ISTANBUL TECHNICAL UNIVERSITY INSTITUTE OF SCIENCE AND TECHNOLOGY
M.Sc. Thesis by İnan KÜÇÜKKAYA
Department : Polymer Science and Technology Programme : Polymer Science and Technology
JANUARY 2010
THE SYNTHESIS OF NEW SULFONAMIDE BASED POLYMERIC SORBENTS FOR SELECTIVE REMOVAL OF MERCURY FROM WATER
ISTANBUL TECHNICAL UNIVERSITY INSTITUTE OF SCIENCE AND TECHNOLOGY
M.Sc. Thesis by İnan KÜÇÜKKAYA
(515001113)
Date of submission : 25 December 2009 Date of defence examination: 28 January 2010
Supervisor (Chairman) : Prof. Dr. B. Filiz ŞENKAL (ITU) Members of the Examining Committee : Assoc. Prof. Dr. Yeşim GÜRSEL( İTU)
Assoc. Prof. Dr. Reha YAVUZ (İTU)
JANUARY 2010
THE SYNTHESIS OF NEW SULFONAMIDE BASED POLYMERIC SORBENTS FOR SELECTIVE REMOVAL OF MERCURY FROM WATER
OCAK 2010
İSTANBUL TEKNİK ÜNİVERSİTESİ FEN BİLİMLERİ ENSTİTÜSÜ
YÜKSEK LİSANS TEZİ İnan KÜÇÜKKAYA
(515001113)
Tezin Enstitüye Verildiği Tarih : 25 Aralık 2009 Tezin Savunulduğu Tarih : 28 Ocak 2010
Tez Danışmanı : Prof. Dr. B. Filiz ŞENKAL (İTÜ) Diğer Jüri Üyeleri : Doç. Dr. Yeşim GÜRSEL (İTÜ)
Doç. Dr. Reha YAVUZ (İTÜ) SUDAN CİVANIN SEÇİMLİ OLARAK GİDERİLMESİ İÇİN YENİ
v
FOREWORD
First of all, I want to thank my advisor, Prof. Dr. Bahire Filiz ġenkal, for her guidance during my research and study at Istanbul Technical University. Her perpetual energy and enthusiasm in research had motivated me.
Res. Assist. Erdem Yavuz, whom I like to give my special thanks for giving me an opportunity to share his knowledge. Also thanks for his friendship.
If you are given second chance in this life, you should use it. Thanks to National Education Ministery for giving second chance to finish my master degree. It will be very useful in rest of my life.
I would like to give my special thanks to Esma AHLATÇIOĞLU for her help, understanding, physical and emotional support.
Finally, I would like to dedicate this thesis to my parents Ġzzet and Nurten KÜÇÜKKAYA, my sisters, Funda, ġeyma; and my brother, Kadir. I owe much to my family for all their self-sacrifice, patience and support during all my education and decisions.
January 2010 Ġnan KÜÇÜKKAYA
vii
TABLE OF CONTENTS
Page
ABBREVIATIONS ... ix
LIST OF TABLES ... xi
LIST OF FIGURES ... xiii
SUMMARY ... xv
ÖZET ... xix
1. INTRODUCTION ... 1
2. THEORETICAL PART ... 3
2.1 Properties and Characterization of Functionallized Polymers ... 3
2.2 Crosslinked Polymers and their Properties ... 6
2.2.1 Classification of grafted and/or crosslinked polymers ... 11
2.3 Mercury and its Harms ... 12
2.3.1 Releases in the environment ... 14
2.3.2 Toxicokinetics of mercury ... 15
2.4 Atom Transfer Radical Polymerization (ATRP) ... 16
2.4.1 Compenents of ATRP ... 21
2.4.2 Monomers ... 21
2.4.3 Initiators and halogen exchange ... 22
2.4.4 Solvents ... 22
2.4.5 Catalysts ... 23
2.4.6 Ligands ... 23
2.4.7 Additives ... 24
2.5 Modification of Polymers ... 25
2.5.1 Characteristics of polymer modification reactions ... 26
2.5.2 Typical chemical polymer modification reactions ... 27
2.5.2.1 Polymer modification reactions involving substitution ... 29
2.5.2.2 Polymer modification reactions involving structure formation ... 30
2.5.2.3 Polymer modification reactions involving degradation ... 31
2.5.2.4 Polymer modification reactions involving addition ... 32
2.5.2.5 Polymer modification reactions involving elimination... 32
2.5.2.6 Polymer modification reactions involving isomerisation ... 33
2.5.2.7 Polymer modification reactions involving exchange ... 33
3. EXPERIMENTAL ... 35
3.1 Materials and Method ... 35
3.2 Preparation of Polymeric Sorbents ... 35
3.2.1 Preparation of GMA-EGDMA copolymer beads (Resin 1) ... 35
3.2.1.1 Modification with ammonia ... 35
3.2.1.2 Determination of amine content ... 36
3.2.1.3 Sulfonamidation of the beads ... 36
3.2.1.4 Sulfonamide content determination ... 36
3.2.2 Preparation of chlorosulfonated poly(styrene) poly(styrene-DVB) beads (Resin 2) ... 36
3.2.2.1 Sulfonamidation of the beads ... 37
3.2.2.2 Preparaton of poly(glycidyl methacrylate) (PGMA) onto beads ... 37
3.2.2.3 Determination of epoxy content ... 38
3.2.2.4 Modification with ammonia ... 38
3.2.2.5 Determination of amine content ... 38
3.2.2.6 Reaction of crosslinked amine-containing beads with benzene sulfonyl chloride ... 38
3.2.2.7 Sulfonamide content determination ... 38
3.3 Mercury Uptake Experiments ... 39
3.4 Kinetics of Mercury Uptake ... 39
3.5 Sorption Tests for Foreign Ions ... 39
3.6 Regeneration of The Resins... 40
4. RESULT AND DISCUSSION ... 41
4.1 Preparation of Sulfonamide Modified Resin (Resin 1) ... 41
4.2 Preparation of Core-shell Structure Sulfonamide Modified Resin (Resin 2) .. 43
4.3 Sulfonamidation of PGMA Grafted Resin ... 47
4.4 Mercury Uptake ... 47
4.5 Kinetics of Mercury Sorption ... 49
4.6 Regeneration of the Resin ... 50
4. CONCLUSION ... 51
REFERENCES ... 53
ix
ABBREVIATIONS
IER : Ion Exchange Resin IEM : Ion Exchange Membrane
ATRA : Atom Transfer Radical Addition ATRP : Atom Transfer Radical Polymerization AIBN : Azobisisobutyronitrile
TMC ATRA : Transition Metal Catalyzed Atom Transfer Radical Addition MMA : Metylmethacrylate
PMDETA : Pentamethyldiethylenetriamine HMTETA : Hexamethyltriethylenetetramine GMA : Glycidyl Methacrylate
EGDMA : Ethylene glycol dimethacrylate EDTA : Ethylenediaminetetraacetic
xi
LIST OF TABLES
Page Table 4.1: Metal uptake characteristics of the resin 1 ... 48 Table 4.2: Metal uptake characteristics of the resin 2 ... 48 Table 4.3: Second order rate constants of the resins ... 50
xiii
LIST OF FIGURES
Page
Figure 2.1 : A. Shematic diagram of (I) physiosorption, (II) grafting to, (III) grafting from. B. Shematic diagram of (I) intermolecular crosslinking
and (II) intramolecular crosslinking ... 7
Figure 2.2 : The general reaction of ATRP ... 8
Figure 2.3 : Shematic presentation of surface grafting on cellulose via controlled living radical polymerization ... 9
Figure 2.4 : Synthesis 2-bromo isobutyrate functional nanoparticles and ps-b-p -Bza hybrid particles using microemulsion and ATRP processes ... 10
Figure 2.5 : A shematic representation for various classifications of grafted and/or crosslinked polymers ... 11
Figure 2.6 : A shematic representation for various types of grafted and/or crosslinked polymers ... 11
Figure 2.7 : An example of radical chain reactions –atom transfer radical addition. 17 Figure 2.8 : Propagation reaction in ATRA ... 18
Figure 2.9 : Transition metal catalyzed atom transfer radical addition ... 19
Figure 2.10: Atom transfer radical polymerization. ... 20
Figure 2.11: Representation of molecular weights and polydispersities with conversion for a living radical polymerization ... 21
Figure 2.12: Examples of ATRP Ligands ... 24
Figure 2.13: Different processes leading to the synthesis of polymers ... 27
Figure 2.14: The set of modification reactions ... 28
Figure 2.15 : The change in the structure of macromolecules as a result of the modification reaction ... 29
Figure 2.16: Schematic representation of a three dimensional polymer ... 31
Figure 2.17: Schematic representation of a possible branching. ... 31
Figure 2.18: Schematic representation of a regularly branched polymer. ... 31
Figure 2.19: An exchange reaction between two polyamides. Variation of the contents (c) of homopolymers (curve 1), block copolymers (curve 2) and statistical copolymers (curve 3) ... 34
Figure 4.1 : Preparation of Sulfonamide based resin (resin 1). ... 42
Figure 4.2 : FT-IR spectra of resin 1. ... 43
Figure 4.3 : Preparation of 2-chloroethyl sulfonamide resin ... 44
Figure 4.4 : Preparation of amine containing core-shell type resin ... 44
Figure 4.5 : Grafting degree – time plot of graft polymerization of glycidyl methacrylate ... 45
Figure 4.6 : FT-IR spectra of resin 2 ... 46
Figure 4.7 : Preparation of sulfonamide containing core-shell type resin (resin 2) .. 47
Figure 4.8 : The mercury uptakes of the resin ... 48
Figure 4.9 : Concentration-time plot of HgCl2 and Hg(CH3COO)2 solution for resin 1 ... 49
Figure 4.10: Concentration-time plot of HgCl2 and Hg(CH3COO)2 solution for resin 2 ... 50
xv
THE SYNTHESIS OF A NEW POLYMERIC SORBENT FOR SELECTIVE REMOVAL OF MERCURY FROM WATER
SUMMARY
In this thesis, two types polymeric sorbents were prepared for removal of mercury ions from aqueous solutions.
These resins were called Sulfonamide pendant crosslinked Glycidyl Methacrylate (GMA) (90%)- Ethylene glycol dimethacrylate (EGDMA) (10%) resin and core-shell type sulfonamide containing resin respectively.
In the present study, copolymer beads were prepared by the reaction of GMA (90 % mole)-EGDMA (10 % mole) with suspension polymerization method. A product 210– 420 µm in size was used in the modification reactions.
Analysis of the bead polymer sample by the pyridine-HCl method found an epoxy content of 4.0 mmol/g.
Reaction with excess ammonia produced an amine containing 3.6 mmol/g of amine functions.
The amine function resin was reacted with benzene sulfonyl chloride to obtain sulfonamide pendant resin.
O O O + O O O O O O O O ( ) ( O O O ) 0.1 0 .9 ( ) P O O P O O + NH3 PP O OH N H2 PP O OH N H2 + S O O Cl PP O OH N H O O S
Crosslinked polystyrene- DVB(10%) resin was chlorosulfonated by using chlorosulfonic acid. Chloride analysis (4.0 mmol g_1) of the first step product corresponds to 70% of chlorosulfonation of the styrenic units. Sulfonamide resin was obtained starting from the reaction of chlorosulfonated polystyrene resin and 2-chloroethyl amine. P P ClSO3H PP S O O Cl P P S O O Cl + HCl.H2N CH2CH2 Cl PP S O O NH Cl
Figure 2: Preparation of 2-chloroethyl sulfonamide resin
Graft polymerization of glycidyl methacrylate can be obtained from the 2-chloroethyl sulfonamide initiator groups on the spherical beads.
P P S O O NH Cl CuBr, Bipyridine 650 C GMA PP S O O NH CH2CH2 ( CH2 C )n CH3 C O CH2 CH2 O CH NH3 P P S O O NHCH2CH2 ( CH2 C )n CH3 C O CH2 CH2 O CH H NH2 O O
Figure 3: Preparation of amine containing core-shell type resin
In the polymerizations [CuBr]/[L] ratio was chosen as 1/2. The grafting degree reaches to 147.8% for 36 h. Epoxy content of the resin was determined as 4.5 mmol g_1. The amine containing resin (resin 2) was reacted with benzene sulfonyl chloride in the presence of NMP as solvent and triethylamine as base. The sulfonamide content of the resin was found as 2.1 mmol/ g resin.
xvii P P S O O NHCH2CH2 ( CH2 C )n CH3 C O CH2 CH2 O CH H NH2 S O O Cl P P S O O NHCH2CH2 ( CH2 C )n CH3 C O NH O O S (C2H5)3N O CH2 CH2 O CH H O
Figure 4: Preparation of sulfonamide containing core-shell type resin
The sulfonamide containing resins were an efficient sorbent to remove mercury from water. Based on the basic reaction of the mercuric ions with sulfonamide groups, which yielded covalent mercury–sulfonamide linkage. The loading experiments indicated a mercury capacity of about 1.34-1.62 mmol/g.
Table 1: Metal uptake characteristics of the sulfonamide pendant crosslinked GMA-EGDMA resin
Metal ion initial concentration [M] Resin capacity (mmol / g.resin) HgCl2 0.074 1.39 Hg (CH3COO)2 0.063 1.62 Cd(II) 0.065 0.027 Mg(II) 0.16 0 Fe(III) 0.05 0.0015
Table 2: Metal uptake characteristics of the sulfonamide containing core-shell type resin
Metal ion initial concentration [M] Resin capacity (mmol / g.resin) HgCl2 0.074 1.52 Hg (CH3COO)2 0.063 1.34 Cd(II) 0.065 0.025 Mg(II) 0.16 0 Fe(III) 0.05 0.008
Sytnhesized polymer resins are efficient sorbents for removal mercury through the sulfonamide groups.
Also, mercury uptake kinetic measurement studies for these resins were performed. To investigate the efficiency of the resins in the presence of trace quantities, we performed batch kinetic sorption experiments with highly diluted HgCl2 and Hg(CH3COO)2 solutions. The sorbed mercury in these resins can be eluted by repeated treatment with HCl without hydrolysis of the sulfonamide groups.
xix
SUDAN CİVANIN SEÇİMLİ OLARAK GİDERİLMESİ İÇİN YENİ POLİMER ESASLI ESASLI RECİNELERİN SENTEZİ
ÖZET
Bu tezde ağır metal olan civanın sulu çözeltilerden giderilmesi için iki polimerik sorbent hazırlanmıĢtır.
Sırasıyla sülfonamit esaslı çapraz bağlı Glisidil Metakrilat (GMA) (90%)-Etilen glikol dimetakrilat (EGDMA) (10%) reçine; ve saçak tipli sülfonamit içeren reçineler olarak adlandırılmıĢlardır.
Bu çalıĢmada, GMA (molce %90) ve EDGMA (molce %10) kopolimer tanecikleri süspansiyon polimerizasyonu yöntemi kullanılarak hazırlanmıĢtır. 210 – 420 mm büyüklükteki ürün tanecikleri, modifikasyon reaksiyonlarında kullanılmıĢtır.
Polimerik kürenin piridin-HCl ile analizi sonucu epoksit miktarı 4.0 mmol/g bulunmuĢtur.
Amonyak fazlasıyla reaksiyon sonucu 3.6 mmol/g amine içeren fonksiyonel amin içeren polimer, fonksiyonel amin grubu içeren reçine benzen sülfonülklorür ile etkileĢtirilerek sülfonamitli reçine elde edilmiĢtir.
O O O + O O O O O O O O ( ) ( O O O ) 0.1 0 .9 ( ) P O O P O O + NH3 PP O OH N H2 PP O OH N H2 + S O O Cl PP O OH N H O O S
Çapraz bağlı polistiren-divinilbenzen(DVB) reçinesi klorsülfonik asit kullanılarak klorsülfonlanmıĢtır. Ġlk aĢama ürününün klorür analizi (4.0 mmolg-1), stiren birimlerinin %70’nin klorsülfonlandığını göstermektedir. Sülfonamit reçine klorsülfonlanmıĢ stiren ile 2-kloretilaminin reaksiyonuyla elde edilmiĢtir.
P P ClSO3H PP S O O Cl P P S O O Cl + H2N CH 2CH2 Cl HCl. PP S O O NH Cl
Şekil 2: 2-kloroetil sülfonamit reçine hazırlanıĢı
GMA’nın graft polimerizasyonu küresel tanelerdeki baĢlatıcı grup, 2-kloroetil sülfonamitten elde edilmiĢtir.
P P S O O NH Cl CuBr, Bipyridine 650 C GMA PP S O O NH CH2CH2 ( CH2 C )n CH3 C O CH2 CH2 O CH NH3 P P S O O NHCH2CH2 ( CH2 C )n CH3 C O CH2 CH2 O CH H NH2 O O
Şekil 3: Amin içeren çekirdek-kabuk tipli reçine
Polimerizasyonda [CuBr]/[L] oranı 1/2 olarak seçilmiĢtir. Graft derecesi 36 saatte, %147,8 e ulaĢmıĢtır. Reçinedeki epoksit miktarı ise 4.5 mmol.gr-1
bulunmuĢtur. Amin içeren reçine, çözücü olarak NMP ve baz olarak ise trietilamin varlığında benzensülfonilklorür ile reaksiyona sokulmuĢtur. Reçinedeki sülfonamit miktarı 2.1 mmol/gr bulunmuĢtur
xxi P P S O O NHCH2CH2 ( CH2 C )n CH3 C O CH2 CH2 O CH H NH2 S O O Cl P P S O O NHCH2CH2 ( CH2 C )n CH3 C O NH O O S (C2H5)3N O CH2 CH2 O CH H O
Şekil 4: Sülfonamit içeren çekirdek-kabuk tipli reçinenin sentezi
Sülfonamit içeren reçineler sudaki civanın giderilmesinde etkili reçinelerdir. Civa iyonunun sulfonamid gruplarla olan temel reaksiyon, kovalent bağlı civa sulfonamit bağı meydan getirmeleredir. Yükleme deneyleri reçinelerin civa tutma kapasitelerinin 1.34-1.62 mmol/g civarında olduğunu göstermiĢtir.
Tablo 1: Sülfonamit saçaklı çapraz bağlı GMA-EDGMA reçinenin metal tutma karakteristikleri
Metal iyon BaĢlangıç
konsantrasyonu [M] Reçine kapasitesi (mmol / g.reçine) HgCl2 0.074 1.39 Hg (CH3COO)2 0.063 1.62 Cd(II) 0.065 0.027 Mg(II) 0.16 0 Fe(III) 0.05 0.0015
Tablo 2: Sülfonamit içeren çekirdek-kabuk tipli reçinenin metal tutma karakteristikleri
Metal ion BaĢlangıç konsantrasyonu [M] Reçine kapasitesi (mmol / g.reçine) HgCl2 0.074 1.52 Hg (CH3COO)2 0.063 1.34 Cd(II) 0.065 0.025 Mg(II) 0.16 0 Fe(III) 0.05 0.008
Sentezlenen bu reçineler, sülfonamit gruplarından dolayı civanın sudan giderilmesinde çok etkindirler.
Ayrıca reçineler için civa kinetik çalıĢmalarıda çok seyreltik civa çözeltileriyle yapılmıĢtır. Reçinelerdeki yüklü civanın salınımı tekrarlanan HCl etkileĢimiyle sülfonamit gruplarının hidrolize olmaksızın gerçekleĢtirilmiĢtir..
1
1. INTRODUCTION
Removal of inorganic pollutants from wastewater is a tedious process compared to the removal of organics because most of the latter can be removed relatively simply by activated carbons without much regard to their origin. Inorganic pollutants, however, need to use sorbents with ligating groups able to bind to them by forming chemical bonds. Ion exchange resins have been widely used in the removal of inorganics. On the other hand, the selective and quantitative separation of metal ions related to water pollution problems has received increasing importance in recent years. Mercury is used in a wide variety of industries such as electrical paints, fungicides, chlor-alkali, paper and pulp, pharmaceutical, etc.. Because of the high toxicity of all mercury compounds, the extraction of mercuric ions from aqueous wastes and drinking water are special environmental importance. One estimate of the total annual global input of mercury to the atmosphere from all sources including natural, anthropogenic, and oceanic emissions is 5500 tons.
Two common ligand types, sulfur and amide are being used currently in the design of polymer sorbents for binding mercuric ions selectively. Amide containing polymers include an iminodiacetamide, a dipridylamide and polythiourea on charcoal are another important for binding highly selective Hg(II). Amide groups form covalent mercury–amide linkages under ordinary conditions.
In this study, new polymeric resins with sulfonamide functions were prepared for the selective extraction of mercuric ions. Mercury sorption characteristics and regeneration conditions of the resulting resins have been studied. Affinity of the resins for other metal ions such as Cd(II), Mg(II), and Fe(III) was also investigated.
3 2. THEORETICAL PART
2.1. Properties and Characterization of Functionalized Polymers
Since the first generation of ion-exchange resins and membranes, the development of functional polymer chemistry and technology has made remarkable progress in recent years. For example, studies related to the preparation and design of several enzyme and nucleic acid models have advanced dramatically, and the development of the technology necessary to use these polymers is of current interest.
Functional polymers, in a broad sense, include a variety of polymeric materials and a number of engineering plastics. These polymer systems often exhibit more specific and better properties if processed as polymer aggregates. For example, organic polymers with polyconjugated double bonds consisting of special structures are known as synthetic metals, which show substantially high electron conductivity in a fiber or film form. On the other hand, ceramic materials with new properties, such as elasticity, have only recently prepared by organic synthetic techniques.
Recently, microporous polymeric materials as well as microcapsules have become of interest in a variety of industrial fields, not only in the general chemical industry, but also in the pharmaceutical, biomedical and electronics industries. For example, fluorine-containing resins are important, particularly, as a safe and durable antithrombogenic biomaterial. Microporous membranes made of vinyl polymers are being applied as separators or filters to concentrate oxygen from air and to manufacture ultrahigh grades of water for the semiconductor industry. Other types of microporous vinyl polymers are being used as highly hydrophilic materials in the fields of cosmetics and environmental hygiene.
The science and technology required for the preparation of microcapsules from different natural and synthetic polymeric materials has made rapid progress. They are being used in various fields for their ability to solidify liquids; to isolate reactive compounds; to remove color, odor, and toxicity; as well as to regulate and control
the release of included compounds. The immobilization of enzymes and the development of polymeric drugs are also playing an important role.
In addition, highly water-absorbing and oil-absorbing resins are of interest. These have developed rapidly in recent years by unique grafting and crosslinking of hydrophilic polymers. Transparent polymeric material with optical functions are also noteworthy. Some are biocompatible, such as poly(2-hydroxethyl methacrylate), which serves as a material for soft contact lenses. Plastic optical fibers are also widely used as substitutes for glass and quartz devices in various fields of technology, especially the biomedical and communication sciences.
The chemistry of so-called electronic functional polymers, in a narrow sense, has developed into a very exciting subject, particularly in the last 10 years. Some of the most attractive materials in this field are the photosensitive and photoresponsive polymers. By using these phenomena, specifically designed polymers undergo reversible crosslinking reactions to become insoluble or soluble. A variety of both negative and positive types of photoresists are being produced. They are initially used in printing, paint, and color industries. The technology to exploit deep ultraviolet (UV) radiation resist with reversible functionality will be one of the most important developments in this industry in the near future. Other subjects of interest in this field, which are under development, are the electronic or X-Ray sensitive resists, as well as the design of more functional photomemory materials.
In connection with biomedical polymers, the chemistry of the polymeric drugs is under continuous advancement. The most effective anticarcinogenic reagent are now targeted by the design of specifically functionalized polymers. The functional polymeric composites are also particularly attractive as implant materials [1].
5
We studied crosslinking polymers in this thesis. The use of crosslinked polymers in chemical applications is associated with some advantages, such as the following. 1-) Since they are in soluble in our solvents, they offer the greatest is of processing. 2-) They can be prepared in the form of spherical beads and can be separated from low molecular weight contamined by simple filtration and washing with very use solvents.
3-) Polymer beads with very low degrees of crosslinking swell extensively, exposing their inner reactive groups to the soluble reagents.
4-) More highly crosslinked resins may be prepared with very porose structures which allow solvents and reagents to penetrate inside of the beads to contact reactive groups.
The following is a classification of the types of crosslinked polymers which are most frequently encouraged with enhanced properties.
a) microporose pore gel-type resins are generally prepared by suspension polymerization using a mixture of vinyl monomer and small amounts( less tan 10% ; in most cases less than 0.5% - 2% ) of a crosslinking agent containing no additional solvent.
Swellable polymers are found to offer advantage over non-swellable polymers of particular interest is their lower fragility, lower sensitivity
b) Macropores and macroreticular resins
The mechanical requirements in industrial applications force the use of higher crosslinking densities for preparing density with enhanced properties. Macropores and macroreticular resins are also prepared by suspension polymerization using higher amounts of crosslinking agents but with the inclusion of an inert solvent as diluents for the monomer phase.
Macroreticular resin is non-swelling and a macro pores a rigid material with a high crosslinking it retains its overall shapes and volume when the precipitate is removed. To sudden shock and their potential to achieve a higher leading capacity during functionalization however, a degree in crosslinking density will increase swelling but will also result in soft gels which generally have low mechanical stability and readily
in fragment even under careful handling. Gels with lower density of crosslinking are difficult to filter and under sever conditions can degrade to produce soluble linear fragments in addition gel type resins that are likely crosslinked may suffer considerable mechanical damage as a result of rubit and extreme change in the nature of the solvating media and can not be subjected to study and high pressures. Macropores resins with less than % 1 crosslinking generally have low mechanical stability while macropores resins with more than % 8 crosslinking are mechanically stable but unfortunately give rise to acute.
2.2. Crosslinked Polymers and Their Properties
Physiosorption, grafting, and crosslinking are the techniques by which the association of monomers and polymers is described. The term physiosorption signifies that it is related to physical attractive forces. The process is a reversible one and is achieved by the end functionalized polymers on to the solid surface or self - assembly of polymeric surfactants, where grafting can be described as the covalent attachment process and irreversible. Grafting can be accomplished by either grafting to or grafting from approaches. In grafting to approaches, functionalized monomers react with the backbone polymer to form the grafted one. On the other hand, grafting from is achieved by treating a substrate with some method to generate immobilized initiators followed by polymerization. High grafting density polymer also can be accomplished using this technique. The schematic presentation of all the processes is depicted in Figure 2.1 A. The crosslinking is the association of polymers through a chemical bond. In most cases, the crosslinking is irreversible. It may be intra - and intermolecular (Figure 2.1 B).
The grafting and crosslinking reactions can be performed by different pathways such as chemical or radiation. There are lots of important modes of reactions are known,
7
Figure 2.1: A. Shematic diagram of (I) physiosorption, (II) grafting to, (III) grafting from. B. Shematic diagram of (I) intermolecular crosslinking and (II) intramolecular crosslinking.
In living radical formation technique, the chain termination step is avoided in two ways: either by increasing the rate of initiation with respect to the rate of propagation or by eliminating chain termination and transfer reactions. The polymer chains grow at a more constant rate, and their chain lengths remain very similar. Thus, it provides polymers that are able to grow whenever an additional monomer is supplied. The conditions are discovered for various types of reactions, e.g., atom transfer, nitroxide mediated, degenerative transfer. Features are given below;
• It proceeds until all monomers have been consumed. Further addition of a monomer results in continued polymerization.
• In these reactions Ri Rp , where, as for the conventional free – radical grafting, RiRp (Ri and Rp are the rate of initiation and propagation).
• Dynamic equilibrium occurs between a propagating radical and dormant species. • Mono and/or narrow dispersities of the products are possible.
• Predetermination of molar mass of the grafted/crosslinked polymer molecules is possible.
Through Atom Transfer, the key molecule is the one that contains halogen, positioned in the α-position with carbonyl, alkoxy - carbonyl, cyano, and phenyl groups. The reversible pseudohalogen homolytic transfer between a dormant species, an added initiator, or the propagating dormant chain end, ( I-X) and a transition metal complex in the lower oxidation state (Mn+ /L), occurs to form the propagating radicals (I* ) and the metal complex in the higher oxidation state with a coordinated halide ligand [X - M(n+1)+/L]. Activation, deactivation, and propagation, as well as termination steps, are present but as the persistent radical effect (PRE), the radical termination is diminished, and the equilibrium is strongly shifted toward the dormant species [3].
Figure 2.2: The general reaction of ATRP
Mn - transition metal, L - complexing ligand, Polymer chain, X = Br or Cl Basic features of ATRP is given below;
• Scope for choosing both the initiator and catalyst complex. • It can occur in solution as well as suspensions.
Polymers prepared by other processes can be functionalized at the terminus or along the backbone and incorporated into an ATRP as a macroinitiator/macromonomer, or simultaneously through use of both a macroinitiator and a macromonomer to
9 Beside there are some limitations like that; • Selection of suitable ligands is critical.
• The reactions preferably conduct in the absence of oxygen, or otherwise some reducing agents [e.g., Cu (0), Sn (EH 2 ), or ascorbic acid] are essential.
Figure 2.3: Shematic presentation of surface grafting on cellulose via controlled living radical polymerization.
Figure 2.4: Synthesis 2-bromo isobutyrate functional nanoparticles and ps-b-p-Bza hybrid particles using microemulsion and ATRP processes.
Grafted and/or crosslinked polymers obtained by the modification of polymers are attractive materials because of their new or enhanced properties compared to corresponding conventional polymers. These new or enhanced properties arise principally from the introduction of new functionalities and/or interlinkages to the original polymers structures by grafting and/or crosslinking reactions. Because their chemical and physical properties may be tailored over a wide range of characteristics, the use of grafted and/or crosslinked polymers has found a permanent place in various fields. This includes separation processes [12,13], energy conversion[14-17], solid state[14], biomedical [18], biological [19-20], and environmental applications[21, 22]. Among all polymers, the contribution of grafted
11
2.2.1 Classification of grafted and/or crosslinked polymers
Grafted and/or crosslinked polymers are available in different forms that vary in their classifications as shown in Figure 2.5. These polymers can be found in various physical forms including gels, resins, fibers, films, and fabrics tha thave widely differing chemical and physical properties and can be chemically active. The majority of these forms are of synthetic origin; i.e., they are made of modified synthetic polymers such as polyethylene, polystyrene, and polyvinyl fluoride, while some of them are obtained from modified natural polymer sources including chitosan, starch, and cellulose. Considering the separation function, grafted and/or crosslinked polymers can be classified into various categories including ion exchangers, chelating adsorbents, hydrogels, and affinity polymers, as shown in Figure 2.6.
Figure 2.5: A shematic representation for various classifications of grafted and/or crosslinked polymers.
Figure 2.6: A shematic representation for various types of grafted and/or crosslinked polymers.
Ion exchange resin (IER) is a network polymer of high molecular weight having fixed ionic groups. Like their membrane counterparts, these resins are capable of exchanging counterions (cations or anions) with the ionic components of a solution with two types of resins that are commercially available: cation and anion exchange. Each resin has a distinct number of mobile ionic sites that set the maximum capcity of exchanges per unit weight of resins. IERs are also available in a strongly or weakly acid form and a strongly or weakly basic form. In addition, IERs are also available in two physical forms: macroporous resins and gel resins. The macroporous resins have major advantages arising from their high degree of crosslinking, which lead to more stability and permanent pore diameter with uniform distribution.
In principle, there is a fundamental difference between the operation mode of IERs and their IEMs counterparts. IERs are used in a flow - by mode whereas IEMs are used in a flow - through mode. Therefore, kinetics of the ion exchange for IEMs is highly favourable relative to IERs, which heavily rely on the diffusion to achieve ion capture in their pores.
Ion exchange resins have found widespread applications in various fields including water and waste water treatment; oil and gas, electronics, electroplating, and tanning industries; sugar decolourizations; starch hydrolysated deionization; acid catalysis of sucrose; steam flooding; and the improvement of soil water retention[24,25].
2.3 Mercury and its harms
Mercury is a naturally occurring element that is found in air, water and soil. It exists in several forms: elemental or metallic mercury, inorganic mercury compounds, and organic mercury compounds. Elemental or metallic mercury is a shiny, silver-white metal and is liquid at room temperature. It is used in thermometers, fluorescent light bulbs and some electrical switches. When dropped, elemental mercury breaks into
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Inorganic mercury compounds have been included in products such as fungicides, antiseptics or disinfectants. Some skin lightening and freckle creams, as well as some traditional medicines, can contain mercury compounds.
Organic mercury compounds, such as methylmercury, are formed when mercury combines with carbon. Microscopic organisms convert inorganic mercury into methylmercury, which is the most common organic mercury compound found in the environment. Methylmercury accumulates up the food chain [26].
Mercury is a naturally occurring element that can be found throughout the environment. Human activities, such as burning coal and using mercury to manufacture certain products, have increased the amount of mercury in many parts of the environment including the atmosphere, lakes and streams. People and animals are exposed to mercury by eating organisms that live in places where microbes have converted some of the natural and human mercury to a more toxic form, methylmercury [26].
In household products, where elemental mercury generally is contained in glass or metal, it does not pose a risk unless the product is damaged or broken and mercury vapors are released. At room temperature, some uncontained mercury can evaporate and become an invisible, odorless toxic vapor. At higher temperatures, these concentrations increase. Very small amounts of elemental mercury (even a few drops) can raise air concentrations of mercury to harmful levels particularly in poorly-ventilated spaces. The longer people breathe the contaminated air, the greater the risk to their health. At high exposures, through inhalation, elemental mercury vapors can produce severe lung, gastrointestinal, and nervous system damage.
Excessive exposure to inorganic and organic mercury compounds can result from misuse or overuse of mercury-containing products. Exposure to mercury compounds is primarily through ingestion, but can occur through other pathways. Organic mercury compounds are more readily absorbed through the gastrointestinal tract and skin than are inorganic compounds. High exposures to mercury compounds can damage the gastrointestinal tract, the nervous system, and the kidneys [26].
Mercury dissolves to form amalgams with gold, zinc and many other metals. Because iron is an exception, iron flasks have been traditionally used to trade mercury. Other metals that do not form amalgams with mercury include tantalum,
tungsten and platinum. When heated, mercury also reacts with oxygen in air to form mercury oxide, which then can be decomposed by further heating to higher temperatures [27].
Since it is below hydrogen in the reactivity series of metals, mercury does not react with most acids, such as dilute sulfuric acid, though oxidizing acids such as concentrated sulfuric acid and nitric acid or aqua regia dissolve it to give sulfate, nitrate, and chloride salts. Like silver, mercury reacts with atmospheric hydrogen sulfide. Mercury even reacts with solid sulfur flakes, which are used in mercury spill kits to absorb mercury vapors (spill kits also use activated charcoal and powdered zinc) [27].
Some important mercury salts include:
Mercury(I) chloride (calomel) is sometimes still used in medicine, acousto-optical filters and as a standard in electrochemistry [28].
Mercury(II) chloride is a very corrosive, easily sublimating and poisonous substance [29].
Mercury fulminate, (a detonator widely used in explosives) [29].
Mercury(II) oxide, the main oxide of mercury.
Mercury(II) sulfide (found naturally as the ore cinnabar, or vermilion which is a high-grade paint pigment) [29].
Mercury(II) selenide, Mercury(II) telluride, Mercury cadmium telluride and mercury zinc telluride are semiconductors and infrared detector materials [30].
2.3.1 Releases in the environment
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65% from stationary combustion, of which coal-fired power plants are the largest aggregate source (40% of U.S. mercury emissions in 1999). This includes power plants fueled with gas where the mercury has not been removed. Emissions from coal combustion are between one and two orders of magnitude higher than emissions from oil combustion, depending on the country [32].
11% from gold production. The three largest point sources for mercury emissions in the U.S. are the three largest gold mines. Hydrogeochemical release of mercury from gold-mine tailings has been accounted as a significant source of atmospheric mercury in eastern Canada [35].
6.8% from non-ferrous metal production, typically smelters.
6.4% from cement production.
3.0% from waste disposal, including municipal and hazardous waste, crematoria, and sewage sludge incineration. This is a significant underestimate due to limited information, and is likely to be off by a factor of two to five.
3.0% from caustic soda production.
1.4% from pig iron and steel production.
1.1% from mercury production, mainly for batteries.
2.0% from other sources [36]. 2.3.2 Toxicokinetics of mercury
The toxicokinetics (i.e., absorption, distribution, metabolism, and excretion) of mercury is highly dependent on the form of mercury to which a receptor has been exposed. The absorption of elemental mercury vapor occurs rapidly through the lungs, but it is poorly absorbed from the gastrointestinal tract. Once absorbed, elemental mercury is readily distributed throughout the body; it crosses both placental and blood-brain barriers. The distribution of absorbed elemental mercury is limited primarily by the oxidation of elemental mercury to the mercuric ion as the mercuric ion has a limited ability to cross the placental and blood-brain barriers. Once elemental mercury crosses these barriers and is oxidized to the mercuric ion,
return to the general circulation is impeded, and mercury can be retained in brain tissue. Elemental mercury is eliminated from the body via urine, feces, exhaled air, sweat, and saliva. The pattern of excretion changes depending upon the extent the elemental mercury has been oxidized to mercuric mercury.
Absorption of inorganic mercury through the gastrointestinal tract varies with the particular mercuric salt involved; absorption decreases with decreasing solubility. Estimates of the percentage of inorganic mercury that is absorbed vary; as much as 20% may be absorbed. Inorganic mercury has a reduced capacity for penetrating the blood-brain or placental barriers. There is some evidence indicating that mercuric mercury in the body following oral exposures can be reduced to elemental mercury and excreted via exhaled air. Because of the relatively poor absorption of orally administered inorganic mercury, the majority of the ingested dose in humans is excreted through the feces.
Methylmercury is rapidly and extensively absorbed through the gastrointestinal tract. Absorption information following inhalation exposures is limited. This form of mercury is distributed throughout the body and easily penetrates the blood-brain and placental barriers in humans and animals. Methylmercury in the body is considered to be relatively stable and is only slowly demethylated to form mercuric mercury in rats. It is hypothesized that methylmercury metabolism may be related to a latent or silent period observed in epidemiological studies observed as a delay in the onset of specific adverse effects. Methylmercury has a relatively long biological half-life in humans; estimates range from 44 to 80 days. Excretion occurs via the feces, breast milk, and urine [37].
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Concurrently, the radical addition reaction possesses intrinsic side reactions, in particular, diffusion controlled radical-radical reactions through coupling or disproportionation. (Figure 2.8) Since these side reactions are extremely fast and unselective, they are difficult to control, and thus the radical addition reaction often becomes a non-exacting method to targeted products. This predicament is more pronounced in the polymerization process, where the addition reaction must be repeated many times, and the precise control of the reaction is even more important. Frequently, the polymers obtained from the radical process do not have well defined structures in terms of molecular weight, molecular weight distribution and functionalities. Therefore the precise control of radical polymerization has been one of the main challenges in polymer synthesis [41].
Fortunately, the side reactions between radicals can be minimized by decreasing the radical concentration ([R*]). While radical - radical reactions are second order in [R*], the addition of radical to alkene is first order in [R*]. A decrease in [R*] suppresses the radical-radical reaction effectively with a smaller reduction in addition rate. In organic synthesis, there have been several useful methods to maintain low radical concentrations, mostly based on chain reactions [38,39] (Figure 2.9) In the chain reactions, only a catalytic amount of radicals is required since the radical is regenerated in the reaction cycle.
One such method is atom transfer radical addition (ATRA) or Kharasch reaction [42-45] (Figure 2.9). In ATRA, the chain reaction is initiated by the formation of radicals 2 from non-radical species 1, promoted either by light, or thermally labile compounds such as AIBN or peroxide. The reaction cycle is composed of addition and transfer reactions. In the addition step, the produced radicals 2 attack alkenes 3 to form adducts 4. The addition should be a fast reaction, in which less reactive radicals 2 are converted to more reactive radicals 4. These reactive radical adducts 4 rapidly abstract atom (or group) X from substrates 5, to form targeted products 6 and to regenerate radicals 2 in the transfer step. Continuous cycles of this chain reaction achieve the addition of I across the double bond of 3 to yield mono-addition products 6 while the radical concentration is kept low, which minimize the radical - radical reactions, termination (kt) (Figure 2.7).
In ATRA, there is another important side reaction, which is the formation of oligomeric or polymeric species, namely, the propagation reaction. Before theradical adducts 4 converts to desired product 6 through atom transfer, radicals 4 can escape from the reaction cycle and add to one or more alkenes 3 to yield oligomers / polymers 7 (Figure 2.10). This side reaction becomes more significant for alkenes with a higher propagation rate constant, kp (i.e. radically homopolymerizable alkenes). The low radical concentration cannot effectively reduce the propagation rate which is also first order in [R*] as the addition reaction is. Instead, the propagation can be suppressed by the correct choice of the alkenes/RX or the increasing the ratio of RX to alkenes.
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The use of a transition metal complex can be an alternative to expedite transfer relative to propagation [46]. Transition metal complexes 8 accelerate transfer of X to 4 without altering the rate of propagation so as to favor the formation of the mono addition product 6 (Figure 2.7). This process is referred to as deactivation, since unreactive products 6 are generated from reactive radicals 4. Transition metal complexes also abstract X from 1 to provide radicals 2 to the reaction cycle, in the activation step. The cycle does not need to be initiated by light or thermal initiator since the activation reaction in the catalytic cycle generates its radicals. The whole reaction cycle in Figure 2.9 will be called transition metal catalyzed atom transfer radical addition (TMC ATRA) to distinguish it from non-metal mediated ATRA or Kharasch reaction.
Figure 2.9: Transition metal catalyzed atom transfer radical addition
The recent adaptation of TMC ATRA to polymerization processes resulted in a new living radical polymerization method, namely, atom transfer radical polymerization (ATRP).[47, 48] Similar to TMC ATRA, ATRP employs transition metal complexes 9 to catalyze atom transfer between radicals 10 and alkyl halides 11 where 10 and 11 can be either monomeric or polymeric species. (Figure 2.10) The atom transfer from alkyl halides 11 to transition metal complexes 9, produces the corresponding alkyl radicals 10 and oxidized metal complexes 8 with the activation rate constant, kact. The reverse reaction yields 11 with reduction of 8, the deactivation rate constant, kdeact•
These activation and deactivation processes are reversible in ATRP, resulting in the reversible formation of radicals 10. The concentration of stationary radical 10 is low since the equilibrium between the activation and deactivation processes is shifted to left hand side, which reduces the termination reactions. The broken arrow in Figure 2.10. represents the small contribution by the termination reactions with k, The addition of monomer with the propagation rate constant, kp, proceeds in a controlled fashion for all chains through these reversible activation and deactivation cycles, and well-defined polymers with predetermined molecular weights and narrow molecular weight distributions can be produced.
It is important to note the concentration of 8 during the reaction (i.e. transition metal complex in higher oxidation state). In radical reactions, the radical - radical termination cannot entirely be eliminated, and their small contribution still exists even in the chain reactions of TMC ATRA / ATRP. This means that oxidized metal complexes 8 in Figure 2.9 and 2.10 are continuously generated throughout the reaction. At some point, the increased concentration of 8 further decreases the radical concentration and slows down the termination reactions and generation of 8. The role of this self-regulating system was first explained by Fischer [49] and later named as the "persistent radical effect (PRE) [50]. The persistent radical refers to stable radicals which only react with transient radicals (e.g. 2 and 4 in Figure 2.9 and 11 in Figure 2.10) and do not react with other species including themselves. Now, it is
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further optimization of current systems and development of new systems. On the other hand, the understanding of the role of transition metal catalysts has been a central question in TMC ATRA and ATRP. The transition metal catalysts may complicate the systems and necessitate more detailed mechanistic and kinetic studies than those for non- metal catalyzed radical reactions.
2.4.1. Components of ATRP
In order to fully understand the ATRP technique, one must understand the roles of the various components. The ATRP system consists of monomer, initiator with a transferable halogen, and a catalyst system composed of a transition metal species and a suitable ligand. Solvent, reaction temperature, and additives are also important. As with any reaction, the conditions under which a particular reaction is to be performed must be optimized to achieve the desired results. Figure 2.11 shows that the relationship between Mw/Mn, Mn with conversion.
Figure 2.11: Representation of molecular weights and polydispersities with conversion for a living radical polymerization
2.4.2. Monomers
A wide variety of monomers have been successfully polymerized by ATRP. These include styrenes [48,51], acrylates [52,53], methacrylates [54,55], vinylpyridine [56] and acrylonitrile [57]. To be polymerizable via ATRP, the monomer must have stabilizing groups (e.g. phenyl or carbonyl) next to the carbon radical in order to have a sufficiently large atom transfer equilibrium and the monomer must not have groups (e.g. acids) that interfere with the catalyst system. The radical polymerization rate is unique to each monomer so that the concentration of propagating radicals and the rate of deactivation need to be adjusted for each particular system in order to
maintain control. The overall equilibrium of the reaction depends on the radical (monomer) and the dormant species as well as on the amount and reactivity of the transition metal catalyst added.
2.4.3. Initiators and Halogen Exchange
ATRP is typically initiated by an alkyl halide, which contains an activating substituent on the a-carbon, stabilizing the resulting radical. The initiation step proceeds by the same process as propagation, and the role of the initiator is to determine the number of growing polymer chains and to provide the head group of the polymer chain. If initiation is fast and transfer and termination reactions are negligible, the number of growing chains is constant and equal to initiator concentration. The theoretical molecular weight or degree of polymerization increases inversely with initial initiator concentration and is given by equation [58]:
(2.1) To achieve control of molecular weight and narrow molecular weight distributions, the halide group must rapidly and selectively migrate between the growing chain and catalyst complex. Molecular weight control has been the most successful when the halogen is either chlorine or bromine [59]. Mixed halide initiation systems (i.e. R-X/Cu-Y (X,Y=C1 or Br) have also been investigated to obtain better control in ATRP of MMA [62].
2.4.4. Solvents
ATRP has been successful in bulk, in solution, and in a heterogeneous system (for example, emulsion or suspension). A good solvent for ATRP will allow minimal chain transfer to solvent and solvent assisted side reactions. Polymerizations are typically carried out in organic non-polar media such as toluene or anisole although a
23 2.4.5. Catalysts
The ideal ATRP catalyst should be highly selective for atom transfer and should not participate in other reactions. The catalyst determines the position of the atom transfer equilibrium and the dynamics of exchange between the dormant and active species. In order to be an efficient catalyst for ATRP, the metal must have at least two readily accessible oxidation states separated by one electron, have reasonable affinity toward a halogen, have an expandable coordination sphere upon oxidation to accommodate the halogen, and be strongly complexed to a ligand. To differentiate ATRP from redox-initiated polymerization, the oxidized transition state should rapidly deactivate the propagating polymer chains to form the dormant species, resulting in a controlled process. Although a variety of metals have been used such as Fe (I) [62], Cu (II) [63-65], and Ni (II) [66,67] this work focuses on ATRP catalyzed by copper, which is superior to other metals in versatility and costs.
2.4.6. Ligands
The ligand serves to solubilize the catalyst in ATRP. Nitrogen ligands, which have been used in copper- and iron-mediated [68] .Multidentate ligands, are the ligands of choice and a variety have been developed. Some examples are shown in Figure 2.7 Derivatives of 2, 2'-bipyridine (bipy) are one example of ligands used in Cu-catalyzed ATRP [69]. The most common bipy ligand is the derivative with solubilizing nonyl groups in the 4, 4' position: 4, 4'-di(5-nonyl)-2, 2'- bipyridine. Haddleton et al. [70-72] reported use of π accepting, chelating imine-based ligands. Other aliphatic multidentate amines such as N N, N. N'. N "-pentamethyldiethylene triamine (PMDETA) 1,1,4,7,10,10 hexamethyltriethylenetetramine (HMTETA), and tris[2 (dimethylamino)ethyl]amine (Me6TREN) [73] have also been reported.
Electronic and steric effects of the ligands affect ligand behavior in ATRP [74,75] Catalytic activity is reduced when there is excessive steric bulk around the metal center or the ligand possesses strongly electron-withdrawing substituents. Activity also decreases with a decrease in coordinating sites and as the number of linking carbons increases. Although the catalyst/ligand species in ATRP may be a complex structure, the generally accepted complex is two bidentate ligands complexed to one copper bromide molecule[76,77] order of decreasing activity for the various types of nitrogen ligands is as follows: R2N- > Pyridines > R-N= > Ph-N= > Ph-NR-.
Aliphatic amine ligands cause faster polymerization rates due to their poorer n-accepting ability, which results in less stabilization of the copper (I) species[73]. Altering the a-donating and π- accepting properties of the ligand changes the redox potential for the copper oxidation and thereby increases the atom transfer equilibrium constant [77].
Proper choice of ligand for the particular ATRP system is very important. Both heterogeneous and homogeneous catalytic systems have been used. Early ATRP experiments comparing bipy ligands with and without solubilizing alkyl groups showed that the soluble catalyst systems resulted in lower polydispersities, similar initiator efficiencies, and slower overall polymerization rates as compared to the heterogeneous bipy systems [78]. The increased control is attributed to the better solubility of the copper (I) species which causes deactivation. Homogeneous systems also offer the advantage of allowing for detailed kinetic and mechanistic studies of the polymerization [79,80] haupt and coworkers [81] have studied ATRP of styrene with various bipy and diimine ligands with CuBr catalyst and showed that ligand
25
using zerovalent metal as an additive. For example, when a small amount of copper (0) (copper powder) is added to styrene and methacrylate polymerizations, polymerization rates increase [82]. Copper (0) is said to reduce "excess" copper (I) forming copper (I). Removal of small amounts of copper (II) enhanced the rate yet left a sufficient amount to maintain control of the polymerization. Addition of CuBr2 has been used as a way to increase the rate of deactivation, in order to shift equilibrium to form the dormant species, which will in turn minimize side reactions and termination.
2.5 Modification of polymers
The modification of polymers is one of the ways of synthesising high-molecular-weight compounds, which, together with polymerisation and polycondensation, has been used for a long time to synthesise polymers. Whereas the polymerisation and polycondensation reactions are processes based on the conversion of monomers into polymers, the third procedure (modification) is a process based on the interconversion of polymers (Figure 2.l3).
Consequently, chemical modification of polymers is a process in which the initial polymer is subjected to physical or chemical influences, as a result of which it is converted into a new polymer with a different chemical structure.
The polymer modification reaction was first achieved by the French worker Braconnot, who obtained nitrocellulose in 1833 by treating cellulose with nitric acid [83]. Similar reactions were later carried out on a series of natural polymers. Different derivatives of cellulose, starch, proteins, rubber, and other natural polymers were obtained [84]. Subsequently synthetic polymers such as poly (vinyl chloride), polystyrene, polyethylene, and others began to be subjected to modification reactions at an increasing frequency.
Such wide-scale use of the modification reactions is due to the fact that the initial polymers do not meet all the practical requirements as regards their properties. Thus many insoluble polymers can be converted by modification into soluble high-molecular-weight compounds. A striking example of this kind is provided by cellulose, whose esters are readily soluble in organic solvents; chlorinated poly (vinyl chloride) and polyethylene become readily soluble. The introduction of carboxy- and amino-groups into polystyrene imparts to it acid and basic properties
respectively, which are essential for ion-exchange resins. The introduction of nitrate ester groups into cellulose imparts explosive properties to the latter. The introduction of phosphate groups as well as chlorine, fluorine, bromine, and certain other elements into polymers renders them incombustible.
Thus the modification of polymers constitutes an extremely universal method whereby it is possible to vary within a wide range their physical and chemical properties in the desired direction. Because of this, the modification of polymers has now become one of the most important procedures whereby a large number of polymers are synthesised both in the laboratory and in industry. In industry modification processes have found extensive applications in the synthesis of many polymers, produced on a large scale, such as nitrocellulose, acetylcellulose, other cellulose esters and ethers, poly(vinyl alcohol), acetals of poly(vinyl alcohol), chlorinated poly (vinyl chloride), chlorinated and sulphochlorinated polyethylene, and others.
2.5.1 Characteristics of polymer modification reactions
A characteristic feature of the polymer modification processes is the fact that all reactions of this kind do not as a rule go to completion and are accompanied by side reactions, which has a significant influence on the course of the process and its results. This is caused by the influence of a number of significant physical and chemical factors, manifested in a specific manner in various polymer modification reactions.
Such factors include the influence of the nature of the reagents and catalysts, the structure and degree of crystallinity of polymers, temperature, the nature of the solvent, and others. Ultimately, the reactions therefore result in a mixture of macromolecules of different structure, each of which contains different units in a
27
The chain effect, the effect of the neighbouring group, and configurational, conformational, electrostatic, and supermolecular effects are usually isolated from such influences as the most significant [85,87].
Figure 2.13: Different processes leading to the synthesis of polymers 2.5.2 Typical chemical polymer modification reactions
Polymer modification processes are based on the use of a series of chemical reactions. In considering the polymer modification reactions known at the present time, one should note their large number and wide variety. Among these reactions, there are both processes based on the interactions of polymers with low-molecular-weight reagents and reactions of polymers with polymers. Figure 2.14 illustrates the fundamental reactions—both normal reactions and secondary reactions—which can occur in the modification of polymers and which we designate as the "set of modifying reactions". Here we encounter both substitution and elimination reactions, on the one hand, and reactions involving addition to multiple bonds, structure formation, and degradation of polymers, on the other. In addition, reactions involving the isomerisation of macromolecules and extremely unusual exchange type reactions of heterochain polymers are used for this purpose.
An interesting feature of all these reactions is that each has a specific influence on the structure of the polymer macromolecule formed as a result of the modification.
Synthesis of polymers Polymerization of monomers Polycondensation of monomers Modifications of Polymers
Complete Substitution of hydroxy-groups in polymers Complete Substitution of hydrogen atoms in polymers Complete Substitution of halogen atoms in polymers
Complete regular structure formation in polymers Regular degradation of macromolecules Complete addition to multiple bonds in macromolecules Reactions involving total elimination from macromolecules
Regular grafting to macromolecules Complete exchange in heterochain polymers
Complete isomerization of macromolecules
No rm al M o d ifi ca ti o n M eth o d s
Incomplete Substitution of hydroxy groups, hydrogen, halogen and other atoms and groups in polymer
macromolecules
Irregular grafting to macromolecules Partial oxidation of macromolecules
Partial irregular structure formation in macromolecules Incomplete addition to multiple bonds in macromolecules
Partial degradation of macromolecules Incomplete exhcange in heterochain polymers
Partial elimination of volatile compounds Incomplete isomerization of macromolecules Statistical distribution of atoms of stable isotopes in polymer
macromolecules
The presence of side groups and branches as well as other anomalous units in initial polymers
Other side reactions
S id e re ac ti o n s a n d o th er ca u se s lea d in g to th e fo rm ati o n o f h etero u n it p o ly m ers Set o f Mod if ic ai ton Me tho ds
29
remaining types of reactions the structure of the macromolecule changes significantly.
The remaining modification reactions cannot therefore be included in the class of polymer-analogue transformations . In considering the modification reactions, one must bear in mind not only their chemical nature but also the way in which they are carried out (in solution, in the melt, in an emulsion, and in the solid), since the result of the process depends significantly on its conditions [86].
2.5.2.1 Polymer modification reactions involving substitution
The substitution reactions can be subdivided into the following subgroups: (1) substitution of a hydroxyl group by acyl and alkyl groups as well as inorganic acid residues; (2) substitution of a hydrogen atom by deuterium, a halogen, halogeno-, nitro-, phosphino-, and sulpho-groups, as well as alkyl groups in grafting reactions; (3) hydrolysis of derivatives of polymeric carboxylic acids; (4) substitution of a halogen and other groups for a hydroxy- or aminogroup; (5) oxidation of side groups.
Figure 2.15: The change in the structure of macromolecules as a result of the modification reaction
These reactions have certain common features consisting in the nature of the alteration of the structure and the properties of the resulting polymers. We may note that the replacement of a hydroxy-group in the polymer by an acid residue or an alkyl group makes the polymer soluble in inorganic solvents; the substitution of hydrogen by a halogen has a similar influence. In contrast, the substitution of an alkyl group in an ether by a hydroxy group impairs the solubility of such a polymer in organic solvents.
The majority of polymer modification reactions involving substitution belong to the type designated as polymeranalogue reactions. Their characteristic feature is retention of the main macromolecular skeleton, only the nature of the substituents surrounding the main chain being altered [86].
2.5.2.2 Polymer modification reactions involving structure formation
The modification of polymers by structure formation is a process in which the macromolecules pass through a stage with branched structures and are ultimately converted into infusible and insoluble three-dimensional structures. In the first stage of this process the polymer interacts with the cross-linking agent, which leads to the formation of bonds between the macromolecules. An industrial structure formation process, known as vulcanisation, is widely used in the processing of polymers, particularly elastomers. Vulcanisation imparts to rubbers resistance to wear and other mechanical influences and resistance to oxidation and the action of solvents.
In addition, in the case of rubbers vulcanisation by sulphur and its derivatives is used. The cross-linking effect of bifunctional reagents and reagents with a larger number of functional groups is used for cellulose, poly (vinyl alcohol), and other polymers. As a result of these reactions, branching takes place initially and is followed by structure formation in polymers with different degrees of cross-linking,
31
macromolecule. A branched polymer, also occupying the entire space, is then formed. The structure of such a polymer branched to the maximum extent is illustrated in Figure 2.18. It is in essence a regularly grafted copolymer
Figure 2.16: Schematic representation of a three dimensional polymer
Figure 2.17: Schematic representation of a possible branching
Figure 2.18: Schematic representation of a regularly branched polymer
The cross-linked three-dimensional polymers formed are stronger, are insoluble, and exhibit a greater thermal stability than the initial linear polymer: this accounts for the wide-scale employment of this modification method, particularly in the rubber industry[86].
2.5.2.3 Polymer modification reactions involving degradation
Degradation is a process directly opposed to structure formation and is used to decrease the molecular weight of polymers.
Degradation is widely used as a polymer modification method, particularly in the processing of natural polymers such as rubber and cellulose. With the aid of degradation by mechanochemical and oxidative methods, one obtains polymers with the optimum size of the macromolecules.
The degradation of carbochain polymers is carried out by a mechanical procedure (on rollers or other machines). In the case of cellulose oxidative degradation by atmospheric oxygen is used. In metathetical reactions in the presence of tungsten- or molybdenum-containing catalysts the carbon carbon double bonds also become extremely labile and capable of degradative transformations [88]. Hydrolysis,
