Polymer grafting on graphene layers by controlled radical polymerization

Polymer grafting on graphene layers by controlled radical polymerization

Advances in Colloid and Interface Science 273 (2019) 102021 Contents lists available at ScienceDirect Advances in Colloid and Interface Science jour...

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Advances in Colloid and Interface Science 273 (2019) 102021

Contents lists available at ScienceDirect

Advances in Colloid and Interface Science journal homepage: www.elsevier.com/locate/cis

Historical perspective

Polymer grafting on graphene layers by controlled radical polymerization Parvaneh Eskandari, Zahra Abousalman-Rezvani, Hossein Roghani-Mamaqani ⁎, Mehdi Salami-Kalajahi ⁎, Hanieh Mardani Faculty of Polymer Engineering, Sahand University of Technology, P.O. Box: 51335-1996, Tabriz, Iran Institute of Polymeric Materials, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran

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Article history: 8 August 2019 Available online 23 August 2019 Keywords: Graphene In situ controlled radical polymerization “Grafting from” “Grafting through” “Grafting to”

a b s t r a c t In situ controlled radical polymerization (CRP) is considered as an important approach to graft polymer brushes with controlled grafting density, functionality, and thickness on graphene layers. Polymers are tethered with chain end or through its backbone to the surface or edge of graphene layers with two in situ polymerization methods of “grafting from” and “grafting through” and also a method based on coupling reactions known as “grafting to”. The “grafting from” method relies on the propagation of polymer chains from the surface- or edge-attached initiators. The “grafting through” method is based on incorporation of double bond-modified graphene layers into polymer chains through the propagation reaction. The “grafting to” technique involves attachment of pre-fabricated polymer chains to the graphene substrate. Here, physical and chemical attachment approaches are also considered in polymer-modification of graphene layers. Combination of CRP mechanisms of reversible activation, degenerative (exchange) chain transfer, atom transfer, and reversible chain transfer with various kinds of grafting reactions makes it possible to selectively functionalize graphene layers. The main aim of this review is assessment of the recent advances in the field of preparation of polymer-grafted graphene substrates with well-defined polymers of controlled molecular weight, thickness, and polydispersity index. Study of the opportunities and challenges for the future works in controlling of grafting density, siteselectivity in grafting, and various topologies of the brushes with potential applications in stimuli-responsive surfaces, polymer composites, Pickering emulsions, coating technologies, and sensors is also considered. © 2019 Elsevier B.V. All rights reserved.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Controlled radical polymerization mechanisms . . . . . . . . . . . . . . . 2.1. Dissociation-combination . . . . . . . . . . . . . . . . . . . . . 2.2. Atom transfer . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Degenerative (exchange) chain transfer. . . . . . . . . . . . . . . 2.4. Reversible chain transfer . . . . . . . . . . . . . . . . . . . . . Polymer-grafting by in situ controlled radical polymerization . . . . . . . . 3.1. Covalent method . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Surface-initiation method . . . . . . . . . . . . . . . . . 3.1.2. Surface-anchoring method . . . . . . . . . . . . . . . . 3.2. Non-covalent “grafting from” method . . . . . . . . . . . . . . . Polymer-grafting by controlled radical polymerization and coupling methods . 4.1. Covalent method . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1. Click chemistry reactions . . . . . . . . . . . . . . . . . 4.1.2. Radical addition reactions . . . . . . . . . . . . . . . . . 4.1.3. Condensation reactions . . . . . . . . . . . . . . . . . . 4.1.4. Other “grafting to” methods . . . . . . . . . . . . . . . . 4.2. Non-covalent method . . . . . . . . . . . . . . . . . . . . . . .

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⁎ Corresponding authors. E-mail addresses: [email protected] (H. Roghani-Mamaqani), [email protected] (M. Salami-Kalajahi).

https://doi.org/10.1016/j.cis.2019.102021 0001-8686/© 2019 Elsevier B.V. All rights reserved.

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5. Applications . . . . . . . . . . . 6. Conclusion, outlook, and challenges Declaration of Competing Interest . . . . Acknowledgement . . . . . . . . . . . Author declaration template . . . . . . References . . . . . . . . . . . . . .

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Nomenclature ACPA AGET AIBN APTES APTMS ATRA ATRP ARGET AT ATNRC BEB BG BIBA BiBB BIRP BHA BPB bpy CBr CD

4,4′-Azobis(4-cyanopentanoic acid) Activators generated by electron transfer 2,2′-Azobisisobutyronitrile (3-Aminopropyl)triethoxysilane (3-Aminopropyl)trimethoxysilane Atom transfer radical addition Atom transfer radical polymerization Activators regenerated by electron transfer Atom transfer Atom transfer nitroxide radical coupling 1-Bromoethylbenzene 1,4-Butylene glycol α-Bromoisobutyric acid α-Bromoisobuyl bromide Bismuthine-mediated radical polymerization O-benzylhydroxylamine 2-Bromopropionyl bromide Bipyridine 4-Hydroxybutyl 2-bromopropionate 3-((4Hydroxybutoxy)dimethylsilyl)propyl methacrylate CDI 1,1′-Carbonyldiimidazole CETP 4-Cyano-4-ethyl-trithiopentanoic acid DOX Doxorubicin hydrochloride CPT 10-Hydroxycamptothecin CRP Controlled radical polymerization CTA Chain transfer agent CTP 4-Cyano-4-(phenylcarbonothioylthio)pentanoic acid CVD Chemical vapor deposition DC Dissociation-combination DDMAT S-Dodecyl-S′-(α,α'-dimethyl-α″-acetic acid) trithiocarbonate DEPN N-tert-butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide DETA Diethylenetriamine DMF Dimethylformamide DMVP Dimethyl vinylphosphonate DSC Differential scanning calorimetry DT Degenerative (exchange) chain transfer DTM 2-(Dodecyl thiocarbonothioyl thio)-2-methyl propionic acid EBiB Ethyl α-bromoisobutyrate EDA Ethylenediamine EG Ethylene glycol EGA Ethylene glycol acrylate EGDMA Ethylene glycol dimethylacrylate EO Ethylene oxide EY Eosine Y f-GO Functionalized GO f-r-GO Functionalized reduced GO FRP Free radical polymerization FTIR Fourier-transform infrared spectroscopy G Graphene

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GC Gas chromatography GMA Glycidyl methacrylate GO Graphene oxide GPC Gel permeation chromatography HEMA 2-Hydroxyethyl methacrylate HOPG Highly-ordered pyrolytic graphite HPLC High-performance liquid chromatography IBU Ibuprofen ICAR Initiators for continuous activator regeneration IIP Ion-imprinted polymer ISET Inner-sphere electron transfer ITO Indium‑tin oxide ITRP Iodide-mediated radical polymerization LCST Lower critical solution temperature LOFX Levofloxacin MA Methyl acrylate MAAM Methacrylamide [MATMA] [BF4] [2-(Methacryloyloxy)ethyl] trimethylammonium tetrafluoroborate MCS (3-Methacryloxypropyl)dimethylchlorosilane METAC Methacroylcholine chloride Me6TREN Tris[2-(dimethylamino)ethyl]amine MIP Molecularly-imprinted polymer MO Methyl orange MPC 2-(Methacryloyloxy) ethyl phosphorylcholine mPEG Methoxypoly(ethylene glycol) MPS 3-(Trimethoxysilyl)propyl methacrylate MPTES 3-Mercaptopropyltriethoxysilane MQ 5-(2-Methacryloyl-ethyloxymethyl)-8-quinolinol NaBH4 Sodium borohydrate NEAM N-ethyleacrylamide NHVB 2,5-Dioxopyrrolidin-1-yl-4-vinylbenzoate NMP Nitroxide-mediated polymerization NP Nanoparticle OBr N-(2-aminoethyl)-2-bromo-2-methylpropanamide OEMA Oxopentanoate ethyl methacrylate OSET Outer-sphere electron transfer PAA Poly(acrylic acid) PAAPBA Poly(3-acrylaminophenylboronic acid) PAEFC Poly(2-acryloxyethyl ferrocenecarboxylate) PAM Polyacrylamide PAMAM Poly(amidoamine) PBA Poly(butyl acrylate) P(BisGMA-UDMA) Poly(dimethacrylate-urethane dimethacrylate) PBO Poly(p-phenylene benzobisoxazole) PCL Poly(ε-caprolactone) PDA Polydopamine PDADMAC Poly(diallyldimethylammonium chloride) PDEA Pyridyl disulfide ethyl acrylate PDEGEEMA Poly[di(ethylene glycol) ethyl ether methacrylate] PDI Polydispersity index PDMA Poly(N,N-dimethyl acrylamide) PDMAEA Poly(dimethyl aminoethyl acrylate)

41 42 43 43 43 43

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PDMAEMA Poly(2-(dimethylamino)ethyl methacrylate) PDMS Poly(dimethylsiloxane) PEDOT Poly(3,4-ethylenedioxythiophene) PEG Poly(ethyl glycol) PEGMA Poly(ethylene glycol) methyl ether methacrylate PES Poly(ether sulfone) PGMA Poly(glycidyl methacrylate) PHEMA Poly(2-hydroxyethyl methacrylate) PHDA Poly(hexadecyl acrylate) P(o-HPMAA) Poly(N-(2-hydroxyphenyl)methacrylamide) P3HT Poly(3-hexylthiophene) PI Polyisoprene P(2-IBO) Poly(2-isopropenylbenzoxazole) PMA Poly(methyl acrylate) PMAA Poly(methacrylic acid) PMAPOSS Poly(methacryloisobutyl polyhedral oligomericsilsesquioxane) PMDETA N,N,N′,N′,N″-pentamethyldiethylenetriamine PMMA Poly(methyl methacrylate) PNAM Poly(N-acrylomorpholine) PNIPAAm Poly(N-isopropylacrylamide) POEGMA Poly(oligo(ethylene glycol) methacrylate) PPDPMA Poly(pentadecyl phenyl methacrylate) PPFMA Poly(pentafluorophenyl methacrylate) PRE Persistent radical effect PS Polystyrene PSF Polysulfone PSS Poly(styrenesulfonate) PTi Polythiophene PT Pyridine-2-thione PtBA Poly(tert-butyl acrylate) PTCDA Perylene-3,4,9,10-tatracarboxylic dianhydride PTFMS Poly{5-bis[(4-trifluoro-methoxyphenyl)oxycarbonyl] styrene} PTHF Poly(tetrahydrofurfuryl methacrylate) PTMA Poly(2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl methacrylate) PVAc Poly(vinyl acetate) PVPBA Poly(4-vinylphenylboronic acid) brushes PVC Poly(vinyl chloride) PVCL Poly(N-vinyl caprolactam) PVDF Poly(vinylidene fluoride) PVK Poly(N-vinylcarbazole) PVP Poly(N-vinyl-2-pyrrolidone) P4VP Poly(4-vinyl pyridine) PU Polyurethane RAFT Reversible addition-fragmentation chain transfer RATRP Reverse atom transfer radical polymerization RB Rhodamine B r-GO or RGO Reduced graphene oxide ROP Ring-opening polymerization RT Reversible chain transfer RTCP Reversible chain transfer polymerization SARA ATRP Supplemental activator and reducing agent atom transfer radical polymerization SBMA Sulfobetaine methacrylate SBRP Stibine-mediated radical polymerization SET LRP Single-electron transfer living radical polymerization SFRP Stable free radical polymerization SI-ATRP Surface-initiated atom transfer radical polymerization SI-NMP Surface-initiated nitroxide-mediated polymerization SI-RAFT Surface-initiated reversible addition-fragmentation chain transfer

SI-RATRP Surface-initiated reverse atom transfer radical polymerization siRNA Small interfering RNA SI-SET LRP Surface-initiated single-electron transfer living radical polymerization SPEEK Acidic sulfonated poly(ether ether ketone) SPM Sulfopropyl methacrylate SPMA 3-Sulfopropyl methacrylate potassium salt SR&NI Simultaneous reverse and normal initiation SS-Na Styrene sulfonate sodium salt TBPT S-4-(Trimethoxysilyl)benzyl S′-propyltrithiocarbonate TDI Toluene-2,4-diisocynate TEFMA 2,2,2-Trifluoroethyl methacrylate TEMPO 2,2,6,6-Tetramethylpiperidinyl-1-oxy TEOS Tetraethyl orthosilicate TERP Tellurium-mediated radical polymerization Tg Glass transition temperature TGA Thermogravimetric analysis THF Tetrahydrofuran TMPM 2,2,6,6-Tetramethyl-piperidin-4-yl methacrylate TNP 2,4,6-Trinitrophenol TREN Tris[2-aminoethyl]amine TRIS Tris(hydroxymethyl) aminomethane UCST Upper critical solution temperature VI Vinyl imidazole

1. Introduction Polymer brushes are a significant type of polymeric materials, which include polymer chains tethered with chain end or through its backbone to a substrate. These ultrathin polymer coatings are in brush forms only at high grafting densities [1]. Surface modification using polymer brushes has largely been interested in tailoring chemical and physical characteristics of surfaces and interfaces. Polymer brushes are commonly prepared by three main methods of “grafting from”, “grafting through”, and “grafting to” strategies [2–4]. The “grafting from” method is based on surface-initiated reactions, where the substrate surface is functionalized with initiator species and subsequently propagation of the polymer chains is accomplished from the initiators attached to the substrate surface. In the “grafting through” method, polymer chains are anchored to the substrate which is already modified with a double bond-containing chemical. Here, polymer chains are incorporated on the substrate from the backbone. The “grafting to” technique is based on the coupling reactions between the polymer chains and functional groups of the substrates, where attachment of the pre-fabricated polymer chains is carried out via physisorption or covalent bond formation. In this method, steric restrictions in grafting of polymer chains directly to a substrate make it difficult to reach polymer brushes with high grafting density and thickness. Also, the reaction of polymer end-groups with the complementary groups on the substrate becomes less efficient with increasing molecular weight of the polymers. However, the “grafting from” method results in polymer brushes with highly controlled grafting density and thickness. In different grafting reactions, functional groups are important sites for grafting of initiator molecules, chemicals with double bond, and complementary groups required for the “grafting to” method. Most importantly, various coupling reactions such as esterification, amidation, click additions, and radical coupling make these polymerizations very important in grafting reactions [5–10]. Graphene has attracted considerable attention because of its applications in thermally-stable composites, supercapacitors, solar cells,

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biosensors, drug delivery, medical imaging, field emission devices, and electrochemical resonators [11–19]. Surface engineering of graphene is inevitable for improvement of its dispersion in various media. Single graphene layers show a strong tendency for agglomeration because of strong π–π interactions and Van der Waals forces between the layers. Lots of strategies have been developed to physically or chemically disrupt the interactions between the layers. Oxidation of graphene results in hydroxyl and epoxy groups at the surface and carboxylic acid functionalities at the edge. These functional groups make the oxidized graphene (GO) dispersible in polar media. Oxygen functionalities can be used as a source for attachment of various functional groups containing initiator, monomer, or other moieties [5,7,20,21]. Therefore, various grafting methods can easily be applied for preparation of polymer brushes at the edge or surface of GO layers. Here, tailoring of the grafted chains functionality, grafting density, molecular weight, polydispersity index (PDI), structure, and topology can be very interesting. Controlled radical polymerization (CRP), which is also known as reversible-deactivation radical polymerization (RDRP), has been considered as a highly applicable method for the synthesis of welldefined polymer chains with controlled structures, site-specific functionalities, and narrow molecular weight distribution [22,23]. Controlling of grafting density, grafting site, functionality, and thickness of the polymer brushes can easily be achieved by using different CRP methods. In this context, four mechanisms of dissociationcombination (DC), atom transfer (AT), degenerative (exchange) chain transfer (DT), and reversible chain transfer (RT) have largely been considered [24]. For example, nitroxide-mediated radical polymerization (NMP) relies on DC, reversible addition-fragmentation chain transfer (RAFT) polymerization is based on DT, atom transfer radical polymerization (ATRP) relies on AT, and reversible chain transfer polymerization (RTCP) is based on RT. Application of CRP methods with different grafting reactions results in selective functionalization of various substrates such as graphene. Therefore, in situ CRP methods have highly been considered in the synthesis of polymer brushes on graphene and its derivatives [2–4]. In summary, in situ CRP methods for preparation of polymer brushes on graphene with two “grafting from” and “grafting through” methods are discussed. Additionally, the “grafting to” method for the preparation of polymer-grafted graphene is also discussed. Here, physical and chemical attachment concepts are considered in the surface- or edge-modification of graphene. For example, π-π and electrostatic interactions are considered as the most important physical modification strategies. Also, condensation, cycloaddition, and addition reactions are considered as the main chemical modification strategies. Finally, this study helps researchers to choose the best method for preparation of polymer-grafted graphene with welldefined macromolecules of controlled molecular weight, thickness, and PDI. Controlling grafting density, site-selectivity in grafting, various topologies of the brushes, and polymer brushes structure are investigated. Because of considerable applications of graphene-attached polymer brushes in the preparation of stimuli-responsive brushes, polymer composites, surfactants, sensors, electro-rheological suspensions, etc., the number of publications on this area has been increased. In different polymer-grafting methods on graphene, “grafting from” has been the most interesting rout. The “grafting through” is the other type of in situ method which has not widely been studied because of its lower content of grafted chains. The “grafting to” method although benefits from various coupling reactions, but it has been restricted because of lower grafting density of polymer chains. Polymer grafting by “non-covalent” methods has recently been interested, since it is a straight-forward method without a need for further graphene oxidation or functionalization. This can result in preservation of the original properties of graphene layers after polymer-modification reactions.

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2. Controlled radical polymerization mechanisms CRP received a large deal of interest in the past few decades because of its versatility in synthesis of well-defined macromolecules with controlled structure, composition, functionality, molecular weight, and molecular weight distribution [22,23]. Although free radical polymerization (FRP) is a highly interested method due to its simple condition and applicability to a wide variety of vinyl monomers, it is poor in the preparation of well-defined polymers. The short life-time of radicals in FRP results in loss of control over chain architecture, molecular weight and its distribution, composition, and site-specific functionalization. Discovery of living anionic polymerization by Michael Szwarc provides such control over molecular architecture and especially nano-structured morphologies [25,26]. Very fast initiation reactions and relatively low propagation rates are required for controlling the molecular weight and its distribution in such polymerizations, which can be achieved by elimination of termination and transfer reactions from the chain-growth polymerization. Following developments in anionic polymerization, new concepts of control over radical polymerization were established by investigation of CRP methods. In these methods, transfer and termination reactions are minimized by lowering the active radical state concentration in the reaction medium. Also, immediate initiation reaction has caused a simultaneous growth of all the polymer chains in a controlled manner [27,28]. Reduction of the number of active radicals in polymerization media can be achieved by their reversible termination or reversible transfer to the other species. Dynamic equilibrium between the propagating radicals as an active state and dormant species is the basic concept for all the CRP methods. CRP requires a quick and adjustable equilibrium between the active and dormant species for controlling the polymer growth reactions. Based on this equilibrium, CRP can be categorized into four common mechanisms of DC, DT, AT, and RT (Fig. 1) [24]. In these mechanisms, radicals are reversibly involved in deactivation/activation equilibriums based on the persistent radical effect (PRE) [27,29] or they can be engaged in reversible transfer processes following typical FRP kinetics.

2.1. Dissociation-combination In the DC-based CRP systems, the cleavage of capping agent (X) from the dormant species (Polymer-X) results in the formation of macroradical active species (Polymer●) (Fig. 1(a)). This type of dissociation can be accomplished thermally or photo-chemically with the rate constant of kd. The rate constant for combination reaction between the persistent radical (X●) and macroradicals is kc. It is clear that X● species are stable enough to undergo no reaction with themselves, monomers, and other species other than macroradicals for preparation of dormant species. To minimize the termination and transfer reactions, it is required to reduce the concentration of Polymer●. Therefore, much lower dissociation rate constant in comparison with the combination rate constant results in higher concentration of dormant species. The X species can be stable free radicals like in stable free radical polymerization (SFRP). In these methods, all the polymer chains are short at the first stages of the polymerization, gradually grow during the course of polymerization, and are generally living. For example, nitroxide radicals are stable free radicals which can mediate polymerization in NMP. The most common nitroxides in NMP are 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) [30] and N-tert-butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide (DEPN) [31]. The first successful NMP is attributed to the polymerization of styrene in the presence of TEMPO, which was reported by Georges in 1993 [30,32]. Other mediators of SFRP are organic nitrogen compounds such as triazolinyl [33], (arylazo)oxy [34], and verdazyl [35]. Borinate [36] radicals, and bulky organic alkanes [37] are the other organic mediators of SFRP. Transition metal compounds

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Fig. 1. The common mechanisms of CRP [24]. Copyright 2008, Reproduced with permission from the Elsevier Ltd.

based on cobalt (Co) [38], molybdenum (Mo) [39], osmium (Os) [40], titanium (Ti) [41], and iron (Fe) [42] can also be considered as SFRP mediators. 2.2. Atom transfer In the AT-based CRP systems, an activator is used for abstraction of capping agent (X) from Polymer-X through an inner-sphere electron transfer (ISET) process and formation of active radicals (Polymer●) and also persistent radical species of XA● (Fig. 1 (b)). The X species can be halogens like chlorine (Cl) and bromine (Br). A halide complex of transition metal in the presence of a ligand is the activator. The transition metal can increase its oxidation number and it is responsible for homolytic cleavage of the alkyl-halogen bond in Polymer-X. The activation and deactivation rate constants are ka and kda, respectively. Similar to the DC method, deactivation rate constants are much higher than the activation rate constants for achievement of lower concentration of radicals in the polymerization medium. All the chains are initially short and grow with the time of polymerization. The main polymerization method based on this mechanism is ATRP [43]. ATRP originates from atom transfer radical addition (ATRA) which is known as a common reaction in organic chemistry [44]. Commercial availability of ATRP reactants such as alkyl halide initiators, transition metal catalysts, and ligands are the main advantages of ATRP. The ATRP equilibrium can be adjusted for different monomers and polymerization media by selection of different ligands. The most efficient transition metal complexes for ATRP are composed from copper (Cu) and nitrogen-containing ligands. The derivatives of bipyridine (bpy) [45], diethylenetriamine (DETA) [46], and tris[2-aminoethyl]amine (TREN) [47] are the most common nitrogen-based ligands in ATRP. In a normal ATRP initiation, the

equilibrium between the dormant species and active radicals is established by using a complex of a transitional metal catalyst in lower oxidation state with a ligand and also an alkyl halide initiator. The presence of oxygen in the reaction medium may result in irreversible oxidation of the activator. However, in a reverse ATRP which is used for prevention from oxidation problems, the equilibrium is formed from a conventional FRP initiator and transitional metal complex in its higher oxidation state [48]. In the reverse initiation system, the amount of catalyst is the same as normal initiation and also this method is inefficient in synthesis of block copolymers. Therefore, simultaneous reverse and normal (SR&NI) initiation is developed, which uses conventional FRP initiator, deactivator, and alkyl halide initiator for establishment of the equilibrium [49]. High amount of the used catalyst in ATRP requires expensive purification techniques after polymerization [50], where environmental problems may also be raised because of toxicity of the transition metals. Therefore, some other initiation systems were developed for reducing concentration of the used catalyst (Fig. 2 (a)). In initiation system based on activators generated by electron transfer (AGET), reducing agents such as tin(II) 2-ethylhexanoate [51] and ascorbic acid [52] are used for generation of the transitional metal complex in lower oxidation state. The most important method for reducing the metal complex concentration is based on activators regenerated by electron transfer (ARGET), where the relative amount of transitional metal complex in its higher oxidation state to the alkyl halide initiator is considerably decreased by using the reducing agent in an excess amount relative to the transitional metal complex [53]. The other method for reducing the metal complex concentration is based on initiators for continuous activator regeneration (ICAR) [54]. In this initiation system, conventional FRP initiators are used for reduction of the very low content of

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2.3. Degenerative (exchange) chain transfer In the DT-based CRP systems, exchange of capping agent (X) is carried out between two growing macroradicals. During this exchange reaction, X● is transferred from Polymer-X to Polymer'● to form Polymer● and Polymer'-X with the equilibrium constant of kex (Fig. 1(C)). The X species can be an atom or a simple group, where it is only transferred from an active radical to another radical. For example, iodide-mediated radical polymerization (ITRP, X is iodine) [63], organotelluriummediated radical polymerization (TERP, X is TeR) [64], organostibinemediated radical polymerization (SBRP, X is SbR2) [65], and organobismuthine-mediated radical polymerization (BIRP, X is BiR2) [66] can be categorized in this mechanism, whereas the DC mechanism coexists in TERP and BIRP with the main DT mechanism [24]. The X species can also be a double-bond containing moieties with the accessibility for active radical addition. The main example for this mechanism is RAFT polymerization [67], where different chain transfer agents (CTAs) can be used as the double-bond containing moieties. In these polymerization methods, the growing radicals are involved in a reversible transfer or degenerative exchange process. Therefore, random activation of dormant species and their deactivation results in uniform growing of polymer chains. In the DT process, new chains are continually produced by a free radical initiator. RAFT polymerization is based on the addition-fragmentation chemistry, where the transfer reaction occurs through an intermediate species generated by the addition of propagating radicals to an unsaturated chain. These intermediate species may fragment back for regeneration of the original chains or fragment from the other side for propagation from the previously unsaturated chain end. The exchange reactions in RAFT polymerization are very fast, which result in polymers with low polydispersities. Successful RAFT polymerization requires proper selection of the leaving and stabilizing substituent of the CTA (R and Z, respectively) for a given monomer. The Z-group affects the stability of the thiocarbonyl group for addition of a radical and also stability of intermediate species, and R-group acts as an initiator for initiation of new chains. RAFT polymerization comprises (I) an initiation step for generation of free radicals, (II) the chain transfer step for addition of the initiator radicals or its oligomeric radicals to the CTA and fragmentation of the same moiety or R-group, (III) reinitiating step for growing of the leaving group radical, (IV) chain equilibrium as the main reaction, and (V) termination reactions of the macroradicals (Fig. 3) [68]. Different CTAs used in RAFT polymerization are dithioesters, dithiocarbamates, trithiocarbonates, and xanthates [69–71]. 2.4. Reversible chain transfer Fig. 2. Schematic illustration for the mechanisms of (a) ATRP with low transitional metal concentration, (b) SET-LRP, and (c) SARA ATRP [62]. Copyright 2013, Reproduced with permission from the American Chemical Society (ACS).

transitional metal complex into its lower oxidation state, which is stopped by consumption of FRP initiators. There are also two polymerization methods in the presence of Cu0, which are known as single-electron transfer living radical polymerization (SET-LRP) and supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP) [55–57]. In SET-LRP, Cu0 and CuII are the main activator and deactivator, respectively. Also, CuI does not participate in the activation of alkyl halides and undergoes an instantaneous disproportionation to Cu0 and CuII (Fig. 2 (b)). Activation of alkyl halide in this reaction occurs by outer-sphere electron transfer (OSET). In SARA ATRP, CuI and CuII are the major activator and deactivator, respectively. Also, Cu0 is a supplemental activator and reducing agent of excess CuII through comproportionation. Activation of alkyl halide in this reaction occurs by ISET (Fig. 2 (c)). There are also other methods based on ATRP, such as eATRP [58] and photoATRP [59], which work without necessity to remove air from the system [60,61].

The RT-based CRP systems rely on the transfer of iodine by using a germanium (Ge), tin (Sn), phosphorus (P), or nitrogen (N) compounds as a deactivator of macroradicals [24,72–74]. As a result of low exchange frequency of iodine in this polymerization and limited control over polydispersity, it is needed to use a catalyst for frequent activation and deactivation of Polymer-I. The radical polymerization method based on this mechanism is reversible chain transfer catalyzed polymerization (RTCP). For example, GeI4 is added to the polymerization system as a deactivator (XA) of Polymer●, and the produced GeI● 3 plays the role of activator (A●) for Polymer-I (Fig. 1(d)) [24]. Therefore, this mechanism is a RT with GeI4 as the CTA, where a catalyst is used for reversible activation and deactivation of Polymer-I. Other CTAs in this method can be based on Sn, P, and N atoms as in SnI4, PI3, and NIS [24,74]. Exchange of polymers between the active and dormant species is the main mechanism to control radical concentration in this method. The main focus of this study is on three methods of ATRP, RAFT, and NMP for grafting polymer chains on graphene layers with four common strategies of “grafting from”, “grafting to”, “grafting through”, and noncovalent method. In Tables 1, 2 and 3, the most important researches on polymer grafting with these CRP systems are presented.

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Fig. 3. Schematic illustration for the general mechanism of RAFT polymerization [68]. Copyright 2005, Reproduced with permission from the John Wiley & Sons Incorporation.

3. Polymer-grafting by in situ controlled radical polymerization

content of PS in GO-g-PS-T and polyisoprene in GO-g-PI-T is 34 and 68% after 7 and 144 h, respectively.

3.1. Covalent method Covalent method is a direct way to form covalent bonding between graphene layers and polymer chains. In this process, hybridization of some carbon atoms may change from sp2 into sp3, which results in disrupting of the conjugated structure of graphene layers [13]. Covalent polymer-grafting was commonly carried out by three methods of “grafting from”, “grafting through”, and “grafting to”, as shown in Fig. 4. In the “grafting from” method, polymerization proceeds from initiator moieties attached to the graphene layers. By the “grafting through” method, a growing chain is anchored to the layers via the graphene-attached monomer moieties. Finally, a preformed polymer chain can be attached to graphene layers by various coupling reactions by the “grafting to” method. 3.1.1. Surface-initiation method 3.1.1.1. Surface-initiated nitroxide-mediated polymerization. NMP has not frequently been used for modification of graphene layers. Ziolo at el [75]. used NMP for grafting polystyrene (PS) and polyisoprene (PI) from 2,2,6,6-tetramethyl-piperidine 1-oxyl (TEMPO)-modified GO (GO-T). They dispersed TEMPO-modified GO in dimethylformamide (DMF) and used it as a multifunctional alkoxyamine to initiate polymerization of styrene and isoprene at 130 °C to yield GO-g-PS-T and GO-g-PI-T, respectively (Fig. 5). They finally showed that the graft

3.1.1.2. Surface-initiated atom transfer radical polymerization. High number of research studies on polymer-functionalization of graphene layers by ATRP and “grafting from” method shows the advantages of surafceinitiated atom transfer radical polymerization (SI-ATRP). In such systems, ATRP initiator is commonly attached at the surface or edge of graphene layers, and sometimes sacrificial ATRP initiator may be used for the proceeding of polymerization in the continuous phase for preparation of free polymer chains. In the case of pristine graphene layers, SI-ATRP of methyl methacrylate (MMA) by using aryl azide-based photo-grafting strategy was reported by Ou and coworkers [76]. They prepared poly(methyl methacrylate) (PMMA)-grafted graphene layers by using 1,3-dipolar cycloaddition reaction. Carbon double bond on the graphene layers reacted with 3,4-dihydroxybenzaldehyde and N-methylglycine through 1,3-dipolar cycloaddition for incorporation of hydroxyl groups on graphene. These hydroxyl groups on the functionalized graphene were used for further functionalization with α-bromoisobuyl bromide (BiBB) and resulted in the grafting of ATRP initiator to the sides of graphene layers. These graphene-based macroinitiators initiated ATRP of MMA at room temperature. The results showed not only a good control over the structure, molecular weight and its distribution, and composition of the attached polymer but also resulted in high grafting densities and well-dispersed PMMA-grafted graphene in organic solvents. The brominated process was also used for introduction of initiator

P. Eskandari et al. / Advances in Colloid and Interface Science 273 (2019) 102021 Table 1 Overview of polymer brushes on graphene and its derivatives prepared via SI-NMP and SIATRP. Graphene type

Polymer

Covalent SI-NMP GO PS and PI

Covalent SI-ATRP G POEGMA G PMMA G

PHEMA

f-G

PMMA

f-G

PNIPAAm

f-G

PMMA

f-G

POEGMA, PMMA, PPFMA, and PMAA

GO GO

PS, PMMA, and PBA PtBA

GO

PNIPAAm

GO

PDMAEMA

GO

PAMAM

GO

PS

GO

PS

GO

PS [83] PGMA [84]

GO

Alkyl polyacrylates

GO

PMMA

GO

PMAPOSS

GO

P(HEMA-g-CL)

GO

PAA and PGMA

GO GO

PDMAEMA [90] PTHF [91] PAA

GO

PPDPMA

GO

PMMA

GO GO r-GO

PGMA [85] PMMA [86] PMAAM PMMA

r-GO

PNHVB

Main concept

Grafting content of 34 and 68% for PS and PI after 7 and 144 h reaction times, respectively. Respectively [75]

Generation of cellular micropatterns [121,122] Direct grafting of bromine to the graphene for using as initiator [77] Increasing dispersibility of graphene and mechanical properties of the PBO matrix [125] High grafting densities without degradation of graphene electronic properties [76] High temperature-sensitivity with application in controlled drug delivery [130] Good control over the initiator and polymer grafting density via electro-polymerization technique [126] Capacity of spatial control over the polymer-functionalization of graphene and graphite due to the photoredox-mediated ATRP nature [124] Improving GO dispersibility [79] Dispersibility of GO-g-PtBA in organic solvents [96] Drug delivery with high drug storage and temperature control in drug release [80] Variation of the LCST behavior of PDMAEMA to UCST behavior via quaternization process [81] Capturing of Fe(III) and treating the environmental contaminants [82] Increasing PDI of free and attached chains by increasing GO content and grafting density [110] Increasing Tg by increasing GO content and grafting density [111] In situ reduction of GO during SI-ATRP [83,84] Higher electro-rheological performance than common graphene-based electro-rheological suspensions [84] Improving tribological properties in terms of considerable friction and wear reduction [88] Improving damping and film formation properties [87] Improving hemocompatibility of starch-polyacrylamide-based semi IPN hydrogel with GO-PMAPOSS in the platelet adhesion test, hemolysis test, and cytotoxicity assay [89] Electrical conductivity, degradability, and biocompatibility [109] Preparation of a new support for trypsin immobilization and efficient proteome digestion [103] Catalysts with excellent activity for degradation studies of organic dyes [90,91] Increasing efficiency in N-glycopeptide enrichment from complex human serum sample [92] Improving adhesion properties [93] Surfactant in PS emulsion polymerization [94] Performance of the membrane with hydrolyzed PMMA-grafted GO additive in the separation of salts, dyes, and heavy metal ions [95] Electrorheological suspensions with higher performance Preparation of GO/MIP composites [120] Increasing thickness of the r-GO layers Application in PVDF matrix as a filler [78] High photocurrent and conversion efficiency of photovoltaic cells fabricated from PNHVB-grafted r-GO in comparison with photovoltaic cells fabricated from PNHVB [138]

9

Table 1 (continued) Graphene type

Polymer

Main concept

f-r-GO

PS

f-r-GO

PMMA and PS

f-r-GO

PMMA [104] PS [140]

f-r-GO

P(o-HPMAA)

f-r-GO

PTMA

f-r-GO

PHEMA

f-r-GO

PMMA [105]

f-GO

PDMAEMA

f-GO f-GO f-GO

PHDA PS PDMAEMA

f-GO f-GO f-GO

pMPC PMMA P[MATMA][BF4]

f-GO

PSSS

f-GO

PS

f-GO

PDMAEMA

f-GO

PVCL

f-GO

PMMA

f-GO

PMMA

f-GO

P4VP

f-GO

PSS

f-GO

PEGMA

f-GO

P(SPM-co-PEGMA)

f-GO

PAAPBA

f-GO

PDMVP-b-PVI

f-GO

PAAPBA

f-GO

PDEGEEMA [142] PNIPAAm [143]

f-GO

P(BisGMA-UDMA)

Increase of tensile strength and Young's modulus of PS matrix [133] Investigation of the grafting ratios of PMMA and PS [119] Preparation of polymer brushes on PDA-coated r-GO by combining mussel-inspired chemistry and SI-ATRP [104,140] Improvement of mechanical properties of PVC matrix [139] Improving dielectric constant of P(2-IBO) matrix [135] High specific capacity of the composite as cathode materials [136] Meeting the biocompatible requirements to support fibroblast cells Application of ARGET ATRP [137] Application of a novel visible light-assisted photoredox catalyst for SI-ATRP of MMA [105] Controllability of dispersibility in water by variation of pH [129] Control of chain length of PHDA brushes [132] Increasing Tg of PS [127] Improving water dispersibility and control of wettability of graphene layers [128] Improving mechanical properties of PU [108] Dispersibility in polymeric dense matrices [118] Dispersibility in water after reduction with hydrazine [112] Catalytic activity in synthesis of isoamyl benzoate [113] High expansion of interlayer distance in high polymer grafting densities [97,98] Low cytotoxicity and high gene transfection [116] High solubility, stability, and drug loading ratio in water and physiological solutions [114] High-density polymer-functionalized GO with higher Tg than pristine PMMA [131] Decreasing PMMA grafting density by increasing graphene loading [104] Drug delivery application of P4VP-grafted GO with high potency for killing cancer cells [117] Higher amount and thickness of the grafted polymer on GO in SI-ATRP than in FRP [101] Preparation of PSF ultrafiltration membrane [99] Proton transport in Nafion-based membranes at low humidity [100] High grafting density of PAAPBA and highly efficient capture of glycoproteins from egg white samples Using thiol-ene click reaction [107] Enhancement of proton conduction within composite membrane by using acid-base block copolymer-functionalized GO [102] Ultrahigh adsorption capacity and water dispersibility of G-PAAPBA as a boron-affinity material [141] Improving dispersibility in various solvents and temperature-responsivity Application of SI-SET LRP [142,143] Improving dispersibility and mechanical properties Application of SI-SET LRP [144]

Non-covalent SI-ATRP GO PDMAEMA GO

PVPBA

r-GO

SPMA, METAC, NIPAAm, and DMAEMA

Application of π-π interaction Showing zwitterionic properties [174] Application of π-π interaction Application in selective enrichment of glycoprotein from biological system complex [176] Application of π-π interaction Using the μCP of initiator molecules through the non-covalent interaction for polymer-functionalization of r-GO [175]

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Table 2 Overview of polymer brushes on graphene and its derivatives prepared via SI-RAFT. Graphene type

Polymer

R-approach G PAA

GO

PMAAM in MIP form

GO GO

PVK PS

GO

PVP

GO GO

PS PMAA in MIP form

GO

PS and PSS-Na

GO

P(OEGMA-co-MQ)

GO GO

PNIPAAm-b-P (GMA-co-MQ) PDADMAC

GO

PS

f-r-GO

PNIPAAm

f-r-GO

PS, PMAA, P4VP, PDMA, and PEG

f-r-GO

PMMA, PNIPAM, and PtBA PA-L-PME

f-r-GO f-r-GO

PNIPAAm and PNEAM

f-GO

P(HEMA-co-AA)

f-GO

PS

r-GO

PTFMS

f-GO

PAA

Z-approach GO PS GO

PNIPAAm, PDMA, PNAM, PMA, and PtBA

Main concept

Improving thermal, mechanical, and elastic properties of the matrix Efficient drug carrier for pH-controlled release of drugs [163] Outstanding affinity, high selectivity, and sensitivity toward 2,4-dichlorophenol in aqueous solution [151] Solubility in organic solvents [146] Using DDMAT-grafted GO in miniemulsion polymerization of styrene [147] Improving dispersibility of the polymer-functionalized GO not only in a variety of solvents but also in PVAc matrix [153] Homogeneous dispersion in PS matrix [148] Three times more histamine absorption capacity of MIP-grafted GO than non-imprinted polymer [152] Effect of SS-Na monomer concentration on the morphology of composites [166] Showing catalytic activity of the Au NPs-GO hybrids for the reduction of 4-nitrophenol in water [149] Quantitative detection of TNP [150] Additive in the preparation of PSF nanocomposite membranes [154] Various grafting densities and various grafting sites [157,158] Combination of click chemistry and RAFT polymerization [161] Optimizing grafting density and grafted polymer chain length by combination of click chemistry and SI-RAFT methods [160] Solubility of polymer-grafted PDA-modified GO layers in several different solvents [165] Tuning the length of the polymer Highly stable dispersions in DMF [164] PNIPAAm- and PNEAM-grafted GO demonstrated very broad LCST in water between 33 and 70 °C [162] Combination of SI-ATRP and SI-RAFT Increasing LCST point of P(HEMA-co-AA) [115] Various grafting densities of PS from the surface of f-GO [156] Increasing the percolation threshold of the nanocomposites with an increase in degree of polymerization [155] PAA grafting on PAMAM dendrimer-modified GO [159]

Using sulfur-functionalized graphenes as macro CTAs [167] Controlling molecular weight in 3980–12,500 g/mol and PDI in 1.11–1.38 Improved solubility and dispersibility in various solvents which confirmed their amphiphilicity [168]

moieties to graphene or GO surface instead of using an ATRP initiator. For example, Shaffer et al. [77] showed direct grafting of bromine to the graphene which was reduced with sodium naphthalide via reacting exfoliated Na-based graphite intercalation compounds with bromine. They showed that the bromide graphene can be a precursor for ATRP of MMA. The results demonstrated that the grafting ratio of this polymer varies between 6 and 25%. The PMMA-attached graphene was more dispersible in organic solvents than the bromine-grafted graphene layers.

GO is decorated with hydroxyl and epoxy groups at the surface and carboxylic acid functionalities at the edge, which are common sources for grafting of various functional groups. In SI-ATRP, the layers are functionalized with initiator moieties from the hydroxyl, epoxy, and carboxylic acid groups. Most of ATRP initiators were used for functionalization of GO from its hydroxyl groups. BiBB as one of the most commonly used initiators in SI-ATRP was coupled at the surface of GO via acylation reaction by Nandi and coworkers [78]. They reported preparation of PMMA brushes at the surface of graphene using ATRP after reduction of BiBBmodified GO with hydrazine hydrate. Polymerization of MMA at the side and basal planes resulted in increase of height of the reduced graphene (r-GO) layers. PMMA-modified r-GO layers were used in preparation of poly(vinylidene fluoride) (PVDF) composites. PVDF has piezo and pyroelectric properties with five different crystalline polymorphic structures (α, β, γ, δ, and ε). Piezoelectric β-polymorph PVDF formation was increased by incorporation of PMMA-modified r-GO layers. They finally showed enhancement in glass transition temperature (Tg), storage modulus, stress at break, and Young's modulus of the nanocomposites. Grafting of PMMA, PS, and poly (butyl acrylate) to the GO surface through SI-ATRP method was also reported by Ruoff and coworkers via a similar method [79]. They showed that polymerfunctionalization of GO improves solubility/dispersibility of GO and also improves its processing potential for application in polymer composite. Poly(N-isopropylacrylamide) (PNIPAAm) as a thermo-responsive polymer were used to functionalize GO layers via SI-ATRP for drug delivery applications. After grafting BiBB on the hydroxyl groups of GO, PNIPAAm was grown from the initiator sites via the “grafting from” method. Ibuprofen (IBU) as the model drug was used for characterization of drug release behavior of PNIPAAm-grafted GO in response to the environmental temperature. Hydrogen bonding between the carboxylic acid functionalities of IBU and NH groups of PNIPAAm-grafted GO below the lower critical solution temperature (LCST) of PNIPAAm and also the large number of internal cavities of the PNIPAAm chains are the main causes of high IBU storage of about 280 wt%. Hydrogen bonding was collapsed by increasing temperature because of PNIPAAm chains variation from hydrophilic to hydrophobic state, and IBU molecules come out of PNIPAAm-grafted GO [80]. Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA)-grafted GO and quaternized PDMAEMA-grafted GO as another thermo-responsive nanocomposite were also synthesized via SIATRP from the hydroxyl groups of GO by Yuan and coworkers [81]. The GO surface was activated by BiBB and then 2-(dimethylamino)ethyl methacrylate (DMAEMA) was polymerized to graft thermo-responsive PDMAEMA on the GO surface via SI-ATRP. The quaternized PDMAEMAgrafted GO was obtained by adding 1,3-propane sultone at room temperature in the presence of tetrahydrofuran (THF), as shown in Fig. 6. In contrast to PDMAEMA-grafted GO which shows LCST-type thermoresponsivity, the quaternized PDMAEMA-grafted GO shows upper critical solution temperature (UCST)-type thermo-responsivity. It means that the quaternization process changed the thermo-responsivity behavior of PDMAEMA. Changing the hydrophilicity of thermo-responsive polymers is the main cause of their dispersion-aggregation behavior in water. Hydrophilicity of PDMAEMA-grafted GO with LCST behavior decreased with increasing temperature of polymer solution. Meanwhile, hydrophilicity of the quaternized PDMAEMA-grafted GO with UCST behavior increased with increasing temperature. This different hydrophilicity and thermo-responsive behavior of PDMAEMA-grafted GO and quaternized PDMAEMA-grafted GO can be efficient in its application in biomedical systems. Polymer-functionalization of GO by attachment of ATRP initiators to its hydroxyl groups was also reported by Liu and coworkers [82]. They reported grafting of magnetite nanoparticles (NPs) to the polymerfunctionalized GO. Accordingly, after grafting of BiBB to the hydroxyl groups of GO, methyl acrylate (MA) has polymerized through SI-ATRP, and the product was used for attachment of the second generation of poly(amidoamine) dendrimers (PAMAM). Fe(III) ions in FeCl3 solution

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Table 3 Overview of polymer brushes on graphene and its derivatives prepared via “grafting through” and “grafting to” methods. Graphene type

Surface grafting method

Polymer

Main concept

“Grafting through” method GO ATRP

PS

GO GO GO

PS PS PVP in IIP form

Increasing molecular weight, PDI of the attached PS chains, polymerization rate, and conversion by increasing grafting density [169,170] Increasing the nanocomposite Tg by increasing grafting density [171] Lower molecular weight and higher PDI of free PS chains than the anchored ones [172] Synthesis of RAFT-IIP with more specific adsorption capacity and faster Ni(II) adsorption kinetics than FRP-IIP [173]

ATRP ATRP RAFT

Covalent “grafting to” method f-GO Radical addition NMP f-GO Azide-alkyne cycloaddition click chemistry ATRP f-GO Azide-alkyne cycloaddition click chemistry ATRP f-GO Azide-alkyne cycloaddition click chemistry RAFT f-GO Radical addition NMP f-GO ATNRC ATRP Non-covalent “grafting to” method r-GO π-π interaction RAFT G π-π interaction RAFT G π-π interaction RAFT G π-π interaction RAFT G π-π interaction and electrostatic interaction RAFT G π-π interaction and click chemistry ARGET ATRP G π-π interaction RAFT GO π-π interaction ARGET ATRP r-GO π-π interaction ATRP r-GO π-π interaction ATRP f-GO Van der Waals interaction RAFT GO Hydrogen bonding and Van der Waals force RAFT

PS

Improvement of PS thermal stability [188]

PS

The mild strategy with high yield [184] Solubility in THF, DMF, and chloroform and excellent control over the layer thickness [185]

PDMAEMA

Controlled release of LOFX [186]

PHPMA

Dispersibility in organic solvents and aqueous media and entering into SMMC-7721 and SH-SY5Y cells [187]

PS

Facile and clean method for polymer-functionalization of GO [189]

PAEFC

Dispersibility in a wide range of organic solvents and potential applications in novel nanoelectronic devices [191]

PNIPAAm

Showing LCST lower than PNIPAAm [196]

PDMAEMA

Variation of the sandwich-like nanostructure into layered nanostructures Phase transfer between aqueous and organic media by variation of pH [195] Application in drug delivery in acidic environment and in the presence of reducing agent [197]

P (OEMA-b-PDEA-b-PEGA) PTFEMA-b-P4VP Improving dispersibility of HOPG [201] PAA, PDMAEMA, and PAM/Ru(bpy)2+ 3

Fabrication of versatile ECL sensors with simplicity, cost-effectiveness, excellent reproducibility, long-term stability, and high sensitivity of tripropylamine, tetracycline, and lysozyme [194]

PMMA

Providing an efficient dispersing agents for exfoliation of few-layered graphene in easily processable low boiling point chloroform [193]

PNIPAAm PMMA-b-PDMS

Application of thermo-responsive composite as a LED light switch to thermo-controlling the light on or off [198] Improvement of mechanical, optical, and thermal properties of the matrix [192]

PTi-g-PMMA

Superior mechanical and electronic properties of the composite [199]

Perylene-PCn

Potential oncology applications of the composite [200]

PDMAEMA

Enhancement of optical properties of GO, high water dispersibility, biocompatibility, easily tunable surface functionalization in siRNA delivery [202] Fabrication of a novel pH-responsive drug delivery nanosystem [203]

CS-g-PMAA

were captured by coordination with primary amino groups of the PAMAM-grafted GO to obtain magnetite-covered GO hybrid composite. The remained Fe(III) was reduced into Fe(II) using sodium borohydrate (NaBH4) in appropriate pH value. The magnetite-decorated GO hybrid can be used as a strong catalyst for degradation of hydroquinone aqueous solutions, which has application in treating the environment contaminants. Monàček et al. [83] prepared electrically conductive PS composites in one step via SI-ATRP and GO reduction. For this purpose, 2bromopropionyl bromide (BPB) was grafted to the GO surface through acylation reaction. They showed that N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA, the tertiary amine ligand for complexing copper catalyst) acts like a reduction agent for GO. Therefore, the conductive graphitic structure of GO could be partially restored in the in

situ reduction process during SI-ATRP reaction. Similarly, poly(glycidyl methacrylate) (PGMA)-grafted GO was prepared with good control over the GO reduction and modification by this group [84]. PGMAgrafted GO has tunable electro-responsive properties resulting in its application in electro-rheological suspensions with higher performance than common graphene-based electro-rheological suspensions. The other covalently polymer-functionalized GO with tunable electrorheological performance of suspensions was synthesized via SI-ATRP by Mrlík and coworkers [85]. In this work, BPB as an ATRP initiator was immobilized on GO and then polymerization of glycidyl methacrylate (GMA) on the surface of GO was performed by using SI-ATRP method. The PGMA-grafted GO was dispersed in silicone-oil electrorheological fluids. Improvement of electro-rheology at steady shear and also oscillatory modes was observed in the case of PGMA-grafted

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Fig. 4. Schematic illustration for the covalent polymer-grafting on graphene surface.

GO. Increase of compatibility between the PGMA-grafted GO with silicone oil and improving of conductivity tuning due to partial reduction of GO were confirmed. Mrlík's group [86] also reported the other study to enhance the conductivity and electro-responsive capabilities in electro-rheological applications by using PMMA-functionalized GO.

Suresh and coworkers grafted PMMA on the BPB-modified and acrylated GO by SI-ATRP and FRP, respectively [87]. They showed that the composites obtained from SI-ATRP showed improved film formation in comparison with the composite prepared from peroxide and acrylated GO. Ray's group [88] used SI-ATRP for grafting alkyl

Fig. 5. Schematic representation for the preparation of TEMPO-modified GO and subsequent graft polymerization of styrene and isoprene [75]. Copyright 2014, Reprinted with permission from the Elsevier Ltd.

P. Eskandari et al. / Advances in Colloid and Interface Science 273 (2019) 102021

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Fig. 6. Schematic illustration for synthesis of PDMAEMA-grafted GO and quaternized PDMAEMA-grafted GO [81]. Copyright 2013, Reprinted with permission from the Elsevier Ltd.

polyacrylates with different lengths (carbon numbers from 10 to 18) on the GO surface with the main aim of controlling grafting density by regulated molecular weight. Alkyl acrylate monomers prepared through esterification reaction between acrylic acid and respective long alkyl alcohols were grown on the GO surface via SI-ATRP after activation of its surface by BiBB from the hydroxyl groups. Results showed that the modification of GO by alkyl polyacrylates resulted in its stable dispersion in both base oil and polyol with improving tribological properties in terms of considerable friction and wear reduction of about 42 and 34%, respectively. Singha's group [89] described covalent functionalization of GO via SI-ATRP of methacryloisobutyl POSS (MAPOSS) after attachment of BiBB at the surface of GO. The poly (methacryloisobutyl POSS) (PMAPOSS)-grafted GO displayed high dispersibility in organic solvents and high hydrophobicity. The hybrid was incorporated into the starch-polyacrylamide based semi-IPN hydrogel, prepared by typical radical polymerization of acrylamide in presence of starch. A hydrophobic path inside the hydrogel was formed because of the self-association of the hydrophobic POSS chains. The prepared hydrogel with significant improvement in hemocompatibility was used for controlled release of ciprofloxacin. Masram et al. reported synthesis of PDMAEMA-grafted GO and in situ immobilization of gold NPs on the PDMAEMA brushes for investigation of catalytic degradation of organic dyes such as rhodamine B (RB), methyl orange (MO), and eosine Y (EY) [90]. The PDMAEMA brushes were synthesized via SI-ATRP on BiBB-modified GO layers. Finally, Au NPs/r-GO/PDMAEMA hybrid with spherical and worm-like morphology was obtained by a simple chemical reduction of PDMAEMA-grafted GO in HauCl4.3H2O and NaBH4 solutions. Formation of worm-like morphology is attributed to the presence of PDMAEMA brushes which is a linker and stabilizing agent in formation of Au NPs. Masram's group [91] used uniformly

distributed lanthanum oxide NPs on poly(tetrahydrofurfuryl methacrylate) (PTHF) brushes spread over the surface of r-GO for catalyst application in the degradation of RB, MO, and EY. Liu et al. [92] reported grafting of hydrazine-functionalized hydrophilic polymer chains on the GO surface for selective N-glycopeptides enrichment from complex human serum samples. Herein, poly(acrylic acid) (PAA) was grafted via SI-ATRP from the BiBB-modified GO. The grafted PAA chains acted as a flexible matrix for hydrazidefunctionalization by adding adipic dihydrazide. Suresh et al. [93] reported grafting of comb-like polymer to graphene via SI-ATRP to improve the adhesion strength of the product on different substrates. The BPB-modified GO was used for SI-ATRP of pentadecyl phenyl methacrylate (PDPMA) monomers for the preparation of the comb-like poly (pentadecyl phenyl methacrylate) (PPDPMA)-grafted graphene. Another amphiphilic comb-like polymer-modified GO was also synthesized and used as a surfactant in the aqueous phase emulsion polymerization of styrene [94] by this group. In this study, PDPMA monomer was polymerized at the surface of GO via SI-ATRP. Then, sulphonation of the resulted brush-like polymer was done to prepare surfactant for emulsion polymerization of styrene, as displayed in Fig. 7. Critical aggregation concentration value of the synthesized surfactant (1.29 mg/mL) was obtained via fluorimetric studies using pyrene as the fluorescent probe. Increase of surfactant concentration from 0.5 to 2.0 wt% resulted in the particle size variation from 160 to 220 nm, respectively. An attractive application of polymer-functionalized graphene layers is their incorporation into the polymeric membranes for separation of salts, dyes, and heavy metals. Mahmoudian et al. [95] described modification of GO with hydrolyzed PMMA via ATRP. They reported preparation of poly(ether sulfone) (PES) mixed matrix membrane with

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Fig. 7. Schematic representation of emulsion polymerization of styrene with amphiphilic polymer-modified graphene surfactant [94]. Copyright 2018, Reprinted with permission from the Springer.

hydrolyzed PMMA grafted on GO. The PMMA-grafted GO was obtained through SI-ATRP of MMA from the surface of BPB-modified GO and hydrolyzed using NaOH and isopropyl alcohol. Finally, the phase inversion process was used for the preparation of the membranes by using PES and Poly(ethyl glycol) (PEG) as the polymer matrix and pore forming agent, respectively. The hydrolyzed PMMA-grafted GO was used as an additive in the presence of N-methyl-2-pyrrolidone as the solvent and distilled water as the non-solvent coagulation bath. The prepared membranes showed performance in separation of salts, dyes, and heavy metal ions. Kang et al. [96] used hydroxyl groups of GO for coupling with trichloro(4-chloromethylphenyl)silane using acylation reaction for the preparation of an ATRP initiator-modified GO (GO-Cl). They prepared organo- and water-dispersible poly(tert-butyl acrylate) (PtBA) grafted GO layers (GO-g-PtBA) by using SI-ATRP, as shown in Fig. 8. The solubility of GO-g-PtBA in organic solvents was higher than GO, and also their dispersion in an electroactive polymer matrix such as poly(3hexylthiophene) (P3HT) was uniform. The sandwich device prepared from Al, the composite thin film of P3HT with five wt% of GO-g-PtBA, and indium‑tin oxide (ITO) showed bistable electrical conductivity switching behavior and non-volatile electronic memory effect. Fieldinduced charge transfers between the electron-donating P3HT matrix and the electron-accepting GO-g-PtBA layers leads to the bistable electrical conductivity switching of the composite. Organic electronic memories and gold NPs-decorated GO-g-PAA nanofilms were also prepared from aqueous dispersions. Converting of PtBA chains of GO-g-PtBA to PAA via hydrolysis changes dispersibility of the functionalized GO in water. Silanization of GO from its hydroxyl groups is one of the other methods for its functionalization. Roghani-Mamaqani and coworkers reported grafting of PS at the surface of initiator-modified GO by using SI-ATRP method [97]. As shown in Fig. 9, (3aminopropyl)triethoxysilane (APTES) was grafted to GO from its hydroxyl groups to yield GONH2. Then, BiBB was anchored to GONH2 via acylation reaction to yield GOHBr, which was used in SI-ATRP of styrene for grafting PS to the graphene surface. By varying the amount of APTES and weight percent of GOHBr, the effect of grafting density along with the graphene loading on the final product properties was evaluated. They showed that interlayer distance of the modified GO was highly expanded in high grafting density samples. In addition, confinement effect of GO on the kinetics of styrene ATRP

was studied by this group using gas and size exclusion chromatography techniques [98]. Functionalization of GO with APTES for introduction of amine functionalities to the GO surface was also reported by Wang and coworkers [99]. The amino-modified GO was reacted with BiBB and the product was used as a precursor for polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA) via SI-ATRP for yielding GO-g-P (PEGMA). The GO-g-P(PEGMA) was applied as a nanofiller in polysulfone (PSF) matrix for improving its surface hydrophilicity and antifouling property in preparation of ultrafiltration membranes. The prepared composite membrane showed excellent resistance to fouling. In addition, excellent filtration and antifouling performance of the membrane surface with pore walls was observed. The other interesting application of GO-g-P(PEGMA) and PSF composite is in water flux and its recovery rate. Similarly, Jiang and coworkers [100] reported synthesis of poly (sulfopropyl methacrylate (SPM)-co-PEGMA) brushes at the surface of GO by SI-ATRP. The copolymer-grafted GO was incorporated into Nafion matrix as a solid-electrolyte for proton-exchange membrane fuel cells. The resulted proton-exchange membrane fuel cells provided an efficient path at the interface of Nafion and GO for proton conduction. The Lewis basic sites on ethylene oxide in the copolymer structure accept proton from sulfonic acid groups and help proton binding based on Grothuss mechanism. Also, ethylene oxide units increase the water retention characteristics of the composite membrane due to the higher proton conductivity in low humidity. The sulfonated acid groups of SPM monomers help the proton hopping in the membrane like oxygen atoms of ethylene oxide. Similarly, poly(4-styrenesulfonate) (PSS)-grafted GO (GO-g-PSS) was synthesized via SI-ATRP by Mahdavi and coworkers [101] after functionalization of GO with APTES and BiBB. GO-g-PSS was also synthesized via FRP method by using 2,2′-azobisisobutyronitrile (AIBN) as the initiator. The results showed that the amount of grafted polymer and thickness of the grafted polymer on GO in SI-ATRP were higher than in FRP. The other study on using SI-ATRP for polymer-grafting on silane-modified GO for application in composite membranes was presented by Wang and coworkers [102]. They reported preparation of phosphoric acid brushes, imidazole brushes, and acid-base or base-acid block copolymer brushes from dimethyl vinylphosphonate (DMVP) and vinyl imidazole (VI) monomers (PDMVP-b-PVI) on GO for incorporation into two polymer matrices of acidic sulfonated poly(ether ether ketone) (SPEEK) and basic chitosan to prepare composite membranes. These polymer matrices were chosen

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Fig. 8. Functionalization of GO with Poly (tert-butyl acrylate) (PtBA) brushes via SI-ATRP [96]. Copyright 2010, Reprinted with permission from the American Chemical Society (ACS).

Fig. 9. Schematic illustration for functionalization of GO with APTES and BiBB [97]. Copyright 2014, Reproduced with permission from the John Wiley & Sons Incorporation.

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because represent anionic polyelectrolyte with phase-separated structure and cationic polyelectrolyte with nonphase-separated structure, respectively. Here, GO was covered with tetraethyl orthosilicate (TEOS) through the sol-gel reaction to introduce hydroxyl groups at its surface, and subsequently reacted with BiBB for grafting of initiator moieties. The acid-basic copolymer-functionalized GO was inserted to the SPEEK and chitosan by electrostatic attractions. These composites with interconnected networks and enlarged free volumes allow to the proton conduction within composite membrane via triggering synergic promotion effect. In some cases, APTES was firstly reacted with ATRP initiator and then the product was used for functionalization of GO. Qin et al. [103] reported preparation of PAA and PGMA brushes on GO by SI-ATRP as new supports for immobilization of trypsin. In this work, 3-(2-bromoisobutyramido) propyl (triethoxy)-silane (BIBAPTES) prepared from the reaction between BiBB and APTES was firstly attached to the GO surface. For preparation of GO-g-PGMA conjugated trypsin, NaIO4 was used with the purpose of converting epoxy groups of the PGMA brushes to aldehyde groups. The aldehyde groups were conjugated with trypsin through Schiff base condensation. Also, condensation reaction between the carboxylic acid groups of PAA and amino groups of trypsin resulted in the conjugation of GO-g-PAA with trypsin. Another polymer-grafting method using silanization reaction from the hydroxyl groups of GO was reported by Roghani-Mamaqani and coworkers [104] using surface-initiated reverse atom transfer radical polymerization (SI-RATRP). Here, azo-anchored GO (GOAzo) was prepared through the attachment of 4,4′-azobis(4-cyanopentanoic acid) (ACPA) to the APTES-functionalized GO (GONH2), as shown in Fig. 10. Then, RATRP of MMA was used to tailor the GO surface. The results showed decreasing PMMA grafting density with increase of graphene content due to decrease of the grafted PMMA chains molecular weight. Ray et al. [105] used 3-aminopropyltrimethoxysilane (APTMS) and ATRP initiator for functionalization of TiO2/r-GO and grafting of PMMA by SI-ATRP using tetrasulfonated copper phthalocyanine as a visible light-assisted photoredox catalyst. They prepared TiO2/r-GO by in-situ hydrothermal treatment of GO in the presence of titanium (IV) isoproxide to prepare a photoactive material for reduction of Cu(II) to Cu(I) complex under irradiation of visible light. This composite demonstrated high dispersibility in the organic solvents. This group used this method for the preparation of ternary hybrid composites through grafting of PS on TiO2/r-GO surface via SI-ATRP [106]. Growth of PS chains on the surface could be adjusted by changing monomer concentration while keeping the amount of initiator-functionalized TiO2/r-GO constant.

Chen et al. [107] used 3-mercaptopropyltriethoxysilane (MPTES) for modification of GO in the preparation of poly(3-acrylaminophenylboronic acid) (PAAPBA)-grafted magnetic GO nanocomposite through thiol-ene click chemistry method and SI-ATRP for capturing glycoproteins from complex biological samples such as egg white. The purpose of MPTES-functionalization was introduction of thiol groups to the surface of GO for application in immobilizing ATRP initiator of allyl 2bromo-2-methylpropionate on GO using thiol-ene reaction. Ning-lin et al. [108] reported synthesis and anticoagulation activities of poly(2(methacryloyloxy) ethyl phosphorylcholine) (pMPC) on the GO via RATRP. Here, GO was modified with (3-chloropropyl)trimethoxysilane and mixed with NaHCO3, butyl hydroperoxide, and 1,4-dioxane to obtain organic/inorganic composite peroxide initiator for the synthesis of polymer-functionalized GO with improved anticoagulation properties especially in blood-contacting materials. The resulted peroxide groups on GO act as RATRP initiator in the synthesis of pMPC-grafted GO. By immersion of pMPC-grafted GO in polyurethane (PU) matrix, resistance to nonspecific protein adsorption was improved. Moreover, the prepared film showed blood compatibility and improved mechanical properties. SI-ATRP and ring-opening polymerization (ROP) approaches were applied for the preparation of poly(2-hydroxyethyl methacrylate)-graftpoly(ε-caprolactone) (PHEMA-g-PCL) [109]. Uniform, conductive, and biocompatible nanofibers were also obtained with gelatin of PHEMAg-PCL. For this purpose, GO were reacted with chloroacetyl chloride through acylation reaction to prepare SI-ATRP macro-initiator, which was used for growing poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(ε-caprolactone) (PCL) brushes through SI-ATRP and ROP, respectively. The fabricated nanofibers showed electrical conductivity, degradability, biocompatibility, and versatile materials morphology, which can be used as scaffolding biomaterials to regenerate medicine. In addition to the grafting reactions from the hydroxyl [97,98,104] and carboxyl groups [110] of GO, ring opening of epoxy groups provides the site for ATRP initiator attachment on the surface of GO. Grafting PS chains from the epoxy groups of GO was reported via SI-ATRP method in various grafting densities [111]. As shown in Fig. 11, N-(2-aminoethyl)2-bromo-2-methylpropanamide (OBr) was prepared by using an amidation reaction between ethylenediamine (EDA) and BiBB. The OBrgrafted GO (GOBr) was prepared through the coupling reaction between amine groups of OBr and epoxy functional groups of GO. Finally, SI-ATRP of styrene with high and low grafting densities has been carried out. The results demonstrated increasing of Tg by the addition of graphene loading and also grafting density of OBr modifier. Edge-functionalization of GO with polymer chains from its carboxylic acid groups using SI-ATRP was also reported. PS-functionalized GO with different grafting densities of PS chains was synthesized via

Fig. 10. Schematic representation for the functionalization of GO with APTES and ACPA [104]. Copyright 2014, Reproduced with permission from the Springer.

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Fig. 11. Schematic representation for the synthesis of OBr, functionalization of GO with OBr, and synthesis of PS-attached graphene [111]. Copyright 2014, Reproduced with permission from the Royal Society of Chemistry (RSC).

SI-ATRP from the edge carboxylic groups [110]. At first, 4-hydroxybutyl 2-bromopropionate (CBr) as a bifunctional modifier was synthesized from 1,4-butylene glycol (BG) and BiBB. The bifunctional CBr modifier was coupled with the edge carboxylic acid groups of GO to yield GCBr as the precursor of styrene polymerization, as shown in Fig. 12. Molecular weight of PS chains and conversion increased with increasing grafting density of CBr. Increase of graphene content and CBr grafting density resulted in higher PDI values of the attached and free chains. SI-ATRP was used for grafting of a poly(ionic liquid) from the edge of GO by Yang and coworkers [112]. Poor dispersion of r-GO in organic solvents and water is the reason for its functionalization with [2-(methacryloyloxy)ethyl]trimethylammonium tetrafluoroborate ([MATMA][BF4]) by using SI-ATRP method. They used EDA as the coupling agent between GO and BiBB, and showed that the covalently grafted hydrophilic groups on r-GO form a colloidal dispersion with high stability in water. Results showed that although electrical

conductivity of poly(ionic liquid)-modified GO is not as high as r-GO, it is higher than the homogenous dispersion of GO in water three degrees of magnitude. EDA was also used as the coupling agent between GO and BiBB for the synthesis of poly(sodium 4-styrenesulfonate) (PSSS)-grafted GO via SI-ATRP by Qiu and coworkers. [113] Similarly, PSSS chains were grafted at the edge of GO after its functionalization with EDA and BiBB. The chemically reduced PSSS-grafted GO was used as a catalyst in isoamyl benzoate synthesis because of its good catalytic activity even after several cycles of usage. The poly(N-vinyl caprolactam) (PVCL)-grafted GO was also prepared similarly and used as an effective cargo for drug delivery [114]. Amidation reaction of diamines with carboxylic acid groups of GO and ATRP initiator (BiBB) was also used in the synthesis of dual temperature- and pH-sensitive polymer-grafted GO, where PHEMA and PAA blocks are temperatureand pH-responsive, respectively [115]. In this work, GO was reacted with thionyl chloride, EDA, and BiBB for the preparation of initiator-

Fig. 12. The overall synthesis pathway for PS-functionalized GO from the edge [110]. Copyright 2014, Reproduced with permission from the Royal Society of Chemistry (RSC).

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grafted GO. Polymerization of 2-hydroxyethyl methacrylate (HEMA) from the surface of modified GO layers was carried out by SI-ATRP method. The second block (PAA) was propagated from the PHEMA-modified GO layers by surface-initiated reversible additionfragmentation chain transfer polymerization (SI-RAFT). For this purpose, bromine chain end functionality of the grafted PHEMA was converted into the RAFT agent and used for polymerization of acrylic acid. One of the other amines used for activation of the carboxyl groups of GO through the amidation is cystamine dihydrochloride containing disulfide bond, which finally yields disulfide bond-containing GO (GOSS-NH2) [116]. In this study, GO-SS-NH2 was used for the preparation of a series of organic-inorganic graphene hybrids for gene/drug delivery applications. As shown in Fig. 13, GO-SS-NH2 was prepared in the presence of 1,1′‑carbonyldiimidazole (CDI), and used for the attachment of α-bromoisobutyric acid (BIBA) by amidation reaction to yield BiBBterminated GO (GO-SS-Br). Finally, bio-cleavable PDMAEMA-grafted GO (SS-GPD) was synthesized by ATRP reaction. The cleavability of PDMAEMA side chains from the GO backbone under reducible conditions, which is a benefit for gene delivery process, was the main reason of choosing this polymer. The aromatic and water-insoluble drug (10hydroxycamptothecin (CPT)) and DNA were loaded in SS-GPD for drug delivery investigation (SS-GPD-CPT) and gene delivery investigation (SS-GPD-pDNA), respectively. The conjugated structure of GO provides the non-covalent Van der Waals interactions between SS-GPD and CPT. The SS-GPDs can be more useful in gene/drug delivery due to its lower toxicity and higher gene-transfection than PDMAEMA. Initiator-modified GO with application in SI-ATRP of 4-vinyl pyridine was prepared by amidation reaction between the carboxylic acid groups of GO and 1,3-diaminopropane, and its subsequent

reaction with BiBB [117]. The poly(4-vinyl pyridine) (P4VP)-functionalized GO can be used in drug delivery applications. Here, camptothecin was used as a cancer drug which had a π-π and hydrophobic interactions with P4VP-functionalized GO. Drug loading and its release were investigated by changing the pH. The resulted nanocomposite shows high potency for killing cancer cells in the body with low cytotoxicity. Marques and coworkers [118] used ethylene glycol (EG) for converting carboxylic acid groups of GO to hydroxyl groups through an esterification reaction. Then, BiBB was anchored to the hydroxyl groups and used for SI-ATRP of MMA. The resulted PMMA-grafted GO was then used as a reinforcing agent in the preparation of PMMA nanocomposite films using the solvent casting method. The PMMA nanocomposite films were prepared with different contents of the pristine and PMMA-grafted GO. Considerable increase of elongation at break for these films showed that the films formed from the composite with 1 wt% of PMMA-grafted GO are tougher than the pure PMMA films. Strong interfacial interactions between PMMA and PMMA-grafted GO resulted in the higher thermal stability of the films. EG was also used as a coupling agent between thermally-reduced GO and BiBB by Liu and coworkers [119]. Here, carboxylic acid groups of thermallyreduced GO were converted to acyl chloride moieties after treatment with thionyl chloride. Successive addition of EG and BiBB to the acyl chloride functionalities resulted in initiator-modified r-GO with application in ATRP of styrene and MMA. A molecularly imprinted (MIP) polymer-GO hybrid was prepared via SI-ATRP of methacrylamide (MAAM) [120]. After modification of GO with thionyl chloride and 2hydroxyethyl 2-bromoisobutyrate, SI-ATRP was conducted in the presence of MAAM as the monomer and 2,4-dichlorophenol as the template

Fig. 13. Schematic illustration showing the preparation processes of bio-cleavable PDMAEMA-grafted GO (SS-GPD) synthesized via SI-ATRP and subsequent loading of CPT and DNA [116]. Copyright 2014, Reprinted with permission from the Royal Society of Chemistry (RSC).

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of MIP. Phenol and 4-monochlorophenol, which have a similar chemical structure to 2,4-dichlorophenol, were chosen as competitive agents. ATRP initiator can also be directly coupled to the graphene surface by aryl-azide based photo-grafting method [121,122]. In this method, the aryl-azide part of the ATRP initiator was activated with irradiation by UV for attachment to the graphene double bond. Grafting of the initiator is based on the perfluorophenyl azide chemistry, where the photochemical activation of perfluorophenyl azide forms the highly reactive singlet perfluorophenylnitrene with the ability of addition to carbon double bonds of graphene. Then, poly(oligo(ethylene glycol) methacrylate) (POEGMA) were propagated from the surface via SIATRP method. Formation of micropatterns of fibroblast and hippocampal neurons on the POEGMA-coated graphene was the main objective of this study. Another study for direct attachment of ATRP initiator to the graphene surface was reported by Liu and coworkers [123]. The ATRA reaction was used for modification of GO surface with initiator moieties for using in SI-ATRP of GMA, sulfobetaine methacrylate (SBMA), and NIPAAm. 1-bromoethylbenzene (BEB), PVDF, and bromine end-functional PS were attached to the double bond of GO surfaces via ATRA reaction to form various polymer architectures on GO surface. Halogen atoms of BEB, PVDF, and bromine end-capped PS were transferred to the modified GO surface and served as initiators for sequential SI-ATRP to obtain PGMA-grafted GO, PVDF- and PNIPAAM-grafted GO, and PS- and PSBMA-grafted GO, respectively (Fig. 14). The method used for grafting of PS on the GO surface is “grating to” method based on radical addition. However, PGMA,

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PNIPAAm, and PSBMA were grafted on the GO surface using “grafting from” method by SI-ATRP. The resultant products showed enhancement of organo-compatibility with dispersion stability in organic solvents. Photoredox-mediated SI-ATRP was also used for covalent polymer-functionalization of graphene by Huang and coworkers [124]. Graphite fluoride and graphene fluoride with higher electrochemical, electronic, and mechanical properties were obtained from fluorination of graphite and graphene, respectively. The POEGMA, PMMA, poly(pentafluorophenyl methacrylate) (PPFMA), and poly (methacrylic acid) (PMAA) brushes were directly grown from the C\\F bonds of graphite fluoride and graphene fluoride as initiating sites in the presence of a photoredox catalyst, which was activated under low-intensity blue LED light strips. Additionally, Ag-decorated PMAA-grafted graphite fluoride was prepared and utilized as a catalyst for the reduction of 4-nitrophenol to 4-aminophenol. Introduction of ATRP initiator via cycloaddition of a diarylcarbene provided a direct and non-destructive method for SI-ATRP of HEMA for preparation of PHEMA-grafted GO, which can be used as a filler of poly(p-phenylene benzobisoxazole) (PBO) matrix [125]. As shown in Fig. 15, BiBB was reacted with 4,4′-dihydroxybenzophenone through esterification reaction, and the tosylhydrazone was used to modify the ketone to prevent side reaction of ATRP initiator segments. Therefore, tosylhydrazone derivatives as carbene precursor were synthesized and deprotonated to create diaryldiazo that can be converted to carbene. By heating the solution containing diaryldiazo and graphene, the diarylcarbene derivatives were prepared and attached to the graphene

Fig. 14. Schematic illustration for initiator-functionalization of GO and its subsequent application in SI-ATRP of GMA, SBMA, and NIPAAm [123]. Copyright 2014, Reprinted with permission from John Wiley & Sons Incorporation.

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Fig. 15. Nondestructive modification of graphene via SI-ATRP and the procedure for composite fibers fabrication [125]. Copyright 2017, Reprinted with permission from American Chemical Society (ACS).

surface by covalent bonding via one-step cycloaddition reaction. Finally, PHEMA brushes were grown from the functionalized GO via SI-ATRP. Consequentially, one-pot in situ copolymerization of p-phenylene benzobisoxazole in the presence of PHEMA-grafted GO was conducted after modification of PHEMA-grafted GO with terephthalic acid. Finally, PHEMA-grafted GO/PBO nanocomposite fiber was prepared via dry-jet wet spinning. The fibers showed improved mechanical properties compared to PBO, such as increasing tensile strength and Young's modulus. Graphene, GO, and r-GO can also be functionalized using the diazonium reaction for anchoring ATRP initiator. Enhancement of hydroxyl groups content of pristine graphene was reported by Daasbjerg and coworkers using diazonium addition [126]. Here, the covalently linked polymer brushes on graphene were prepared by using electrochemical technique. Accordingly, the 4-(2-hydroxyethyl) benzenediazonium tetra fluoroborate was electro-grafted on chemically vapor-deposited graphene on Ni (Ni-G) film through the diazonium addition reaction. The corresponding initiator films were prepared through nucleophilic acyl substitution reaction between BiBB and Ni-G. Finally, MMA was used as monomer to prepare surface-attached PMMA brushes on Ni-G surface via SI-ATRP. The advantage of electrochemical technique is good control over the initiator and polymer grafting densities on the Ni-G surface by varying number of volumetric cycles. Direct functionalization of carbon atoms of GO or r-GO with sp2 hybridization by diazonium addition can result in high loading of ATRP initiator on the surface of the graphene. Lu’s group [127] reported preparation of PS-modified GO via combination of diazonium addition and SI-ATRP method. By using different concentrations of diazonium compound, different hydroxyl-functionalized GO samples with various densities of hydroxyl groups were prepared. After surface activation with BPB, SI-ATRP was carried out in the presence of different concentrations of styrene monomer for preparation of PS brushes with different chain lengths. The grafting density and chain length of PS chains were changed by varying the concentration of diazonium compound and styrene monomer, respectively. Distribution of the PS chains on the single-layer graphene for the high grafting density samples was more uniform than the low grafting density ones. Other interesting work for introduction of hydroxyl groups on the GO with diazonium salt was reported by Lee

and cowerks [128]. Here, hydroxyl groups were applied on thermallyreduced GO via diazonium addition between 2-(4-aminophenyl) ethanol and r-GO. After further modification of the hydroxylfunctionalized r-GO with BiBB, thermo-responsive PDMAEMA was grown from the surface of modified r-GO, which can potentially be used in advanced electronic, energy, and sensor applications. One year later, this group [129] applied this method for the synthesis of PAA-grafted r-GO as a pH-responsive hybrid material. After SI-ATRP of tert-butyl acrylate (tBA) and hydrolysis of the tert-butyl groups, water-dispersible PAA-grafted r-GO was prepared. They also claimed that this pH-responsive hybrid material has potential applications in graphene-based switching devices and sensors. Liu et al. [130] prepared thermo-responsive hybrid materials by grafting PNIPAAm on the surface of aminophenol-attached GO by ATRP in water. Surfacefunctionalization of GO was carried out by diazonium reaction and subsequent ATRP initiator attachment. The thermo-responsive properties provide interesting applications such as in environmental devices and controlled drug delivery systems. Hur and coworkers [131] reported another method for polymer attachment on the GO surface using SI-ATRP and diazonium addition. As shown in Fig. 16, GO was prepared via microwave's thermal expansion of graphite. For increasing the interfacial interactions between GO and PMMA, GO was diazotized with paminobenzoic acid to obtain DGO-COOH with high density of carboxyl groups. Quaternization following by esterification were applied to enhance the number of anchoring sites of BiBB for increasing grafting density of polymer brushes. They showed that Tg and thermal stability of the PMMA-grafted GO nanocomposites were higher than the pristine PMMA. Combination of diazonium addition and SI-ATRP was also reported for the synthesis of thermo-responsive poly(hexadecyl acrylate) (PHDA)-functionalized GO by Xingxiang and coworkers [132]. GO was reacted with 4-aminobenzyl alcohol through the diazonium addition and then coupled with BPB for preparation of the graphene precursor for conduction of ATRP. They showed that chain length of PHDA brushes could be controlled by tuning of the molar ratio of monomer to initiator. This composite could potentially be applied in temperature-sensitive drug carriers, solar energy storage, and temperature sensors.

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Fig. 16. Schematic representation for the synthesis of PMMA-grafted GO by using diazotization and SI-ATRP method [131]. Copyright 2014, Reprinted with permission from Springer.

Diazonium salts were also added to the r-GO prepared by reduction of GO with hydrazine hydrate by Lu’s group [133]. They reported using diazonium addition by 2-(4-aminophenyl)ethanol, grafting of BPB, and SI-ATRP of styrene to prepare PS brushes at the surface of r-GO. Effect of functionalized graphene layers on Tg, tensile strength, and Young's modulus of non-polar polymers such as PS was studied. The diazonium compound of 2-(4-aminophenyl)ethanol was also applied by Wang and coworkers [134] for grafting azo polymer brushes on the surface of r-GO through SI-ATRP. To prepare solution-processable nanocomposites with improved dielectric properties, poly(N-(2-hydroxyphenyl) methacrylamide) (P(o-HPMAA))-grafted r-GO/poly(2-isopropenylbenzoxazole) (P(2-IBO)) hybrid material was prepared [135]. As shown in Fig. 17, the GO reduced by hydrazine hydrate was functionalized with BiBB using diazonium addition and acylation reactions. The o-HPMAA monomer was prepared through the reaction of oaminophenol and methacryloyl chloride. The prepared rGO-grafted P(o-HPMAA) was used as a filler for P(2-IBO) hybrid material via solution blending. The P(2-IBO) synthesized via FRP of heterocyclic 2-isopropenylbenzoxazole possesses higher dielectric constant than conventional main-chain polybenzoxazoles with strictly limited processability and dielectric performance due to the conjugated benzoxazole groups on the backbone. High mobility of the dipole (benzoxazole ring) on the side chains and the flexible backbone of P (2-IBO) is the main reason of high dielectric constant and good solubility in the ordinary organic solvent, respectively. Huang et al. [136] used 2-(4-aminophenyl)ethanol for the introduction of hydroxyl groups on the r-GO surface via diazonium addition. Then, SI-ATRP was used for

polymerization of 2,2,6,6-tetramethyl-piperidin-4-yl methacrylate (TMPM) and preparation of poly(2,2,6,6-tetramethylpiperidin1-oxyl-4-yl methacrylate) (PTMA)-grafted r-GO. The product was used as nanofiller in the preparation of graphene-based high-energy density cathode materials through a simple dispersing-depositing process. Combination of diazonium addition and ARGET ATRP method for growing polymer brushes from the surface of r-GO was reported by Turng and coworkers [137]. 2-(4-aminophenyl)ethanol was used for adding extra hydroxyl groups to r-GO through the diazonium addition reaction, and subsequently BiBB was attached to obtain the precursor for ARGET ATRP of HEMA in the presence of tin(II) 2-ethylhexanoate as the reducing agent. The PHEMA-grafted GO changed the interfacial interaction between GO and proteins, which is critical for regulating cellular adhesion and proliferation behavior on the graphene substrate surface. Additionally, biocompatible requirements to support fibroblast cells, even human cells, attachment and proliferation of the PHEMAgrafted GO were proved via NIH-3 T3 fibroblast cells and human umbilical vein endothelial cells viability tests. Meyer et al. [138] used hydroxyl groups of r-GO for modification with methyl αbromoisobutyrate and subsequent SI-ATRP of 2,5-dioxopyrrolidin-1yl-4-vinylbenzoate (NHVB) to graft polypyridylruthenium derivatized PS brushes (PNHVB). In the next step, amidation reaction between the product and amine-derivatized ruthenium (II) polypyridyl complex was carried out, where ruthenium (II) polypyridine complexes act as light-harvesting chromophores. They showed that one polymer chain has attached per 100 graphene carbons and the length of polymer brushes are 30 repeating units with PDI of about 1.2. It is

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Fig. 17. Schematic illustration for the synthesis of P(o-HPMAA)-grafted r-GO and preparation of the nanocomposite from P(o-HPMAA)-grafted r-GO and P(2-IBO) [135]. Copyright 2015, Reprinted with permission from the American Chemical Society (ACS).

expected that the final product has potential for light-harvesting applications in optoelectronic tools. Hydroxyl groups of r-GO was used by Jia and coworkers to graft polymer chains [139]. Dopamine (DA) was used not only for the reduction of GO but also for coating of polydopamine (PDA) at the surface of GO by the bio-adhesive dopamine self-polymerization. Hydroxyl groups at the surface of PDA-modified GO provide the site for attachment of BiBB as the ATRP initiator. Finally, grafting PMMA was carried out by combination of SI-ATRP of MMA and mussel-inspired chemistry. The PVC films containing PMMA-modified graphene showed a higher storage modulus than nanocomposites containing GO and r-GO. Combination of mussel-inspired chemistry of DA and SI-ATRP was also used in the synthesis of PS brushes on r-GO [140]. Firstly, BiBB was grafted to DA, and then DA-BiBB was attached to the surface of GO for its simultaneous reduction and generation of the graphene-attached initiator. In this step, DA was added for coating of PDA at the surface of GO by bioadhesive dopamine self-polymerization. Finally, PS-grafted r-GO was prepared by SI-ATRP of styrene, as shown in Fig. 18. The PS-grafted rGO was dispersible in different solvents including DMF, toluene, and 1,4-dioxane. Boron-affinity material with ultrahigh binding capacity for cis-diols was prepared by Wang and coworkers [141]. Here, magnetic GO was prepared by incorporation of Fe3O4 on the surface of GO and subsequent deposition of PDA on its surface through the π–π interaction. BiBB was grafted on the PDA-coated magnetic GO, and PAAPBA brushes were attached from the active surface by SI-ATRP for preparation of a new sort of boron-affinity material. The final product was employed for selective enrichment of cis-diols such as nucleosides,

saccharides, and catecholamines from biological media. Adding salts to facilitate complexation improved the enrichment process through the electrostatic interaction between cis-diols and the boronic acid ligand mechanism. Functionalization of GO with polymer chains was rarely done by surface-initiated SET LRP (SI-SET LRP). Li at el [142]. reported covalent grafting of poly[di(ethylene glycol) ethyl ether methacrylate] (PDEGEEMA) on the surface of GO by in situ SI-SET LRP. This method is a mild and effective strategy for growing thermo-sensitive polymers directly from the surface of GO. The exfoliated GO layers were grafted with tris(hydroxymethyl) aminomethane (TRIS) through the epoxy ring opening reaction with the purpose of increasing the number of reactive hydroxyl groups at room temperature. The hydroxyl groups were reacted with BiBB for preparation of SI-SET LRP initiating groups at the surface of GO for the synthesis of the thermo-responsive PDEGEEMA brushes in the presence of CuBr/tris[2-(dimethylamino)ethyl]amine (Me6TREN) as the catalyst (Fig. 19). The prepared TRIS-GO-PDEGEEMA hybrid materials demonstrated good dispersibility in different solvents in comparison with GO. TRIS-GO-PDEGEEMA showed temperature switching assembly and disassembly behavior in water by the hydrophilic and hydrophobic shifting of the grafted PPEGEEMA chains at about 34 °C. Deng et al. [143] also reported controlled grafting of thermo-responsive PNIPAAm brushes at the surface of GO via in situ SI-SET LRP with a similar mechanism. Polymer-functionalization of GO by SI-SET LRP under mild conditions was also reported by Hemmati and coworkers [144] in 2018. The number of hydroxyl groups of GO was firstly increased by in situ diazonium addition reaction, and

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Fig. 18. Schematic representation for surface-functionalization of GO with ATRP initiator followed by PS-grafting via a combination of mussel-inspired chemistry and SI-ATRP [140]. Copyright 2016, Reprinted with permission from the Elsevier Ltd.

Fig. 19. Schematic illustration for grafting of PPEGEEMA chains from the surface of “TRIS”-modified GO through SET LRP [142]. Copyright 2011, Reprinted with permission from the John Wiley & Sons Incorporation.

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BiBB as the SET LRP initiator was introduced to the surface. Poly(dimethacrylate-urethane dimethacrylate) (P(BisGMA-UDMA))functionalized GO layers were prepared via SI-SET LRP in the presence of Cu/Me6TREN as the catalyst. After polymer grafting, dispersibility of the layers was significantly improved. 3.1.1.3. Surface-initiated reversible addition-fragmentation chain transfer polymerization. The RAFT agent can be attached to graphene materials from its leaving (R) or stabilizing groups (Z). In the R-group approach, the leaving group of RAFT agent is grafted to the surface or edge of graphene layers, and polymer-grafting is carried out similar to the “grafting from” method. However, in the Z-group approach, the stabilizing group of RAFT agent is attached to the substrate, and polymer-grafting is carried out similar to the “grafting to” method. In the Z-group approach, hindrance effect of the grafted polymer chains results in difficult control over molecular weight and its distribution in the case of the grafted polymer chains. In contrast to the Zgroup approach, control of molecular weight and PDI of polymer chains in the R-group approach is simple, and high grafting density is obtained as a result of easy diffusion of monomers to the surface for the propagation of the graft chains. Additionally, free radical initiator sources can also be attached at the surface of graphene layers for polymer grafting. 3.1.1.3.1. Initiator-attached approach. Voylov and co-workers [145] reported free radical and RAFT polymerization of sodium 4-vinylbenzenesulfonate induced by GO without any conventional thermal initiators. They showed that GO produces C-centered radicals and free radicals resulting in the polymerization of sodium 4vinylbenzenesulfonate radical polymerization. RAFT polymerization using GO as a radical initiator showed excellent control over polymer chains molecular weight and is distribution with a narrow PDI in between 1.01 and 1.03. 3.1.1.3.2. Chain transfer agent-attached approach. R-group approach: In 2011, S-1-dodecyl-S′-(α,α'-dimethyl-α″-acetic acid)trithiocarbonate (DDMAT)-functionalized GO was used for grafting of poly(Nvinylcarbazole) (PVK) from the surface of GO by the R-group approach [146]. Here, molecular weight and PDI of the grafted PVK on GO was 8050 g.mol−1 and 1.43, respectively. Also, the resulting PVK-grafted GO shows good solubility in organic solvents. RAFT-mediated miniemulsion polymerization was also used for the preparation of PS-grafted GO layers [147]. DDMAT was anchored on the GO surface via esterification reaction for controlling polymerization of styrene via R-group approach. Different amounts of the modified GO were dispersed in water and sonicated with styrene monomer after addition of sodium dodecylbenzene sulfonate as the surfactant and hexadecane as the co-surfactant to form miniemulsion systems. PS-grafted GO with core-shell morphology was obtained by starting the RAFT polymerization. As expected for RAFT-mediated polymerization, molar mass and PDI of PS decreased by increasing the RAFT-functionalized GO concentration. By grafting PS chains from the surface of GO, thermal stability and mechanical properties of the nanocomposites were improved in comparison with the neat PS. Ding et al. [148] prepared PS-grafted graphene via RAFT polymerization and “grafting from” approach. Here, 4-cyano-4-[(dodecylsulfanylthiocarbonyl) sulfanyl] pentanoic acid as the RAFT agent was introduced at the surface of thermally-reduced GO and used as a core material. RAFT polymerization was conducted for growing PS chains from the surface of r-GO. Grafting density of RAFT agent on r-GO was calculated as 0.84 functional groups per 100 carbon atoms, and the grafting density of PS was about 0.18 chains per 100 carbons. They showed that thermal properties of the composites were effectively improved as a result of homogeneous dispersion of graphene layers in the PS matrix. Therefore, this composite can be used in thermal interface materials and other high-performance thermal applications. In another research study [149], in situ synthesis of gold (Au) NPs on 8hydroxyquinoline ligand-containing copolymer brushes at the surface

of GO were reported. DDMAT as the RAFT agent was attached on GO by the esterification reaction. Then, RAFT polymerization of OEGMA and 5-(2-methacryloyl-ethyloxymethyl)-8-quinolinol (MQ) resulted in water-soluble copolymer brushes of P(OEGMA-co-MQ) at the surface of GO (Fig. 20 (a)). Subsequently, in situ reduction of Au3+ by alkali resulted in the generation of Au NPs, where the 8-hydroxyquinoline units acted as the capping agent to stabilize Au NPs (Fig. 20 (b)). This method was applied for imprinting the Au NPs on GO which exhibits high catalytic activity for the reduction of 4-nitrophenol. A similar work has been carried out for preparation of PNIPAAm-b-P(GMA-co-MQ)-grafted GO by SI-RAFT polymerization of NIPAAm, MQ, and GMA from the RAFT agent-modified GO with the aim of preparation of a fluorescent sensing platform for 2,4,6-trinitrophenol (TNP) [150]. The 8-hydroxyquinoline units coordinate with Al3+ to form green luminescent copolymer brushes on GO. Quenching of the green fluorescence of the polymergrafted GO by TNP can be used for detection of TNP quantitatively. Also, the fluorescent composite showed temperature-responsive behavior because of the presence of PNIPAAm chains. MIP and GO hybrid composites were prepared via RAFT polymerization for detecting endocrine disrupting chemicals [151]. Here, 2hydroxylethyl-2′-bromoisobutyrate was attached on the surface of GO, and then RAFT agent was grafted on the modified layers by reaction of RAFT agent-modified GO with phenylmagnesium bromide. Subsequently, the MIP film on GO was synthesized via SI-RAFT polymerization. The average thickness of the grafted polymer on GO was about 3.70 nm. The MIP and GO hybrid composite showed an outstanding affinity, high selectivity, and sensitivity toward 2,4-dichlorophenol in aqueous solution. Peeters et al. [152] prepared other MIP-GO hybrids via RAFT polymerization for thermal detection of histamine. GO was functionalized with a RAFT agent in a simple two-step process. Coupling of the RAFT agent to the surface of GO was accomplished via acylation reaction between the hydroxyl groups of GO and BiBB followed by synthesis of trithiocarbonate from carbon disulfide and dodecylmercaptane. Subsequently, a MIP layer was formed on the activated surface by RAFT polymerization using MAA and ethylene glycol dimethylacrylate (EGDMA) as the monomer and crosslinker, respectively. They used heat-transfer method for sensing histamine in the nanomolar regime. Indeed, this is a fast and low-cost method for detection of small organic molecules in relevant biological samples. The RAFT polymerization of vinyl pyrrolidone from xanthate-modified GO is used to synthesize PVP-grafted GO by Nandi and coworkers [153]. GO was modified with BPB and then potassium ethyl xanthate as the RAFT agent. RAFT polymerization of vinyl pyrrolidone from GO was conducted using AIBN to prepare PVP-grafted GO, as shown in Fig. 21. PVP-grafted GO and poly(vinyl acetate) (PVAc) films were formed by a solvent-casting method. By attachment of PVP on GO layers, dispersibility of the layers was improved in a variety of solvents of Hansen solubility parameter in the range 6.3–58. PVP-grafted GO layers in PVAc matrix indicated a homogeneous dispersion, although both GO and PVP are individually immiscible with PVAc. Mechanical properties of PVAc considerably improved because of homogeneous dispersion of PVP-grafted GO in the composite films. This PVP-grafted GO material can be used in biotechnological applications since PVP is a biocompatible polymer. By a similar mechanism, Ghasemi Kochameshki et al. [154] grafted poly (diallyldimethylammonium chloride) (PDADMAC) at the surface of GO by using O-ethyl xanthate as the RAFT agent and used the product as an additive in the preparation of nanocomposite membranes from PSF. Luo and co-workers [155] reported the modification of r-GO with APTES and coating of the modified layers by a rigid liquid-crystalline fluoride polymer (poly{5-bis[(4-trifluoro-methoxyphenyl)oxycarbonyl]styrene} (PTFMS)) with different shell thicknesses by an SI-RAFT polymerization method. They studied the interfacial thickness effect on dielectric behavior of the nanocomposites with r-GO and modified rGO. The percolation threshold of the nanocomposites increased from 0.68 to 1.69 vol% with an increase in shell thickness, which is controlled

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Fig. 20. Schematic illustration for (a) preparation of copolymer brushes on GO, (b) preparation of Au NPs and GO hybrid and its catalytic reduction of 4-nitrophenol [149], and (C) quenching of the green fluorescence of the polymer-modified GO in aqueous solution by TNP [150]. Copyright 2016, Reprinted with permission from the Royal Society of Chemistry (RSC). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 21. Schematic representation for the synthesis of PVP-grafted GO by RAFT polymerization [153]. Copyright 2013, Reprinted with permission from the Royal Society of Chemistry (RSC).

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Fig. 22. General scheme for preparation of (a) GOHA and GOHR [156], (b) GOR [157], and (c) GCR [158]. Copyright 2016, 2016, and 2017, Reproduced with permission from the Elsevier Ltd and Springer.

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by the degree of polymerization. The composite with thicker PTFMS shell presented a higher breakdown strength and lower dielectric constant. Nature of the rigid-polymer shell and morphology variation of rGO are the reasons of lower dielectric constant in the case of thick shell, as proven by the interfacial polarization and micro-capacitor model. PS chains were grafted from hydroxyl groups of GO layers via RAFT polymerization in various grafting densities by Roghani-Mamaqani and coworkers [156]. They functionalized GO by two different contents of APTES to yield GOHA in low and high grafting densities. Subsequently, DDMAT was grafted on GOHA to obtain GOHR with various grafting densities. PS chains were grown from the surface of GOHR by RAFT polymerization using R-group approach, as shown in Fig. 22 (a). Based on their results, the number-average molecular weight of PS chains was decreased with increasing of GOHR content. Similar works have been carried out by this group, where grafting of PS chains was conducted from epoxy [157] and carboxylic acid [158] groups of GO. As shown in Fig. 22 (b), EDA and DDMAT were coupled by an amidation reaction to yield OR. Then, OR in two different contents was grafted on GO to obtain OR-modified GO (GOR) in two different grafting densities. GOR was used in SI-RAFT polymerization of styrene with two different grafting densities. As shown in Fig. 22 (c), butadiene and DDMAT were coupled by an esterification reaction to yield CR, which was grafted at the edge of GO in two different contents to yield CR-

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modified GO (GCR) in two different grafting densities. GCR was used in SI-RAFT polymerization of styrene with two different grafting densities. Salami-Kalajahi and co-workers [159] reported the modification of GO with third-generation of PAMAM dendrimer to obtain PAMAMgrafted GO with lots of amine functional groups at the surface. Then, the amine functionalities were used for attachment of S-(thiobenzoyl) thioglycolic acid as the CTA. Finally, PAA was grafted on GO via SIRAFT polymerization. Hwang et al. [160] prepared polymer-modified graphene layers via “grafting to” and “grafting from” methods by using RAFT polymerization and click chemistry, as shown in Fig. 23. Alkyne groups were firstly introduced at the surface of graphene by diazonium functionalization, and the alkyne-functionalized graphene (Alkyne-FG) was used as a core material. Then, “grafting to” and “grafting from” methods were used to functionalize graphene layers via RAFT polymerization and click chemistry. In the case of “grafting to” approach, azido-terminated RAFT agent (CTA\\N3) and different monomers were used for the preparation of various azido-terminated polymers (polymer-N3) via RAFT polymerization. Then, polymer chains were covalently attached to the graphene layers using azide-alkyne click chemistry. In the “grafting from” approach, RAFT agent-modified graphene layers (CTA-FG) were obtained by click reacting between the CTA-N3 and alkyne groups of the alkyne-functionalized graphene (Alkyne-FG). Then, polymer chains were grown from the graphene surface via RAFT polymerization using

Fig. 23. Schematic representation for two types of grafting reactions used in the synthesis of polymer-functionalized graphene by RAFT method [160] Copyright 2012, Reprinted with permission from the American Chemical Society (ACS).

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Fig. 24. Schematic illustration for the preparation of CTA-modified r-GO and in situ RAFT polymerization of NIPAAm on the surface of r-GO [161]. Copyright 2012, Reprinted with permission from the John Wiley & Sons Incorporation.

R-group approach. The advantage of “grafting to” method is the preparation of well-defined polymer chains with high grafting density, where grafting density decreases by increasing chain length of the grafted polymers. The advantage of “grafting from” method is introduction of high molecular weight polymer chains to the graphene surface. In addition, higher grafting density of polymer chains can be obtained via the “grafting from” method. Synthesis of high molecular weight polymer chains by the “grafting from” method effectively improves solubility of the grafted polymer. However, processability of this method decreases because of the extra steps and proximity of initiating groups. Combination of RAFT polymerization and click chemistry reactions is a common tool for fabrication of PNIPAAm-grafted r-GO [161], as shown in Fig. 24. Here, diazonium salt of propargyl p-aminobenzoate was prepared and used for introduction of alkyne groups on r-GO. Azidoterminated RAFT agent was grafted on r-GO by click reaction, and in situ polymerization of NIPAAm on r-GO was conducted via RAFT polymerization using R-group approach. Click chemistry and RAFT polymerization was also used for the synthesis of water-soluble NIPAAm and N-ethyleacrylamide (NEAM) copolymer brushes on r-GO using “grafting from” method [162]. GO was modified with azide groups by a reaction with sodium azide. Alkyne-terminated RAFT agent was prepared by the reaction of propargyl alcohol and DDMAT, and then alkyne-terminated RAFT agent was grafted on GO layers using a click chemistry reaction. Finally, NIPAAM and NEAM were polymerized on graphene layers via SI-RAFT polymerization. Liu et al. [163] used RAFT polymerization in the presence of a crosslinking agent for the preparation of PAA and graphene hydrogel, as shown in Fig. 25. Aryl diazonium salt was attached on graphene layers for application of carboxylic acid groups at the surface of graphene. RAFT agent of 2-cyano-7-hydroxy-5-oxoheptan-2-yl ethyl carbonotrithioate was covalently coupled on carboxylic acidfunctionalized graphene layers via the esterification reaction. Subsequently, G-PAA hydrogels were prepared via in situ polymerization in the presence of ethylene glycol dimethacrylate as a crosslinking agent. A pH-controlled porous structure was observed for this hydrogel, which was mechanically strong and thermally stable. Because of pH-

sensitivity of hydrogel, it was used for controlled drug delivery. For this purpose, doxorubicin was loaded into the hydrogels as a model drug. As a result of in vitro experiments, this hydrogel can be used for drug release control in the neutral intestine environment. A nano-initiator for the synthesis of amino acid-based polymer nanohybrids was synthesized by the Stadler's group [164]. After reduction of GO layers using hydrazine hydrate, 2-(4-aminophenyl) ethanol was used for attachment of hydroxyl groups on its surface. The layers were crosslinked by dicarboxylic acid-functionalized RAFT-agent. Polymerization of N-acryloyl-L-phenylalanine methyl ester (A-L-PME) monomer was conducted using nano-initiator via RAFT polymerization with different monomer/initiator ratios, as shown in Fig. 26. Increasing monomer/initiator ratio resulted in increasing molecular weight of the polymers and also decreasing PDI. Charpentier and coworkers [165] reported preparation of PDAcoated r-GO and its reaction with DDMAT by an esterification reaction. Surface-initiated polymerization of MMA, NIPAAm, and tBA from the DDMAT moieties was carried out by RAFT polymerization via the R-group approach to yield PMMA, PNIPAAm, and PtBA-grafted PDAmodified GO layers. PS-grafted GO was synthesized using RAFT agent-modified GO in emulsifier-free emulsion polymerization by Jana and coworkers. [166] RAFT agent-modified GO was obtained by modification of GO with 3-benzylsulfanylthiocarbonylsulfanyl-propionic acid. Then, in situ emulsifier-free emulsion polymerization was carried out for the preparation of nanocomposites using RAFT agent-modified GO, styrene sulfonate sodium salt (SS-Na) as comonomer, and styrene as the monomer. They showed that concentration of SS-Na as the ionic comonomer plays a vital role in the formation of the composite. When the concentration of SS-Na was zero, proper surface interaction between the polymer particles and GO layers does not exist. Small and stable NPs with core-shell morphology were formed at moderate SS-Na concentration, where PS is the core and PSS-Na is the shell. In the absence of PS, colloidal stability is limited because of no particle formation. Z-group approach: Grafting RAFT agent on graphene surface is carried out from its Z-group in some cases, which is known as the Z-group approach in polymer grafting by in situ RAFT polymerization.

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Fig. 25. Modification of graphene with aryl diazonium salt, RAFT agent, and the subsequent preparation of the G-PAA hydrogels via in situ polymerization [163]. Copyright 2014, Reprinted with permission from the Elsevier Ltd.

In 2012, sulfur-functionalized graphenes were used as macro CTAs for grafting PS brushes by free radical grafting methods by Mülhaupt and coworkers [167]. As shown in Fig. 27, thiol-functionalized and RAFT-agent modified graphenes were used for grafting PS on GO, stearylamine-functionalized GO (Stearyl-GO), and thermally-reduced GO (TRGO). RAFT-mediated polymerization was carried out using dithiourethane-, dithioester-, and dithiocarbonate-functionalized graphenes. RAFT agents were prepared via deprotonation of hydroxyl groups of functionalized graphenes and their reacting with carbon disulfide to afford dithiocarbonate salts followed by alkylation with benzyl bromide to yield benzyldithiocarbonate groups (pathway A). In pathway B, the Grignard reagents were reacted with carbon disulfide to prepare dithiocarboxylates reacting with the surface epoxy groups of GO. Thiol-functionalized graphenes were prepared by esterification reaction between the hydroxyl groups of Stearyl-GO with 3mercaptopropionic acid (pathway C). In pathway D, deprotonation of the hydroxyl groups of Stearyl-GO and TRGO and their reaction with propylene sulfide yields thiol groups via nucleophilic ring-opening of the thiiran rings. PS grafting of thiol-functionalized graphenes was initiated either with AIBN or styrene thermal self-initiation. They showed

that both thiol-functionalized and RAFT-agent modified graphenes were highly effective as CTAs in FRP, initiated either with AIBN or by styrene thermal self-initiation. By using stearyl-modified GO highly effective grafting can be achieved. Growing of PS chains in pathway A can be accounted in Z-group approach method; however, the propagation method in pathway B is R-group approach. Simultaneous coupling reaction and RAFT polymerization were used by Zhao and co-workers [168] for the synthesis of homopolymersand diblock copolymers-grafted GO using PS, PNIPAAm, PDMA, poly(Nacrylomorpholine) (PNAM), poly(methyl acrylate) (PMA), and PtBA. They used alkoxysilane-functionalized RAFT agents of S-methoxycarbonylphenylmethyl S′-3-(trimethoxysilyl)propyltrithiocarbonate (MPTT) and S-4-(trimethoxysilyl)benzyl S′-propyltrithiocarbon-ate (TBPT) as Z-functionalized and R-functionalized couplable CTA in their well-tuned procedure, as shown in Fig. 28. In similar conditions, MPTTbased grafting reaction resulted in polymer brushes with lower molecular weight, narrower molecular weight distribution, and lower grafting density in comparison with the TBPT-based grafting reaction because of the increased shielding effect and also different grafting processes. The grafted polymers showed controlled molecular weight in the range of

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Fig. 26. Schematic illustration for crosslinking hydroxyl-functionalized GO via dicarboxylic acid-functionalized RAFT-agent and subsequent RAFT polymerization of N-acryloyl-Lphenylalanine methyl ester (A-L-PME) monomer with different monomer/initiator ratios [164]. Copyright 2017, Reprinted with permission from the Elsevier Ltd.

Fig. 27. Synthetic strategies toward PS brushes on GO by RAFT polymerization (pathways A and B) and also chain-transfer grafting by thiols (pathways C and D) [167]. Copyright 2012, Reprinted with permission from the American Chemical Society (ACS).

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Fig. 28. Synthesis of homopolymers- and diblock copolymers-grafted GO by (a) MPTT- and (b) TBPT-mediated RAFT polymerization [168]. Copyright 2012, Reprinted with permission from the American Chemical Society (ACS).

3980–12,500 g/mol, PDI in the range of 1.11–1.38, and molar grafting ratio of 73.6–220 μmol/g. The products showed high dispersibility in hexane and water, which confirms its amphiphilicity. 3.1.2. Surface-anchoring method The “grafting through” method is one of the interesting methods for grafting polymer chains on various substrates from the backbone. This grafting method is based on incorporation of the propagating polymer chains in the double bond-containing surfaces from the backbone. This method finally resulted in anchoring a propagating chain to the surface and its subsequent propagation from that surface. RoghaniMamaqani's group reported the “grafting through” method for grafting PS chains on the GO surface via ATRP in 2014 [169]. As shown in Fig. 29, GO was functionalized with 3-(trimethoxysilyl)propyl methacrylate (MPS) from the hydroxyl groups to yield GOHD. Polymerization of styrene was conducted in the presence of GOHD and ethyl αbromoisobutyrate (EBiB) as the ATRP initiator. In the propagation step, active radicals were engaged with the double-bond of GOHD, and surface-anchored PS chains were obtained. PS chains with different grafting densities at the surface of graphene were obtained by using

GOHD with different modifier content [170]. By increasing the grafting density of PS chains, Mn and PDI of the attached chains, the polymerization rate, and monomer conversion increased. They also reported functionalization of GO with PS chains using “grafting through” method from the edge carboxyl groups [171]. As shown in Fig. 30, 3-((4hydroxybutoxy)dimethylsilyl)propyl methacrylate (CD) was prepared using a coupling reaction between BG and (3-methacryloxypropyl)dimethylchlorosilane (MCS). Then, GO was modified with CD in different amounts from the edge carboxylic acid groups to yield GCD with different modifier contents. Using ATRP in the presence of styrene monomer and EBiB as the initiator, PS chains were anchored at the edge of GO layers by the “grafting through” method. They showed that Tg of composites increased by increasing grafting density of PS chains. Roghani-Mamaqani [172] and coworkers have also reported grafting of PS with various densities from the surface epoxy groups using ATRP and “grafting through” method. As shown in Fig. 31, 3-(((2 aminoethyl)amino)dimethylsilyl)propyl methacrylate (OD) was obtained through the acylation reaction between EDA and MCS. Then, OD in different amounts was coupled with the GO surface via ring opening of

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Fig. 29. Schematic illustration for synthesis of PS-grafted GO with different grafting densities using ATRP [170]. Copyright 2015, Reproduced with permission from the Springer.

epoxy groups to reach the double bond-containing GO (GOD) with different grafting density of OD. Finally, GOD was used to anchor PS chains in different grafting densities using ATRP and the “grafting through” method. Through the polymerization, free polymers with lower molecular weight and broader molecular weight distributions in comparison with the grafted PS chains were also obtained. Incorporation of higher density of PS chains in the interlayer of GOD resulted in higher Tg values and interlayer distances. Liu and coworkers [173] used “grafting through” method for the synthesis of a two-dimensional nickel ion-imprinted polymer (RAFT-IIP) based on GO/SiO2 composite through the RAFT polymerization and surface-imprinting method. As shown in Fig. 32, there are four steps for the preparation of such composite: 1) Sol-gel reaction was used to prepare GO/SiO2, where SiO2 coated the surface of GO through the covalent bonding between TEOS and the GO functional groups; 2) The GO surface was silanized with MPS for introduction of vinyl groups, which is prerequisite of “grafting through” method (GO/SiO2-MPS); 3) RAFT polymerization was performed in the presence of AM monomer,

EGDMA cross-linker, AIBN initiator, Ni(II) template, and CTA to cover the GO surface with a thin polymer layer; and 4) RAFT-IIP with a thin polymer layer on GO/SiO2 was obtained after eluting the product for removing the template. According to the results, the RAFT-IIP showed more specific adsorption capacity and faster Ni(II) adsorption kinetics than FRP-IIP. The RAFT-IIP demonstrated not only more splendid recognizing ability but also large adsorption capacity than non-imprinted polymer. 3.2. Non-covalent “grafting from” method Changing functionality of graphene layers by oxidation and also further covalent functionalization processes results in variation of its properties. Therefore, non-covalent methods were investigated for prevention from variation in the structure of graphitic layers upon the polymer-functionalization processes. Here, non-covalent polymerfunctionalization of graphene is reviewed where polymers were synthesized by CRP. The most important mechanism for non-covalent

Fig. 30. Schematic illustration for the synthesis procedure of CD and preparation of CD-modified GO [171]. Copyright 2014, Reproduced with permission from the John Wiley & Sons Incorporation.

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Fig. 31. Schematic representation for synthesis of OD and preparation of PS-grafted GO with various grafting densities using ATRP [172]. Copyright 2017, Reproduced with permission from the John Wiley & Sons Incorporation.

polymer-functionalization of graphene is π–π stacking interactions. The π–π stacking interaction is established between π-donors of the especial structures such as hydroquinone, benzene rings of graphene, resorcinol, or dioxynaphthalene residues and also π-accepting rings such as bipyridinium, pyrene, or π-extended viologen units. Pyrene is the most commonly used functional group which is a polycyclic aromatic hydrocarbon including four fused benzene rings. Pyrene-end polymers which were synthesized via CRP are used in the most reports of graphene non-covalent polymer-functionalization. In some cases, the pyrene-including initiator is non-covalently attached on graphitic layers, and then polymer brushes are grown from

the surface with the “grafting from” method. Pei et al. [174] synthesized PDMAEMA-attached GO via π-π stacking interactions. They prepared pyrene-terminated initiator and attached it on GO layers via π-π stacking interaction. Then, the polymer brushes were grown from the surface of the modified GO. The PDMAEMA-attached GO was loaded by palladium and gold NPs through ion exchange and in situ reduction processes. Uniformly distributed Pd\\Au NPs on PDMAEMA-attached GO was used as a catalyst in reduction of 4-nitrophenol by an excess of NaBH4. Zhou and coworkers [175] reported polymer-functionalization of graphene by SI-ATRP and using π-π interaction. To preserve the substrate properties, non-covalent interaction is more desirable than the

Fig. 32. Schematic representation for preparation of ion-imprinted polymer with excellent thermal stability based on GO/SiO2 composite by RAFT polymerization [173]. Copyright 2015, Reprinted with permission from the American Chemical Society (ACS).

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Fig. 33. Procedures for fabricating patterned polymer brushes on r-GO substrate assembled on silicone wafer by using SI-ATRP [175]. Copyright 2013, Reproduced with permission from the American Chemical Society (ACS).

covalent attachment. They prepared patterned polymer brushes on GO film by micro-contact printing of initiator moieties and subsequent SIATRP method, as shown in Fig. 33. Four different cationic, anionic, temperature-responsive, and pH-sensitive monomers were used in SIATRP reaction which are 3-sulfopropyl methacrylate potassium salt (SPMA), methacroylcholine chloride (METAC), NIPAAm, and DMAEMA, respectively. In this work, GO was assembled on silicon wafers, and the prepared GO film was chemically reduced for reaching excellent electrical and mechanical properties. Two different types of ATRP initiators with pyrene and catechol end groups were used for introduction to the r-GO surface via π-π stacking. In 2016, Zhang et al. [176] reported preparation of polymer-functionalized magnetic GO nanocomposites by a solvothermal reaction and SI-ATRP. The solvothermal reaction was firstly used for the preparation of magnetic layers by attachment of Fe3O4 to the GO surface. Then, a pyreneterminated ATRP initiator, which was fabricated through the reaction between 1-pyrenebutanol and BiBB, was introduced on the surface of magnetic GO by the π-π interaction between the pyrene and GO layers. Subsequently, well-defined and high-density poly(4-vinylphenylboronic acid) (PVPBA) brushes were propagated from the surface of modified GO by SI-ATRP. The boronic acid polymer brushes immobilized on the magnetic GO was used for selective enrichment of glycoprotein from biological systems. SI-RAFT was also used to synthesis of pyrene-functionalized polymer for non-covalent modification of graphene or GO by π-π interaction. For instance, Liu et al. [177] reported functionalization of graphene by positively-charged poly(dimethylaminoethyl acrylate) (PDMAEA), negatively-charged PAA, and neutral PS synthesized via SI-RAFT. Polymerization proceeds from graphene surface via a pyrenefunctionalized RAFT agent attached to the surface by π-π interaction. The composites with all three types of polymer brushes and the same monomer conversion showed a similar conductivity. However, in the case of the composites including random copolymer brushes of these monomers, conductivity was significantly decreased. In addition, the grafted polymers with longer chains showed lower conductivity. 4. Polymer-grafting by controlled radical polymerization and coupling methods Polymer-functionalization of graphitic layers by coupling methods has largely been considered in the recent years. Most commonly, well-

defined polymers were synthesized by different CRP methods and subsequently coupled at the edge or surface of graphene and its derivatives. Here, the most recent reports on polymer-functionalization of graphitic layers by the combination of CRP and coupling methods are reviewed. 4.1. Covalent method 4.1.1. Click chemistry reactions Synthesis of 1,4-disubstituted regioisomers is carried out by a copper-catalyzed reaction; however, a ruthenium-catalyzed reaction results in the different regioselectivity with the establishment of 1,5-disubstituted triazoles [178–182]. These catalyzed reactions can be defined as click chemistry by azide-alkyne cycloaddition reaction. Thiol-ene and thiol-yne chemistries are the other important click chemistry reactions, which are highly efficient without any side products, need simple reaction conditions, and result in high-yield products [107,183]. Feng et al. [184] proposed a facile methodology for immobilization of well-defined PS chains on graphene layers using azide-alkyne cycloaddition, where the resulting layers were dispersible in common organic media. Accordingly, azido-terminated PS chains were synthesized by ATRP using 3-azidoethyl 2-bromoisobutyrate as the initiator. Herein, the GO layers were modified with propargyl alcohol using an acylation reaction. The click reaction of alkyne-modified GO layers with azidoterminated PS chains proceeds at room temperature. Similar work has been carried out by Tu and coworkers [185] in 2011, where alkyneterminated PS chains were synthesized by using ATRP. Then, the alkyne-terminated PS samples with various chain lengths were grafted at the surface of azido-functionalized GO via a [3 + 2] Huisgen cycloaddition and formation of 1, 2, 3-triazoles ring in the presence of CuBr/ PMDETA complex as the catalyst. Here, alkyne-terminated PS chains were prepared from 3-azidoethyl 2-bromoisobutyrate as the ATRP initiator. The azido-functionalized GO layers were also prepared by the modification of GO with BiBB and its subsequent reaction with NaN3. This composite showed high dispersibility in THF, DMF, and chloroform. In addition, the interlayer distance of GO layers was increased by increasing the length of the intercalated PS chains. A ternary composite based on GO, Fe3O4 NPs, and PDMAEMA was prepared by Chen and coworkers [186] by anchoring Fe3O4 NPs on GO layers and subsequent grafting of PDMAEMA to the surface by click chemistry. As shown in Fig. 34, alkynyl groups were attached at the surface of GO layers by an

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Fig. 34. Schematic illustration for the synthesis of Fe3O4/PDMAEMA-GO hybrid composite and loading of LOFX drug [186]. Copyright 2014, Reprinted with permission from the Royal Society of Chemistry (RSC).

esterification reaction between hydroxyl groups of propargyl alcohol and carboxylic acid functionalities of GO. Then, in situ decoration of the alkynyl-modified GO layers (alkynyl-GO) with Fe3O4 NPs was performed and azido-terminated PDMAEMA prepared by ATRP was grafted on the Fe3O4/alkynyl-GO via an azide-alkyne cycloaddition. This hybrid composite displayed good dispersion stability in water depending on the density of polymer grafting and also length of the grafted chains. Loading and release properties of levofloxacin (LOFX) showed that this hybrid composite had high loading capacity and controlled release by pH trigger. Combination of RAFT polymerization and click chemistry for the synthesis of graphene-based polymer brushes was developed by Hwang and coworkers [160]. As shown in Fig. 23, alkyne groups were introduced on graphene via diazonium addition method. Different azido-terminated polymers with low PDI values were obtained by RAFT polymerization and grafted on the graphene layers via click chemistry. This powerful strategy controlled the grafted polymer's molecular weight, architecture, grafting content, and also grafting density on the graphene, and finally resulted in relevant dispersibility and processability. The other research study for covalent polymer-modification of graphene by RAFT polymerization and click chemistry was

reported by Huang and coworkers [187]. Azido-terminated poly(N-(2hydroxypropyl) methacrylamide) (PHPMA-N3) homopolymer was prepared using RAFT polymerization and then grafted on the ynecontaining graphene (G-yne) by click chemistry, where CuBr/PMDETA was used as the catalyst to yield G-PHPMA composites. As shown in Fig. 35, the G-yne was prepared by diazonium functionalization of GO layers with but-3-ynyl 4-aminobenzoate and isoamyl nitrite. The PHPMA-N3 was obtained from RAFT polymerization of N-(2-hydroxypropyl) methacrylamide with S-1-dodecyl-S′-(α,α’-dimethyl-α”-3azido-1-propyl acetate) trithiocarbonate as the azido-functionalized CTA. Finally, the covalent bonding between PHPMA and modified GO layers was formed by azide-alkyne click chemistry. The resulting nanocomposite showed excellent dispersion in organic solvents and water and was able to enter into SMMC-7721 and SH-SY5Y cells, which shows its potential applications in biomaterials. 4.1.2. Radical addition reactions Free radicals are species which contain one or even higher amounts of unpaired electrons. These species are usually uncharged and therefore show different chemistry in comparison with the electrondeficient species like cations and carbenes. Grafting of styrene polymers

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Fig. 35. Schematic illustration for the synthesis methodology of the G-PHPMA nanocomposite [187]. Copyright 2014, Reprinted with permission from the Royal Society of Chemistry (RSC).

on the graphene by radical method and without the need for initiator was reported in 2013 [188], as shown in Fig. 36. The organophilic stearylamine-modified GO which includes both olefinic unsaturation and stable graphene radicals is the key intermediate. By exploring of the C-centered graphene radicals, their concentration increases at the early stage of polymerization and decays with increasing styrene conversion. At the first times of polymerization, the PS radicals formed by thermal polymerization of styrene are added to the graphene double bond like a “grafting to” process (pathway A) or abstract hydrogen from stearylamine-modified GO (pathway B) which finally resulted in increase of the concentration of C-centered radicals on graphene. However, depletion of the C-centered graphene radicals was observed by

“grafting from” reaction of polystyrene initiated by C-centered radicals (pathway C) and also the “grafting to” process with PS radicals (pathway D). Hence, hydrogen transfer and the attachment of PS radicals to graphene must be dominant in comparison with the initiation of styrene polymerization from the surface during the first times of the reaction. High grafting contents were obtained by the addition of PS radicals to graphene (the “grafting to” method) and graphene-initiated polymerization (the “grafting from” method). The addition of TEMPO results in controlled graft contents by controlling the radical concentration and therefore “grafting from” polymerization. With increasing the reaction time and thereupon the reaction medium viscosity, rate of the “grafting to” reaction is reduced in comparison with the “grafting from”. This in

Fig. 36. Schematic illustration for the in situ styrene polymerization: (A) The “grafting to” reaction by the addition of PS radicals to graphene, (B) hydrogen transfer from graphene layers to PS Radicals, (C) the “grafting from” by graphene radical-initiated styrene polymerization and (D) the “grafting to” by radical recombination. [188] Copyright 2013, Reprinted with permission from the American Chemical Society (ACS).

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Fig. 37. Schematic representation for the preparation of GO-GMA-g-PS. Copyright 2016 [189], Reprinted with permission from the John Wiley & Sons Incorporation.

situ polymerization process makes it possible to graft styrene copolymers with butyl acrylate and acrylonitrile to the graphene surface. Results shows that both stearyl-modified GO and PS-grafted graphenes considerably increase the PS thermal stability. Saldívar-Guerra and coworkers [189] prepared well-defined PSfunctionalized GO, which was synthesized via NMP. As shown in Fig. 37, the polymer-functionalization was done in two steps. In the first step, epoxy ring of GMA was reacted with the hydroxyl groups of GO for double bond addition onto the GO surface (GO-GMA). In the next step, the PS synthesized via NMP was anchored to the double bond of grafted GMA to yield GO-GMA-g-PS. This method provided the facile and clean rout for polymer-functionalization of GO.

4.1.4. Other “grafting to” methods There are also other ways to graft polymer chains on graphitic layers. For example, Huang at el [191]. reported an interesting work, where Br-terminated poly(2-acryloxyethyl ferrocenecarboxylate) (PAEFC) homopolymers were prepared by ATRP, and grafted on the surface of TEMPO-functionalized graphene layers (GS-TEMPO) via atom transfer nitroxide radical coupling (ATNRC) to obtain ferrocene-including graphene (GS-PAEFC) composites, as shown in Fig. 39. They showed that GS-PAEFC nanocomposites are dispersible in a wide variety of organic solvents. They claimed that this study has potential applications in novel nanoelectronic devices.

4.1.3. Condensation reactions PVK was synthesized by RAFT polymerization using DDMAT. Then, PVK-functionalized GO was synthesized by the reaction of DDMATterminated PVK with toluene-2,4-diisocyanate (TDI)-modified GO [190]. Here, TDI was attached at the edge and surface of GO layers by amidation reaction to obtain TDI-modified GO. Finally, DDMATterminated PVK was coupled at the edge and surface of TDI-modified GO by amidation reaction (Fig. 38). The resulted GO layers are readily dispersible in organic solvents because of the presence of PVK chains, where they can be used for the fabrication of optoelectronic and nonvolatile rewritable memory devices.

4.2. Non-covalent method In some cases, polymers including pyrene moieties on the backbone attach to the surface of graphene layers like “grafting to” method. These polymers could be synthesized by different polymerization methods. Zhang et al. [192] reported the synthesis of the pyrene-functionalized copolymer of MMA and dimethylsiloxane by ARGET ATRP for noncovalent functionalization of GO, as shown in Fig. 40. Pyrenefunctionalized ARGET-ATRP initiator (Py-Br) was synthesized by a coupling reaction between 1-pyrenemethanol and BiBB. Then, pyrene end-capped PMMA (Py-PMMA-Br) was obtained via ARGET-ATRP in

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Fig. 38. Schematic illustration for preparation of GO-TDI, DDAT-PVK, and GO-PVK [190]. Copyright 2010, Reprinted with permission from the John Wiley & Sons Incorporation.

the DMF solution of CuBr2 and PMDETA. The pyrene-functionalized copolymers of MMA and dimethylsiloxane (Py-PMMA-b-PDMS) were synthesized through the same route. Py-PMMA-b-PDMS was used for modification of GO using π-π interaction between the layers and pyrene moieties. Finally, the Py-PMMA-b-PDMS-grafted GO and PMMA composites were obtained by the solution blending method. The Py-PMMA-b-PDMS-grafted GO improved thermal, mechanical, and optical, characteristics of PMMA, and shows potential applications in organic light-emitting diodes. Tsitsilianis and coworkers [193] reported preparation of non-covalent PMMA-functionalized graphene. Pyrene end-capped PMMA chains were synthesized through the combination of ARGET ATRP and click chemistry. The PMMA chains with pyrene

Fig. 39. Schematic illustration for preparation of GS-PAEFC nanocomposite [191]. Copyright 2018, Reprinted with permission from the Royal Society of Chemistry (RSC).

functionalities were synthesized by using pyrene-capped initiator through ARGET ATRP. Hetero-telechelic PMMA chains with pyrene and azide functionalities were prepared by end-group transformation of pyrene-modified PMMA with azide groups using NaN3. In the next step, the azide-alkyne click reaction between pyrene-modified PMMA with azide groups and the alkyne-functional pyrene derivative resulted in the bifunctional pyrene-capped PMMA. The product was used as an efficient dispersing agent for graphene through the π-π interaction between the pyrene and graphene benzene rings for the exfoliation of graphene in low boiling point chloroform. Also, bridges, loops, dangling ends, and free chains conformations of graphene-grafted PMMA were confirmed by molecular dynamics simulations. RAFT polymerization was also applied to prepare pyrene-capped polymers for grafting on graphene layers by π-π interaction. Xu and coworkers [194] synthesized pyrene-capped PAA, PDMAEMA, and PAM (Polyacrylamide) via RAFT polymerization for fabrication of polymer-modified highly-ordered pyrolytic graphite (HOPG) as a solid-state tris(2,2-bipyridyl)ruthenium(II) (Ru(bpy)2+ 3 ) electrochemiluminescent (ECL) sensors with different functions. The π-π stacking interactions between the polymers and HOPG provided the HOPG polymer-modification. These ECL sensors, fabricated by using the π-π stacking interaction between the HOPG surface and the pyrene moieties of PAA and also a stable electrostatic interactions between the negatively charged polymers and Ru(bpy)2+ 3 , were applied for tripropylamine, tetracycline, and lysozyme detection. Davis et al. [195] reported non-covalent modification of graphene by pyrene-capped polymers to give pH-sensitive

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Fig. 40. Schematic illustration for the synthesis of Py-PMMA-b-PDMS [192]. Copyright 2015, Reprinted with permission from the Royal Society of Chemistry (RSC).

sandwich-like nanostructures. They synthesized pyrenefunctionalized RAFT agent and used it for mediation of DMAEMA and AA polymerization, as shown in Fig. 41. The hydroxylfunctionalized RAFT agent was prepared by a condensation reaction between an acid-functional trithiocarbonate (1-pyrenebutyric acid) and hydroxyl ethyl disulfide of 3-[hydroxylethyl disulfide] ethyl. Finally, the pyrene-functionalized RAFT agent was synthesized by another condensation reaction between pyrene butyric acid and the prepared hydroxyl-functionalized RAFT agent. By varying the solution pH, the sandwich-like nanostructure changed into layered nanostructures via self-assembly, and phase transfer between the aqueous and organic media was observed. Davis et al. [196] also reported the synthesis of thermo-sensitive sandwich-like composites by attachment of pyrene-capped PNIPAAm synthesized via RAFT polymerization on the graphene surface. The pyrene-functionalized RAFT agent was used to control the polymerization of NIPAAm through the RAFT polymerization to achieve PNIPAAm with pyrene end groups. The pyrene-functionalized PNIPAAm was attached to the conjugated basal plane of graphene via π-π stacking interaction. Results showed that LCST of the resulted composite is lower than that of the pyrene-functionalized PNIPAAm. RAFT polymerization was also used for the synthesis of pyrenecapped tri-block copolymers to functionalize graphene by Liu and

coworkers [197]. As shown in Fig. 42, pyrene-capped RAFT agent was synthesized by an esterification reaction between 4-cyano-4ethyl-trithiopentanoic acid (CETP) and 1-pyrenemethanol. Then, oxopentanoate ethyl methacrylate (OEMA) which contains ketone groups was polymerized through RAFT reaction. In the next step, pyridyl disulfide ethyl acrylate (PDEA) containing mimic drug pyridine-2-thione (PT) was introduced at the end of POEMA in the presence of AIBN to prepare P(OEMA-b-PDEA) via RAFT polymerization. Subsequently, ethylene glycol acrylate (EGA) was copolymerized with P(OEMA-b-PDEA) to yield P(OEMA-b-PDEA-b-PEGA) tri-block copolymer. O-benzylhydroxylamine (BHA) as another drug was conjugated to the copolymer through the oxime coupling chemistry. Finally, the drug loaded tri-block copolymer was attached to the graphene surface through π-π stacking interaction. The BHA and PT released under acid environment and in the presence of the reducing agent of dithiothreitol or glutathione, respectively. Increasing of biocompatibility, solubility of hydrophobic drugs, and stability of graphene in the physiological environment were the main results. This group also modified chemically vapor deposited (CVD) graphene with pyrene-capped PNIPAAm synthesized by RAFT polymerization. This thermo-responsive graphene-polymer film was prepared through non-covalent π-π stacking interactions [198].

Fig. 41. Schematic representation for the synthesis of pH-sensitive pyrene-polymer composites by combination of RAFT polymerization and π-π Stacking [195]. Copyright 2010, Reprinted with permission from the American Chemical Society (ACS).

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Fig. 42. Schematic illustration for preparation of pyrene-capped tri-block copolymer and its grafting to the surface of graphene by π-π stacking interaction [197]. Copyright 2015, Reprinted with permission from the Elsevier.

In addition to the pyrene group, there are some other π-accepting agents attached to the graphene surface or end of the polymer backbone, and used for non-covalent functionalization of graphene. Nandi et al. [199] used polythiophene (PTi) as a π-accepting agent for polymer-functionalization of r-GO through the π-π interaction. The π-electrons of PTi with the ability of π-stacking interactions with graphene layers are the main reason for choosing this πaccepting agent. In this study, PMMA-grafted PTi was prepared by grafting of PMMA on the PTi backbone via ATRP, added to r-GO for preparation of PMMA-grafted PTi and r-GO composites. The thiophene units of PTi chain interacted with the r-GO surface through the π-π interactions, and superior mechanical and electronic properties were observed for the composite. Zhang and coworkers [200] reported preparation of the phosphorylcholine oligomer-grafted perylene (Perylene-PCn) as a water-soluble and biocompatible polymer, which was grafted to the r-GO surface through π-π stacking

interactions between the graphene layers and perylene moieties of the polymer. As shown in Fig. 43 (a), perylene-3,4,9,10tatracarboxylic dianhydride (PTCDA) reacted with ethanol amine to yield perylene-3,4,9,10-tetracarboxylic acid bisimide derivatives (PBI-OH). Then, PBI-Br as an ATRP initiator was prepared through the coupling reaction between PBI-OH and BiBB. Finally, ATRP of MPC was conducted in the presence of PBI-Br, MPC, and CuBr to synthesize Perylene-PCn. The synthesized polymer attached to the chemically reduced GO surface by π-π stacking interaction between the r-GO and perylene moieties of polymer (Fig. 43 (b)). The resulted composite was used as an anti-tumor agent delivery device with potential oncology applications. Non-covalent polymer-functionalization of graphene by using RAFT polymerization was also reported by Cheong and coworkers [201]. HOPG was used as the template in this work. Accordingly, RAFT polymerization of 2,2,2-trifluoroethyl methacrylate (TEFMA)

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Fig. 43. Schematic illustration for (a) synthesis of Perylene-PCn and (b) non-covalent functionalization of of r-GO with Perylene-PCn [200]. Copyright 2016, Reprinted with permission from the Royal Society of Chemistry (RSC).

in the presence of 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (CTP) as the CTA and AIBN as the free radical initiator resulted in the TEFMA macro-RAFT agent. Then, PTFEMA-b-P4VP copolymer was synthesized through the subsequent RAFT polymerization of 4VP by using TEFMA macro-RAFT agent. Then, the PTFEMA-b-PVP block copolymers were used for anchoring at the basal plane of graphene through the adsorption of PVP blocks via π-π stacking interactions. The results showed that the dispersion stability of HOPG was improved by its non-covalent polymer functionalization. Noncovalent polymer-functionalization of graphene with Van der Waals interactions between polymer and GO was reported for small interfering RNA (siRNA) delivery [202]. Methoxypoly(ethylene glycol) (mPEG) with NH2 end group was covalently grafted to GO from its carboxylic acid groups of via an amidation reaction (mPEG-GO). In addition, PDMAEMA chains were synthesized via RAFT polymerization in the presence of 2-(dodecyl thiocarbonothioyl thio)-2-methyl propionic acid (DTM) as the CTA. Finally, PDMAEMA was introduced to GO via the physical interaction between DTM and GO using a sonication process. Results of siRNA delivery showed efficient gene transfection and also low cytotoxicity. Grafting of mPEG and PDMAEMA on the GO surface caused enhancement of optical properties of GO with high dispersibility in water,

good biocompatibility, and easily tunable surface-functionalization in siRNA delivery. Non-covalent copolymer-functionalization of graphene with physical interactions between polymer and GO was also reported for doxorubicin hydrochloride (DOX) delivery as an anticancer drug [203]. Here, chitosan was functionalized with phthalic anhydride followed by 4-cyano,4-[(phenylcarbothioyl) sulfanyl] pentanoic acid to obtain CTA-grafted chitosan. Then, polymerization of MAA via RAFT polymerization in the presence of the CTA-grafted chitosan was conducted for obtaining PMAA-grafted chitosan as a pH-responsive product. Strong physical interactions between the functional groups of PMAA-grafted chitosan and GO resulted in PMAA-grafted chitosan/GO composites. The π–π stacking between GO and DOX provided possibility for drug loading in this composite. 5. Applications General applications of modified graphene layers and its derivatives with polymers synthesized via CRP are summarized. The specific applications of such systems are already explained in the preceding sections. The potential applications of polymer-functionalized graphene materials include drug delivery, biomedical materials, high-

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performance materials, sensors, catalysts, membranes, adsorbents, water treatment, actuators, etc. Noticeable variations in properties of stimuli-responsive polymers could be observed by changing the environmental condition, like temperature, pH, etc. Synthesis of graphene-anchored thermo-responsive polymers via ATRP [80,81,128,132,196], SET LRP [142,143], and RAFT polymerization [150,198], pH-responsive polymers [129,163,195], and also electro-responsive [84] polymers were reported in the literature. These polymers can be used in the preparation of smart drug delivery systems. In some cases, the drug was connected to the polymergrafted graphene by π-π stacking and hydrophobic interactions. For example, Park et al. [117] reported the synthesis of pH-sensitive P4VPfunctionalized GO by ATRP, which was loaded by CPT as a cancer drug via π-π stacking and hydrophobic interactions. The important point is the biocompatibility of P4VP-grafted GO which is required for drug delivery systems. They [114] also reported a similar work but a different pH-responsive polymer (PVCL). The prepared PVCL-grafted GO was loaded by CPT via π-π stacking and hydrophobic interactions. It was highly biocompatible and stable in physiological solutions. Chen and coworkers [186] loaded the LOFX drug within the Fe3O4/PDMAEMAgrafted GO by the π-π stacking interaction. The prepared composite exhibits extra loading capacity and also pH-controlled release properties for LOFX. In addition, thermo-responsive polymer-functionalized graphene can be used as drug carriers. For instance, Zhu et al. [80] reported the preparation of a thermo-responsive drug delivery system based on PNIPAAm-grafted GO. Herein, IBU drug was loaded within the product via π-π stacking interaction. Because of two-dimensional structure of GO and the hydrogen interaction between the polymer and drug, the resulted nanocomposite showed high IBU storage of about 280 wt%. A pH-sensitive graphene/PAA hydrogel was obtained via RAFT polymerization to control the release of DOX [163]. Efficiently controlled drug release of this hydrogel by swelling in the intestine environment makes it a good candidate in drug delivery applications. Singha and coworkers [89] also reported GO/PMAPOSS hydrogel through the SI-ATRP, which could entrap ciprofloxacin as an antibiotic drug. Dual controlled drug delivery through pH-stimulation and biodegradation has also been reported by Liu and coworkers [197]. Poly (OEMA-b-PDEA-b-PEGA) tri-block copolymer with two kinds of mimic drug was prepared by RAFT polymerization. First mimic (PT) was added to the tri-block by PDEA block. The second one (BHA) was conjugated with tri-block copolymer via efficient oxime coupling chemistry and π-π stacking interaction. BHA was released in the acid environment, whereas the PT could be released in the presence of dithiothreitol or glutathione as the reducing agent. To investigate the effect of antitumor drug (PTX), Perylene-PCn was attached on the r-GO substrate [200]. Polymer-functionalized graphene containing disulfide linkages in initiation sites of ATRP have been designed for gene delivery [116]. Further graphene-based polymer nanohybrids were synthesized for small interfering RNA (siRNA) delivery into targeted cells [202]. The cell behavior regulation capacity of the polymer-functionalized graphene nanomaterials can be beneficial for the future biomedical applications. In 2014, the human hepatoma cell line (SMMC-7721) and neuroblastoma cell line (SH-SY5Y) were cultured for cellular uptake of PHPMA-grafted graphene synthesized via the combination of RAFT polymerization and click chemistry via the “grafting to” method [187]. Other cell behavior testing of polymer-functionalized graphene was done using the NIH-3 T3 fibroblast cells and human umbilical vein endothelial cells viability assays by Turng's group [137]. In addition, generation of cellular micropatterns from NIH-3 T3 fibroblast and hippocampal neurons on a POEGMA-coated graphene film was studied [121,122]. Fetal bovine serum protein absorption assay experiment was performed to testify the biocompatibility of the obtained hybrid nanomaterials. Enrichment of biomolecules (proteins, peptides, etc.) via the polymer-functionalized graphene materials is one of the interesting bio-applications. For example, PAA-grafted GO was synthesized for efficient N-glycopeptide enrichment, which is demanded with

biomedical sciences, and in particular biomarker research, in complex human serum samples [92]. Chen's group studied the N-glycopeptide enrichment via both covalent [107] and non-covalent [176] polymerfunctionalized graphene. Catecholamines were enriched in the urine sample by PAAPBA grafted on the polydopamine-coated magnetic GO [141]. Polymer-functionalized graphene materials were developed as new supporting materials for improved proteases immobilization, leading to open a gateway to new fields in biotechnology. Qin et al. [103] synthesized PAA and PGMA brushes on GO by SI-ATRP as a support for trypsin immobilization and efficient proteome digestion. Polymer-functionalized graphene is an efficient nanofiller in polymer composites to improve its mechanical [108,153,192,199], electrical [135,155], and thermal [148] properties. Graphene as a compatibilizer in a polymer matrix can be resulted in high reinforcement, Young's modulus, and tensile strength. The amphiphilic nature of the polymerfunctionalized graphene is the main reason of improving mechanical properties of the polymer nanocomposite [153]. Noncovalent polymer-functionalized GO in PMMA matrix causes dramatical enhancement of the elongation at break and tensile toughness of the polymer nanocomposite [192]. Young's modulus was also increased by the addition of non-covalent polymer-functionalized graphene to the polymer matrix [199] Increasing tensile strength and elongation at break by the addition of polymer-functionalized graphene was also reported by Jun et al. [108] for PU matrix. Entanglement of the grafted polymer on graphene surface with the polymer chains of matrix affected the interfacial adhesion between r-GO and the matrix, and finally resulted in higher dielectric constant [155]. Incorporation of polymer-grafted graphene as a conductive filler into the polymer matrix has an ability to enhance the degree of orientational freedom of dipole in polymer dielectrics [135]. Increasing thermal conductivity of the matrix by the addition of polymer-grafted graphene was also reported [148]. Application of polymer-functionalized graphene as a filler in polysulfone membranes [95,99,154] for water treatment and Nafionbased membranes [100,102] for proton conduction has been investigated in 2017 and 2018. Polysulfone ultrafiltration membrane surface hydrophilicity and antifouling properties were improved with using PEGMA-grafted GO [99]. Another modification of polysulfone membranes with polymer-functionalized graphene with water treatment application was reported by Ghasemi Kochameshki's group [154]. Ionimprinting technology was used for the preparation of ion-imprinted polymer-functionalized graphene for selective adsorption of nickel ions [173]. Other water contaminants which could be eliminated with polymer-functionalized graphene are dyes. For example, Au/ PDMAEMA/r-GO acted as a catalyst for RB, MO, and EY dyes degradation [90]. La2O3/PTHF/r-GO was applied as a catalyst for these dyes degradation via the same group [91]. In addition, the catalytic performance of polymer-functionalized graphene was studied for the reduction of 4-nitrophenol [149,174]. Other catalyst applications of polymerfunctionalized graphene are in isoamyl benzoate synthesis [113] and hydroquinone degradation [82]. The molecularly-imprinted polymerfunctionalized graphene was used for detection of histamine [152] and endocrine disrupting chemicals [151]. One of the interesting applications for polymer-functionalized graphene was reported by Kattimuttathu [94]. They used amphiphilic comb-like polymer-functionalized GO as a surfactant in the preparation of styrene emulsion polymerization. There are also some electrical applications for polymerfunctionalized graphene such as the photovoltaic cell [138], actuators [198], and sensors [128,129,194]. 6. Conclusion, outlook, and challenges Surface engineering of graphene is inevitable for improvement of its dispersion in different solvents and polymer matrices. Lots of strategies were developed to disrupt the interaction between the layers physically or chemically, and increase its compatibility with the host matrix. Graphene oxidation results in the appearance of hydroxyl and epoxy

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groups at the surface and carboxylic acid functionalities at the edge. These functional groups are a source for attachment of various functional groups containing initiator, monomer, or other moieties. Therefore, various grafting methods are used for the preparation of polymer brushes at the edge or surface of GO. In this review, application of the in situ CRP methods to prepare polymer brushes on graphene with two grafting methods of the “grafting from” and “grafting through” was explained. Also, the “grafting to” method for the preparation of polymer-grafted graphene layers is also discussed. Here, physical and chemical attachment concepts were considered in surface- or edge-modification of graphene layers. For example, π-π interaction and electrostatic interaction are considered the most important physical modification strategies and condensation, cycloaddition, and addition reactions are considered as the main chemical modification strategies. An important challenge is tailoring of the grafted chain's functionality, grafting density, molecular weight, PDI, structure, and topology. Graphene functionalization by CRP methods results in well-defined polymer brushes with a high degree of control over the grafting density, functionality, and thickness of the grafted polymer brushes. These characteristics of polymer brushes are very important in its final properties. For reaching controlled and desired characteristics of polymer brushes, it is required to control the number of brushes in a surface unit and also the thickness of the grafted polymer on the surface. In addition, it is interesting that the polymer chains tethered to the graphene and the free chains are somewhat different. Also, grafting of prefabricated polymer chains to a substrate is confined by steric restrictions, which finally results in difficult production of densely grafted polymer brushes. Efficiency of reaction between the polymer functional groups with the substrate functionalities decreases with increasing polymer molecular weight. However, grafting density and thickness of polymer brushes can be precisely controlled in “grafting from” method. Finally, covalent-functionalization of graphene and its derivatives ensures a strong bonding between the polymer and graphene. However, the weaker non-covalent methods prevent from generation of defect sites on the surface of graphene. These non-covalent polymerfunctionalized graphenes show good physical, mechanical, and electronic properties. We believed in this study can help to the researchers to choose the appropriate method for preparation of polymer-grafted graphitic substrates with well-defined polymers of controlled molecular weight, thickness, and PDI. Control of grafting density, site-selectivity in grafting, various topologies of the brushes, and polymer brushes structure are achievable. This study has potential applications in the preparation of stimuli-responsive polymer brushes, polymer composites, Pickering emulsion surfactants, coating technologies, sensors, supercapacitators, etc. Declaration of Competing Interest None. Acknowledgement Financial support of Iran National Science Foundation (INSF) is highly appreciated (Grant Number: 96013220). Author declaration template We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us.

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