A starlike amphiphilic graft copolymer with hydrophilic poly(acrylic acid) backbones and hydrophobic...

A Starlike Amphiphilic Graft Copolymer with HydrophilicPoly(acrylic acid) Backbones and HydrophobicPolystyrene Side Chains

DAN PENG,1,2 CHUN FENG,1 GUOLIN LU,1 SEN ZHANG,1 XIAOHUAN ZHANG,1 XIAOYU HUANG1

1Laboratory of Polymer Materials and Key Laboratory of Organofluorine Chemistry, Shanghai Institute of OrganicChemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, People’s Republic of China

2New Material Research Institute of Shandong Academy of Sciences, 19 Keyuan Road, Ji’nan, Shandong 250014,People’s Republic of China

Received 5 January 2007; accepted 21 March 2007DOI: 10.1002/pola.22118Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A well-defined starlike amphiphilic graft copolymer bearing hydrophilicpoly(acrylic acid) backbones and hydrophobic polystyrene side chains was synthesizedby successive atom transfer radical polymerization followed by the hydrolysis of poly-(methoxymethyl acrylate) backbone. A grafting-from strategy was employed for thesynthesis of a graft copolymer with narrow molecular weight distribution. Hydropho-bic polystyrene side chains were connected to the backbones through stable C��Cbonds. The poly(methoxymethyl acrylate) backbones can be easily hydrolyzed withHCl without affecting the hydrophobic polystyrene side chains. This kind of amphi-philic graft copolymer can form stable sphere micelles in water. The sizes of themicelles were dependent on the ionic strength and pH value. VVC 2007 Wiley Periodicals,

Inc. J Polym Sci Part A: Polym Chem 45: 3687–3697, 2007

Keywords: amphiphilic; ATRP; graft copolymers; starlike; synthesis

INTRODUCTION

Self-assembly behaviors of amphiphilic copoly-mers have attracted much attention during thepast decade,1–3 due to the potential applicationsin the fields of catalysis,4 drug delivery,5,6 bio-reactor7 and others.8,9 Various micelle morpholo-gies, including spheres, cylinders, bilayers andvesicles, and so forth, have been observed foramphiphilic block and linear-graft copolymers inselective solvents.10–12 A starlike polymer has dif-ferent properties in comparison with a linearpolymer of similar composition because of the

branched architecture and high degree of chainend functionality.13 Recently, many efforts weremade to effect the synthesis of well-defined starpolymer, which was significant for the explorationof the relationship between the structure and theproperty.14–18 Compared with block and linear-graft copolymers, starlike amphiphilic graftcopolymers possess additional complexity becauseof the complicated and confined structure con-taining multiple arms connected to the centralcore and dense branches grafting from the arms.

Generally, three strategies, including grafting-through, grafting-onto, and grafting-from, wereused to synthesize the graft copolymers. Theproducts of the first strategy usually have a broaddistribution of chain-length because they wereprepared by the conventional radical polymeriza-tion of macromonomers.19–21 A coupling reaction

Correspondence to: X. Huang (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 3687–3697 (2007)VVC 2007 Wiley Periodicals, Inc.

3687

was employed to connect the side chains onto thebackbone in the second strategy, which often suf-fered from the insufficient grafting efficiency dueto the low reactivity of functional groups of thepolymer chains.22–24 Since the pioneering work ofMatyjaszewski, Percec, and Sawamoto, atomtransfer radical polymerization (ATRP) has beenextensively developed.25–28 ATRP is a particu-larly attractive controlled/living radical poly-merization and has been proved to be a highlyversatile method to synthesize the polymers withwell-defined structure including the controllablemolecular weight and narrow molecular weightdistribution. The last strategy, grafting-from, hasseveral advantages such as high grafting densityand narrow molecular weight distribution of theresulting polymers because the side chains can beformed via ATRP initiated by the pendant initiat-ing groups on the backbone.29,30

According to the features of the syntheticapproaches of starlike polymers, two differentmethods including ‘‘arm-first’’ and ‘‘core-first,’’were employed to synthesize the starlike poly-mers. For the arm-first strategy, a coupling reac-tion31–33 or a multi-vinyl crosslinking agent34–36

was employed to form the starlike polymer afterthe preparation of the linear arms. Recently, thesynthesis of 3-arm and 4-arm starlike polymerwas reported by the combination of ATRP andclick reaction, which was characteristic of highspecificity and had almost quantitative yield.31

However, slow diffusion of the polymer chainsand ‘‘dead’’ polymer chains made some coresincompletely coupled with the arms so that theresulting product contained the starlike poly-mers with different numbers of arms. On theother hand, when a multi-vinyl crosslinkingagent was used to construct the starlike polymer,the number of the arms per starlike polymer wasjust a statistical average value. For the core-firststrategy, multi-initiating groups of the core mole-cule initiated the polymerization to form thearms and the number of the arms was deter-mined by the number of the initiating sites ofthe core molecule.37–39

In 2003, Matyjaszewski and his coworkersreported the first preparation of nonlinearbrushes with three- and four-arm stars, whichwere synthesized by grafting n-butyl acrylatefrom poly(2-bromopropionyloxyethyl methacry-late) macroinitiator.40 Both the backbone and theside chains were hydrophobic. In addition, theside chains were connected to the backbonethrough the ester groups, which could be cleaved

under acidic and basic conditions. Now, wereport the preparation of novel starlike amphi-philic graft copolymers with hydrophilic poly(acrylic acid) arms and hydrophobic polystyrenebranches via the combination of ATRP, grafting-from strategy and core-first technique (Scheme1), whose branches were connected to the back-bone through stable C��C bonds instead of esterconnections.

Moreover, the critical micelle concentrations(cmc) in aqueous solution were determined bythe fluorescence spectroscopy. The micellar mor-phologies and sizes were studied through trans-mission electron microscopy (TEM). Recently, ithas been shown that the architecture of thecopolymer is the key to tune the micelle mor-phologies.13,41–44 This study may present moreinformation about the relationship between thestructure and the self-assembly behaviors ascompared with those of the amphiphilic blockand the linear-graft copolymers, which willimprove our ability to tune the polymer’s proper-ties through chemical design.

EXPERIMENTAL

Materials

Styrene (St, Aldrich, 99%) was washed with 5%aqueous NaOH solution to remove inhibitor, thenwith water, dried over CaCl2 and distilled twiceover CaH2 under reduced pressure just before use.Copper(I) bromide (CuBr, Aldrich, 98%) was puri-fied by stirring overnight over CH3CO2H at roomtemperature, followed by washing the solid withethanol, diethyl ether and acetone prior to dryingat 40 8C under vacuum for 1 day. Diisopropyl-amine (Aldrich, 99.5%) was dried over KOH forseveral days followed by distilling from CaH2

under N2 atmosphere. Tetrahydrofuran (THF)was dried over CaH2 for several days and distilledfrom sodium and benzophenone under N2 atmos-phere before use. n-Butyllithium (n-BuLi, Aldrich,1.6 M in hexane), N,N,N0,N0,N@-pentamethyldie-thylenetriamine (PMDETA, Aldrich, 99%) and a-bromoisobutyryl bromide (TCI, 98%) were used asreceived. Chloromethyl methyl ether45 and penta-erythritol tetrakis(2-bromopropionate) (4BrPr)14

were synthesized according to literatures.

Measurements

FT-IR spectra were recorded on a Nicolet AVA-TAR-360 FT-IR spectrophotometer with 4 cm�1

3688 PENG ET AL.

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

resolution. All 1H-NMR and 13C-NMR analyseswere performed on a Varian Mercury 300 spec-trometer (300 MHz) in CDCl3 with TMS (1H-NMR) or CDCl3 (13C-NMR) as internal standard.Relative molecular weights and molecular weightdistributions were determined by gel permeationchromatography (GPC) system equipped with aWaters 1515 Isocratic HPLC pump, a Waters2414 refractive index detector (RI), a Waters2487 dual absorbance detector and a set ofWaters Styragel columns (HR3, HR4 and HR5,7.8 3 300 mm). GPC measurements were carried

out at 35 8C using THF as eluent with a 1.0 mL/min flow rate. The system was calibrated withpolystyrene standards. Bromine content wasdetermined by titration with Hg(NO3)2. Conver-sion of St was determined by GC using HP 6890system with SE-54 column. TEM images wereobtained by Philips CM120 operated at 80 kV.

Synthesis of Methoxymethyl Acrylate

Triethylamine (30 mL, 216 mmol) and acrylicacid (13 mL, 189 mmol) were added to 200 mL

Scheme 1. Synthesis of starlike amphiphilic graft copolymer.

SYNTHESIS OF AN AMPHIPHILIC GRAFT COPOLYMER 3689


dried dichloromethane at room temperature.Next, chloromethyl methyl ether (15 mL, 200mmol) was added dropwise and the solution wasstirred at room temperature until thin layerchromatography (TLC) showed the completion ofthe reaction. The solvent was removed by rotaryevaporation and the residue was purified by dis-tillation under reduced pressure to give the prod-uct (16.7826 g, 73% yield).

FT-IR (film): m (cm�1): 3003 (¼¼C��H), 2960(��C��H), 1732 (C¼¼O), 1299, 1264, 1202, 1153,1095, 1025(C��O��C), 932. 1H-NMR (CDCl3): d(ppm): 3.50 (s, 3H, CH3O), 5.33 (s, 2H, COO-CH2O), 5.90 and 6.50 (dd, 1H CH2 ¼ CH), 6.17(dd, 1H, CH2¼¼CH).

Polymerization of MOMA by ATRP

Starlike poly(methoxymethyl acrylate) (PMOMA)1 was prepared by ATRP of MOMA initiated by4BrPr. A typical procedure is listed as follows:CuBr (0.0920 g, 0.64 mmol) was firstly added toa 25 mL Schlenk flask (flame-dried undervacuum just before use) sealed with a rubberseptum for degassing and kept under N2 atmos-phere. Next, MOMA (3.0 mL, 25.7 mmol) andPMDETA (0.13 mL, 0.70 mmol) were introducedvia a gas-tight syringe and the mixture wasstirred for several minutes. Finally, 4BrPr(0.1079 g, 0.16 mmol) was charged via a gas-tight syringe. The solution was degassed bythree cycles of freeze-pump-thaw followed byimmersing the flask into an oil bath thermo-stated at 50 8C to start the polymerization. Thepolymerization was terminated by putting theflask into liquid nitrogen after 3 h. THF wasadded to dissolve the viscous crude product andthe solution was filtered through a short Al2O3

column to remove the copper catalyst. Theresulting solution was concentrated and precipi-tated in hexane. After repeated purification bydissolving in THF and precipitating in hexanefor thrice, 0.7363 g glassy solid was obtainedwith a yield of 25%, Mn ¼ 11,000, Mw/Mn ¼ 1.17.

FT-IR (film): m (cm�1): 2958 (��C��H), 1740(C¼¼O), 1146, 1088 (C��O��C), 927. 1H-NMR(CDCl3): d (ppm): 1.09 (d, 12H, CH3CH), 1.51,1.68, 1.96 (br, 2H, CH2��CH), 2.35 (br, 1H,CH2��CH), 3.41 (s, 3H, OCH3), 3.95, 4.07 (s, 8H,OCH2C), 4.27 (t, 1H, CHBr), 5.14 (s, 2H, COO-CH2O). 13C��NMR (CDCl3): d (ppm): 33.3-36.9(CH2CH), 41.4 (CH2CH), 56.8 (��OCH3), 92.6(COOCH2O), 173.9 (COOCH2). ELEM. ANAL: Br%:2.13%.

Synthesis of PMOMA-Br Macroinitiator 2

Dried THF (20 mL) and diisopropylamine(1.3 mL, 9.0 mmol) were added to a sealed250 mL three-neck flask. The solution was cooledto �78 8C and n-BuLi (1.6 M, 4.4 mL, 7.0 mmol)was added slowly. After 1 h, the mixture wastreated with starlike PMOMA 1c (0.6221 g,Mn ¼ 11,000, Mw/Mn ¼ 1.17) in 100 mL driedTHF under �78 8C. The reaction lasted for 7 h,followed by the introduction of a-bromoisobutyrylbromide (1.1 mL, 9.0 mmol). After 2 h, the reac-tion was terminated by water. The organic phasewas washed with water and brine, and driedover MgSO4 overnight. After filtration, the fil-trate was concentrated and precipitated in hex-ane. The product was dried under vacuum togive 0.5581 g yellow powder (Mn ¼ 8200, Mw/Mn

¼ 1.25).FT-IR (film): m (cm�1): 2961, 1740, 1260, 1214,

1158, 1093, 926. 1H-NMR (CDCl3): d (ppm):1.01–3.13 (br, protons on PMOMA backbone),1.97 (s, 6H, C(CH3)2Br), 3.48 (s, 3H, ��OCH3),5.26 (s, 2H, COOCH2O). 13C-NMR (CDCl3): d(ppm): 19.1, 21.2 (C(CH3)2Br), 24.6–36.3(CH2CH), 39.1, 41.2 (CH2CH), 47.9 (tert-C onPMOMA backbone), 55.0 (C(CH3)2Br), 57.8(OCH3), 93.2 (COOCH2O), 170.5, 174.0(O��C¼¼O), 207.4 (C��C¼¼O). ELEM. ANAL: Br%:10.49%.

The structure of PMOMA -Br macroinitiator 2is as indicated in Scheme 2. The molecularweight of x part (incorporated with ATRP initia-tion group) is 265 and the molecular weight of ypart is 116. The bromine content was 10.49%,the ratio of x/y is calculated to be 1/4 accordingto eq 1, which means the approximate graftedATRP initiation group density of PMOMA-Brmacroinitiator 2 is 1/5.

80x=ð265xþ 116yÞ ¼ 0:1049 ð1Þ

Scheme 2. Chemical structure of macroinitiator 2.

3690 PENG ET AL.


Synthesis of Starlike Graft CopolymerPMOMA-g-PS 3

A dried 25-mL Schlenk flask was charged withCuBr (0.0056 g, 0.04 mmol). The flask wasdegassed and CuBr was kept under N2 atmos-phere. Next, PMDETA (0.01 mL, 0.05 mmol) andSt (3.0 mL, 26.2 mmol) were introduced via agas-tight syringe. Finally, PMOMA-Br 2 (0.0304g, Mn ¼ 8200, Mw/Mn ¼ 1.25, Br% ¼ 10.49%)was added. The flask was degassed by threecycles of freeze-pump-thaw and followed byimmersing into an oil bath thermostated at110 8C. After 30 min, the polymerization was ter-minated by putting the flask into liquid N2. Thereaction mixture was diluted by THF andfiltered through a short Al2O3 column to removethe copper catalyst. After concentration, starlikePMOMA-g-PS 3 was obtained by precipitationin methanol and dried under vacuum (0.0793 g,Mn ¼ 46,000, Mw/Mn ¼ 1.39).

FT-IR (film): m (cm�1): 3082, 3060, 3026, 2962,2924, 2850, 1736, 1601, 1493, 1452, 1366, 1155,1086, 1028, 907, 757, 699. 1H-NMR (CDCl3): d(ppm): 1.44 (br, 2H, CH2), 1.86 (br, 1H, CH),2.32, 2.70 (CH on PMOMA backbone), 3.42 (s,3H, OCH3), 4.49 (br, 1H, CH(Ph)Br), 5.22 (s, 2H,COOCH2O), 6.58, 7.05 (br, 5H, C6H5).

Hydrolysis of Starlike PMOMA-g-PS 3

Starlike PMOMA-g-PS 3 was treated with excess1 M HCl in THF for 2 h at room temperature.The reaction mixture was washed with wateruntil the aqueous phase became neutral andthen washed with brine and dried over MgSO4.After filtration, the filtrate was concentrated andprecipitated in hexane. The final white solid wasdried under vacuum.

FT-IR (film): m (cm�1): 3400, 3082, 3060, 3026,2923, 2849, 1713, 1601, 1493, 1452, 757, 699.

1H-NMR (CDCl3): d (ppm): 1.43 (br, 2H, CH2),1.85 (br, 1H, CH), 2.32, 2.70 (CH on PAA back-bone), 4.45 (br, 1H, CH(Ph)Br), 6.59, 7.09 (br,5H, C6H5).

For GPC analysis, PAA backbone was repro-tected to poly(methyl acrylate) backbone byCH2N2, as shown in Scheme 3.

Determination of Critical Micelle Concentration

Pyrene was used as fluorescence probe. The ace-tone solution of pyrene (60 mg/L, 2.97 3 10�4

mol/L) was added to water until the concentra-tion of pyrene reached 5 3 10�7 mol/L. Next, dif-ferent amounts of starlike PAA-g-PS 4 (hydroly-sis product of starlike PMOMA-g-PS 3) solutionsin THF (1 mg/mL) were added to water contain-ing pyrene. All fluorescence spectra wererecorded at 25 8C.

TEM Images

To prepare micelles, starlike PAA-g-PS 4 solutionin THF (1 mg/mL) was added dropwise to waterwith vigorous stirring until the final concentra-tion was 0.05 mg/mL. The solution was stirredfor another several hours for the evaporation ofTHF. For TEM studies, a drop of micellar solu-tion was deposited on an electron microscopycopper grid coated with carbon film and thewater evaporated at room temperature.

RESULTS AND DISCUSSION

Synthesis of PMOMA Backbone

We selected methoxymethyl acrylate as the mono-mer to build the backbone due to its mild acidichydrolysis conditions. It was reported that mono-

Scheme 3. Reprotection of poly(acrylic acid) backbone.



functional initiator (e.g., methyl 2-bromopionate)can initiate ATRP of MOMA to prepare linearPMOMA.46,47 In this work, we used a kind of tet-rafunctional initiator, pentaerythritol tetrakis(2-bromopropionate), to initiate ATRP of MOMA toform four-arm starlike PMOMA backbone.

The molecular weight distributions were keptnarrow (Mw/Mn < 1.20) as shown in Table 1.However, a shoulder peak appeared in GPCcurve of 1a, which resulted from the side reac-tions at high conversion. When the conversionwas kept below 40%, symmetrical and narrowdispersed peaks were obtained (1b and 1c).

Figure 1 shows 1H-NMR spectrum of four-armstarlike PMOMA; ATRP mechanism was con-firmed by the peaks ‘a,’ ‘e,’ and ‘f,’ which belongto the protons of the initiation groups and thechain end groups, respectively.

We calculated PMOMAs’ molecular weightsas shown in Table 1 from 1H-NMR data accord-

ing to eq 2 (S is the peak area, and 116 is themolecular weight of MOMA):

Mn;NMR ¼ 4 3Sg

Se

� �3 116 ð2Þ

For PMOMA 1b, every arm has 28 MOMArepeating units; for PMOMA 1c, every arm has36 MOMA repeating units.

Synthesis of Macroinitiator

Generally, graft copolymers prepared by ATRPhad a hydrophobic backbone and hydrophilicside chains because the ester groups of the back-bone were easy to be converted to the halogen-containing ATRP initiation groups.29,48 To retainester groups so that they can be hydrolyzed tocarboxylic groups, another method was used toconnect ATRP initiation groups to the a-carbonof the ester groups of polyacrylate backboneusing lithium diisopropylamide (LDA) and a-bro-moisobutyryl bromide as shown in Scheme 1.Ester groups of polyacrylate backbone and C-Brgroups of ATRP initiation groups were supposed

Table 1. ATRP of MOMA Initiated by 4BrPr

PMOMA[MOMA]:[CuBr]:[PMDETA]:[In]a

T(8C)

t(h)

Mnb

(g/mol) Mw/Mnb

Mn,NMRc

(g/mol)Yield(%) Br%d

1a 17:1:1:1 50 3 8200 1.16 – 60 –1b 34:1:1:1 50 2 11,000 1.08 13,000 33 2.341c 40:1:1:1 50 3 11,000 1.17 17,000 25 2.13

a [In] ¼ 43[4BrPr].b Measured by GPC.c Obtained by 1H NMR.d Obtained by titration with Hg(NO3)2.

Figure 1. 1H-NMR spectrum of starlike PMOMA 1.Figure 2. 1H-NMR spectrum of PMOMA-Br 2 mac-roinitiator.

3692 PENG ET AL.


to be not affected during the reaction, asevidenced from the previous similar reactionshown below.49

By this approach, starlike PMOMA 1 was suc-cessfully transformed into the macroinitiatorand ATRP initiation groups were connected topolyacrylate backbone through stable C��Cbonds instead of ester connections.

1H-NMR spectrum of PMOMA-Br 2 is shownin Figure 2. We can find all the correspondingsignals of the protons of PMOMA backbone. Itwas found that the integration area of the peakat 1.97 ppm obviously increased because of thenewly introduced 6 protons of COC(CH3)2Br ofATRP initiation groups. Also, no signal of alkenewas found in the region between 4.5 ppm to 7.0ppm, which meant that the possible eliminationreaction of CHBr end group did not occur duringthe chemical modification with LDA and a-bro-moisobutyryl bromide.

13C-NMR spectrum of PMOMA-Br 2 is shownin Figure 3. The appearance of a new peak at207.4 ppm (the ketone carbon of ��CO(CH3)2Br)also indicated the successful synthesis of four-arm macroinitiator 2.

As we can see from Figure 4, only a unimodalpeak was found in GPC curve after the reactionwith LDA and a-bromoisobutyryl bromide. Themolecular weight distribution kept narrow (Mw/Mn ¼ 1.25), which meant the architecture ofpolymer chain was not destroyed. The Mn

of PMOMA-Br 2 macroinitiator (Mn ¼ 8200)was a little smaller than that of PMOMA 1c(Mn ¼ 11,000), due to the newly branched ATRPinitiation groups, which was similar to our pre-vious studies.46,47

The successful introduction of ATRP initiationgroups to PMOMA backbone was also confirmedby the increase of bromine content of PMOMA 2

Figure 3. 13C-NMR spectrum of PMOMA-Br 2 macroinitiator.

Figure 4. GPC traces of starlike PMOMA 1c,PMOMA-Br 2, starlike PMOMA-g-PS 3, and repro-tected starlike PAA-g-PS 4 in THF.



(2.13% to 10.49%) prepared from PMOMA 1c af-ter the reaction. Since the peak of the protons ofCOC(CH3)2Br overlapped with that of PMOMAbackbone, it was difficult to calculate the densityof the grafted ATRP initiation group from 1H-NMR. From the result of bromine content, weobtained the approximate grafted ATRP ini-tiation group density (1/5), which meant ATRPinitiation groups were introduced to one fifth ofrepeating units of PMOMA backbone. Becauseevery arm of PMOMA 1c has 36 MOMA repeat-ing units, PMOMA-Br 2 had about 28 newCOC(CH3)2Br ATRP initiation groups after thereaction with LDA and a-bromoisobutyryl bro-mide. Also every arm of PMOMA 1c has a CHBrend group which should be kept during the reac-tion, PMOMA-Br macroinitiator 2 should haveabout 32 ATRP initiation groups in total.

Graft Copolymerization of Styrene

The starlike graft copolymer, PMOMA-g-PS 3,was synthesized by ATRP of styrene in bulk at110 8C initiated by PMOMA-Br 2 (Table 2). Thestarlike graft copolymer’s molecular weight was

much higher than that of PMOMA-Br 2, whichmeant styrene was initiated for polymerization.

The starlike graft copolymer showed a unimo-dal and symmetrical GPC curve, which is char-acteristic of ATRP25 and also indicated thatintermolecular coupling reactions could be ne-glected.46 A high feed ratio of monomer to ATRPinitiation group (500:1) and a low conversion ofmonomer (2.60%) were necessary to suppress theintermolecular coupling reactions in our system,which was same in the synthesis of linear graftcopolymer.29,46,50,51

The structure of starlike PMOMA-g-PS 3 wascharacterized by FT-IR and 1H-NMR, respec-tively. Figure 5 (plot A) shows FT-IR spectrum ofstarlike PMOMA-g-PS 3. The typical signals ofpolystyrene side chains were found to appear at3026, 1601, 1493 and 1452 cm�1. Also, the sharpbands at 757 and 699 cm�1 confirmed the mono-substituted benzene ring of polystyrene segment.The peaks at 1736 and 1155 cm�1 can be attrib-uted to polyacrylate segment.

1H-NMR signals of the corresponding protonsof both PMOMA backbone and PS side chainsalso convinced us of the structure of starlikePMOMA-g-PS 3 (Fig. 6). A new peak appearedat 4.40 ppm (peak a), which is the signal of pro-ton of ��CH(Ph)Br end group, also confirmingATRP mechanism of the polymerization of St.

Table 2. Synthesis of PMOMA-g-PS Starlike Graft Copolymer Initiated by 2a

Time Mnb (g/mol) Mw/Mn

b Conv.c (%) Mn,NMRd NMOMA/NSt

d

3 30 min 46,000 1.39 2.60 181,000 1/11

a [St]:[Br group]:[CuBr]:[PMDETA] ¼ 500:1:1:1, polymerization temperature: 110 8C.b Measured by GPC.c Determined by GC.d Obtained by 1H NMR.

Figure 5. FT-IR spectra of starlike PMOMA-g-PS 3(A) and starlike PAA-g-PS 4 (B).

Figure 6. 1H-NMR spectrum of starlike PMOMA-g-PS 3.

3694 PENG ET AL.


The ratio of repeating units of PMOMA back-bone to PS side chains (NMOMA/NSt) was calcu-lated according to eq 3 (S is the peak area):

NMOMA=NSt ¼ ðSb þ ScÞ=Sphenyl ð3Þ

From the value of NMOMA/NSt and the molecu-lar weight of PMOMA, the molecular weight ofstarlike graft copolymer was also calculatedaccording to eq 4 (Mn,NMR of PMOMA backboneis 17,000), which was much higher than thatdetermined by GPC because the molecularweight of branched polymer is usually underesti-mated by GPC due to the lower hydrodynamicradius.52

Mn;NMR ¼ 17; 000þ ðNSt=NMOMAÞ 3 104 3 34 3 4

ð4Þ

Hydrolysis of Starlike PMOMA-g-PS

Hydrolysis of PMOMA backbone with HCl wascarried out at room temperature for 2 h. Figure7 shows 1H-NMR spectrum of the hydrolyzedproduct. The disappearance of the signals of theprotons of methoxymethyl assured the completehydrolysis of PMOMA backbone. 1H-NMR sig-nals of the corresponding protons of PS sidechains still existed. Additionally, the peak of theproton of ��CH(Ph)Br end group appeared at4.40 ppm (peak a).

The appearance of a new broad peak of��COOH at 3400 cm�1 and the shift of C¼¼O to1713 cm�1 in FT-IR spectrum of the productafter hydrolysis [Fig. 5 (plot B)] indicated theformation of PAA backbone. The typical signalsof polystyrene side chains were also foundto appear at 3026, 1601, 1493,1452, 757, and699 cm�1.

Because 4 arms were connected to the centercore with 4 ester groups, the effect of hydrolysisconditions on these ester groups should be stud-ied. PAA backbone was reprotected by CH2N2 tobe transformed into poly(methyl acrylate) back-bone for GPC measurement. A unimodal GPCcurve, whose shape and position (Mn ¼ 45,000,Mw/Mn ¼ 1.36) were almost identical with thatof PMOMA-g-PS before hydrolysis (Mn ¼ 46,000,Mw/Mn ¼ 1.39), was observed in Figure 4. So,the structure of starlike graft copolymer was notdestroyed during the hydrolysis process. All thisevidence confirmed the structure of starlikePAA-g-PS 4.

Critical Micelle Concentration of Morphologyof Micelles

Fluorescence probe technique53,54 was used tomeasure the critical micelle concentration (cmc)of starlike PAA-g-PS 4 in water. In the presenceof micelles, pyrene is solubilized within the inte-rior of the hydrophobic part. The intensity ratios(I1/I3) against the logarithm of polymer concen-trations were plotted to determine cmc as theonset of micellization (Fig. 8).

The cmc value is 3.79 3 10�7 g/mL, which iscomparable with that of linear PAA-g-PS graftcopolymer with similar composition.46 It seemsthat further star-branch would not facilitate theformation of micelles because linear graft copoly-mer had a high degree of branch.

The micelles formed by PAA-g-PS 4 werespheres [Fig. 9(A)], which was same as that of

Figure 7. 1H-NMR spectrum of starlike PAA-g-PS 4.

Figure 8. Dependence of fluorescence intensityratios of pyrene emission bands on the concentrationsof PAA-g-PS 4.



linear PAA-g-PS with similar composition.46

Since poly(acrylic acid) backbone is a kind of pol-yelectrolyte, the self-assembly behavior in aque-ous solution should be related with the ionicstrength and pH values. We studied the micellarmorphologies and sizes in NaCl and NaOH solu-tions, respectively.

Figure 9(B) presents TEM image of micelles ofPAA-g-PS 4 in 5% NaCl solution. With theincrease of ionic strength, the sphere micellesbecame bigger (� 180 nm) than that in water(� 50 nm). It can be explained that Naþ screenedthe electronic repulsion among the hydrophilicpoly(acrylic acid) backbone, so the hydrophobicside chains were easier to aggregate to form bigmicelles.55 The addition of NaOH to water pro-moted the ionization of PAA and the charge den-sity on the hydrophilic backbone increased,which prevented the aggregation of side chains,so the smaller micelles [� 30 nm, Fig. 9(C)]formed in basic solution compared with that inneutral solution.56

CONCLUSIONS

In summary, the synthesis of well-definedamphiphilic star-graft copolymer with hydro-philic poly(acrylic acid) backbone and hydropho-bic polystyrene side chains was reported. Thehydrophobic side chains were connected to thebackbone through stable C��C bonds. The armnumber can be regulated by the multifunctionalinitiator. The micelles formed by starlike PAA-g-

PS were spheres, the addition of NaCl andNaOH to water had no effect on the micellarmorphology, but increased the sizes.

The authors thank the National Natural ScienceFoundation of China (grant no.: 20404017) for finan-cial support.

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Figure 9. TEM images of micelles of starlike PAA-g-PS 4 in water (A), in 5% NaClsolution (B) and in NaOH solution, pH ¼ 12.25 (C).

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A starlike amphiphilic graft copolymer with hydrophilic poly(acrylic acid) backbones and hydrophobic...

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