Konvansiyonel ve kontrollü radikal polimerizasyonları için yarı iletken nanoparçıkların heterojen fotobaşlatıcı olarak kullanımı

Semiconducting materials have had a great impact in academia and industry. They offer many potential applications in environmental, analytical, and energetic matters. Semiconductors are highly photosensitive and can promote redox reactions by the photogenerated charge carriers. Electron-hole pairs are released from the conduction and valence band of semiconductor, respectively, upon light illumination. On the other hand in (photo)polymerization techniques there is a growing demand for more convenient and efficient systems so that polymerization can be conducted straightforwardly under facile conditions with low energy consumptions. Accordingly, a lack of control in conventional photopolymerizations entails much work to devolop or adapt controlled polymerization techniques with emerging or existing photocatalytic concepts. This prompted us to explore and take the advantages of semiconductor zinc oxide nanoparticles in different photopolymerization systems. We designed a set of reactions in conventional and controlled radical photopolymerizations: free radical polymerization and atom transfer radical polymerization. In the free radical technique, polymerizations were investigated in aqueous and organic media, in which oxygen inhibition problem can partially be suppressed in aqueous environment by including redox reactions of oxygen and water molecules in the formation of initiating radicals. Whereas, in organic media, the process was accomplished with the use of co-initiators. Amine and iodonium salt co-initiators were used to realize redox reactions with the photogenerated holes and electrons, respectively, to form initiating radicals and initiate the polymerization. Several control experiments were carried out to have a clear understanding of the proposed initiation mechanism of each individual system. The heterogeneous nature of the nanoparticles also offers additional advantage of reusability in further reactions. That is nanoparticles were easily separated off the system and reused for several times under the same experimental conditions without any loss of activity. Doping zinc oxide nanopaticles with iron enhanced the photoactivity of the nanopaticles and gave better results in term of high conversion and rapid reaction time. There has been a growing interest in developing ATRP systems offering higher and eliminating some drawbacks in conventional ATRP that uses copper(I), including low-stability and high loadings of the copper catalyst. In the controlled photoinitiated atom transfer radical polymerization (photo ATRP), the required copper(I) catalyst was achieved by reduction of copper(II) species with the aid of electrons releasing from the semiconductor under light. Nanoparticles were able to form in situ the copper(I) catalyst by reducing initially loaded copper(II) species. Poly(methyl methacrylate)s were obtained in a controlled manner with well-controlled molecular weights and narrow molecular weight distributions employing ethyl α-bromoisobutyrate as the ATRP initiator, copper(II)Br2/PMDETA (PMDETA: N,N,N′,N′′,N′′-pentamethyldiethylenetriamine)as the catalyst and zinc oxide nanoparticles as the photocatalyst. Kinetics studies showed all the characteristics of living/controlled polymerization with well-agreement of molecular weights with the theoretical values and high chain end fidelity as approved by 1H NMR spectroscopy and chain extension experiments. Additionally the system exhibited temporal control over the polymerization so that the chain growth could be manipulated by simply switching the light source On/Off.