ISSN 1759-9954

Polymer Chemistry

1759-9954(2013)4:1;1-M

www.rsc.org/polymers Volume 4 | Number 1 | 7 January 2013 | Pages 1–196

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Showcasing research from the Functional Polymer Materials and Polymerization Reaction Engineering Lab at Xiamen University, Xiamen, PR China.

Title: Facile synthesis of gradient copolymers via semi-batch

copper(0)-mediated living radical copolymerization at ambient

temperature

Performance of the Semi-Batch Cu(0)-mediated LRP of tBA and

MMA employing conventional ATRP ligands at 25 °C produces

gradient copolymers with well-defi ned microstructure. The addition

of N2H4 into the reaction system allows the reaction proceeding in

the oxygen tolerant system.

As featured in:

See Yin-Ning Zhou and Zheng-Hong Luo,

Polym. Chem., 2013, 4, 76.

www.rsc.org/polymersRegistered Charity Number 207890 PAPER

Pierre-Henri Lanoë, Arnaud Favier, Yann Leverrier et al. Biocompatible well-defi ned chromophore–polymer conjugates for photodynamic therapy and two-photon imaging

PolymerChemistry

PAPER

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aDepartment of Chemical Engineering, Sh

200240, P. R. China. E-mail: luozh@sjtu.edbDepartment of Chemical and Biochemical

361005, P. R. China. E-mail: luozh@xmu.ed

Cite this: Polym. Chem., 2013, 4, 76

Received 27th July 2012Accepted 28th August 2012

DOI: 10.1039/c2py20575k

www.rsc.org/polymers

76 | Polym. Chem., 2013, 4, 76–84

Facile synthesis of gradient copolymers via semi-batchcopper(0)-mediated living radical copolymerization atambient temperature

Yin-Ning Zhouab and Zheng-Hong Luo*ab

In this work, we report an example of the facile synthesis of methyl methacrylate/tert-butyl acrylate (MMA/

tBA) gradient copolymers (poly(MMA-grad-tBA) using the Cu(0) and conventional ATRP ligands as catalysts

in DMF solvent at 25 �C. Semi-batch copper(0)-mediated living radical copolymerization technique (Cu(0)-

mediated LRP) was used for achieving the chain gradient microstructure of the resulting copolymers. We

also compared copolymerizations with two different ATRP ligands at ambient temperature allowing

control over the molecular weight and polydispersity with a quarter of catalyst concentration versus a

conventional ATRP in dipolar protic solvent (i.e. DMF), while the reaction temperature up to 80 �C in a

non-polar medium (i.e. toluene) in order to reach the above polymerization efficiency. The addition of

a small amount of reducing agent (i.e. hydrazine hydrate) into the reaction system allows the reaction

proceeding in the oxygen tolerant system without losing control and decreasing total conversion such

as using the reagents without deoxygenating.

Introduction

Gradient copolymer, in which the instantaneous composition ofcopolymer changes continuously from one end of the chain tothe other, constitutes a relatively new class of polymer with amolecular structure that differs from those of random and blockcopolymers.1,2 As a novel type of chain microstructure, synthesisof gradient copolymers and evaluation of their properties havereceived increasing interest recently.3–8 On the other hand, boththeoretical and experimental investigations demonstrated thatthe gradient composition change of the gradient copolymers canresult in less intrachain and interchain repulsion compared toblock copolymers, leading to unique behaviors, especiallyunique thermal properties in bulk gradient copolymers.2,9–13

Arising from their unique structures and properties, thebehavior of gradient copolymers is useful for some featuredapplications including designing biosensors, membranes, andsubstrates for separation of biomolecules,14 compatibilizers inpolymer blends15 and damping materials,16 etc.

The recent advent of controlled/“living” radical polymeriza-tion (CLRP) techniques makes the control of chain structure nolonger a formidable task.17 Currently, the most used CLRPinvolves nitroxide mediated polymerization (NMP),18 atomtransfer radical polymerization (ATRP),19,20 reversible additionfragmentation chain transfer polymerization (RAFT),21 and

anghai Jiao Tong University, Shanghai

u.cn

Engineering, Xiamen University, Xiamen

u.cn

single-electron transfer and single-electron transfer degenera-tive chain transfer living radical polymerization (SET-LRP andSET-DTLRP).22,23 More recently, more attention has been paid tothe reductions in the amount of copper required for ATRPthrough the techniques of activators (re)generated by electrontransfer (A(R)GET), initiators for continuous activator regener-ation (ICAR).24,25 In addition, although the mechanism of SET-LRP is still under debate in the literature,22,23,26–32 the SET-LRPwith Cu(0) as catalyst has shown some distinct advantages overother CLRPs since its emergence in 2006,23 including lowtemperature polymerization conditions, a small amount ofcatalyst, an ultrafast polymerization rate and obtaining highmolecular weight polymers with narrow molecular weightdistribution. Putting the undened mechanism aside, thedirect use of Cu(0) as the catalyst is attractive since Cu(0) ischeaper and easier to handle than Cu(I) and Cu(II) complexes.Moreover, the SET-LRPs of acrylate monomers can be easilyperformed in these dipolar protic and aprotic solvents (i.e.dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF)) atambient temperatures,33–39 while Perrier et al. reported theCu(0)-mediated LRP of methyl methacrylate (MMA) in tolueneat 30 �C with well-controlled behavior based on the use of anactive ligand.40 In addition, Perrier et al.’s recent report impliedthat the Cu(0)-mediated polymerization of methacrylate (MA)may be an intricate control process and the early loss of controlin polymerization can be prevented by delaying the addition ofmonomer or addition of a small amount of 2,2,6,6-tetrame-thylpiperidine-1-oxyl (TEMPO).41 Percec et al. reported adetailed kinetic investigation and structural analysis of the

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polymer products prepared by the Cu(0)-mediated LRP insolvents mediating different degrees of disproportionation.26

Additionally, Percec et al.’s previous work,42 which is animportant inspiration signicance and reference value totoday’s research, showed that the addition of reducing agentssuch as phenol and phenolates as well as hydrazine or ascorbicacid reduces the amount of Cu(II) species. The potential benetof these additives was believed to be a regeneration of activeCu(I) species from Cu(II) species via a redox process.42–44

In general, introducing a comonomer in CLRP offers thepossibility of preparing tailor-made copolymers with specicdesired characteristics. However, in a batch process, composi-tion dri is very common due to differences in the reactivityratios of the monomers. To synthesize forced gradient copoly-mers, a semi-batch operation is commonly used, which is amethod without limitation of reactivity of co-monomers. It wasexperimentally and theoretically demonstrated that, thesecopolymers with uniform composition or linear gradientcomposition can be successfully designed and controlled.45–48 Inthis contribution, we reported the facilely controlled synthesisof MMA/tBA gradient copolymers mediated by copper powder atambient temperature in a polar solvent (i.e. DMF), using thecommercially available ligand N,N,N0,N0 0,N0 0-pentamethyldiethylene triamine (PMDETA) via semi-batch Cu(0)-mediatedLRP (see Scheme 1). The process of reaction was also investi-gated in non-polar solvent (toluene), using the same ligand(PMDETA). In particular, all the reagents used were not deoxy-genated, which was instead of the addition of a small amount ofreducing agent hydrazine hydrate to the reaction mixture.

Experimental sectionMaterials

t-Butyl acrylate (tBA, 99%, Sinopharm Chemical Reagent Co.,Ltd (SCRC)) and methyl methacrylate (MMA, 99%, SCRC) wererinsed with 5 wt% aqueous NaOH solution to remove inhibitor,dried with MgSO4 over night and distilled before use. Ethyl 2-bromoisobutyrate (Eib-Br, 98%) was obtained from A BetterChoice for Research Chemicals GmbH & Co. KG. (ABCR). 4,40-Dinonyl-2,20-bipyridyl (diNbpy, Nanjing Chemzam Pharmtech,99%) was recrystallized three times from ethanol. 1,1,4,7,7-Pentamethyldiethylenetriamine (PMDETA, Aldrich, 98%).

Scheme 1 Experimental apparatus for the semibatch Cu(0)-mediated LRP.

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Copper powder (75 mm, 99%, Sigma-Aldrich). Hydrazine mon-ohydrate (Alfa Aesar, 98+%) dimethyl sulfoxide (DMSO, 99.5%,SCRC) and N,N-dimethylformamide (DMF, 99.5%, SCRC). Allother reagents and solvents were obtained from SCRC and usedwithout further purication.

Measurements

The copolymer compositions were determined by nuclearmagnetic resonance (1H NMR) spectroscopy (Bruker AV400MHz) in CDCl3 and tetramethylsilane (TMS) as an internalstandard. To obtain the relative amounts of the co-monomersincorporated in polymer chains were estimated from the areasunder assigned peaks of the spectra. The copolymer composi-tion was determined by comparing the integrated intensities ofthese resonance signals as above mention. The molecularweight (Mn) and molecular weight distribution (Mw/Mn, PDI) ofthe polymer was determined at 40 �C by gel permeation chro-matography (GPC) equipped with a waters 1515 isocratic HPLCpump, three Styragel columns (Waters HT4, HT5E, and HT6)and a waters 2414 refractive index detector (set at 30 �C), usingTHF as the eluent at the ow rate of 1.0 mL min�1. A series ofpoly(methyl methacrylate) (PMMA) narrow standards were usedto generate a universal calibration curve.

Typical procedures for the synthesis of gradient copolymerpoly(tBA-grad-MMA) via semi-batch Cu(0)-mediated LRPtechnique

Solvent (DMF, 4.5 ml), catalyst (copper powder, 4.77 mg, 0.075mmol), and hydrazine hydrate (3.6 ml, 0.075 mmol) were rstadded to a 25 ml dried round-bottom ask, and then themixture was stirred for 5 min in a water bath with a thermostatat 25 � 0.1 �C. Aer that, tBA (4.35 ml, 30 mmol), ligand(PMDETA, 31.35 ml, 0.075 mmol) and initiator (Eib-Br, 43.89 ml,0.3 mmol) were added. In addition, MMA (3.21 ml, 30 mmol)was transferred into an airtight syringe and assembled to asyringe pump. Synchronously, the continuous addition of MMAto the ask was started at an optimized rate (0.642 ml h�1),which corresponds to targeted monomer conversion (“TestReaction” method49). Samples were taken of about 0.5–0.6 mLevery 1 h. Aer 5 h, the MMA addition was complete. Thereaction was stopped aer 5.5 h by exposing the reaction

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mixture to the air. All of the above polymer solutions werediluted with CHCl3 and passed over an alumina column toremove the catalyst. The solvent was distilled off under vacuumusing a rotary evaporator at 25 �C, and then the polymer wasprecipitated twice in the mixed solvent of methanol and water(v/v, 1 : 1). The puried product was dried under vacuum untilconstant weight was reached.

Fig. 1 Copolymer cumulative composition in gradient copolymer as a functionof the degree of polymerization.

Results and discussion

It is well-known that the reactivation of the oxidized metalcatalyst, such as Cu2O, is oen performed with hydrogen in thelaboratory.50 On the other hand, it has been reported that Cu2Oon Cu(0) surface can be reduced effectively by hydrazine.51

Furthermore, the Cu(0) powder can be synthesized by the in situreduction of Cu(II) salts with hydrazine.52,53 Percec et al. reportedthat a successful SET-LRP can also be carried out using Cu2O ascatalyst, nevertheless, the process is less reactive.23

In order to assure the stringent anaerobic experimentalconditions (deoxygenated reagents) and eliminate the timeconsuming deoxygenating cycles before the semi-batch Cu(0)-mediated LRP process for the synthesis of gradient copolymers,the reducing agent hydrazine hydrate was added into the reactiveask, which therefore resulted in an activation of the copperpowder by reducing Cu2O to Cu(0), and then the fresh Cu(0) actsaneffective scavenger for oxygen in reagents. Thus,webelieve thatthe benet of hydrazine hydrate addition in Cu(0)-mediated LRPclearly stands on its own. Herein, an equimolar (with respect toCu(0)) amount of hydrazine hydrate was employed for producingthe gradient copolymers with linear gradient composition.

For the sake of monitoring the sequence distribution ofmonomers in products, during the synthesis of gradient copol-ymers, reaction aliquots were taken from reactor to verify thechange in cumulative composition (Fcum) of feeding monomer(M2) as a function of chain length. It should be pointed out thatthe instantaneous composition (Finst) of the copolymer chains isnot experimentally measurable. Fcum is the one that is measuredby 1H NMR. And then the Finst of feeding monomer was in turncalculated according to the following equation: Finst ¼[Conv.total,i� Fcum,M2,i� Conv.total,i�1� Fcum,M2,i�1]/[Conv.total,i�Conv.total,i�1], where Conv.total is the total conversion of bothmonomers (tBA and MMA).54–56 Note that the Finst curve offeeding monomer as a function of polymerization degree virtu-ally shows a true gradient prole along a chain in contrast to thatof Fcum (Fig. 1). The linear relationship shows that an idealgradient composition is formed clearly and gradually. Thecompositions of gradient copolymers are summarized in Table 1.

The overall ratio of incorporated monomer in the resultingcopolymer poly(tBA-grad-MMA) and the monomer conversionwere determined using the 1H NMR spectra of pure copolymerand polymerization solution respectively at an interval time.The changing characteristic signals for PtBA and PMMA inFig. 2–4 show that the MMA units are gradually incorporatedinto the polymer chains. Firstly, the overall ratio of incorporatedmonomer in the resulting copolymer was determined using 1HNMR (Fig. 2, taking entry 3 for example) measurement bycomparing the peak area ratio of characteristic signals for PtBA

78 | Polym. Chem., 2013, 4, 76–84

(1.350–1.550 ppm 9H, –OC(CH3)3) and PMMA (3.500–3.750ppm, 3H, –OCH3). The equations for calculation are as follows:

Fcum;tBA ¼Dtert-butyl=

9

Dtert-butyl=9 Dmethoxyl=3þ

where Dtert-butyl and Dmethoxyl are peak areas of the total protonsignals of PtBA and PMMA in the pure copolymer, respectively.Secondly, the monomer conversion was calculated from thesignals of the double-bond protons in 1H NMR spectrum ofpolymerization solution (Fig. 3 and 4, taking entry 3 forexample, d tBA ¼ 5.650–5.800, 5.975–6.085, 6.225–6.350 ppm, dMMA ¼ 5.600–5.525, 6.080–6.130 ppm). And the signal for the–OC(CH3)3 group of tBA/PtBA protons (d ¼ 1.350–1.550 ppm)was used as standard. The equations for calculation of conver-sion of tBA are as follows:

ln

�½MtBA�0.½MtBA�t

�¼ ln

�A0

.At

�

Conv:tBA ¼ 1� ½MtBA�t½MtBA�0

¼ 1� At

A0

where, At¼ Dpropenyl,t/Dtert-butyl,t,Dpropenyl,t andDtert-butyl,t are peakareas of the double-bond and tert-butyl in the 1H NMR spectra atdifferent times, respectively. A0 ¼ Dpropenyl,0/Dtert-butyl,0 is thenormalized peak areas of double-bond in 1H NMR spectra, hereDtert-butyl,t ¼ Dtert-butyl,0. The conversion of MMA is also obtainedfrom above equations based on the same standard.

Fig. 5 describes the number-average molecular weight(Mn,GPC) and molecular weight distribution (Mw/Mn) as functionof the overall monomer conversion. The relatively high value ofMw/Mn, which declines gradually with the increase of theconversion indicates the slight broadening of the polydispersityand also exhibits a slight tailing towards low molecular weight,which possible due to the poor initiation efficiency caused byrecombination. Furthermore, the copolymerization kineticsdata were also recorded (see Fig. 6 and 7). The kinetic plots for

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Table 1 Summary the composition of gradient copolymers

Entry MMA (ml) Adding time (h)Conv.tBA(%)

Conv.MMA

(%) Fcum,MMA Gradient composition

1 1.284 2 69.31 37.32 0.35 Poor2 3.210 5 72.05 58.95 0.45 Good3 3.210 5 72.30 65.51 0.47 Excellent4 3.210 5 70.39 43.14 0.38 General5 3.210 5 81.97 56.97 0.41 General6 1.926 3 68.16 38.34 0.36 Poor7 1.284 2 84.89 43.73 0.34 Poor8 3.210 5 83.95 90.94 0.52 None (random)9 3.210 5 92.70 85.57 0.48 Excellent

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the copolymerization in DMF initiated with Eib-Br in the pres-ence of Cu(0)/PMDETA at 25 �C (Table 2, entry 3) can be observedin Fig. 6. The approximate linearity of the kinetic plot with aninducing period of about 30 min may results from Cu2O on thesurface of Cu(0) powder, which indicates that a successful Cu(0)-LRP in the presence of limited amount of air with hydrazineadditive requires the complete consumption of oxygen and rapidregeneration of Cu(0) by reduction of the resulting Cu2O duringthe early stage of the polymerization in this system.57 In addition,from Fig. 7, the plots show excellent linearity, which proves anexcellent living polymerization. Fig. 7 also demonstrates that agood agreement between experimental and theoretical molec-ular weight is obtained at the interval time of reaction and thepolymer molecular weight distribution index is in the range of1.78–1.39. We propose the cause of the poor control observedduring the early stages of the reactions is that Cu(I)X/L species donot instantaneously disproportionate, thus resulting in a highradical concentration.44 The relatively broader distribution athigh conversionmay stem from the highly reactive Cu(0) surface

Fig. 2 1H NMR spectrum of the tBA/MMA gradient copolymers at different react

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and high concentration catalyst (in the following discussion)which facilitates a rapid initiation with an increasing radicalconcentrationwhich favors irreversible termination;meanwhile,the reduction of Cu(II) by an excess of hydrazine leads to thedecrease of deactivator concentration.38,42 As a whole, all theabove data showed that the gradient copolymers were synthe-sized successfully via the semi-batch Cu(0)-mediated LRP.

A large quantity of catalyst used in ATRP, which is difficult toremove and results in the catalyst contaminated polymericmaterials, is the main limitation for the industrialization ofATRP. In this work, a series of experiments were performed withdifferent amounts of Cu(0) powder in DMF with VtBA/Vsol. ¼ 1/1and [MMA]0/[tBA]0/[Eib-Br]0 ¼ 100/100/1 at 25 �C. From theresults summarized in Table 2 (entries 1–4) and Fig. 8, it isfound that the total conversion increases rst and thendecreases with the increase of the amount of the Cu(0) powder,while the value of Mw/Mn raises from 1.38 to 1.48. In practice,the reaction solution became exceedingly viscous just aer 2 hand the reactants experienced diffusion limitation, which

ion time.

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Fig. 3 1H NMR spectrum of the polymerization solutions at different reaction time.

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results in the decline of the total conversion at the highestconcentration of catalyst ([Cu(0)]0/[initiator]0 ¼ 1/1, entry 1);accordingly, it causes the poor gradient composition. Thereason may be that the increase of the amount of Cu(0) causes ahigher concentration of Cu(II)/N-ligand produced by thedisproportionation of in situ Cu(I) species, leading to a fasterpolymerization.20 However, the higher radical concentrationleads to more pronounced termination and the formation of the

Fig. 4 Partial enlarged details for 1H NMR spectrum of the polymerization solutio

80 | Polym. Chem., 2013, 4, 76–84

increasing levels of persistent radicals (XMtn+1/L),58 which

reduces the radical concentration and self-regulates the system.These phenomena imply that the lower concentration of cata-lyst gives rise to the improvement in control of molecular weight(MW) and polydispersity, but there exists a maximum value oftotal conversion. Therefore, a suitable catalyst concentrationshould be chosen to provide a balance between polymerizationrate and controllability.

ns at different reaction time from 5.50 to 6.50 ppm.

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Fig. 5 Number-average molecular weight (Mn,GPC) and molecular weightdistribution (Mw/Mn) as function of the overall monomers conversion at differentreaction time.

Fig. 6 Kinetic plot for the semi-batch copolymerization of tBA and MMA usingCu(0) and PMDETA as catalyst system in DMF. (Table 2, entry 3).

Fig. 7 Dependence of poly(tBA-grad-MMA) gradient copolymer molecularweight (Mn) and molecular weight distribution (Mw/Mn) on the total conversionfor the copolymerization in DMF (Table 2, entry 3).

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From Tables 1 and 2 (entries 1–4), these results demonstratethat this polymerization proceeded by the Cu(0)-mediated LRPat a relatively lower concentration of catalyst ([Cu(0)]0/[initiator]0 ¼ 1/4, entry 3) features the better control of MW andits distribution with a “living”/controlled manner, which leadsto form the good gradient composition in polymer chains.

Two different ligands widely used in ATRP were chose toinvestigate the effect of ligand on the Cu(0)-mediated copoly-merization. The ligand 4,40-dinonyl-2,20-bipyridine (diNbpy) hasbeen used extensively in ATRP to control synthesis of gradientcopolymers.48,49,59 PMDETA is a commercially available ligandwith intermediate activity,37,38,40 and also it was veried by Per-rier et al. that PMDETA as a SET-LRP ligand was superior todiNbpy and hexamethylene tris(2-aminoethyl)amine (Me6T-REN) in toluene.60 The comparison experiments were carriedout with 0.25 equimolar of Cu(0) relative to initiator in DMF at25 �C (Table 2, entries 3 and 5), which is inspired by Perrieret al.’s contribution.60 Although the overall conversion for abovetwo systems at the end of copolymerization remains the nearlysame, using diNbpy, the polymerization can give broader poly-dispersity (Fig. 8). Uncontrolled free radical polymerizationduring the early stages of the reaction as a result of insufficientconcentration of Cu(II) deactivator can be regarded as rationalexplanation for this consequence. Therefore, the generalgradient composition is introduced in this reaction system.

There are many achievements about the effect of solvents onthe process of Cu(0)-mediated “living”/controlled radical poly-merization.26,27,41,61–64 In this work, the attempts launched inDMF, DMSO, toluene in combination with PMDETA wereapplied to analyze the inuence of solvents on the polymeriza-tion and controllability. The results are shown in Tables 1 and 2,entries 3, 6–9, and Fig. 8. First, the proportion of solvent inreaction mixture was reduced from VtBA/Vsol. ¼ 1/1 to VtBA/Vsol. ¼2/1. During the polymerization, we found that the reactionsolution became exceedingly viscous just aer 3 h. Comparingwith the results at entry 3, one can nd that the total conversiondeclines, polydispersity becomes slightly broader at entry 6 andthe gradient composition of resulting copolymers is also poor,which may be the same as the explanation for the increase ofcatalyst concentration that causes the similar consequencementioned above. Second, a widely used disproportionatingsolvent, DMSO was chosen as the solvent instead of DMF. Theextraordinary viscous reaction appeared just aer 2 h. From theresults summarized in Table 1, entry 7, the polymerization rateis signicantly elevated comparison with DMF as solvent underthe same reaction conditions (entry 3), and the conversion oftBA monomer soars to 84.9% at the end of reaction, while it ofMMA just arrives 43.7%. The value of Fcum,MMA (0.34) indicatesthe poor gradient composition. In addition, the value of Mw/Mn

is larger than the result of entry 3 at the comparative totalconversion. The highly polymerization rate obtained by SET-LRP has been attributed to the self-regulated generation ofCu(0) activator and Cu(II)X2/L deactivator from the dispropor-tionation of Cu(I)X/L produced in situ during activation inDMSO.26 Finally, we performed the polymerization in toluenewith the same experimental conditions (entry 8). It can beexpected that the polymerization rate is extremely low at 25 �C.

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Table 2 Synthesis of poly(tBA-grad-MMA) by Cu(0)-mediated LRPa

Entry

[MMA]0/[tBA]0/[Eib-Br]0/[Cu(0)]0/[N2H4$H2O]/[Ligand] Solvent Time (h)

Conv.(%) Mn

b (g mol�1) (theo)Mn (g mol�1)(GPC) Mw/Mn (GPC)

1 100/100/1/1/1/1 DMF 2.5 53.3 12 620 12 957 1.482 100/100/1/0.5/0.5/0.5 DMF 5.5 65.5 15 136 15 269 1.453 100/100/1/0.25/0.25/0.25 DMF 5.5 68.9 15 825 16 015 1.394 100/100/1/0.1/0.1/0.1 DMF 5.5 56.8 13 341 13 767 1.385c 100/100/1/0.25/0.25/0.25 DMF 5.5 69.5 16 210 16 409 1.896d 100/100/1/0.25/0.25/0.25 DMF 3.5 53.2 12 575 13 681 1.667 100/100/1/0.25/0.25/0.25 DMSO 2.5 64.3 15 258 16 617 2.088 100/100/1/0.25/0.25/0.25 Toluene 20.0 87.4 19 865 20 308 1.589e 100/100/1/0.25/0.25/0.25 Toluene 5.5 89.1 20 449 21 059 1.29

a Temperature ¼ 25 �C, ligand ¼ PMDETA, VtBA/Vsol. ¼ 1/1. b Mn (theo) ¼ ([tBA]0/[Initiator]0) � conv.tBA � 128.17 + ([MMA]0/[Initiator]0) �conv.MMA � 100.12. c Ligand ¼ diNbpy. d VtBA/Vsol. ¼ 2/1. e Temperature ¼ 80 �C.

Fig. 8 Number-average molecular weight (Mn,GPC) and molecular weightdistribution (Mw/Mn) at different entries (corresponding to Table 2).

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The reaction time was prolonged to 20 h aer the commonerwas added. Therefore, the composition of resulting copolymershould be random. It seems not to be the same catalyst acti-vation mechanism with SET-LRP reported by some researchgroups.26,41 They proposed that Cu(0) is oxidized to Cu(I) whileactivating the initiator, which triggers uncontrolled polymeri-zation. Cu(I) then reacts with the alkyl halide initiator togenerate active species and Cu(II), leading to the establishmentof the equilibrium between Cu(I) and (II). However, interest-ingly, when the reaction temperature was raised up to 80 �C, thepolymerization rate was noticeably accelerated and the poly-dispersity index was narrowed to 1.29, which is lower than thatin the other experiments carried out in this work. The values ofFcum,MMA(0.48) and conversion of both comonomers (85.6% and92.7% for MMA and tBA respectively) manifest the goodgradient composition. The reason is not clear now, moreintensive investigations should be needed.

Conclusion

In conclusion, the semi-batch Cu(0)-mediated LRP techniqueusing Cu(0) and PMDETA as a catalyst system in DMF in the

82 | Polym. Chem., 2013, 4, 76–84

presence of hydrazine hydrate at 25 �C was used to producelinear gradient copolymers. For this method, we observed thatthe addition of an equimolar (with respect to ligand) amount ofhydrazine hydrate to the reaction mixture allowed the reactionto proceed in the reagents without deoxygenating. Besides, theresults regarding the effect of the reaction conditions on thecopolymerization demonstrated that the suitable catalystconcentration was required to balance the polymerization ratewith its controllability. The commercially available ligandPMDETA was regarded as a promising one, with moderatepolydispersity and relatively high conversion. Also, the differentkinds of solvents had great impact on the copolymerization;using DMF, the polymerization gave a more appropriate poly-merization rate for controlling synthesis of gradient copolymerthan DMSO and toluene at 25 �C, but the copolymerizationperformed in toluene at 80 �C featuring low polydipersity andhigh yield had excellent prospects. Accordingly, we thought thatthe above method was a facile and efficient one for the synthesisof gradient copolymer poly(tBA-grad-MMA).

Acknowledgements

The authors thank the National Natural Science Foundation ofChina (no. 21076171, 21276213).

References and notes

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