Piridinyum tuzlarının serbest radikal fotobaşlatıcı olarak kullanımı
Photosensitized free radical polymerization using pyridinium salts
- Tez No: 39764
- Danışmanlar: PROF.DR. YUSUF YAĞCI
- Tez Türü: Yüksek Lisans
- Konular: Kimya, Chemistry
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1994
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 53
Özet
ÖZET Piridinyum tuzlan fotokimyasal katyonik polimerizasyon başlatıcısı olarak kullanılmaktadır. Bu polimerizasyonlarda piridinyum tuzu doğrudan fotokimyasal parçalanmaya uğrayarak aktif katyonları oluşturduğu gibi dolaylı olarak da aktive edilebilir. Bu çalışmada N-Etoksi-2-Metil Piridinyum Hekzaflorofosfat (EMP+) tuzu değişik fotouyancılar varlığında serbest radikal polimerizasyon başlatıcısı olarak kullanılmıştır. Fotouyancı olarak; antrasen, tioksanton, fenotiazin ve perilen kullanılmıştır. Serbest radikal polimerleşebilen monomer olarak da Metilmetakrilat, Süren, Etil Akrilat ve 2-Vinil Piridin kullanılmıştır. Uygun karışımlar piridinyum tuzunun absorplamadığı dalga boylarında \>350 nm ışmlandınldığmda serbest radikal polimerleşme gerçekleştirilmiştir. Polimerizasyon, piridinyum tuzunun elektron transferi ile parçalanarak etoksi radikali vermesiyle başlatılmıştır. Polimerizasyonda aydınlatma süresi, fotouyancı yapısı ve konsantrasyon etkileri incelenmiştir. Pratik uygulamaya örnek oluşturacak şekilde çapraz bağlanabilen iki fonksiyonlu bir monomer içeren kanşımda aynı başlatıcı sistemin etkinliği diğer klasik başlaücı sistemlerle kıyaslanarak incelenmiştir.
Özet (Çeviri)
PHOTOSENSITIZED FREE RADICAL POLYMERIZATION USING PYRTDINIUM SALTS SUMMARY Photopolymerization is practically applied in the fabrication of electronic devices such as printed circuits and in surface coatings. It may be induced by free radicals or cationic species generated by irradiation of initiators. Although the photoinitiated cationic polymerization has several advantages over the free radical polymerization, the former is still early stage of development. Most industrial applications involving photochemical curing processes are based on free radical polymerization. The relatively well advanced state of free radical photoinitiated polymerization is mainly due to the availability of a wide range of photoinitiators with properties fulfilling practical requirements. Thermally stable onium salts, such as iodonium salts and sulphonium salts (Ar2I+X“ and Ar3I+X”) with non-nucleophilic counter anions (X“ : PF6', SbF6”, AsF6“, BF4”etc.) have been widely used as photoinitiators for cationic polymerization. Generally these compounds do not absorb light at \>350 nm and are, therefore, of little technical importance. Quite recently pyridinium salts are found. N-Alkoxy-Pyridinium and N-Alkoxy quinolinium salts of the general structure CO ^x^x-N- OEt OEt OEt PF6“ are capable of acting as photoinitiators for cationic polymerization of cyclic ethers VIsuch as cyclohexeneoxide and vinyl ethers such as n-butyl-vinyl ether. Pyridinium salts act directly as photoinitiators provided by the irradiation of the performed, at wavelengths corresponding to their absorption bands. If the pyridinium ring is appropriately substituted, the absorption band is shifted to longer wavelengths; e.g. comparing pyridinium with biphenyl pyridinium, the absorption spectra of the latter is red shifted. A mechanism put forward to explain the ability of pyridinium salts such as EMP+PF6”(N-Ethoxy-2-Methyl Pyridinium Hexafluorophosphate) to induced the polymerization appropriate monomers is shown below. EMP+ -M. ( EMP+)* -»? Polymer (i) (ii) &? -*? I ( ) I j. R, H (iii) % ?+ I H + H (iv) M Polymer (v) Notably radical cations, formed upon irradiation can be responsible for polymerization. Their mechanism was substantiated by recent flash photolysis studies. In practical applications photoinitiators need to show some absorption at greater than 350 nm. It appears that pyridinium salts have no absorptivity at desired wavelengths. Several systems have been developed to extent their photosensitivity to longer wavelengths (X>350 nm). In these cases other molecules are present VHwhich participate in the reaction sequences to yield reactive species capable of initiating cationic polymerization. Depending on their role in the process they may be free radical sources or sensitizers. Pyridinium salts may also be used as agents for the oxidation of free radicals. Carbocations formed with the aid of pyridinium salts, e.g. EMP+, are capable of initiating the polymerization of butyl vinyl ether and cyclohexene oxide. R- + EMP+PFfi- > R+ PFfi- + EMP- (vi) Photosensitization with the aid of appropriate compounds absorbing light at longer wavelengths \>350 nm can help to extent the applicability of pyridinium salts as photoinitiators. For this purpose, the reactions of EMP+ with excited states of thioxanthone, anthracene, perylene and phenothiazine were studied. In the ground state these compounds strongly absorb light between 350-400 nm. The sensitization action of the compounds used can be based on either energy transfer from excited sensitizer (PS*) to the pyridinium ion followed by decomposition of the excited pyridinium ion (I+)* PS* + I+ > PS + I+* (vii) or electron transfer resulting in the formation of the radical cation of the sensitizer according to reaction (viii). PS* + l+ > PS+ + I- (viii) Apart from cationic polymerization, pyridinium salts may also be used as photoinitiators for free radical polymerization of vinyl compounds. In this work the photosensitized free radical polymerization using N-Etoxy-2-Methyl Pyridinium ions (EMP+) by anthracene, thioxanthone, phenothiazine and perylene is studied. These photosensitizers strongly absorb light between 350 and 400nm. Dichloromethane solutions containing monomer, photosensitizer and EMP+PF6“ were irradiated in pyrex tubes in an Annular type photoreactor. A possible mechanism for the initiation of free radical polymerization applies to electron transfer between photoexited sensitizer and the pyridinium salt. VUlPS PS* + hv PS' OEt PS,,* a J PS+ OEt - r OEt (ix) (x) (xi) r OEt kNA + EtO' (xii) EtO« + M Polymer (xiii) The polymerization of vinyl monomers was initiated effectively upon irradiation of solutions of monomers in dichloromethane containing EMP+ and anthracene. At the irradiation wavelength (\>350 nm) EMP+ is transparent and initiation is caused by light absorbed by anthracene. Notably, the conversion of monomer to polymer was significantly low in the absence of the pyridinium salt. Ethoxy radical formed according to reaction xii is capable of initiating free radical polymerization. Photosensitized polymerization of MMA in the presence of EMP+ using various concentration of anthracene and thioxanthone was studied. Irradiations were carried out at constant irradiation time 4 h. It can be seen a maximum sensitization effect on photosensitizer concentration. As the concentration of photosensitizer further increased, the percentage conversion drops. This behavior may be attributed to aggregation in solution and resultant self quenching. IXThe dependence of the rate of polymerization on EMP+ concentration was studied. It turned out that the polymerization rate was increased with increasing pyridinium ion concentration. The photosensitized polymerization of MMA by thioxanthone in the absence and presence of EMP+ was also examined. There is a considerable increase on polymerization conversion in the case of presence of EMP+. Two mechanisms for the generation of ethoxy radicals are feasible. The first one involves electron transfer between pinacol type radical, formed by hydrogen abstraction by triplet thioxanthone and EMP+. Subsequent decomposition of pyridinyl radical would than generate ethoxy radical. An alternative mechanism occurs similar to anthracene sensitization. Although distinction between these two alternative mechanisms is not possible at this stage, by laser flash photolysis studies it was found that polimerizable monomers were rather reactive towards triplet excited thioxanthone. The sensitization effect of perylene and phenothiazine was also studied. When the time of polymerization is increased, the percentage of conversion increases linearly. It's interesting to note that the sensitization effect of phenothiazine in the absence of EMP+ ion is greater than that of in the presence of pyridinium salt. Table 1 shows the sensitization effect of photosensitizers on the free radical polymerization of various monomers initiated by pyridinium salt. As a very simple demonstration of the possible value of photosensitized free radical polymerization by pyridinium salts in UV curing applications, several experiments were performed using a monomer mixture containing ethylene glycol dimethacrylate. Because of the presence of two methacrylate groups, this monomer ultimately forms gelled systems. The results shows that anthracene-EMP+ system exhibits intermediate behavior.o e o (S es em s ”3? o g o es > i. ss w "?3 s: ü s es £ XI
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