Novel Complex Polymers with Carbazole Functionality by Controlled Radical Polymerization

This review summarizes recent advances in the design and synthesis of novel complex polymers with carbazole moieties using controlled radical polymerization techniques. We focus on the polymeric architectures of block copolymers, star polymers, including star block copolymers and miktoarm star copolymers, comb-shaped copolymers, and hybrids. Controlled radical polymerization of N-vinylcarbazole (NVC) and styrene and (meth)acrylate derivatives having carbazole moieties is well advanced, leading to the well-controlled synthesis of complex macromolecules. Characteristic optoelectronic properties, assembled structures, and three-dimensional architectures are briefly introduced.


Introduction
Polymers with carbazole groups are of considerable scientific and industrial interest because of their attractive features, such as their hole-transporting, high charge-carrier, and electroluminescent properties [1].The hole-transporting ability of carbazole-containing polymers makes them especially useful for applications in organic electronics.Numerous studies have been devoted to carbazole-containing polymers as a result of the success of poly(N-vinylcarbazole), poly(NVC), in electrophotographic applications [1].Recent developments in this field are mostly connected to applications in polymeric light-emitting diodes, organic photorefractive materials, and photovoltaic devices.For example, conjugated poly (3,6-carbazole) and poly(2,7-carbazole) derivatives having carbazole moieties in the main chain have been employed for applications in solar cells and white-lightemitting diodes [2][3][4].Carbazole-containing polymers can be divided into two general groups: polymers containing carbazole moieties in the main chain, or those containing carbazole moieties in the side chains.Poly(NVC) belongs to the group of polymers having carbazolyl groups in the side chains.A variety of polymers with pendant carbazolyl groups has been reported, including polyacrylate [5,6], polymethacrylate [7][8][9][10], and polystyrene [11,12] derivatives.Many factors play crucial roles in the manipulation of their properties and practical applications, such as their chemical structures, polymer architectures, conformation, location, and stacking of carbazole units.However, it was difficult to control their molecular weights and architectures, because most of these polymers with pendant carbazolyl groups have been traditionally synthesized by conventional radical polymerization.To manipulate their unique electronic and photonic functions, it is desirable to establish precise synthetic methods of controlling molecular weight, polydispersity, topology, composition, and functions.This can be achieved by using controlled polymerization systems.
In the past few decades, considerable attention has been given to the self-assembly of block copolymers, because of the feasibility of using them to generate nanostructured materials and their numerous potential applications in separation technology, controlled drug delivery and release, and smart catalyst separation technology [13][14][15][16][17]. Traditional amphiphilic block copolymers containing chemically connected hydrophilic and hydrophobic segments provide a great variety of morphologies in selective solvents, for example, water, in which micellar aggregates occur as a result of the association of the insoluble blocks.Block copolymers incorporating semiconducting segments and characteristic optoelectronic functions have attracted significant research interest [18][19][20][21][22][23].Block copolymers are emerging as a promising class of materials for optoelectronic applications, such as organic light-emitting devices, photovoltaics, and organic field-effect transistors, because of their ability to form a variety of ordered structures via self-assembly processes.Depending on the chemical nature of the functional segments and their composition, block copolymers provide a great opportunity to tune their chemical, physical, and optoelectronic properties and assembled structures.
Controlled radical polymerization (aka controlled/"living" radical polymerization or reversible deactivation radical polymerization) combines the benefits of the robust nature of conventional radical polymerization with the capability to prepare well-defined macromolecular architectures common to living polymerization techniques.This method has facilitated the synthesis of various functional polymers with predetermined molecular weights, narrow molecular weight distribution, and controlled architectures, such as star and branched polymers, block and graft copolymers, using a facile approach.The synthesis of functional polymers with controlled molecular weights, low polydispersity, and complex architectures has been possible because of recent significant progress in controlled radical polymerization techniques, such as nitroxide-mediated polymerization (NMP) [24,25], atom transfer radical polymerization (ATRP) [26,27], reversible addition-fragmentation chain transfer (RAFT) polymerization [28][29][30][31][32][33][34][35][36][37][38][39][40][41], single electron transfer-living radical polymerization (SET-LRP) [42], organoheteroatom-mediated living radical polymerization [43], and organometallic-mediated radical polymerization [44].All these systems are based on establishing a rapid dynamic equilibration between a minute amount of growing free radicals and a large majority of dormant species and are more tolerant of functional groups and impurities.Such controlled radical polymerization methods have become key tools for polymer synthesis, especially for synthesizing complex polymers with well-defined structures.
This review highlights recent developments in the design and synthesis of novel complex polymers with carbazole moieties using controlled radical polymerization techniques of carbazole-containing monomers, as shown in Scheme 1.We mainly focus on three classes of carbazole-containing polymers, as follows: (i) self-assembled block copolymers including amphiphilic block copolymers, block copolymers composed of "more activated" and "less activated" monomers, rod-coil-type block copolymers, and block copolymers possessing a metal ligating functionality (ii) branched polymers, such as star block copolymers, miktoarm star copolymers, star polymers having a metallocore, and comb-shaped copolymers; (iii) hybrids obtained by controlled radical polymerization of carbazole-containing monomers.

Carbazole-Containing Block Copolymers
Block copolymers incorporating carbazole functionality have attracted significant research interest, mainly as a result of the feasibility of using them to create highly ordered structures and their potential applications in the optoelectronic fields.Depending on the ordered structures and stacking of the carbazole moiety, the three-dimensional hierarchical structures formed by self-organization of the carbazolecontaining block copolymers may lead to unique electronic and photonic functions.There has been much research for the synthesis of carbazole-containing block copolymers using controlled radical polymerization of NVC.Styrene and (meth)acrylate derivatives having carbazole moieties have also been used for the controlled synthesis of carbazolecontaining block copolymers.

Controlled Radical Polymerization of N-Vinylcarbazole.
Poly(NVC) was the first and most widely studied polymeric photoconductor.NVC, a strong basic monomer, undergoes facile cationic polymerization with almost all cationic initiators to give poly(NVC).The living cationic polymerization of NVC with hydrogen iodine has also been reported [45].
Free radical initiators, such as azo compounds and peroxides, readily polymerize NVC, and radical polymerization was used in manufacturing the polymer [1].Several attempts to synthesize poly(NVC) using controlled radical polymerization systems have been reported in the literature.For example, Fukuda et al. demonstrated that the control of NVC homopolymerization could not be achieved by a typical NMP, whereas the copolymerization with styrene under the same conditions proceeded in a living fashion [46].Baethge et al. also reported the controlled NMP of styrene and NVC, while they demonstrated that the synthesis of the homopolymer, poly(NVC), with controlled molecular weights and a narrow polydispersity, was still problematic [47,48].Nowakowska et al. reported that the synthesis of poly(sodium styrenesulfonate-b-NVC) by NMP was possible only when the polymerization of NVC was conducted in the presence of acetic anhydride as an accelerator [49].ATRP (C 60 Cl n /CuCl/2,2 -bipyridine) was also used for the synthesis of poly(NVC) having a narrow polydispersity (M w /M n = 1.33).However, the actual structure of the product was a star-like architecture with a C 60 core because of multiple chlorine atoms in one initiator molecule, C 60 Cl n , and there was no information on each poly(NVC) and its detachments from the surface of C 60 [50].They also demonstrated that the attempt to synthesize the block copolymer, poly(NVCb-styrene), using the ATRP system provided products with bimodal molecular weight distributions [51].These previous studies failed to provide convincing proof of the controlled character of the homopolymerization of NVC, such as a kinetic investigation, evaluation of the end groups, and a chain extension experiment.The difficulties in adapting controlled radical polymerization to NVC may also be based on the fact that the NVC propagating radical is relatively unstable and thus highly reactive, mainly because of the electron-donating carbazolyl pendant (Q = 0.26, e = −1.29)[52], leading to a tendency to undergo chain transfer and chain termination reactions.In other words, systems suitable for inducing a fast interconversion between the dormant and the reactive radical species having a high electron density at the radical center derived from NVC were difficult to find.However, recent advances in the field of controlled radical polymerization have resulted in successful controlled polymerization of NVC.Mori et al. recently synthesized poly(NVC) with predetermined molecular weights (M n in the range of 3000-48000) and low polydispersities (M w /M n in the range of 1.15-1.20)by xanthate-mediated controlled radical polymerization [53].The RAFT process is generally accomplished by performing a radical polymerization in the presence of a thiocarbonylthio compound, such as a dithioester, dithiocarbamate, trithiocarbonate, or xanthate, all of which act as reversible chain transfer agents (CTAs).When xanthates are employed, the terminology MADIX (macromolecular design via the interchange of xanthates) is frequently used to describe the process [54][55][56].In general, the controlled radical polymerization of the N-vinyl and O-vinyl monomers was considered difficult, since the generated radical species are highly reactive due to their nonconjugated nature and strong electron-donating pendant groups.However, dithiocarbonates (xanthates) were recently reported to be useful for controlling the radical polymerization of highly reactive O-vinyl and N-vinyl monomers, such as vinyl acetate [57][58][59][60], N-vinylpyrrolidone [61][62][63], Nvinylcarbazole (NVC) [53], N-vinylindole derivatives [64], N-vinylphthalimide [65][66][67], N-vinylnaphthalimide [66], and N-vinylimidazolium salts [68].
Both the MADIX and RAFT processes are based on the generally accepted reversible addition-fragmentation chain transfer mechanism between an active and a dormant species.To achieve control of the radical polymerization via the RAFT/MADIX process, a delicate balance of the forward and reverse rates of addition (k add and k −add ) and fragmentation (k β and k −β ), together with the rates of reinitiation (k i ) and propagation (k p ), is required.Because the NVC propagating radical is a poor homolytic leaving group, the fragmentation of the RAFT-adduct radical (intermediate radical) is thought to be very slow when dithioesters are used, resulting in insufficient control of the polymerization.In contrast, the xanthate-type CTA is useful for achieving controlled radical polymerization of NVC, because it increases electron density at the radical center, which may lead to the destabilization of the RAFT-adduct radicals relative to the normal dithioester-type CTA and an increase in the fragmentation rate.The electron-donating O-alkyl substituents may lead to stabilization of the thiocarbonyl product of fragmentation through their conjugation with the C=S double bond, as shown in Scheme 2. This lowers the rate of addition of the propagating radicals to the sulfur atom, and consequently the overall rate of chain transfer [30,57,[69][70][71]; whereas the introduction of electron-withdrawing groups in the Z moiety leads to an increase in the rate of addition [70].
The blocking order is crucial for the synthesis of welldefined block copolymers by the RAFT process.The first dithioester-terminated polymer, S=C(Z)S-A, in which the A block corresponds to the first polymer and Z is the stabilizing International Journal of Polymer Science group, should have a high transfer constant in the subsequent polymerization of the second monomers to give the B block [72,73].The rapid conversion of macro-CTA to a block copolymer is also required to achieve a block copolymer with low polydispersity, which allows all the second blocks to be initiated at approximately the same time [74,75].The xanthate-mediated controlled radical polymerization of NVC was also applied for the synthesis of welldefined block copolymers involving a poly(NVC) segment.For example, a well-defined block copolymer involving a poly(lithium vinyl sulfonate) segment was synthesized by RAFT polymerization of a vinyl sulfonate ester, neopentyl ethenesulfonate, using xanthate-terminated poly(NVC) macro-CTA, followed by deprotection (Scheme 3) [76].The resulting product can be regarded as an amphiphilic block copolymer, comprising poly(lithium vinyl sulfonate) as a strong anionic polyelectrolyte and poly(NVC) as a hydrophobic segment.Amphiphilic block copolymers, poly(NVC)-b-poly(N-vinylpyrrolidone), were prepared by xanthate-mediated RAFT polymerization (Scheme 4) [77].The amphiphilic block copolymers dissolved in several organic solvents; however, depending on their composition, formed either micelles or large aggregates in methanol.The presence of globular aggregates was confirmed by tapping mode atomic force microscopy.The xanthatemediated RAFT polymerization of NVC was applied for the synthesis of three types of block copolymers, poly(ethylene glycol)-b-poly(NVC), poly(NVC)-b-poly(vinyl acetate), and poly(NVC)-b-poly(vinyl alcohol), as shown in Scheme 5 [78].
Well-defined pH-and thermomultiresponsive fluorescent micelles based on the self-assembly of diblock copolymers, poly(N-isopropylacrylamide-co-NVC)-b-poly[2-(dimethylamino)ethyl acrylate], were obtained by RAFT copolymerization of N-isopropylacrylamide and NVC followed by chain extension in the presence of 2-(dimethylamino)ethyl acrylate [79].The micelles were formed in aqueous solutions in a wide range of temperatures, and their sizes increased from 40 to 65 nm when the pH was varied from basic to acidic.Cross-linking of the poly[2-(dimethylamino)ethyl acrylate)]-containing shell with 1,2-bis(2-iodoethoxy)ethane results in spherical soft nanoparticles.The presence of NVC in concentrations as low as 4% in the core of the micelles was reported to allow the nanoparticles to be tagged by fluorescence, making them well suited to therapeutic applications.
The synthesis of poly(NVC)-based block copolymers functionalized with rhenium diimine complexes or pendant terpyridine ligands was reported [80].The copolymers were synthesized by RAFT polymerization, and they exhibited interesting morphological properties as a result of the phase separation between different blocks.It was demonstrated that the rhenium complex polymer block could function as a photosensitizer, while the terpyridine-containing polymer block can be used as a template for nanofabrication by selective deposition of zinc complexes.
Generally, the selection of the RAFT agent is dictated by the types of monomer being polymerized.The RAFT agents (dithioesters, trithiocarbonates) suitable for controlling polymerization of "more activated" monomers (e.g., styrene, acrylates, and methacrylates) tend to inhibit polymerization of "less activated" monomers (e.g., NVC, vinyl acetate, and N-vinylpyrrolidone).Similarly, RAFT agents suitable for polymerizations of "less activated" monomers tend to give little or poor control over polymerizations of "more activated" monomers.Hence, the synthesis of block copolymers composed of "less activated" and "more activated" monomers was difficult.Recently, switchable RAFT polymerization using N-(4-pyridinyl)-N-methyldithiocarbamates was found to provide excellent control over polymerization of "less activated" monomers and, after addition of 1 equivelant of a protic or Lewis acid, become effective in controlling polymerization of "more activated" monomers [81].Switchable RAFT polymerization allowed the synthesis of poly(methyl acrylate)-b-poly(NVC) with narrow molecular weight distributions.The preparation of unimodal low dispersity block copolymers of poly(N, N-dimethylacrylamide) with NVC was also conducted by switchable RAFT polymerization [82].The controlled radical polymerizations of "less activated" monomers, NVC, vinyl acetate, and N-vinylpyrrolidone, were successfully achieved in the presence of a disulfide, isopropylxanthic disulfide, using AIBN as the initiator [83].The use of RAFT polymerization for the synthesis of optoelectronic polymers was recently reviewed by Moad et al [22].The polymerization proceeded via MADIX process, where xanthate was formed in situ from the reaction of AIBN and isopropylxanthic disulfide.Organoheteroatom-mediated living radical polymerization using organotellurium, organostibine, and organobismuthine chain transfer agents were also useful methods for achieving the controlled character of polymerization of conjugated and nonconjugated monomers [43].
A rod-coil block copolymer consisting of poly(3-hexylthiophene) and poly(NVC) was synthesized by RAFT polymerization of NVC using trichiocarbonate-teminated poly(3-hexylthiophene) as a macro-CTA (Scheme 6) [84].The rod-coil block copolymer was employed for a white organic electroluminescence device.It is possible to suppress energy transfer from poly(NVC) as wide bandgap units to poly(3-hexylthiophene) as low bandgap blocks by obtaining the phase-separated domains from thin films of the resulting block copolymer, yielding dual emissions for white electroluminescence with a CIE coordination of (0.34, 0.33).The homopolymerization of NVC was performed with ATRP with Cu(I)/Cu(II)/2,2 -bipyridine as the catalyst system at 90 • C in toluene [85].N-2-Bromoethyl carbazole was used as the initiator, and the optimized ratio of Cu(I) to Cu(II) was found to be 1/0.3.The resulting poly(NVC) was used as a macroinitiator for the ATRP of methyl methacrylate, and this resulted in the formation of a block copolymer.The order of blocking is considered to be also crucial for ATRP or NMP as RAFT process.The polymerization of NVC and 3-(9H-carbazole-9-yl)propyl methacrylate (CzPMA) was carried out using various controlled radical polymerization methods, such as ATRP, single electron transfer-living radical polymerization (SET-LRP), and single electron transfer initiation followed by RAFT (SET-RAFT) [86].In the case of NVC, well-controlled polymerization with narrow molecular weight distribution was achieved by high-temperature ATRP, while SET-RAFT polymerization was relatively slow and controlled at ambient temperature.In the case of CzPMA, SET-RAFT was more suitable for ambient temperature polymerization.The synthesis of diblock copolymers was achieved from poly(NVC) and poly(CzPMA) macroinitiators using a flavanone-based methacrylate as the second monomer.The emission spectra of these block copolymers showed higher intensities compared with homopolymers.
N-ethyl-3-vinylcarbazole (E3VC) was also employed as a carbazole-containing monomer, which can be regarded as a styrene derivative.The controlled RAFT polymerization of E3VC was performed using benzyl 1-pyrrolecarbodithioate as a suitable CTA to afford well-defined poly(E3VC) in which the carbazole unit is directly bound to the polymer main chain [91].Well-defined block copolymers with poly(E3VC) segments have been synthesized using dithiocarbamateterminated polystyrene as a macro-CTA (Scheme 7(a)).Amino-acid-containing amphiphilic block copolymers composed of poly(E3VC) as a hydrophobic segment and poly(Nacryloyl-L-proline methyl ester) [92,93] as a hydrophilic segment were prepared by RAFT polymerization (Scheme 7(b)) [94].Formation of micelles consisting of a hydrophobic core of poly(E3VC) and a hydrophilic shell of poly(N-acryloyl-L-proline methyl ester) was observed in methanol, which is a selective solvent for the poly(N-acryloyl-L-proline methyl ester) segment.The chiroptical properties were affected by the assembled structure induced by the selective solvent and were evaluated by fluorescence and circular dichroism spectra.
N-Ethyl-2-vinylcarbazole (E2VC) is a monomer structurally similar to the styrenes, which are known to polymerize well by NMP [95].Polymerization of E2VC from a unimolecular alkoxyamine was reported to show a molecular weight that increases steadily with time, whereas termination or decomposition of the "living" radical ends was clearly evident.Nevertheless, a sufficient fraction of the chains remained living that poly(E2VC)-b-polystyrene could be formed by chain extension of the poly(E2VC) macroinitiator with styrene.

Controlled Radical Polymerization of (Meth)Acrylate
Derivatives Having Carbazole Moieties.Photorefractive active block copolymers were successfully synthesized via ATRP of acrylate containing liquid crystalline moieties and hole transporting carbazole moieties [96].The photorefractive characteristics of the copolymers indicated a higher diffraction efficiency and larger coupling gain for the block copolymer, compared to the corresponding statistical random copolymer.The controlled radical polymerization of 2-(N-carbazolyl)ethyl methacrylate (CzEMA) and 4-(5-(4tert-butylphenyl-1,3,4-oxadiazol-2-yl)phenyl) methacrylate via the RAFT process has been studied for the synthesis of functional polymers with hole-or electron-transfer ability [97].RAFT polymerization of these monomers using cumyl dithiobenzoate as a suitable CTA and AIBN as an initiator afforded well-defined polymethacrylates having hole-and electron-transporting groups.Monomers containing a chromophore group, that is, 9-anthrylmethyl methacrylate and CzEMA, were copolymerized with methyl methacrylate using ATRP, resulting in two series of copolymers with various amounts of included chromophore units [98].Luminescence comparison demonstrated more intense light emission by polymethacrylates with carbazolyl groups than by copolymers with anthryl groups.The wavelength of emitted light also differed in the range of blue-violet and blue-green fluorescence for carbazolyl and anthryl copolymers, respectively.A methacrylate-functionalized phosphorescent Ir(III)complex was copolymerized with a methacrylate-functionalized carbazole-derivative using ATRP (Scheme 8), with the aim of applying this type of host-guest-system to organic light-emitting devices [99].In the system, the emission originated almost exclusively from the complex, suggesting efficient intrachain energy transfer.

Star Polymer/Star Block Copolymer/Miktoarm Star Copolymer.
There are two basic routes for synthesizing star polymers [100]: the "core first" method (polymerization from multifunctional initiators or microgels), and the "arm first" method, in which growing polymer chain ends are reacted with a multifunctional terminating agent or a divinyl compound.The use of multifunctional initiators or terminators leads to stars with a well-known (though often, low) number of arms; while the use of microgels or divinyl compounds leads to a rather broad arm number distribution, with a high average arm number.The design and synthesis of novel star copolymers having characteristic architectures, such as star block copolymers and heteroarm or miktoarm (mixed) star polymers, are attracting significant attention because of their interesting structures and properties [17,[101][102][103][104][105][106].Amphiphilic star block copolymers and miktoarm star polymers can form a variety of superstructures as a result of self-organization, and the resulting assembled structures should be governed by the branched architecture, as well as the chemical nature of the components, their composition, and molecular weight.
The xanthate-mediated RAFT polymerization of NVC was also applied for the synthesis of star polymers [107].RAFT polymerization was employed for the purpose, using three different xanthate-type tetrafunctional CTAs: two Z-designed CTAs having different R (leaving) groups in which the Z (stabilizing) groups are linked to the core, and one R-designed CTA.The R-group approach was found to be the most efficient for the controlled synthesis of four-arm poly(NVC) stars having low polydispersities and controlled molecular weights.Amphiphilic star block copolymers were synthesized by the polymerization of NVC using the poly(acrylic acid) star, which was prepared from the R-designed tetrafunctional CTA (Scheme 9) [108].DLS measurements of the amphiphilic stars in THF solution, which is a good solvent for both components, suggested the existence of unimolecular star micelles.In contrast, the formations of the micelles and inverse micelles were observed in water, which is a good solvent only for the poly(acrylic acid) segment, and in CHCl 3 , which is a good solvent only for poly(NVC) segment.The absorbance and fluorescence spectra indicated that specific conformations of amphiphilic stars, like micelles and inverse micelles, affected the characteristic optoelectronic properties.
Star polymers containing chemically different arms are designated heteroarm or miktoarm (mixed arm) star polymers when they comprise two or more different kinds of arms, respectively.Their unique architectures cause them to reveal interesting properties in the solid state and in solution [109].A novel amphiphilic A 3 B miktoarm star copolymer, poly(N-isopropylacrylamide) 3 -poly(NVC), was successfully synthesized by a combination of singleelectron transfer living radical polymerization and RAFT polymerization (Scheme 10) [110].First, the well-defined three-armed poly(N-isopropylacrylamide) was prepared via SET-LRP of N-isopropylacrylamide using a tetrafunctional bromoxanthate iniferter (Xanthate-Br 3 ) as the initiator and Cu(0)/PMDETA as a catalyst system.Secondly, the target amphiphilic A 3 B miktoarm star copolymer was prepared via RAFT polymerization of NVC employing three-armed poly(N-isopropylacrylamide) having a xanthate moiety as the macro-CTA.The amphiphilic A 3 B miktoarm star copolymer showed an increase in the fluorescence intensity of micelle with temperature and had good temperature reversibility.
Styrene-type monomer, VBCz, and methacrylate-type monomer, 2-(9H-carbazole-9-yl)ethyl methacrylate(CzEMA), were polymerized to star polymers via ATRP using zinc 5,10,15,20-tetrakis(4-(2-methyl-2-bromopropoxy) phenyl) porphyrin as an initiator (Scheme 11) [111].The emission spectra of the two star polymers, poly(VBCz) and poly (CzEMA) stars, displayed red light emission in the solid state, while those of the two monomers showed blue light emission, suggesting the effective energy transfer from the carbazole to the Zn porphyrin core.It was demonstrated that the flexible arms of star polymers can effectively hinder π-π interaction of the porphyrin cores and present aggregation, which would lead to fluorescent self-quenching in the solid state.
A derivative Ru(II) complex with radical initiating sites was employed in the ATRP of functional N-(ωalkylcarbazoly) methacrylates to provide linear metallopolymers with the metal chromophores at one terminus of the polymer chain [112].Similar Ru(II) complexes with one and three initiating sites were also employed as metalloinitiators for ATRP of N-(ω -alkylcarbazoly) methacrylates with NiBr 2 (PPh 3 ) 2 as a catalyst (Scheme 12) [113].Starlike three-armed polymers could be obtained from the metalloinitiators with three initiating sites.

Comb-Shaped Copolymers.
There have been very few reports of the synthesis of comb-shaped copolymers with carbazole-containing segments.These comb-shaped copolymers can be regarded as cylindrical polymer brushes having the same number of side chains as the degree of polymerization of the main chain.These cylindrical polymer brushes with carbazole-containing segments are architecturally interesting for both experimental and theoretical chemists because of the possibility of forming extended chain conformations, based on the intramolecular excluded-volume interactions between side chains densely grafted to the backbone.The homopolymerization of macromonomers, "grafting onto" and "grafting from" can be used in much the same way as conventional graft copolymers and combshaped copolymers.The "grafting from" method was mainly employed for the synthesis of comb-shaped copolymers with carbazole-containing segments.
The comb-shaped copolymer having poly(VBCz) side chains was prepared via NMP of VBCz using a macro-TEMPO agent, which was synthesized by anion ringopening polymerization of 4-glycidyloxy-2,2,6,6-tetrameth-ylpiperidine1oxyl (Scheme 13) [114].The fluorescence, ultraviolet intensities, and cyclic voltammetries of the combshaped copolymers with different molecular weights showed a regular order.It was reported that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels accompanied by bandgaps were adjustable via molecular weights of the polymers, which should be favorable to the application of these polymers in electronic devices.
A new light-emitting and hole transporting combshaped copolymer was prepared via ATRP of an acrylate monomer, 3-(N-carbazolyl)propyl acrylate (CzPA), initiated by a macroinitiator with ATRP initiating sites distributed along a polythiophene backbone (Scheme 14).This polymer can be regarded as a rod-coil-type comb-shaped copolymer, in which the polythiophene rod is located in the backbone and the poly(CzPA) coil is located in the side chain.The energy transfer process from the poly(CzPA) side chains to the polythiophene backbone was evidenced in both the solution and film states.Electroluminescence devices fabricated from the copolymer demonstrated that the copolymer could act as both a light emissive and hole-transporting material [115].

Carbazole-Containing Hybrids
Carbazole-containing polymer chains attached to planar and spherical surfaces have recently attracted much interest as candidates for various optoelectronic industrial applications.Electroactive substrates, such as carbon nanotubes, fullerene, graphene, and quantum dots, were mainly employed.The so-called "grafting onto," and "grafting from" methods can be used for the synthesis of carbazole-containing hybrids.In the "grafting from" process, the side chains of the brush are formed via controlled radical polymerizations involving ATRP and NMP initiated by the pendant initiating groups on the surface.When RAFT polymerization was employed, the CTA moiety was attached to the surface.Well-defined polymer brushes with high grafting density and rather narrow distributions can be obtained using this method.The "grafting onto" technique was frequently used for the synthesis of the hybrids, in which the carbazole-containing polymers were attached to the substrates by adsorption or chemical reaction of end-functionalized polymers or block copolymers onto the surfaces.
The poly(NVC)-grafted multiwalled carbon nanotubes hybrid materials were synthesized by RAFT polymerization of NVC in the presence of a trithiocarbonate-type CTA functionalized covalently onto multiwalled carbon nanotubes (Scheme 15) [116].Incorporation of poly(NVC) onto the surfaces of the multiwalled carbon nanotubes can considerably improve their solubility and processability.Hybrid materials that are suitable candidates for viable optical limiting devices exhibit significant nonlinear optical responses.Poly(NVC) chemically modified graphene oxide was obtained by RAFT polymerization of a trithiocarbonate type-CTA functionalized on graphene oxide [117,118].The resulting hybrid material showed good solubility in organic solvents and a significant energy bandgap in the vicinity of 2.49 eV [118].The poly(NVC)-C 60 composites were synthesized by ATRP (C 60 Cl n /CuCl/2,2 -bipyridine) of NVC [50,119,120].The photoconducting properties of poly(NVC)-C 60 composites were also investigated.
The CdSe-polymer composite was prepared via ATRP of NVC on functionalized CdSe quantum dots (Scheme 16) [121,122].It was revealed that grafting poly(NVC) onto the surface of CdSe nanocrystals would reduce the bandgap of poly(NVC) and cause a red shift in the emission peak.The nanocrystal-polymer nanocomposite was employed as the electron acceptor in polymer bulk heterojunction solar cells.Ultrasound-assisted bulk synthesis of CdS-poly(NVC) nanocomposites was reported via RAFT polymerization [123].Poly(methyl methacrylate)/zinc oxide (ZnO) or carbazole-containing polymers, poly(CzEMA)/ZnO nanocomposites, which are composed of high molecular weight polymers with narrow molecular weight distributions and ZnO quantum dots, were prepared by ATRP [124].2-Bromo-2-methylpropionyl group was introduced onto the ZnO nanoparticle surfaces, which was employed for the surface-initiated ATRP.
The styrene derivative having a carbazole moiety (VBCz) was employed for the synthesis of bifunctional nanoparticles with fluorescence and magnetism by surfaceinitiated iron(III)-mediated ATRP with activators generated by electron transfer (AGET ATRP) [125].VBCz was grafted from magnetic nanoparticles (ferroferric oxide) via AGET ATRP, using FeCl 3 * 6H 2 O as the catalyst, tris(3,6dioxaheptyl)amine as the ligand, and ascorbic acid as the reducing agent.Another interesting system involves the synthesis of multistimuli-response hybrid nanoparticles with magnetic cores and thermoresponsive fluorescencelabeled shells by surface-initiated RAFT polymerization using carbazole-containing CTA-functionalized magnetic silica nanoparticles [126].
Polymer brushes refer to an assembly of polymer chains, which are tethered by one end to a surface or an interface.Surface-initiated polymerization has the advantage of allowing easy modification of surface properties by varying the composition of the polymer brush, grafting density, and the degree of polymerization.Electrochemically crosslinked surface-grafted poly(NVC) brushes have been demonstrated as hole transport layers on a photovoltaic device using surface-initiated polymerization [127,128].The pendant carbazole group made the poly(NVC) an electrochemically cross-linkable precursor, capable of forming conjugated polymer network films.The covalent linkage of the poly(NVC) brush allowed for direct electroluminescent device preparation on modified ITO, giving the advantage of strong adhesion to ITO with possible long-term stability against acid dopants and oxygen.Surface-initiated ATRP of 9-(2-(4-vinyl(benzyloxy)ethyl)-9H-carbazole (VBOCz) was also employed for the synthesis of carbazole-based polymer brushes on silicon surfaces [129].Conductance switching at a voltage of about 2.1 V was observed in the memory device based on the brushes.

Conclusion
This review has summarized the approaches to synthesis of a variety of tailor-made polymers containing carbazole functionality.The combination of different polymerization mechanisms, self-organization of block copolymers, and surface-initiated polymerization have allowed great advances in the design and synthesis of various nano-and mesoscale polymeric materials with sophisticated structures, in addition to recent progress in controlled radical polymerization techniques.The synthetic methodologies for novel architectures, characteristic bulk, and solution properties related to the self-organization process, and a wide range of applications of block copolymers containing poly(NVC) segments have been extensively investigated in the last decade.Controlled radical polymerization of NVC, styrene, and (meth)acrylate derivatives having the carbazole moiety were well advanced, leading to the well-controlled synthesis of complex macromolecules, such as star polymers, star block copolymers, and polymer brushes with the carbazole functionality.The carbazole-containing complex macromolecules can be combined with a broad range of functional groups, such as fluorophores, electroactive groups, dyes, and other biorelated materials.These carbazole-containing polymers with well-defined architectures can provide viable tailored materials with unique electronic and photonic properties for a wide range of applications, including polymeric light-emitting materials, and organic photorefractive materials.

Scheme 1 :
Scheme 1: Representative examples of carbazole-containing vinyl monomers used for controlled radical polymerizations.

Scheme 7 :
Scheme 7: Synthesis of block copolymers by RAFT polymerization of E3VC.

Scheme 9 :
Scheme 9: Synthesis and assembled structures of amphiphilic star block copolymers.

Scheme 11 :
Scheme 11: Synthesis of star polymers via ATRP using zinc porphyrin as a tetrafunctional initiator.