Optimized water-based ATRP of an anionic monomer: Comprehension and properties characterization

Optimized Water-Based ATRP of an Anionic Monomer:Comprehension and Properties Characterization

ROSICA MINCHEVA,1,2 DILYANA PANEVA,1 LAETITIA MESPOUILLE,2 NEVENA MANOLOVA,1

ILIYA RASHKOV,1 PHILIPPE DUBOIS2

1Laboratory of Bioactive Polymers, Institute of Polymers, Bulgarian Academy of Sciences, Acad. G. Bonchev str., Bl.103A, 1113 Sofia, Bulgaria

2Laboratory of Polymeric and Composite Materials, Center of Innovation and Research in Materials and Polymers(CIRMAP), University of Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium

Received 14 October 2008; accepted 1 December 2008DOI: 10.1002/pola.23222Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The effect of pH and the ligand nature over the atom transfer radicalpolymerization (ATRP) of the anionic monomer sodium 2-acrylamido-2-methylpropa-nesulfonate (AMPSNa) was investigated in aqueous medium by using x-halogenatedpoly(ethylene oxide) and CuBr, as macroinitiator and catalyst, respectively. The sta-bility of both catalytic complexes and macroinitiator was investigated in function ofpH, that is, fixed between 7.5 and 12. UV-VIS spectroscopy confirmed a good catalyticcomplex stability in the studied conditions. Hydrolysis of the macroinitiator estergroup at pH higher than 7.5 was detected by 1H NMR and GPC, yielding ill-definedpolymer samples when ATRP is performed in alkaline conditions. 2,20-Bipyridyl(Bpy), 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA), and tris(2-methyl-aminoethyl)amine (Me6-TREN)-based complexes were compared at the optimal pH(pH 7.5). When polymerization was carried out in the presence of CuBr � 2Me6-TRENcomplex block copolymers with narrow molecular weight distribution (1.1 � MW/Mn

� 1.3), and good agreement between theoretical and experimental molar masses wasobtained. Moreover, increasing the PAMPSNa polymerization degrees (n) did notaffect the control over the polymerization. Preliminary characterization of the diblockcopolymers behavior in aqueous medium revealed a strong polyelectrolyte effect inde-pendently of n. Interestingly, occurrence of interactions between the PEO andPAMPSNa-blocks was also evidenced by differential scanning calorimetry and ther-mogravimetric analyses. VVC 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47:

1108–1119, 2009

Keywords: atom transfer radical polymerization (ATRP); diblock copolymers;PAMPS; polyelectrolytes; poly(ethylene oxide)-b-poly(sodium 2-acrylamido-2-methyl-1-propane sulfonate) (PEO-b-PAMPSNa)

INTRODUCTION

Among all the controlled radical polymerizationmethods, atom transfer radical polymerization(ATRP) is one of the most commonly used owingto its specific features as the use of commerciallyavailable and inexpensive catalytic complexes,

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 1108–1119 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: P. Dubois (E-mail: [email protected])

1108

facility to synthesize initiators, and wide range ofaccessible monomers, as well as mild reaction con-ditions. Beyond these advantages, ATRP allowsthe preparation of (co)polymers with various top-ologies and presenting a good control over themacromolecular parameters.1 Generally, the poly-merization is based on reversible oxidation of thetransition metal complex of low valence afterabstraction of a halogen atom from an alkylhalideinitiator, leading to the formation of free radicals.As this equilibrium is mostly shifted toward dor-mant species, the resulting (co)polymers contain ahalogen end group, which can be either reacti-vated later to insert further monomers or gener-ate end-functional polymers [Scheme 1(A)].

Over the last decade, a new tendency to replacethe rather toxic organic solvents by so-called‘‘green’’ solvents as water has appeared,2–4 whichallows an expansion of the range of monomerspolymerizable by ATRP in a controlled mannerand in the most environmentally friendly andinexpensive solvent. Numerous studies haveexploited aqueous ATRP, motivated by the possi-bility to reach new kind of polymers as polyelec-trolytes and polymers with hydrophilic groups.3–8

However, despite the great potential of this mech-anism to reach such hydrophilic polymers in acontrolled manner, the number of water-solublepolymers obtained by aqueous ATRP is still lim-ited because of occurrence of side reactions

[Schemes 1(B–E)], as disproportionation of themost investigated Cu(I)-based ATRP activator,substitution of the halide ligand by water of theCu(II)-based ATRP deactivator, and hydrolysis ofthe alkyl halide bond issued from the initiator.2

In the particular case of methacrylamide mono-mers, aqueous ATRP appears more problematicbecause conventional linear amine- and pyridine-based ligands are characterized by inadequate re-dox potential, generally leading to low monomerconversions. In addition, the control over the poly-merization reaction is compromised also by theloss of the halide atom from the end groupthrough a cyclization reaction and/or concurrentcomplexation of the polymerizing methacryla-mides with the catalytic complex.5 Masci et al.first established the control over the aqueousATRP of N-isopropylacrylamide in a dimethylfor-mamide/water (1:1, v/v) mixture solvent at 20 �C,6

using ethyl-2-chloropropionate and CuCl � tris(2-dimethylaminoethyl)amine (Me6-TREN) as initia-tor and catalytic complex, respectively.6,7 AqueousATRP is critical for anionic monomers because ofstrong interactions between the monomer andcatalytic complex, formation of metal carboxylatesor protonation of the ligands leading in both casesto a loss of control over the polymerization pro-cess.8 The formed metal carboxylate complexproved to be an inefficient deactivator and cannotbe reduced into Cu(I)-based ATRP active catalyst,whereas the protonation of the ligand leads to theprecipitation of Cu(I). These side reactions can beavoided by transforming the acidic monomer inits salt form as evidenced by Armes and cow-orkers.9 This strategy affords the preparation ofwell-defined low-molecular weight poly(sodiummethacrylate)-based copolymers without any pro-tection reaction of the carboxylic group. Moreinterestingly, the use of a monomer in its saltform can significantly moderate the polymeriza-tion kinetics because of charge density variationresulting from acid–base equilibrium. However,because amino-based ligands are also sensitive toprotonation, the reaction pH has to competeligand protonation.9 Neutralization of the acidicmonomer to its salt form was also used by Masciet al. who reported the first successful ATRP of so-dium 2-acrylamido-2-methylpropanesulfonate(AMPSNa) performed in aqueous solution.7 Thepropagation reaction proved to follow a first-orderkinetic plot when using Me6-TREN as a ligand.Moreover, it was demonstrated that addition of asmall amount of CuCl2 improved the controlledpolymerization of AMPSNa. In a recent

Scheme 1. Schematic representation of the basicequilibrium (A) and the side reactions (B, C, D, andE) observed in aqueous ATRP. (B) Br-atom substitu-tion and/or hydrolysis of the initiator, (C) complexa-tion of the Cu(II) complex with water molecules and/or polar monomers, (D) disproportionation of theCu(I) complex, and (E) termination (transfer) reac-tion. R � X - alkyl halide (X ¼ Br, Cl), kact, kdeact,and kt represent activation, deactivation, and termi-nation kinetic constants, respectively.

OPTIMIZED WATER-BASED ATRP OF AN ANIONIC MONOMER 1109

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

communication, some of us have reported the syn-thesis of double hydrophilic diblock copolymersbased on PEO and PAMPSNa via ATRP ofAMPSNa as initiated by a-methoxy-x-(2-methyl-bromoisobutyrate) poly(ethylene oxide) (MeO-PEO-BriB).10 The ATRP of AMPSNa was con-ducted in a methanol/water (1:3 v/v) solvent mix-ture at a pH of 7.5, yielding double hydrophilicPEO-b-PAMPSNa diblock copolymers built up ona strong polyelectrolyte block with anticoagulantproperties11,12 and a nonionic biocompatibleblock.13

This investigation is a continuation of our pre-liminary studies and aims at comprehensive eval-uation of the effect of some experimental parame-ters on the aqueous ATRP of AMPSNa. Indeed, inour first communication,10 the influence of thetemperature, solvent composition, and the pres-ence of a low-molecular-weight electrolyte wasevaluated on the polymerization rate and degreeof control. To precisely control the macromolecularparameters of the prepared block copolymers, wecontinued our efforts on optimization of the reac-tion conditions (converting of AMPS into its saltform followed by freeze-drying, precisely tuningthe medium pH value as well as evaluation of theeffect of the ligand nature and reaction tempera-ture) on the ATRP. In this contribution, the poly-merization has been performed in the pH rangefrom 7.0 to 12.0 with thorough characterization ofthe recovered copolymers. 2,20-Bipyridine (Bpy),1,1,4,7,10,10-hexamethyltriethylenetetramine(HMTETA), and tris(2-methylaminoethyl)amine(Me6-TREN) have been considered as ligands forcopper(I) bromide. Furthermore, the propertiesof the synthesized double hydrophilic diblockcopolymers were investigated by viscometry, dif-ferential scanning calorimetry (DSC), and ther-mogravimetric analysis (TGA).

EXPERIMENTAL

Materials

2-Bromo-2-methylpropionylbromide (BriBBr, 98%,MW ¼ 229.90 g/mol, d ¼ 1.86, Aldrich), 2,20-bipyr-idine (Bpy, [99%, MW ¼ 140.19 g/mol, Aldrich),1,1,4,7,10,10-hexamethyltriethylenetetramine(HMTETA, 97%, MW ¼ 156.18 g/mol, Aldrich),tris(2-aminoethyl)amine (96%, MW ¼ 146.23g/mol, d ¼ 0.976, Aldrich), and phenylisocyanate(�98%, MW ¼ 119.12 g/mol, d ¼ 1.096, Aldrich)were used as received. Me6-TREN was synthe-sized as described elsewhere.14 Copper(I) bromide

(CuBr, [99%, MW ¼ 143.45 g/mol, Aldrich) waspurified by three consecutive washings with aceticacid and methanol. Triethylamine (99%, MW¼ 101.19 g/mol, d ¼ 0.726 g/mol, Fluka) was driedover barium oxide for 48 h and distilled beforeuse. 2-Acrylamido-2-methylpropanesulfonic acid(AMPS, acid form, 99%, MW ¼ 207.25 g/mol,Aldrich) was turned into salt form (AMPSNa) bytitration with saturated sodium hydroxide to pH7.0 and with 0.1 M NaOH to pH 7.5, 8.0, 9.0, 10.0,or 12.0 and freeze-dried. The monomer was storedunder N2 at �20 �C. a-Methoxy-x-hydroxy poly(ethylene oxide)s, MeO-PEOm-OH, m ¼ 12 (Mn

¼ 500 g/mol, Mw/Mn ¼ 1.06, Aldrich) and MeO-PEOm-OH, m ¼ 114 (Mn ¼ 5000 g/mol, Mw/Mn

¼ 1.06, Fluka) were purified by azeotropic distilla-tion with toluene and dried to constant weightbefore use. Tetrahydrofuran (THF, Labscan, 99%)was dried under sodium/benzophenone complexand distilled just prior use. All the other reagentswere of analytical grade and used as received.

Synthesis and Characterizationof MeO-PEO114-Br

iB

The macroinitiator was synthesized using aknown procedure.10 In the typical experiment,MeO-PEO114-OH (30 g, 6 mmol, 1 eq) and trie-thylamine (1.25 mL, 9 mmol, 1.5 eq) were dis-solved in 50 mL anhydrous THF. To the obtainedsolution, a solution of BriBBr (1.11 mL, 9 mmol,1.5 eq) in 30 mL anhydrous THF was added drop-wise for 30 min. After 24 h stirring at 50 �C, theprecipitated ammonium salts were filtrated outand charcoal was added to the supernatant toadsorb BriBBr in excess. After a second filtration,the purified MeO-PEO114-Br

iB was recovered bydrying at 40 �C and reduced pressure up to con-stant weight (recovery yield, 95%). The degree ofesterification was calculated by 1H NMR spectros-copy from the relative intensity of the methyleneprotons of ethylene oxide repetitive units at 3.65ppm and the methyl protons of isobutyrate end-groups at 1.89 ppm. 1H NMR (Bruker AMX-300apparatus operating at 300 MHz, room tempera-ture, CDCl3): d ¼ 1.89 (BrAC(CH3)2ACO), 3.34(CH3OA(CH2ACH2AO)114), 3.65 (CH3OA(CH2ACH2AO)114-1ACH2ACH2AOACO), 4.30(CH3OA(CH2ACH2AO)114-1ACH2ACH2AOACO).

Synthesis and Characterization of PEO-b-PAMPSNa

The ATRP of AMPSNa was performed in distilledwater/methanol mixtures (3:1, v/v) for 3 h using

1110 MINCHEVA ET AL.


MeO-PEOm-BriB (m ¼ 12, 114) as macroinitiator

and in the presence of small amount of low-molec-ular-weight salt (NaCl) as extra halide ions aspreviously described.10,11 Copper(I) bromideligated either with HMTETA, Bpy, or Me6-TREN(L) was used as ATRP catalytic complex. The ini-tial molar ratio [AMPSNa]/[MeO-PEOm-Br

iB]/[CuBr]/[L]/[NaCl] was 45/1/2/1/2, 70/1/2/1/2, or140/1/2/1/2 ([AMPSNa]0 ¼ 1.2 mol/L). The reac-tion temperature was maintained at 60 or 30 �C.For the synthesis of diblock copolymers 1–3 in Ta-ble 3, a dry round-bottom flask fitted with a rub-ber septum and a magnetic stirrer was chargedwith 0.07 g CuBr (4.85 � 10�4 mol, 1 eq) and 0.22g Me6-TREN (9.7 � 10�4 mol, 2 eq) . With aflamed syringe, 0.5 mL degassed distilled waterwere added to ensure homogeneous conditions.The catalytic complex was then degassed by threerepetitive freeze-thaw cycles. A second round-bottom flask was charged with 5 g AMPSNa (2.18� 10�2 mol, 45 eq), 2.5 g MeO-PEO114-Br

iB (4.85� 10�4 mol, 1 eq), and 0.056 g NaCl (9.7 � 10�4

mol, 2 eq). The flask was fitted with a rubber sep-tum, and 5 mL methanol and 14.5 mL distilledwater were added to dissolve the monomer, themacroinitiator, and NaCl. The obtained solutionwas degassed by bubbling with N2 for 15 min. Asdegassed solution was transferred under nitrogenflow into the first flask and the ATRP was carriedout under stirring at 30 �C for 3 h. The ratiowater/methanol in the final volume was tuned tobe 3:1 (v/v). Reaction was stopped by dilution withdistilled water and freeze-drying of the product.The pH value before and after AMPSNa polymer-ization was 7.5. Copper catalyst was removed byfiltration of an aqueous copolymer solutionthrough a silica gel column. The obtained copoly-mer was purified from unreacted monomer by di-alysis against water for 24 h and characterized by1H NMR using a Bruker AMX-300 apparatus atroom temperature in D2O. 1H NMR (300 MHz,D2O): d ¼ 1.12 (NHAC(CH3)2ACH2 and CH2ACHfrom PAMPSNa-block), 1.51 (C(CH3)2ACO frommacroinitiator), 3.25 (NHAC(CH3)2ACH2 fromPAMPSNa-block), 3.58 (CH3OA(CH2ACH2AO)mfrom macroinitiator), 3.65 (CH3OA(CH2ACH2AO)m-1ACH2ACH2AOACO from macroini-tiator), 4.19 (CH3OA(CH2ACH2AO)m-1ACH2ACH2AOACO from macroinitiator). The index ofpolydispersity of the obtained diblock copolymerswas determined by gel permeation chromatogra-phy (GPC) in 0.05 M CH3COOH/CH3COONaaqueous solution at 30 �C using an Agilent 1100Series liquid chromatograph (flow rate ¼ 1 mL/

min, loop volume 100 mL, and solution concentra-tion ¼ 1 mg/mL) equipped with two PL aquagel-OH mixed 8 mm columns. For calibration, poly(ethylene oxide) standards (MW ¼ 600–50000,Polymer Laboratories) were used.

The viscometric measurements of the diblockcopolymers were performed at 25 � 0.1 �C usingUbbelohde viscometer (inner diameter of the cap-illary ¼ 0.46 mm). The diblock copolymer solu-tions were prepared by dissolving 0.2 g of polymerin 20 mL buffer solutions of pH 4.0 and ionicstrength (I) 0.1 to obtain concentration of 10 mg/mL 24 h before measurements. All measurementswere repeated at least five times. DSC was per-formed on DSC Q 100 (TA Instruments) equip-ment using hermetically closed pans in the tem-perature range from �90 to 250 �C for a heatingrate of 10 �C/min under N2 flow. TGA were per-formed using TGA Q50 V6.1 Build 181 (TAInstruments) equipment in the temperaturerange from 25 to 700 �C under N2 flow with aheating rate of 10 �C/min.

Effect of pH on the Catalytic Complex Stability

The stability of the catalytic complex was investi-gated for CuBr � 2Bpy at pH 7.5, 8.5, 9.5, and 12.0in water/methanol (3:1, v/v). In a typical experi-ment, a dry round-bottom flask was charged with0.04 g CuBr (3.5 � 10�4 mol, 1 eq), 0.11 g Bpy (7� 10�4 mol, 2 eq), and 0.04 g NaCl (7 � 10�4 mol,2 eq). The flask was closed with a three-way cock-stop fitted with a rubber septum and degassed bythree repetitive vacuum-nitrogen cycles. Twentymilliliters of degassed water/methanol (3:1, v/v)solution with pH 7.5, 8.5, 9.5, or 12.0 were added,and the catalytic complex was allowed to react for3 h at 60 �C under N2 flow. The pH value wasmeasured by a pH meter (PHM 210, Radiometer).After 3 h under N2, the samples were left in airfor 24 h under stirring. Samples collected at thebeginning, at the end of the reaction, and after24 h in air were analyzed with a Cary 50 ScanUV-VIS spectrophotometer (Varian, Australia) atwavelength (k, nm) in the range of 800–300 nm.

Effect of pH on the Macroinitiator Stability

The stability of the macroinitiator was investi-gated for MeO-PEO114-Br

iB at pH 7.5, 9.0, 10.0,and 12.0 in water/methanol (3:1, v/v) for 3 h at60 �C. The pH value was measured by a pH meter(PHM 210, Radiometer). The obtained productswere extracted with chloroform, dried to constant



weight under reduced pressure, and analyzed by1H NMR (300 MHz, CDCl3) and GPC in THF atdouble detection (RI and UV). For GPC analysis,6 mg of each product were dissolved in 5 mL tolu-ene, and 10 molar excess of phenylisocyanate wasadded. The reaction was performed for 150 min atroom temperature under stirring. For sake ofcomparison, a sample from MeO-PEO114-OH wasalso reacted with phenylisocyanate. The excess ofunreacted phenylisocyanate was neutralized withmethanol, and the products were dried to con-stant weight under reduced pressure.

RESULTS AND DISCUSSION

Influence of pH

The 2-acrylamido-2-methylpropanesulfonic acid(AMPS) monomer is a strong acid, which whendissolved in water gives rise to significant autoini-tiation leading to spontaneous polymerization.15

Moreover, AMPS could not be used as such inATRP because of protonation of the amine func-tionalities in the ligand, leading thereby to cata-lyst deactivation. In fact, with the acid form ofAMPS, ATRP did not take place and thereforecomplete neutralization of the monomer to its so-

dium salt (AMPSNa) before ATRP to be conductedis needed.7,10 In our previous investigations, itwas shown that the ATRP of AMPSNa, initiatedby a-methoxy-x-(2-methylbromoisobutyrate) poly(ethylene oxide) (MeO-PEO-BriB) proceeds with agood control in water–methanol (3:1, v/v) mixedsolvent at pH 7.5 (Scheme 2).10,11 Small amountof NaCl as extra halide ions was added, as it isknown to impede the complexation of acidic mono-mer with Cu(I), thus suppressing catalyst deacti-vation.16 However, the control over the ATRPproved to be strictly dependent upon the pH valueof the reaction medium.

To clarify the impact of pH over the final mac-romolecular parameters, the aqueous ATRP ofAMPSNa in water–methanol (3:1, v/v) initiatedby MeO-PEO-BriB was examined at pH valuesranging from 7.5 to 12.0. The degree of polymer-ization (n) of the AMPSNa-block was fixed at n¼ 70 and 2,20-bipyridine (Bpy) was chosen asligand for CuIBr. The choice of Bpy was motivatedby its commercial availability and the formationof water-soluble and hydrolytically stable complexwith CuIBr. In a first experiment, ATRP polymer-ization of AMPSNa was carried out without con-trolling the pH of the reaction medium (entry 1,Table 1). The monomer AMPS was neutralized to

Scheme 2. ATRP of AMPSNa, as initiated by MeO-PEO-BriB in water–methanol(3:1, v/v) mixed solvent.

Table 1. Effect of the Reaction Media pH on the Conversion and Molar Mass Parametersfor PEO114-b-PAMPS70 Copolymers (Reaction Time: 3 h; 60 �C)

Entry pH Conversion (%)a

Molecular Parameters

MNMRn MW/Mn

b GPC Trace

1 c 100 10,300 7.31 Multimodal2 12.0 71 8,400 2.13 Multimodal3 10.0 95 2,800 2.37 Multimodal4 9.5 80 10,500 1.45 Multimodal5 8.5 92 5,800 2.07 Multimodal6 7.5 56 16,000 1.21 Monomodal

aConversion (%) as calculated by the 1H NMR spectra of the crude copolymers.b Polydispersity index as determined by GPC in 0.05 M CH3COOH/CH3COONa aqueous solution.c pH was measured to be 11.0 after the polymerization reaction.



form AMPSNa by addition of a slight excess (1.2eq) of NaOH to the AMPS aqueous solution. Theobtained AMPSNa solution was purged with N2

for 15 min and transferred into the initiator andCuBr � 2Bpy solution. After 3 h of polymerizationat 60 �C, the reaction was diluted with distilledwater and the product was recovered by freeze-drying. As demonstrated by the broad polydisper-sity index and the multimodal GPC trace of theas-obtained copolymer (entry 1, Table 1), the poly-merization proceeded rapidly with complete lackof control over the macromolecular parameters.

In a next series of experiments (entries 2–6, Ta-ble 1) the neutralization of AMPS to AMPSNawas carefully performed by titrating AMPS aque-ous solution with NaOH to reach pH values of 7.5,8.5, 9.5, 10.0, and 12.0 and the neutralized mono-mer was isolated by freeze-drying. It is to be men-tioned that the pH value of the reaction mixturemeasured after polymerization strictly corre-sponded to the value obtained after AMPS neu-tralization. Interestingly, all GPC chromatogramsof PEO114-b-PAMPSNa70 block copolymer presentbi- to multimodal distribution contaminated bytraces of unreacted macroinitiator for all polymer-izations carried out at pH higher than 7.5 (entries2–5, Table 1, Fig. 1). The observed multimodaldistribution evidenced a loss of control over theAMPSNa ATRP upon increasing pH. This lossmight be ascribed to side reactions including com-plexation of the Cu(II) complex with polar mole-cules [Scheme 1(C)], disproportionation of theCu(I) complex [Scheme 1(D)], and/or displacementof the Br atom from the macroinitiator and/orelimination of HX of the latter [Scheme 1(B)] asalready observed for ATRP in aqueous medium.2

Besides, side reactions as thermal-initiation ofAMPSNa homopolymerization, elimination, and/

or transfer reactions might occur, thus being thereason for multimodality in GPC. It might beassumed that pH plays an important role over thecontrol of aqueous AMPSNa ATRP favoring inbasic conditions the occurrence of such side-reactions (see Scheme 1). Indeed, as shown in Fig-ure 1, decreasing the pH value allowed enhancingthe macroinitiator consumption (determined fromthe ratio of the first and the second elution peakareas). Thus, an almost monomodal GPC trace(with only a small shoulder at 18.65 mL elutionvolume and issued from residual macroinitiator)was obtained for the PEO114-b-PAMPSNa70 blockcopolymer at pH 7.5, characterized by a narrowpolydispersity index (PI ¼ 1.21) (entry 6, Table 1,Fig. 1). Nevertheless, the kinetics of the polymer-ization reaction slowed down on decreasing pH toreach 56% of monomer conversion, and the finalcomposition was found to differ from the theoreti-cal one, as calculated from the 1H NMR spectrum.

To clarify the role of pH on the AMPSNa poly-merization through aqueous ATRP, the stability ofthe CuBr � 2Bpy complex was examined in water/methanol ¼ 3:1 (v/v) for different pH values andrecorded after 1 min and 3 h at 60 �C under N2

flow. Furthermore, the samples were left in anopen air system for 24 h under stirring and thenexamined by UV-VIS. The assays were performedin absence of monomer as because of the presenceof NaCl, it was not expected to complex the cata-lyst.16 The spectra were recorded from 800 to300 nm (Fig. 2). The UV-VIS spectra of CuBr �2Bpy before and after 3 h at 60 �C and pH rang-ing from 7.5 to 12.0 showed a single peak at k ¼434 nm, characteristic of the CuBr � 2Bpy com-plex, whereas the spectrum of the catalystexposed to air during 24 h presented a shift of the

Figure 1. GPC of the aqueous AMPSNa ATRPcrude products obtained at different pH. n ¼ 70,CuBr � 2Bpy, 60 �C, water/methanol ¼ 3:1 (v/v).

Figure 2. UV-VIS spectra of the catalytic complex(CuBr � 2Bpy) before (1) and after 3 h (2) at 60 �Cunder N2, and after 24 h in air (3); pH 8.5.



peak to 690 nm indicative for oxidation of Cu(I) toCu(II).17 It might be assumed that under theinvestigated experimental conditions, the CuBr �2Bpy activating catalytic complex was notaffected by the pH value.

Another possibility to explain the lack of con-trol observed at pH higher than 7.5 may relyupon partial or complete Br-substitution and/orhydrolysis of the macroinitiator at higher pH[Scheme 1(B)]. If mostly substitution occurs, thereplacement of Br-end group leads to inactivemacroinitiator and NaBr. Therefore, one mayassume for a spontaneous polymerization of themonomer because of self-initiation. On the otherhand, if hydrolysis of the ester bond is predomi-nant, a mixture of macroinitiator, a-methoxy-x-hydroxy PEO (PEO-OH) precursor, and 2-bromo-isobutyric acid is mostly obtained. Although there-formed precursor (PEO-OH) is unable to initi-ate the ATRP, the residual amount of intact mac-roinitiator as well as the formed 2-bromoisobuty-ric acid may still initiate AMPSNa ATRP, leading(together with the amount of re-formed precursor)to a mixture of (co)polymers with significantly dif-ferent molar masses and multimodal GPC tracedistribution. Macroinitiator Br-end group substi-tution and hydrolysis might be expected toincrease on increasing pH, thus leading to a lossof control over the initiation step of AMPSNaATRP and favoring eventually the thermoinducedpolymerization of the monomer. The possible sidereactions occurring with the macroinitiator onincreasing medium pH are depicted in Scheme 3.

Therefore, the stability of MeO-PEO114-BriB

macroinitiator at various pH has been studied.Practically, MeO-PEO114-Br

iB was maintained inwater/methanol ¼ 3:1 (v/v) mixture at pH 7.5, 9.0,10.0, and 12.0 for 3 h at 60 �C under inert atmos-phere. The product was recovered by extractionwith chloroform and dried to constant weightunder reduced pressure. The 1H NMR spectra(Fig. 3) revealed that the relative intensity ofmethyl protons from the macroinitiator activecenter (BrAC(CH3)2ACO) at d ¼ 1.89 ppmdecreased on increasing the pH from 7.5 to 9.0and completely disappeared at pH 12.0.

These observations might be indicative of thealkaline hydrolysis of the ester group, leading tothe formation of MeO-PEO-OH, and therefore aloss of ATRP x-bromo macroinitiator. To confirmthe hydrolysis of the macroinitiator in basic me-dium, the obtained products were dissolved in tol-uene, reacted with phenylisocyanate, and charac-terized by GPC with both RI and UV detection atk ¼ 260 nm. For the sake of comparison, MeO-PEO114-Br

iB and MeO-PEO114-OH used for themacroinitiator synthesis were also reacted withphenylisocyanate and analyzed by GPC with dou-ble detection. The results have shown that after 3h at 60 �C and at pH 7.5, only 3% of OH end-groups are detected (as calculated from the areaof the peak in UV detection) with respect to thestarting MeO-PEO114-Br

iB macroinitiator. As sus-pected, the relative hydroxyl group content wasfound to increase to 20% and even 80% at pH 10.0and 12.0, respectively, undoubtedly confirmingthe alkaline hydrolysis of the macroinitiator esterfunction under basic conditions. Displacement ofthe bromide atom from the macroinitiator has not

Scheme 3. Side reactions likely occurring withinthe MeO-PEO114-Br

iB macroinitiator on increasingpH.

Figure 3. 1H NMR spectra of MeO-PEO114-BriB

and the products recovered after 3 h at 60 �C inwater/methanol ¼ 3:1 (v/v) at different pH.



been observed but one cannot preclude that ittakes place.

High monomer conversion was furthermorerecorded at higher pH (70–100% at pH 8.5–12,when compared with 56% at pH 7.5), which can-not be explained by the as-obtained results de-spite their significance. One might argue for aspontaneous polymerization of the monomerbecause of self-initiation. However, as alreadyreported, the fully neutralized AMPS monomerdoes not produce self-initiation.15 The fastAMPSNa polymerization recorded at higher pHmight be assigned to some hydrolysis of theCu(II)-halide complex, presumably followed bycoordination of water molecules to Cu(II) [asdepicted in Scheme 1(C)]. The coordination ofwater molecules to Cu(II) has been reported todecrease significantly the deactivator concentra-tion, leading to faster and uncontrolled polymer-ization.2 An improvement of the control over theaqueous ATRP of AMPSNa as initiated by ethyl 2-chloropropionate was already observed by Masciet al.7 by adding one equivalent of Cu(II) withrespect to the Cu(I) species.

Influence of the Ligand Type

It is well known that the position of the dynamicequilibrium between the dormant and active spe-cies in ATRP is determined by the nature of the

catalytic complex.18 To tune up the position of theATRP equilibrium in aqueous medium, the choiceof the ligand is of prime importance. A suitableligand should have sufficient binding affinity to-ward the metal to compete with water and/or po-lar monomer as potential coordinating mole-cules.19 Moreover, the ligand should be able todrive and solubilize the transition metal to the po-lymerization phase and to maintain the fast equi-librium between the growing radicals and the dor-mant species. It was stated that ligands as Bpy,which can form water-soluble copper complexes,are poorly efficient for promoting aqueous ATRPbecause of a failure in adjusting the reaction equi-librium.19 Therefore, effect of the ligand on theaqueous ATRP of AMPSNa has been studied.

Two tetradentate (HMTETA and Me6-TREN,Fig. 4) ligands have been employed for the ATRPof AMPSNa, initiated by MeO-PEO114-Br

iB, andthe results compared with those previouslyobtained with Bpy. The ATRP has been performedin a mixed solvent water/methanol (3:1, v/v), pH7.5 using a catalyst-to-ligand (L) molar ratio([CuBr]:[L]) of 1:2.

As far as Bpy was considered as ligand, lowmonomer conversion (56%) was obtained after 3 hat 60 �C (entry 1, Table 2), whereas under thesame conditions, the conversions obtained withHMTETA and Me6-TREN reached 100 and 98%,respectively. The observed difference in monomerconversions might be ascribed to different propa-gation rates depending on the redox potentialand/or solubility of the catalytic complexes. Maty-jaszewski and coworkers have demonstrated thatthe activity of tetradendate ligand-based com-plexes is higher with respect to bidentate ligand-based complex leading to better control inATRP at least for styrene and methacrylic

Figure 4. Structure of the used ligands: Bpy (A),HMTETA (B), and Me6-TREN (C).

Table 2. Effect of Ligand on the Conversion and Molar Mass Parameters for PEO114-b-PAMPSNa70 Copolymers([CuBr]0/[ligand]0 ¼ 1:2, pH ¼ 7.5, Reaction Time ¼ 3 h; 60 �C)

Entry Ligand Conversion (%)a

Molecular Parameters

MNMRn MW/Mn

b GPC Trace

1 Bpy 56 16,000 1.21 Monomodal2 HMTETAc 100 22,700 1.96 Monomodal3 Me6-TREN 98 11,700 1.59 Monomodal4 Me6-TREN

d 99 16,000 1.29 Monomodal

aConversion (%) as calculated from the 1H NMR spectra of the crude copolymers.b Polydispersity index as determined by GPC in 0.05 M CH3COOH/CH3COONa aqueous solution.cMeO-PEO12-Br

iB was used as macroinitiator.dATRP performed at 30 �C.



monomers.20,21 In the present case, broader poly-dispersities were obtained with HMTETA andM6-TREN most probably because of irreversibletermination (and transfer) reactions [Scheme1(E)], which generally occur at such high mono-mer conversions. The ATRP of AMPSNa promotedby CuBr � 2HMTETA was characterized by ashoulder in the GPC trace at lower retention vol-ume, which can be also ascribed to irreversibletermination reactions [Scheme 1(E)]. The polydis-persity index displayed by the PEO114-b-PAMPSNa70 copolymer synthesized in the pres-ence of CuBr � 2Me6-TREN as catalytic complex(MW/Mn ¼ 1.59) was lower but still too high, sug-gesting an unsatisfactory control over the ATRPof AMPSNa. Taking into account the high reac-tion temperature (60 �C), one can suggest thatslowing the propagation rate may improve thecontrol over the macromolecular parameters.Therefore, the ATRP of AMPSNa was performedat 30 �C. Clearly, lowering the temperature from60 to 30 �C allowed for narrowing the molecularweight distribution (PDI of 1.29 instead of 1.59)while the monomer conversion remained almostquantitative (99%) (entry 4, Table 2). A block co-polymer with monomodal molecular weight distri-bution was recovered confirming a good controlover the aqueous ATRP of AMPSNa as initiatedby MeO-PEO114-Br

iB and catalyzed by CuBr �2Me6-TREN in water/methanol ¼ 3:1 (v/v) at pH7.5 and 30 �C for 3 h.

These experimental conditions have been fur-ther validated by targeting different degrees ofpolymerization (n ¼ 45, 70, and 140; Table 3). Asshown by GPC analysis (Fig. 5), whatever the tar-geted length of PAMPSNa-block, the polymeriza-tion reaction remains under perfect control. Theas-recovered diblock copolymers were all mono-modal with narrow molar mass distribution andnumber-average molecular weight close to the

theoretical predictions. From Figure 5, a clearshift of the GPC traces toward lower elution vol-umes is observed by increasing the initial mono-mer-to-macroinitiator molar ratio attesting forthe ‘‘living’’/controlled character of the ATRP reac-tion. In sharp contrast with Figure 1, no trace ofremaining macroinitiator could be detected.

Solution Properties of PEO-b-PAMPSNaDiblock Copolymers

To study the conformation of PEO-b-PAMPSNadiblock copolymers (Table 3) in aqueous solutions,viscometric analyses were performed. The solu-tion behavior of the synthesized PEO114-b-PAMPSNan (n ¼ 41, 66, and 150) diblock copoly-mers was evaluated by the dependence of the vis-cosity number on polymer concentration (Fig. 6).

As clearly evidenced by Figure 6, the viscositynumber of PEO114-b-PAMPSNan (n ¼ 41, 66, and

Figure 5. GPC chromatograms of PEO114-b-PAMPSNan (n ¼ 41, 66, and 150) diblock copolymersas obtained by aqueous ATRP initiated by MeO-PEO114-Br

iB in water/methanol ¼ 3:1 (v/v), pH 7.5,[CuBr]/[Me6-TREN] ¼ 1:2 at 30 �C for 3 h.

Table 3. Conversion and Molar Mass Parameters for PEO114-b-PAMPSNan Copolymers (n ¼ 45, 70, and 140([CuBr]0/[Me6-TREN]0 ¼ 1:2, pH ¼ 7.5, Reaction Time ¼ 3 h; 30 �C)

Entry n Conversion (%)a

Molecular Weight Parameters

Mthn

b MNMRn MW/Mn

c GPC Trace

1 45 100 10,300 9,400 1.10 Monomodal2 70 99 15,900 16,000 1.29 Monomodal3 140 98 31,400 34,300 1.21 Monomodal

aConversion (%) as calculated from 1H NMR spectra of the crude copolymers.b Theoretical number-average molar mass, as calculated by Mth

n ¼ Conv. � DP � MWAMPSNa.c Polydispersity index as determined by GPC in 0.05 M CH3COOH/CH3COONa aqueous solution.



150) aqueous solutions increases on decreasingpolymer concentration even for the copolymercontaining the shortest PAMPSNa-block (n ¼ 41).The obtained values increase also on increasingthe PAMPSNa-block length (from n ¼ 41 to 150).This behavior is indicative for a strong polyelec-trolyte effect because of an increase in polyelectro-lyte block Debye length resulting in an increase ofintramolecular repulsive interactions leading tochain expansion. It could be supposed that for thePEO114-b-PAMPSNan (n ¼ 41, 66, and 150), theeffective charge density is high enough to provokeexpansion of the PAMPSNa-block even at n ¼ 41.Thus, the conformation of PAMPSNan (n ¼ 41, 66,and 150) block in PEO-b-PAMPSNa water solu-tions with I ¼ 0.1 appears highly extended. Theobtained results are in agreement with thoseobtained for PAMPS homopolymers in salt-free22

and salt-containing23 aqueous solutions, as wellas for other polymers, containing permanentcharges along their chains.24,25 It might be con-cluded that the presence of PEO block does notsignificantly influence the aqueous solutionbehavior of the PAMPSNa-block. Moreover,screening of the negative charges by the low-molar-mass salt was not observed at the studiedionic strength (I ¼ 0.1). This value of I was chosenfor its proximity to those of the body fluids (I¼ 0.15). These observations might be explainedby the presence of permanent charges (not cre-ated during polymer dissolution) spread alongthe polyelectrolyte block chain in agreement withthe literature.

It might be assumed that the diblock copoly-mers overlapping concentration (C* at which poly-

electrolyte macromolecules begin to overlap) liesin the investigated concentration range. The C*value might be calculated using eq 1:

C� ¼ 1=½g (1)

where [g] is the intrinsic viscosity.24 The intrinsicviscosity value can be obtained using the Fuosseq 2:

gsp=C ¼ A=ð1þ BC1=2Þ (2)

where A is the intrinsic viscosity and B is aconstant. The C* values, calculated from eq 1,were 0.31, 0.54, and 0.59 g/dL for PEO114-b-PAMPSNa150, PEO114-b-PAMPSNa66, and PEO114-b-PAMPSNa41, respectively.

Thermal Properties of PEO-b-PAMPSNaDiblock Copolymers

The thermal properties of the double hydrophilicPEO-b-PAMPSNa diblock copolymers (Table 3)have been studied by DSC and TGA. No Tg wasdetectable in the performed DSC analyses for theMeO-PEO114-Br

iB macroinitiator, while a sharpmelting peak at 66 �C (DHm ¼ 48.97 J/g) wasobserved. The Tg of PAMPSNa-block (Tg ofPAMPSNa-block from 72 to 124 �C depending onthe synthesis conditions26,27) was also notobserved, most probably because of the appear-ance of a wide peak around 140 �C assigned to theexistence of water molecules bound to the polymerchains. The obtained results are in agreementwith the literature data showing that each mono-mer unit on the PAMPSNa chain is able to boundup to eight water molecules.28 The DSC analysesof the PEO-b-PAMPSNa diblock copolymers (Ta-ble 3) have shown displacement of the Tm

recorded for the starting PEO macroinitiator to-ward lower temperatures (35 �C for PEO114-b-PAMPSNa150) with an enthalpy of meltingdecreasing on increasing the PAMPSNa-blockchain length (from 48.97 to 2.16 J/g for the macro-initiator and PEO114-b-PAMPSNa150, respec-tively). Furthermore, an appearance of a widepeak between 120 and 140 �C assigned to the exis-tence of water bound to the polymer chains wasobserved. The intensity of this peak increasedwith the PAMPSNa-block length more likelymeaning an increase of the bound water amount.This might be indicative for the presence of inter-actions between the PAMPSNa and PEO blocksimpeding the crystallization of PEO sequences.

Figure 6. Dependence of the viscosity number ofPEO114-b-PAMPSNan (n ¼ 41, 66, and 150) on poly-mer concentration in aqueous solutions at pH 4.0(ionic strength ¼ 0.1): (!) n ¼ 150; (l) n ¼ 66; (~) n¼ 41. 25 � 0.1 �C.



Figure 7 shows TGA of MeO-PEO114-BriB,

PEO114-b-PAMPSNa41, PEO114-b-PAMPSNa66,and PAMPSNa. Up to 200 �C, the weight lossrecorded for the macroinitiator did not reach 4 wt% and was assigned to the evaporation of water.The main decomposition peak started at 296 �Cand ended at 450 �C with a maximum at 365 �Cleading to a weight loss of 95 wt %. The thermaldecomposition of PAMPSNa and the diblockcopolymers was also characterized by a first stageof water evaporation in the temperature range50–200 �C. The weight losses were 6, 9, and12 wt % for PEO114-b-PAMPSNa41, PEO114-b-PAMPSNa66, and PAMPSNa, respectively. Thereal thermal decomposition of PAMPSNa wascharacterized by a first stage starting at 330 �Cand ending at 375 �C with a maximum at 359 �Cand a weight loss of 34 wt %. The second stageended at 483 �C with a maximum at 421 �C and9 wt % of weight loss, and the third stage endedat 650 �C with a maximum at 557 �C and a weightloss of 19 wt %. Noteworthy, the thermal decom-position of the diblock copolymers started athigher temperatures (336 �C) in comparison withthose of MeO-PEO114-Br

iB and PAMPSNa (296and 330 �C, respectively). Hence, the thermal sta-bility of the polymers increased in the followingorder: MeO-PEO114-Br

iB\ PAMPSNa\ PEO114-b-PAMPSNan (n ¼ 41 and 66). Surprisingly, thethermal decomposition of the diblock copolymersPEO114-b-PAMPSNan (n ¼ 41 and 66) was charac-

terized by a single decomposition peak, startingat 336 �C and ending at 650 �C.

The positions of the peak maximum (and theweight loss) were found to depend on thePAMPSNa-block length and were 384 (58%) and412 �C (56%) for n ¼ 41 and 66, respectively,meaning that the thermal stability of the diblockcopolymers increases on increasing thePAMPSNa-block length. It might be supposedthat the increased thermal stability of thePEO114-b-PAMPSNan (n ¼ 41 and 66) comparedwith those of PAMPSNa and MeO-PEO114-Br

iB,as well as the appearance of a single decomposi-tion peak in the thermal degradation of thediblock copolymers confirmed the interactionexisting between the PEO and PAMPSNa-blocks.These observations might be explained by coordi-nation of the Naþ from the AMPSNa monomerunits in the PAMPSNa-block by the oxygen atomsin the PEO block chain.29 By analogy with the for-mation of covalent bonds and/or polyelectrolytecomplex,30 Naþ coordination would lead to someincrease of the diblock copolymer thermal stability.

CONCLUSIONS

In conclusion, the effect of pH and ligand natureon the aqueous ATRP of AMPSNa as initiated byMeO-PEO-BriB was examined and explained.Based on the obtained results, it comes out that

Figure 7. TGA thermograms of MeO-PEO114-BriB, PEO114-b-PAMPSNan (n ¼ 41

and 66) diblock copolymers, and PAMPSNa.



the loss of control over the polymerizationobserved in basic conditions is due to side reac-tions (disproportionation and alkaline hydrolysis)involving the MeO-PEO-BriB macroinitiator. Theoptimal conditions for a well-controlled aqueousATRP of AMPSNa in a water/methanol solventmixture (3:1 in volume, pH 7.5, room tempera-ture, Me6-TREN as a ligand) have been deter-mined. Finally, the solution properties of theresulting diblock copolymers have been investi-gated by viscometry, and their thermal behaviorhas been evaluated by DSC and TGA.

The authors are much indebted to the bilateral coopera-tion between the Bulgarian Academy of Sciences andCommissariat General aux Relations Internationales(CGRI, Brussels). The authors are grateful to ‘‘RegionWallonne’’ and European Community (FEDER, FSE)for general support in the frame of ‘‘Objectif 1-Hainaut:Materia Nova’’. R. Mincheva is grateful to SSTC for thefinancial support of her Research Fellowship (SSTC-PAI 6/27).

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Optimized water-based ATRP of an anionic monomer: Comprehension and properties characterization

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