İSTANBUL TECHNICAL UNIVERSITY INSTITUTE OF SCIENCE AND TECHNOLOGY M.Sc. Thesis by Başak BULBA Department : Chemistry Programme : Chemistry JANUARY 2010
THE SURFACE MODIFICATION OF SILICA
İSTANBUL TECHNICAL UNIVERSITY INSTITUTE OF SCIENCE AND TECHNOLOGY
M.Sc. Thesis by Başak BULBA
(509081206)
Date of submission : 25 December 2009 Date of defence examination: 25 January 2010
Supervisor (Chairman) : Prof. Dr. Gürkan HIZAL (ITU) Members of the Examining Committee : Prof. Dr. Ümit TUNCA (ITU)
Prof. Dr. Nergis ARSU (YTU)
JANUARY 2010
THE SURFACE MODIFICATION OF SILICA
OCAK 2010
İSTANBUL TEKNİK ÜNİVERSİTESİ FEN BİLİMLERİ ENSTİTÜSÜ
YÜKSEK LİSANS TEZİ Başak BULBA
(509081206)
Tezin Enstitüye Verildiği Tarih : 25 Aralık 2009 Tezin Savunulduğu Tarih : 25 Ocak 2010
Tez Danışmanı : Prof. Dr. Gürkan HIZAL (İTÜ) Diğer Jüri Üyeleri : Prof. Dr. Ümit TUNCA (İTÜ)
Prof. Dr. Nergis ARSU (YTÜ) ABC TİPİ FARKLI KOLLU YILDIZ POLİMER İLE
v FOREWORD
This master study has been achieved at Istanbul Technical University, Chemistry Department.
I would like to express my deep appreciation and thanks for my thesis supervisor, Prof. Dr. Gürkan HIZAL and co-supervisor Prof. Dr. Ümit TUNCA for offering inestimably help in all possible ways, permanent encouragement and helpful critics during this research.
I wish to express my special thanks to Hakan DURMAZ for his leading during my research. Also, I would like to express my appreciation to him for his helpful and understanding attitudes throughout my laboratory and thesis study. It has been a pleasure to work with him.
I would like to also extend my sincere gratitude to my friend Aydan DAĞ for her friendly and helpful attitudes and unbelievable sensibility during my laboratory works. In addition, I would like to thank my group member Eda GÜNGÖR for her support and sincerity during my laboratory study.
I would like to offer the most gratitude to my family Orhan BULBA, Kezban BULBA, Berrin BULBA and Okan Anıl BULBA, and my friends Ġpek ÖSKEN, Sevcan AYAKSIZ, Duygu GÜRSOY, Volkan KIRMIZI, Çiğdem BĠLĠR, Elif ERDOĞAN, Ceyda Önen YALÇIN, and Dila KILIÇLIOĞLU for their patience, understanding and moral support during all stages involved in the preparation of this research.
December 2009 Başak BULBA
vii TABLE OF CONTENTS
Page
FOREWORD ... v
TABLE OF CONTENTS ... vii
ABBREVIATIONS ... ix
LIST OF FIGURES ... xi
SUMMARY ... xiii
ÖZET ... xv
1. INTRODUCTION ... 1
1.1 Purpose of the Thesis ... 3
2. THEORETICAL PART ... 5
2.1 Conventional Free Radical Polymerization ... 5
2.2 Conventional Living Polymerizations ... 6
2.3 Controlled/ „„Living” Free Radical Polymerizations ... 8
2.3.1 Nitroxide-Mediated Living Free Radical (NMP) ... 9
2.3.2 Atom Transfer Radical Polymerization (ATRP) ... 10
2.3.2.1 Monomers ... 12
2.3.2.2 Initiators ... 12
2.3.2.3 Catalysts ... 13
2.3.2.4 Ligands ... 14
2.3.2.5 Solvents ... 15
2.3.2.6 Temperature and Reaction Time ... 15
2.3.2.7 Molecular weight and molecular weight distribution... 15
2.3.3 Reversible-Addition Fragmentation Chain Transfer (RAFT) ... 16
2.4 Star Polymers ... 18
2.4.1 Stars by the Arm-First Method ... 18
2.4.2 Stars by the Core-First Method ... 20
2.5 ABC Terpolymers ... 21
2.6 Click Chemistry ... 24
2.7 Surface-immobilized macromolecules ... 30
2.7.1 Synthesis of surface-immobilized macromolecules (polymer brushes) ... 30
2.7.2 “Grafting to” approach to fabricate polymer brushes ... 31
2.7.3 “Grafting from” approach to synthesize polymer brushes ... 32
2.7.3.1 Synthesis of tethered polymer brushes by conventional radical polymerizations ... 32
2.7.3.2 Synthesis of tethered polymer brushes by controlled radical polymerization ... 34
3. EXPERIMENTAL WORK ... 37
3.1 Materials ... 37
3.2 Instrumentation ... 37
3.3 Synthesis of Initiator ... 38
3.3.1 Synthesis of 2,2,5-trimethyl-[1,3]dioxane-5-carboxylic acid ... 38
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3.3.3 Synthesis of prop-2-ynyl
3-hydroxy-2-(hydroxymethyl)-2-methylpropanoate ... 39
3.3.4 Synthesis of prop-2-ynyl 3-(2-bromo-2-methylpropanoyloxy)-2-(hydroxymethyl)-2-methylpropanoate ... 39
3.3.5 Synthesis of monocarboxylic acid terminated PEG (PEG-COOH) ... 40
3.3.6 Synthesis of PEG-macroinitiator ... 40
3.4 Synthesis of PEG-PMMA Copolymer with Alkyne at the Junction Point via ATRP of Methyl Methacrylate (Alkyne-PEG-PMMA) ... 40
3.5 Synthesis of 3-(trimethylsilyl)prop-2-ynyl 2-bromo-2-methylpropanoate ... 41
3.6 Synthesis Bromo End-functionalized Sillyl Protected PS via ATRP of Styrene ... 41
3.7 Synthesis of azide end – functionalized PS ... 42
3.8 Click reaction between Alkyne-PEG-PMMA and Sillyl protected PS-N3 (ABC Type Miktoarm Star Polymer) ... 42
3.9 Deprotection reaction of ABC Type Miktoarm Star Polymer (Hydrolysis Reaction ... 43
3.10 Synthesis of Azide Functionalized Silica Nanoparticles ... 43
3.10.1 Synthesis of azido ethanol ... 43
3.10.2 Synthesis of teos azide ... 43
3.10.3 The reaction between silica and TEOS azide (Si-N3) ... 44
3.11 Click Reaction Between Silica Azide and Alkyne PS-PEG-PMMA ... 44
4. RESULTS AND DISCUSSION... 45
4.1 Synthesis of Initiator... 45
4.2 Synthesis of PEG-PMMA Copolymer with Alkyne at the Junction Point via ATRP of Methyl Methacrylate (Alkyne-PEG-PMMA ... 50
4.3 Synthesis of Azide End–Functionalized PS ... 52
4.4 Synthesis of ABC Type Miktoarm Star Polymer ... 55
4.5 Silica Modification of PS-PEG-PMMA via Click Reaction ... 58
5. CONCLUSION ... 65
REFERENCES ... 65
ix ABBREVIATIONS
ATRP : Atom Transfer Radical Polymerization : Atom Transfer Radical Polymerization NMP : Nitroxide Mediated Polymerization : Stable Free Radical Polymerization CRP : Controlled/Living Radical Polymerization : Styrene St : Styrene : Styrene
MMA : Methyl methacrylate : Methyl methacrylate
PS : Polystyrene
PMMA PEG
: Poly(methyl metacrylate) : Poly(ethyleneglycol)
LFRP : Living Free Radical Polymerization TEMPO : 2, 2', 6, 6'- Tetramethylpiperidinyloxy PDI : Polydispersity Index
PRE : Persistent Radical Effect Mtn : Transition metal
L Mn Mw
: Ligand
: Number Average Molecular Weight : Weight Average Molecular Weight Mw/Mn : The Molecular Weight Distribution ka : Rate constant of activation
kd : Rate constant of deactivation kp : Rate constant of propagation THF : Tetrahydrofuran : Tetrahydrofuran DMAP : 4-dimethylaminopyridine
PMDETA : N,N,N’,N’’,N’’- pentamethyldiethylenetriamine GPC : Gel Permeation Chromotography
xi LIST OF FIGURES
Page
Figure 2.1 : General free radical polymerization mechanism ... 6
Figure 2.2 : The general outline of the free-radical mechanism ... 10
Figure 2.3 : General scheme of transition-metal-catalyzed ATRP ... 12
Figure 2.4 : Derivatives of 2,2-bipyridine and nitrogen based ligands ... 14
Figure 2.5 : Mechanism of RAFT polymerization ... 17
Figure 2.6 : Scheme of star formation by the “arm-first method” ... 19
Figure 2.7 : Schematic presentation of all possible arrangements for an ABC terpolymer ... 23
Figure 2.8 : Regioselectivity mechanism of triazole forming cycloaddition. ... 27
Figure 2.9 : Proposed catalytic cycle for the Cu(I)-catalyzed ligation ... 28
Figure 2.10: Preparation of polymer brushes by “physisorption”, “grafting to” and “grafting from” ... 31
Figure 2.11: Schematic description of the concept for the preparation of cleavable polymer brushes by “grafting from” approach ... 33
Figure 4.1 : The 1H NMR spectrum of 2,2,5-trimethyl-[1,3]dioxane-5-carboxylic acid in CDCl3 ... 45
Figure 4.2 : The 1H NMR spectrum of prop-2-ynyl 2,2,5-trimethyl-1,3-dioxane-5-carboxylate in CDCl3 ... 46
Figure 4.3 : The 1H NMR spectrum of prop-2-ynyl 3-hydroxy-2-(hydroxymethyl)-2-methylpropanoate in CDCl3 ... 47
Figure 4.4 : The 1H NMR spectrum of prop-2-ynyl 3-(2-bromo-2 methylpropanoylo- xy)-2-(hydroxymethyl)-2-methylpropanoate in CDCl3 ... 48
Figure 4.5 : The 1H NMR spectrum of PEG-COOH in CDCl3 ... 49
Figure 4.6 : The 1H NMR spectrum PEG-macroinitiator in CDCl3... 50
Figure 4.7 : The 1H NMR spectrum (Alkyne-PEG-PMMA) in CDCl3 ... 52
Figure 4.8 : The 1H NMR spectrum of 3-(trimethylsilyl)prop-2-ynyl 2-bromo-2-methylpropanoate in CDCl3 ... 53
Figure 4.9 : The 1H NMR spectrum of bromo end-functionalized sillyl protected PS in CDCl3 ... 54
Figure 4.10: The 1H NMR spectrum of azide end-functionalized sillyl protected PS in CDCl3 ... 54
Figure 4.11: The 1H NMR spectrum of ABC type miktoarm star polymer in CDCl3 ... 56
Figure 4.12: The 1H NMR spectrum of alkyne functionalized ABC type miktoarm star polymer in CDCl3 ... 57
Figure 4.13: GPC chromatogram of the alkyne-PEG-PMMA, Si-p-PS-N3 and ABC type miktoarm star polymer ... 57
Figure 4.14: The 1H NMR spectrum of 2-azido ethanol in CDCl3 ... 58
Figure 4.15: The 1H NMR spectrum of prop-2-ynyl 3- (triethoxysilyl) propylcarbamate in CDCl3 ... 59
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Figure 4.16: TGA of Silica, azide, Alkyne-PS-PEG-PMMA and
Silica-Alkyne-PS-PEG-PMMA ... 62 Figure 4.17: The 1H NMR spectrum of silica-PS-PEG-PMMA in CDCl3 ... 62
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THE SURFACE MODIFICATION OF SILICA NANOPARTICLES VIA ABC TYPE MICTOARM STAR POLYMER
SUMMARY
In recent years surface modification of silica nanoparticles has attracted considerable attention. Silica nanoparticles could be used for a diversity of applications depending on their porosity and hardness. Mechanical, structural and thermal properties of particle and the polymer could be altered via chemical modification of a silica nanoparticle surface. Moreover, surface modification improves the stability of the compound.
Star polymers have attracted much attention in research over the years due to their unique-three dimensional shape and highly branched structure. There are two general strategies used to produce star polymers: the arm-first and core-first techniques. In the arm-first strategy, a polymer with a proper end-group functionality is reacted with an appropriate multifunctional core to give a star polymer. In the second strategy (core-first), the polymer chain is simultaneously grown from a multifunctional initiator. In recent years, the use of controlled/living radical polymerization techniques in the synthesis of star polymers has quickly increased because of the variety of applicable monomers and greater tolerance to experimental conditions in comparison with living ionic polymerization routes.
The development of controlled free radical polymerization is among the most important advances in polymer chemistry. Living polymerization techniques afford control over molecular weight distribution, architecture and functionalities of the resulting polymer. Among the CRP processes, atom transfer radical polymerization (ATRP), nitroxide mediated polymerization (NMP) and reversible addition fragmentation techniques (RAFT), are the most efficient methods for the synthesis of special polymers with complex architectures. Sharpless et al. Popularized the 1,3-dipolar cycloaddition of azides and terminal alkynes, catalyzed by copper(I), in organic synthesis. These reactions have a very high thermodynamic driving force which makes them one of the efficient reactions available. Such reactions were proven to be very practical because they can be performed in high yield, in multiple solvents (including water), and in the presence of many other functional groups. Both, ATRP and NMP methods based on the fast equilibrium between active and dormant chains, actually it is the main effect to obtain controlled structure. One of the advantageous of controlled radical polymerization techniques such as ATRP and NMP is that the molecular weight and the chain end functionality can be controlled. The wide range of functionality can be introduce into the polymer chain and this leads to the synthesis of well-defined copolymers by a sequential two-step or one pot method without any transformation or protection of initiating sites.
Recently, Huisgen type 1,3-dipolar cycloaddition reactions between azide and alkyne functionalities yielding 1,2,3-triazole derivatives which are conducted at elevated temperatures is an alternative route to carbon-carbon bond. In 2001, Sharpless and
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coworkers reported that the reaction between azide and alkyne functionalities in the presence of Cu(I)/Ligand as catalyst leaded 1,2,3-triazole derivatives with high yield and high regioselectivity. Because of their efficiency and simplicity, these cycloadditions were calssified as “click” reactions. Later, click chemistry strategy was successfully applied to macromolecular chemistry, affording polymeric materials varying from block copolymers to complex macromolecular structures. The surface modification of silica nanoparticles via click chemistry has acquired much attention in last years. There are several methods have been suggested to surface modification of silica nanoparticles in literature but none of them includes miktoarm star polymer. From this point of view, in this study was aimed to surface modification of silica nanoparticles via ABC type miktoarm star polymer.
In this work, first of all well-defined azide end functionalized poly(styrene) and alkyne functionalized poly(ethyleneglycol)-poly(methylmethacrylate) were obtained. Then, ABC type miktoarm star polymer synthesized via click chemistry. Finally, azide functionalized silica nanoparticles reacted with alkyne functionalized miktoarm star polymers by using click chemistry. The obtained products were characterized by Gel Permeation Chromatography (GPC), Nuclear Magnetic Resonance Spectroscopy (NMR) and Thermal Gravimetric Analysis (TGA).
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ABC TİPİ FARKLI KOLLU YILDIZ POLİMER İLE SİLİKA NANOPARTİKÜLLERİNİN YÜZEY MODİFİKASYONU
ÖZET
Son yıllarda, nanopartiküllerin yüzey modifikasyonu önemli derecede ilgi çekmektedir. Silika nanopartiküller, gözenek yapılarına ve sertliklerine göre değişik uygulamalarda kullanılabilmektedirler. Silika nonopartiküllerin modifikasyonu ile polimerin mekanik, yapısal ve termal özellikleri değiştirilebilmektedir. Bunun yanında, yüzey modifikasyonu bileşiğin kararlılığını da arttırmaktadır.
Yıldız polimerler araştırmalarda üç boyutlu ve çok dallanmış yapılarından dolayı yıllardır ilgi çekmektedirler. Yıldız polimerlerin elde edilmesinde kullanılan iki genel yöntem vardır: kol öncelikli ve çekirdek öncelikli yöntemleri. Kol öncelikli yönteminde, uygun uç grup fonksiyonalitesine sahip polimer ona uygun çok fonksiyonlu bir çekirdekle yıldız polimer elde etmek için reaksiyona sokulur. Ġkinci yöntemde (çekirdek öncelikli ) ise, polimer zinciri çok fonksiyonlu bir başlatıcıdan eşzamanlı bir şekilde büyümektedir.Son yıllarda, yaşayan iyonik polimerizasyon yöntemine kıyasla uygulanabilir monomerlerin çeşitliliğinin ve deneysel koşullardaki toleransın daha fazla olması gibi sebeplerden dolayı, yıldız polimerlerin sentezinde kontrollü/yaşayan polimerizasyon yönteminin kullanımı hızla artmaktadır.
Kontrollü serbest radikal polimerizasyonunun gelişimi polimer kimyasındaki en önemli ilerlemeler arasındadır. Yaşayan polimerizasyon teknikleri, polimerin moleküler kütle dağılımı, geometrik yapısı ve fonksiyonelliğini kontrol edebilmemizi sağlamaktadır. CRP işlemi sırasında, atom transfer radikal polimerizasyonu (ATRP), Nitroksit Ortamlı Radikal Polimerizasyonu (NMP) ve tersinir eklenme-parçalanma zincir transferi (RAFT) teknikleri kompleks geometrik yapıya sahip özel polimerleri sentezlemek için kullanılabilecek en etkili yöntemlerdir. Sharpless ve arkadaşları, azid ve uç gruptaki alkinlerin bakır(I) katalizli 1,3-dipolar siklokatılma reaksiyonlarını organik sentezlerde tercih edilir hale getirmişlerdir. Bu reaksiyonlar sahip oldukları yüksek termodinamik itme gücü sayesinde mümkün olan en etkili yöntemler olarak görülmektedir. Click reaksiyonları, yüksek verim ile, su dahil olmak üzere farklı çözücü kullanılabilmesine olanak tanıyarak ve birçok fonksiyonel yapının varlığında uygulanabilir olduğu için son derece pratik oldukları kanıtlanmıştır. ATRP ve NMP yöntemleri, aktif ve durağan zincirlerin üzerindeki hızlı dengeye dayanmaktadır, esasında bu kontrollü bir yapı elde etmenin temelidir. ATRP, NMP gibi kontrollü radikal polimerizasyonu tekniklerinin avantajlarından biri moleküler ağırlığın ve zincirin uç fonksiyonelliğinin kotrol edilebilir olmasıdır. Bu teknikler sayesinde polimer uç gruplarına çok çeşitli fonksiyonellikler kazandırılabilir bu da herhangi bir transformasyon reaksiyonu gerektirmeden iyi tanımlı polimerlerin eldesine izin verir.
Son dönemlerde, 1,2,3-triazole türevlerini veren azid ve alkin fonksiyonaliteleri arasındaki Huisgen tipi 1,3-dipolar siklokatılma reaksiyonları , yüksek sıcaklıklarda karbon-karbon bağının oluşması için alternatif bir yöntemdir. 2001 yılında, Sharpless
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ve arkadaşları azid ve alkin fonksiyonalitelerinin Cu(I)/Ligand katalizör varlığındaki reaksiyonunun yüksek verim ve alan-seçiciliği ile 1,2,3-triazole türevlerine yönlendiğini bildirmişlerdir. Verimliliğinden ve basitliğinden dolayı, bu siklo katılma reaksiyonları “click” reaksiyonları olarak sınıflandırılmışlardır. Daha sonra, click kimyası yöntemi blok copolimerlerden kompleks makromoleküler yapılara kadar değişen makromoleküler kimyaya başarılı bir şekilde uygulanmıştır.
Click kimyası ile yüzeye silika nanopartiküllerinin modifiye edilmesi son yıllarda oldukça dikkat çekmiştir. Yüzeye silika nanopartiküllerinin modifiyesi için birçok yöntem önerilse de, bu yöntemlerden hiçbiri farklı kollu yıldız polimeri içermemektedir. Bu bakış açısı ile, bu çalışma silika nanopartikül yüzeyinin ABC tipi farklı kollu yıldız polimer ile modifiye edilmesini amaçlamıştır.
Bu çalışmada, ilk olarak iyi tanımlanmış azid uç grup fonksiyonlu poli(stiren) ve alkin fonksiyonlu poli(etilenglikol)-poli(metilmetakrilat) elde edilmiştir. Daha sonra, click kimyası kullanılarak ABC tipi farklı kollu yıldız yıldız polimer sentezlenmiştir. Son olarak, azid fonksiyonlu silika nanopartiküller click kimyası reaksiyonu ile alkin fonksiyonlu farklı kollu yıldız yıldız polimerleri ile modifiye edilmiştir. Elde edilen ürünler, Jel Geçirkenlik Kromatografisi (GPC), Nükleer Manyetik Rezonans Spektroskopisi (NMR) ve Termal Gravimetrik Analiz (TGA) ile karakterize edilmişlerdir.
1 1. INTRODUCTION
Silica nanoparticles are used for a variety of applications depending on their porosity and hardness. However, the main challenge is to control the interparticle aggregation. Aggregation can be controlled by covalently grafting polymer chains onto the particle. The chemical modification of a silica nanoparticle surface with a polymer not only improves the stability but can also alter the mechanical, structural, and thermal properties of particle and the polymer [1]. Such hybrid organic–inorganic materials find a number of applications in optics and electronics [2]. Both „„grafting-to‟‟ and „„grafting-from‟‟ methods have been explored for the synthesis of hybrid nanomaterials from preformed silica nanoparticles. The grafting-to technique involves the chemical reaction of a reactive polymer end group to the surface [3]. A star structure is defined as a nonlinear polymer that consists of multiple backbone chains existing from junction points [4]. Star polymers show different crystalline, mechanical, and viscoelastic properties in comparison with their corresponding linear analogues. Interest in star polymers arises from their compact structure and globular shape, which predetermines their low viscosity when compared to linear analogues and makes them suitable materials for several applications.
Much of the academic and industrial research on living polymerization has focused on anionic, cationic, coordination, and ring-opening polymerizations. The development of controlled/living radical polymerization (CRP) methods has been a long-standing goal in polymer chemistry, as a radical process is more tolerant of functional groups and impurities and is the leading industrial method to produce polymers [5]. Synthesis of star polymers, which began in the 1950s with living anionic polymerization, has recently received increased attention due to the development of controlled/living radical polymerization (CRP) [6,7].
Typically, star polymers are synthesized via CRP by one of two strategies: core-first and arm-first. The arm-first strategy can be further subcategorized according to the procedure employed for star formation. One method is chain extension of a linear
2
arm precursor with a multivinyl crosslinking agent, and the other is coupling linear polymer chains with a multifunctional linking agent or “grafting-onto” a multifunctional core. Although both methods were successfully used for star synthesis in anionic polymerization, to date only the former procedure, using a multivinyl cross-linking agent, has been applied in CRP for synthesis of star polymers containing multiple arms and/or functionalities [8,9].
Atom transfer radical polymerization (ATRP) is a particularly attractive CRP process for synthesis of chain-endfunctionalized polymers [10,11]. The polymers produced by ATRP preserve terminal halogen atom(s) that can be successfully converted into various desired functional chain-end groups through appropriate transformations, especially nucleophilic substitutions. The modified chain-end group, such as a hydroxyl group or an amino group, cannot react with itself but can react with an appropriate functional group on the multifunctional coupling agent, such as a carboxylic acid group by esterification, to form a star polymer. However, a commonly encountered drawback, when using such a method, is a low yield of star products due to the slow and inefficient reactions between the modified polymer chain ends and multifunctional linking agents. In other words, highly efficient site-specific organic reactions are required in order for star synthesis to be highly successful [9].
In the past few years, “click reactions”, as termed by Sharpless et al. (2004), have gained a great deal of attention due to their high specificity, quantitative yields, and near-perfect fidelity in the presence of most functional groups. The most popular click chemistry reaction is the copper-catalyzed Huisgen dipolar cycloaddition reaction between an azide and an alkyne leading to 1,2,3-triazole [12-15].
Lutz et al. (2005) used click chemistry to prepare an end-functionalized ATRP polymer [16]. The bromine chain ends of the polymer were first transformed into an azide end group and subsequently reacted with terminal alkynes to create different functional end groups. van Hest et al. prepared terminal azide and alkyne functionalized ATRP polymers to modularly synthesize block copolymers [17]. Matyjaszewski et al. (2005) synthesized an alkyneterminated ATRP initiator to polymerize well-defined α-alkyne-ω-bromo-terminated polystyrene (PS). These polymers prepared by ATRP were coupled via a step growth mechanism using click coupling to yield PS containing triazole linkages in the repeat units [18].
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Matyjaszewski et al. also synthesized α,ω-dihydroxypolystyrene and star polymers via ATRP and click chemistry. Macrocyclic polymer, neoglycopolymer, and first-generation dendritic copolymers have also been synthesized by combination of ATRP and click chemistry [1, 19-20].
There are a number of reports on the synthesis of hybrid silica nanoparticles both by grafting-to and grafting-from techniques. Procedures for the grafting-from technique commonly involve covalent attachment of a suitable atom transfer radical polymerization (ATRP) initiator or reversible addition fragmentation transfer (RAFT) agent to silica. The grafting-to technique involves the chemical reaction of a reactive polymer end group to the surface [3]. However the grafting density, which controls the final properties of the hybrid nanomaterial, is low due to steric hindrance. This drawback is overcome by using a grafting-from technique, in which the polymer chain is grown from the surface through a covalently linked monomer [21] or an initiator [22].
1.1 Purpose of the Thesis
The target of the present work was to modify the silica nanoparticles‟ surface by the combination of ATRP and click reactions based on the arm-first method. For this purpose, at first PEG-macroinitiator, (6), was obtained. On the other hand, well defined azide-end-functionalized PS, (10), was synthesis and click reaction strategy was followed between (6) and (10). Consequently, ABC type miktoarm star polymer poly(styrene)-poly(ethylene glycol)-poly(methylmethacrylate), (12), in which alkyne functionality at the junction point was obtained. Then, azide functionality on the silica surface was obtained via simple sol-gel process. Finally, click reaction was performed between ABC miktoarm star polymers and azide containing silica nanoparticles in the presence of copper catalyst. Then, target compound was obtained successfully. The characterization and efficiency of all reactions has been investigated by Gel Permeation Chromatography (GPC), Nuclear Magnetic Resonance Spectroscopy (NMR) and Thermal Gravimetric Analysis (TGA) measurements.
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5 2. THEORETICAL PART
2.1 Conventional Free Radical Polymerization
Free radical polymerization has been an important technological area for seventy years. As a synthetic process it has enabled the production of materials that have enriched the lives of millions of people on a daily basis. Free radical polymerization was driven by technological progress, and its commercialization often preceded scientific understanding. Free radical polymerization is (relatively) easy to introduce on an industrial plant, it is compatible with water, and it could accommodate a wide variety of functional monomers [6].
As chain reactions, free radical polymerizations proceed via four distinct processes: Initiation: The first step in producing polymers by free radical
polymerization is initiation. This step begins when an initiator decomposes into free radicals in the presence of monomers. The initiator-derived free radicals that initiate polymerization are generated by thermal or photochemical homolytic cleavage of covalent bonds, or by a redox process. In this first step, a reactive site is formed, thereby “initiating” the polymerization.
Propagation: Primary radicals generated by the decomposition of initiator add to monomer to yield primary propagating radicals. This is followed by a succession of rapid propagation steps that proceed with high regioselectivity to form radical centers bearing a substituent. The reactive site is regenerated after each addition of monomer.
Transfer: Transfer occurs when an active site is transferred to an independent molecule such as monomer, initiator, polymer and solvent or transfer agent. This process results in both a terminated molecule (see step four) and a new active site that is capable of undergoing propagation.
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Termination: In this final step, eradication of active sites leads to “terminated,” or inert, macromolecules. Termination refers to the bimolecular reaction of propagating radicals by combination or disproportionation that leads to the deactivation of propagating radical chain ends.
The free radical chain process is demonstrated schematically below at figure 2.1: R· represents a free radical capable of initiating propagation; M denotes a molecule of monomer; Rm and Rn refer to propagating radical chains with degrees of polymerization of m and n, respectively; AB is a chain transfer agent; and Pn + Pm represent terminated macromolecules.
Figure 2.1 : General free radical polymerization mechanism.
Free-radical polymerization proceeds via a chain mechanism, which basically consists of four different types of reactions involving free radicals: (1) radical generation from nonradical species (initiation), (2) radical addition to a substituted alkene (propagation), (3) atom transfer and atom abstraction reactions (chain transfer and termination by disproportionation), and (4) radical–radical recombination reactions (termination by combination). It is clear that a good process and product control (design) requires a thorough knowledge of the respective rates of these reactions and preferably, knowledge about the physics governing these rates [23].
2.2. Conventional Living Polymerizations
Living polymerization was first defined by Szwarc (1956) as a chain growth process without chain breaking reactions (transfer and termination) [24]. Such a polymerization provides end-group control and enables the synthesis of block
7
copolymers by sequential monomer addition. However, it does not necessarily provide polymers with molecular weight (MW) control and narrow molecular weight distribution (MWD). Additional prerequisites to achieve these goals include that the initiator should be consumed at early stages of polymerization and that the exchange between species of various reactivities should be at least as fast as propagation [25-27]. It has been suggested to use a term controlled polymerization if these additional criteria are met [28]. This term was proposed for systems, which provide control of MW and MWD but in which chain breaking reactions continue to occur as in RP. However, the term controlled does not specify which features are controlled and which are not controlled. Another option would be to use the term „„living‟‟ polymerization (with quotation marks) or „„apparently living,‟‟ which could indicate a process of preparing well-defined polymers under conditions in which chain breaking reactions undoubtedly occur, as in radical polymerization.
LRP requires all chains to begin growing (reversibly via exchange processes) at essentially the same time and retain functionalities until the very end of the reaction. This is in contrast to RP, where all chains terminate and initiation is never completed, even when all monomer is consumed. Therefore, the three basic prerequisites for LRP are
Initiation should be completed at low monomer conversions.
Relatively low MW (DP < 1000) should be targeted to avoid transfer effects. This requires high concentration of growing chains, (e.g., > 10-2M for bulk polymerization).
Concentration of propagating radicals ([Po] < 10-7 M) should be sufficiently low to enable growth of chains to sufficiently high MW, before they terminate.
In addition, there is not a general mechanism of polymerization on which to base one‟s experiment: initiation may occur in some systems before complete dissociation of initiator. Knowledge of the initiating mechanism must be determined a priori to ensure a successful reaction. Despite the advantage of molecular control of living systems, the experimental rigor involved in ionic polymerization is often too costly for industrial use and free radical routes are preferred.
8
2.3. Controlled/ ‘‘Living” Free Radical Polymerizations
A special feature of LRP, and many other new living polymerization systems, such as carbocationic, ring opening, group transfer, and ligated anionic polymerization of acrylates, is the existence of equilibrium between active and dormant species [29]. The exchange between the active and dormant species allows slow but simultaneous growth of all chains while keeping the concentration of radicals low enough to minimize termination. This exchange also enables quantitative initiation necessary for building polymers with special architectures and functionalities, presently accessible in classic living polymerizations. The term controlled/living could also describe the essence of these systems [28].
Ideally, living systems lead to polymers with degrees of polymerization predetermined by the ratio of concentrations of consumed monomer to the introduced initiator DPn=Δ[M]/[I]0, with polydispersities close to Poisson distribution (DPw/DPn ≈ 1 + 1/DPn), and with complete end functionalization. Experimentally, the best way to evaluate such systems is to follow the kinetics of polymerization and the evolution of molecular weights, polydispersities, and functionalities with conversion.
Free radical processes have been recently developed which allow for both control over molar masses and for complex architectures. Such processes combine both radical techniques with living supports, permitting reversible termination of propagating radicals. In particular, three controlled free radical polymerizations have been well investigated. Each of these techniques is briefly presented below and all are based upon early work involving the use of initiator-transfer-agent-terminators to control irreversible chain termination of classical free radical process.
Living polymerization is defined as a polymerization that undergoes neither termination nor transfer. Polymer chains all grow at the same rate, decreasing the polydispersity. The propagating center at 100 % conversion still exists and can be further reacted, which can allow novel block, graft, star, or hyperbranched copolymers to be synthesized. Living polymerizations have been realized in anionic processes where transfer and termination are easy to suppress. Due to the favorable coupling of two radical propagating centers and various radical chain transfer reactions, the design and control of living radical processes is inherently a much more challenging task. The living process of radical polymerization involves the
9
equilibration of growing free radicals and various types of dormant species. By tying up a great deal of the reactive centers as dormant species, the concentration of free radicals decreases substantially and therefore suppresses the transfer and termination steps. These reactions are also denoted as controlled /living polymerizations rather than as true living polymerizations because transfer and termination are decreased but not eliminated. The methods at the forefront fall into one of three categories: nitroxide mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer (RAFT) [30].
2.3.1. Nitroxide-Mediated Living Free Radical (NMP)
Nitroxide–mediated living free radical polymerization (NMP) belongs to a much larger family of processes called stable free radical polymerizations. This pioneering work was one of the seminal contributions that provided the basis for the development of living free radical polymerization (LFRP), and it is interesting to note the similarity between the iniferter mechanism and the general outline of a living free-radical mechanism (Figure 2.2). In this general mechanism, the reversible termination of the growing polymeric chain is the key step for reducing the overall concentration of the propagating radical chain end. In the absence of other reactions leading to initiation of new polymer chains (i.e., no reaction of the mediating radical with the vinylic monomer), the concentration of reactive chain ends is extremely low, minimizing irreversible termination reactions, such as combination or disproportionation. All chains would be initiated only from the desired initiating species and growth should occur in a pseudoliving fashion, allowing a high degree of control over the entire polymerization process with well-defined polymers being obtained.
The identity of the mediating radical, X., is critical to the success of living free radical procedures and a variety of different persistent, or stabilized radicals have been employed [1,16,18,31]. However the most widely studied and certainly most successful class of compounds are the nitroxides and their associated alkylated derivatives, alkoxyamines. Interestingly, the development of nitroxides as mediators for radical polymerization stems from pioneering work by Solomon, Rizzardo, and Moad into the nature of standard free-radical initiation mechanisms and the desire to efficiently trap carbon-centered free radicals [32].
10
Figure 2.2 : The general outline of the free-radical mechanism
A NMP run can be done in two ways. In one way, polymerization is initiated with a model alkoxyamine like S-TEMPO and BS-TEMPO, which is prepared and purified independently. In the other way, the initiating alkoxyamine is prepared in situ [33]. Specifically, a conventional initiator such as benzoyl peroxide (BPO) is mixed with a nitroxyl like TEMPO in monomer in a suitable ratio, for instance, [TEMPO]/[BPO]=1.2, and the mixture is heated at a high temperature so that all BPO molecules decompose in a short time to produce adducts of the type B-Mn- TEMPO, where B and M denote the BPO fragment and the monomer moiety with n=1 or 2 in most cases, [the adduct B-TEMPO (n=0) is unlikely to be formed]. These adducts will work as an initiating alkoxyamine. For kinetic studies, the use of a purified model alkoxyamine is obviously preferable to avoid unnecessary complexities.
2.3.2. Atom Transfer Radical Polymerization (ATRP)
The name atom transfer radical polymerization (ATRP) comes from the atom transfer step, which is the key elementary reaction responsible for the uniform growth of the polymeric chains. ATRP originates in atom transfer radical addition (ATRA) reactions, which target the formation of 1 : 1 adducts of alkyl halides and alkenes, which are also catalyzed by transition metal complexes [34]. ATRA is a modification of the Kharasch addition reaction, which usually occurs in the presence
11
of light or conventional radical initiators [35]. Because of the involvement of transition metals in the activation and deactivation steps, chemo-, regio-, and stereoselectivities in ATRA and the Kharasch addition may be different.
ATRP has some links to transition-metal-catalyzed telomerization reactions [36]. These reactions, however, do not proceed with efficient exchange, which results in a nonlinear evolution of the molecular weights with conversions and polymers with high polydispersities. ATRP is also related to transition metal initiated redox processes and inhibition with transition metal compounds [37,38]. These two techniques allow for either an activation or deactivation process, however, without efficient reversibility.
ATRP was developed by designing a proper catalyst (transition metal compound and ligands), using an initiator with an appropriate structure, and adjusting the polymerization conditions, such that the molecular weights increased linearly with conversion and the polydispersities were typical of a living process [39-41]. This allowed for an unprecedented control over the chain topology (stars, combs, branched), the composition (block, gradient, alternating, statistical), and the end functionality for a large range of radically polymerizable monomers [41-46].
A general mechanism for ATRP is given below (Figure 2.3). The radicals, i.e., the propagating species Pn*, are generated through a reversible redox process catalyzed by a transition metal complex (activator, Mnt –Y=ligand, where Y may be another ligand or a counterion) which undergoes a one-electron oxidation with concomitant abstraction of a (pseudo)halogen atom, X, from a dormant species, Pn–X. Radicals react reversibly with the oxidized metal complexes, X–Mtn+1/ligand, the deactivator, to reform the dormant species and the activator. This process occurs with a rate constant of activation, ka, and deactivation kda, respectively. Polymer chains grow by the addition of the free radicals to monomers in a manner similar to a conventional radical polymerization, with the rate constant of propagation, kp. Termination reactions (kt) also occur in ATRP, mainly through radical coupling and disproportionation; however, in a well-controlled ATRP, no more than a few percent of the polymer chains undergo termination. Other side reactions may additionally limit the achievable molecular weights. Typically, no more than 5% of the total growing polymer chains terminate during the initial, short, nonstationary stage of the polymerization. This process generates oxidized metal complexes, the deactivators,
12
which behave as persistent radicals to reduce the stationary concentration of growing radicals and thereby minimize the contribution of termination at later stages [47]. A successful ATRP will have not only small contribution of terminated chains but also uniform growth of all the chains; this is accomplished through fast initiation and rapid reversible deactivation.
Figure 2.3 : General scheme of transition-metal-catalyzed ATRP.
As a multicomponent system, ATRP includes the monomer, an initiator with a transferable (pseudo)halogen, and a catalyst (composed of a transition metal species with any suitable ligand). Both activating and deactivating components of the catalytic system must be simultaneously present. Sometimes an additive is used. For a successful ATRP, other factors, such as solvent, temperature, concentrations and solubility of all components, and sometimes the order of their addition must be also taken into consideration.
2.3.2.1. Monomers
A variety of monomers have been successfully polymerized using ATRP. The most common monomers are methacrylates, acrylonitriles, styrenes, acrylates and (meth)acrylamides, which contain substituents that can stabilize the propagating radicals. Even under the same conditions using the same catalyst, each monomer has its own unique atom transfer equilibrium constant for its active and dormant species. In the absence of any side reactions other than radical termination by coupling or disproportionation, the magnitude of the equilibrium constant (Keq=kact/kdeact) determines the polymerization rate.
2.3.2.2. Initiators
The main role of the initiator is to determine the number of growing polymer chains. Two parameters are important for a successful ATRP initiating system. First, initiation should be fast in comparison with propagation. Second, the probability of the side reactions should be minimized. In ATRP, alkyl halides (RX) are typically
13
used as initiators. To obtain well-defined polymers with narrow molecular weight distributions, the halide group, X, should rapidly and selectively migrate between the growing chain and the transition metal complex. Thus far, when X is either bromine or chlorine, the molecular weight control is best. Iodine works well for acrylate polymerizations in copper-mediated ATRP [48] and has been found to lead to controlled polymerization of styrene in ruthenium and rhenium-based ATRP [49,50]. Some pseudohalogens, specifically thiocyanates and thiocarbamates, have been used successfully in the polymerization of acrylates and styrenes [48,51-52].
Initiation should be fast and quantitative with a good initiator and proper selection of group R. Any alkyl halide with activating substituents on the a-carbon, such as aryl, carbonyl, or allyl groups, can potentially be used as ATRP initiators, polyhalogenated compounds (e.g., CCl4 and CHCl3), and compounds with a weak R–X bond, such as N–X, S–X, and O–X, can also be used as ATRP initiators. When the initiating moiety is attached to a macromolecule, macroinitiators are formed, and can be used to synthesize block or graft copolymers [41]. However, the efficiency of block/graft copolymerization may be low if the apparent rate constant of cross-propagation is smaller than that of the subsequent homopolymerization.
Many different types of halogenated compounds are potential initiators and their different structures. For examples; halogenated alkanes, benzylic halides, α-haloesters, α-haloketones, α-halonitriles and sulfonyl halides are uses as initiators. 2.3.2.3. Catalysts
Perhaps the most important component of ATRP is the catalyst. It is the key to ATRP since it determines the position of the atom transfer equilibrium and the dynamics of exchange between the dormant and active species. There are several prerequisites for an efficient transition metal catalyst.
The metal center must have at least two readily accessible oxidation states separated by one electron.
The metal center should have reasonable affinity toward a halogen.
The coordination sphere around the metal should be expandable on oxidation to selectively accommodate a (pseudo)halogen.
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The position and dynamics of the ATRP equilibrium should be appropriate for the particular system.
The most important catalysts used in ATRP are; Cu(I)Cl, Cu(I)Br, NiBr2(PPh3)2, FeCl2(PPh3)2, RuCl2(PPh3)3/ Al(OR)3.
2.3.2.4. Ligands
The main roles of the ligand in ATRP is to solubilize the transition metal salt in the organic media and to adjust the redox potential and halogenophilicity of the metal center forming a complex with an appropriate reactivity and dynamics for the atom transfer [53,54]. The ligand should complex strongly with the transition metal. It should also allow expansion of the coordination sphere and should allow selective atom transfer without promoting other reactions.
The most widely used ligands for ATRP systems are the derivatives of 2,2 bipyridine and nitrogen based ligands such as N,N,N’,N’’,N’’ -pentamethyldiethylenetriamine (PMDETA), tetramethylethylenediamine (TMEDA), 1,14,7,10,10-hexamethyltriethylenetetraamine (HMTETA), tris[2-(dimethylamino) ethyl]amine (Me6-TREN) and alkylpyridylmethanimines.
15 2.3.2.5. Solvents
ATRP can be carried out either in bulk, in solution, or in a heterogeneous system (e.g., emulsion, suspension). Various solvents, such as benzene, toluene, anisole, diphenyl ether, ethyl acetate, acetone, dimethyl formamide (DMF), ethylene carbonate, alcohol, water, carbon dioxide, and many others, have been used in the polymerization of different monomers. A solvent is sometimes necessary, especially when the polymer is insoluble in its monomer (e.g., polyacrylonitrile). ATRP has been also successfully carried under heterogeneous conditions in (mini)emulsion, suspension, or dispersion. Several factors affect the solvent choice. Chain transfer to solvent should be minimal. In addition, potential interactions between solvent and the catalytic system should be considered. Catalyst poisoning by the solvent (e.g., carboxylic acids or phosphine in copper-based ATRP) and solvent-assisted side reactions, such as elimination of HX from polystyryl halides, which is more pronounced in a polar solvent, should be minimized [55,56].
2.3.2.6. Temperature and Reaction time
The rate of polymerization in ATRP increases with increasing temperature due to the increase of both the radical propagation rate constant and the atom transfer equilibrium constant. As a result of the higher activation energy for the radical propagation than for the radical termination, higher kp/kt ratios and better control („„livingness‟‟) may be observed at higher temperatures. However, chain transfer and other side reactions become more pronounced at elevated temperatures [56,57]. In general, the solubility of the catalyst increases at higher temperatures; however, catalyst decomposition may also occur with an increase in temperature [58,59]. The optimal temperature depends mostly on the monomer, the catalyst, and the targeted molecular weight.
2.3.2.7. Molecular weight and molecular weight distribution
The average molecular weight of the polymer can be determined by the ratio of consumed monomer and the initiator as in a typical living polymerization (DPn=∆[M]/[I]o , DP=degree of polymerization) while there is a narrow molecular weight distribution (1.0 < Mw/Mn < 1.5).
16
The molecular weight distribution or polydispersity Mw / Mn is the index of the polymer chain distribution. In a well-controlled polymerization, Mw / Mn is usually less than 1.1.
Polydispersity defined as;
Mw / Mn = 1 + [[RX]o kp / kd [D]] . [(2/p) – 1] (2.1) Where, D: Deactivator, kp: Propagation rate constant, kd: Deactivation rate constant, p: Monomer conversion
When a hundred percent of conversion is reached, in other words p=1, it can be concluded that;
Polydispersities (molecular weight distributions) decrease, if the catalyst deactivates the chains faster (smaller kp / kd )
For the smaller polymer chains, higher polydispersities are expected to obtain because the smaller chains include little activation-deactivation steps resulting in little control of the polymerization.
Polydispersities decrease as the concentration of the deactivator decreases. (For example, the addition of a small amount of Cu(II) halides in copper-based ATRP decreases the reaction rate thus leads to better controlled polymerizations)
2.3.3. Reversible-Addition Fragmentation Chain Transfer (RAFT)
RAFT polymerization has received increasing attention in recent years. Among available controlled free radical polymerization techniques, RAFT has arguably the most important commercial significance because it works with the greatest range of vinyl monomers and under a wide variety of experimental conditions [60].
The RAFT process is a versatile method for conferring living characteristics on radical polymerizations which provides unprecedented control over molecular weight, molecular weight distribution, composition and architecture [60-64]. It is suitable for most monomers polymerizable by radical polymerization and is robust under a wide range of reaction conditions. RAFT polymerizations of styrene were described in the first communication of RAFT polymerization in 1998 [65] and have been the subject of many subsequent papers. The mechanism of the RAFT process is
17
shown in Figure 2.6. Ideally, since radicals are neither formed nor destroyed as a consequence of the RAFT equilibria, they should not directly affect the rate of polymerization. RAFT agents can behave as ideal chain transfer agents [66-68].
Figure 2.5 : Mechanism of RAFT polymerization.
Reversible addition-fragmentation chain transfer (RAFT) is achieved by performing a free radical polymerization in the presence of dithio compounds, which act as efficient reversible addition-fragmentation chain transfer agents. Much like the first two routes, the rapid switching mechanism between dormant and active chain ends affords living polymerization character [69].
RAFT incorporates compounds, usually dithio derivatives, within the living polymerization that react with the propagating center to form a dormant intermediate. The dithio compound can release the alkyl group attached to the opposite sulfur atom which can then propagate with the monomer. The greatest advantage to RAFT is the incredible range of polymerizable monomers. As long as the monomer can undergo radical polymerization, the process will most likey be compatible with RAFT. However, there are many major drawbacks that arise when using this process. The dithio end groups left on the polymer give rise to toxicity, color, and odor and their removal or displacement requires radical chemistry. Also, the RAFT agents are
18
expensive and not commercially available. Another drawback is that the process requires an initiator, which can cause undesired end groups and produce too many new chains which can lead to increased termination rates [30].
2.4. Star Polymers
A star structure is defined as a nonlinear polymer, which consists of multiple arms existing from junction points [70]. Star polymers show different properties from the point of view: crystalline, mechanical, and viscoelastic properties compared to their linear counterparts. Miktoarm star polymers constitute at least one arm that is chemically different than others [71]. The miktoarm star polymers have been obtained from living ionic polymerization methods, mainly anionic polymerization until recently [72-74]. The types of miktoarm star polymers, A2B, A3B, A2B2, AnBn, ABC are the most common examples found in the literature. Other less common structures like AB5, AB2C2, and ABCD are now also available. However, more recently different methods have been reported for the preparation of miktoarm star polymers, ranging from the combinations of the controlled/living radical polymerization systems (atom transfer radical polymerization (ATRP), nitroxide-mediated radical polymerization (NMP), and reversible addition fragmentation chain transfer (RAFT) polymerization), to the combinations of them with the other living polymerization method (ring opening polymerization (ROP)) [75-84]. Therefore, these methods gave rise to a variety of miktoarm stars like AB2, A3B3, A(BC)2, and ABC. Star polymers have found applications in various areas like rheology modifiers, pressure sensitive adhesives, etc.
2.4.1. Stars by the Arm-First Method
This technique involves the synthesis of preformed arms, usually through living polymerization followed by reaction with a multifunctional linking agent [85-86]. Schematic representation of star formation by the “arm-first method” is shown in Figure 2.7.
19
where; X linear polymer chains
Y Y Y
multifunctional linking agent
Figure 2.6 : Scheme of star formation by the “arm-first method”.
Star formation by using the arm first technique also involves the use of divinylcoupling reagents such as divinylbenzene (DVB) as a multifunctional linking agent. Initially, a few units of the divinyl coupling reagents are added to the macroinitiator chain ends to form short block copolymers.
The block copolymers containing the divinyl units then start to react with each other to form cross-linked cores, and this leads to the formation of star polymers. Finally star-star coupling can occur, leading to the formation of higher molecular weight stars.
Coupling of monofunctional living chains with a difunctional reagent was first applied to living anionic polymerization. A similar approach has also been succesful with ATRP. There are several parameters in an ATRP that should be controlled carefully in order to maximize the yield of stars and prevent star-star coupling reactions. Some detailed studies have been carried out on the coupling of monofunctional polystyrenes and polyacrylates with (DVB) and di(meth)acrylates to prepare star polymers and the following guidelines have been developed:
The ratio of difunctional reagent to growing chains seems to be optimal in the range of 10-20
20
Monomer conversion (or reaction time) has to be carefully controlled and stopped before star-star coupling occurs.
Higher yields of stars are observed for polyacrylates than for polystyrenes. This may be attributed to a higher proportion of terminated chains in styrene polymerization.
The choice of the difunctional reagent is important and reactivity should be similar to, or lower than that of the arm-building monomers.
Halogen exchange slightly improves efficiency of star formation. Solvent, temperature, catalyst concentration should be also optimized.
Some of the recent studies on star synthesis by the arm-first method are described below: An original study based on the arm-first approach was reported by Fraser et al., who synthesized 2,2-bipyridyl- carrying PS and PMMA chains by ATRP, which they managed to chelate onto a hexadendate Fe(II)- based complex to form corresponding star-like polymers, thus containing a metallic core.
To derive their PS stars, Matyjaszewski et al. used a preformed PS macroinitiator obtained by ATRP that was allowed to react with various divinylic monomers, in the presence of Cu/Br dipyridyl in anisole at 110oC. A ratio of 5:15 between divinylbenzene and PS macroinitiator was found to be optimal for the star formation. Other experimental parameters such as the choice of solvent, the addition of Cu(II), and the reaction time were found to be crucial for the formation efficient star formation.
2.4.2. Stars by the Core-First Method
The core-first approach has come to maturity after it was shown in the1990s that stars of precise functionality could be obtained from multiionic initiators.
The core-first method involves the use of a multifunctional initiator, and the number of arms in the star polymer can be determined by the number of initiating sites on the initiator. In this technique multifunctional initiators are used to grow chains from a central core resulting in macromolecules with well-defined structures in terms of both arm number and length. Furthermore the reaction consists solely of stars in the absence of linear polymers.Most of the star polymers were prepared by this technique.
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The first report of the core-first technique described the hexakis (bromomethyl) benzene-initiated ATRP of styrene, methyl acrylate, and methylmethacrylate [44], but its use was rather limited due to poor solubility in the reaction media.
2.5. ABC terpolymers
In recent years ternary triblock terpolymers have attracted increasing interest owing to their rich variety of bulk morphologies [87].
Emerging technologies in medicine, microelectronics and optics require the availability of novel polymeric materials with ever more sophisticated properties and performances. Living and controlled/living polymerization methods have allowed for the synthesis of tailor-made macromolecules of varying chemical structure, composition, molecular characteristics and architecture. Among the different architectures, block extended is the work dedicated to the synthesis, solution and bulk properties of triblock terpolymers of the ABC type [88].
Linear ABC triblock terpolymers represent a relatively new class of polymeric materials with an increasing interest for their properties in the bulk and in solution. The three chemically different components of these materials, each placed in a separate block, can confer to the terpolymer three different functions. Another similar, but more novel, and equally interesting class of polymeric materials is that of ABC heteroarm or miktoarm star terpolymers, bearing three arms, each of which is a different homopolymer [89].
The presence of three different monomers placed in different blocks confers to these polymers, three rather than two functions [90]. It is well known that the addition of a third block leads to a much richer variety of phases (over 30 phases have been identified to date in bulk). These materials have the potential to generate a variety of well controlled multiphase microdomain structures with nanosized structural units in bulk and thin films and to provide supramolecular structures in solution with a mesoscopic length scale. Therefore, numerous applications such as multifunctional sensors, multiselective catalysts for sequential or simultaneous chemical reactions, separation membranes, filters, etc., are possible [88].
The purpose of this investigation was to further extend the synthetic work on three-component polymers and prepare a new structure of star terpolymers whose arms are
22
not different homopolymers but ABC triblock terpolymers. A combination of two hydrophilic and one hydrophobic monomers was chosen, leading to water-soluble, amphiphilic materials [89].
ABC triblock copolymers comprised mostly of diene-, styrene-, metacrylate-, or pyridine-based monomers have been studied extensively. These well-defined structures have elicited fascination not only for theoreticians modeling phase behavior but also in the physical realm for studying morphological transitions. The phase behavior of these systems is governed by the Flory interaction parameter between two domains, , and is strongly influenced by the weight fraction of the various blocks present in the copolymer. The morphological possibilities for these copolymers can range from a basic lamellar structure to highly complex core-shell gyroid morphology and even to a unique knitting pattern. Blending these types of copolymers definitely play a central role in polymer science.
Following the intense interest in the study of diblock and ABA triblock copolymers, the polymer community starts now to focus on a new type of block copolymers, that of ABC triblock copolymers comprising three blocks, each made of a different monomer repeat unit [90]. In bulk, four different ordered structures can be obtained (alternating lamellae, cylinders, body-centered cubic arrays of spheres and gyroid) depending on the copolymer composition and architecture. Considerably less
block copolymers with other copolymers enables additional manipulation of the morphological patterns. Until now, however, the monomers comprising the ABC triblock copolymers have been limited to those that can be polymerized either anionically or by group transfer polymerization. Recently, examples of inorganic/organic hybrid ABC triblock copolymers synthesized by combining living anionic ring-opening polymerization with atom transfer radical polymerization (ATRP) have been presented, in addition to ABC triblock copolymers synthesized wholly by ATRP or through reversible addition fragmentation chain transfer (RAFT). Kelly and Matyjaszewski demonstrated that ABC triblock copolymers of various chain architectures and monomer combinations can be successfully prepared using ATRP methods [91].
The key to the controlled synthesis of block copolymers in ATRP is to maintain high chain end functionality, i.e., limit termination and side reactions, and to balance the
23
reactivity of the end group with that of the monomer, i.e., avoid slow initiation. While the latter consideration is not as problematic as it is in anionic or carbocationic polymerizations and can be overcome through a careful choice of the block order, radical termination cannot be completely avoided due to the nature of the polymerization process. It can be limited, however, through the careful choice of the polymerization conditions and through adjustment of the equilibrium between the active and dormant species, often by adding a "persistent radical" in the form of a higher oxidation state metal. Kelly and Matyjaszewski‟s report focuses on the preparation of copolymers using these approaches to obtain well-defined multiblock copolymers. Several different catalyst systems, based predominantly on linear amine ligands, as well as different synthetic methodologies (i.e., the halogen exchange technique) were utilized to successfully prepare these copolymers [91].
Recently, the co-terpolymerization reactions, involving two or three monomers for the synthesis of synthetic polymers, have been commonly used. The properties of available polymers can also be changed by these reactions and novel polymers can be obtained by co-terpolymerization reactions. Thus, several useful terpolymers have been synthesized and used for various purposes [16].
Figure 2.7 : Schematic presentation of all possible arrangements for an ABC terpolymer. (a–c) Linear triblock terpolymer, ABC, BAC, CBA,
respectively (d). Miktoarm star terpolymer (e), Cyclic terpolymer (f– h). One of the chains is cyclic (starts and ends at the junction point) and the other two linear (i-k). One chain is linear and the two are cyclic (l). All chains are cyclic (o).
24
As an important illustration, interesting results have been recently obtained with SBM Nanostrengthw block terpolymers produced on an industrial scale. These triblocks copolymers combine polystyrene (PS), 1–4 polybutadiene (PBu) and polymethylmetacrylate (PMMA) segments. These engineering polymers can, for instance, be used as additives, allowing a much better solubility between incompatible commodity or technical plastics and fine tuning between toughness and stiffness of the host matrix. Detailed characterization of these new block copolymers obtained both by controlled radical polymerization and anionic polymerization represents a real challenge due to their increasing complexity [88].
2.6. Click Chemistry
The click chemistry as a modular approach was introduced by Sharpless, which has important features including high yields, functional group tolerance, and selectivity [92].
Although demand for new chemical materials and biologically active molecules continues to grow, chemists have hardly begun to explore the vast pool of potentially active compounds. The emerging field of “click chemistry,” a newly identified classification for a set of powerful and selective reactions that form heteroatom links, offers a unique approach to this problem [88]. “Click chemistry” is a term used to describe several classes of chemical transformations that share a number of important properties which include very high efficiency, in terms of both conversion and selectivity under very mild reaction conditions, and a simple work up. It works well in conjunction with structure based design and combinatorial chemistry techniques, and, through the choice of appropriate building blocks, can provide derivatives or mimics of „traditional‟ pharmacophores, drugs and natural products. However, the real power of click chemistry lies in its ability to generate novel structures that might not necessarily resemble known pharmacophores.
A concerted research effort in laboratories has yielded a set of extremely reliable processes for the synthesis of building blocks and compound libraries:
• Cycloaddition reactions, especially from the 1,3-dipolar family, but also hetero-Diels-Alder (DA) reactions.
