Kontrollü polimerizasyon ve çapraz eşleşme metodlarıyla poli(p-fenilen) sentezi

Synthesis of poly (p-phenylene)s by controlled polymerization and cross-coupling methods

Özet

Polifenilenler (PF) ve türevleri, mükemmel termal ve mekanik özelliklerinden dolayı yüksek teknoloji polimerlerin önemli bir sınıfını oluştururlar. PF lerin birçok organik çözücüde çözünmemesi işlenebilirliklerini kısıtlayıcı rol oynar. Bu yüzden, ana zincire hareketli alkil yan zincirlerinin eklenmesi, çözünür ve işlenebilir yüksek molekül ağırlıklı polimerlerin kontrollü sentezine izin vermesi yönünden önem kazanmıştır. Ana zincirin uzunluğu ve yan zincirlerin esnekliğinden dolayı bu tür polimerler “hairy-rod” olarak adlandırılırlar. Sert, çözünmez, çubuk şeklindeki PF lere yumuşak bir Polistiren (PSt) grubunun eklenmesiyle tamamen yeni özelliklere sahip bir polimerin sentezi mümkün olabilir. Son yıllarda PF lerin sentezi Ni ve Pd katalizli çapraz-kenetlenme reaksiyonları ile yapılmaktadır. Bu çalışmada, 2,5-Dibromo-1,4-(dihidroksimetil)benzen, kalay oktoatın (Sn(Oct)2 ) katalizör olduğu є-kaprolaktonun (KL) halka açılma polimerizasyonunda (ROP) başlatıcı olarak kullanılarak poli(є-kaprolakton)(PKL) makromonomeri sentezlendi. Makromonomer Suzuki eşleşme tepkimesinde ve $NiCl_2/bpy/PPh_3/Zn$ sisteminin kullanıldığı Yamamoto tipi polimerizasyonda kullanılarak, oda sıcaklığında genel organik çözücülerde çözünebilen PKL yan zincirli poli(p-fenilen) ler (PPF) elde edildi. Ayrıca PKL makromonomeri 4-bromometil benzoil klorür ile reaksiyona sokularak Atom Transfer Radikal Polimerizasyonu (ATRP) için uygun çift fonksiyonlu makrobaşlatıcı sentezlendi. Makrobaşlatıcının Stiren (St) ile ATRP reaksiyonundan iyi tanımlanmış, düşük polidispersitede bir polimer elde edildi. Bu polimerin Suzuki eşleşme tepkimesinde kullanılmasıyla alternatif hekzil ve PKL-b-PSt yan zincirleri içeren PPP ler sentezlendi. Polimerler spektral, GPC, DSC ölçümleriyle analiz edildi.

Abstract

Poly(p-phenylene) (PPP), one of the most structurally simple of all the linear, rigid-rod polymers, is arguably one of the potentially most useful engineering materials. Its primary properties include high mechanical strength, excellent thermal and thermal oxidative stability, insolubility in all solvents, intractability, and the ability to conduct electricity upon oxidative or reductive doping. Despite its deceptively simple structure and its unique combination of attractive materials properties, PPP has traditionally been a very difficult polymer to synthesize and fabricate. The struggle to find efficient methods for producing PPP with both a regioregular linear structure and high molecular weight, together with the inherent lack of processability of the material, have retarded the development of PPP as a viable, useful material. Only within the past five to ten years has the problematic synthesis of high molecular weight, structurally regular PPP been solved through a number of ingenious strategies. In addition, a number of methods have been recently found to overcome the processing difficulties commonly associated with this polymer. Furthermore, non-traditional roles have also been found for $PPP_s$ of all qualities in applications far removed from the traditional roles of the polymer as a high-performance structural material and organic conductor. Polyphenylene (PP) is used as a coating material in the packaging industry to protect integrated circuits from breakage, humidity, and corrosion. Other interesting and important properties that $PPP_s$ exhibit include liquid crystallinity and photo- and electroluminescence. As $PPP_s$ are insoluble in many organic solvents, which limit their processability, attachment of conformationally mobile alkyl side chains to the backbone has been important because it has allowed the controlled synthesis of soluble and processable polymers with high molecular weight. In view of the expected large persistence length of the main chain and of the flexibility of the side chains, such molecules have been termed “hairy-rod” polymers. On combining a stiff, insoluble, rod-like polymer such as PPP with a soft coil, for example polystyrene (PSt), it is possible to form a new polymer with novel and interesting properties. Taking in account the considerable interest not only in the synthesis of new types of plastic materials, but also in the modification of the commodity polymers to improve their properties to meet the requirements for high-tech applications, PSt or poly(methyl methacrylate) (PMMA) were used, in which nanostructured photoactive conjugated oligo(phenylene vinylene)s segments are attached as side chains to the backbone. Living polymerizations are chain growth reactions that the final average molecular weight of the polymer can be adjusted by varying the initial monomer/ initiator ratio, with a narrow molecular weight distribution. Atom transfer radical polymerization (ATRP), appears to be a powerful tool for the polymer chemists, providing new possibilities in structural and architectural design. Current methodologies for the direct synthesis of derivatized $PP_s$ are primarily based upon nickel- and palladium- mediated cross-coupling reactions due largely to their preservation of regiochemistry and nearly quantitative yields. The present work described the synthesis and characterization of PPs with well-defined polymeric sidechains by cross-coupling methods. 2,5-Dibromo-1,4- (dihydroxymethyl)benzene was used as initiator in ring opening polymerization (ROP) of ε-caprolactone (CL) in the presence of stannous octoate $(Sn(Oct)_2)$ catalyst. As this polymer was intended to be used in further polymerization reactions, the efforts were directed toward obtaining a low molecular weight combined with a low polydispersity. The crosscoupling of aryl halides and aryl boronic acids (Suzuki coupling) and Yamamoto method is one of the most common methods for the synthesis of polyarylenes so,the resulted PCL macromonomer, with a central 2,5-dibromo-1,4-diphenylene group, was used in combination with 1,4-dibromo-2,5-dimethylbenzene for a Suzuki coupling in the presence of $Pd(PPh_3)_4$ as catalyst or using the system $NiCl_2/bpy/PPh_3/Zn$ for a Yamamoto type polymerization. The obtained polyphenylenes, with PCL side chains, have high solubility in common organic solvents at room temperature similar with the starting macromonomer.On the other hand, (PCL) macromonomer with OH end groups and having a central 2,5 dibromo-1,4-phenylene moiety was reacted with 4-bromomethyl benzoyl chloride to give a bifunctional macroinitiator for ATRP. The polymerization of styrene (St) in the presence of benzyl bromide functionalized PCL furnished a well defined block-copolymer having 25 repeating units of $varepsilon$- caprolactone and 19 of styrene with a low polydispersity. The new polymers were analyzed by $^1H-NMR$, UV spectroscopy, GPC and DSC measurements.