Theses supervised by Prof. Dr. Yusuf Yağcı

Functional polymersPhotoactivityCopolymers+1

Functional high performance polybenzoxazines and their properties

Phenolic resins are leading the polymer market due to their widespread applications such as adhesives, structural applications materials in aerospace, printed circuit boards, conductive polymer elements, and encapsulation materials for electronic applications. Phenolic resins, acrylates, bismaleimides, polyesters, epoxy resins and isocyanate polymers are the best known members of the thermosets. High processing temperatures and void formation during curing are the main challenges must be overcome regarding application of these materials. Lately, benzoxazine based phenolic resins has been developed and attracted significant attention as a novel type of phenolic resin. Polybenzoxazines have various outstanding properties, including good thermal stability, high glass transition temperature, high char yield, no need of catalysts for curing, near-zero volume changes during curing, low moisture absorption and no volatile release. Recently, owing to the design flexibility of benzoxazine and related polymeric benzoxazine precursors, various smart materials were synthesized by innovative strategies including self-healing materials, electrochemically activated smart coatings, smart sorbents for heavy metals, hydrophobic surface applications and porous polybenzoxazine resins. Taking account of the unique advantages of these strategies and attractive characteristics of polybenzoxazines, in this thesis, we focused on the combination of various functional groups with benzoxazine precursors and then investigated the resulted products properties. In the first part of the thesis, a self-healing strategy for poly(propylene oxide)s bearing benzoxazine units (PPO-Benz) through supramolecular attractions is described. Poly(propylene oxide) bisamine (PPO) with a molecular wieght of 2000 Da were reacted with formaldehyde and bisphenol A to yield desired PPO-Benz with 12360 Da. The cross-linked polymer films were then prepared by solvent casting of suitable compositions of PPO-Benz and carboxlic acid containing benzoxazine monomer (Carb-Benz) in chloroform followed by thermal ring opening reaction of benzoxazine groups at 200 °C. Thermal curing and thermal stability of the film and final products were investigated. It was demonstrated that the self-healing capacity of the films were improved by employing Carb-Benz in the formulation. Figure 1 : Self-healable film preparation from carboxylic acid containing benzoxazine monomer and PPO-Benz. In the second part of the thesis, a novel strategy to obtain sulfur rich polybenzoxazine copolymers by reacting allyl functional benzoxazine (BA-ala) and elemental sulfur was described. Simultaneous inverse vulcanization and ring-opening reactions of benzoxazine generated soluble copolymers in specific feed ratios. Parameters such as monomer structure and feed ratios on the polymerization were studied. The thermal stability of the copolymers was investigated and compared to that of polybenzoxazines derived from neat BA-ala by using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The surface properties of the materials as examined by scanning electron microscope (SEM) confirmed that elemental sulfur and benzoxazine copolymers can be produced without a phase separation at micrometer level. Moreover, a sponge like insoluble macroporous polybenzoxazine networks was obtained at the 20%-wt feed ratio of sulfur. Figure 2 : A novel strategy to obtain sulfur rich polybenzoxazine copolymers by reacting allyl functional benzoxazine (BA-ala) and elemental sulfur. Finally, side-chain benzoxazine functional polybutadienes was synthesized by photoinduced hydrogen abstraction process. First, photosensitive benzoxazine compounds possessing both chromophoric carbonyl and hydrogen donating sites in the structure were synthesized using vanillin or 4-hydroxybenzophenone in the conventional benzoxazine synthesis. Irradiation of neat polybutadiene (PB) in the presence of the corresponding benzoxazines, namely benzophenone benzoxazine (BPh-ptol) and vanillin benzoxazine (Van-a) under 300–350 nm light gave PBs with approximately 4-5 benzoxazine units per chain. Successful modification was confirmed by the spectral and thermal investigations. It is shown that benzoxazine modified PBs undergo thermally activated curing in the absence of any catalyst forming polybutadiene thermoset with high char yield. Figure 3 : Photoactive benzoxazines having both chromophoric carbonyl and hydrogen donating sites were synthesized using vanillin or 4-hydroxybenzophenone by conventional benzoxazine synthesis methodology.

Mustafa Arslan

İstanbul Technical University · Institute of Graduate Studies in Science

2016

İstanbul Technical University · Institute of Graduate Studies in Science

Photoinitiated metal-free controlled/living radical polymerization using polynuclear aromatic hydrocarbons

Recently, photoinitiated polymerization has received revitalized interest as it congregates a wide range of economic and ecological anticipations. Because of the excellent advantages, photoinduced polymerization has widely been applied in coatings, adhesives, inks, printing plates, optical waveguides and microelectronics. Theadvantegesofthismethodarehighrateofpolymerizationatambienttemperatures, low energy consumption, solvent-free polymerization, spatial and temporal control over the processes. There currently exist a few methods, where photo induced polymerization can be applied including controlled/living radical polymerizations such as iniferter, nitroxide mediated radical polymerization(NMRP), reversible addition-fragmentation chain transfer polymerizations (RAFT) and atom transfer radical polymerization (ATRP). Among them, ATRP became the most commonly used method as it has higher range of tolerance not only to the initiators but also to various number of monomers. Traditional ATRP requires a low-oxidation state transition metal complex (commonly CuX/L, X = Cl or Br and L = ligand) in conjunction with an appropriate alkyl halide (R-X). The photochemical initiation both enables the easy control of the polymerization under ambient temperature even for heat-sensitive monomers and tends to minimize side reactions like chain transfer or depolymerization. In this thesis, photo-initiated metal-free controlled living radical polymerization of (meth)acrylates, and vinyl monomers was investigated using the polynuclear aromatic compounds,pyreneandanthracene. Fluorescencespectralanalysesalongwithnuclear magneticresonancestudieswereperformedtodeterminetherateconstantsofinitiator radical formation and investigate the mechanisms of polymerization. The obtained polymers were analyzed by spectral and chromatographic methods. Results show that the excited state anthracene undergoes a faster electron transfer reaction with the alkyl halide initiator than the excited state of pyrene. Pyrene excimers, which are formed at higher concentrations, also react with alkyl halides to form initiator radicals. Although pyrene monomers and excimers are acting slower, polymers with higher control over the chain end functionalities and molecular weight characteristics are obtained in comparison to anthracene as sensitizer.

Andrıt Allushı

2016

Atom transfer radical polymerizationGraft copolymersElectrochemical polymerization+7

Poli(Fenilen vinilen) türevlerinin görünür bölgede katyonik polimerizasyon için fotobaşlatıcı olarak kullanımı

Conjugated polymers have been widely studied as materials due to their interesting optical and nonlinear optical properties and electronic conductivity that lead to a wide range of practical applications such as information storage, optical signal processing, substitutes for batteries, solar energy conversion, and optical signal processing. Moreover, electroluminescence from conjugated polymers is a rapidly increasing a great deal of research has gone into research interest since the first report of polymeric lightemitting diodes (PLEDs) based on poly(phenylene vinylene) (PPVs). Current interest in among a variety of conjugated polymers, In particular, PPV and its derivatives have attracted much attention for the most promising class of high performance polymers due to their relatively high photoluminescence (PL) and electroluminescence (EL) quantum efficiencies as well as good colour tunability through molecular structure designs. They also show high chemical and excellent mechanical properties, thermal stability that allows the fabrication of electronic devices such as flexible LEDs. PPV itself has a rigid structure and is inherently insoluble, thus making it impossible to process these materials into thin films required for the most applications. As a result, considerable effort has been directed toward the preparation of well-defined conjugated polymers with improved solubility, processability and stability. Incorporation of conformationally mobile, relatively long and flexible side chains onto the polymer backbone has been important for synthesizing fusible and soluble rigid-rod conjugated polymers. Taking into consideration the substantial interest not only in the synthesis of new types of polymers, but also in the modification of commodity polymers to improve their properties to meet the requirements for high-tech applications, with a soft coil polystyrene (PSt) or poly(ε-caprolactone) (PCL) has been used in which nanostructured photoactive conjugated oligo(phenylene vinylene) segments are attached as side chains to the backbone. It is possible to form a new polymer with novel and interesting properties. Utilization of several conjugated aromatic compounds for the sensitization of onium salts has prompted us to apply PPVs as photosensitizers for cationic polymerization. Therefore, in order to overcome solubility problems of bare PPVs associated with the rigid structure, PPVs with polystyrene and poly(ε-caprolactone) grafts (PPV-g-PSt and PPV-g-PCL, respectively) were synthesized. The first step in the synthesis of the desired PPVs was to obtain precursor macromonomers. In the preparation of macromonomers, controlled polymerization methods were delibaretly employed so as to obtain polymers with desired functionalities at the chain ends, and combine with proper molecular weights and polydispersities Dibromo-benzene functional polystyrene (DBB-PSt) was synthesized by Atom Transfer Radical Polymerization (ATRP) of styrene by using 1,4-dibromo-2-(bromomethyl)benzene as initiator in the presence of CuBr/bpy as catalytic system. The corresponding poly(ε-caprolactone) derivative (DBB-PCL) was prepared by Ring Opening Polymerization (ROP) of ε-caprolactone by using [2,5-dibromo-(4-hydroxymethyl) phenyl)]methanol as bifunctional initiator, in the presence of stannous octoate. In the next step, these polymers were coupled with (4-formylphenyl)boronic acid by Suzuki coupling to give aldehyde functional macromonomers. Finally, the obtained polymers were coupled with p-xylylenebis(triphenylphosphonium bromide) by the Wittig reaction in the presence of potassium tert-botoxide to yield the desired conjugated polymers. The resulting main chain conjugated graft copolymers were soluble as were the starting macromonomers and PPVs showed blue or green fluorescence in solution. Even tough photopolymerization can be initiated radically, cationically and anionically, much effort has been devoted to free radical and cationic systems mainly due to the availability of a wide range of photoinitiators and the great reactivity of monomers. The majority of industrial applications of photoinitiated polymerizations for various techniques deal with free-radical systems. However, there are some drawbacks associated with this type polymerization such as the inhibition effect of oxygen and post-cure limitations which may affect the properties of the final product. Therefore, photinitiated cationic polymerization holds considerable promises in the future, particularly as a means of overcoming these limitations. Some important examples of these initiators are iodonium and sulfonium salts. These onium salts give an irreversible photochemical reaction in response to UV excitation due to their thermal stability, solubility in monomers and efficiency in generating reactive species. However, these onium salts only absorb in the region of 220–300 nm. This means, in visible region, they are either poorly responsive or completely insufficent. Thus, their use in applications requiring long wavelength light emitting sources is limited. This drawback can be evaded by using an activator that broadens the spectral sensitivity of the onium salt photoinitiators. These activators are photosensitizers and they can initiate polymerization in the near-UV or visible wavelengths of light. In this work, we report the use of PPVs derivatives for electron transfer photosensitization of onium salts. For this purpose, PPV-g-PSt and/or PPV-g-PCL were used as photosensitizers. The excited state emission characteristics of the compounds were investigated by means of fluorescence and phosphorescence spectroscopic measurements. The cationic polymerization of typical monomers, such as cyclohexene oxide (CHO), isobutyl vinyl ether (IBVE) and N-vinylcarbazole (NVC) with the described initiating system was also tested at room temperature at appropriate wavelengths in the presence of oxidizing salts such as diphenyliodonium hexafluorophosphate (DPI), and N-ethoxy-2-methylpyridinium (EMP). The initiation mechanism, as correlated with optical absorption and fluorescence spectroscopic measurements, free energy changes (ΔG) and proton scavenging studies, involves formation of exciplex by the absorption of light in the first step. Subsequently, an electron transfer from the excited PPV-g-PSt or PPV-g-PCL to oxidizing salt occurs to yield radical cations of the PPV backbone. Thus, formed radical cations abstract hydrogen from the surrounding monomer or the solvent to release a Bronsted acid, which initiates the polymerization. Triphenylsulphonium hexafluoroarsenate (TPS) was found to be inefficient in the photoinduced electron transfer process due to the unfavorable thermodynamic conditions.

Semih Erdur

İstanbul Technical University · Institute of Graduate Studies in Science

Activated clayAtom transfer radical polymerizationComposite polymers+4

Polimer/kil nanokompozitlerin hazırlanmasında yeni yöntemler

Polymer/clay nanocomposites have exhibited immensely enhanced properties and higher performance as compared to both their conventional polymer composites and pure polymers. Currently polymer/clay nanocomposites can be prepared by three ways such as solution mixing, melt blending, and in-situ polymerization. Solution mixing method consists to solubilize polymer in an organic solvent, then the clay is dispersed in the obtained solution and subsequently either the solvent is evaporated or the polymer precipitated. The large quantities of volatile solvent necessary for this approach make it less attractive as an industrial process. Melt blending is a solvent-free method to enable mixing of the layered silicate with the polymer matrix in the molten state. However, very careful attention has to be paid to finely tune the processing conditions to increase the compatibility of clay layer surfaces with the polymer matrix. In the in-situ polymerization technique, the monomer, together with the initiator and/or catalyst, is intercalated within the silicate layers and the polymerization is initiated by external stimulation such as thermal, photochemical or chemical activation. The chain growth in the clay galleries triggers the clay exfoliation and hence the nanocomposite formation.The in-situ polymerization can be initiated by externally stimulation such as thermal, photochemical or chemical activation. The growth of polymer chains within the clay galleries may lead to the clay exfoliation and hence the nanocomposite formation.Recently, a highly efficient method, namely copper (I) catalyzed azide/alkyne cycloaddition (CuAAC) "click" reaction, in which exfoliation is rooted in the functional groups of the intercalant that readily react with the antagonist groups of the preformed polymershas been established. To take advantage of click chemistry, azide and alkyne partners could each be incorporated in either the clay surface or polymer chain. The quantitative efficiency of coupling reaction coupled with tolerance to a wide variety of functional groups and reaction conditions make this coupling process highly attractive for the nanocomposite preparation. However, there are limited examples in the literature to date regarding the preparation of polymer/clay nanocomposites via the CuAAC click reactions. In the first part of thesis,an efficient protocol, atom transfer radical nitroxide coupling chemistry (ATNRC), for the preparation of polymer/clay nanocomposites via grafting-onto strategy with well-defined polymer synthesized via atom transfer radical polymerization has been described. The highly efficient ATNRC chemistry was based on mixing a nitroxide-containing organoclay with corresponding halide-containing polystyrene in the presence of Cu(I)Cl/PMDETA catalytic system was developed.The radical coupling, taking place between the clay layers, not only leads to attach the polymer chain but also successful nanocomposite formation with highly exfoliated morphology. Spectroscopic and microscopic investigations revealed that successful nanocomposite formation has been achieved by this method. By addition of small amounts of layered silicate loadings resulted in remarkable improvements of thermal properties of nanocomposites. In the second part of thesis,a series of A3-type star poly(methylmethacrylate)/clay nanocomposites has been prepared by in-situ atom transfer radical polymerization (ATRP) initiated from organomodified montmorillonite containing quaternary trifunctional ATRP initiator. The first order kinetic plot showed a linear behavior, indicating the controlled character of the polymerization. The resulting nanocomposites were characterized by spectroscopic, thermal and microscopic analyses. Spectroscopic and microscopic investigations revealed a complex morphology, with partial intercalation/exfoliation, which depends on the concentration of clay.The exfoliated nanocomposite was obtained when polymerization was conducted with 1% of organic clay loading.However, with increasing the clay loading to 3, 6 and 10%, the degree of exfoliation of the nanocomposites decreased, which confirmed by both spectroscopic and microscopic analyses. Thermal analyses show that all nanocomposites had higher glass transition values and thermal stabilities compared to neat polymer. In the third part of thesis, the preparation of polymer/clay nanocomposites by specific hydrogen bonding interactions between surface functionalized silica nanoclays and 2-ureido-4[1H]pyrimidinone-bonded supramolecular poly(ethylene glycol) or poly(ɛ-caprolactone)s, which has self-association capability through quadruple hydrogen bonds was described. An 2-ureido-4[1H]pyrimidinone (UPy) motif with self-association capability (through quadruple hydrogen bonds) was successfully anchored onto montmorillonite clay layers. Polymer/clay nanocomposites were prepared by specific hydrogen bonding interactions between surface functionalized silica nanoclays and UPy-bonded supramolecular poly(ethylene glycol) or poly(ɛ-caprolactone). The mixed morphologies including intercalated layers with a non-uniform separation and exfoliated single layers isolated from any stack were determined by combined spectroscopic and microscopic analyeses. Thermal measurements showed that all nanocomposites have higher decomposition temperatures and thermal stabilities comparedto neat polymer. Thedifferential scanning calorimetry data implied that thecrystallinity of polymers did not show essential changes upon introduction of organomodified UPy clays.

Muhammed Aydın

İstanbul Technical University · Institute of Graduate Studies in Science

İstanbul Technical University · Institute of Graduate Studies in Science

Organik ve sulu ortamlarda serbest radikal polimerizasyonu için tek bileşenli tip II fotobaşlatıcı olarak poli(vinil alkol)-tiyoksanton

In recent years, photoinitiated polymerization has received revitalized interest as it congregates a wide range of economic and ecological anticipations. The most important elements of photoinitiated polymerization are the photoinitiators, which absorps the light to yield the species responsible for the initiation of polymerization. Thus, development of new photoinitiating systems became an important subject for the synthetic researchers in the past few years. Both free radical and cationic photoinitiators are intensively examined and their initiating mechanisms are examined in detail. Free radical photoinitiators can be divided into Type I (α or less common β cleavage) and Type II (H-abstraction). Some of the most influential Type I initiators are benzoin ether derivaties, benzil katals, hydroxylalkylphenones, a-aminoketones and acylphosphine oxides. Besides that typical Type II photoinitiators include aromatic carbonyl ssuch as benzophenone and derivatives thioxanthone (TX), benzil, quinones, and organic dyes, however alcohols, ethers, amines, and thiols are used as hydrogen donors. Type II photoinitiators are the most widely used in many technologically important UV curing applications because of their excellent light absorption characteristics. The radicals formed on the thioxanthone moiety do not actually participate in the initiation process on account of the steric hindrance and delocalization of the single electron on the π-system. Although they exhibit excellent initiation efficiency, the hydrogen donor compounds at high concentrations, have several intrinsic disadvantages such as offensive odor, toxicity, and migration in UV-curing technology. Several strategies have been developed to overcome these problems. The usual approach is to chemically incorporate the hydrogen-donating sites into TX chromophores. In this current work, TX moiety was incoperated to the side chains of commercially available poly(vinyl alcohol) (PVA) by a simple acetalization reaction. Initially, the designed thioxanthone structure with aldehyde functionality (TX-A) was facilely synthesized. Afterwards, the obtained TX-A was linked to the PVA counterpart by a simple addition reaction to yield the desired PVA-TX macroinitiator. PVA-TX polymeric photoinitiator exhibits typical TX absorption characteristics and soluble in water and some polar organic solvents such as DMF and DMSO. Moreover, PVA-TX displays one-component nature and initiates the polymerization in both organic and aqueous media without the necessity of additional hydrogen donor donor due to the presence of remaining hydrogen donating hydroxyl groups in the structure.

Senem Körk

BiopolymersConducting copolymers

Fonksiyonel konjuge polimerler

The considerable role of conjugated polymers (CPs) in emerging synthetic bio-applications, optical, electronic, diodes, and display technologies, represents the promising way in which considerable endeavors have been directed towards the realization of many aspects of conducting organic polymers. Conducting polymers are known as "synthetic metals", making them fundamental materials in many research fields because of their outstanding characteristics. They exhibit both excellent conductivity and high mechanical strengths and processability. The use of CPs as an excellent immobilization platform for biomolecules led to the development of efficient biosensors, which were easy to process, had the ability to conduct electricity at any desired level, had a low cost, and straightforward preparation techniques. CPs provide high surface area, adjustable morphology by arranging the thickness of the polymer film and offer extensive stability of the enzymes incorporated in them. Furthermore, these materials also facilitate structural and electronic modifications which can be used in detecting a target compound in any test solution. The polymer structure can be tuned to accomplish the desired properties, producing a sensitive and reproducible microenvironment for biological reactions to mimic the naturally occurring environments of the biological molecules. Among vast number of CPs, poly(p-pheneylene)s (PPP)s and polythiophenes (PT)s are the most promising class of polymer in terms of relatively high photoluminescence, electroluminescence quantum efficiencies and thermoxidative stability and so on. Conjugated structure of CPs can create excellent one-dimensional surface for energy transport of electrons and strong UV absorption. Especially, their fluorescence feature is one of the most susceptible to environmental change and this allows for eminent selectivity in signaling reporter group materials and provides advantages for CPs to be used in sensor technologies including pH sensors, temperature sensors and recently developed biosensors. Recent innovative strategies for the syntheses of functional polymeric structures bearing polypeptides, amino-rich and water soluble groups have received enormous interest in the fields of biomedicine, drug delivery, biomineralization, bioconjugations, and tissue engineering. Taking account of the unique advantages of these strategies, in this thesis, we focused on the combination of PPPs and PTs with polypeptide, amino and water soluble groups in the same conjugated backbone. In the first part of thesis, a simple and efficient approach for the electrochemical deposition of polypeptides as bio-based covering materials for surface design is described. The method involves N-carboxyanhydride (NCA) ring-opening polymerization from its precursor to form a thiophene-functionalized polypeptide macromonomer (T-Pala), followed by electropolymerization. The obtained conducting polymer, namely polythiophene-g- polyalanine (PT-Pala), was characterized and utilized as a matrix for biomolecule attachment. The biosensing applicability of PT-Pala was also investigated by using glucose oxidase (GOx) as a model enzyme to detect glucose. The designed biosensor showed a very good linearity for 0.01–1.0 mM glucose. Finally, the antimicrobial activities of newly synthesized T-Pala and PT-Pala were also evaluated by using the disc diffusion method. In the second part of thesis, we report a novel approach for fabrication of multifunctional conjugated polymers, namely poly(p-phenylene)s (PPPs) possessing polypeptide (poly-L-lysine, PLL) and hydrophilic poly(ethylene glycol) (PEG) side chains. The approach is comprised of the combination of Suzuki coupling and in situ N-carboxyanhydride (NCA) ring-opening polymerization (ROP) processes. First, polypeptide macromonomer was prepared by ROP of the corresponding NCA precursor using (2,5-dibromophenyl) methanamine as an initiator. Suzuki coupling reaction of the obtained polypeptide and PEG macromonomers both having dibromobenzene end functionality using 1,4-benzenediboronic acid as the coupling partner in the presence of palladium catalyst gave the desired polymer. A different sequence of the same procedure was also employed to yield polymer with essentially identical structure. In the reverse sequence mode, low molar mass monomer (2,5-dibromophenyl)methanamine, and PEG macromonomer were coupled with 1,4- benzenediboronic acid in a similar way followed by ROP of the L-Lysine NCA precursor through the primary amino groups of the resulting polyphenylene. In the third part of thesis, a novel approach for bioconjugation associated with a fluorescent conjugated polymer is demonstrated. For this purpose, a conjugated polymer, poly(p-phenylene) (PPP), with lateral substituents, namely primary amino groups and poly(ethylene glycol) (PEG) chains, as a potential building block for polymer bioconjugates was synthesized and characterized. The synthesis was achieved through Suzuki polycondensation reaction in the presence of Pd(PPh3)4 catalyst by using independently prepared PEG and amino functionalized dibromo benzenes in conjunction with benzene diboronic acid. For the evaluation of the bioactive PPP labeled with folic acid (FA) as a potential targeted cell imaging probe, HeLa and A549 cancer cells were used. Cytotoxicity assay showed that the polymer was not toxic to either of the cells. Additionally, the fluorescence images showed that, depending on the level of the FA receptors on the cell surfaces, the fluorescent intensity in HeLa cells was obviously higher than A549 cells when treated with FA conjugated PPP-NH2-g-PEG polymer. The resulting FA/PPP-NH2-g-PEG conjugate was successfully used as a bioconjugate for targeting and specifically imaging FA receptor positive HeLa human cervical cancer cells. Summary, we successfully achieved the systhesis, characterization and some bioapplications of complex macromolecular architecture based on conjugated polymers bearing polypeptide, water soluble pendant ,PEG, and primary amine groups. The systhesis of polypeptide squences were achieved by NCA-ROP technique based on transformation of α-amino acids to urethane derivatives which have possibility to obtain polypeptides in the well-designed and desicive control of molecular weight with one spot intra-molecular cyclization reaction in the presence of primary amino functional initiators. We succesfully used the primary amino functional thiophene and phenyl compounds as NCA-ROP initiator to obtain macromonomers having electropolymerization ability and possibility to give Suzuki coupling polymerization. After the obtaining of conjugated polymers bearing polypeptide and primary amine groups based on PPP and PTs, some biofunctional material was used in the step of bioapplications.

Hüseyin Akbulut

İstanbul Technical University · Institute of Graduate Studies in Science

Atom transfer radical polymerizationBlock polymersPhotoactivity+6

Makromoleküler sentezler için fotokimyasal yöntemler

Photo-induced reactions have been receiving a revitalized interest as it congregates a wide range of academical, commercial, economical and ecological anticipations. In the area of synthetic polymer chemistry, most of the research intensively focused on photopolymerizations, which is typically the process that transforms a monomer into a polymer by the help of the incident photon energy. Even when monomers are exposed to light of appropriate frequencies, polymers can be obtained. However such reactions are generally low-yielded reactions and almost always an additive is required to increase the efficiency. This additive is known as a photoinitiator which absorbs the light and decomposes to give the reactive species which are responsible for initiation of polymerization. Both free radical and cationic polymerizations are known and their mechanisms were investigated in detail. Both polymerization mechanisms have its own advantages and the researchers are generally interested in the development of new photoinitiating systems for both polymerizations. The reason behind the research for such developments lies on the fact that no photoinitiating system can be perfectly applied to all industrial applications. There has been tremendous effort to develop photoinitiators with enhanced absorption characteristics, solubility, low odor, low migration, and non-yellowing properties. Specifically, the development of photoinitiators absorbing light in the near UV-visible light range of the electromagnetic spectra is a key challenge in the last decade due to the fulfillment of green chemistry demands. Utilization of photochemistry to the synthetic polymer chemistry is not only based upon the photopolymerization methodologies, but also the use of photon energy for the triggering of various reactions. In such processes, the photo-inductions provide spatial and thermal control over the processes under mild conditions compare to the thermal modes. Recently, numbers of papers reported the use of photochemistry to induce the very well-known atom transfer radical polymerization and click chemistry reactions. Both processes require first oxidation state copper salts (CuCl or CuBr) and by photochemical reductions, it was demonstrated that one can generate Cu(I) starting from Cu(II) salts by using various photoinitiators. Thus, the necessity of preserving easily oxidizable Cu(I) species by cumbersome reaction conditions (i.e. inert atmosphere requirements, freeze-thaw pump cycles etc.) can be eliminated as a result of the stability of Cu(II) salts. In this thesis, three different publications are presented, which are mainly based on the photo-induced processes for macromolecular syntheses. In the first part, the synthesis, characterization and initiation efficiency of a novel polymeric Type II photoinitiator, namely poly(ethylene oxide) dimethylammonium thioxanthonemethyl-carboxylate (PEO-NH+(CH3)2 TX-CH2COO-) possessing both photochromophoric and hydrogen donating groups in the structure is demonstrated. In contrast to existing Type II photoinitiators, the present photoinitiator initiates free radical polymerization by counter anion excitation even in the absence of added hydrogen donors. The photoinitiator can be applied to the polymerization of monomers soluble in organic solvents or water. Thus both hydrophobic and hydrophilic polymers can be easily obtained using this photoinitiator developed. In the second part of the thesis, a novel visible light sensitive photoinitiating system for the cationic polymerization of typical monomers, e.g. of oxiranes such as cyclohexene oxide, vinyl ethers such as iso-butyl vinyl ether, and other vinyl monomers such as N-vinylcarbazole, using fullerene derivatives is described. The cationic polymerization of these monomers was initiated at room temperature upon irradiation in the visible region (λinc > 400 nm) in bulk or chlorobenzene solutions with polystyrene-C60 (PS-C60) adduct or bare C60, respectively in the presence of oxidizing salts such as silver hexafluorophosphate (AgPF6) and diphenyliodonium hexafluorophosphate (Ph2I+PF6-). A feasible mechanism, as correlated with optical absorption measurements, free energy changes (ΔG), and proton scavenging studies, involves formation of exciplex by the absorption of light in the first step. Subsequent electron transfer from excited C60 or PS-C60 to oxidizing salt yields radical cations of the fullerene derivatives. Both radical cations and strong Bronsted acid derived by hydrogen abstraction initiate the cationic polymerization of variety of monomers. Finally, development of a long wavelength photoinduced "Copper (I) Catalyzed Azide-Alkyne Cycloaddition (CuAAC)" click reaction based on the electron transfer reactions of excited states of polynuclear aromatic compounds such as anthracene, pyrene and phenothiazine and Cu(II), is presented. Although at different rates, all the sensitizers were shown to efficiently reduce Cu(II) to Cu(I) that catalyzes click reaction between model organic compounds. The applicability of the method for the construction of various macromolecular architectures including telechelic polymers and block copolymers was demonstrated. Spectroscopic and chromatographic investigations revealed that successful macromolecular syntheses were achieved.

Ali Görkem Yılmaz

İstanbul Technical University · Institute of Graduate Studies in Science

İstanbul Technical University · Institute of Graduate Studies in Science

ZnO yarı-iletken nanopartikülleri kullanılarak heterojen fotokataliz sistemi ile azid-alkin çıt-çıt reaksiyonu

In the past decade, selectivity, efficiency and diversity are the most important and required subjects for various reaction systems. "Click chemistry" can be given as an example for these systems offering remarkable advantages. Copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) reactions between azides and terminal alkynes which developed by the groups of Sharpless and Meldal, have become the most popular click reactions to date. These reactions are versatile, regiospecific, and exhibit a high efficiency under mild reaction conditions with little or no by-products. Although there are a variety of metals that can be used as a catalyst for the azide-alkyne cycloaddition reactions, copper stands out as the only metal for the reliable, simple, fast and 1,4-regiospecific catalysis. With all these excellent features, CuAAC has found precious applications in different fields such as drug discovery, biochemistry, polymer chemistry and materials science. Recently, there is an explosive growth of research in the field of nanomaterials due to their operation for materials and devices using different techniques at nanometer scale. Nanoparticles are a part of nanomaterials that are defined as a single particles 1–100 nm in diameter and they have been a common material to develop new cutting-edge applications in communications, energy storage, optics, transmission, environmental protection, cosmetics, biology, and medicine due to their attractive optical, electrical, and magnetic properties. Moreover, nanoparticles can be combined with a wide range of metals and semiconductor core materials that bring in advantageous properties such as fluorescence and magnetic treatment. Amongs all the various types, zinc oxide - semiconductor metal oxide nanoparticles- are the most preferred type with their own importance due to their vast area of applications, such as, gas sensor, chemical sensor, bio-sensor, cosmetics, storage, optical and electrical devices, solar cells, and drug-delivery. In this thesis, a visible light-induced copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) strategy is carried out benefiting from the electron transfer reactions of zinc oxide nanoparticles (ZnO NPs) to achieve the required catalyst for the coupling process. Semiconductor ZnO NPs as a heterogeneous photocatalyst releases electrons on UV/VIS irradiation resuting in the photoreduction of air-stable copper(II) ions to copper(I) which is the catalyst of the CuAAC reaction A variety of azide and alkyne components has been examined and the system tolerates different substrates in the click reaction well. 1H-NMR and FTIR spectroscopy are used for the characterization of the reactions and it was determined that a triazole forms with the consumption of the starting azide and alkyne molecules.

Kadriye Özde Yetişkin

İstanbul Technical University · Institute of Graduate Studies in Science

Uç alkinler ile tiyollerin Cu(I) bağlanmış Schiff baz ağ polimeri katalizörü eşliğinde oksidatif çapraz dehidrojenatif birleştirme reaksiyonları

Alkynes are an important class of organic molecules because of their versatile applications in materials science and organic synthesis as building blocks. Sulfur-containing compounds have also a crucial role in synthetic organic chemistry since sulfur moiety acts as an important auxilary functional group. There has been an effort to combine these alkynes with sulfur-containing compounds by using metal-acetylides and sulfuryl halides or disulfides. However, these methods require a prefunctionalization progress of terminal alkyne or sulfur-containing coupling partners. This situation causes large number of side product formations which results in a dramatic decrease in the yields of reactions. Hence, the discovery of a new method to perform these reactions in high yields and better conditions gains a considerable attention. There are lots of studies on designing an useful catalyst to remove prefunctionalization processes and provide high yields and resusability. The term "catalysis" was first employed by Bezelius in 1836 tı ¸sdentify a new entity capable of promoting the occurrence of a chemical reaction by a "catalytic contact". In his view, the catalyst was seen as something that is added to the reaction to speed up the rate of the reactin (catalytic force) without being consumed or produced in the process. It is important to recognize that the catalysis can be traced back to the ancient terms. However, catalysis started to play a major impact on the chemical industry starting from the begining of twentieth century, nowadays more than 95 percent of chemicals being produced via a process that is at least includes at least one catalytic step. Traditionally, catalysts were distinguished into homogeneous and heterogeneous; subsequently, heterogenized catalysts were also introduced. This distinction is linked to the fact that the catalyst operates respectively in the same phase where the reaction occurs (homogenous catalysts) or in a different phase (heterogeneous or heterogenized catalysts). The main difference between a homogenous and heterogeneous catalyst is the fact that in case of homogeneous catalysts, every single catalytic entity can act as a single active site. This makes homogeneous catalysts more active and selective compared to traditional heterogeneous catalysts such as oxides or supported metal particles. A major drawback of the homogeneous catalysts is the difficulty of their recovery from the reaction medium. Precipitation with subsequent recovery or distillation of the reaction products, which is an energy intensie process, are typically needed in order to re-utilize homogeneous catalysts. Such operations may often deactivate the catalyst. Despite of these considerable advances, the problem with the homogeneous catalysis still remains to be unsolved. It is difficult to seperate catalyst from reaction mixture and reuse it. In contrast, design of an heteregenous catalysis should take increasingly more attention due to its possible advantages such as reusability, waste minimization derived from reaction workup and help to the development of green chemistry concept. xix Zhao et al. reported application of a hexagonally-ordered mesoporous material (MCM-41) supported bidentate nitrogen copper(I) complex [MCM-41-2N-CuCl] as a highly efficient and recyclable copper catalyst for the direct oxidative cross-dehydrogenative coupling of terminal alkynes with thiols using O2 as the sole oxidant under mild conditions to selectively afford a variety of alkynyl sulfides in good to excellent yields. According to their results, mesoporous material (MCM-41) has a extremely high surface area, large and uniform pore size, bidentate nitrogen copper complex can be easily formed and this complex can catalyze the coupling reaction between an alkyne and a thiol. In this thesis, the copper(I) incorporated microporous polymers' catalytic activity towards thiol-alkyne oxidative cross-dehydrogentavie coupling reactions without any prefunctionalization process was studied. In the first part of the thesis, the microporous network polymer was synthesized by melamine and terephtalaldehyde monomers through Schiff base chemistry. As known, the porous nature comes from the unique properities of monomers and frame of the polymer. In general, microporous organic polymers includes functional polar pendant groups such as amines, phenols or carboxylic acids since these groups can coordinate with metals. The rigid frame is also important for a ordered microporous structure. For that reason, aromatic conjugated structures or kinked aromatic high-performance polymers are used to have a rigid frame. Our choice of melamine and terephtalaldehyde as monomers is compatible with these criteria. Afterwards, the copper ions were incorporated into this microporous polymers and these materials were characterized. In the second part of the thesis, copper incorporated microporous Schiff base network polymer was used as a catalyst in the alkyne-thiol oxidative cross-dehydrogenative coupling reactions. The spectral and molecular weight analyses were applied to characterize the products.

Organic chemistry

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