II.tip serbest radikal fotopolimerizasyonunda poli(etilen oksit)'in hidrojen verici olarak kullanılması

Poly(ethylene oxide) as hydrogen donor in type II photoinitiated free radical polymerization

Özet

Fotopolimerizasyon bilimi, uygulama alanlarındaki artış nedeniyle gerek endüstri gerekse akademik çalışmalarda gittikçe artan bir öneme sahip olmaktadır. Basitçe ışıkla başlatılmış polimerizasyon reaksiyonlarına fotopolimerizasyon denir. Genellikle mor ötesi veya görünür bölge ışık kaynakları kullanılır. Fotobaşlatıcının uygun bir dalga boyundaki ışık absorpsiyonu sonucunda oluşan primer radikaller tek fonksiyonlı monomerlerin polimerizasyonunu sağlarken çok fonksiyonlu monomerlerinde çapraz bağlı yapılara dönüştürülmesini sağlar. Fotobaşlatıcılar, radikal oluşturma mekanizmalarına göre ( I.tip) ve ( II. tip) fotobaşlatıcılar olmak üzere iki ayrı sınıfa ayrılır. Birinci tip fotobaşlatıcılar, radikal vermek üzere doğrudan fotoparçalanmaya uğrayan çeşitli fonksiyonel gruplar içeren aromatik karbonil bileşiklerdir. İkinci tip sistemlerde, polimerizasyonun başlaması hidrojen verici molekül üzerinde oluşan radikaller vasıtasıyla gerçekleşirken etkin olmayan ketil radikalleri birbirleriyle birleşerek ortamdan kaybolur. Bu çalışmada poli(etilen oksit)’in (PEO) II. tip serbest radikal fotopolimerizasyonunda hidrojen verme kabiliyeti polimerizasyon ve spektroskopik yöntemlerle incelenmiştir. PEO’in molekül ağırlığının ve fotobaşlatma etkisi ayrıca incelenmiştir. PEO ve benzofenon çözeltisi monomersiz ortamda bir radikal tutucu (2,2,6,6-tetrametilpiperidinil-1-oksi, TEMPO) varlığında fotolize uğratılmaktadır. Bu işlem sonunda TEMPO molekülü PEO’e bağlanmaktadır. Böylelikle uyarılmış benzofenon molekülünün PEO anazinciri üzerindeki metilen gruplarından hidrojen kopartabileceğinin ispatıdır. Uygulanan fotobaşlatma sistemi ayrıca çok yönlü bir aşı kopolimerizasyon yöntemine dönüştürülebilir. Fotokimyasal yollarla TEMPO bağlanan PEO 110 oC ısıtılarak stiren monomerinin kararlı serbest radikal polimerizasyonu sonucu poli(etilen oksit-g-stiren) aşı kopolimerleri elde edilir. Ayrıca PEO’in diş dolgusu formülasyonundaki potansiyel kullanımıda gösterilmiştir. Polimerik doğası, suda çözünürlüğü ve toksik olmayan özellikleri PEO’i diş dolgularında gelecek vaat eden bir hidrojen verici adayı yapmaktadır.

Abstract

Photoinitiated free radical polymerization has been widely used in research and industrial applications during the past few decades. Photopolymerization offers compelling advantages over traditional thermal polymerization, including low energy consumption, room temperature curing, spatial and temporal control of initiation, and solvent-free polymerization. These advantages have lead to tremendous growth in the use of photopolymerization in a variety of applications, including coatings on a variety of substrates, adhesives, flexographic printing plates, soft contact lenses, and dental materials. Photopolymerizations are simply polymerization reactions initiated by light, typically in the ultraviolet or visible region of the light spectrum. Photopolymerizations are initiated by certain types of compounds which are capable of absorbing light of a particular wavelength. The wavelength or range of wavelengths of the initiating source is determined by the reactive system including the monomer(s), the initiator(s), and any photosensitizers, pigments or dyes which may be present. Photoinitiated radical polymerization may be initiated by both cleavage (Type I) and H-abstraction type (Type II) initiators. Because of their vital role in photopolymerization, photoinitiators are the subject of particularly extensive research. Most of this research has focused on type I photoinitiators, which upon irradiation which undergo an α- cleavage process to form two radical species. Type II photoinitiators are a second class of photoinitiators and are based on compounds whose triplet excited states are reacted with hydrogen donors thereby producing an initiating radical. Because the initiation is based on bimolecular reaction, they are generally slower than type I photoinitiators which are based on unimolecular formation of radicals. On the other hand, type II photoinitiators possess better optical absorption properties in the near-UV spectral region. Moreover type I compounds give raise to volatile photodecomposition products due to the cleavage mechanism adding to migration the problem of release of odour. In this respect the type II photoinitiators have a more favourable profile because the ketyl radical either is re-oxidised back to the ketone or gives rise to recombination products with formation of higher molecular weight derivatives with a lower volatility than parent compounds. Typical type II photoinitiators include benzophenone, thioxanthones, benzil, and quionones while alcohols, ethers, amines and thiols are used as hydrogen donors. The selection of a coinitiator (hydrogen donor) is undoubtedly of great importance. Tertiary amines are more reactive coinitiators than are alcohols or ethers. In type II photoinitiating system, the unreacted photoinitiator and amine coinitiator, as well as the photolysis products, tend to cause discoloration of the cured composite. Furthermore, the practical application of amines suffers from their usage in large amounts which is particularly important for curing applications since formulations containing amine at high concentrations causes a decrease in the pendulum hardness of the cured films due to the plasticizing effect of amines. In addition, the amine is known to be both toxic and mutagenic. Hydrogen donating capability of poly(ethylene oxide) (PEO) in Type II photoinitiated free radical polymerization was demonstrated by polymerization and spectroscopic studies. The effect of molecular weight of PEO on the photoinitiation efficiency was investigated. Photolysis of solutions containing benzophenone and PEO in the presence of a radical scavenger namely, 2,2,6,6-tetramethylpiperidine-Noxyl free radical (TEMPO) revealed that photoexcited benzophenone readily abstracts hydrogen from methylene groups present in PEO backbone. It was demonstrated that such photoinitiating system can be converted to a versatile grafting process. PEO possessing photochemically attached TEMPO units initiates the Nitroxide Mediated Radical Polymerization (NMP) of styrene upon heating at 110 °C leading to the formation of poly(ethylene oxide-gstyrene) graft copolymer. Potential use of the photoinitiating system in dental formulations was also demonstrated. The polymeric nature, water solubility and nontoxicity make PEO a promising candidate as hydrogen donor in dental formulations.