Atom transfer radikal polimerizasyonda aktif bakır (I) kompleks katalizinin reaksiyon ortamında üretilmesi

Özet

Atom Transfer Radikal Polimerizasyon (ATRP) yakın dönemde birçok akademik araştırmaya konu olmuş kontrollü/”yaşayan” radikal polimerizasyon (LRP) yöntemlerindendir. Yüksek maliyetli ve hazırlık esnasında kolaylıkla oksitlenebilen düşük oksidasyon basamağındaki metal tuzları katalizliğinde gerçekleşmesi ATRP’nin en büyük dezavantajlarındandır. Bu çalışmada stiren (St) ve metil metakrilatın (MMA) ATRP’si Cu (II)/ N,N,N’,N”,N”-pentametildietilentriamin (PMDETA)/ p-metoksitiyofenol ya da sodyum tiyofenolat (PhSNa) katalizliğinde belirli miktarda hava varlığında sırasıyla 110 ve 90 oC’de gerçekleştirilmiştir. Normal ATRP’den farklı olan bu yeni yaklaşım, tiyofenol türevi gibi elektron transfer bileşiklerinden Cu (II)’ye (deaktivatör) elektron transferi sonucu Cu(I)’in (aktivatör) in situ oluşumuna dayanmaktadır. In situ Cu(I) oluşumu CuCl2/PMDETA/PhSNa’nın N,N-dimetilformamid içindeki karışımından belirli zaman aralıklarında alınan örneklerin UV-VIS ölçümleriyle izlenmiştir. Bunun yanında p-metoksitiyofenol ve PhSNa’nın elektron transfer bileşiği olarak polimerizasyon kinetiği üzerine etkileri incelenmiştir. Polimerizasyonların monomer konsantrasyonuna göre 1. dereceden kinetik izlediği ve sayıca ortalama molekül ağırlıklarının % dönüşümle orantılı olarak arttığı gözlenmiştir. Ayrıca CuBr2/PMDETA/p-metoksitiyofenol katalizliğinde belirli miktarda hava varlığında gerçekleştirilen ATRP ile sentezlenen polistiren (PS) homopolimeri MMA ile zincir uzatma polimerizasyonunda makrobaşlatıcı olarak kullanılıp PS-PMMA blok kopolimeri sentezlenmiştir. Yapılan zincir uzatma polimerizasyonu ve kinetik çalışmalar ile Cu(II)/PMDETA/p-metoksitiyofenol ya da PhSNa katalizli polimerizasyonların yaşayan karakterde olduğu kanıtlanmıştır. ATRP'de in situ Cu(I) oluşumu metodu normal ATRP’nin bütün avantajlarını taşımasının yanı sıra düşük maliyetli ve hava koşullarında kararlı Cu(II) tuzlarının kullanılmasına ve polimerizasyonun sınırlı miktarda hava varlığında gerçekleştirilmesine olanak tanımaktadır. Böylelikle ATRP’nin uygulanabilirliğini kısıtlayan en önemli dezavantajlardan biri ortadan kaldırılmıştır.

Abstract

Radical polymerization is industrially the most widespread method to produce polymeric materials. This is due to its tolerance to protic compounds (such as water), a high reaction rate, convenient temperature range and very minimal requirements for purification of monomers, solvents. Furthermore, radical polymerizations can be carried out in bulk, in solution, aqueous suspension, emulsion, and dispersion. The major drawback of conventional radical polymerization is the lack of control over polymer structure. Due to slow initiation fast propagation, and subsequent transfer or termination, polymers with high molecular weight and broad molecular weight distribution are generally produced. These features are reflected in the physical and mechanical properties of the produced polymers. Living polymerization is free from side reactions such as termination and chain transfer and can thus generate polymers of welldefined architectures and molecular weights. However, ionic polymerization is limited to a handful of monomers; it also requires very stringent drying, exclusion of moisture, and also very low temperatures. The recent development of the living (or controlled/“ living”) radical polymerization (LRP) opened up a new and versatile route to the synthesis of well-defined, polymers with low polydispersities and various possible architectures. Among them atom transfer radical polymerization (ATRP) is the most convenient and useful method to synthesize polymers with well-controlled molecular weight and molecular weight distribution from a wide range of monomers. The advantages of ATRP, in comparison with other LRP processes, include the large range of available monomers and (macro)initiators, the simplicity of reaction setup, and the ability to conduct the process over a large range of temperatures, solvents, and dispersed media. However, major disadvantages of ATRP are high cost and easy oxidation of metal salt in lower oxidation state. These can be overcome by in situ Cu(I) generation via an electron transfer from a reducing agent to a more stable higher oxidation state metal salt. This approach is different than the normal ATRP in two respects: (i) In this system, reactive Cu(I) species are formed in situ via redox process between Cu(II) and reducing agents. Therefore, the small concentration of Cu(I) promotes the low concentration of transient radicals, in addition, the initial presence of Cu(II) (persistent radical) facilitates the deactivation process and thus suppresses undesirable radical-radical coupling reaction. (ii) The unavoidable oxidation of catalyst or the oxygen induced polymerization by the diffused oxygen as it was reported in normal ATRP does not play detrimental role in this system. In this study ATRP of styrene (St) and methylmethacrylate (MMA) were conducted in the presence of limited amount of air using Cu(II)/PMDETA as catalyst, sodium thiophenolate (PhSNa) or p-methoxythiophenol as reducing agent. Electron transfer to higher oxidation state metal salts from reducing agents such as PhSNa or p-methoxythiophenol provided the in situ formation of copper (I) species which is activator in copper catalyzed LRP. During the in situ generation of activator, thiophenol derivative is oxidized to yield disulfide. In this respect, UV-VIS measurements were performed to prove the electron transfer reaction between PhSNa and Cu(II)/PMDETA complex. Secondly, polymerization kinetics were followed to compare the efficiency of pmethoxythiophenol and PhSNa as reducing agent. Besides, to investigate chain-end functionality of synthesized polymers, PS macroinitiator prepared by this method was used as macroinitiator for chain extension polymerization with MMA under similar conditions. In summary a new method for conducting ATRP was developed. The procedure used to generate copper (I) species relies on electron transfer from reducing agents to copper (II) species rather than reduction by organic radicals. This novel procedure has all the benefits of a normal ATRP process combined with the additional benefit of adding the catalyst complex to the reaction mixture in its more stable higher oxidation state. It is anticipated that in situ copper (I) formation method will facilitate the commercial application of ATRP.