bout, David A.
Since their discovery, semiconducting conjugated polymers have shown great promise as active materials for a range of electronic devices. Initially desired for their high quantum yield, conjugated polymers have become popular due to their low cost and potential to be transferred to existing technology. Conjugated polymers are liquid crystalline, packing into well ordered domains upon thermal annealing of the films, which often leads to complex polymer interactions that can affect their semiconducting properties such as charge transport, emission color and ultimately device efficiency. Film morphology is difficult to characterize, with the order often varying on the nanoscale within a film. Near field scanning optical microscopy (NSOM) combined with Atomic Force Microscopy (AFM) can probe the degree of order of a film on the nanoscale and correlate it to topography; this can then be related to changes in luminescence emission and device characteristics to infer how charges are moving within a film. The effect of morphology on device function can vary between polymer systems; for example, di-alkyl polyfluorenes (PFs), a popular blue emitter for LEDs, undergo fluorescence degradation from ketone-based defects. Ordering of PF films containing some chemical defects increased the energy transfer from pristine chains to defects, increasing the defects’ degrading effect on the film emission. In comparison, the air-stable di-alkyl polyphenylene ethynylenes (PPEs) have numerous chain interactions in the amorphous pristine film, but show evidence of fewer interactions between these chains after ordering the film rather than more interactions. PPE polymers with varied lengths of sidechains produce dissimilar electroluminescence intensities, due to differences in their morphologies that affected how charges moved and recombined within the films. Understanding the effect of changes in polymer film morphology on luminescence and charge movement will help future efforts in understanding more complex polymer interactions, such as seen in blended polymer films.
Synthesis of conjugated polymers and block copolymers via catalyst transfer polycondensation
(2013-08) Ono, Robert Jun; Bielawski, Christopher W.; Sessler, Jonathan L.
Conjugated polymers hold tremendous potential as low-cost, solution processable materials for electronic applications such organic light-emitting diodes and photovoltaics. While the concerted efforts of many research groups have improved the performance of organic electronic devices to near-relevant levels for commercial exploitation over the last decade, the overall performance of organic light-emitting diode and organic photovoltaic devices still lags behind that of their traditional, inorganic counterparts. Realizing the full potential of organic electronics will require a comprehensive, molecular-level understanding of conjugated polymer photophysics. Studying pure, well-defined, and reproducible conjugated polymer materials should enable these efforts; unfortunately, conjugated polymers are typically synthesized by metal-catalyzed step-growth polycondensation reactions that do not allow for rigorous control over polymer molecular weight or molecular weight distribution (i.e., dispersity). Chain-growth syntheses of conjugated polymers would not only allow for precise control over the aforementioned polymer metrics such as molecular weight and dispersity, but could also potentially create new applications by enabling the preparation of more advanced macromolecular structures such as block copolymers and surface grafted polymers. Our efforts toward realizing these goals as well as toward exploiting chain-growth methodologies to better understand fundamental conjugated polymer photophysics and self-assembly will be presented.
Understanding of conjugated polymer morphology formation and the structure-property relationships from the single chain level to the bulk level
(2012-08) Adachi, Takuji; Vanden Bout, David A.
Morphology is the origin of life and function. Defining and designing morphology, understanding the relationship between morphology and function, is an essential theme in a number of research areas. In conjugated polymer research, the major obstacles to achieving these goals are the heterogeneity and complexity of conjugated polymer films. In the study presented in this dissertation, various single molecule spectroscopy techniques were used as an approach to minimize the complexity of these problems. By using excitation polarization spectroscopy, it was discovered that single chains of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) assume a highly ordered rod conformation despite the fact that the morphology of bulk films is known to be amorphous. The comparison of results from experiments and a coarse grained bead-on-a-chain simulation suggested that single chains have the ability to use a thermally induced defect to maximize [pi]-[pi] stacking and adopt a rod conformation as a stable conformation. Bias-induced centroid spectroscopy (BIC) on highly ordered single chains demonstrated that the energy transfer scale could be an order of magnitude larger than the value typically measured for bulk films. It was further demonstrated that such an extraordinary long energy transfer was not a unique property of single chains but was also observed in aggregates as long as the morphology was ordered. These studies were extended to another model compound poly(3-hexylthiophene) (P3HT) to generalize the mechanism of morphology formation and the structure-property relationship. For P3HT, it was shown that side-chains were a very important factor in determining single chain conformation, while the conformation of MEH-PPV was not affected by side-chains. By controlling the side-chains, both ordered and disordered P3HT chains were obtained. The comparison of results from experiments and an energy transfer model simulation quantified that energy transfer was at least twice as efficient in ordered chains as in disordered chains. In aggregates, the difference between the energy transfer efficiency of ordered and disordered morphology was even larger than that in the case of single chains. These results could suggest that there is a very fast energy transfer mechanism that occurs through interchain interactions when chains are packed in ordered fashion.
The use of new reactions for novel polymerizations, polymers and architectures
(2008-12) Coady, Daniel Joseph; Bielawski, Christopher W.
The design, synthesis and characterization of novel conjugated polymers are described. Using a coupling reaction recently developed within our labs, polymers were constructed through triazene linkages generated by joining N-heterocyclic carbenes (NHCs) with organic azides. This triazene reaction produced polymer of sufficiently high molecular weight as to be spin-coated and rendered conductive upon doping with iodine. The reaction also has potential for executing post-polymerization modifications. This was evidenced through rapid functionalization of poly(4-methylazido-styrene) via triazene formation using a commercially available N-heterocyclic carbene (NHC). A formal anion metathesis of benzobis(imidazolium)s was used to transform neutral block copolymers into block ionomers. Further investigation of the block ionomers revealed their solvent mediated self assembly. The gradual change of organic to aqueous media caused the adoption of a three-dimensional micelle conformation as determined by transmission electron microscopy and dynamic light scattering. Through the exploitation of carbene-carbon disulfide adducts, new chain transfer agents were generated. After 2-dithiocarboxylate-imidazolium adduct formation, alkylation was performed with benzyl bromide. The resulting charged chain transfer agent was tested for its ability to moderate radical addition fragmentation (RAFT) polymerizations of styrene. A considerable increase in transfer kinetics as compared to that of commonly used RAFT agents was observed whilst retaining low polydispersity and molecular weight control. The rate enhancement is presumably due to the electron withdrawing imidazolium activating the thionyl towards the nucleophilic radical while retaining effective fragmentation. Ion coordinating macrocycles were affixed to a poly(methacrylate) scaffold for employment as electrolyte extractants. Polymer bound calix[4]pyrrole was found to complex fluoride and chloride with sufficient strength as to extract tetrabutylammonium salts from water. Enhanced extraction abilities were observed when calix[4]pyrrole was used in conjunction with benzo-15-crown-5. Methacrylate polymers containing both macrocycles affected the removal of aqueous potassium fluoride from a biphasic water/dichloromethane mixture. To provide evidence for the presence of potassium fluoride within the dichloromethane layer, ¹⁹F NMR and flame emission spectroscopy were used.