In Situ Controlled Radical Polymerization: A Review on Synthesis of Well-defined Nanocomposites

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In Situ Controlled RadicalPolymerization: A Review on Synthesis ofWell-defined NanocompositesHossein Roghani-Mamaqani a , Vahid Haddadi-Asl a & Mehdi Salami-Kalajahi ba Department of Polymer Engineering and Color Technology,Amirkabir University of Technology, Tehran, Iranb Department of Polymer Engineering, Sahand University ofTechnology, Tabriz, Iran

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To cite this article: Hossein Roghani-Mamaqani, Vahid Haddadi-Asl & Mehdi Salami-Kalajahi (2012):In Situ Controlled Radical Polymerization: A Review on Synthesis of Well-defined Nanocomposites,Polymer Reviews, 52:2, 142-188

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Polymer Reviews, 52:142–188, 2012Copyright © Taylor & Francis Group, LLCISSN: 1558-3724 print / 1558-3716 onlineDOI: 10.1080/15583724.2012.668153

In Situ Controlled Radical Polymerization: A Reviewon Synthesis of Well-defined Nanocomposites

HOSSEIN ROGHANI-MAMAQANI,1 VAHID HADDADI-ASL,1

AND MEHDI SALAMI-KALAJAHI2

1Department of Polymer Engineering and Color Technology, AmirkabirUniversity of Technology, Tehran, Iran2Department of Polymer Engineering, Sahand University of Technology,Tabriz, Iran

Conducting various kinds of controlled radical polymerization (CRP) methods in thepresence of specific nanofillers to prepare nanomaterials with excellent properties isreviewed. The addition of a small amount of such fillers results in excellent physical,mechanical, and thermal properties; therefore, the synthesis and structure of suchproducts are investigated in detail. Nanofillers which are capable of carrying functionalgroups are classified by their dimensions. Also, their surface modification methodsto attach polymer chains on their surfaces are debated. Additionally, various CRPmethods with their particular features in different mediums are discussed. Also, thedetailed examples of the preparation of such nanofiller loaded nanomaterials by variousgrafting techniques namely the “grafting from” and the “grafting through” techniquesare drawn from the literature. Finally, the “grafting onto” method by a combinationof CRP and mainly click chemistry to prepare polymer grafted nanosubstrates arediscussed.

Keywords In situ controlled radical polymerization, nanocomposite, surface modifi-cation, grafting techniques, click chemistry

Nomenclature

10-carboxylic acid-10-dithiobenzoate-decyltrimethylammonium CDDAbromide

11-(2-Bromo-2-methyl)propionyl-oxy-undecyl trichlorosilane BMPUS1-phenylethyl dithiobenzoate FEDB2-(2- cyanopropyl)dithiobenzoate CPDB2,2,6,6-tetramethylpiperidinyloxyl TEMPO2,2′-azo-bisisobutyronitrile AIBN2-bromo-2-methyl propionate BMP2-bromoisobutyryl bromide EBiB2-hydroxyethyl 2-bromopropionate HEBP2-hydroxyethyl-2α-bromoisobutyrate HEBrB

Received November 5, 2011; accepted January 15, 2012.Address correspondence to Vahid Haddadi-Asl, Department of Polymer Engineering and Color

Technology, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran. Fax: +98 2144739501. E-mail: [email protected]

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In Situ Controlled Radical Polymerization 143

2-methacryloyloxyethylhexadecyldimethylammonium bromide MHAB3-(trimethoxysilyl)propyl methacrylate MPS3-aminopropyldimethylethoxysilane APDMS3-methacryloxypropyldimethylchlorosilane MPDCS4-cyano-4-(dodecylsulfanylthiocarbonyl) sulfanyl pentanoic acid CDMPAcrylic acid AAcActivators generated by electron transfer AGETActivators regenerated by electron transfer ARGETAlkyl halide bond R-XAtom transfer nitroxide radical coupling ATNRCAtom transfer radical polymerization ATRPAtomic force microscopy AFMBromoisobutyrate functionalized MWNT MWNTBrButyl acrylate BACarbon nanotube CNTCation exchange capacity CECCationic ring opening polymerization CROPControlled radical polymerization CRPDormant species PnXFree radical polymerization FRPGel permeation chromatography GPCGrowing radicals R∗

Initiators for continuous activator regeneration ICARMercaptopropyl triethoxysilane MPTMSMetal in its normal state Mt0

Methyl 2-bromopropionate MBPMethyl methacrylate MMAMontmorillonite MMTMulti-walled carbon nanotubes MWCNTMulti-walled nanotubes MWNTN,N ′-dimethyl(methylmethacryloyl ethyl) ammonium propane DMAPS

sulfonateN-acryloyl glucosamine AGAN-butyl methacrylate n-BMAN-isopropylacrylamide NIPAAmNitroxide-mediated radical polymerization NMPN-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] DEPNPhenyldithiobenzoate magnesium bromide PhC(S)SMgBrPoly(2-hydroxyethyl methacrylate) PHEMAPoly(2-vinyl pyrolidone) P2VPPoly(4-vinylpyridine) P4VPPoly(methyl acrylate) PMAPoly(methyl methacrylate) PMMAPoly(N-isopropylacrylamide) PNIPAAmPolyvinylimidazole PVIReverse atom transfer radical polymerization RATRPReversible addition-fragmentation chain transfer RAFTReversible deactivation radical polymerization RDRPS-benzyl S′-trimethoxysilylpropyltrithiocarbonate BTPTSingle electron transfer living radical polymerization SET-LRP

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144 H. Roghani-Mamaqani et al.

Single-walled carbon nanotubes SWCNTS-methoxycarbonylphenylmethyl S′-trimethoxysilylpropyltrithio- MPTT

carbonateSodium dodecyl sulfate SDSSurface initiated atom transfer radical polymerization SI-ATRPTert-butyl acrylate t-BATert-butyldimethylsilyl methacrylate MASiTetraethylorthosilicate TEOSTitania nanoparticles TiO2Transition metal complex (activator) MtnY/LTransition metal in its higher oxidation state (deactivator) X-Mtn+1Y/LX-ray photoelectron spectroscopy XPS

1. Introduction

In the past few decades, organic/inorganic composite nanomaterials have attracted a greatdeal of attention. This is mostly because of the superior properties of the resulted nano-materials in comparison with the neat polymer matrix. In general, these nanomaterials areorganic polymer matrix in which nanoscale inorganic fillers are dispersed uniformly. Thesenanofillers are like nanosized phase separations in the polymer matrix which bring abouta combination of inorganic phase and organic polymer matrix properties. In essence, acombination of rigidity and thermal stability of inorganic phase and the ductility, flexibil-ity, and processability of the polymer phase make these nanomaterials to be industriallyattractive. In addition, a high interfacial area between the phases because of the ultrafinesize of nanofillers leads to a significant fraction of interfacial polymer. This also results inadvanced properties of these materials in comparison with the neat polymer matrix and con-ventional composite materials at even very low nanofiller loadings. These nanofillers canbe categorized by their dimension. One-dimensional nanofillers consist of layered silicates(e.g., montmorillonite and saponite), and also other planar materials like graphene. In thecase of nanotubes and whiskers, nanosize is limited to two dimensions. Three-dimensionalnanofillers refer to the nanoparticles such as metals (e.g., silver and germanium), metaloxides (e.g., Al2O3, Fe2O3, and ZnO), semiconductors (e.g., PbS and CdS), and othercompounds like Mg(OH)2, SiO2, carbon black, and so forth.1–5

Nowadays, polymer/clay nanocomposites have attracted much attention as advancedpolymeric materials. Relatively low clay loading can result in significant improvementsin mechanical properties,6 thermal stability and flame retardancy,7 magnetic and electricproperties,8 gas permeation,9 and enhanced modulus10 for the synergism between the com-ponents. On the contrary, the degree of dispersion of clay platelets into the polymer matrixdetermines the structure of nanocomposites and affects the above-mentioned properties.Based on the reactants, processing system, and interaction between clay layers and thehost polymer, melt intercalation, solution blending, and in situ polymerization have beenemployed to prepare a nanocomposite. The latter consists of polymerization of monomersin the presence of clay layers.11–14 In situ polymerization almost results in an exfoliatedstructure which is caused by easy intercalation of monomers into the interlayer gallery ofnanoclay because of their low viscosity. A variety of polymerization systems and meth-ods brings about in situ polymerization to be one of the most interesting techniques fornanocomposite preparation.

Polymers containing a small amount of carbon nanotube (CNT) exhibit consider-able improvements in mechanical,15 electrical,16 and magnetic17 properties. CNTs withnanoscale diameter and high aspect ratio are an ideal reinforcing agent for polymer

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nanocomposites. However, it is extremely difficult to disperse and align CNTs in a polymermatrix since they form stabilized bundles. For dispersion of nanotubes in a polymer matrixseveral methods have been employed which are solution mixing, melt mixing, electrospin-ning, in situ polymerization, and their chemical functionalization.18 Chemical modificationof CNTs has been carried out by attaching modifier molecules to their wall19 and also attheir defect sites.20

Among the numerous nanomaterials, polymer/silica nanocomposite has been reportedthe most. These nanohybrid materials can be prepared by various routes. The organic com-ponent can be introduced as a monomer, preformed linear polymer, or a polymer networkwhich is physically or chemically cross-linked. The mineral part can be a precursor orpreformed nanoparticles. The linear polymer could be in the molten, solution, or emulsionstate. Physically crosslinked polymers are semicrystalline linear polymers; however, ther-mosets and elastomers are examples of chemically crosslinked matrices.21 Thus, accordingto the starting moieties, three major ways are employed to prepare nanohybrid systemswhich are blending, the sol-gel process, and in situ polymerization.5

This review will discuss about the most recent researches on the preparation of nano-materials via in situ reversible deactivation radical polymerization (RDRP)22 which is com-monly known as controlled radical polymerization (CRP). To this point, various kinds ofnanofilers with different ways of their surface modification are presented and subsequently,some of the important polymerization techniques are discussed. An important considera-tion in this review involves the study of CRP methods, and consequently discussing therobustness of each technique in the synthesis of nanomaterials. In this context, chain endfunctionality and functionilization of nanofillers and polymers are investigated. Also, var-ious grafting methods namely “grafting from,” “grafting through,” and “grafting on,” areinvestigated. Some important reactions to graft a polymer onto substrates like “click chem-istry” are also discussed. As will be noted, nanoscale is considered where the dimensionsof the particle, platelet (thickness), or fiber (diameter) are in the range of 1–100 nm.

2. Surface Modification of Nanofillers

Poor interfacial interaction between polymer matrices and nanofillers often results inphase separation. Therefore, to increase the compatibility between a polymer matrix andnanofillers, various methods have been employed. Surface modification of nanofillers is themost commonly used technique which can be carried out by either chemical or physicalmethods. Surface modification of nanofillers, in addition to the enhancement of interfacialinteraction between the two phases, improves nanofiller dispersion in the matrix. Addi-tionally, well-defined polymer nanohybrids by using various grafting techniques in thevicinity of nanofillers can be synthesized. This is mainly obtained by using functionalmodifiers which can simply initiate polymerization from the surface of the nanofiller or theattached polymer chains onto its surface through the functionalities. Surface modificationof nanofillers is commonly carried out by either physical or chemical processes.23

2.1 Chemical Modification

Because of the formation of covalent bond between the modifier molecule and nanofillersurface, chemical modification results in much stronger interactions of these two species.Although chemical modification includes modification of nanofiller surfaces either by mod-ifier agents or by polymer chains, modification with modifier agents is the most interestingmethod. Covalent attachment of polymers on the nanofillers surface is important because

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polymer chains improve dispersion of nanofillers in a matrix. There are three commonmethods for the covalent attachment of polymers to the surface of nanofillers, which are“grafting from,” “grafting through,” and “grafting to” methods. The “grafting from” methodrelies on the surface fictionalization of nanosubstrates by a polymer precursor and subse-quently propagation of polymer chains from the surface attached initiators. In the “graftingto” technique, end group transformed polymer chains with specific characteristics can reactcovalently with the functionalities of the nanofillers and therefore forms polymer attachedsubstrates. The “grafting through” methodology is based on the attachment of a moietywith double bond on the surface of nanofillers and subsequently incorporating this doublebond into the polymer chains during the propagation reaction.

The hydroxyl group on the surface of nanosilica can react chemically with variousmolecules with functionalities of hydrolizable moieties such as chloro, ethoxy, or methoxygroups. In this context, the most commonly used coupling agents are special silane com-pounds which benefits two functional groups on its structure; one for the coupling reactionwith the hydroxyl group of nanosilica and the other for engagement of polymer chains withsilica nanoparticles. This engaging moiety can be a double bond to progress a “graftingthrough” reaction24–25 or it can be an initiator moiety to proceed with a grafting reactionfrom the surface of nanosilica.26 In addition, this moiety can be a functional group whichcan react irreversibly with polymer backbone functionalities to graft the polymer chainsonto nanosilica.27 Two latter routes of surface modification result in polymer chain attach-ment on nanosilica surface. Therefore, an increase in the hydrophobicity of nanoparticlescauses higher filler and matrix miscibility. Also, because of the entanglement of the polymerchains attached to the nanosilica surface and polymer chains of the matrix, the interfacialinteraction between nanosilica and matrix increases.

Nanotubes, because of Van der Waals forces, could hardly be dispersed and aligned ina polymer matrix. Therefore, surface modification is necessary to obtain a nanocompositein which nanotubes dispersed appropriately in the matrix. Defect and covalent functional-ization are the most commonly used chemical modifications of nanotube surface. Carbonnanotubes are subjected to oxidative reactions to remove metal particles or amorphouscarbon; this causes some defects at their open ends. For instance, carbon nanotubes containoxidized carbon atoms in the form of -COOH and they can be used for covalent attachmentof organic groups by converting them into acid chlorides and subsequently reacting withamines or alcohols. Since the portion of these defected carbons in nanotubes is under 5%,this type of surface modification cannot result in well-enhanced interfacial interactionsbetween nanotubes and the host polymer matrix. Covalent functionalization of nanotubesalmost carried out through its surface bounded carboxylic acid groups. Also, direct func-tionalization of nanotubes using sulfuric acid, nitric acid, and aqueous hydrogen peroxideresults in functional groups of carboxylic acid or hydroxyl groups. By acid treating of thesurface of a nanotube, some defects in the carbon-carbon bonds are formed; this finally leadsto the formation of carboxylic acid groups on their surface. The presence of carboxylic acidor hydroxyl groups on the nanotubes surface facilitates attachment of organic or inorganicmaterials through a reaction with amines or alcohols.28 In addition, similar to the graftingof polymer chains onto silica nanoparticles, three major routes of polymer grafting can beapplied to the surface of nanotubes through their carboxylic acid or hydroxyl groups.29

2.2 Physical Modification

Physical surface modification is often obtained by adsorpsion of surfactants on the surfaceof nanofillers. Electrostatic interactions play a key role in the adsorption process. In the case

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of silica nanoparticles, a surfactant reduces the physical attraction of silica particles in itsagglomerates and consequently increases the interfacial interaction between the nanopar-ticles and polymer matrix. In addition, such an adsorption can be adjusted to verify thehydrophobicity of silica nanoparticles to the required content. In this process, the formationof hydrogen bonds between the surfactant and silanol groups of nanosilica can result inhigher interfacial interactions between nanosilica and the polymer matrix.23 Additionally,physical functionalization of nanotubes mainly involves surfactants, biomacromolecules,or wrapping with polymers. The interaction becomes stronger when the hydrophobic partof the surfactant contains an aromatic group.29

Silicate nanolayers consist of alkali counter-ions on their surfaces which make them im-miscible in hydrophobic polymers. The alkali cations are not structural and can be replacedby other positively charged molecules. Therefore, ion-exchange of the alkali counter-ionwith a cationic-organic surfactant such as ammonium, phosphonium, or sulfonium is nec-essary to reach nanolayers with lowered surface energy and organophilic specificity.30

Moreover, with intercalation of these surfactants with long organic chain into the claygalleries, the interlayer distance of the clay platelets increases. Then, it will be possiblefor polymer chains or even monomers to diffuse more easily into the galleries and subse-quently push the galleries apart by shear forces or polymerization pressure. Sometimes,these surfactants are accompanied with an initiator moiety on its tail which can initiatepolymerization of diffused monomers in the gallery and therefore grafting polymer chainsfrom clay layers can be continued by diffusion of monomers.31 Moreover, some surfactantscan be engaged in the polymerization process through its double bond and formed graftedpolymer chains on the clay layers via a “grafting through” process.32,33

2.3 Plasma Modification

In the case of carbon nanotubes, covalent functionalization can also be achieved by plasmatreatment. The plasma treatment method is important since it has the advantage of beingnonpolluting and can be scaled up to produce large quantities for commercial applications.The available species, radicals, electrons, ions, and UV light within plasma strongly interactwith or affect the surfaces of carbon nanotubes and thereby breaking the sp2-hybridizedgraphite-like carbon bonds (C=C) within the carbon nanotubes lattice and creating activesites for binding functional groups. Rather, it is molecules containing groups that caninteract with the plasma-generated surface-bound radicals. Furthermore, reaction of theprocess gas or monomer fragments with the sample surfaces results in functional groupsgenerated by the plasma treatment itself. Plasma modification has been used to reduceagglomeration by modifying the surface of the nanotubes. The exerted functional groupselectrostatically prevent agglomeration and can also enhance interactions with the polymermatrix. Utilizing plasma methods can increase the dispersion of nanofillers in polymermatrices and enhance the interfacial bonding between the host matrix and nanotubes.34–39

3. Nanocomposite Synthesis via In situ controlled Radical Polymerization

In this technique, nanoparticles are dispersed in liquid monomer or monomer solution,and subsequently, the resulting mixture is polymerized by various CRP methods. The mainadvantage of this method is its potential to graft polymer chains onto nanoparticles surfaces.This method is more useful for the preparation of nanocomposites with polymers that cannotbe processed by solution or melt mixing because of their insolubility or thermal instability.40

Moreover, because of the lower viscosity of monomer molecules, the homogeneity of

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the resulting nanocomposite is much higher than the products of melt intercalation andsolution blending techniques. This advantage is more peered, especially in layered silicatenanocomposites where monomer penetrates into the clay interlayer galleries, and afterwardsby initiation of polymerization reaction, the clay platelets can easily be pushed apart.41–43

The most important factors that affect the properties of nanocomposites are dispersion ofnanoparticles which is related to the affinity of polymer and filler interfaces. Inorganicparticles are dispersed homogeneously in a polymer matrix when they are modified byan appropriate coupling agent.44 These coupling agents, in addition to the increase of theaffinity of nanoparticle surfaces and polymer matrices, can be used to graft polymer chainsto the nanofiller surfaces via thee major methods of “grafting from,” “grafting through,” and“grafting onto.” In this section, three most common nanofillers, namely nanoclay, carbonnaotube, and silica are described structurally. In addition, synthesis of nanocompositesby various CRP methods using different kinds of modified nanofillers especially silicananoparticles, clay nanoplatelets, and naotubes are presented. An effort has been made tostudy various grafting methods in the case of each substrate to modify nanofillers and alsosynthesize a nanocomposite.

Layered silicates are nanolayers with thicknesses are in the range of one nanometer.These nanolayers building blocks consist of silica tetrahedral sheets and metal octahedralsheets. In tetrahedral sheets, silicon is surrounded by four oxygen atoms, and in octahedralsheets a metal is encircled by eight oxygen atoms. For example, in kaolinite, a tetrahe-dral sheet is fused with an octahedral sheet, whereby the oxygen atoms are shared. Theother example is montmorillonite (MMT), in which the crystal structure consists of two-dimensional layers formed by fusing two silica tetrahedral sheets to an octahedral sheet ofAl(OH)3. The layer thickness is around one nm, and the lateral dimensions of these layersmay vary from 30 nm to several microns or larger, depending on the particular layeredsilicate. There is a gap between layers which is called gallery. These layers are usuallybound together with counterions. Isomorphic substitution within the layers in which someof the aluminum atoms are replaced with magnesium or ferrous in MMT, or some of themagnesium atoms are replaced with lithium in hectorite, causes negative charges to bedistributed within the surface of the platelets that are balanced by positive counterions,typically alkali and alkaline cations, located between the platelets or in the galleries.45

Layered silicates are characterized by their cation exchange capacity (CEC) which is theirmoderate surface charge and generally expressed as meq/100 g. This charge varies fromlayer to layer; therefore, the CEC value needs to be considered as an average value over thewhole crystal. Tetrahedral substituted and octahedral substituted structures are two types oflayered silicates. In the case of tetrahedral substitution, the negative charge is located on thesurface of the silicate layers; on the other hand, in the case of octahedral substitution, thenegative charge is located in the gallery.46 Dispersion ability of silicate layers into individuallayers and their capability of surface modification through an ion-exchange reaction withvarious types of cations make them an important field of study in polymer nanocomposites.

Silica nanoparticles were first formed by sol-gel and microemulsion methods.47 Fromhydrolysis of dilute tetraethylorthosilicate (TEOS) solution in methanol, Stober and co-workers could prepare monodisperse silica nanospheres.48 By improving this method in aninverse microemulsion system, nanoparticles of silica with higher monodispersity were ob-tained through controlled hydrolysis of TEOS.49 In these techniques, diffusion of a siliconalkoxide, typically TEOS, into the water droplets results in hydrolysis of the alkoxide andformation of oxy-hydroxy-silicate species and alcohol. However, industrial silica nanopar-ticles are available in two forms of powder and colloid. The powder is produced by fumingand precipitation methods. Fumed silica is synthesized by hydrolyzation of SiCl4 in a flame

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of oxygen-hydrogen;50 but precipitated silica is formed by treating silicates with mineralacids to obtain fine hydrated silica particles during the precipitation.51 Colloidal silicaspheres are dispersed in the medium of water or alcohol. Silanol and siloxane functionalityon the surface of three-dimensional silica nanoparticles make it naturally hydrophilic. Freeor isolated, hydrogen-bonded or vicinal, and geminal silanols are three types of surfacesilanol functionality in silica nanoparticles.52 Hydrogen bonding between the silanol groupsof adjacent silica particles can result in aggregates which remain intact even under highmixing rates. To overcome these interactions and prevent from aggregate formation, surfacemodification of nanoparticles seems to be necessary.

Carbon nanotubes are tiny tubes of carbon hexagons which have diameters about fewnanometers and their lengths are about up to several centimeters. There are two kindsof carbon nanotubes—single-walled carbon nanotubes (SWCNT) which are individualcylinders of 1–2 nm in diameter and formed by rolling a single graphene sheet and multi-walled nanotubes (MWNT) in which additional graphene sheets wrapped around the SWNTcore by weak Van der Waals forces.53 About 5% of carbon atoms were located at somedefective sites. Some kinds of defects in the structure of nanotubes which can be used forits surface modification are topological, rehybridization, and incomplete bonding defects.54

Topological defects like non-hexagonal carbon rings can be formed during the nanotubegrowth process.

In free radical polymerization (FRP), the initiation reaction is slow and all the chainsare dead at any time of the reaction. The active species are free radicals which can bestabilized by resonance and polar effects. Radicals can be electrophilic or nucleophilicand in some cases possess a moderate tendency to alternate during copolymerization. FRPcannot control molecular weight or molecular weight distribution and therefore cannot yieldblock copolymers because of a very short lifetime of their growing chains. Due to theirsp2 hybridization, they show poor stereoselectivity. However, a high degree of head-to-tailstructures and formation of high molecular weight polymers is evidence of good regio- andchemoselectivity respectively.55

However, in CRP, the amount of chain breaking reactions is minimized and alsoby instantaneous initiation, all chains can propagate simultaneously. The elimination oftransfer and termination reactions forms the basis of Szwarc’s discovery of living anionicpolymerization. These chain breaking processes were avoided with reducing traces ofmoisture and air in the anionic polymerization of non-polar vinyl monomers.56 In thispolymerization method, active sites are active ionic species. Polymerization can proceedby discrete or solvatee counter ions which mainly depends on the type of solvent. Thesimilar electrostatic charge of the active sites prevents them from the combination reactions.Also, a higher degree of system purity lessens the probability of transfer reactions in theanionic polymerization systems. Therefore, polymerization proceeds until the completeconsumption of monomers. Chain end functionality and the living nature of polymerchains make it possible to synthesize block copolymers by this technique. A combinationof propagation via radical mechanism and instantaneous initiation results in a recentlydeveloped polymerization methods known as CRP. Mechanistically, CRP is based uponthe existence of an equilibrium reaction which swiftly switches growing chains betweenactive and dormant states. Consequently, the instantaneous concentration of free radicals islessened and thereby the bimolecular irreversible termination reactions are suppressed.55

3.1 Atom Transfer Radical Polymerization

3.1.1 Mechanistic Study. Atom transfer radical polymerization (ATRP) uses a transi-tion metal complex as its controlling agent in the equilibrium process. Transition metal

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complex (activator, (MtnY/L)) is responsible for homogeneous cleavage of alkyl halidebond (R-X) through a reversible redox process. As a result, it undergoes an inner-sphereelectron oxidation with abstraction of a radically transferable atom (X) from the dormantspecies (PnX). The growing radicals (R∗) react reversibly with transition metal in its higheroxidation state (deactivator, (X-Mtn+1Y/L)) to form dormant species. This equilibrium isconducted with the rate constants of kact and kdeact for activation and deactivation reactionsrespectively. In the case that kdeact� kact and the rate of initiation is much more than the rateof propagation, the probability of bimolecular termination reactions is reduced and poly-merization resembles a living system (Scheme 1). In this scheme, kt and kp represent thetermination and propagation rate constants respectively. Also, R-R, R= , and RH symbolizethe dead chains resulting from the combination of radicals, disproportionation betweengrowing radicals, and abstraction of hydrogen atom by a growing radical.11,12,57

Scheme 1. Reversible equilibrium in ATRP reaction reproduced from Roghani-Mamaqani et al.11

by permission of John Wiley & Sons, Inc.

The application of various initiation systems is an advantage of ATRP over the otherCRP systems. Reverse atom transfer radical polymerization (RATRP) is a suitable methodfor circumventing oxidation problems of normal ATRP. The reactants of this technique areless sensitive to oxygen and therefore it can easily be handled. Additionally, due to thetoxicity of most transition metals, removal or disposal of large quantities of these catalystsmay have environmental issues.58 Consequently, several other initiation systems have beeninvestigated to overcome to this deficiency. For example, activators generated by electrontransfer (AGET),13,14,59–61 activators regenerated by electron transfer (ARGET),62,63 andinitiators for continuous activator regeneration (ICAR)64,65 have been investigated recently.In the AGET initiation system, an oxidatively stable transition metal complex goes quicklythrough a reduction reaction and forms a catalyst in its lower oxidation state. This iscarried out by a reducing agent, and then polymerization reaction is mainly continued bynormal ATRP mechanism. Tin(II) 2-ethylhexanoate and ascorbic acid are the most commonreducing agents used in AGET ATRP.59 According to the CRP mechanism, in the case thatconcentration of active radical species is sufficiently low (ka� kda), the probability ofbimolecular termination reactions is reduced.66 The ARGET ATRP method provides acontinuous controlled polymerization with a significant reduction of the amount of copperbased catalyst complex (down to 10 ppm) due to a constant regeneration of the activatorspecies by environmentally acceptable reducing agents, which compensate for any loss ofactivator by termination. In ICAR ATRP, free radicals are slowly and continuously generatedby conventional radical initiators (e.g., 2,2′-azo-bisisobutyronitrile (AIBN)) throughout apolymerization to constantly reduce and regenerate the metal complex that accumulates asa persistent radical (Scheme 2).

Additionally, there is a living radical polymerization known as single electron transferliving radical polymerization (SET-LRP) which follows the same mechanism as ARGETATRP according to the reports of Matyjaszewski et al.60 SET-LRP is catalyzed by extremelyreactive metal in its normal state (Mt0) that is formed by outer-sphere single-electron trans-fer. The reaction is controlled or deactivated by metal in its higher oxidation state species

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Scheme 2. Proposed equilibrium between dormant and active species in an AGET, ARGET, andICAR ATRP system reproduced from Braunecker and Matyjaszewski 55 by permission of ElsevierScience Ltd., UK.

that are formed via the same process. In the ATRP process, radicals and the deactivatorspecies are formed through the homolytic atom transfer of the halogen radical atom fromthe dormant species to the activator. This process proceeds via an inner-sphere electrontransfer mechanism. However, in SET-LRP, radicals and activator complex are formed bythe heterolytic outer-sphere single-electron transfer reaction between the electron-donorand electron-acceptor species. Under SET-LRP conditions, the resulting MtnX/L complexis unstable and spontaneously disproportionate into extremely reactive Mt0 atomic species(activator) and Mtn+1 X2/L complex (deactivator). It has been reported that SET-LRP is veryeffective at room temperature and that extremely high molecular weights can be obtainedin conjunction with a low PDI. Even in the presence of typical radical inhibitors such asphenol, SET-LRP shows control over the molecular weight distribution and exhibits a highreaction rate.68,69

3.1.2 Nanocomposites by In situ ATRP.3.1.2.1 Non-Grafting Method. Synthesis of finely well-defined exfoliated polystyrene

nanocomposites by ATRP method at 110◦C using cloisite 30B was investigated by ourresearch group. The living nature of ATRP reaction was employed to in situ synthesizetailor-made polystyrene chains with narrow molecular weight distribution in the presence ofinorganic fillers.3,11 We also prepared clay-dispersed polystyrene nanofibers with diametersin the range of 500 nm by combination of ATRP and electrospinning.57 In addition, Dattaet al.70 carried out an in-depth study on the structure and properties of a series of poly(ethylacrylate)/clay nanocomposites prepared by in situ ATRP. They also showed that interactionbetween the carbonyl groups of poly(ethyl acrylate) and hydroxyl groups of cloisite 30Bresults in an increase of polymerization rate.

In emulsion polymerization, monomer is insoluble in the medium, but it can be emul-sified in the medium by various emulsifiers. Emulsification divides monomers into twoparts in the reaction medium. A portion of monomers is in the form of micelles with 5–10nm diameter and the other part forms droplets with 1–10 µm diameter. Initiator is solublein the medium and initiation occurs in the micelles by diffusion of the initiator. Someindividual monomer molecules present in the reaction medium transferred from dropletsinto the micelles by the reduction of monomer concentration in the micelles by progress ofpolymerization. The radicals formed in the medium by decomposition of the initiator canbe surrounded by the dissolved monomer and emulsifier molecules, or it can be absorbed by

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the micelles. Finally, micelles grow to 50–300 nm by the diffusion of monomer moleculesuntil the monomer is completely consumed. These particles which remain suspended in themedium are called latex. Emulsifier adsorbed at the interface and stabilizes the particlesby either electrostatic effect (anionic and cationic surfactant), steric effect (nonionic sur-factants), or both of them (polyelectrolytes).71 There is not any direct relation between thesize of the latex particles in emulsion polymerization and the initial monomer droplet andmicelle sizes. The fraction of the dissolved monomer, the amount of the modifier, and thereaction temperature are important factors which determine the final size of the particles.Providing that the monomer droplets become small to be able to enter the initiator moietiesand take part in the polymerization process, the complex conditions of nucleation in theemulsion process can be eliminated. These conditions can be achieved by strong shearforces like ultrasonication and resulted in a new way of polymerization which is calledminiemulsion. Therefore, micellar nucleation can be avoided by using the surfactant inconcentrations of lower than that of the critical micelle concentration value. In this tech-nique, the monomer droplets of 50–500 nm in size formed by the addition of a co-surfactantsuch as hexadecanol or an ultra hydrophobe like hexadecane. The co-surfactant limits thecollision and adhesion of monomer droplets by forming a barrier on their surfaces. Inaddition, the co-surfactant or an ultra hydrophobe prevents from the diffusion of monomermolecules from the small to large particles (Ostwald ripening) by building up an osmoticpressure between the monomer droplets. The capability of highly hydrophobe monomerspolymerization and making particles containing additives are the advantages of miniemul-sion over emulsion polymerization; however, the requirement of removing hydrophobefrom the product is its disadvantage.71–73 Chen et al.74 synthesized super-paramagneticmagnetite/polystyrene nanocomposite particles by inverse emulsion polymerization withwater-based magnetic ferro-fluid as dispersing phase and organic solvent and styrene as con-tinuous phase. They described the possible mechanism for the formation of the compositeparticles. Khezri et al.75 reported the synthesis of well-defined clay-dispersed poly(styrene-co-methyl methacrylate) nanocomposites via RATRP in miniemulsion system was (Scheme3). They used the reverse initiation technique for its lesser sensitivity to oxygen. Addition-ally, they consider appropriate selection of hydrophobic ligand and also a surfactant which

Scheme 3. General procedure for preparation of poly(styrene-co-methyl methacrylate) nanocom-posites reported by Khezri et al.75 by permission of John Wiley & Sons, Inc.

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is stable at higher temperatures to conduct a successful miniemulsion ATRP. They ob-tained monodisperse droplets and particles with sizes in the range of around 200 nm inwhich increasing the clay content results in a decrease in the molecular weight of theirnanocomposites. However, the polydispersity index of the extracted polymer chains arerelatively high and increases by the addition of nanoclay content. They ascribed the highvalue of PDI to the decrease of deactivator concentration in the organic phase at highertemperatures which is the result of variation of the deactivator partitioning behavior athigher temperatures. Also, Khezri et al.14 reported the synthesis of poly(styrene-co-methylmethacrylate) nanocomposites using AGET ATRP in miniemulsion. Additionally, Hatamiet al.13 discussed the synthesis of poly(styrene-co-butyl acrylate) nanocomposite latexesvia AGET ATRP in the miniemulsion system.

A microemulsion system is a dispersion of oil in water which is thermodynamically sta-ble and optically transparent. Unlike a miniemulsion, domains in microemulsion stabilizedby suitable surfactants spontaneously without external shear forces. Microemulsion poly-merization consists of monomer-swollen micelles of 5–10 nm in size. In this process, someof the micelles are converted to polymer particles and the remaining monomer-swollenmicelles, similar to emulsion polymerization systems, supply monomer to the particles.The final product is stable latex composed of polymer particles with particle sizes in therange of 10–100 nm which displays high molecular weight and broad molecular weight dis-tribution.65,66,76 Reddy et al.77 presented a noncovalent method for coating MWCNT withpolyaniline nanospheres using a microemulsion polymerization method. In their well-tunedmethod, aniline polymerization is performed with MWCNT in the presence of sodium do-decyl sulfate (SDS) as a surfactant and also a dopant. Composites prepared without thesurfactant were found to be in core-sheath type cable structures. Coating of a very smallsize of polymer nanospheres onto MWCNTs in less polymerization time, nanocompositeswith good thermal and electrical properties through the synergic interaction between CNTsand polyaniline, and nanocomposites with excellent colloidal stability are some advantagesof their method over the other chemical methods.

3.1.2.2 “Grafting from” Technique. Botcher et al.78 were the first who reported thesurface functionalization of silicate layers by ATRP initiator. They intercalated an ATRPinitiator, consisting of a quaternary ammonium salt moiety and a 2-bromo-2-methyl pro-pionate (BMP) moiety, into the interlayer spacings of the layered silicate. Zhao et al.79

employed their functionalization procedure to modify MMT layers and subsequently syn-thesize polystyrene chains on the layers by “grafting from” technique. Datta et al.80 graftedATRP initiator onto cloisite 30B platelets via transesterification reaction. Subsequently,they employ surface-initiated atom transfer radical polymerization (SI-ATRP) to synthe-size tailor-made poly(ethyl acrylate)/clay nanocomposites (Scheme 4). Extensive exfoli-ation was facilitated by using these initiator modified clay platelets. Poly(ethyl acrylate)chains with controlled polymerization and narrow polydispersities were forced to be grownfrom the clay gallery (intergallery) as well as from the outer surface (extragallery) of theclay platelets. Molecular weights of all the surface bound polymers are lower and they havebroad PDI compared to the corresponding free polymers generated outside. This is attributedto the slower diffusion of monomer inside the clay galleries and the competition betweenthe initiator sources. They showed that in the case of 40% initiator grafting on the surface ofcloisite 30B, a weight loss of about 21% is obtained from the results of thermogravimetricanalysis (TGA) at temperatures higher than 600◦C. In addition, the weight percentage ofthe corresponded polymer binding to this clay is approximately 56% estimated from theweight loss of about 77% after the complete degradation of the nanocomposite at hightemperatures.

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Scheme 4. Attachment of ATRP initiator on the cloisite 30B surface via esterification and thesubsequent formation of polyacrylate chains by SI-ATRP reproduced from Datta et al.78 by permissionof John Wiley & Sons, Inc.

The substrate geometry and graft density play key roles in the kinetics of SI-ATRP.For instance, in densely grafted spherical particles, as the brush thickness increases, propa-gating radicals spatially diverge from one another and recover bulk kinetic rates. However,the surface initiated polymerization from planar substrates is expected to exhibit confine-ment effects irrespective of the brush thickness. Behling et al.81 studied the kinetics of theSI-ATRP of styrene from the surface of functionalized MMT as a function of graft density.Compared with ATRP reactions with the free initiator, they observed a seven-fold increasein the polymerization rate at the highest graft density of about one chain per nm2. However,bulk kinetics is recovered as the graft density is reduced. They related this phenomenon toa consequence of local concentration heterogeneities that shifts the ATRP equilibrium infavor of the active state. Karesoja et al.82 also reported the attachment of 11-(2-Bromo-2-methyl)propionyl-oxy-undecyl trichlorosilane (BMPUS) ATRP initiator on MMT plateletsvia silylation reactions. Subsequently, the initiator-attached clay was used to polymerizebutyl acrylate (BA) and methyl methacrylate (MMA) on the clay surface. Yenice et al.83

reported the synthesis of poly(styrene-b-tetrahydrofuran) block copolymer on the surfacesof silicate layers by mechanistic transformation. Polystyrene/MMT nanocomposite wassynthesized by in situ ATRP from initiator moieties immobilized within the silicate gal-leries. Then, poly(styrene-b-tetrahydrofuran) nanocomposite was prepared by mechanistictransformation from ATRP to the cationic ring opening polymerization (CROP). Theyshowed that degradation thermograms of the nanocomposites have two stages. The firstone is in the temperature window of 230–320◦C and the weight loss is about 25% corre-sponding to poly(tetrahydrofuran) chains. The second one is in the temperature window of320–440◦C and the weight loss is about 65% corresponding to the polystyrene chains. Theyalso presented that these values are in close agreement with the composition of the blockcopolymer. The improved decomposition temperature of poly(tetrahydrofuran) comparedto polystyrene segment may be related to some physical interaction of polyether with theSi-O-Si layer of nanoclay. Also, Li et al.84 reported the synthesis of covalently bonded lay-ered silicate/polystyrene nanocomposites via ATRP in the presence of the initiator-modifiedlayered silicate.

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Qin et al.85 studied the attachment of n-Butyl methacrylate (n-BMA) brushes on thesurface of SWNT by a “grafting from” method. Carboxylic acid groups on SWNT wereformed by nitric acid oxidation. The ATRP initiator was covalently attached to the SWNT byesterification of 2-hydroxyethyl 2-bromopropionate (HEBP) with carboxylic acid groups.Methyl 2-bromopropionate (MBP) was added as free initiator during the brush preparationto control the growth of brushes and to monitor the polymerization kinetics (Scheme 5). Ac-cording to their results, the molecular weight of free poly(n-butyl methacrylate) increaseslinearly with monomer conversion. And also, poly(n-butyl methacrylate) cleaved from theSWNTs after high conversion had the same molecular weight as poly(n-butyl methacrylate)produced in solution. They showed that the amount of PnBMA grown from the SWNTincreased linearly with the molecular weight of the free PnBMA. Using atomic force mi-croscopy (AFM) images they showed that the contour lengths of the SWNT brushes ona mica surface range from 200 nm to 2.0 µm and an average height of the backbone isin the range of 2–3 nm, indicating that the bundles of original SWNT were broken intoindividual tubes by functionalization and polymerization. Also, Kong et al.86 carried outa similar work on functionalization of MWNTs and synthesized poly(methyl methacry-late) (PMMA) grafted MWNTs by the “grafting from” technique. They experienced that10-fold increase of the feed monomer results in an increase of graft polymer thicknessfrom 3.8 to 14 nm. Therefore, the thickness of the polymer layer can be well controlledthrough the in situ ATRP approach by varying the feed ratio. The corresponding weightloss as a function of temperature estimated to increase from 31.9% to 82% at tempera-tures higher than 600◦C by 10-fold increase of monomer in the feed. They also synthesizedPoly(N-isopropylacrylamide) (PNIPAAm) grafted MWNT from initiators attached onto theMWNTs by ATRP. These studies result in covalently linked core-shell molecular nanorodswith various amounts of polymer shell. The resulting hybrid molecular nanorods showed

Scheme 5. General procedure for preparation of poly(n-butyl methacrylate) nanocomposites reportedby Qin et al.85 by permission of the American Chemical Society, USA.

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temperature responsive behaviors in water, because of the hydrophilic and hydrophobictransformation of the bonded PNIPAAm chains at 30–35◦C.87 In addition, Yao et al.88

functionalized SWCNT along their sidewalls with phenol groups using the 1,3-dipolarcycloaddition reaction. Phenol groups were further derivatized with 2-bromoisobutyrylbromide (EBiB) and results in the attachment of ATRP initiators to the sidewalls of thenanotubes. These initiators conducted ATRP of MMA and tert-butyl acrylate (t-BA) fromthe surface of the nanotubes. They observed extremely high molecular weights (higher than300000 g.mol−1) after relatively short polymerization times (2 h). Also, the polydispersityindex of the recovered polymer chains was consistently greater than 1.6. In addition, at-tempts to chain extension of the grafted polymers did not result in any significant massenhancement. These results indicate that the polymerizations were not living and had ter-minated irreversibly. Using an approximate nanotube molecular weight of 668000 g.mol−1

and an initial sample mass of 4 mg, they estimated that the number of attached polymerchains varies between 7 and 21 per nanotube, depending on the sample.

Chen et al.89 reported the growth of lightly crosslinked poly(2-hydroxyethyl methacry-late) (PHEMA) brushes and subsequent capsule formation using Pickering emulsion inter-face initiated ATRP. Initiator-immobilized silica nanoparticles assembled at the interfaceof oil and water and finally stable Pickering emulsions were formed (Scheme 6). Poly-merization was conducted in the water phase of Pickering emulsions from the part of thesurfaces of initiator-coated silica nanoparticles exposed to water. Authors showed that asPHEMA has a character of lightly crosslinking when the polymerization occurs in water,novel hybrid capsules can be obtained. These hybrid hollow capsules were obtained be-cause of the partial crosslinking of PHEMA brushes and have an attractive character ofsemipermeable membranes. They calculated that ∼7.06 × 105 initiator sites are loadedper nanoparticle and therefore the grafting density of initiators on the surface of silicananoparticles is ∼2.5 initiator.nm−2, which is nearly the same as the TGA analysis result(∼1.5 initiator.nm−2). They also showed that referring to the char value at 600◦C obtainedfor 2-bromoisobutyrate-functionalized silica nanoparticles as a reference, the PHEMAweight content relative to that of silica particles was calculated to be 55.2% that increasesto the value of 70.3% by increasing the polymerization time and monomer concentration.

Scheme 6. (a) Attachment of ATRP initiator on the silica nanoparticles; (b) preparation of Pickeringemulsions; (c) grafting PHEMA chains from silica nanoparticles reproduced from Chens et al.89 bypermission of John Wiley & Sons, Inc. (Color figure available online).

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Scheme 7. Attachment of ATRP initiator on the surface of silica nanoparticles reproduced fromCarrot et al.90 by permission of John Wiley & Sons, Inc.

Carrot et al.90 used initiator modified silica nanoparticles to synthesize poly(n- buty-lacrylate) by combination of ATRP and “grafting from” method (Scheme 7). This methodensures the covalent binding of all the polymer chains to the nanoparticles because poly-merization is initiated from the moieties previously bound to the surface and thereforenanocomposites with hard core and soft shell are formed by their well-tuned process. Thismethod has the advantage of avoiding nongrafted chains and results in individual coatedparticles. Also, the possibility of controlling molar mass of the grafted polymer chains isprovided using this method. This factor is important for the control of SiO2 nanoparticledispersion in the polymer matrix. Moreover, the small size of the particles leads to theirbetter dispersion in the polymer matrix. The percentage of grafted initiator molecules ascompared with the total amount of starting material was determined from elemental anal-ysis on the basis of carbon. The weight percentage grafting is described as the proportionof the weight fraction of carbon in the sample and initiator and the grafting efficiencyis estimated from the percentage of hydroxyl groups initially available at the surface ofthe particles (following equations). Elemental analysis gave a percentage of carbon in thesample of about 2.80 wt%. This corresponds to a grafting percentage of 6.23 wt% and agrafting efficiency of 61%. It represents 61% of grafted initiator segments as comparedwith the initial amount of OH groups on the surface of the SiO2 particles. This percentageis translated into a grafting density of 1.4 initiators per nm2.

In similar research, Harrak et al.91 reported the grafting of polymer chains from silicaparticles via ATRP. They functionalized silica surface by silanization with a mercaptopropyltriethoxysilane (MPTMS) and then reacting the thiol with 2-bromoisobutyryl bromidegenerates the halogen-functional ATRP initiator. Then, the polymerization of styrene wasconducted. The number of initiator molecules grafted onto the surface of the nanoparticlesis correlated to the number of bromine obtained from elemental analysis according to thefollowing equation:

no. of molecules per nm2 = (nBr × NA)/(A × 1018)

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where, NA is the Avogadro number, A is the specific area of the silica particles in nm2,and nBr is the number of moles of bromine determined from elemental analysis relative tothe amount of silicone atoms. They debated that a monolayer was formed by considering1 silane per nm2, that is, 6 wt% of MPTS. However, by increasing this amount to 5 silanesper nm2, that is, 30 wt% of MPTS, the polycondensed silanes layer is obtained. Quantitativemeasurements obtained from elemental analysis of sulfur showed the grafting efficiency of100%. The amount of polystyrene grafted onto the particles has been estimated from TGAand was found to be 49.36% which corresponded to a total of 55 chains per nanoparticles.

Wang et al.92 also prepared polymer grafted silica nanoparticles by introducing silicasurface with peroxide groups for RATRP. This was implemented by the reaction of hy-drogen peroxide with chlorosilyl groups, which were introduced by the treatment of silicawith thionyl chloride. RATRP from silica nanoparticles was then performed to attach ho-mopolymer chains to an inorganic core. Additionally, Bottcher et al.93 reported the covalentattachment of (11′-Chlorodimethylsilylundecyl)-2-chloro-2-phenylacetate as an ATRP ini-tiator to the surface of silica and subsequent grafting of styrene monomer from its surfaceby ATRP. They proved the living nature of their system by the second addition of styreneinto the system. By this reaction the number-average molecular weight of their polystyreneincreases from 21000 to 70000 g.mol−1 which corresponds to an increase of graft densityfrom 1.2 to 3.8 g polystyrene/g SiO2.

3.1.2.2 “Grafting through” Technique. Surveying literature also shows that in the caseof nanolayer loaded nanocomposites, lots of studies have been carried out. For instance,ATRP was employed by our group to synthesize polystyrene nanocomposite with two foldcharacteristic chains by a “grafting through” method.32 In this work, ATRP was applied to amixture of monomer and modified MMT, which is ion-exchanged with a special chemicalhaving a double bond on its structure. In this process, a mixture of clay-attached and freepolystyrene chains were obtained (Scheme 8). Not only the effect of nanoclay layers onthermal properties and kinetic of polymerization, but also the discrepancies in characteristicthermal and kinetics behaviors of free and attached polymer chains in the same matrix havebeen investigated in this study.

Scheme 8. Preparation of polystyrene nanocomposite with two fold characteristic chains by in situATRP reported by Roghani-Mamaqani et al.32 by permission of John Wiley & Sons, Inc.

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According to the results of this work, nanoclay exerts a positive effect on the rate ofthe polymerization of free polystyrene chains. The polydispersity index of polymer chainsincreases by the addition of nanoclay. Additionally, the number- and weight-average molec-ular weights of clay-attached polystyrene chains are lower than that of the freely dispersedpolystyrene chains. Also, polydispersity index of the attached chains is higher compared tothe free polystyrene chains. Ahmadian-Alam et al.94 also synthesized poly(styrene-co-butylacrylate) nanocomposite latexes via in situ AGET ATRP by employing a clay-anchored reac-tive modifier. Using 1H NMR spectra, they showed that the molar ratio of styrene in attachedpolymer chains is somewhat higher than the free polymer chains. They attributed this mainlyto the feed mole fraction and reactivity ratio of styrene and butyl acrylate (rst = 0.74 andrBA = 0.24). More reactivity ratio and easy diffusion of styrene into the nanoclay gallerieson account of its higher concentration increase the incorporation of styrene comonomerinto the attached copolymer chains. Zeng et al.95 synthesized poly(methyl methacrylate)and polystyrene/clay nanocomposites via in situ bulk polymerization. They used a polymer-izable group containing surfactant, 2-methacryloyloxyethylhexadecyldimethylammoniumbromide (MHAB) to modify the clay surface. Introducing polymerizable groups onto theclay surface improved clay dispersion significantly, and exfoliated poly(methyl methacry-late) and polystyrene/clay nanocomposites were successfully synthesized with a clay con-centration of 5 wt%.

3.2. Reversible Addition-Fragmentation Chain Transfer

3.2.1 Mechanistic Study. Fast exchange reactions in this technique lead to well controlledsystems. Successful application of reversible addition-fragmentation chain transfer (RAFT)process requires appropriate selection of a RAFT reagent for a particular system. The RAFTtechnique uses dithioester, dithiocarbamate, trithiocarbonate, and xanthate compounds astransfer agents to mediate the polymerization via a reversible chain transfer process.96

These RAFT agents include an appropriate leaving group (R′) that is able to re-initiatepolymerization and a Z-group that influences the stability of the intermediate macroradicalspecies.97 Employment of an appropriate RAFT agent with suitable leaving and stabilizingagents depends on the monomer and is a crucial factor in RAFT-mediated polymerization.Both the R and Z groups of a RAFT agent should be carefully selected to provide appropriatecontrol. Generally, R• needs to be more stable than Pn

• to efficiently fragment and initiatepolymerization. The stability of the dormant species and rate of addition of R• to a givenmonomer are important factors in the selection of the R group. Mechanistically, afterpropagating macroradicals (R•) add to the carbon-sulfur double bond of a RAFT reagentwith a rate constant of ka, the formed radical adduct undergoes β-scission and either yieldsback the reactants (with a rate constant of ka) or releases another initiating radical with arate constant of fragmentation kf. In this way, equilibrium between the dormant and theactive species is established in which all the chains propagate simultaneously (Scheme 9).55

3.2.1 Nanocomposite Synthesis by In situ RAFT3.2.1.1 Non-Grafting Method. Qu et al.98 synthesized amphiphilic poly(styrene-

block-acrylamide) organic-MMT nanocomposites using dithioester as a chain-transferagent and azobisisobutyronitrile as an initiator by RAFT-mediated polymerization method.They used cetyltrimethyl ammonium chloride as a clay surface modifier. Subsequently, theypolymerized styrene and acryl amide in the presence of modified clay layers and obtainedexfoliated nanocomposites.

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Scheme 9. Reversible equilibrium in RAFT reaction reproduced from Braunecker and Maty-jaszewski55 by permission of Elsevier Science Ltd., UK.

3.2.1.2 “Grafting from” Technique. To graft a polymer chain from an inorganic sub-strate, two main approaches can be envisaged: (a) use of a surface-anchored initiator witha free RAFT agent in solution; and (b) use of a surface-anchored RAFT agent and anadequate initiation method. Initiators can be anchored onto inorganic surfaces by severaltechniques such as chemical modification, plasma discharge, and high energy irradiation.3.2.1.2.1 Surface-Attached Initiators. Baum et al.99 reported the formation of polymerbrushes by RAFT-mediated polymerization initiated by a surface-anchored azo-initiator.They synthesized styrene, methyl methacrylate, and N, N-dimethylacrylamide brushes onthe silicate surface. They also produced block copolymer brushes that display reversiblesurface properties upon treatment with block-selective solvents. Both Mn and PDI werecomparable for free polymer vs. cleaved polymers. Also, some “grafting to” reactionsmay occur by bimolecular coupling of the active free polymer chains in solution andactive radicals on the surface. The continuous stepwise increase observed in brush filmthicknesses proves that there are no significant termination reactions occurring during thebrush polymerization. Zhai et al.100 carried out controlled grafting of polybetaine brushesonto hydrogen-terminated Si(100) substrates (Si–H substrates) via surface-initiated RAFT-mediated polymerization. The azo initiator was immobilized on the Si–H surface by cou-pling of a ω-unsaturated alkyl ester to the Si–H surface under UV irradiation. Subsequently,the reduction of the alkyl ester into a hydroxyl group by LiAlH4 and esterification of thehydroxyl group with the initiator 4,4′-azo-bis(4-cyanopentanoic acid) after acid chlorina-tion resulted in the initiator functionalized substrate (Scheme 10). In the presence of thechain-transfer agent, RAFT-mediated polymerization of the sulfobetaine monomer N,N ′-dimethyl(methylmethacryloyl ethyl) ammonium propane sulfonate (DMAPS) was initiatedfrom the surface-immobilized azo initiator to produce DMAPS polymer (PDMAPS) brusheson the silicon substrate. This approach presents a very important disadvantage. Because ofthe living character of the surface-initiated RAFT process, the ellipsometric results showedthat the thickness of the PDMAPS film on the Si-g-PDMAPS surface increased linearlywith the time of polymerization of the polymer brush. In this method, the number of initi-ating species arising from the initiator must be smaller than those resulting from the chaintransfer agent to achieve a good control of polymer chain growth. As a consequence, thegrafting density will be low and a great amount of ungrafted polymer chains will be formed,which may require additional separation and purification of the resulting nanohybrids.96

Wang et al.101 reported the surface functionalization of Fe3O4 magnetic nanoparticleswith styrene and acrylic acid (AAc) via RAFT-mediated polymerization. Peroxides and

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Scheme 10. Surface functionalization of the silicon substrate and synthesis of polymer brushesreproduced from Zhai et al.100 by permission of the American Chemical Society, USA.

hydroperoxides generated on the surface of Fe3O4 nanoparticles via ozone pretreatmentfacilitated the thermally initiated graft polymerization in the RAFT process.3.2.1.2.2 Surface-Attached RAFT Agents. RAFT agent attachment to a nanofiller surfacecan be carried out via either the Z or R groups (Scheme 11). The R-group approach aremost like the “grafting from” technique; however, the Z-group approach is similar to the“grafting to” approach. The number of graft polymer chains in the latter method is lower.The reason is that the grafted polymer chain acts as a barrier to the attachment of subsequentchains. Therefore, by addition of surface attached chains, control of molecular weight andpolydispersity of polymer chains in the “grafting to” method becomes difficult. On the otherhand, using the “grafting from” approach allows higher graft density and better control ofmolecular weight and polydispersity of the polymer chains, as monomer molecules caneasily diffuse to the surface of the particles.102 The Z-group approach is not a “grafting

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Scheme 11. Two different approaches of surface attachment where RAFT agent attached to thesurface via its Z or R group reported by Hojjati and Charpentier102 by permission of John Wiley &Sons, Inc.

from” method, because polymer chains do not grow from the substrate surface. However,this section is discussed here for the sake of coherency.

3.2.1.2.2.1 R-Group Approach. Zhang et al.103 intercalated 10-carboxylic acid-10-dithiobenzoate-decyltrimethylammonium bromide (CDDA) as a RAFT agent betweenclay layers. Consequently, the CDDA-intercalated MMT was used as a RAFT agent inthe in situ RAFT-mediated polymerization for preparation of polystyrene nanocomposites(Scheme 12). The theoretical loaded amount of CDDA on MMT is 29%. This value is abit higher than the experimental result of 25% determined by TGA. Appropriate controlof molecular weight and narrow molecular weight distribution demonstrates that the in-tercalated MMT does not restrict diffusion of propagating radicals and dormant chains.Growth of polystyrene chains on the surface of layered silicates results in polymer brushes.

Scheme 12. General procedure for preparation of polystyrene nanocomposites reported by Zhanget al.103 by permission of John Wiley & Sons, Inc.

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Their similar molecular weight to the chains formed in the solution indicates their sameprobability of propagation. In situ RAFT-mediated polymerization with a RAFT agent-intercalated clay layers results in nanocomposites with exfoliated structure and higherthermal stability in comparison with the neat polystyrene. Additionally, Samakande etal.104–106 studied the synthesis of RAFT-agent anchored ammonium bromide compoundsto synthesize polystyrene/clay nanocomposites in bulk and miniemulsion system.

Cui et al.107 successfully grafted polystyrene chains from the surface of MWNTs viaRAFT process by using RAFT agent immobilized on MWNTs. To functionalize MWNTs,carboxylic groups on MWNT were formed by nitric acid oxidation. Then, bromoisobutyrategroups were covalently attached to the MWNT by esterification of 2-hydroxyethyl-2α-bromoisobutyrate (HEBrB) with carboxylic groups, forming bromoisobutyrate functional-ized MWNT (MWNTBr). RAFT agent functionalized MWNT was produced by substitutereaction of MWNTBr with phenyldithiobenzoate magnesium bromide (PhC(S)SMgBr).In addition, Hong et al.108 synthesized MWNTs with temperature-responsive shells bygrafting PNIPAAm from MWNT via surface RAFT-mediated polymerization using RAFTagent functionalized MWNT as chain transfer agent (Scheme 13). They showed that theamount of PNIPAAm attached to MWNTs determined by TGA varies from 35 to 87 wt%when the polymerization time increased from 9 to 36 h. Also, according to the results ofTGA, about 0.76 RAFT agent functions attached per 100 carbon atoms calculated from theweight loss from the RAFT agent modified MWNT in temperature window of 280–420◦C.In addition, referring to the results of X-ray photoelectron spectroscopy (XPS) analysis,the mole content of the RAFT agent functions on the surface of MWNT is about 0.8% withrespect to carbon, which is similar to the result obtained from TGA.

Scheme 13. General procedure for preparation of PNIPAAm shells on MWCNTs reported by Honget al.108 by permission of John Wiley & Sons, Inc.

Tsujii et al.109 also studied the mechanism and kinetics of RAFT-mediated graft poly-merization of styrene initiated on silica particles. They synthesized polystyrene graftedsilica particles by SI-ATRP technique, and then the terminal halogen atom of the graft

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polymer was converted to the RAFT agent by the reaction with 1-phenylethyl dithioben-zoate (FEDB). The graft polymerization of styrene was carried out in the presence of a freeRAFT agent. The authors showed that since the RAFT-mediated polymerization proceededthrough equilibrium between non-anchored chains that were thermally initiated and thegrafted polymer chains, the added RAFT agent could control the polymerization occur-ring in bulk and simultaneously allow the exchange of a propagating grafted chain with adormant free chain. They also proved that the enhanced recombination is specific to theRAFT-mediated graft polymerization and is due to the effective migration of radical onthe surface by sequential degenerative (exchange) chain transfer. They showed that graftdensity decreases with polymerization time especially at first and then reaches about 0.08chains.nm−2. This means that termination reactions occur at the early stages of polymeriza-tion, while no termination occurs after graft density reaches its stable value. In this system,surface migration of the graft radical can occur via degenerative chain transfer reactions athigh density of surface grafted dormant chains. Surface migration narrows the chain lengthdistribution of the graft chains. However, it allows the graft radicals to undergo bimolecu-lar termination reactions especially at large graft densities and consequently broadens thechain length distribution. There is a critical value of graft density below which the surfacemigration of graft radicals hardly occurs. For graft densities below this limit, a RAFTsystem would behave similarly to an ATRP system.

Hua et al.110 reported the controlled graft modification of silica particles using RAFT-mediated polymerization under ultrasonic irradiation. They dispersed silica particles indry toluene under ultrasonic irradiation. Subsequently, silica particles were reacted with3-aminopropyldimethylethoxysilane (APDMS), and S-1-dodecyl-S′-(α,α′ dimethyl-α′′-acetic acid) trithiocarbonate (DDCAT). They carried out graft polymerization of methylacrylate under ultrasonic irradiation (Scheme 14). According to their findings, there is atrithio-group concentration of 0.30 mmol.g−1 for SiO2-RAFT agent and also graft poly-merization was well controlled and poly(methyl acrylate) (PMA) modified silica particles

Scheme 14. Modification and graft polymerization of silica by RAFT-mediated polymerizationreproduced from Hua et al.110 by permission of Elsevier Science Ltd., UK.

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were synthesized successfully. The weight loss increased from 24.8 to 34.9% when thepolymerization time increased from 2 h to 6 h, which indicates that the amount of PMAgrafted on the silica gel increased with the polymerization time.

Other methods for attaching the RAFT agent to silica or other substrates are surfacemodifications with hydroxyl or amine groups followed by esterification or amidation witha RAFT agent bearing a carboxylic acid or an activated ester.111–113

3.2.1.2.2.1 Z-Group Approach. This method has been reported less than the R-groupapproach. In this method, grafting density is low and bonds linking the inorganic surfaceto the polymer brushes are weak. Growing chains are hindered by the other chains toreach the surface and undergo reversible chain transfer reactions. Besides, the RAFT agentis sensitive to hydrolysis, oxidation, and nucleophilic attack and loss of grafted polymerchains can occur. However, this strategy enables the preparation of nanohybrids containingexclusively dormant chains.96

Nguyen et al.114 reported RAFT-mediated polymerization of styrene and methylmethacrylate using cumyl dithiobenzoate as the RAFT agent immobilized via Z-group to sil-ica nanoparticles. The procedure of generating sulfur-free polymers is presented in Scheme15. Immobilization of the RAFT agent, formation of tethered polymer chains, achievingtethered living polymer via flushing the system with AIBN-derived 2-cyanoisopropyl rad-icals in the absence of monomer, releasing of these radicals via a single addition fragmen-tation chain transfer step, and finally, termination by another small radical yields the finalsulfur-free polymeric material. They concluded that tethering of a RAFT agent to a solidsubstrate via the Z-group allows for effective separation of the living and the terminateddead polymeric material generated during RAFT-mediated polymerization.

Scheme 15. Attachment of polymer brushes on the surface of silica reproduced from Nguyen et al.114


Stenzel et al.115 prepared glycopolymer brushes composed of N-acryloyl glucosamine(AGA) and N-isopropylacrylamide (NIPAAm) using RAFT-mediated polymerization. TheRAFT agent was immobilized on the surface of a treated silicon wafer via covalent at-tachment using the Z-group. They showed that the polymers generated in solution confirmthe living behavior with the molecular weight increasing linearly with monomer conver-sion while the molecular weight distribution remains narrow. They found that brusheswith length of 5 to 30 nm depending on the monomer conversion were achieved. At the

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same conversion, PNIPAAm brushes were typically 5 nm bigger than PAGA brushes. ThePNIPAAm polymer brush with a thickness of 26 nm at a monomer conversion of 0.47 corre-sponds to a molecular weight of the free PNIPAAm polymer in solution of 21620 g.mol−1.Therefore, a grafting density of 0.67 chains.nm−2 was estimated which indicates thatthese brushes are very dense. The linear increase between monomer conversion and brushthickness reveal that the process is controlled. Also, Peng et al.116 presented the sur-face modification of Si(100) with covalently bonded polymer brushes by interface RAFT-mediated polymerization, with the RAFT agent anchored on the substrate surface. Thesimple approach of immobilizing the thiocarbonyl-thioether compound on a hydrogenterminated Si(100) surface via the radical stabilizing group (Z-group) provided the stable,Si-C bonded RAFT agent for the subsequent RAFT-mediated polymerization process. Theypresented that the degrees of polymerization are estimated to be 64 and 70 respectively forthe Si-grafted PHEMA with thickness of 6 nm and Si-grafted PMMA with thickness of5 nm, based on the grafting efficiency of 36% and bulk densities of 1.15 and 1.17 g.cm−3

for PHEMA and PMMA. The molecular weights of the grafted PHEMA and PMMA arecalculated to be about 8300 and 7000 g.mol−1, which are comparable to the values mea-sured by gel permeation chromatography (GPC) for the PHEMA (Mn = 8600 g.mol−1)and PMMA (Mn = 7200 g.mol−1) homopolymers at the same time of 8 h for RAFTpolymerization. Thus, the “grafting to” efficiency is comparable to that of the “graftingfrom” process at least in the initial stage. Zhao et al.117 also investigated the Z-supportedRAFT-mediated polymerization to prepare silica hybrids. Two RAFT agents, S-benzylS′-trimethoxysilylpropyltrithiocarbonate (BTPT) and S-methoxycarbonylphenylmethyl S′-trimethoxysilylpropyltrithiocarbonate (MPTT), were synthesized and covalently attachedto the surface of the fumed silica. The resultant silica supported RAFT agents were usedto mediate RAFT-mediated polymerization of vinyl monomers such as methyl acrylate,butyl acrylate, N,N-dimethylacrylamide, N-isopropylacrylamide, methyl methacrylate, andstyrene in the presence of a free RAFT agents to synthesize polymer grafted silica particles.They evaluated the effects of RAFT agents loading on solid supports, types of free RAFTagents, and reaction media on graft polymerization. They concluded that by using 2-(2-cyanopropyl)dithiobenzoate (CPDB) as a free RAFT agent and at low conversions (lessthan 40%), well-defined polymer chains with polydispersity indices less than 1.2 and chainlengths similar to those of free polymers could be successfully grafted onto the surface offumed silica, and relatively high grafting ratios could be achieved. The graft polymerizationof methyl acrylate in heterogeneous media such as methanol can result in longer graftedchain lengths and higher grafting densities due to the heterophase reaction environmentand smaller shielding effect.

The R-group approach can result in a higher molecular weight of grafted polymers andgrafting density, but the molecular weight distribution may be broadened by the possiblechain coupling. However, the Z-group approach can yield better-defined grafted polymerswith monomodal molecular weight distribution, although the grafting density is liable todecrease due to the shielding effect.117

3.2.1.3 “Grafting through” Technique. Salem and Shipp118 performed RAFT-mediated polymerizations in the presence of organically modified clay and consequentlypolystyrene, poly(methyl methacrylate), and poly(n-butyl acrylate) nanocomposites weresuccessfully synthesized (Scheme 16). Well-defined molecular weights and low polydisper-sities are the characteristics of the polymers. According to their findings, linear evolution ofpolystyrene molecular weight with monomer conversion and agreement between the actualand expected molecular weights are observed. In addition, molecular weights of both thetethered and free polystyrene chains are the same, which shows that they grow under the

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Scheme 16. Surface modified clay with double bond containing ammonium salt and use of RAFTreaction to synthesize exfoliated nanocomposites reproduced from Salem and Shipp118 by permissionof Elsevier Science Ltd., UK.

same conditions. They also determined that 20 wt% of the polymer chains are attached tothe silicate layers. In the case of n-butyl acrylate polymerization, molecular weight evolu-tion against conversion shows a good match between the expected and the actual molecularweights and also the polydispersities are low. However, polymerization kinetic data ofpoly(n-butyl acrylate) show an induction period of about 2–3 h before the polymerization,and a limiting conversion of approximately 60% after 36–48 h. They demonstrated thatthese results are due to differences in addition rates of the initiating radical to monomercompared with the addition rates of the propagating radicals to monomer. Also, they showedthat excellent control over the polymerization was achieved up to high conversion in thecase of poly(methyl methacrylate) and there was no apparent induction period during itspolymerization.

Yang et al.119 reported the covalent attachment of MWCNT by polyvinylimidazole(PVI) via the methods of chemical modification and emulsion polymerization. Chemicalbonded carbon nanotubes by heterocyclic polymer material significantly improve theirdispersion in organic solvent. The detailed procedure is presented in Scheme 17. Refer-ring to MWNT filled nanocomposites, Rahimi-Razin et al.120 synthesized PMMA/MWNTnanocomposites by a double modified MWNT and also studied the kinetics of free andattached PMMA chains formation.

Chinthamanipeta et al.121 synthesized poly(methyl methacrylate) coated silica nanopar-ticles. In this study, the surface of silica nanoparticles is modified covalently by attach-ing the methacryl group to the surface using 3-methacryloxypropyldimethylchlorosilane(MPDCS). By polymerization of methyl methacrylate (MMA) using the 4-cyano-4-(dodecylsulfanylthiocarbonyl) sulfanyl pentanoic acid (CDMP) RAFT agent, PMMA–SiO2

nanocomposites were prepared (Scheme 18). They reported that according to the resultsof TGA, the amount of methacrylate groups on the silica particles was determined to be263.6 µmol.g−1 which corresponds to 1.1 methacrylate groups.nm−2 with the assumptionthat the density of the methacrylate-functionalized silica particles was comparable to thatof bulk silica (2.07 g.cm−3) and no weight loss occurs from silica before grafting themethacrylate group. A similar procedure was also presented by Semsarilar et al.122 to syn-thesize poly(methyl methacrylate) coated silica nanoparticles via the “grafting through”technique. Salami-Kalajahi et al.123–124 also synthesized PMMA/silica nanocomposites byin situ RAFT polymerization and studied the kinetics of free and attached chains.

Ngo et al.125 modified the surface of titania nanoparticles (TiO2) by a coupling agentas 3-(trimethoxysilyl)propyl methacrylate (MPS) to form TiO2-MPS polymerizable par-ticles (Scheme 19). MMA and tertbutyldimethylsilyl methacrylate (MASi) were radically

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Scheme 17. Surface modified carbon nanotubes with double bond containing compound and useof RAFT reaction to synthesize nanocomposites reproduced from Yang et al.119 by permission ofElsevier Science Ltd., UK.

polymerized through the immobilized vinyl bond on the surface in the presence of theRAFT agent 2-cyanoprop-2-yl dithiobenzoate using AIBN as an initiator. Thermogravi-metric analysis and elemental analysis revealed a surface coverage of the coupling moleculeof 2.0 wt%. Therefore, higher amounts of free homopolymers were formed than anchored

Scheme 18. General procedure for preparation of poly(methyl methacrylate) coated silica nanopar-ticles reported by Chinthamanipeta et al.121 by permission of Elsevier Science Ltd., UK.

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Scheme 19. General procedure for preparation of poly(methyl methacrylate) and poly(tert-butyldimethylsilyl methacrylate) coated titania nanoparticles reported by Ngo et al.125 by permissionof Elsevier Science Ltd., UK.

onto the surface of titania particles. TGA measurements showed that grafted PMMA andPMASi were accounted for 10% and 4.8% of the particle mass respectively. In addition,the grafting efficiency is higher for the PMMA than for the PMASi. The hindered effect ofthe trialkylsilyl groups of the silylated monomer seems to decrease the ability of growingradicals to be linked to the surface in comparison with MMA.

3.3 Nitroxide-Mediated Radical Polymerization

3.3.1 Mechanistic Study. The nitroxide-mediated radical polymerization (NMP) techniqueuses alkoxyamines as dormant species to control the polymerization process via a dynamicequilibration between them and propagating radicals. Stable free radicals like 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) should be stable to some extent and also should notparticipate in side reactions such as the abstraction of β-H atoms. In addition, they shouldnot react with themselves or monomers. Mechanistically, after decomposition of dormantalkoxyamines by the rate constant of kd, the formed active radicals can propagate by addingmonomer units or recombinated with the stable nitroxide radicals to form dormant specieswith the rate constant of kc (Scheme 20). However, side reactions such as thermal initia-tion, biradical termination, and alkoxyamine decomposition to yield terminally unsaturatedpolymer and hydroxylamine are possible.126–128

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Scheme 20. Equilibrium of NMP reproduced from Braunecker and Matyjaszewski55 by permissionof Elsevier Science Ltd., UK.

3.3.2 Nanocomposite Synthesis by In situ NMP3.3.2.1 Non-Grafting Method. Luo and Sen129 studied the kinetics of NMP and also

RAFT-mediated polymerization of n-butyl acrylate in the presence of alumina. They foundthat the rate of polymerization increased in the presence of alumina for both of the poly-merization methods. Furthermore, they showed that the polymerization rate increased withincreasing amount of added alumina and finally reached an asymptote due to saturation co-ordination at the alumina surface. Polydispersity of the polymers in the presence of aluminaremained low (1.10–1.15) which indicates that the polymerization is well-controlled andthat alumina was increasing the reactivity of the propagating radicals rather than increasingtheir concentration. However, the rate of NMP of styrene was nearly unaffected by the addi-tion of alumina since styrene cannot coordinate to a Lewis acid. This shows that there is nochange in propagating radical concentration through the interaction of alumina with NMP.

3.3.2.2 “Grafting from” Technique. Konn et al.130 reported polystyrene chains growthfrom the surface of synthetic Laponite clay platelets by NMP using N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] (DEPN) as mediator. A novel water-soluble qua-ternary ammonium alkoxyamine was first synthesized and intercalated into the clay gal-leries by cation-exchange reaction. Polystyrene chains with controlled molecular weightsand narrow polydispersities were then grown from the organoclay surfaces (Scheme 21).

Scheme 21. Preparation of laponite nanocomposite by NMP reproduced from Konn et al.130 bypermission of the American Chemical Society, USA.

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They showed that the amount of attached polymer increases with increasing the polymermolecular weight. However, the amount of polymer binding is higher than the theoreticalvalues. This difference which is more important in lower molecular weights can be at-tributable to physisorption of the free polymer chains; the lower is the polystyrene chainlength, the more important is the adsorption. Adsorption occurs through hydrogen bondingbetween the end groups of the free polymer chains and the hydroxyls located at the edgesof the clay platelets.

Chang et al.131 activated carboxyl groups on MWNT with thionyl chloride to giveacyl chloride derivative. Then, the NMP initiator is introduced to MWNT by esterifi-cation of 1-Hydroxy-2-phenyl-2-(2′,2′,6′,6′-tetramethyl-1′-piperidinyloxy)ethane (HO-PE-TEMPO) with the acyl halide group of MWNT. The obtained product was then used forbulk polymerization of styrene, yielding polystyrene-grafted MWNT.

Parvole et al.132 synthesized hybrid silica particles comprising an inorganic coreand an organic polymer shell by NMP of butyl acrylate in the presence of a graftedalkoxyamine. The layer of the initiator molecules attached to the surface of silica based onan acyclic β-phosphonylated nitroxide (Scheme 22). With this stable radical as chain growth

Scheme 22. Attachment of NMP agent on the surface of silica reproduced from Parvole et al.132 bypermission of John Wiley & Sons, Inc.

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moderator tethered to the inorganic core, it is demonstrated that the “grafting from” poly-merization exhibits a controlled character with a very low polydispersity index (PDI <

1.1). For this grafted alkoxyamine, the value of the grafting density was determined to be0.86 µmol.m−2 corresponding to 0.5 molecule.nm−2. A polymer density of 0.1 µmol.m−2

(0.06 molecule.nm−2) decreasing to 0.05 µmol.m−2 (0.03 molecule.nm−2), with the in-crease of the molecular weight from 20000 to 100000 g.mol−1 is observed respectively.This low efficiency (5–10%) could be correlated with cleavage of the macromolecularchains with time at high temperatures. Also, it can be due to a crowding effect of theinitiator groups on the surface. In addition, Chevigny et al.133 used silica nanoparticleswith a covalently bonded alkoxyamine to synthesize polystyrene coated nanoparticlesby NMP. By reaction between the aminopropylsilane and silica, particles with aminogroup coated surfaces were obtained. Subsequently, the initiation-controlling alkoxyaminemoiety is introduced via an overgrafting reaction between the amino group and the N-hydroxysuccinimide-based MAMA-SG1 activated ester. Finally, these nanoparticles withnitroxide moieties were used to initiate the polymerization of styrene from its surfaces.The two steps of grafting results in the high grafting density of first, the initiator and, sec-ond, the grafted chains—a mean value of 400 chains per particle, close to 0.2 chain.nm−2.Bartholome et al.134 also grafted polystyrene chains from the surface of silica nanoparti-cles. According to their resultss, the maximum grafting density of the attached polystyrenechains was estimated to be 0.35 µmol/m2, which corresponds to more than 45% initiationefficiency.

3.3.2.2 “Grafting through” Technique. Diaconu et al.135 modified MMT with 2-methacryloylethylhexadecyldimethylammonium bromide (cationic macromonomer), andacrylic oligomer end-capped with a nitroxide group (cationic macroinitiator) by a cation-exchange reaction (Scheme 23). They showed that employing these reactive clays inminiemulsion polymerization resulted in synthesis of acrylic polymer nanocompositeswith partially exfoliated structures. Low molecular weight polymer chains attached tothe clay were extracted from the clay interlayer of the nanocomposites. They could notprove the end-tethering of the acrylic polymer to the clay layers and related this phe-nomenon to the point that the organic cationic modifier could have left the clay interlayerspace during the miniemulsion preparation or during the initial step of polymer recov-ery, the polymer chains formed within the clay interlayer space were extracted in thecase of the cationic macroinitiator, and that polymerization in the interlayer space pro-ceeded only to a very limited extent in the case of the cationic macromonomer modifiedMMT.

Scheme 23. Chemical structure of the cationic macroinitiator and macromonomer used to modifyNa-MMT reproduced from Diaconu et al.135 by permission of the American Chemical Society,USA.

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4. Nanocomposite Synthesis via Combination of CRPand “grafting to” Method

In this method, the substrate surface is modified by organic species by the mentioned sur-face modification processes and further coated with polymer chains previously synthesizedvia CRP methods. In comparison with the in situ preparation method, this approach usuallyyields low graft densities. However, the possibility of the preparation of precise nanostruc-tures with defined sizes which can be modified with different polymers without significantalteration of substrates properties is its advantage.

4.1 Click Chemistry

The term click chemistry is defined as a reaction that is not sensitive to oxygen or water,uses readily available starting materials and reagents, requires no solvent or a solvent thatis easily removed or benign like water, enables simple product isolation and simple reac-tion conditions, has a high thermodynamic driving force and goes rapidly to completion,generates only inoffensive byproducts that can be removed by nonchromatographic meth-ods, and be stereospecific. Most of the click chemistry reactions are carbon–heteroatombond forming reactions, for example: (i) Cycloadditions of unsaturated species, especially1,3-dipolar cycloaddition reactions, but also the Diels–Alder family of transformations(ii) Nucleophilic substitution chemistry, especially ring-opening reactions of heterocyclicelectrophiles that have high ringstrain such as epoxides, aziridines, aziridinium ions, andepisulfonium ions, (iii) carbonyl chemistry of the “non-aldol” type, such as the forma-tion of ureas, thioureas, aromatic heterocycles, oxime ethers, hydrazones, and amides, and(iv) Oxidizing reactions like aziridination, dihydroxylation, epoxidation, and sulfenyl halideaddition, but also Michael additions of Nu-H reactants.68,136–139

4.2 Combination of ATRP and “grafting to” Method by Click Chemistry

The ATRP and click chemistry combination is very popular because both of them canbe carried out with the same copper catalyst and the halogen terminus of polymer chainsobtained via ATRP can easily be converted into the corresponding azide derivative. Qinet al.140 synthesized polystyrene functionalized SWNTs by the “grafting to” method. Theysynthesized polystyrene-N3 with designed molecular weight and narrow molecular weightdistribution by ATRP of styrene followed by end group transformation (Scheme 24). Bygrafting of polystyrene-N3 to the nanotubes, well-defined nanocomposites were synthe-sized. According to their results, the degree of functionalization was about 1 in 48 SWNTcarbon atoms and polystyrene cleaved from the SWNT after high conversion had the samemolecular weight as the polystyrene produced in solution. Wu et al.141 employed anothersimilar way to attach polystyrene to the MWCNTs. In their process, polystyrene chainswere directly reacted with MWCNTs under ATRP conditions using CuBr/2,2′-bipyridineas catalyst and therefore polymer attached MWCNTs with about 50 wt% of polyetyrenechains were obtained. Additionally, Lou et al.142 also presented that heating of TEMPO-terminated poly(2-vinyl pyrolidone) (P2VP) chains causes the TEMPO group to dissociateand results in radical terminated chains which are capable of grafting onto CNTs. Ac-cording to their results, a grafting ratio of 6–12% was obtained. In this context, Ranjanand Brittain143 synthesized a RAFT agent which provides the possibility of using RAFT-mediated polymerization and click chemistry together for surface modification of silicananoparticles with polystyrene and polyacrylamide brushes via the “grafting to” approach.They presented that this click reaction was used to attach polymers onto the surface which

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Scheme 24. Functionalization of SWNTs with polystyrene via “grafting to” method reproducedfrom Qin et al.140 by permission of the American Chemical Society, USA.

produced relatively high grafting density. Li et al.144 functionalized SWCNT with alkynes.Then styrene that was polymerized via ATRP, was azide-functionalized and clicked onto thecarbon nanotube. They presented that according to the TGA results, polymer-functionalizedSWNTs which consisted of 45% polymer, amounting to approximately one polymer chainfor every 200−700 carbons of the nanotubes, depending on the polymer molecular weight.Li et al.145 also employed a similar procedure to synthesize well-defined thermoresponsivewater-soluble diblock copolymer and homopolymers functionalized with controlled num-bers of C60 moieties at predetermined positions via the combination of ATRP and clickchemistry. Also, Zhang et al.146 reported the covalent coating of MWCNT by layer-by-layerclick chemistry. Poly(2-azidoethyl methacrylate) and poly(propargyl methacrylate) weresynthesized by ATRP and reverse RAFT-mediated polymerization respectively. The twopolymers were then alternately coated on alkyne-modified MWCNTs using Cu(I)-catalyzedclick reaction.

Wang et al.147 reported the modification of silica nanoparticles surface with polystyrenebrushes via the “grafting from” approach. They subsequently substituted the terminal bro-mides of polystyrene grafted silica nanoparticles with azido groups. Then, these azido-terminated polystyrene brushes on the nanoparticle surface were reacted with variousalkyne-terminated functional end groups via click reactions to form the polymer coatednanoparticles (Scheme 25). Huang and Chang148 synthesized thermoresponsive microcap-sules of PNIPAAm copolymers through covalent layer-by-layer assembly of azido- andacetylene-functionalized copolymers by click chemistry on the surface of azido-modifiedsilica particles. Subsequently, the silica core was dissolved by treatment with HF to yieldthe microcapsules.

Atom transfer nitroxide radical coupling (ATNRC) is a reaction in which TEMPOderivatives are used to end-cap polymer chains. ATNRC also falls under the category ofclick chemistry. Deng et al.149 combined ATNRC chemistry with the “grafting onto” strategyand obtained an efficient way to functionalize graphene sheets with presynthesized poly-mer (Scheme 26). A radical scavenger species, TEMPO, was first anchored onto -COOHgroups on graphene oxide. After that, TEMPO functionalized graphene sheets reactedwith Br-terminated well-defined PNIPAAm homopolymer, which was presynthesized bySET-LRP to form PNIPAAm-graphene sheets nanocomposite in which the polymers were

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Scheme 25. Synthetic routes to modification of silica nanoparticles via commutative combination ofATRP and click coupling reactions reproduced from Wang et al.147 by permission of Elsevier ScienceLtd., UK.

covalently linked onto the graphene via the alkoxyamine conjunction points. PNIPAAm-graphene sheets nanocomposite shows excellent dispersibility in various solvents includingnonpolar and polar organic solvents and even water because of the introduction of thehydrophilic PNIPAAm segments. Interestingly, the dispersibility of PNIPAAm-graphenesheets in water shows a temperature responsive behavior around 37◦C due to the presenceof thermoresponsive PNIPAAm chains. The grafting ratio of the TEMPO group was calcu-lated to be 7.33% (0.429 mmol.g−1) via the data of nitrogen content. From the data of XPS,the weight percentage of grafted PNIPAm homopolymer and ATNRC grafting efficiencywere calculated to be 34.0 and 22.0% respectively, which were similar to those obtainedfrom elemental analysis.

4.3 Combination of RAFT and “grafting to” Method by Click Chemistry

The most common way of RAFT and click chemistry combination is achieved by usingalkene and azide-functionalized polymers and substrates. These polymers can be syn-thesized by post-polymerization modification of polymer synthesizedvia RAFT-mediatedpolymerization. Then, these can be utilized as a precursor in the click reactions. Zhanget al.150 reported the post-modification route for preparation of smart hybrid gold nanopar-ticles with poly(4-vinylpyridine) (P4VP) based on RAFT and click chemistry. An azideterminated ligand was first synthesized to modify gold nanoparticles by ligand exchange

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Scheme 26. Preparation of PNIPAAm-functionalized graphene sheets via ATNRC chemistry com-bining with grafting-onto strategy reproduced from Deng et al.149 by permission of John Wiley &Sons, Inc.

reaction, and then click reaction was used to graft alkyne terminated P4VP which wasprepared by RAFT onto the surface of gold nanoparticles (Scheme 27). They showed thatthe hybrid gold nanoparticles have a very high grafting density. The as-synthesized hybridgold nanoparticles exhibited a pH responsive behavior at about pH 5. TGA measurementsof P4VP coated gold nanoparticles showed that the weight loss percentage correspondingto the decomposition of molecular chains was 81.8%. The residual mass percentage was8.8%. From the TGA data of P4VP coated gold nanoparticles, it indicated that roughly76% azide groups were converted during the click reaction. The surface grafting densityof the polymer was also calculated to be about 6 chains.nm−2. They also applied thismethod to prepare hybrid gold nanoparticles with PNIPAAm.151 TGA results of PNIPAAmcoated gold nanoparticles indicate that the weight loss percentage corresponding to thedecomposition of molecular chains was 76.9%. The residual mass percentage was 17.5%.The TGA data of PNIPAAm coated gold nanoparticles indicate that roughly 70.2% of theazide groups were converted during the click reaction. They concluded that PNIPAAm was

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Scheme 27. Schematic fabrication and pH responsive behaviour of AuP4VP nanoparticles repro-duced from Zhang et al.150 by permission of Elsevier Science Ltd., UK.

immobilized onto gold nanoparticles with grafting densities of 5.8 chains per nm2 by a clickreaction. The hybrid gold nanoparticles showed a temperature responsive phenomenon asthe temperature changed between 20 and 45◦C.

Ranjan and Brittain143,152,153 reported the use of an alkyne-functionalized RAFT agent.The obtained polymers were used for surface modification of silica. This RAFT agent wasused in different ways. First, the RAFT agent was used in the polymerization of acryamide.The obtained polymer was “clicked” on azide-functional silica nanoparticles. Second, theRAFT agent was “clicked” on the azide-functional silica nanoparticles and styrene waspolymerized onto the RAFT agent functionalized particles.152 In addition, the “graftingfrom” technique was used by this group to graft the second block, poly(metyl acrylate).Third, this RAFT agent was clicked and polymerized in a one-pot synthesis of polystyrenegrafted silica nanoparticles. In this procedure, polystyrene grafted silica nanoparticleshave been prepared by a tandem process by simultaneously employing RAFT-mediatedpolymerization and click chemistry.153 Azide-modified silica, an alkyne functionalizedRAFT agent, and styrene are combined to produce the desired product. As it resulted fromthermal gravimetric and elemental analysis, the grafting density of polystyrene on the silicain this process is intermediate between the “grafting to” and “grafting from” techniques forpreparing polystyrene brushes on silica. The tandem process of RAFT and click chemistryrepresents an intermediate case in terms of steric aspects consistent with the intermediategrafting density. The detailed procedure is provided in Scheme 28. With high specificity ofclick reaction and versatility of RAFT-mediated polymerization, the tandem approach caneasily be used to do many functionalization and postmodification reactions for a variety ofpolymers. By changing the relative rates of the click reaction and RAFT polymerization, thistandem method can be shifted between the “grafting to” and the “grafting from” methods.The grafting density of tethered chains can be easily adjusted by changing their relativerates. In addition, Huang et al.154 synthesized silica-polymer hybrids by combination ofRAFT-mediated polymerization and azide-alkyne cycloaddition click reactions. When the

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Scheme 28. Attachment of polymer brushes on the surface of silica reproduced from Ranjan et al.153


grafted polymer chains had a molecular weight varying between 5640 and 25200 g.mol−1,the grafting density of 0.017–0.085 chains.nm−2 could be achieved by their incrementalmethod.

4.4 Combination of NMP and “grafting to” Method

Liu et al.155 reported the synthesis of polystyrene and poly[(tert-butyl acrylate)-b-styrene]with well-defined molecular weights by NMP. Subsequently, the homopolymers and blockcopolymers were used to functionalize shortened SWNTs through a radical coupling re-action (Scheme 29). These polymers were covalently attached to SWNTs through ther-molytic release of the nitroxide end groups in the presence of SWNTs, which led toan efficient radical coupling of the polymers to the nanotube sidewalls. The resultingpolymer-functionalized nanotubes were highly soluble in a range of organic solvents, andthis solubility could be controlled by varying the chemical composition of the appendedpolymers.

Parvole et al.156 proposed two different ways of “grafting from” and “one-step method”to synthesize polymer coated silica particles (Scheme 30). At first, alkoxyamine initiator

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Scheme 29. Preparation of polystyrene by NMP and its utilization for the functionalization ofSWNTs reported by Liu et al.155 by permission of the American Chemical Society, USA.

Scheme 30. Grafting polymer chains by (a) “grafting from” and (b) one-step approach via NMPreproduced from Parvole et al.156 by permission of Elsevier Science Ltd., UK.

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based on the nitroxide SG1 (N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)]nitroxide) and bearing an N-succinimidyl ester function was used to synthesize a varietyof well-defined polymers with the activated ester group at the α-end. Grafting of polymerchains onto amine functionalized silica particles was performed at room temperature. Theyreported that polymer grafting densities were in the range of 0.1–0.2 chain per nm2. Toincrease the grafting density, they proposed a direct one-step method. According to theirreport, the amine modified silica, the N-succinimidyl ester functionalized alkoxyamine andthe monomer were all introduced into the reaction medium at once. This technique allowedgrafting and chain growth to take place simultaneously at the polymerization temperatureand produced hybrid particles with grafting density of about 0.9 chain per nm2 and longpolymer chains.

5. Summary

In situ CRP methods were studied in detail to synthesize well-defined polymer nanocom-posites. In this context, atom transfer radical polymerization (ATRP), reversible additionfragmentation chain transfer (RAFT), and nitroxide mediated polymerization (NMP) indifferent polymerization systems and in the presence of nanoparticles like silica, car-bon nanotubes, and nanolayers like clay as three-, two-, and one-dimentional nanofillersare reviewed. Compared to ATRP, RAFT polymerization is compatible with most vinylmonomers, requires similarly mild reaction conditions, but is free from metal contamina-tion. Low grafting density and a difficulty in the synthesis of surface bound RAFT CTA aretwo major problems which hinder the use of RAFT polymerization for surface modification.NMP also benefits from the mild reaction conditions and no need for catalyst. “Graftingfrom,” “grafting through,” and “grafting onto” as the three common ways of polymer graft-ing are also debated in detail. In the “grafting from” method, only grafted chains to thenanofillers are formed; however, attached and free polymer chains with different or similarcharacteristics are synthesized in the “grafting through” method. Additionally, end func-tionalized polymer chains are used to synthesize graft polymers to the inorganic surfacesby “grafting onto” method. Finally, nanocomposites with controlled molecular weight andnarrow molecular weight distribution of its matrix polymer were formed through thesewell-tuned processes. Comparing two common ways of direct grafting, “grafting through”has some benefits over the “grafting from” approach. One of the benefits of the “graftingthrough” method is that the dispersion of surface-modified nanoparticles with monomer-likespecies are quite stable, thereby reducing the possibility of aggregation which is sometimesobserved in the “grafting from” approach. A secondary benefit is that by using the “graftingthrough” approach, the same nanofiller content can be used to get various desired molecularweights. This is in contrast to the “grafting from” method where the nanofiller content andinitiator or transfer agent concentration are intrinsically linked. This means that, for exam-ple, samples of the RAFT agent anchored nanoparticles with various graft densities mustbe synthesized to obtain the same molecular weights, or alternatively, an extra untetheredRAFT agent may be added. However, heterogeneity of the product from the viewpoint oftethered and free chains with their different characteristics in the same product might be anexample of its disadvantages. Also, the “grafting from” technique results in brushes withhigher grafting density but contamination of the product by residual metals is its deficiency.The “grafting to” technique does not have a high grafting density but in comparison withthe two other mentioned grafting techniques, it does not require tedious synthesis and pu-rification. Also, in the “grafting to” technique, presynthesized polymer chains with specialcharacteristics can be attached onto the substrates.

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In Situ Controlled Radical Polymerization: A Review on Synthesis of Well-defined Nanocomposites

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