Gorkem Yilmaz

Synthesis of ABC type miktoarm star copolymers by triple click chemistry

An ABC type miktoarm star copolymer possessing polystyrene (PS), poly(e-caprolactone) (PCL) and poly(ethylene glycol) (PEG) arms was synthesized by combining existing triple click chemistries, namely thiol–ene, copper catalyzed azide–alkyne cycloaddition (CuAAC) and Diels–Alder (DA) reactions. For this purpose, a core (1-(allyloxy)-3-azidopropan-2-yl (anthracen-9-ylmethyl) succinate) with allyl, azide and anthracene functionalities was synthesized in two steps with high yields. Then, polymers with corresponding clickable sites, namely ω-thiol polystyrene (PS-SH), α-alkyne poly(e-caprolactone) (alkyne-PCL) and ω-maleimide poly(ethylene glycol) (Me-PEG-MI), were independently prepared. As the first step of the grafting onto process, PS-SH was thiol–ene clicked onto the core to yield PS-N3-Ant. Finally, alkyne-PCL and Me-PEG-MI were bonded to PS-N3-Ant either in a sequential or a one-pot in situ manner using CuAAC and DA click reactions. All intermediates, related polymers at different stages and final PS-PCL-PEG miktoarm star copolymer were characterized by 1H NMR, FT-IR, and GPC analyses.

Poly(vinyl alcohol)–Thioxanthone as One‐Component Type II Photoinitiator for Free Radical Polymerization in Organic and Aqueous Media

A novel one-component type II polymeric photoinitiator, poly(vinyl alcohol)-thioxanthone (PVA-TX), is synthesized by a simple acetalization process and characterized. PVA-TX enables photopolymerization of methyl methacrylate and acrylamide in both organic and aqueous media. Photopolymerization proceeds even in the absence of a co-initiator since PVA-TX possesses both chromophoric and hydrogen donating sites in the structure.

LED and visible light-induced metal free ATRP using reducible dyes in the presence of amines

A new photoinitiating system involving electron acceptor dyes, namely, eosin Y and erythrosin B, in conjunction with alkyl halides and amines for photoinduced ATRP of (meth)acrylates and vinyl monomers in the absence of inorganic catalysts is reported. The polymerizations were efficiently activated by the photomediated redox processes to produce polymers with controlled chain end functionality and narrow molecular weight distribution. The dye/amine system was shown to be efficient under various colors of LED and industrially available visible light irradiation. The livingness nature of the polymerization was proved by GC analyses and the irradiation dependency of polymerization was confirmed by light on/off experiments.

Conventional Type II photoinitiators as activators for photoinduced metal-free atom transfer radical polymerization

A novel methodology for photoinduced metal-free Atom Transfer Radical Polymerization (ATRP) by using conventional Type II photoinitiators such as benzophenone, thioxanthone, isopropyl thioxanthone and camphorquinone as sensitizers is presented. These sensitizers efficiently activate the ATRP of vinyl monomers when used in conjunction with co-initiators and alkyl halides under light irradiation. DFT calculations were performed to reveal mechanistic aspects and showed that the initiation of the controlled ATRP reaction pathway from the triplet state is energetically favoured for all sensitizers. In the case of benzophenone, however, the hydrogen abstraction pathway, which leads to free radical polymerization, is a possible side reaction. The strategy applied was proved to yield polymers with narrow molecular distribution. The chain-end fidelity of the polymer obtained was approved by chain extension and block copolymerization experiments, whereas the irradiation dependency of polymerization was confirmed by light on/off experiments.

Block copolymer synthesis in one shot: concurrent metal-free ATRP and ROP processes under sunlight

A completely metal-free strategy was developed by combining Atom Transfer Radical Polymerization (ATRP) and Ring Opening Polymerization (ROP) for the syntheses of block copolymers. These two different metal-free controlled/living polymerizations, which have no effect on each other, were concurrently realized in one reaction medium. Using a specifically designed bi-functional initiator, possessing primary hydroxyl and tertiary bromide functionalities, vinyl and lactone monomers were simultaneously polymerized under sunlight, by metal-free strategies. Spectral and chromatographic results reveal that the synthetic approach applied produces block copolymers in a controlled manner, free from homopolymers, and with narrow molecular weight distributions.

Mono‐addition Synthesis of Polystyrene–Fullerene (C60) Conjugates by Thiol–Ene Chemistry

Although fullerene (C60) and derivatives were discovered more than two decades ago, they still receive extensive interest due to their excellent photochemical, electrochemical, and magnetic properties. However, industrial-scale manufacture of new devices based on the C60 molecule is extremely limited as a result of its poor processability. The low-solubility characteristics and perhaps problems associated with the possible migration effect of bare fullerene in the composite materials are additional limitations, which need to be overcome. Grafting of polymers onto C60 is one way to overcome such limitations. However, synthesis of mono-addition products is difficult to perform due to the multifunctional nature of the C60 nuclei. Multi-addition processes usually yield unavoidable insoluble network products and the target C60 end-capped polymer can only be obtained after cumbersome isolation processes. As such, previous attempts focused on the development of new ways for the synthesis mono-addition products. The known strategy involves the use of living-polymerization techniques. Fullerene-endcapped polymers can be synthesized through living radical or anionic polymerizations by using C60 as a terminator, in which the C60 reacts with the propagating radical or anionic species to form polymers with C60 moiety at the w terminus. However, this approach usually yields a mixture of products requiring rigorous purification processes. Thus, the inherent contamination of the product makes these systems somewhat problematic for most applications. In complete contrast to radicaland anionic-polymerization-based approaches, the corresponding living cationic polymerization was reported to give desired polymers with higher purity. In this case, the living cationic polymerization of vinyl ethers through ion-coupling reactions or functional initiator afforded C60-end-capped polymers with well-controlled architecture. In the latter case, the obtained polymer exhibited solvatochromism, indicating self-assembly in the media. Other strategies include reaction of azide-chain-end functionality of polymers and click reactions, namely Cu-mediated 1,3dipolar Huisgen and Diels–Alder-coupling reactions involving ligation of azideand diene-functional polymers, respectively. These reactions yield mono-addition conjugates with relatively low polydispersity values. Recently, thiol–ene reactions, originally developed for UV-curing applications, have been introduced as a versatile route for the synthesis of numerous polymeric structures. b, c,d] Due to its easy handling, efficient and robust characteristics, the reaction is referred as a click reaction. Recently, thiol–ene reactions have also been utilized for the synthesis of telechelic polymers, dendrimers, gels and functionalization of polymer pendants, hyperbranched polymers, functional cross-linked networks, and glass surfaces. These reactions can also be combined with the other click processes for the synthesis of more complex macromolecular structures. Very recently, two examples of thiol–ene reactions were described in publications from this laboratory that are combined with 1,3-dipolar Huisgen and Diels–Alder click reactions for the synthesis of miktoarm star polymers. Another successful combinations involving thiol–ene chemistry has been recently shown by Schubert and co-workers. With these considerations in mind, we aim to explore the possibility of using thiol–ene chemistry to prepare C60 polystyrene conjugates in a way that only mono-addition product is formed without any contamination of ill-defined derivatives. The first step in the synthesis of C60 polystyrene conjugate was to obtain the precursor click component, polystyrene with thiol–end group (PS SH). PS SH was synthesized by a two-step procedure by using bromo-functional polystyrene prepared by atom-transfer radical polymerization (ATRP). 21] Subsequently, PS SH was grafted onto the C60 nuclei through thiol–ene click reaction in the presence of a known thermo-labile radical source, 2,2-azobisisobutyronitrile (AIBN). Thermal pathway rather than the photochemical protocol is deliberately selected so as to prevent side reactions arising from the absorption of C60. Obviously, C60 would have shielded the absorption of the photoinitiator due to its high photosensitivity in the UV/Vis region. The overall process is depicted in Scheme 1. In the process, the use of excess amount of C60 would essentially yield mono-adduct and unmodified C60. The insolu[a] B. Iskin, G. Yilmaz, Prof. Dr. Y. Yagci Department of Chemistry Istanbul Technical University 34469, Maslak, Istanbul (Turkey) Fax: (+90) 212-285-6386 E-mail : yusuf@itu.edu.tr [b] Prof. Dr. Y. Yagci King Abdulaziz University, Faculty of Science Chemistry Department, Jeddah (Saudi Arabia) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201200534.

Polysulfone/Pyrene Membranes: A New Microwell Assay Platform for Bioapplications

The use of PSU-Py prepared by click chemistry as a platform in membrane-bottom microwell plates for oxidase and hydrolase/oxidase-based enzyme assays is studied. For the GOx assay, the postulated fluorescence mechanism is based on the consumption of glucose by dissolved oxygen and GOx in the microwell plates covered with the PSU-Py membrane. For the AG-GOx assay, maltose is used as AG substrate and hydrolyzed to glucose which is then oxidized by the GOx activity. It is shown that the PSU-Py membrane acts as a fluorescence indicator of the enzymatic reactions, and both GOx and AG/GOx enzyme assays are successfully applied for glucose, maltose and acorbose analysis in the range 0.125-2.0 × 10(-3) M glucose, 0.05-0.5 × 10(-3) M maltose, and 0.0125-0.1 mg · mL(-1) acorbose, respectively.

Dibenzoyldiethylgermane as a visible light photo-reducing agent for CuAAC click reactions

A highly active, versatile and photoresponsive system for copper catalyzed azide–alkyne cycloaddition (CuAAC) click reaction has been developed using the dibenzoyldiethylgermane (DBDEG) photoinitiator with the copper(II) chloride (CuCl2)–N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) ligand under visible light. Selected azide and alkyne compounds with various functional groups are used to conduct the click reaction and almost quantitative yields are attained. The approach pertains to the visible light generation of germyl radicals capable of reducing Cu(II) ions to Cu(I) species to catalyze the CuAAC reactions. This strategy has been applied in the achievement of various macromolecular reactions including polymer end-group functionalization, block copolymer formation and step-growth polymerization.

Unconventional Sulfur Chemistries for Macromolecular Syntheses

GRAPHICAL ABSTRACT Abstract Sulfur-based reactions play an increasing role in macromolecular syntheses. The present article focuses on recent work from this laboratory that has led to advances in several areas of the field of sulfur chemistry for the synthesis of macromolecular structures with various topologies ranging from linear to networks. Two different classes of photopolymerizations, namely radical and cationic photopolymerizations can be initiated by sulfur-based thioxanthone and triaryl sulfonium salts, respectively. Also described is the structural variation that may affect the spectral sensitivity and efficiency in the related photopolymerizations. An independent part of the article consists of a birds eye view of various modification processes of polythiophenes. A special emphasis is devoted to the incorporation of polypeptides into these sulfur-based conjugated polymers and their biosensing applications. Thiol-ene chemistry is also discussed in terms of its effect and utilization for the synthesis of telechelic polymers and star copolymers. Finally, recently developed thiol-benzoxazine chemistry offering a facile and an efficient route to exploring the many possibilities in macromolecular synthesis is described.

Polymeric Thioxanthones as Potential Anticancer and Radiotherapy Agents

Thioxanthone (TX) and its derivatives, which are widely used as photoinitiators in UV curing technology, hold promising research interest in biological applications. In particular, the use of TXs as anticancer agent has recently been manifested as an outstanding additional property of this class of molecules. Incorporation of TX molecules into specially designed polymers widens their practical use in such applications. In this study, two water-soluble, biocompatible, and stable polymers, namely poly(vinyl alcohol) and poly(ethylene glycol), possessing TX moieties at the side chains and chain ends, respectively, are prepared and used as anticancer and radiotherapy agents. The findings confirm that both polymers are potential candidates for therapeutic agents as they possess useful features including water-solubility, radiosensitizer effect, and anticancer activity in a polymeric scaffold.