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.