reversible-deactivation radical polymerization in the presence of metallic copper. activation of...

PolymerChemistry

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Reversible deact

DSodTSpwXZSsm

main research interests include Curonmentally friendly catalysts in R

aJiangsu Key Laboratory of Advanced Func

Department of Polymer Science and Engin

Engineering and Materials Science, Sooch

Suzhou 215123, China. E-mail: zhangzhen

Fax: +86-512-65112796bSchool of Chemistry and Materials Science,

Cite this: Polym. Chem., 2014, 5, 3533

Received 7th October 2013Accepted 8th January 2014

DOI: 10.1039/c3py01398g

www.rsc.org/polymers

This journal is © The Royal Society of C

ivation radical polymerization inthe presence of zero-valent metals: fromcomponents to precise polymerization

Wenxiang Wang,ab Junfei Zhao,a Nianchen Zhou,a Jian Zhu,a Wei Zhang,a

Xiangqiang Pan,a Zhengbiao Zhang*a and Xiulin Zhu*a

Typically, reversible deactivation radical polymerization (RDRP) in the presence of a zero-valent metal

involves a monomer, initiator, zero-valent metal, ligand and solvent. RDRP in the presence of a zero-

valent metal demonstrates many advantages, including well-controlled behavior, low reaction

temperatures, good retention of chain-end functionality, and the ready recyclability of the metal. The

development of zero-valent metal-mediated RDRP has had a profound impact on precise polymer

synthesis due to its preparative “green” characteristics, while still allowing excellent control over

molecular weights and molecular weight distributions. Herein, we highlight recent work from the advent

of zero-valent metal-mediated RDRP looking at advances in its components and the synthesis of well-

defined polymers.

r Wenxiang Wang was born inhandong, China, in 1986, andbtained his B. Eng. from Qing-ao University of Science &echnology in June 2008. Sinceeptember 2008, he has beenerforming his PhD researchorking on RDRP with Professoriulin Zhu and Zhengbiaohang at Soochow University inuzhou, China. In June 2013,he got her PhD degree of Poly-er Chemistry and Physics. Her(0)-mediated RDRPs and envi-DRPs.

tional Polymer Design and Application,

eering, College of Chemistry, Chemical

ow University, Suzhou Industrial Park,

[email protected]; [email protected];

Ludong University, Yantai 264025, China

hemistry 2014

1. Introduction

Over the past 30 years, controlled/“living” radical polymeriza-tion (CRP) (denoted as reversible-deactivation radical polymer-ization, RDRP by IUPAC)1 has become the most important androbust toolbox for polymer chemistry and macromolecularsynthesis, combining the advantages of traditional free radical

Prof. Zhengbiao Zhang receivedhis PhD under the direction ofProf. Xiulin Zhu at SoochowUniversity (Suzhou, Jiangsu,China) on the initiation systemof RAFT polymerization andthe mechanistic aspects ofcontrolled radical polymeriza-tions in 2007. He joinedProfessor E. T. Kang at theNational University of Singapore(Singapore) as a ResearchFellow on surface modication/

functionalization of organic/inorganic substrates for diverse bio-related applications in 2007–2008. He was appointed as a VisitingScholar with Prof. Stephen Z. D. Cheng at The University of Akron(USA) in 2012–2013. His current research interests include themethodological study of controlled radical polymerization, thedesign and synthesis of macromolecules with well-regulatedmolecular weight, stereospecicity, monomer sequence andarchitecture.

Polym. Chem., 2014, 5, 3533–3546 | 3533

http://dx.doi.org/10.1039/c3py01398g

http://pubs.rsc.org/en/journals/journal/PY

http://pubs.rsc.org/en/journals/journal/PY?issueid=PY005011

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polymerization and living ionic polymerization, i.e., greatermonomer diversity and less stringent reaction conditions.2

Since the discovery of RDRP by Otsu in 1982,3 many powerfulRDRP techniques have been developed, including, but notlimited to: nitroxide-mediated polymerization (NMP),4 revers-ible addition–fragmentation transfer (RAFT) polymerization,5

atom transfer radical polymerization (ATRP)6,7 and zero-valentmetal-mediated RDRP.8–10 In all RDRP techniques, an effectivereversible deactivation between the propagating radicals anddormant species is necessary to ensure a low radical concen-tration, thereby largely reducing bimolecular termination andother side reactions.11,12 Based on these well developed conceptsover the past 30 years, RDRP can enable not only the productionof polymers with precisely controlled molecular weight (MW)and narrow molecular weight distribution (Mw/Mn), but alsoprepare every aspect of the macromolecular architecture, i.e.,the composition, topology and functionality.

Metal-mediated RDRP, like ATRP, is a robust and versatilesynthetic technique for the preparation of polymers with well-dened architecture and site-specic functionality.13,14 Thepolymeric chain growth of ATRP, as a typical metal-mediatedRDRP (a transition metal-catalyzed living radical polymeriza-tion),7,15 is mediated by a transition-metal complex (such asCu,14 Ru,7 Fe,16 etc.17). As shown in Scheme 1, a low radicalconcentration, which facilitates the minimization of termina-tion reaction, is maintained by the dynamic equilibriumbetween the radical species (Pnc) and the dormant species (Pn–X)as well as between the lower oxidation state (Mt

n–X/L) and thehigher oxidation state (X–Mt

n+1–X/L) metal.18 For metal-medi-ated RDRP, zero-valent metal-mediated RDRP, which uses

Scheme 1 The typical ATRP mechanism.

Prof. Xiulin Zhu received his PhDunder the direction of Prof. ZurenPan at Zhejiang University (Zhe-jiang province, China) on poly-merization engineering in 1987.He worked in Soochow University(Jiangsu province, China) as alecturer in 1988, AssociateProfessor in 1990 and Professorin 1994. He was appointed as theVice President of SoochowUniversity in 1993 and the Pres-ident of Soochow University in

2006 (until now). His current research interests include the meth-odology of polymer synthesis/reactions and the design/synthesis ofprecisely-dened macromolecules with diverse architectures andfunctions.

3534 | Polym. Chem., 2014, 5, 3533–3546

abundantly available and readily reusable zero-valent metals ascatalysts, is the preferential choice for the synthesis of well-dened polymers.

In the 1970s, many polymer chemists reported radical poly-merizations in the presence of activated metal powders (e.g.,Ni(0), Co(0), Cu(0), Fe(0), etc.), but the ability to control thesereactions was unknown.18–20 In 1990, Otsu et al. also obtained anill-dened poly(styrene) (PSt) with a very high Mw/Mn, initiatedby reduced Ni(0)/benzyl halide mixtures.21 Since the emergenceof atom transfer radical addition in organic synthesis,22–25 anumber of zero valent metals such as Cu(0),26,27 and Fe(0)28–30

were found to be particularly active in ATRA processes. Aer theinvention of ATRP, which was inspired by the ATRA reaction,the utilization of Cu(0) in conjunction with a suitable ligand toscavenge excess metal halide was reported for the rst time byMatyjaszewski et al. in 1997.8 This work reported an ATRP with asignicant reduction of the amount of catalyst required tomaintain reasonable polymerization rates.31 This reduction inthe soluble catalyst concentration was a signicant advance-ment in metal-mediated RDRP, affording simplied posttreatment and many other desirable benets. When a smallamount of Cu(0) powder was added to ATRP of styrene (St) and avariety of (methyl)acrylates, a dramatic rate increase wasobserved. Looking toward practical applications and potentiallarge-scale production, ATRP has witnessed signicant devel-opment and can now be successfully conducted with very lowamounts of catalyst, for example, methods such as initiators forcontinuous activator regeneration (ICAR) ATRP,32 activatorsregenerated by electron transfer (ARGET) ATRP,33,34 and elec-trochemically mediated atom transfer radical polymerization(eATRP).35 In ARGET ATRP (Scheme 2),33,34,36,40,41 an excess ofreducing agent, which does not form molecules capable ofinitiating ATRP, is used. Zero-valent metals, which can reactwith alkyl halides and initiate polymerization, can play the dualrole of a supplemental activator and reducing agent. Activatorsgenerated by electron transfer (AGET) ATRP has also37,38 beenproposed by Matyjaszewski et al., in which a reducing agent is

Scheme 2 The mechanism of ARGET ATRP with a zero-valent metalas both a supplemental activator and reducing agent. (Reprinted withpermission from Y. Zhang, Y. Wang and K. Matyjaszewski, Macromol-ecules, 2011, 44, 683–685. Copyright 2011, American ChemicalSociety).37

This journal is © The Royal Society of Chemistry 2014


Scheme 4 The mechanism of SET-LRP proposed by Percec et al.(Reprinted with permission from V. Percec, T. Guliashvili, J. S. Ladislaw,A. Wistrand, A. Stjerndahl, M. J. Sienkowska, M. J. Monteiro andS. Sahoo, J. Am. Chem. Soc., 2006, 128, 14156–14165. Copyright2006, American Chemical Society).9

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used to reduce the higher oxidation state transition metalcomplex (Scheme 3).39,40 The addition of Cu(0) to the self-condensing vinyl polymerization of novel (meth)acrylic mono-mers using ATRP catalyst (Cu(II)) systems has also enabledsuccessful polymerization.41 Similar to the case of Cu(II)/Cu(0),Fe(III) halides have also been used in the presence of Fe(0) toprepare in situ Fe(II).31 The use of a zero-valent metal wire42,43 asa heterogeneous reducing agent facilitates the handling of thereducing agent and simplies the purication of the polymer.

In 2006, single-electron transfer living radical polymerization(SET-LRP) proposed by Percec et al. demonstrated good controland relatively fast polymerization by using Cu(0) and tris-(2-(dimethylamino)ethyl)amine (Me6TREN) in dimethyl sulfoxide(DMSO).9 Percec et al. (Scheme 4) claimed that, in SET-LRP, theCu(0) species acts as an electron donor, while the initiator/dormant propagating species acts as an electron acceptor. Cu(0)donates an electron to Pn/P–X, resulting in a radical-anion. Pnc/Pcis then generated from the heterolytic cleavage of the radicalanion intermediate [Pn/PX]

�c. The Cu(I) species generated duringthe formation of the radicals spontaneously disproportionatesinto extremely reactive nascent Cu(0) atoms and a Cu(II)X2/Lspecies, which mediate the initiation and the reversible termi-nation, respectively. Although many pieces of excellent work44–46

have been published based on SET-LRP, the mechanismproposed by Percec et al. disagreed with that for ARGET ATRP(Scheme 2) and a recently rened mechanism, named supple-mental activator and reducing agent atom transfer radical poly-merization (SARA ATRP) (Scheme 5).47 According to the SARAATRP mechanism, Cu(I) acts as the primary activator of alkylhalides with Cu(0) acting as a supplemental activator. An inner-sphere electron transfer occurring during the activation step wasproposed, while disproportionation is very slow and limited inDMSO, Cu(0) is a very slow activator and Cu(I) is the major acti-vator. Meanwhile, the slow activation of alkyl halides by Cu(0)and the comproportionation of Cu(II) with Cu(0) account for thesmall number of radicals lost to termination reactions in SARAATRP.47–51 As for clarication of the mechanism, there has been alarge body of mechanistic work conducted, focusing on thefollowing key questions: (a) whether or not the alkyl halide formsa radical anion intermediate during the activation process,9,52,53

(b) whether the role of Cu(0) is to act as a supplemental activatorof alkyl halides and a reducing agent38,49–51 or if Cu(0) is theexclusive activator of alkyl halides,9,54,55 and (c) whether Cu(I)/Lspecies participate in the activation of alkyl halides38,50,51 or if theyundergo instantaneous and complete disproportionation.9,56

Putting aside the mechanism, RDRP in the presence of Cu(0) or

Scheme 3 Mechanism of AGET ATRP.


other zero-valent metals has become an important RDRP tech-nique and has demonstrated many desirable advantages, such ashigh polymerization rates at low temperature for monomers withrelatively large kp and good controllability under appropriateconditions. This mini-review will give a brief overview of RDRP inthe presence of zero-valent metals covering three aspects thatinclude the components, advantages and precise polymersynthesis.

2. Components of zero-valent metal-mediated RDRP

When using a typical zero-valent metal as a catalyst in RDRP,the monomer, catalyst system, initiator and solvent are essen-tial. The following section will elaborate on the monomers,catalyst systems, initiators and solvents that have been used.

2.1 Monomer

Various monomers have been successfully polymerized usingzero-valent metals as catalysts in RDRP, including (meth)acry-lates, (meth)acrylamides, acrylonitrile, vinyl chloride, styrene,etc. The rapid progress and proliferation of RDRP has allowed a

Scheme 5 The mechanism of SARA ATRP proposed by Matyjaszewskiet al. (Reprinted with permission from D. Konkolewicz, Y. Wang,M. Zhong, P. Krys, A. A. Isse, A. Gennaro and K. Matyjaszewski,Macromolecules, 2013, 46, 8749–8772. Copyright 2013, AmericanChemical Society).47

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broad scope of vinyl monomers to be polymerized into well-dened polymers with controlledmolecular weights and narrowMWDs. In this section, a variety of monomers for RDRP usingzero-valent metal as a catalyst conducted at ambient tempera-ture are described.

Acrylates and acrylamides. Copper-based systems have beenthe most extensively studied and are effective for controlling theMWDs (Mw/Mn < 1.30) of poly(acrylate)s. Zero-valent metal-mediated RDRP has been used for the typical acrylate andacrylamide monomers, such as methyl acrylate (MA), ethylacrylate (EA), n-butyl acrylate (nBA), t-butyl acrylate (tBA), sol-ketal acrylate (SA) [(2,2-dimethyl-1,3-dioxolan-4-yl)methyl acry-late],57 2-methoxyethyl acrylate (MEA),58 poly(ethylene glycol)methyl ether acrylate (PEGMEA),59 2-ethylhexyl acrylate(2-EHA),60 and N-isopropylacrylamide (NIPAM).60,61 As anexample, a,u-di(bromo) PMA, PEA, and PBA with Mn rangingfrom 8500–35 000 g mol�1 have been synthesized using Cu(0)/tris[2-(dimethylamino)ethyl] amine (Me6TREN) at 25 �C inDMSO. These results accelerated strategies based on theTERMINI concept, the synthesis of block copolymers, tele-chelics, and other polymers with complex architecture.62 Theapplication of a copper(II) deactivator in the Cu(0)-mediatedRDRP of tBA has been achieved.63 A one-pot reduction/conju-gation reaction enables post polymerization modication withfunctionalised MA and acrylamides (AM).64 The polymerizationof N,N-dimethylacrylamide (DMAA), NIPAM in protic65–67 ordipolar aprotic solvents and the surface-initiated RDRP ofPNIPAM68 can also be achieved using Cu(0)-mediated RDRP at25 �C. Cu(0)-mediated RDRP of MA in the presence of a classical4-methoxyphenol free radical inhibitor has been investigated.69

The Cu(0)/Me6TREN-mediated RDRP of MA in hydrophobicmedia was enhanced using phenols.70 A series of functionalwater-soluble polymers were successfully synthesized via RDRPmediated by the prior disproportionation of CuBr/Me6TREN inpure water affording polymers with controlled chain lengthand narrow molecular weight distributions (Mw/Mn � 1.10),including PNIPAM, PDMAA, poly(ethylene glycol) acrylate(EGA), 2-hydroxyethyl acrylate (HEA), and an acrylamido glycomonomer.71 Moreover, Cu(0) wire-mediated RDRP of HEA canbe performed using methanol (MeOH) and DMSO as solventswith ethyl 2-bromoisobutyrate (EBiB) and Me6TREN as aninitiator and ligand, respectively.72

Methyl methacrylate. Cu(0)-mediated RDRP of MMA inDMSO has been performed.73–76 Self-regulated Cu(0)-catalyzedRDRP has been shown to be compatible with other methacrylatemonomers such as butyl methacrylate (BMA). Due to the lowerintrinsic kp of methacrylates, zero-valent metal-mediated RDRPof MMA is about 5–10 times slower than the zero-valent metal-mediated RDRP of MA under identical conditions. As shown inour research, the RDRP of MMA is also feasible using 2-cyano-prop-2-yl 1-dithionaphthalate (CPDN), a typical RAFT agent, asan initiator with Cu(0)/N,N,N0,N0 0,N0 0-pentamethyldiethylenetri-amine (PMDETA)76 or Fe(0) as the catalyst.77 Meanwhile, Cu(0)/PMDETA catalyzed RDRP of MMA initiated with EBiB has alsobeen reported with 1,1,1,3,3,3-hexauoro-2-propanol (HFIP),78

which provided both an enhanced rate of polymerization withgood molecular weight evolution, even at low temperatures

3536 | Polym. Chem., 2014, 5, 3533–3546

ranging from�18 to 0 �C, and improved syndiotacticity (z0.77)compared to the free radical polymerization. The active Cu(0)species formed by the disproportionation of the Cu(I) species inaqueous medium probably plays a vital role in the possible air-initiatied polymerization of isobornyl methacrylate (IBMA) viaCu(0)-mediated RDRP, however, there is poor initiator efficiencydue to the heterogeneous nature of the polymerizationmedium.79 Methacrylate monomers have also been successfullypolymerized by ATRP utilizing copper wire as an in situ reducingagent at near-ambient temperatures (35 �C) in anisole.80

4-Vinylpyridine. In the presence or absence of a ligand, well-controlled poly(4-vinylpyridine) (P4VP) has been successfullyobtained via Cu(0)-mediated RDRP using HFIP, a uoroalcohol,as a solvent, which can strongly interact with 4-vinylpyridine(4VP) via hydrogen bonding. Using equimolar amounts of 4VPand HFIP, and room temperature (25 �C) favored a betterhydrogen bonding interaction.81 The presence of Me6TREN canfacilitate faster polymerizations with more predictable number-average molecular weights (Mn,SEC) and narrower molecularweight distributions (Mw/Mn < 1.25) compared to those poly-merized without the hydrogen bonding interaction. Moreover,the hydrogen bonding had a profound impact on stereo-regulation during polymerization, which enabled the synthesisof highly syndiotactic (60.2%) P4VP with a higher glass transi-tion temperature (Tg).82 Furthermore, reversible-deactivationradical copolymerization of 4VP and St was also successfullyperformed in HFIP, exhibiting a better controlled behavior anda predominantly alternating monomer sequence. This well-regulated polymerization behavior as well as the monomersequence were ascribed to electron induction and steric repul-sion effects from the hydrogen bonding interaction between4VP and HFIP.83 Moreover, the alternating copolymer of 4VPand St obtained in HFIP showed a slightly higher glass transi-tion temperature with respect to that of the random copolymerobtained in 2-propanol.

Styrene. Generally speaking, higher temperatures are oenneeded to realize controlled polymerization due to the lowpropagation rate coefficient of St. In this regard, the controlledpolymerization of St is very slow via zero-valent metal catalyzedRDRP at ambient temperature. For example, Subramanian et al.reported a feasible Cu(0)/PMDETA catalyzed RDRP of St with1-bromoethyl benzene (1-PEBr), EBiB, or diethyl-2-bromo-2-methyl malonate (DEBMM) used as the initiator in DMSO at25 �C, however, a relatively broad polydispersity (Mw/Mn > 1.40)was obtained.84 They also claimed that combining Cu(0)-medi-ated RDRP and RAFT polymerization could improve the livingcharacter of the reaction. The Cu(0)/PMDETA catalyzed RDRP ofSt initiated with DEBMM or p-toluenesulfonyl chloride (TsCl)combined with (2-ethoxy carbonyl)prop-2-yl-pyrrole-1-carbodi-thioate (CTA) in DMSO at 25 �C provided PSt with a lowerpolydispersity (Mw/Mn z 1.20–1.26). We recently reported asuccessful Cu(0)/PMDETA-mediated RDRP of St at roomtemperature with methyl-2-bromopropionate (MBP) or EBiB asan initiator in DMF, but the polymerization rate was very low.85

The reversible-deactivation radical copolymerization of St andother monomers could be easily realized. We have explored theCu(0)/PMDETA mediated copolymerization of MMA and St in




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DMF at 25 �C.86 The Cu(0)/Me6TREN mediated copolymeriza-tion of 4VP and St was also successfully performed at 25 �C inour group, with HFIP used as a solvent.83 Perrier et al. havereported the controlled polymerization of St using Cu(0)/PMDETA as a catalyst at 90 �C in toluene.87 In our work, Fe(0)(either powder or wire) and elemental bromine (Br2) have beenused as the catalyst for RDRP of St without any additionalinitiator at 110 �C. The polymerization occurred with control atan appropriate molar ratio of Fe(0)/Br2.88

Acrylonitrile. Polyacrylonitrile (PAN) and its copolymers,such as that formed from St and AN (SAN), are commerciallyimportant polymers due to their versatility and good perfor-mance parameters including hardness and rigidity, chemicalresistance, compatibility with polar substances, and low gaspermeability. Well-dened PAN and SAN with predictablemolecular weights, structures, and a high number of chain-endfunctionalities would offer unique high performance polymers.Therefore, the synthesis of controlled PAN and SAN has greatsignicance for industrial applications. The feasibility of thehomo-polymerization of AN and copolymerization of St and ANusing RDRP were investigated. The preparation of well-denedPAN at room temperature was explored by our group using[EBiB]0/[Cu(0)]0/[bipyridine (bpy)]0 in DMSO.89 RDRP of ANinitiated by 2-bromopropionitrile (BPN) using Cu(0)/Me6TRENas a source of catalyst in DMSO have also been studied in detailby Li et al.90 Chen et al. reported that RDRP of AN can be cata-lyzed by Cu(0) powder with carbon tetrachloride (CCl4) as aninitiator and hexamethylenetetramine (HMT) as a ligand inN,N-dimethylformamide (DMF) or a mixed solvent.91 In addition,Zn(0)/CuBr2 (10–50 ppm) was used, for the rst time, to catalyzethe radical polymerization of AN at ambient temperature asreported by Liu et al.92

Vinyl chloride. RDRP of vinyl chloride (VC) has been anotable challenge to polymer chemistry, because of the rela-tively stable gem-dihalide end groups of the monomer, whichhave proven inert to most RDRP conditions.93 The developmentof a Cu(0)/tris(2-aminoethyl)amine (TREN)/CuBr2 catalyzedRDRP of VC initiated with CHBr3 in DMSO at 25 �C was reportedby Percec et al.9,94 The use of CuBr2 as an additive enabled therst RDRP of VC with a controlled molecular weight, low Cu(0)powder loading and improved Ieff with a targeted degree ofpolymerization (DP) of 1400. Without CuBr2, the RDRP of VC inDMSO could be optimized by increasing the amount ofTREN. Under the optimized conditions, a target DP of 350 wasachieved within 360 minutes, producing PVC with Mn,SEC ¼25 900 gmol�1 (conversion¼ 86%,Mw/Mn¼ 1.53). Additionally,it has been shown that Cu(0) wire can be used as a simpler andmore easily removable and recyclable catalyst for the polymer-ization of VC at 25 �C.

N-vinylcarbazole. Carbazole-related polymers have attractedsignicant attention for their useful physical properties. Forexample, a N-vinylcarbazole (NVC) based polymer was the rstand remains the most widely studied polymeric photo-conductor and has been polymerized successfully by cationic orfree radical polymerization at high temperatures. The rstreport on the polymerization of carbazole-based monomers viazero-valent metal-mediated RDRP was reported by Haridharan


et al.95 The ambient-temperature polymerization of NVC via aCu(0)/PMDETA-mediated RDRP method was possible in thepresence or absence of CTA yielding a Mw/Mn < 1.25, howeverthe rate of polymerization was very slow. In the case of carbazolemethacrylate (CMA), a well-controlled polymerization giving anarrow molecular weight distribution (Mw/Mn ¼ 1.21–1.32) wasachieved in the presence of CTA, with moderate conversion.66

The zero-valent metal-mediated RDRP method is a useful andsimple tool for the synthesis of carbazole-based methacrylatepolymers, although it results in poorer conversion comparedwith the zero-valent metal-mediated RDRP of other commonmonomers. This may be attributed to the presence of CTA,which not only enables control, but also decrease the poly-merization rate.

2.2 Catalyst system

Cu(0) powder and wire. Cu(0)-mediated RDRP is signicantlyfaster than other metal-catalyzed RDRP processes. A typicalzero-valent metal-mediated RDRP of MA in DMSO at 25 �C([MA]0/[MBP]0/[Cu(0)]0/[Me6TREN]0 ¼ 222/1/0.1/0.1) can achievecomplete conversion within 50 min with an approximate Mn of20 000.9 Cu(0) powder combined with Me6TREN, PMDETA,TREN or bpy has been used in the zero-valent metal-mediatedRDRP of MA, EA, BA, MMA, AN and VC initiated with chloro-,bromo-, or iodo-containing compounds.9,10,55,96–99 The catalyticactivity depends on the structure of the ligand used. The generalorder is as follows: tetradentate (cyclic-bridged) > tetradentate(branched) > tetradentate (cyclic) > tridentate > tetradentate(linear) > bidentate ligands. The nature of the N atoms is alsoimportant and follows the order pyridine $ aliphatic amine >imine. Ethylene is a better linkage for N atoms in the ligandthan propylene.100 A series of RAFT chain transfer agents can beefficiently synthesized using a one-step atom transfer radicaladdition–fragmentation (ATRAF) technique in the presence ofCu(0) powder.101

The effect of Cu(0) particle size on the kinetics of zero-valentmetal-mediated RDRP in DMSO at 25 �C has also investigatedby Percec et al., using the Cu(0)/Me6TREN catalyzed polymeri-zation of MA initiated with MBP.54 Decreasing the Cu(0) particlesize results in a marked increase in the apparent rate constantof propagation (kappp ). For example, decreasing the Cu(0) particlesize from 425 nm to 50 nm increases kappp by almost an order ofmagnitude. Regardless of the Cu(0) particle size used, an almostperfect zero-valent metal-mediated RDRP occurs with a rst-order polymerization in growing species with up to 100%conversion in DMSO. However, it should be noted that a rela-tively lower number of end-group functionalities should resultfrom the faster polymerization.

The combined advantages of easier catalyst preparation,handling, uniformity, and simple recovery/recycling makeCu(0)-wire catalyzed RDRP an ideal RDRP methodology(Scheme 6). RDRP in the presence of Cu(0) wire demonstrates aform of RDRP with greater precision, allowing for accuratedetermination of the external rate order for heterogeneousCu(0) catalysis and the accurate prediction of kappp from the wiredimensions. Cu(0) wire also exhibited signicantly greater

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Scheme 6 Cu(0) wire mediated RDRP (Reprinted with permissionfrom Brad M. Rosen and Virgil Percec. Chem. Rev., 2009, 109, 5069–5119. Copyright 2009 American Chemical Society).10


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control over the molecular weight distribution than Cu(0)powder (Scheme 6).9,102–105 Cu(0) wire could also be used as asupplemental activator and reducing agent to accurately quan-tify the kinetics of the RDRP process.47,49–51 High-molecular-weight (co)polymers can be formed using ARGET ATRP with analkyl pseudohalide simultaneously acting as both an initiatorand chain transfer agent and Cu(0) wire can be used as areducing agent several times without additional treatment.106

Copper tubular reactor. The use of copper tubing as both thereactor and as a catalyst source has been demonstrated for thecontinuous RDRP of MA at ambient temperature and using alow solvent content of 30%.107 The polymerization proceedsquickly, reaching 67% conversion at a residence time of 16 min.Lower ligand levels could be used without a precipitousdecrease in the reaction rate, offering the potential to balancethe increased reactor volume (residence time) required with thedecreased amount of rawmaterial and post process puricationcosts.107

In situ Cu(0). RDRP has been catalyzed by in situ Cu(0)generated from copper sulphate pentahydrate (CuSO4$5H2O)and hydrazine hydrate (N2H4$H2O) at 25 �C. The polymerizationoccurred smoothly with moderate controllability. The poly-merization rates increased with the increasing amount ofN2H4$H2O, and there was an optimal value of the initiatorconcentration for the polymerization rate.Mn,SEC increased withmonomer conversion and Mw/Mn was below 1.40. Mn,SEC devi-ated signicantly from the theoretical values with about 50%polymer chain-end delity.108 The in situ-Cu(0) mediated RDRPof hydrophilic monomers was also exploited with full dispro-portionation of CuBr/Me6TREN to Cu(0) powder and CuBr2conducted in water prior to the addition of both monomer andinitiator.71 The in situ Cu(0)-mediated RDRP of MMA initiatedwith a Ni(0)/EBiB/CuBr2/PMDETA system exhibited an almostoptimal “living”/controlled nature and generated polymers witha polydispersity index as low as 1.04 at 75.3% conversion andcontrolled molecular weights close to the theoretical values.109

In addition, a successful example of RDRP with CuSO4$5H2O asa catalyst was reported in the presence of Fe(0) or ascorbic acidas a reducing agent. CuSO4$5H2O produced a more controllablepolymerization compared with the widely used CuBr2.110

Other zero-valent metals. In 1999, Sawamoto et al. demon-strated the successful RDRP of methyl methacrylate (MMA)catalyzed with Ni(0)/PPh3 using an organic bromide as an

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initiator in the presence of Al(Oi-Pr)3 for the rst time.111 In ourwork, Fe(0) powder was used to catalyze the polymerization ofMMA in the presence of a RAFT agent, CPDN. Chen et al.synthesized PAN via RDRP using Fe(0) as a catalyst and itsdesorption properties towards Hg2+ were investigated aermodication with NH2OH$HCl.105 The same group also repor-ted the successfully continuous RDRP of AN with iron tubes as acatalyst source without any ligand in N-methyl-2-pyrrolidone(NMP) and DMF.112 Recently, the feasibility of using an Fe(0)powder-mediated polymerization of MMA and St with theaddition of CuBr2 as a deactivator was investigated by our groupat 25 �C, in which Fe(0) played the dual role of the activator forthe generation of active radicals and the reducing agent forCuBr2.113,114 Fe(0) (either powder or wire) has been demon-strated to remove/modify the thiocarbonylthio end groups ofRAFT-made polymers.115 Matyjaszewski and co-workers repor-ted the polymerization of MA mediated by a zero-valent metalwire (zinc, magnesium, and iron) combined with CuBr2 inDMSO at 25 �C.37 Chen et al. also have dedicated researchtoward a series of other zero-valent metal catalyzed RDRPs. Forexample, RDRPs of MMA with ytterbium powder [Yb(0)]116 andgadolinium powder [Gd(0)]117 as catalyst, and RDRPs of ANcatalyzed with samarium powder [Sm(0)],92 tin powder [Sn(0)],118

lanthanum powder [La(0)],119,120 magnesium powder [Mg(0)]121

and so on. These studies have all played a crucial role inextending the applications of RDRP.

2.3 Initiators

As with all metal-catalyzed RDRP processes, the appropriatechoice of initiator is critical. A variety of monofunctional,bifunctional, multifunctional, and macro initiators have beenused in zero-valent metal-mediated RDRP, much like ATRP. Theleaving group is more or less exclusively a halogen atom in zero-valent metal-mediated RDRP, and thereby halogen compounds(R–X, where X ¼ halogen) are widely employed as initiators.Obviously, Chlorine (Cl), bromine (Br), and iodine (I) are allactive as the halogen, however uorine (F) is inactive for metal-catalyzed RDRP. The component R should contain someradical-stabilizing groups in order to facilitate radical genera-tion, such as esters [–C(]O)OR], ketones [–C(]O)R], amides[–C(]O)NR2], cyano groups (–C^N), phenyl groups (–Ar), etc.

a-Haloesters. The most common initiator for the polymeri-zation of monofunctional acrylates is MBP, which has also beenused as an initiator in the Cu(0)/Me6TREN catalyzed RDRP ofMA in DMSO.9 Cu(0)-mediated RDRP of MMA can be initiatedwith the alkyl chloride methyl-2-chloropropionate (MCP) andcatalyzed by Cu(0)/Me6TREN/CuCl2 in DMSO at 25 �C.122 EBiBhas been used in the Cu(0)/bpy catalyzed RDRP of MMA inDMSO9 and the Cu(0)/PMDETA catalyzed RDRP of MMA inHFIP.78 Using EBiB as an initiator, comproportionation wasachieved in the initiation step of Cu(0)-mediated RDRP.123

Additionally, alkyl 2-bromo-2-phenylacetates, a type of initiatorwith synergistic phenyl and ester groups, such as ethyl 2-bromo-2-phenylacetate (EBPA), has been reported as an effectiveinitiator in the metal-catalyzed RDRP of MMA.124,125 Recently,ligand-free Cu(0)-mediated polymerization of MMA with EBPA




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selected as an initiator has been realized.74 The higher activa-tion rate constant (kact) of EBPA resulted in good control overthe polymerization even without any ligand. 2,2-Dichlor-oacetophenone (DCAP) has also been reported as an effectivealternative initiator for the Cu(0)/PMDETA-catalyzed RDRP ofMMA in DMSO or HFIP at 25 �C in work from our group.78

Haloforms and carbon tetrachloride. Haloforms, when usedas initiators for Cu(0)-mediated RDRP can undergo subsequentfunctionalization to provide strategies for the synthesis ofdifferent block copolymers and other complex architectures.The haloforms, CHCl3,9,122 CHBr3,9,126 and CHI3,9,126 have beenemployed as initiators for the zero-valent metal-mediated RDRPof MA in DMSO at 25 �C. Among these compounds, CHCl3 andCHBr3 are always monofunctional initiators, however, CHI3 isonly a monofunctional initiator for PMA at low conversion andtransitions to a bifunctional initiator at high conversion. CHBr3has also been demonstrated as an effective initiator for the zero-valent metal-mediated RDRP of VC in DMSO at 25 �C by Percecet al.9,14 The RDRP process has been evaluated for the Cu(0) wirecatalyzed polymerization of MMA using inexpensive and readilyavailable CCl4 as an initiator, with the reaction driven tocompletion in 10 h at 25 �C.127 Chen et al. have also reportedother zero-valent metal (such as Yb(0),116 Sn(0)118 and La(0)119,120)catalyzed RDRPs of MMA or AN with CCl4 as an initiator.

a-Halonitriles. a-Halonitriles are fast radical generators inmetal-catalyzed RDRPs, due to the presence of the strong elec-tron-withdrawing cyano group. Moreover, the radical formedaer halogen abstraction is sufficiently reactive, which leads tofast initiation through rapid radical addition to the monomer.Using BPN resulted in polymers with the lowest polydispersitiesand is an effective initiator for Cu(0)-mediated RDRP of MA orAN in DMSO at 25 �C.90 It was subsequently determined that aniodine-based initiator, 2-iodo-2-methylpropionitrile (CPI) wasalso an effective initiator for the Cu(0)-mediated polymerizationof MMA in the absence of ligand with DMSO as solvent.75

Sulfonyl halides. TsCl has been demonstrated to be anexcellent radical initiator for the process of Cu(0)-mediatedRDRP of MMA at 50 �C.128 The rst order kinetic dependence ofthe polymerization was preserved up to complete conversion.Polymerizations in MeOH could be signicantly enhanced bythe addition of small amounts of water. The molecular weightsincreased linearly with conversion and very narrow molecularweight distributions were obtained. In addition, phenoxy-benzene-4,40-disulfonyl chloride (PDSC) was used as anbifunctional initiator in conjunction with Cu(0)/bpy as a catalystin the RDRP of MMA at 25 �C in NMP.9

2.4 Solvents

DMSO. DMSO is one of most commonly used solvents forATRP and zero-valent metal-mediated RDRP, because it has theadvantage of promoting the reaction. DMSO has a particularlyhigh freezing point (18 �C), which is conducive to the freeze–pump–thaw process. It also enhances the polarity of the reac-tion system, thereby contributing to the electron transferprocess during the activation of alkyl halides by Cu(0).9,129–132

Matyjaszewski et al. evaluated the effect of solvents via a


thermodynamic scheme that represents the ATRP equilibriumas the sum of an equilibrium involving carbon–halogenhomolysis and three thermodynamic contributions relatedto the catalyst: the reduction/oxidation of (i) the metalcomplex and (ii) the halogen atom as well as (iii) the affinity ofthe higher oxidation state of the catalyst for halide anions(or it’s “halidophilicity”).132 The experimental data agreed withthe thermodynamic calculations, and DMSO was found to havea greater ATRP equilibrium constant than acetone under iden-tical conditions.132

DMF. It has been demonstrated that zero-valent metal-mediated RDRP can be performed effectively in a variety ofsolvents including DMF, NMP, propylene carbonate (PC),ethylene carbonate (EC), and dimethylacetamide (DMAC).58 Theaddition of 5–10% H2O to these solvents results in a linearenhancement of kappp and oen a modest improvement in themolecular weight distribution. Interestingly, it has been shownthat a DMF–H2O mixture could be used as solvent for the zero-valent metal-mediated RDRP of hydrophilic monomers, such asNIPAM, PEGMEA and 2-EHA.61 PAN has good solubility in DMF,which makes DMF the preferred solvent for the zero-valentmetal-mediated RDRP of AN. Using DMF, the RDRP of AN canbe catalyzed by Cu(0) powder or Cu(0) wire.89–92,133

Alcohols & water. Alcohols including methanol, ethanol,1-propanol, tert-butanol and methoxyethanol have been shownto be effective solvents for the Cu(0)/Me6TREN catalyzed RDRPof MA at 25 �C.9,58,134 The discovery that ethyl acetate–MeOHmixtures can act as effective solvents for Cu(0)-mediated RDRPis of particular interest for large-scale applications due to thelow cost of ethyl acetate, as well as its distinct solubility prolecompared to typical RDRP solvents.135 Additionally, HFIP is asuitable solvent for the extremely rapid zero-valent metal-mediated RDRP of MMA78 and AN92 at very low temperatures,providing dual control of the molecular weight distribution andpolymer tacticity. Recently, Percec et al. reported the Cu(0)-mediated RDRP of a range of hydrophobic and hydrophilicacrylates, including MA, nBA, tBA, EHA and HEA using MBPand EBiB as initiators, Me6TREN as ligand and 2,2,2-tri-uoroethanol (TFE) as an efficient solvent.136

Water is an environmentally friendly reaction medium. Theapparent rate constants of polymerizations mediated by Cu(0)exhibit a linear increase with the addition of H2O. According toPercec's theory, H2O exhibits the highest equilibrium constantfor disproportionation of Cu(I)X. Therefore, higher equilibriumconstants are observed for disproportionation generated by theaddition of H2O to organic solvents (DMSO, DMF, DMAC, EC,PC, EtOH, MeOH, methoxyethanol, NMP, acetone).58 However,it was predicted by the Kamlet–Ta relationship that H2Oshould have a high ATRP equilibrium constant (KATRP,log KATRP z �4).132 This result can be also used to explain therate enhancement witnessed aer the addition of H2O into anorganic solvent.132 Cu(0)-mediated RDRP of MEA was achievedin H2O catalyzed with sodium dithionate, which was the rstreport dealing with the synthesis of PMEA using a RDRPapproach in an aqueous medium.137 The synthesis of a zwit-terionic polymer was achieved in aqueous solution at 25 �Cusing 2-chloropropionamide as an initiator and Cu(0)

Polym. Chem., 2014, 5, 3533–3546 | 3539



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powder/Me6TREN as the catalyst system at room temperature.67

Recently, Haddleton et al. carried out an interesting piece ofwork, in which NIPAM was polymerized in a range of interna-tional beers, wine, ciders and spirits utilizing Cu(0)-mediatedRDRP.138

Non-polar solvents. Perrier et al. demonstrated the forma-tion of well-dened PMMA homopolymer and block architec-tures via Cu(0)-mediated RDRP in toluene at 30 �C. UnreactedCu(0) in toluene is easily removed by ltration, centrifugation orsedimentation, which is likely to result in even lower copperconcentrations. This provides a facile low-temperature route tothe preparation of well-controlled poly(methacrylate)s usingsignicantly less copper.139 Subsequently, Harrisson et al. havedemonstrated that it is possible to control the polymerization ofMA using elemental copper and a conventional ATRP ligand/initiator at room temperature in a non-polar solvent. Thismethod provides high chain end delity, as the polymersgenerated can be chain extended with 99% efficiency.140

Ionic liquids. Ionic liquids are organic salts with meltingpoints at or near room temperature. They have been extensivelystudied, even as solvents for polymerization processes becauseof their non-volatility, high stability, high ion conductivity, andwide electrochemical activity. A rapid Cu(0)/bpy catalyzed RDRPof MMA initiated by PDSC at 70 �C has been reported using theionic liquid 1-butyl-3-methylimidazolium hexauorophosphate(BMIMPF6) as solvent.73 All of the polymerizations in BMIMPF6showed excellent predictability in terms of their molecularweight evolution and distribution (Mw/Mn z 1.11–1.26).

Acetic acid. Percec et al. investigated the inuence of acidityon Cu(0)-mediated RDRP of MMA in protic and dipolar aproticsolvents. They demonstrated that Cu(0)-mediated RDRP toler-ated the addition of acetic acid to very high levels and that goodcontrol over the polymerization process was preserved, whichshed new light on the Cu(0)-mediated RDRP under acidicconditions.141

3. Towards precise control via zero-valent metal-mediated RDRP

Some of the distinguishing characteristics and intrinsicadvantages of normal RDRPs, such as ATRP and RAFT, arealso exhibited by zero-valent metal-mediated RDRP. Despitesignicant development in zero-valent metal catalyzed RDRPtechniques, the pursuit of more precise control over themacromolecular structure of the resulting polymers, includingchain end functionality, tacticity and compatibility with otherRDRP techniques, never stops. In addition, the utilization ofzero valent metals as catalysts has some intrinsic advantagesthat arise from the variable physical form of the metals and thepolymerization behavior.

3.1. Good retention of chain-end functionality

In RDRPs the complete elimination of side reactions is difficult,and this becomes worse for polymerizations with low concen-trations of initiator or high monomer conversions duringpolymerization. However, good retention of chain-end

3540 | Polym. Chem., 2014, 5, 3533–3546

functionality is necessary to achieve high molecular-weightpolymers and allows for the synthesis of macroinitiators inblock-copolymerization. The reaction temperature plays animportant role in RDRPs. On the one hand, a higher tempera-ture may improve the ratio of the propagation to terminationrate coefficient, which is favorable for high chain end func-tionality. On the other hand, lowering the temperature would beuseful for the suppression of undesirable side reactions, such asbackbiting in acrylates. Therefore, the temperature should beappropriately selected by balancing the two concernsmentioned above. The synthesis of a polymer with high chain-end functionality is very important for successful zero-valentmetal-mediated RDRP.51,142,143 From these initial reports, aseries of in-depth NMR, MALDI-TOF, and chain extensionstudies conrmed high chain-end functionality.9,126,144 In addi-tion, the polymers from zero-valent metal-mediated RDRP canbe used as macro-initiators in chain extension reactions.65

Improving initiation efficiency in the polymerization processcan improve the chain-end functionality.145 However, it shouldbe noted that the formation of Cu(II)X2–ligand complexesinevitably leads to a loss of halogen-end group functionalitybased on the mass–balance relationship.146 Furthermore, anincrease of CuBr2–ligand complexes generated during thepolymerization process has been experimentally observed in theliterature.51,97

3.2. Simultaneous control of molecular weight and tacticity

The control of multiple characteristics of primary structures,such as molecular weight, stereochemistry, and monomersequence, in synthetic polymers has lead to the development ofmore sophisticated functional polymers that could rival naturalmacromolecules that possess uniform molecular weight,stereoregularity, and regulated monomer sequences. Recentyears have witnessed a large number of novel RDRP methodsand catalysts that can precisely control either the molecularweight or the tacticity, or occasionally both, during a variety oftypes of polymerization.147 Using a low polymerization temper-ature, zero-valent metal-mediated RDRP may be considered tobe an appropriate and effective method for the simultaneouscontrol of molecular weight and tacticity for various types ofmonomer.

The controllability of molecular weight and tacticity ofPMMA with Cu(0)/PMDETA as a catalyst system and uo-roalcohol HFIP as a solvent was rst achieved by our group.78 Ahigher concentration of HFIP and a lower reaction temperatureproduced a higher syndiotactic ratio. Using HFIP as solvent,Chen et al. reported the Sm(0) powder catalyzed RDRP of ANwith N,N,N0,N0-tetramethylethylenediamine (TEMED) as aligand and BPN as an initiator resulting in the simultaneouscontrol of the molecular weight and tacticity at 35 �C.133 First-order kinetics of polymerization with respect to the monomerconcentration, a linear increase of the molecular weight withmonomer conversion, and highly syndiotactic PAN wereobtained, indicating that the molecular weight and tacticity ofPAN could be simultaneously controlled via zero-valent metal-mediated RDRP.




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Recently, we illustrated simultaneous control over themolecular weight and tacticity of P4VP via a well-chosen RDRPtechnique with hydrogen bonding interactions. The optimalconditions for the hydrogen bonding interaction between 4VPand HFIP were determined using UV-vis spectroscopy. Theresults demonstrated that equimolar amounts of 4VP andHFIP and a temperature of 25 �C favored a better hydrogenbonding interaction.81 Guided by these preliminary results, thisroom-temperature Cu(0)-mediated RDRP was selected for thecontrolled polymerization of 4VP in the presence of an equimolaramount of HFIP. The subsequent polymerizations producedpolymers with more predictable number-average molecularweights (Mn,SEC) and narrower molecular weight distributions(Mw/Mn < 1.25) in comparison with those without hydrogenbonding interactions. Moreover, the hydrogen bonding had aprofound impact on stereoregulation during the polymerization,which resulted in highly syndiotactic (60.2%) P4VP and gave riseto a higher glass transition temperature (Tg).81,82

3.3. Compatibility with other RDRP techniques

Since the catalyst systems in zero-valent metal-mediated RDRPcan also be used as catalysts in “click” chemistry, zero-valentmetal-mediated RDRP and “click” chemistry can take place atthe same time in the same reaction system. This methodprovides a simple way to generate two different polymer chainswith the same backbone. A clickable alkyne monomer, prop-argyl methacrylate (PgMA), was successfully polymerized by ourgroup in a well-controlled manner via single electron transferinitiation and propagation using EBiB/CTA/Cu(0)/PMDETA.148

The living nature of the polymerization was conrmed by therst-order kinetic plots, the linear relationship between themolecular weight and the monomer conversion while main-taining a relatively narrow molecular weight distribution(Mw/Mn � 1.55), and the successful chain-extension with MMA.Moreover, a one-pot/one-step technique was successfullyemployed to prepare side-chain functionalized polymers. PSt–b-PEO-b-PtBA triblock copolymers were successfully synthesizedvia a combination of zero-valent metal-mediated RDRP and“click” chemistry using Cu(0)/PMDETA as a catalyst system in aone-pot strategy.149 Zero-valent metal-mediated RDRP has beensuccessfully employed for the synthesis of tunable thermores-ponsive protein–polymer conjugates by combining with nucle-ophilic thiol–ene “click” chemistry.150 Discrete oligo(ethyleneglycol) methacrylates were polymerized directly from a salmoncalcitonin macroinitiator and readily synthesized in a one-potprotocol utilizing thiol–ene chemistry to yield well-denedconjugates. It should be noted that most of these structureshave also been successfully produced using classical ATRP.15

A combination of zero-valent metal-mediated RDRP and anitroxide radical coupling reaction at ambient temperature wasrecently developed by Huang et al., in which the macroradicalsgenerated in the zero-valent metal-mediated RDRP mechanismare trapped by nitroxide radicals. This reaction is an effectivealternative approach for coupling reactions under ambientconditions, which is applicable to a variety of halogen-containingpolymers, including polymers made from PSt, PMA and PMMA


with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). In addition,the use of room temperature inhibits side reactions such asthermal crosslinking, chain transfer and b-H transfer seenin poly(methacrylate ester) macroradicals.151 Moreover, thesynthesis of ABC triblock copolymers has been accomplishedusing a Cu(0)-catalyzed one-pot strategy that combines thisreaction with “click” chemistry.152 First, the precursors, a,u-het-erofunctionalized poly(ethylene oxide) (PEO) with a TEMPOgroup and an alkyne group, PSt, and PtBA with a bromine orazide end group, were designed and synthesized. The one-potcoupling reaction between these precursors was then carried outusing the Cu(0)/Me6TREN catalyzed system. The reactionbetween the bromine and the nitroxide radical group combinedwith the simultaneous click coupling between the azide and thealkyne group produced ABC triblock copolymers. It was notice-able that Cu(I) generated from Cu(0) in the zero-valent metal-mediated RDRP mechanism was utilized to catalyze the clickchemistry. In order to estimate the effect of Cu(0) on the one-potreaction, a comparative analysis was performed in presence ofdifferent Cu(0) species. The results showed that Cu(0) with amore active surface area could signicantly accelerate the one-potreaction. Our group have also described the facile removal/modication of thiocarbonylthio end groups of RAFT-madepolymers (especially PMMA) by utilizing Cu(0) powder/TEMPOunder mild conditions. The utilization of Cu(0) wire also effec-tively removed the thiocarbonylthio end group. Fe(0) (eitherpowder or wire) instead of Cu(0)/ligand can be applied to remove/modify the thiocarbonylthio end group giving comparableresults. This work provided a promising alternative approachtowards an adjustable end group removal/modication of RAFTpolymers, and would eventually strengthen and facilitate thepotential large-scale application of RAFT-related polymers.115

4. Functional polymers prepared byzero-valent metal catalyzed RDRP4.1 Controlled copolymerization and functional copolymersvia zero-valent metal catalyzed RDRP

Zero-valent metal-mediated RDRP has been developed as areliable, robust and straightforward method for the construc-tion well-dened copolymers (Scheme 7). Percec et al. investi-gated the copolymerization of MMA andmethacrylic acid (MAA)using a zero-valent metal as a catalyst. Kinetic plots of poly-merizations with different MAA content were recorded andevaluated in order to estimate the rate of copolymerization ofMAA in MeOH–H2O.153 Our group investigated the copolymeri-zation of MMA and St catalyzed by Cu(0) at room temperature.The reactivity ratios of MMA and St were calculated andcompared to systems mediated using other RDRP methods.86

The Cu(0)-mediated RDRP of vinyl acetate (VAc) and AN at 25 �Cproceeded smoothly in DMSO. The polymerization exhibitedmoderately controlled features at low VAc feed ratios. Thereactivity ratios of VAc and AN in the system were found tobe 0.003 and 1.605, respectively. This resulted in the copoly-merization of AN and VAc, wherein VAc was an unavailablemonomer for the Cu(0)-mediated RDRP.88

Polym. Chem., 2014, 5, 3533–3546 | 3541


Scheme 7 Functional polymers designed using zero-valent metal-mediated RDRP.


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Native cellulose esters or ethers, which are soluble inselected organic solvents, can be relatively easy to functionalizewith various haloacyl groups, giving rise to polyfunctionalmacroinitiators. Vlcek et al. reported the rst example of theapplication of a Cu(0)-mediated RDRP process to the controlledgraing of cellulose esters, cellulose diacetate (CDA), andcellulose acetate butyrate (CAB).154 Chloro and bromo-func-tionalized macroinitiators were successfully prepared from thesowood hemicellulose O-acetylated galactoglucomannan via aCu(0)-mediated RDRP.155 A multi-functional macroinitiatorfor Cu(0)-mediated RDRP was designed from acetylated gal-actoglucomannan by functionalization of the anomerichydroxyl groups on the heteropolysaccharide backbone witha-bromoisobutyric acid.156

Whittaker et al. reported a new approach for the facilesynthesis of high-order multiblock copolymers. The approachentailed the sequential addition of different monomers via aniterative Cu(0)-mediated RDRP technique, allowing near perfectcontrol over the copolymer microstructure. It was possible tosynthesize high-order multiblock copolymers with unprece-dented control, i.e., a P(MA-b-nBA-b–EA-b-2EHA-b-EA-b-nBA)copolymer, without any purication between iterative 24 h blockformation steps.157 Cu(0)-mediated RDRP was also successfullyused to produce well-dened linear and star homo- and diblock-copolymers of poly(methyl acrylate-b-solketal acrylate) (PMA-b-PSA), and poly(methyl acrylate-b-glycolic acid) (P(MA-b-GA)n)(where n ¼ 1 or 4). Such vesicle structures have potential appli-cations as nanoscale delivery devices for drugs and otherimportant biomolecules. Novel AB2-type amphiphilic blockcopolymers of poly(ethylene glycol) (PEG) and PNIPAM, PEG-b-(PNIPAM)2 were successfully synthesized through Cu(0)-medi-ated RDRP using a bifunctional macroinitiator. Differentialscanning calorimetry (DSC) measurements of PEG-b-(PNIPAM)2indicated that the block copolymer molecular architecture had asignicant inuence on the phase transition.158 Huang et al. havepresented the synthesis of well-dened amphiphilic graingcopolymers containing a poly(N-isopropylacrylamide)-b-poly-(ethyl acrylate) (PNIPAM-b-PEA) backbone and hydrophobic

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poly(2-acryloyloxyethyl ferrocenecarboxylate) (PAEFC) side chainsvia a combination of Cu(0)-mediated RDRP and ATRP.159 Triblockcopolymers of polystyrene-block-poly(ethylene oxide)-block-poly-(tert-butyl acrylate) (PSt-b-PEO-b-PtBA) have been prepared via acombination of Cu(0)-mediated RDRP and “click” chemistryusing Cu(0)/PMDETA as the catalyst system, wherein the Cu(I)generated in situ was used to directly to catalyze the “click”chemistry.149

Meanwhile, Cu(0)-mediated RDRP has shown promise inachieving comparable polymer layer thicknesses at lower reac-tion temperatures and shorter reaction times. A pH-responsiveamphiphilic A2B2 miktoarm star-like block copolymer, poly-(acrylic acid)2-poly(vinyl acetate)2 [(PAA)2(PVAc)2], with acontrolled molecular weight and well-dened structure wassuccessfully synthesized by our group via a combination ofCu(0)-mediated RDRP and RAFT polymerization methods.160

The well-dened miktoarm star block copolymer that wasobtain was further utilized to prepare pH-responsive amphi-philic A2B2 miktoarm star-like block copolymers, which canself-assemble into spherical micelles possessing a PVAc coreand PAA shell. Recently, the synthesis of well-dened multi-block star copolymers via Cu(0)-mediated RDRP has beenreported.161 The technique involves a core rst approach using amulti-functional initiator in combination with iterative Cu(0)-mediated RDRP steps. It is noteworthy that tedious puricationis not required between successive chain extension steps sincecomplete monomer conversion can be achieved before theaddition of each consecutive monomer type.161

Well-dened graing copolymer poly[poly(ethylene glycol)methyl ether acrylate]-g-poly[poly(ethylene glycol) ethyl ethermethacrylate] (PPEGMEA-g-PPEGEEMA) comprising twodifferent hydrophilic side chains was synthesized via thecombination of Cu(0)-mediated RDRP, ATRP, and a “graing-from” strategy. Themolecular weights of the backbone and theside chains were both controllable, and the molecular weightdistributions were between 1.15 and 1.20.162 A series of well-dened double hydrophilic gra copolymers, consisting of aPNIPAM-b-PEA backbone and poly(2-vinylpyridine) side chains[PNIPAM-b-(PEA-g-P2VP)], were synthesized by successiveCu(0)-mediated RDRP and ATRP at 25 �C. Unimolecularmicelles with a PNIPAM core formed in an acidic environment(pH ¼ 2) at elevated temperature (T > 32 �C), although theaggregates turned into spheres with a PEA-g-P2VP-coreaccompanying the an increase in pH value (pH > 5.3) at roomtemperature.163 A versatile zero-valent metal-mediated RDRPprocess was developed and successfully used for the prepara-tion of a series of hemicellulose-based gra-copolymers withtunable hydrophilicity. Acetylated galactoglucomannan(AcGGM), when used as a macroinitiator, can be graed ontoMMA, NIPAM and acrylamide (AcAm).164 Cellulose nano-crystals (CNCs) can be graed onto PNIPAM brushes via asurface-initiated RDRP under various conditions at roomtemperature. It expected that the suspension stability, inter-facial interactions, friction, and other properties of graedCNCs can be controlled by changing the temperature and canprovide a unique platform for the further development ofstimuli-responsive nanomaterials.165




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The amphiphilic heterogra copolymer poly(methyl meth-acrylate-co-2-(2-bromoisobutyryloxy)ethyl methacrylate)-gra-(poly(acrylic acid)/polystyrene) (P(MMA-co-BIEM)-g-(PAA/PSt))has been synthesized successfully using a combination of Cu(0)-mediated RDRP, Cu(0)-mediated RDRP-NRC, ATRP, and NMPvia the “graing from” approach. The self-assembly behavior ofthe amphiphilic heterogra copolymer P(MMA-co-BIEM)-g-(PAA/PSt) in aqueous solution was investigated using atomicforce microscopy (AFM) and dynamic light scattering (DLS), andthe results demonstrated that the morphology of the formedmicelles was dependent on the graing density.166

4.2 Dendrimers and surface-conned zero-valent metalcatalyzed RDRPs

Dendrimers are monodisperse branched non-biological macro-molecules constructed via divergent or convergent synthesis thathold great promise for a variety of applications. In combinationwith a terminal thio-bromo “click” reaction and acylation with2-bromopropionyl bromide, the Cu(0)-mediated RDRP of MA hasbeen shown to provide access to a three-step “branch” and “grow”divergent approach for preparing dendritic macromolecules,wherein PMA connects the branching subunits.62 This facilemethodology can provide a diversity of dendritic macromoleculartopologies and will ultimately provide the means to develop self-organizable dendriticmacromolecules.Moreover, four poly(amino(meth)acrylate) brushes, poly(2-(dimethylamino)ethyl methacry-late) (PDMAEMA), poly(2-(diethylamino)ethyl methacrylate)(PDEAEMA), poly(2-(dimethylamino)ethyl acrylate) (PDMAEA),poly(2-(tert-butylamino)ethyl methacrylate) (PTBAEMA), have beensynthesized by Walters et al. via surface-conned RDRPs using asurface-conned initiator derived from silane self-assembledmonolayers (SAMs) on silicon wafer substrates.167

5. Conclusions

Zero-valent metal-mediated RDRP has developed dramaticallyand becomes a robust and versatile synthesis technique for thepreparation of polymers with well-dened architecture and sitespecic functionality, such as stars, bottle brushes and blockand gra copolymers. The intrinsic advantages of using a zero-valent metal as a catalyst can signicantly simplify the poly-merization procedure and facilitates its potential applicationfor large-scale production.

Acknowledgements

Financial support from the National Nature Science Foundationof China (21174094, 21374068), the Nature Science Key BasicResearch of Jiangsu Province for Higher Education(12KJA150007), the Priority Academic Program Development(PAPD) of Jiangsu Higher Education Institutions, and theProgram of Innovative Research Team of Soochow University isgratefully acknowledged.


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reversible-deactivation radical polymerization in the presence of metallic copper. activation of...

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