PPEGMEA-g-PDEAEMA: Double hydrophilic double-grafted copolymer stimuli-responsive to both pH and salinity

PPEGMEA-g-PDEAEMA: Double HydrophilicDouble-Grafted Copolymer Stimuli-Responsiveto Both pH and Salinity

LINA GU,1 CHUN FENG,1 DONG YANG,2 YAOGONG LI,1 JIANHUA HU,2 GUOLIN LU,1 XIAOYU HUANG1

1Key Laboratory of Organofluorine Chemistry and Laboratory of Polymer Materials, Shanghai Institute of OrganicChemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China

2Key Laboratory of Molecular Engineering of Polymers (Ministry of Education), Laboratory of Advanced Materialsand Department of Macromolecular Science, Fudan University, 220 Handan Road, Shanghai 200433,People’s Republic of China

Received 15 February 2009; accepted 16 March 2009DOI: 10.1002/pola.23405Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A series of well-defined double hydrophilic double-grafted copolymers,consisting of polyacrylate backbone, hydrophilic poly(2-(diethylamino)ethyl methacry-late) and poly(ethylene glycol) side chains, were synthesized by successive atomtransfer radical polymerization. The backbone, poly[poly(ethylene glycol) methylether acrylate] (PPEGMEA) comb copolymer, was firstly prepared by ATRP of PEG-MEA macromonomer via the grafting-through route followed by reacting with lith-ium diisopropylamide and 2-bromopropionyl chloride to give PPEGMEA-Br macroini-tiator of ATRP. Finally, poly[poly(ethylene glycol) methyl ether acrylate]-g-poly(2-(diethylamino)ethyl methacrylate) graft copolymers were synthesized by ATRP of2-(diethylamino)ethyl methacrylate using PPEGMEA-Br macroinitiator via the graft-ing-from route. Poly(2-(diethylamino)ethyl methacrylate) side chains were connectedto polyacrylate backbone through stable CAC bonds instead of ester connections,which is tolerant of both acidic and basic environment. The molecular weights ofboth backbone and side chains were controllable and the molecular weight distribu-tions kept relatively narrow (Mw/Mn � 1.39). The results of fluorescence spectroscopy,dynamic laser light scattering and transmission electron microscopy showed this dou-ble hydrophilic copolymer was stimuli-responsive to both pH and salinity. It can ag-gregate to form reversible micelles in basic surroundings which can be convenientlydissociated with the addition of salt at room temperature. VVC 2009 Wiley Periodicals, Inc.

J Polym Sci Part A: Polym Chem 47: 3142–3153, 2009

Keywords: ATRP; double hydrophilic; graft copolymers; stimuli-responsive;synthesis

INTRODUCTION

Amphiphilic block copolymer can self-assembleinto micelles due to the micro-phase precipitationof insoluble block and the affinity of soluble blockto solvent, which is similar to that of small

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 3142–3153 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: J. Hu (E-mail: [email protected]) orX. Huang (E-mail: [email protected])

3142

molecular surfactant.1 Recently, water-solubledouble hydrophilic block copolymer (DHBC) wasfound to exhibit intriguing micellization behaviorof self-assembling into one or more types of aggre-gates in water2–21 as the external conditions suchas pH,2–5 temperature7–11 and ionic strengths3,5

are properly tuned, which can selectively renderone block of DBHC water-insoluble while theother block is still solvated to stabilize the formedcolloidal aggregates. Due to the promising appli-cations in diverse fields such as coating, pharma-ceutics, colloidal stabilization and template forthe preparation of nano-materials, DHBC hasreceived great attentions.2–21 Typical hydrophilicsegments employed in double hydrophilic copoly-mers include poly(acrylic acid),9,12 poly(ethyleneglycol)2,17 and poly(N-alkylaminoethyl methacry-late).7,12,14

Theoretically, the architecture of copolymerplays an important role in determining the micel-lization properties.22,23 Previous studies showedthat nonlinear block copolymers exhibited funda-mentally different micellization properties com-pared to those of linear copolymers.24–28 Thus,studies of self-assembly behavior of copolymerswith complex architecture have attracted muchattention due to their key roles in understandingthe correlation of the structure and the propertiesand exploring new materials as well. Double-grafted copolymer, which possesses two differentside chains connected to the backbone, is a newclass of branched polymer with interesting andcomplex structure. Polystyrene-based homocenti-pedes (PS-g-PS2) and co-centipedes containing PSand polyisoprene (PI) segments (PI-g-PS2) weresynthesized by step-growth polymerization of(PS)2SiCl2 with bifunctional PS or PI.29 In addi-tion, Hirao et al. reported the synthesis of double-grafted copolymers via styrene and isoprene.30

These interesting works disclosed the relationshipbetween the complex structure and the propertiesof new grafting architecture, however, all thesecentipede-like copolymers were water-insoluble,which limited the studies of the self-assemblybehavior of these interesting structures.

Generally, three different strategies includinggrafting-through, grafting-onto and grafting-fromcan be employed to synthesize the graft copoly-mers.31 The development of atom transfer radicalpolymerization (ATRP),32–34 single-electron-trans-fer living radical polymerization (SET-LRP)35 andreversible addition-fragmentation chain transfer(RAFT)36 has enabled the preparation of copoly-mers with well-defined molecular architectures.

In our previous study,37 we firstly reported thesynthesis of amphiphilic double-grafted copolymercontaining hydrophilic poly(ethylene glycol)(PEG) and hydrophobic PS side chains by succes-sive ATRP via both grafting-through and graft-ing-from strategies. Unfortunately, only fewreports concerned with the synthesis of doublehydrophilic graft copolymers, as well as self-assembly behavior.38 In current work, we reportthe synthesis of poly[poly(ethylene glycol) methylether acrylate]-g-poly(2-(diethylamino)ethyl meth-acrylate) (PPEGMEA-g-PDEAEMA) doublehydrophilic double-grafted copolymer by succes-sive ATRP (Scheme 1). PEG is a kind of hydro-philic polymer with great biocompatibility andability to complex with mono-valence metallic cat-ion.39,40 PDEAEMA is soluble in acidic solution asa weak cationic polyelectrolyte due to the protona-tion of tertiary amine group; however, it will pre-cipitate at around neutral pH, indicatingPDEAEMA is very sensitive to pH.41 The synthe-sized graft copolymer is expected to be stimuli-responsive to both pH and salinity and its aggre-gation behavior was studied.

EXPERIMENTAL

Materials

Poly(ethylene glycol) methyl ether acrylate (PEG-MEA, Mn ¼ 454, Aldrich, 99%) was passedthrough a column filled with basic alumina. Cop-per(I) bromide (CuBr, Aldrich, 98%) was purifiedby stirring overnight over CH3CO2H at room tem-perature, followed by washing the solid with etha-nol, diethyl ether and acetone prior to drying invacuo at 40 �C for 1 day. 2-(Diethylamino)ethylmethacrylate (DEAEMA, 99%, Aldrich) waspassed through basic alumina column and distilledin vacuo from CaH2 and then stored at �20 �Cbefore use. Diisopropylamine (Aldrich, 99.5%) wasdried over KOH for several days and distilledfrom CaH2 under N2 before use. Tetrahydrofuran(THF) was dried over CaH2 for 7 days anddistilled from sodium and benzophenone under N2

before use. Pyrene (Aldrich, 99%) was recrystal-lized from methanol and stored in darkness.n-Butyllithium (n-BuLi, Aldrich, 1.6 M in hex-ane), hexamethyltriethylenetetramine (HMTETA,Aldrich, 99%), methyl 2-bromopropionate (2-MBP,Acros, 99%), tris(aminoethyl)amine (TREN,Aldrich, 96%) and 2-bromopropionyl chloride (2-BPC, Acros, 99%) were used as received. Tris

SYNTHESIS OF PPEGMEA-g-PDEAEMA COPOLYMER 3143

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

(2-(dimethylamino)ethyl)amine (Me6TREN) wasprepared from TREN according to previous litera-ture.42

Measurements

FT-IR spectra were recorded on a Nicolet AVA-TAR-360 FT-IR spectrophotometer with a resolu-tion of 4 cm�1. All NMR analyses were performedon a Bruker Avance 500 spectrometer (500 MHz)in CDCl3, DMSO-d6 and D2O, TMS (1H NMR)and CDCl3 (13C NMR) were used as internalstandards. Elemental analysis was carried out ona Carlo-Erba 1106 system. Bromine content wasdetermined by the titration with Hg(NO3)2. Con-version of DEAEMA was determined by 1H NMRin CDCl3. Relative molecular weights and molecu-lar weight distributions were measured by con-ventional gel permeation chromatography (GPC)system equipped with a Waters 1515 Isocratic

HPLC pump, a Waters 2414 refractive index de-tector and a set of Waters Styragel columns (HR3,HR4 and HR5, 7.8 � 300 mm). GPC measure-ments were carried out at 35 �C using THF as elu-ent with a flow rate of 1.0 mL/min. The systemwas calibrated with linear polystyrene standards.Steady-state fluorescent spectra of pyrene weremeasured on a Hitachi F-4500 spectrofluorometerwith the band width of 2.5 nm for excitation emis-sion, where kex was 339 nm. Hydrodynamic diam-eter (Dh) was measured by dynamic laser lightscattering (DLS) with a Malvern Nano-ZS90 Zeta-sizer. Transmission Electron Microscope (TEM)images were obtained by a JEOL JEM-1230instrument operated at 80 kV.

Homopolymerization of PEGMEA

ATRP of PEGMEA was carried out in H2O/THFunder N2 using 2-MBP as initiator and CuBr/

Scheme 1. Synthesis of PPEGMEA-g-PDEAEMA double-grafted copolymer.

3144 GU ET AL.


Me6TREN as catalytic system. To a 25 mLSchlenk flask (flame-dried in vacuo before use)sealed with a rubber septum, CuBr (0.1722 g,1.2 mmol) was added for degassing and keptunder N2. PEGMEA (10 mL, 24 mmol) and Me6T-REN (0.162 mL, 1.2 mmol) were introduced via agastight syringe with stirring. Next, the mixedsolvent, H2O/THF (v:v ¼ 10:1, 10.3 mL) whichwas purged with N2 for 10 min and mixed underN2, was added via a gastight syringe. Finally, 2-MBP (0.134 mL, 1.2 mmol) was charged via a gas-tight syringe. The solution was degassed by threecycles of freezing-pumping-thawing followed byimmersing the flask into an oil bath preset at40 �C to start the polymerization. The flask wascooled by liquid nitrogen to terminate the poly-merization after 5 h. THF was added for dilutionand the solution was filtered through an aluminacolumn to remove the copper catalyst. The result-ing solution was concentrated and precipitatedinto n-hexane. After repeated purification by dis-solving in THF and precipitating in n-hexane forthree times, 9.403 g of poly[poly(ethylene glycol)methyl ether acrylate] (PPEGMEA) 1 wasobtained with a yield of 86.3%.

GPC: Mn ¼ 5400, Mw/Mn ¼ 1.09. FI-IR (film), m(cm�1): 2869, 1728 (C¼¼O), 1455, 1109 (CAOAC),952. 1H NMR: d (ppm): 0.86 (3H, CH3CH), 1.26,1.65 (2H, CH2CH), 2.18 (1H, COCH(CH3)CH2),2.22–2.53 (1H, CH2CH), 3.39 (3H, OCH3), 3.68(4H, OCH2CH2O), 3.88 (3H, COOCH3), 4.18 (2H,COOCH2CH2O), 4.33 (1H, CHBr). 13C NMR: d(ppm): 26.0, 29.7 (CH2CH), 35.2, 38.9 (CH2CH),55.1 (CH2OCH3), 59.4–72.6 (OCH2CH2O), 174.0(COOCH2).

The conversion of PEGMEA macromonomerwas determined by 1H NMR according to previousliterature.43 The procedure was same as theafore-mentioned polymerization except that themixed solvent was changed to D2O/THF (v:v¼ 10:1). The conversion was 100% since that thesignal of double bond of PEGMEA macromonomerwas not found in 1H NMR spectrum.

Preparation of PPEGMEA-Br Macroinitiator

Lithium diisopropylamide (LDA) and 2-bromopro-pionyl chloride were used to transform PPEG-MEA 1 comb homopolymer into the macroinitia-tor. Dried THF (40 mL) and diisopropylamine(1.12 mL, 8 mmol) were added to a 500 mL sealedthree-neck flask under N2. The solution wascooled to �78 �C and n-BuLi (5 mL, 8 mmol) wasadded slowly for 1 h. Next, the mixture reacted

with PPEGMEA 1 (3.632 g, Mn ¼ 5400, Mw/Mn

¼ 1.09) in 160 mL of dried THF under �78 �C andthe reaction lasted 3 h. Finally, 2-bromopropionylchloride (0.8 mL, 8 mmol) in 10 mL of dried THFwas introduced. The reaction was terminated bywater after 3 h. The solution was concentratedand dialyzed in water for 1 day. The aqueous solu-tion was extracted by CHCl3 and dried againstNa2SO4 overnight. After filtration, the solutionwas concentrated and precipitated into n-hexane.The product was dried in vacuo to give 3.8279 g ofPPEGMEA-Br 2 macroinitiator.

GPC: Mn ¼ 5000, Mw/Mn ¼ 1.03. FT-IR (film): m(cm�1): 2871, 1734, 1453, 1351, 1105, 951, 855. 1HNMR: d (ppm): 0.86 (3H, CH3CH), 1.26, 1.71 (2H,CH2CH), 1.90 (3H, CH(CH3)Br), 2.28 (1H,COCH(CH3)CH2), 3.38 (3H, OCH3), 3.67 (4H,OCH2CH2O), 3.89 (3H, COOCH3), 4.19 (2H,COOCH2CH2O), 4.33 (1H, CHBr). 13C NMR: d(ppm): 18.7–21.0 (CH(CH3)Br), 26.2–34.0 (CH2 onPPEGMEA backbone), 41.0, 42.3 (CH on PPEG-MEA backbone), 47.1 (CH(CH3)Br), 52.2 (tert-Con PPEGMEA backbone), 54.7 (CH2OCH3), 58.6–72.4 (OCH2CH2O), 169.7-174.0 (OAC¼¼O), 211.1(C¼¼O). ELEM. ANAL. C% ¼ 50.06%, Br% ¼ 10.06%.

Graft Copolymerization of DEAEMA

PPEGMEA-g-PDEAEMA double hydrophilic dou-ble-grafted copolymer was synthesized by ATRPof DEAEMA initiated by PPEGMEA-Br 2 macro-initiator using CuBr/HMTETA catalytic system.CuBr and PPEGMEA-Br 2 macroinitiator (Mn

¼ 5000, Mw/Mn ¼ 1.03) in dried THF were addedto a 25-mL Schlenk flask (flame-dried in vacuobefore use) sealed with a rubber septum underN2. After three cycles of evacuating and purgingwith N2, DEAEMA, diphenyl ether (internalstandard of 1H NMR measurement for the conver-sion of DEAEMA) and HMTETAwere charged viaa gastight syringe. The flask was degassed bythree cycles of freezing-pumping-thawing followedby immersing the flask into an oil bath preset at60 �C. The polymerization was quenched by put-ting the flask into liquid nitrogen after certaintime. The reaction mixture was diluted by THFand passed through an alumina column to removethe residual copper catalyst. The solution wasconcentrated and precipitated into cold n-hexane(�78 �C). The white viscous solid was dried invacuo overnight to obtain the final product,PPEGMEA-g-PDEAEMA 3 graft copolymer.

FT-IR (film): m (cm�1): 2968, 2871, 1730, 1455,1383, 1267, 1239, 1150, 1068. 1H NMR: d (ppm):



0.88 (3H, CCH3), 1.05 (6H, N(CH2CH3)2), 1.17–2.08 (2H, CH2 and 1H, CH), 2.58 (4H,N(CH2CH3)2), 2.79 (2H, COOCH2CH2N), 3.39(3H, OCH3), 3.65 (4H, OCH2CH2O), 4.00 (2H,COOCH2CH2N). The data of relative molecularweights and molecular weight distributions arelisted in Table 1.

Determination of Critical Micelle Concentration

Pyrene was used as fluorescence probe to measurethe critical micelle concentration (cmc) of PPEG-MEA-g-PDEAEMA 3. Acetone solution of pyrene(1.25 � 10�3 mol/L) was added to a large amountof water until the concentration of pyrene reached6 � 10�7 mol/L. Different amounts of acetone sol-utions of graft copolymer 3 (1, 0.1 or 0.01 mg/mL)were added to water containing pyrene ([pyrene]¼ 6 � 10�7 mol/L). All fluorescence spectra withdifferent pHs and ionic strengths were recordedat 20 �C.

TEM Images

To prepare micelles, THF solution of PPEGMEA-g-PDEAEMA 3 graft copolymer (10 mg/mL) wasadded dropwise to water with vigorous stirringuntil the concentration of graft copolymer 3 was0.1 mg/mL. For TEM studies, 10 lL of micellar so-lution was deposited on an electron microscopycopper grid coated with carbon film.

RESULTS AND DISCUSSION

Preparation of PPEGMEA-Br Macroinitiator

PPEGMEA 1 comb homopolymer was synthesizedvia ATRP initiated by 2-MBP and detailed charac-

terization of PPEGMEA 1 can be found from ourprevious report.37 In current case, Me6TREN wasused as ligand instead of PMDETA so that thedisproportionation of CuBr could be neglected.Successful ATRP of PPEGMEA macromonomerwas confirmed by the unimodal and symmetricalGPC curve with narrow molecular weight distri-bution (Mw/Mn ¼ 1.09). Real molecular weight ofPPEGMEA 1 was calculated to be 9100 from 1HNMR, which is in accordance with the theoreticalvalue estimated from the feed ratio of PEGMEAto 2-MBP (20:1) and the conversion of PEGMEA(100%). Thus, every PPEGMEA 1 chain possessesabout 20 poly(ethylene glycol) side chains.

PPEGMEA-Br 2 macroinitiator was preparedby connecting ATRP initiation groups to a-carbonof ester groups using LDA and 2-BPC since estergroups of polyacrylate backbone have beenemployed to link PEG side chains. CHBr groupsof ATRP initiation groups, ester groups and CHBrend groups of PPEGMEA 1 were not affected dur-ing the reaction, which had been evidenced in pre-vious reports.44–46 Therefore, all ATRP initiationgroups were connected to the backbone throughstable CAC bonds instead of environment-sensi-tive ester linkages. The chemical structure ofPPEGMEA-Br 2 was examined by 1H and 13CNMR. The signal of three protons of newly intro-duced ACH(CH3)Br group appeared at 1.90 ppmin 1H NMR spectrum after the reaction, whichwas overlapped with the signals of polyacrylatebackbone. The possibility of elimination reactionof CHBr end group was excluded since that nosignal of alkene was detected in the regionbetween 4.5 ppm and 7.0 ppm. Moreover, a newpeak attributed to the ketone carbon ofACOCH(CH3)Br group located at 211.1 ppm in13C NMR spectrum. All these evidences verified

Table 1. Synthesis of PPEGMEA-g-PDEAEMA 3 Graft Copolymera

Sample Time (h) Conv.b (%) NDEAEMAc Mn,NMR

d (g/mol) Mw/Mne cmcf (g/mL)

3a 4 10.92 8.6 34,900 1.25 4.12 � 10�7

3b 9 16.46 15.8 54,800 1.26 3.95 � 10�7

3c 12 27.91 21.6 70,900 1.36 3.84 � 10�7

3d 15 32.86 24.4 78,700 1.39 3.62 � 10�7

a Initiated by PPEGMEA-Br 2 (Mn ¼ 5000, Mw/Mn ¼ 1.03, grafted ATRP initiation group density: 0.75/1) at 60 �C, feed ratio:[DEAEMA]:[Br group]:[CuBr]:[HMTETA] ¼ 400:1:1:1.

b Conversion of DEAEMA measured by 1H NMR.c The number of DEAEMA repeating unit per PDEAEMA side chain determined by 1H NMR.dObtained by 1H NMR.eMeasured by GPC in THF.f Critical micelle concentration determined by fluorescence spectroscopy using pyrene as probe at 15 �C when pH ¼ 12.

3146 GU ET AL.


the successful incorporation of ATRP initiationgroups.

The approximate grafted ATRP initiation groupdensity, In%, is calculated from the contents ofcarbon (C%) or bromine (Br%) determined by ele-ment analysis according to the following equa-tions. In eq 1, 48.4 and 54.9% are carbon contentsof repeating units with or without ATRP initiationgroup, respectively. In% is calculated to be 74.5%according to eq 1. In eq 2, 590 and 454 are the mo-lecular weights of repeating units with or withoutATRP initiation group, respectively. Thus, In% iscalculated to be 68.9% according to eq 2. Thesetwo results were quite close with each other, butthe first one calculated from C% is more reliabledue to the definition of analysis method. There-fore, about 15 (¼ 20 � 74.5%) ATRP initiationgroups were introduced to polyacrylate backbone.

C% ¼ 48:4%� In%þ 54:9%� ð1� In%Þ (1)

Br% ¼ 80� In%=½590� In%þ 454� ð1� In%Þ�(2)

GPC curve of PPEGMEA-Br 2 macroinitiatorshowed only a unimodal and symmetric peak withnarrow molecular weight distribution (Mw/Mn

¼ 1.03), which demonstrated that the architectureof the backbone was kept during the reac-tion.37,44–46 Real molecular weight of PPEGMEA-Br 2 was calculated to be 11,100 (¼ 9,100 þ 15 �(136-1), 136 is the molecular weight of ATRP ini-tiation group) from 1H NMR.

Synthesis of PPEGMEA-g-PDEAEMADouble-Grafted Copolymer

PPEGMEA-g-PDEAEMA 3 double-grafted copoly-mer was synthesized by ATRP of DEAEMA inbulk at 60 �C initiated by PPEGMEA-Br 2 macro-initiator through the grafting-from strategy.CuBr/HMTETA was as catalytic system for ATRPand the results are summarized in Table 1. Allgraft copolymers showed unimodal and symmetri-cal GPC curves with relatively narrow molecularweight distributions (Mw/Mn � 1.39), which illus-trated that the intermolecular coupling could beneglected.47 In current case, high feed ratio of themonomer to the initiator and low conversion ofthe monomer were performed to suppress theintermolecular coupling as reported in previousstudies.31,47–50

1H NMR signals of the corresponding protonsof both PPEGMEA and PDEAEMA segments

assured us of the structure of PPEGMEA-g-PDEAEMA 3 graft copolymer as shown in Fig-ure 1. The peaks at 3.39 ppm (terminal OCH3)and 3.65 ppm (OCH2CH2 repeating unit) belongto PEG side chains. The peaks ‘b’ (2.58 ppm) and‘c’ (2.79 ppm) are attributed to four protons ofN(CH2CH3)2 and 2 protons of COOCH2CH2Nof PDEAEMA side chains, respectively. The signalof two protons of COOCH2CH2N group (peak ‘f ’)is found to appear at 4.00 ppm.

Since the molecular weight of graft copolymermeasured by GPC is very different from the ‘real’value,51 the lengths of PDEAEMA side chains weredetermined by 1H NMR instead of GPC. The num-ber of DEAEMA repeating unit per PDEAEMAside chain (NDEAEMA) was calculated according toeq 3 (S is the peak area and 15 is the number ofATRP initiation site) as listed in Table 1. Thus, themolecular weights of graft copolymer (Mn,NMR)were obtained according to eq 4 (11,000 and185 are the molecular weight of PPEGMEA-Br 2macroinitiator and DEAEMA, respectively).

NDEAEMA ¼ ½ð20� 8:4Þ � 2Sf=Se�=15 (3)

Mn;NMR ¼ 11;000þ 15� 185�NDEAEMA (4)

Semilogarithmic plot of Ln([M]0/[M]) versustime is depicted in Figure 2 according to the dataof conversions of DEAEMA listed in Table 1,which shows the conversions of DEAEMAincreases with the time and a linear dependence

Figure 1. 1H NMR spectrum of PPEGMEA-g-PDEAEMA 3 in CDCl3.



of Ln([M]0/[M]) on the time when the feed ratio ofDEAEMA to ATRP initiation group is 400:1. Inaddition, the apparent rate coefficient (kapp ¼ 7.20� 10�6 s�1) is determined from the slope of the ki-netic plot. It is obvious that the apparent polymer-ization rate is first order with respect to the con-centration of DEAEMA, which indicated a con-stant number of propagating species during thepolymerization of DEAEMA. This phenomenonaccorded with the characteristic of ATRP.32 Fromthe afore-mentioned results, it is clear that thesynthesis of the backbone and side chains areboth controllable. The synthesized double-graftedcopolymers, PPEGMEA-g-PDEAEMA 3, possesswell-defined structure: a polyacrylate backbone(20 repeating units) with two different sidechains: one is shorter PEG (8.4 repeating unitsper chain) at each grafting site, the other is longerPDEAEMA (8.6–24.4 repeating units per chain)at most grafting sites.

pH-Responsive Self-Assembly Behavior ofPPEGMEA-g-PDEAEMA 3

PDEAEMA homopolymer is a kind of weak poly-base and pKa of its conjugated acid is 7.3.41 It iswater-insoluble in neutral or basic surroundingsand turns to be soluble in aqueous media as aweak cationic polyelectrolyte in acidic environ-ment due to the protonation of tertiary aminegroups.41 pH-regulated water was used to test thesolubility of PPEGMEA-g-PDEAEMA 3 graft co-polymer. It was found that the graft copolymerdissolved well in acidic aqueous solution (pH\ 7);however, it showed poor solubility in basic aque-

ous solution (pH [ 7). It has been reported that1H NMR can be conveniently utilized to study themicellization of stimuli-responsive copolymers,providing the structural information of which seg-ment in the copolymer is serving as the core ofthe micelle since that the mobility of the chainsacting as the core of the micelle decreased com-pared to that of free chains in solution.41 1H NMRspectrum of homogeneous acidic solution (pH ¼ 2)shows the signals of both PDEAEMA and PEGside chains, indicating complete dissolution ofboth segments in the copolymer [Fig. 3(A)]. WhenpH of the solution was increased to 12, only sig-nals of PEG segment are clearly visible in 1HNMR spectrum of the heterogeneous basic solu-tion and the signals of PDEAEMA segment cannot be observed any more [Fig. 3(B)] since thatPDEAEMA segments were ‘‘frozen’’ by the micel-lar shell (PEG segments), which testified the for-mation of micelles containing PDEAEMA coreand PEG shell.

pH-responsive self-assembly behavior of PPEG-MEA-g-PDEAEMA 3 graft copolymer was ana-lyzed by DLS, fluorescence spectroscopy andTEM. The hydrodynamic diameter (Dh) of PPEG-MEA-g-PDEAEMA 3b is obviously affected by pHof aqueous solution as shown in Figure 4. WhenpH is below 7, 3b graft copolymer molecularly dis-solved with a low Dh around 14 nm. Upon increas-ing of pH with the addition of 1 M NaOH aqueous

Figure 2. Kinetic plot for bulk ATRP of DEAEMAinitiated by 2 at 60 �C.

Figure 3. 1H NMR spectra of PPEGMEA-g-PDEAEMA 3 in D2O, pH ¼ 2 (A) and 12 (B), inser-tions are their solutions’ photos, respectively.

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solution, the micellization took place in basic envi-ronment (pH[ 7) with a much higher micelle sizeranging from 165 to 180 nm.

Pyrene was used as fluorescence probe toexplore the self-assembly behavior of PPEGMEA-g-PDEAEMA 3 graft copolymer in aqueous media.As it is well-known, fluorescence spectrum of py-rene is sensitively to the environment, especiallythe polarity of its surroundings.52 In the presenceof micelles, pyrene is solubilized within the inte-rior of the hydrophobic region, a more favorablehydrophobic microenvironment for the probe,thus the values of I1/I3 of the emission spectrum

changed sharply as a result.53 Figure 5 revealsthe effect of pH on the fluorescence intensity ratioI1/I3 of PPEGMEA-g-PDEAEMA 3b. In acidicenvironment (pH\ 7), I1/I3 ratio kept constant ataround 1.7, which indicated that pyrene probeslocated in a hydrophilic environment. This to-gether with DLS result clearly indicated thatPPEGMEA-g-PDEAEMA 3b dissolved molecu-larly in acidic environment and no micelle wasformed. When pH of the solution was raised to 7,I1/I3 ratio decreased rapidly to 1.2–1.3 denotingthe formation of micelles in basic surroundings(pH[ 7).

The cmcs of PPEGMEA-g-PDEAEMA 3 graftcopolymers were measured in basic environment(pH ¼ 12) as listed in Table 1. The values of cmcare as low as 10�7 g/mL compared to those of tra-ditional surfactants or block copolymers,54 whichare related with the branched structure and theasymmetric compositions of these double-graftedcopolymers. It is clear that PPEGMEA-g-PDEAEMA 3 formed highly stable micellar aggre-gates with low rates of dissociation in vivo due tosuch low cmc values.55 Moreover, the cmc valuesof PPEGMEA-g-PDEAEMA 3 decreased slightlywith the increasing of the molecular weights,which corresponds to the ascending content ofhydrophobic PDEAEMA side chains in basic sur-roundings. The cmc of 3b graft copolymer wasalso measured in different environments. Figure 6show the variation of the fluorescence intensity ra-tio I1/I3 as a function of the concentration of

Figure 4. Hydrodynamic diameter (Dh) of PPEG-MEA-g-PDEAEMA 3b as a function of pH measuredat 25 �C with a concentration of 3 mg/L.

Figure 5. Dependence of fluorescence intensity ratioI1/I3 of PPEGMEA-g-PDEAEMA 3b on pH at 15 �C,[pyrene] ¼ 6 � 10�7 M and [3b] ¼ 10 mg/L.

Figure 6. Dependence of fluorescence intensity ratioof pyrene emission bands on the concentration ofPPEGMEA-g-PDEAEMA 3b in different environ-ments.



PPEGMEA-g-PDEAEMA 3b in acidic (pH ¼ 2)and basic (pH ¼ 8, 10, and 12) surroundings,respectively. I1/I3 ratio fluctuated slightly between1.7 and 1.8 throughout the whole concentrationregion when pH was 2, implying that pyreneprobes located in a hydrophilic environment and3b graft copolymer dissolved well without anyaggregates. In contrast, I1/I3 ratio decreasedquickly in basic aqueous media when the concen-tration was above a certain value which corre-sponds to the cmc. In particular, the cmcmeasuredin weak basic environment (pH ¼ 8) is almost 30times higher than those measured in strong basicsurroundings (pH ¼ 10 and 12). This differencewas led by the different deprotonation degree ofPDEAEMA side chains in different environments:the fully deprotonated PDEAEMA segments instrong basic solution (pH ¼ 10 and 12) are morehydrophobic than the partially deprotonatedPDEAEMA segments in weak basic solution (pKa

of its conjugated acid is just 7.3).41

The micellar structures formed by PPEGMEA-g-PDEAEMA 3b in different environments werevisualized by TEM (Fig. 7). Large compoundmicelles (about 150–700 nm) with visible interiorstructure were formed in weak basic solution (pH¼ 8). However, the aggregates formed in strongbasic environment (pH ¼ 10 and 12) are coexistedlarge compound micelles (about 200–700 nm)with blurry interior structure and tubal micelles(about 50–100 nm). This difference was alsocaused by the different deprotonation degree ofPDEAEMA side chains in different environments:fully deprotonated PDEAEMA segments withhigher hydrophobicity formed the micelles withcompact structure in strong basic solution (pH¼ 10 and 12) and the lower hydrophobicity of par-tially deprotonated PDEAEMA segments inweak basic solution (pH ¼ 8) resulted in the

micelles with loose structure. All these evidencesclearly pointed to pH sensitivity of PEGMEA-g-PDEAEMA 3 graft copolymer.

Effect of Salinity on Micellization Behavior ofPPEGMEA-g-PDEAEMA 3

To confirm whether PPEGMEA-g-PDEAEMA 3graft copolymer can form aggregates in purewater, fluorescence spectrum of pyrene in aqueoussolution of PPEGMEA-g-PDEAEMA 3 preparedby pure water was measured. It was found fromFigure 8 that pH of aqueous solution began toincrease slightly when the concentration of graftcopolymer exceeded 10�6 g/mL and I1/I3 ratiodecreased sharply when pH was above 6, whichshowed the formation of aggregates. Thus, thesalt effect on the self-assembly behavior of PPEG-MEA-g-PDEAEMA 3 graft copolymer can be

Figure 7. TEM images of aggregates formed by PPEGMEA-g-PDEAEMA 3b in dif-ferent environments, [3b] ¼ 0.1 mg/mL, (A) pH ¼ 8, (B) pH ¼ 10, and (C) pH ¼ 12.

Figure 8. Fluorescence intensity ratio and pH ver-sus the concentration of PPEGMEA-g-PDEAEMA 3bat 20 �C.

3150 GU ET AL.


examined by fluorescence probe technology inNaCl-added aqueous solution, which can excludethe influence of pH.

Figure 9 shows the relationship between thecmc of PPEGMEA-g-PDEAEMA 3b and the con-centration of NaCl. When the concentration ofsalt was below 0.2 M, the coordination of tertiaryamine groups of PDEAEMA side chains with Naþ

led to the less hydrophobicity and cmc increasedwith the elevating of the concentration of NaCl.The cmc of 3b graft copolymer can not be meas-ured when the concentration of salt exceeded 0.2M, implying the complete dissociation of aggre-gates and 3b graft copolymer molecularly dis-solved because that PDEAEMA side chainsturned to be hydrophilic caused by the completeionization with Naþ. This deduction was con-firmed by the appearance of the signals of both

PDEAEMA and PEG side chains in 1H NMR spec-trum.

From the above results, we can conclude thatPPEGMEA-g-PDEAEMA 3 graft copolymer issensitive to both pH and salinity as shown inScheme 2. It dissolved molecularly in acidic envi-ronment (pH\ 7) and aggregated to form reversi-ble micelles in basic surroundings (pH[ 7) whichcan be conveniently converted to the unimers byadding salt at room temperature.

CONCLUSIONS

In summary, PPEGMEA-g-PDEAEMA doublehydrophilic graft copolymer bearing two differenthydrophilic side chains was synthesized by thecombination of ATRP and the grafting-from strat-egy. PDEAEMA polyelectrolyte side chains wereconnected to the backbone through stable CACbonds instead of ester connections, which is toler-ant of both acidic and basic environment. Thiskind of double hydrophilic double-grafted copoly-mer was sensitive to both pH and salinity. It canform reversible aggregates in basic environment.In particular, the addition of NaCl led to the easydissociation of the aggregates into the unimers atroom temperature.

PPEGMEA-g-PDEAEMA double hydrophilicgraft copolymer would be particularly interestingfor different applications including drug carrier,biological vectors and protective shells for sensi-tive enzymes. Most importantly, this kind of stim-uli-responsive polymer is extra attractive for bio-logical applications due to the large amount of pHvariations in normal or infected tissues.

Figure 9. Dependence of cmc of PPEGMEA-g-PDEAEMA 3b on the concentration of NaCl.

Scheme 2. Reversible micellization behavior of PPEGMEA-g-PDEAEMA doublehydrophilic double-grafted copolymer.



The authors thank the financial support from NationalNatural Science Foundation of China (50873029),Shanghai Rising Star Program (07QA14066), ShanghaiScientific and Technological Innovation Project(08431902300) and Knowledge Innovation Program ofChinese Academy of Sciences.

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PPEGMEA-g-PDEAEMA: Double hydrophilic double-grafted copolymer stimuli-responsive to both pH and salinity