synthesis of amphiphilic copolymers by atrp initiated with a bifunctional … · 2007-12-12 ·...

REACTIVE

Reactive & Functional Polymers 67 (2007) 1027–1039

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&FUNCTIONALPOLYMERS

Synthesis of amphiphilic copolymers by ATRP initiated witha bifunctional initiator containing trichloromethyl groups

P. Ritz a,*, P. Latalova a, M. Janata a, L. Toman a, J. Krız a, J. Genzer b, P. Vlcek a

a Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republicb Department of Chemical and Biomolecular Engineering, North Carolina State University,

911 Partners Way, Raleigh, NC 27695-7905, USA

Received 22 September 2006; received in revised form 28 February 2007; accepted 2 June 2007

Available online 29 June 2007

Abstract

Bifunctional polystyrene macroinitiators, having various molecular weights, were prepared by atom transfer radicalpolymerization (ATRP), initiated with bifunctional initiator 1,3-bis{1-methyl-1[(2,2,2-trichloroethoxy) carbonyl-amino]ethyl}benzene in conjunction with CuCl catalyst and polyamine ligands. These macroinitiators were subsequentlyused for ATRP of tert-butyl acrylate (t-BuA), giving BAB triblocks poly[(t-BuA)-b-(Sty)-b-(t-BuA)] as precursors ofamphiphilic copolymers. Both the polymerization steps proceeded as controlled processes with linear semi-logarithmicconversion plots and lengths of the blocks following theoretical predictions. Hydrolysis of outer poly(t-BuA) blocks ledto triblock copolymers with the central polystyrene block and outer blocks of poly(acrylic acid), the molecular weightsof which ranged from ca. 5 � 103 to almost 1 � 105 Da.� 2007 Elsevier Ltd. All rights reserved.

Keywords: ATRP; Block copolymer; Polyhalogenated initiator; Amphiphilic copolymer; Surfmer

1. Introduction

A three-component initiating system for atomtransfer radical polymerization (ATRP) containshalide-type true initiator, catalyst – a transitionmetal salt in the lower oxidation state, and a com-plexing ligand, based on amine-type or organophos-phorus compounds [1,2]. In addition to alkyl or arylhalides, esters of haloorganic acids, also polyhalo-genated compounds of the general formula R–CX3

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doi:10.1016/j.reactfunctpolym.2007.06.007

* Corresponding author. Tel.: +420 296 809 258; fax: +420 296809 410.

E-mail address: [email protected] (P. Ritz).

can serve as ATRP initiators, in which –R can behydrogen, halogen, or various organic substituents.However, due to their chemical structure these com-pounds can, at least theoretically, act as bifunc-tional initiators. Initiating behavior of variousR–CCl3 in combination with a CuCl catalyst anda bipyridine (bpy) ligand was studied in detail fromboth the kinetic and mechanistic point of view byDestarac et al. [3]. It was shown that these initiatorscan be, with respect to their initiating efficiency,divided into two basic groups: (i) those, containingactivating electron-withdrawing group adjacent tothe –CCl3 (e.g., esters of trichloroacetic acid or(dichloromethyl)benzene) and (ii) those, in which

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1028 P. Ritz et al. / Reactive & Functional Polymers 67 (2007) 1027–1039

–R is alkyl, not activating alkylhalide group (e.g.,1,1,1-trichlorononane). Kinetic studies documentedthat the polymerization rate of styrene depends onthe type of the R-substituent. Specifically, when ini-tiated with activated initiators, the polymerizationproceeds with a higher rate when initiated with anon-activated type. Moreover, size exclusion chro-matography (SEC) and nuclear magnetic resonance(NMR) studies showed that non-activated initiatorsare clearly monofunctional, whereas some of theactivated types can behave as bifunctional. ATRPof MMA initiated with R–CCl3 compounds underthe same catalysis was presented in another workof the same authors [4], offering similar conclusions.Moreover, it was established here that the numberaverage molecular weight (Mn) of PMMA, formedin polymerization initiated with CCl4 does notincrease linearly with monomer conversion. Thisbehavior could be caused by formation of branches,initiated with a chlorine atom from the central CCl2group. However, model polymerizations initiatedwith Cl2CHCH2OH or CH3CCl2CH3 did not verifythis idea because the process was very slow. Thenon-linearity of the dependence of Mn on monomerconversion was explained by a possible b-scission ofthe chains containing the central CCl2 group, lead-ing to a non-active polymer and a new macroradi-cal. However, when initiator with non-activatedtrichloromethyl groups is used instead of CCl4,b-scission of the chains was not detected as has beenshown in our recent work [5]. In addition, the func-tionality of polyhalogenated macroinitiators canalso depend on the type of monomer, as has beendocumented by Hoecker et al. in polymerizationsof styrene and MMA, initiated with (dichloro-methyl)benzene [6].

We have synthesized recently a bifunctionalATRP initiator, 1,3-bis{1-methyl-1[(2,2,2-trichloro-ethoxy) carbonylamino]ethyl}benzene with twotrichloromethyl groups, by condensation of 2,2,2-trichloroethan-1-ol and aromatic diisocyanate1,3-bis(1-isocyanato-1-methylethyl)benzene (BI-1,Scheme 1) [7]. This compound was successfully used

Scheme 1.

in combination with a CuCl catalyst and amine-typeligands in controlled ATRP synthesis of PMMAand PMMA/t-BuA block copolymers [5] withtailorable block lengths. Here, we present an applica-tion of this initiator in preparing poly[(tert-butylacrylate)-block-styrene-block-(tert-butyl acrylate)](poly[(t-BuA)-b-(Sty)-b-(t-BuA)]) and poly(tert-butylacrylate)-block-[styrene-co-(2-hydroxyethyl metha-crylate)]-block-(tert-butyl acrylate) (poly[(t-BuA)-b-(Sty-co-HEMA)-b-(t-BuA)]) triblock copolymersand their corresponding amphiphilic analogues,which possess outer poly(acrylic acid) blocks.

2. Experimental

2.1. Materials

Styrene (Sty, >99%, Fluka), tert-butyl acrylate(t-BuA, >98%, Fluka) were distilled from calciumhydride under reduced pressure and stored in a free-zer under argon atmosphere. 2-Hydroxyethyl meth-acrylate (HEMA, >99%, Aldrich) was passedthrough a column filled with alumina in order toremove inhibitor and stored under argon in freezer.Toluene, acetone, and dichoromethane (>99%,Fluka) were distilled from calcium hydride andstored under argon at room temperature. 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA,97%, Aldrich), N,N0,N00,N00-pentamethyldiethylene-triamine (PMDETA, 99%, Aldrich), copper (Cu,powder 200 mesh, >98%, Aldrich), trifluoroaceticacid (TFA, >99%, Aldrich), 3-isopropenyl-a,a-di-methylbenzyl isocyanate (>95%, Aldrich), and dibu-tyltin dilaurate (DBTDL, >95%, Aldrich) were usedas received. CuCl (>99%, Aldrich) was purified bywashing with glacial acetic acid, absolute ethanol,and finally with diethyl ether. The bifunctional initi-ator containing trichloromethyl groups (BI-1) wassynthesized in the same way as described earlier byToman et al. [7].

2.2. Polymer synthesis

2.2.1. Polystyrene homopolymers (1A) and randomcopolymers with HEMA (1B)

In a typical experiment, CuCl (0.1368 g, 1.05mmol) and bifunctional initiator BI-1 (1.1187 g,2.09 mmol) were added to a flask, equipped with amagnetic stirring bar. After sealing with a three-way stopcock, the flask was degassed and backfilledwith argon three times. Subsequently, styrene(12 ml, 104.73 mmol) and toluene (11 ml) as a

P. Ritz et al. / Reactive & Functional Polymers 67 (2007) 1027–1039 1029

solvent, were added. Then, the complexing ligand,HMTETA (0.29 ml, 1.05 mmol) or PMDETA(0.22 ml, 1.05 mmol), was added and the solutionwas stirred until the complex was formed. The mix-ture was frozen in dry-ice ethanol bath and degassedby three freeze–pump–thaw cycles. The flask wassubsequently placed in an oil bath and stirred at110 �C for 10 h. Then, the reaction was quenchedby intensive cooling and diluted with tetrahydrofu-ran (THF). This crude reaction mixture was ana-lyzed by gas chromatography (GC) and SEC; theformed polymer was recovered by precipitation inacidic methanol/water mixture = 80/20 (v/v) anddried in vacuum at 40 �C to constant weight.

Random copolymers of styrene with HEMA(mole ratio 1:50) were prepared by following thesame polymerization procedure.

2.2.2. Synthesis of poly[(t-BuA)-b-(Sty)-b-

(t-BuA)] (2A) and poly[(t-BuA)-b-(Sty-co-HEMA)-b-(t-BuA)](2B) triblock copolymers

A reaction flask, equipped with a three-way stop-cock and a magnetic stirring bar was charged withmacroinitiator 1A or 1B (1.0068 g, 0.205 mmol,Mn = 4900, Mw/Mn = 1.19), CuCl (20.27 mg, 0.205mmol), copper powder (200 mesh, 39.04 mg, 0.614mmol), and flushed with argon. The flask was thenevacuated and backfilled with argon three times.Afterwards, t-BuA (3 ml, 20.48 mmol), PMDETA(42.8 ll, 0.205 mmol), and dried acetone (2 ml) wereadded with gas-tight syringes. The reaction mixturewas repeatedly frozen in dry-ice/ethanol bath andbackfilled with argon for 30 min in order to removeoxygen. The polymerization was carried in oil bathat 70 �C for 16 h. Quenching, isolation, and analysisof such prepared block copolymers followed the sameprocedure as that for polystyrene homopolymers.

2.2.3. Modification of 2B with 3-isopropenyl-a,a-

dimethylbenzyl isocyanate (3B)

A mixture of 2B copolymer containing 2-hydroxy-ethyl groups from the HEMA units (12.25 g,2.4 � 10�6 mol of OH groups), and 3-isopropenyl-a,a-dimethylbenzyl isocyanate (50 ll, 0.025 mmol)in dichloromethane (50 ml) was treated with a cata-lytic amount of dibutyltin dilaurate. The solutionwas stirred at ambient temperature for 24 h. Thereaction mixture was subsequently evaporated todryness; the crude polymer was dissolved in THF,precipitated into a methanol/water mixture (80/20v/v) and, finally, the product was dried at room tem-perature under vacuum.

2.2.4. Hydrolysis of 2A and 3B

A round-bottom flask fitted with magnetic stir-rer was charged with 2A or 3B (10 g, 72.44 mmolof tert-butyl ester groups), followed by dichloro-methane (50 ml). The mixture was subsequentlystirred for 10 min at room temperature in orderto dissolve the polymer. Trifluoroacetic acid(26.9 ml, 0.36 mol, i.e., fivefold excess over thetert-butoxy groups) was then added and the mix-ture was stirred at room temperature for 24 h.The solvent and unreacted trifluoroacetic acid werethen removed from the resulting heterogenous mix-ture by decantation, and the residual solid productwas washed with dichloromethane three-times fol-lowed by drying under vacuum at room tempera-ture to constant weight.

2.3. Measurements

2.3.1. Size exclusion chromatography (SEC)All polymers were characterized by SEC (Labora,

Czech Rep.) in THF at room temperature at a flowrate of 1 ml/min using a set of two PSS-SDV col-umns (5 lm; 103 and 105 A) (PSS, Germany),equipped with RI and UV detectors. Polystyrenestandards and the Mark–Houwink constants forthe PS/THF system (K = 1.17 � 10�4 mL g�1, a =0.717) were used to establish the number averagemolecular weights (Mn) and molecular weight distri-butions (MWD). Obviously, for block copolymersonly their apparent Mn values were obtained in thisway.

2.3.2. Nuclear magnetic resonance (NMR)1H NMR spectra of 10% (w/w) solutions in

CDCl3 or DMSO-d6 were measured at 330 K witha Bruker Avance DPX300 spectrometer at 300.13and 75.45 MHz resonance frequency, respectively.The spectra were measured in a quadrature mode.For 1H NMR, 32 k points were collected in 32 scans,the repetition time being 12 s. The integral intensitiesof characteristic signals were measured using thesoftware supplied with the NMR spectrometer.

3. Results

3.1. Homopolymerization of styrene

Styrene was polymerized in toluene with a CuCl/HMTETA (or PMDETA) catalyst system and abifunctional initiator with 2,2,2-trichloroethylfunctional groups (BI-1) at 110 �C. The choice of

Fig. 1. Plot of ln([M]o/[M]) vs. time for Run 1 (solid circles) andRun 2 (empty circles).


a suitable catalyst/ligand system is a key parameterin establishing the dynamic equilibrium between thedormant and active species as the basic conditionsfor well-controlled ATRP [8]. Malinowska et al.[5] reported that ATPR of MMA initiated withthe abovementioned initiator and catalysed withthe HMTETA/CuCl was well-controlled, whichindicates a high activity of the catalytic com-plex [9]. The results from the polymerization ofstyrene catalyzed with the CuCl/HMTETA orCuCl/PMDETA complexes are summarized inTable 1. The experiments were performed using var-ious catalyst concentrations (with respect to that ofthe initiator), depending on the Sty/BI-1 mole ratio.If this ratio was low, i.e., 50 (Runs 1 and 2) and thepolymerizations took place in toluene; better resultswere obtained if the concentrations of both theCuCl and HMTETA equalled one half of the initi-ator content (Run 1). The dependence of ln[M]o/[M] on the reaction time was linear (cf. Fig. 1), indi-cating a constant equilibrium concentration ofactive radicals and, therefore, a controlled process.However, changing the BI-1/CuCl/HMTETA moleratio to 1/1/1 (Run 2) resulted in the product with abroad MWD. Also, the dependence of ln[M]o/[M]on the reaction time was curved upward in this case(cf. Fig. 1). Obviously, the lower catalyst concentra-tion favorably lowers the concentration of activeradicals in the system. Nevertheless, higher concen-trations of a catalyst and ligand, is recommendedfor synthesis of polystyrenes with higher molecularweights (Runs 3–6). As can be seen from these

Table 1ATRP of styrene with the HMTETA and the PMDETA ligand

Run No. Ligand Feed compositiona Reaction time (

1 HMTETA 1:0.5:0.5:50b 92 1:1:1:50b 103 1:1:1:100b 74 1:1:1:300c 125 1:2:2:500c 206 1:2:2:1000c 207 PMDETA 1:0.5:0.5:50b 248 1:1:1:50c 79 1:1:1:50d 10

10 1:1:1:100b 2411 1:2:2:200b 2412 1:2:2:500b 18

a BI-1:CuCl:ligand:Sty mole ratio.b Sty/toluene = 55/45 v/v.c Bulk polymerization.d Sty/toluene = 68/32 v/v.

results, Mn of the prepared polymers follows thetheoretical values in the range of almost two ordersof magnitude.

Bulk polymerizations, under otherwise the sameconditions, proceed also to high conversions (Runs4–6). However, the MWDs of the polymers areslightly broadened and their molecular weights donot exactly follow theoretical predictions; in partic-ular, if the monomer/initiator mole ratio is as highas 500 or even 1000. It can be assumed, that termi-nation reactions are non-negligible under theseconditions.

In further experiments (Runs 7–12), the PMDETAligand was tested under similar reaction conditions.

h) GC conversion (%) Mn,calc SEC

Mn Mw/Mn

76.64 4400 4600 1.2394.51 5400 6500 1.4396.26 10600 11000 1.2878.85 25000 23300 1.2199.02 52000 41300 1.4299.45 104500 93700 1.5166.28 3800 3600 1.1973.52 4200 4200 1.1562.53 3600 2900 1.2598.00 10800 9600 1.2578.43 16800 16200 1.2484.80 44100 41400 1.23

Fig. 2. Plot of ln([M]o/[M]) vs. time for Run 8 (solid rhombs) andRun 9 (empty rhombs).

Fig. 3. Plot of ln([M]o/[M]) vs. time for Run 16.


With a low catalyst concentration and a low excessof styrene over the initiator, i.e., for the BI-1:CuCl:ligand:Sty mole ratio equal to 1:0.5:0.5:50, the poly-merization in toluene proceeds slowly resulting in aproduct with a narrow MWD (Run 7). An increasein the concentration of the catalyst components (BI-1:CuCl:PMDETA = 1:1:1) led to a higher reactionrate in toluene as well as in the bulk (Runs 8 and9). Moreover, the linear dependence of ln[M]o/[M]on the reaction time indicated a well-controlledpolymerization under these conditions (cf. Fig. 2).Also, experiments with a higher Sty/initiator moleratio (Runs 10–12) resulted in narrow MWD prod-ucts whose molecular weights corresponded to the

Table 2Synthesis of poly[(t-BuA)-b-(Sty)-b-(t-BuA)] in various solvents at 70 �

Run No. Feed compositiona GC conversion (%) React

13 1:1:1:100b 24.90 1714 1:1:1:100c 91.41 1715 1:1:1:100d 49.97 1716 1:1:3:1:50e 95.80 817 1:1:3:1:100e 93.20 11.518 1:1:3:1:200e 95.45 1819 1:1:3:1:300e 93.20 1820 1:2:6:2:500f 93.39 1621 1:2:6:2:1000f 73.47 18

Note: For Run 13–19, PSty-macro with: Mn,SEC = 4900, Mw/Mn = 1.1Mw/Mn = 1.22 was used.

a Mole ratio.b PSty-macro:CuCl:PMDETA:t-BuA, 50% (v/v) DMF.c PSty-macro:CuCl:PMDETA:t-BuA, 50% (v/v) anisole.d PSty-macro:CuCl:PMDETA:t-BuA, 50% (v/v) acetone.e PSty-macro:CuCl:Cu:PMDETA:t-BuA, 40% (v/v) acetone.f PSty-macro:CuCl:Cu:PMDETA:t-BuA, 60% (v/v) acetone.

theoretical values. Thus, PMDETA is a more con-venient ligand in this polymerization, leading to acontrolled process under a wide range of concentra-tion conditions. This can probably be due to a lowerredox potential of the CuCl /PMDETA complex,compared with CuCl/HMTETA [10].

3.2. Block copolymers

Since polystyrenes prepared by ATRP initiatedwith the bifunctional polyhalogenated initiatorBI-1 mentioned above, are linear and symmetric,having C–Cl bonds at both ends, they can be used

C

ion time (h) Block copolymer characteristics

Mn,calc Mn,NMR Mn,SEC Mw/Mn

8100 19800 21500 1.5416600 17400 18200 1.2211300 12800 12500 1.1311000 15900 18400 1.3216500 18600 19100 1.1729200 31900 30500 1.2240700 41600 38300 1.3565000 70000 57000 1.2797800 92300 73600 1.30

9 was used. For Runs 20–21, PSty-macro with: Mn,SEC = 5000,

Fig. 4. SEC eluograms of copolymer (2) and macroinitiator (1),Run 15.

Fig. 5. SEC eluograms of copolymer (2) and macroinitiator (1),Run 21.


as a,x-bifunctional macroinitiators (PSty-macro)for ATRP of other monomers.

As documented recently, ATRP of tert-butylacrylate proceeds in a controlled manner underCuBr/PMDETA catalysis in polar solvents such asDMF, acetone and 1,4-dimethoxybenzene [11], oranisole [12]. The results of our experiments, per-formed at 70 �C in various solvents, are summarizedin Table 2. The polymerization of t-BuA in DMF(Run 13) proceeds slowly, reaching only 25% con-version after 17 h. Moreover, the Mn of the productis much higher than its theoretical value and theMWD is rather broad. This is in contrast withresults by Davis and Matyjaszewski which describedDMF as a good solvent for ATRP of t-BuA [11].Perceptibly better results were obtained when ani-sole or acetone were used as solvents (Runs 14and 15). Polymerization in anisole reached almostquantitative conversion (91%) after 17 h, Mn ofthe prepared polymer corresponds to the expectedvalue and also MWD is acceptably narrow. Poly-merization in acetone (Run 15) is faster than thatin DMF, but slower than that in anisole; the mono-mer conversion reaches nearly 50% within the sametime interval. The molecular weight of the productis in a good agreement with the theoretical valueand its PDI is even lower than that of the polymerprepared in anisole even in the presence of Cu0 pow-der (Run 17). Thus, under these reaction conditions,the lengths of the poly(t-BuA) blocks in copolymerscan be controlled stoichiometrically over a widerange, as documented by Runs 16–21. Both of thesesolvents were already successfully used for ATRP oft-BuA [11,12], however, we observed lower poly-merization rate. The differences can originate fromdifferent starting monomer concentrations and dif-ferent catalysts (CuCl vs. CuBr).

The semilogarithmic plot of monomer consump-tion vs. time is also linear, suggesting a constantconcentration of active radicals during the polymer-ization (cf. Fig. 3, Run 16 in Table 2). In all exper-iments listed in Table 2, there were no detectableamounts of unreacted macroinitiator in resultingblock copolymers. SEC eluograms of macroini-tiators and the corresponding block copolymers ofRuns 15 and 21 (cf. Figs. 4 and 5) are symmetricand narrow, without a hint of bimodality or eventailing.

3.3. Hydrolysis of block copolymers

For hydrolysis of the ester groups of thepoly(t-BuA) blocks of the copolymers, trimethylsilyliodide (TMSI) was proposed as an effective dealky-lation reagent under neutral and mild conditions[13]. Thus, the modified Olah’s method was appliedto hydrolysis of the copolymer 2A and TMSI wasgenerated in situ by a reaction of trimethylsilyl chlo-ride (TMSCl) and sodium iodide (NaI). However,the final product, obtained after a hydrolysis withHCl, remained brown colored regardless of therepeated treatment with sodium thiosulfate. This iswhy this method was abandoned.

Consequently, trifluoroacetic acid (TFA) in non-polar aprotic solvent such as CH2Cl2 under mildconditions was used, according to literature [14].The resulting amphiphilic block copolymers thenprecipitated from the solution mixture duringhydrolysis and could be easily separated and puri-fied by decantation. The products were character-ized by 1H NMR in DMSO-d6 see Fig. 6. Thepeak corresponding to methyl protons in t-butylester groups (1.43 ppm) disappeared after hydroly-sis, whereas the peak at 9.53 ppm, typical for a

Fig. 7. SEC eluograms of the polystyrene macroinitiator(Mn,SEC = 41400, Mw/Mn = 1.23) before (1) and after (2)hydrolysis.

Fig. 6. 1H NMR spectra of: (A) poly[(t-BuA)-b-(Sty)-b-(t-BuA)]; (B) poly[(AA)-b-(Sty)-b-(AA)].


hydroxyl group, arose, documenting completehydrolysis of the ester.

The bifunctional initiator (BI-1), incorporated inthe chains of both the polystyrene macroinitiatorsand block copolymers, contains urethane bonds(cf. Scheme 1), which could also undergo hydrolysisunder alkaline or acid conditions [15]. Thus, it wasnecessary to verify its stability in a solution ofTFA in dichloromethane used for hydrolysis oft-Bu ester groups. The test was performed with apolystyrene macroinitiator, (Mn,SEC = 41400, Run12 in Table 1), which was treated with a mixtureof TFA and dichloromethane, then isolated andagain analyzed by SEC (cf. Fig. 7). Both the traceshave the same shape and position, without a hintof low-molecular weight shoulder which wouldappear as a consequence of hydrolysis of the ure-thane bond in polystyrene chains. Therefore, TFA

in CH2Cl2 is a suitable agent for hydrolysis oftert-butyl ester groups as it produces no scissionof urethane bonds in the polymer chains.

Scheme 2.


Fig. 8. 1H NMR spectra of: (A) poly[(Sty)-co-(HEMA)]; (B) poly[(t-BuA)-b-(Sty-co-HEMA)-b-(t-BuA)]; (C) poly[(t-BuA)-b-(Sty-co-HEMA)-b-(t-BuA)] modified with vinyl double bonds.



3.4. Modification of block copolymers with vinyl

double bonds

Amphiphilic block copolymers based on styreneand acrylic acid can be used, after neutralizationof acid groups, as polymeric surfactants capable ofstabilizing polymeric dispersions in emulsion poly-merizations [16–18]. For better stabilizing efficiency,the hydrophobic block of the copolymer can bechemically anchored on a particle surface whilethe hydrophilic blocks extend into the aqueousphase and create a well-defined hydrophilic shell.In order to achieve this, it is suitable to introducependant vinyl double bonds on the hydrophobicblock of the copolymer, which can subsequentlycopolymerize with monomer(s) present on anymicellar surface [19,20].

Fig. 9. 1H NMR spectra poly[(Sty)-co-(HEMA)] modified with

To obtain such a ‘‘polymerizable” surfactant (cf.Scheme 2), we first prepared a styrene/HEMA ran-dom copolymer by ATRP of styrene with HEMA(mole ratio 1:50), initiated with BI-1 and catalyzedwith the CuCl/PMDETA system under the sameconditions as mentioned above (Run 8, cf.Table 1). The product (1B) was analyzed by SECand 1H NMR: Mn,NMR = 5100, Mn,calc = 5200,Mw/Mn = 1.23; HEMA/Sty mole ratio (�1:56) wasestablished from integrated intensities of the methy-lene groups of the HEMA (4.3 ppm) units and theprotons of aromatic rings of styrene (6.5–7.3 ppm)(cf. Fig. 8A). Further, this hydroxy-functionalizedpolystyrene was used as a macroinitiator for ATRPof t-BuA under the standard conditions presentedabove (Run 20, cf. Table 2). The mole ratio oft-BuA monomer to the poly[(Sty-co-HEMA)]

vinyl double bonds, before (A) and after (B) hydrolysis.


macroinitiator was equal to 500. SEC analysis of theresulting block copolymer (2B) (not shown) revealeda narrow MWD without a hint of bimodality or thepresence of any unreacted macroinitiator. However,1H NMR spectrum of the block copolymer 2B (cf.Fig. 8B) shows no signals of methylene protons at4.3 ppm assigned to HEMA. This discrepancy canbe explained by a very low concentration of HEMA(3.6 � 10�3 mol%) in the block copolymer chainswith Mn,NMR = 70100, which is below the resolutionlimit of 1H NMR. The 2B copolymer then reactedwith 3-isopropenyl-a,a-dimethylbenzyl isocyanateunder DBTDL catalysis, which introduced doublebonds onto the HEMA units. The missing signalsat 5.0 and 5.3 ppm (the methylene protons) and at2.13 ppm (the methyl protons from AC(CH3)@CH2

groups) in the 1H NMR spectrum of the blockcopolymer 3B (cf. Fig. 8C) can also be interpretedin the same way as above. Finally, poly(t-BuA)block of this copolymer was hydrolyzed with TFA(see above), giving an amphiphilic triblock with pen-dant isopropenyl double bonds present on the lipo-philic block (4B) (cf. Scheme 2).

Scheme

Since the concentrations of the vinyl bonds and2-hydroxyethyl groups in the final modified poly-[(t-BuA)-b-(Sty-co-HEMA)-b-(t-BuA)] were belowthe detection limit of 1H NMR, a separate testwas performed with the poly[(Sty-co-HEMA)]macroinitiator 1B mentioned above; HEMA/Stymole ratio was 1:56, with Mn,NMR = 5100 andMw/Mn = 1.23. This precursor was condensed withthe unsaturated aromatic isocyanate in the sameway as for copolymer 2B. The obtained modifiedmacroinitiator was then analyzed by 1H NMR; thespectrum in Fig. 9A clearly indicates the presenceof signals of both the methylene (5.0 and 5.3 ppm)and methyl protons (2.13 ppm), which arise fromthe AC(CH3)@CH2 groups. The degree of function-alization of HEMA hydroxy groups with the isopro-penyl double bonds was established from integratedintensities of the methylene groups of the HEMA(4.3 ppm) units and methylene protons of the iso-propenyl groups. The mole ratio of isopropenylgroups to styrene units in the polymer was ca.1:40, which was slightly higher than that for HEMAunits (1:56) in the copolymer. This discrepancy

3.


likely originates from the formation of allophanatestructures (cf. Scheme 3C) by the reaction ofANHA groups in urethane groups (cf. Scheme 3B)with the isocyanate, which was used in a tenfoldmolar excess with respect to hydroxy groups. More-over, a long reaction time of the modification (seeExperimental section) can also play a non-negligiblerole in this respect. This modified polystyrenemacroinitiator was also treated with TFA in orderto verify hydrolytic stability of the urethane bond.The 1H NMR spectra, collected before (cf.Fig. 9A) and after (cf. Fig. 9B) the treatment, clearlyshow the same intensities of the signals of the iso-propenyl protons (5.0; 5.3 and 2.13 ppm), thus indi-cating that scission of the urethane group does nottake place to a detectable extent.

Since the modification of HEMA units in therandom copolymers proceeds smoothly with highyields, one can speculate about the same behaviorin the case of modification of high-molecular weighblock copolymers.

Stabilizing efficiency of the prepared amphiphilictriblock copolymer, functionalized with isopropenyldouble bond was preliminarily tested in a simpleemulsion polymerization of the Sty/BuA/HEMAmonomer mixture under conventional conditions.The experiment, performed in the presence of ratherlow concentration of the surfactant (1–2 mol% withrespect to the monomers) gave stable latex of thecorresponding copolymer. Let’s note that the latexremains stable and without visible change of viscos-ity within more than two months. Of course,detailed independent study would be needed to opti-mize the emulsion polymerization system includingstructure, molecular parameters and concentrationof the triblock copolymer; nevertheless, remarkablestabilizing effect of the amphiphilic triblock hasbeen clearly indicated.

4. Conclusion

Bifunctional initiators with trichloromethylactive groups were successfully used to synthesizepolystyrene homopolymers and a styrene/HEMArandom copolymer using ATRP catalyzed withCuCl in the presence of amine-type ligands. Withboth the ligands, HMTETA or PMDETA, theATRP proceeds in a ‘living’ manner, giving polysty-renes with narrow MWDs and Mn, which are nearto the theoretical values. The molecular weight ofpolystyrenes can be controlled over more than twoorders of magnitude while keeping MWDs narrow.

The polystyrenes thus prepared were then used asbifunctional macroinitiators for the polymerizationof t-BuA under CuCl/Cu/PMDETA catalysis, lead-ing to corresponding triblock copolymers with var-iable block lengths. Polymerizations in acetone ledto copolymers with the narrowest MWDs and pre-dictable molecular weights.

The poly[(t-BuA)-b-(Sty)-b-(t-BuA)] copolymersthus synthesized were hydrolyzed using TFA indichloromethane to yield corresponding amphi-philic block copolymers with outer poly(AA)blocks. The chosen method of hydrolysis couldquantitatively and selectively convert the tert-butylester to carboxylic groups under mild conditions,as confirmed by SEC and 1H NMR.

The polystyrene macroinitiator containingHEMA units can be modified, after the synthesisof block copolymers, by introducing the vinyldouble bonds. Subsequent hydrolysis of the poly(t-BuA) blocks results in amphiphilic block copoly-mers containing polymerizable vinyl bonds in thehydrophobic polystyrene block. The amphiphilicblock copolymers modified in this way can be used,after neutralization of poly(AA) blocks, as poly-meric surfactants and steric stabilizers in emulsionpolymerizations.

Acknowledgements

The authors thank the Ministry of Education,Youth and Sports of the Czech Republic for spon-soring this work (Grant # 1P05ME753). J.G.acknowledges the support from the US NationalScience Foundation (CTS and InternationalPrograms).

References

[1] K. Matyjaszewski, J. Xia, Chem. Rev. 101 (2001) 2921.[2] M. Kamigaito, T. Ando, M. Sawamoto, Chem. Rev. 101

(2001) 3689.[3] M. Destarac, J.M. Bessiere, B. Boutevin, J. Polym. Sci., Part

A: Polym. Chem. 36 (1998) 2933.[4] M. Destarac, K. Matyjaszewski, B. Boutevin, Macromol.

Chem. Phys. 201 (2000) 265.[5] A. Malinowska, P. Vlcek, J. Krız, L. Toman, P. Latalova,

M. Janata, B. Masar, Polymer 45 (2005) 5.[6] A. Neumann, H. Keul, H. Hoecker, Macromol. Chem.

Phys. 201 (2000) 980.[7] L. Toman, M. Janata, J. Spevacek, B. Masar, P. Vlcek, P.

Policka, Polym. Prepr. Am. Chem. Soc., Div. Polym. Chem.43 (2002) 18.

[8] K. Matyjaszewski, J.L. Wang, T. Grimaud, D.A. Shipp,Macromolecules 31 (1998) 1527.


[9] J. Xia, K. Matyjaszewski, Macromolecules 30 (1997) 7697.[10] J. Qiu, K. Matyjaszewski, L. Thouin, C. Amatore, Macro-

mol. Chem. Phys. 201 (2000) 1625.[11] K.A. Davis, K. Matyjaszewski, Macromolecules 33 (2000)

4039.[12] T. Liu, R. Casado-Portilla, J. Belmont, K. Matyjaszewski, J.

Polym. Sci., Part A: Polym. Chem. 43 (2005) 4695.[13] G.A. Olah, S.C. Narang, Tetrahedron 38 (1982) 2225.[14] Q.G. Ma, K.L. Wooley, J. Polym. Sci., Part A: Polym.

Chem. 38 (2000) 4805.[15] A. Vigroux, M. Bergon, C. Bergonzi, J. Am. Chem. Soc. 116

(1994) 11787.

[16] M. Bouix, J. Gouzi, B. Charleux, J.P. Vairon, P. Guinot,Macromol. Rapid Commun. 19 (1998) 209.

[17] C. Burguiere, S. Pascual, B. Coutin, A. Polton, M. Tardi, B.Charleux, K. Matyjaszewski, J.P. Vairon, Macromol. Symp.150 (2000) 39.

[18] C. Burguiere, S. Pascual, C. Bui, B. Charleux, J.P. Vairon,K. Matyjaszewski, K.A. Davis, I. Betremieux, Macromole-cules 34 (2001) 4439.

[19] J.M. Asua, H.A.S. Schoonbrood, Acta Polym. 49 (1998) 671.[20] E. Aramendia, M.J. Barandiaran, J.C. De La Cal, J. Grade,

T. Blease, J.M. Asua, J. Polym. Sci., Part A: Polym. Chem42 (2004) 4202.

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