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.