Synthesis of Amphiphilic Block Graft Copolymers [Poly(styrene-b-ethylene-co-butylene-b-styrene)-g- Poly(acrylic acid)] and Their Aggregation in Water

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1 Synthesis of Amphiphilic Block Graft Copolymers [Poly(styrene-b-ethylene-co-butylene-b-styrene)-g- Poly(acrylic acid)] and Their Aggregation in Water FANGLIN NING, MING JIANG, MINFANG MU, HONGWEI DUAN, JINGWEI XIE Department of Macromolecular Sciences and Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai, , People s Republic of China Received 9July 2001; accepted 18 January 2002 Published online 00 Month 2002 in Wiley InterScience ( DOI: /pola ABSTRACT: Novel block graft copolymers [poly(styrene-b-ethylene-co-butylene-b-styrene)-g-poly(tert-butyl acrylate)] were synthesized by the atom transfer radical polymerization (ATRP) of tert-butyl acrylate (tba) with chloromethylated poly(styrene-bethylene-co-butylene-b-styrene) (SEBS) as amacromolecular initiator. The copolymers were composed of triblock SEBS as the backbone and tba as grafts attached to the polystyrene end blocks. The macromolecular initiator (chloromethylated SEBS) was prepared by successive hydrogenation and chloromethylation of SEBS. The degree of chloromethylation, ranging from 1.6 to 36.5 mol %according to the styrene units in SEBS, was attained with adjustments in the amount of SnCl 4 and the reaction time with aslight effect on the monodispersity of the starting material (SEBS). The ATRP mechanism of the copolymerization was supported by the kinetic data and the linear increase in the molecular weights of the products with conversion. The graft density was controlled with changes in the functionality of the chloromethylated SEBS. The averagelengthofthegraftchain,rangingfromafewrepeatunitstoabouttwohundred, wasadjustedwithchangesinthereactiontimeandalterationsintheinitiator/catalyst/ ligandmolarratio.incompleteinitiationwasdetectedatalowconversion;moreover,for initiators with low functionality, sluggish initiation was overcome with suitable reaction conditions. The block graft copolymers were hydrolyzed into amphiphilic ones containing poly(acrylic acid) grafts. The aggregation behavior of the amphiphilic copolymerswasstudiedwithdynamiclightscatteringandtransmissionelectronmicroscopy, and the aggregates showed avariety of morphologies Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: , 2002 Keywords: amphiphilic block graft copolymers; atom transfer radical polymerization (ATRP); macromolecular architecture; graft density; average graft length; incomplete initiation; morphology INTRODUCTION Correspondence to: M. Jiang ( mjiang@fudan.edu.cn) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 40, (2002) 2002 Wiley Periodicals, Inc. Well-defined block or graft copolymers with different molecular architectures have been extensively studied because they not only are academically interesting for exploring the architectural effects on phase separation, morphologies, order disorder transitions, and so on but also are quite significant for developing new materials based on the self-assembly and microphase separation of copolymers Traditionally, such well-defined macromolecules were only produced through living polymerizations, including anionic, cationic, and group- 1253

2 1254 NING ET AL. transition procedures and their combinations. However, such approaches are associated with the limitations of the available monomers and, in some cases, very rigorous demands for reagent purification In recent years, living free-radical polymerization, especially copper-based atom transfer radical polymerization (ATRP), has received a great deal of attention in the literature because of its monomer versatility, simplicity in practice, controllability with respect to molecular parameters, and so forth. So far, ATRP has successfully been used to prepare a wide range of products with various chain architectures, such as linear, star, block star, dendritic, highly branched, and comb architectures, 34 and various compositions. We have been interested in the relationship between polymer architectural structures and their properties both in solution and in bulk. In a recent communication, we briefly reported on the synthesis of diblock graft copolymers in bulk copper-based ATRP. 40 In this article, we report the ATRP synthesis of copolymers with a complicated and definite architecture, that is, a monodisperse poly(styrene-bethylene-co-butylene-b-styrene) (SEBS) as the backbone carrying grafts of poly(t-butyl acrylate) at the two end blocks, with an emphasis on the controllability of the molecular parameters, including the graft density and average length of the graft chain, by the selection of the starting material and the adjustment of reaction conditions. The motivation for producing such block graft copolymers is as follows. First, the solution properties, micellization behavior, morphologies, and so forth of such block graft copolymers would be interesting and valuable to explore for their architecture effects because related studies in the literature are mostly on linear and star copolymers only. Second, poly(tert-butyl acrylate) (PtBA) branches can be hydrolyzed into hydrophilic poly(acrylic acid) (PAA) with ease, leading to amphiphilic block graft copolymers. The aggregation behavior of such amphiphilic copolymers with complicated architectures in water is particularly of interest. EXPERIMENTAL Materials The monodisperse triblock copolymer SEBS (PS 72 -b-eb 714 -b-ps 72 ; Kraton G1652, Shell Co.), having a number-average molecular weight (M n ) of 55,000, a molecular weight distribution [weight-average molecular weight/number-average molecular weight (M w /M n )] of less than 1.06, and 28.6 wt % styrene units and composed of two polystyrene (PS) end blocks and a poly(ethyleneco-butylene) central block, was used as the starting material. tert-butyl acrylate (tba; Aldrich) was dried and purified as follows. It was washed three times with a 5 wt%sodium hydroxide solution and then with deionized water until a neutral ph was attained. After being dried with CaCl 2 and CaH 2 successively overnight, it was distilled over CaH 2. The purified monomer was stored in the dark at 0 C. 2,2 -Bipyridine (bipy) was purified by recrystallization in acetone. CuCl was washed with acetic acid, ethanol, and ethyl ether successively for several cycles and then dried in vacuo at 60 C. Diphenyl ether was distilled under CaH 2. Stannic chloride, trioxane, and chlorotrimethylsilane were purified with routine methods. Synthesis of the Poly(styrene-b-ethylene-cobutylene-b-styrene)-g-Poly(acrylic acid) (SEBS-g-PAA) Amphiphilic Copolymers Hydrogenation and Chloromethylation To avoid crosslinking and degradation over the course of chloromethylation and graft copolymerization, the double-bond residual in the parent polymer SEBS first needed to be saturated completely. The details of this procedure, which consisted of successive bromination and hydrogenolysis with Br 2 and lithium aluminum hydride, can be found in a previous article. 44 The chloromethylation of the PS blocks was performed on saturated SEBS according to procedures basically similar to those for PS as suggested by Itsuno et al. 45 Typically, 5.0 g of saturated SEBS and 5.4 g (60 mmol) of trioxane were added to 250 ml of chloroform. After the polymer and trioxane were dissolved completely, 22.8 ml of chlorotrimethylsilane (180 mmol) was added, followed by 3 ml of SnCl 4 (25.8 mmol), at 0 C. The mixture was stirred at 0 C for 30 min and then for another 6 h at room temperature, unless indicated otherwise. The reaction was stopped by the addition of 100 ml (50/50 v/v) of methanol/water to the reaction mixture. The product was purified by several dissolution/precipitation cycles with chloroform/ methanol and then dried in vacuo. Care was exercised over the course of drying to avoid

3 AMPHIPHILIC BLOCK GRAFT COPOLYMERS 1255 Table 1. Chloromethylation of SEBS a Run SnCl 4 Equivalent b (h) Time Cl Content (mol %) c Graft Sites (per PS Block) d PDI a The reaction was performed at 0 C for 30 min and then for another several hours at room temperature; the polymer concentration was 0.02 g/ml. b Based on the styrene units in SEBS. c The molar ratio of -CH 2 Cl to the styrene units in SEBS. d The product of the styrene unit numbers per PS block and the Cl content. crosslinking. The degree of chloromethylation (DC) was adjusted by changes in either the quantity of SnCl 4 or the reaction time. The content of chloromethyl (OCH 2 Cl) was measured by 1 H NMR according to the ratio of the peak area of chloromethyl 1 H to that of phenyl ring 1 H. The products chloromethylated SEBS are denoted xclsebs, where x represents the molar ratio of chloromethyl groups to styrene repeat units. The reaction conditions and characterization data of xclsebs are summarized in Table 1. The product yields for hydrogenation and chloromethylation were almost 100%. Synthesis of the Poly(styrene-b-ethylene-cobutylene-b-styrene)-g-Poly(tert-butyl acrylate) (SEBS-g-PtBA) Triblock Graft Copolymers The block graft copolymers SEBS-g-PtBA were synthesized by solution ATRP. In a typical solution polymerization, a Schlenk flask was charged with 0.5 g of 9.2ClSEBS (0.133 mmol), g of CuCl (0.133 mmol), and g of bipy (0.399 mmol) and was capped with a rubber septum. tba (4 ml, 17 mmol) in a biphenyl ether solution (50/50 v/v) previously degassed three times by a freeze pump thaw technique was added to the flask with a syringe. After the polymer was completely dissolved by stirring (ca. 30 min or more), the reaction mixture was heated at 130 C and then kept there for a designated time interval. The reaction was then quenched in liquid nitrogen, and the crude product was dissolved with tetrahydrofuran (THF), filtered, and precipitated in a large excess of methanol. This purification procedure was repeated three times for the removal of CuCl and bipy. The product was dried in vacuo. Hydrolysis of SEBS-g-PtBA The amphiphilic block graft copolymers SEBS-g- PAA were produced by the hydrolysis of SEBS-g- PtBA. The reaction was performed in solutions of SEBS-g-PtBA (0.5 wt %) in dioxane/hcl (10/1 v/v) via refluxing for 2 days at 90 C. The solvent and HCl were then removed with a rotary evaporator. The products were further dried at 40 C in vacuo for 3 days. Preparation of the Aggregate Solution The aggregate solutions of SEBS-g-PAA were prepared as follows. 46 Typically, desired amounts of the copolymers were dissolved in THF to form a solution with concentration of 0.5 or 1.0 mg/ml, and this was followed by the dropwise addition of water to the solution with mild stirring until the content of water reached about 30 wt %. Then, the aggregate solution was dialyzed first against a mixture solvent rich in water (70 wt %) and second against pure water for 1 week for the removal of THF. Characterization Methods Size exclusion chromatography (SEC) measurements were performed at room temperature with an instrument consisting of a Waters model 510 pump, an Erma ERC-7512 differential refractometer, a variable-wavelength UV detector, and a set of Standards Service columns with THF as the

4 1256 NING ET AL. solvent. The flow curve was recorded at a rate of 1 ml/min. 1 H NMR spectra were recorded on a Bruker MSL300 NMR spectrometer with CDCl 3 as the solvent. Fourier transform infrared (FTIR) spectra of thin polymer films on potassium plates were obtained with a PerkinElmer 983G spectrophotometer. Dynamic light scattering (DLS) measurements were performed to obtain the z-average distribution of the apparent hydrodynamic diameter (D h ) of the aggregates with a commercial laser light scattering spectrometer (Malvern 4700 autosizer) equipped with multi- digital time correlation (Malvern PCS7132), a PCS stepper motor controller, and a solid-state laser (ILT 5500QSL; output power 100 mw at nm). In DLS, the line-width distribution G ( ) can be calculated from the Laplace inversion of the intensity intensity time correlation function G (2) (q,t). 47,48 In this study, the correlation functions were analyzed by the CONTIN program, which can give access to the distribution of the relaxation times in the experimental time correlation functions; then, the z-average distribution of D h was calculated. All the DLS measurements were made at 25 C ata scattering angle of 90 and a concentration of 0.5 or 1 mg/ml. Transmission electron microscopy (TEM) was performed with a Philips EM400 microscope at an accelerating voltage of 80 kv. The experimental samples were prepared by the immersion of a copper grid, which was coated with thin films of Formvar and carbon successively, in a water solution of the amphiphilic copolymers; it remained immersed for 20 min for sufficient absorption of the aggregates on the carbon film. The copper grids were immediately transferred into a metal cell, subsequently frozen by liquid nitrogen, and then freeze-dried in vacuo for 2 days. RESULTS AND DISCUSSION Preparation of the Macromolecular Initiators Because commercially available SEBS contains few residual double bonds, further saturation is necessary for preventing the starting materials from crosslinking or degrading in the subsequent chloromethylation and graft copolymerization (Scheme 1). Our work showed that using a 10-fold excess of bromine ensured the complete saturation of the residual double bond. The chloromethylation of SEBS was performed according to a procedure suggested by Itsuno et al. 45 that was originally used for the functionalization of either crosslinked or linear PS. The remarkable advantages of the procedure are as follows. The reaction is performed in a one-pot manner, so contact with the toxic chloromethylation agent generated in the reaction can be avoided and the crosslinking of PS can be prevented. The products of chloromethylated SEBS listed in Table 1 display different DC values ranging from 1.6 to 36.5 mol %. The DC could be readily adjusted with changes in the amount of stannic chloride or the reaction time. Using a low concentration of SEBS was important for avoiding interchain crosslinking. In fact, when the concentration was greater than g/ml, crosslinking occurred, leading to gelation. For obtaining a rather high DC, which required a large amount of stannic chloride, the further lowering of the polymer concentration to 0.02 g/ml was necessary. Under this reaction condition, ClSEBS was obtained with a DC as high as 36.5 mol % based on the styrene units. This means that there is about one potential graft point per three styrene units. In addition, by a combination of less stannic chloride addition and a shorter reaction time, we were able to produce ClSEBS with a very low DC, such as 1.6ClSEBS, in which, on average, only one potential graft point per PS block existed. Figure 1 shows the SEC curves of the starting materials and C1SEBS with respective functionalities of 9.2 and 36.5 mol %. The curves, along with their polydispersity indices (PDI) listed in Table 1, confirmed that the chloromethylation procedure was successful in preparing the triblock macromolecular ATRP initiators with controllable amounts of potential grafting sites without distinctly spoiling the monodispersity of the starting material SEBS. We also noticed that, for the samples with higher DC values, a small shoulder in a higher molecular weight region appeared that might have arisen from a slight interchain reaction. The FTIR spectra of the starting material and xclsebs are presented in Figure 2. For the higher DC sample of ClSEBS, a rather strong band of COCl stretching (1263 cm 1 ) appears. On the 1 H NMR spectrum of ClSEBS shown in Figure 3, the adsorption at 4.49 ppm, the characteristic resonance of 1 HinCH 2 ClO connected to a benzene ring, also confirms the successful chloromethylation. The DC was calculated by the ra-

5 AMPHIPHILIC BLOCK GRAFT COPOLYMERS 1257 Scheme 1 tio of the 1 H peak area of OCH 2 Cl to that of the benzene ring 1 H. In our samples, the ratio ranges from (1.6ClSEBS) to (36.5ClSEBS). ATRP Graft Copolymerization of ClSEBS with tba The reaction conditions of the grafting copolymerization of tba with ClSEBS as the macromolecular initiator and the product characterization data are listed in Table 2. The data show that most of the graft products retained relatively low polydispersity. Generally, the results indicated that with changes in the DC values of ClSEBS, the catalyst compositions, and the reaction times, we were able to produce block graft copolymers (Scheme 2) with controllable graft densities and average PtBA graft lengths over broad ranges. Figure 4 presents the kinetics of the solution graft copolymerization of tba at 130 C with 9.2ClSEBS as the ATRP initiator and a COCl/

6 1258 NING ET AL. Figure 1. SEC chromatograms of (a) saturated SEBS, (b) 9.2ClSEBS, and (c) 36.5ClSEBS. CuCl/bipy molar ratio of 1/1/3. The data shown in the semilogarithmic plot of ln[m] 0 /[M] versus time, where [M] 0 is the initial monomer concentration and [M] is the monomer concentration at time t, indicated that the concentration of growing radicals was nearly a constant but with an obviously negative deviation at low conversion. This deviation from the first-order plot at low conversion may imply that this ATRP initiation with polyfunctional initiators did not occur simultaneously for all of the initiation sites, which could lead to grafts of different lengths. This phenomenon was carefully studied and reported recently for ATRP with dendritic multiarm initiators. 49 SEC chromatograms of 9.2ClSEBS and the corresponding block graft copolymers obtained at different reaction times are given in Figure 5. The SEC spectra show that the molecular weight of the graft copolymers increased with the reaction time and monomer conversion. At the same time, a shoulder at a high molecular weight region can be seen, and its fraction increases with the copolymerization. This shoulder, in our opinion, can probably be attributed to the product species terminated by radical coupling because of the higher local radical concentration associated with the block graft copolymer architecture and the solution copolymerization condition, leading to increased mobility of reactive chain ends. 50,51 The presence of such high molecular species noticeably altered the monodispersity of the products along with the procession of the reaction but not seriously; in most cases, the PDIs were not greater than By comparing the kinetic data in Figure 4 with those in our previous work for the grafting copolymerization of chloromethylated diblock poly- (styrene-b-ethylene-co-propylene) with ethyl methacrylate 40 or tert-butyl methacrylate, 52 we found that under similar reaction conditions, tba showed a much slower reaction rate and, consequently, a much lower conversion. This difference is similar to that found in the ATRP homopolymerization of the monomers. In general, the ATRP polymerization of styrene or alkyl acrylate is much slower than that of alkyl methylacrylate but more controllable The block graft products were characterized by 1 H NMR and FTIR. A typical FTIR spectrum of the block graft copolymer shown in Figure 2 clearly illustrates the occurrence of the graft copolymerization as the characteristic peak at 1731 cm 1, assigned to the stretching absorption of the carbonyl group in the graft chain, appears. The 1 H NMR spectra of 9.2ClSEBS and SEBS-g-PtBA with different average graft segment lengths are shown in Figure 3. A remarkable feature of the spectra of the block graft copolymers at the higher conversion [Fig. 3(C); E3 with an average polymerization degree of the graft segments of 22] is the disappearance of the resonance at 4.49 ppm. This demonstrates that all OCH 2 Cl bonds were disassociated to initiate the polymerization at the higher conversion, at least at the NMR resolution. However, for sample E1, a copolymer with a shorter average graft length of 12, a tiny Figure 2. FTIR spectra of saturated (a) SEBS, (b) 32.5ClSEBS, and (c) SEBS-g-PtBA for E6 in Table 2.

7 AMPHIPHILIC BLOCK GRAFT COPOLYMERS 1259 Figure 3. 1 H NMR spectra of (A) 9.2ClSEBS and (B,C) SEBS-g-PtBA samples of E1 and E3, respectively, in Table 2. resonance peak at 4.5 ppm was detected, an indication of incomplete initiation. The resonance at 1.4 and ppm is assigned to the contribution of protons in the tert-butyl group in tba and the contribution of protons in the benzene ring, respectively. Therefore, the ratio of the peak area of the latter to that of the former can be used to calculate the copolymer composition and the molecular weight M n,nmr. The average length of the graft segments can be further estimated on the following basis: all COCl bonds fission and participate in the initiation at the higher conversion. Besides M n,nmr, the molecular weight of the block graft copolymers can be measured directly by SEC (M n,sec ) and calculated (M n,cal ) from the amount of the consumed monomer ( [M]), the

8 1260 NING ET AL. Table 2. Synthesis of SEBS-g-PtBA by Solution ATRP a Run Reaction Time (h) ln[m] 0 /[M] M cal /M SEC ( 10 4 ) l M NMR ( 10 4 ) j PDI 1 W g /W 0 Graft Length (N) k E1 b,h / (12) E2 b,h / E3 b,h / (22) E4 b,h / (34) E5 b,h / (46) E6 c,f / (190) E7 c,f 3 7.3/ E8 d,g 7 / E9 e,g / E10 b,g 3 9.5/ a The temperature was 130 C, and the monomer/diphenyl ether ratio was 1/1 (v/v). b The macromolecular initiator used was 9.2CISEBS c The macromolecular initiator used was 1.6CISEBS. d The macromolecular initiator used was 4CISEBS. e The macromolecular initiator used was 36.5CISEBS. f The stoichiometry of [C-Cl]/[CuCl]/[bipy] was equal to 1/2/6. g The stoichiometry of [C-Cl]/[CuCl]/[bipy] was equal to 1/1.5/4.5. h The stoichiometry of [C-Cl]/[CuCl]/[bipy] was equal to 1/1/3. i M cal was calculated according to eq 1. j M NMR was estimated from the ratio (R) of the peak area of the benzene ring 1 H in SEBS-g-PtBA to that of the methyl 1 Hof tert-butyl. k The values outside parentheses were calculated from M cal, and those in parentheses were estimated from NMR by N (500 x)/(r x), where x (mol %) represents the Cl content of CISEBS. l The ratio of the weight of PtBA branches to that of the initiator CISEBS. amount of the macromolecular initiator xclsebs ([I]), and the apparent molecular weight of xclsebs (M n,app ) as measured by SEC and the following equation: M n,cal [M]/[I] M n,app (1) Figure 6 compares the calculated and experiment molecular weights M n,cal, M n,nmr, and M n,sec. Both series of data for M n,cal and M n,nmr show a linear increase with the reaction time and monomer conversion. This is anticipated by the mechanism of living polymerization. M n,sec is always less than both M n,cal and M n,nmr, and the Scheme 2 larger the conversion is, the larger the difference is. The result is reasonable if the following facts are taken into account. Because the M n,sec values were based on the calibration of PS standards, block graft copolymers with complicated molecular architectures and small contents of PS (28.6 wt %) were expected to cause substantial error. Specifically, such graft copolymers always present smaller hydrodynamic volumes and, consequently, smaller apparent molecular weights than the corresponding linear polymers. The larger the total molecular weight is, the larger the difference is. In our experiments, much attention was paid to the case of preparing the block graft copolymers from ClSEBS with low-potential grafting sites. For example, for both 1.6ClSEBS and 4.0ClSEBS, with a common molar ratio of 1/1/3 COCl/CuCl/bipy, almost no copolymerization occurred even in several days. However, with the amounts of CuCl and bipy increasing (i.e., 1/2/6 COCl/CuCl/bipy), the reaction rate increased abruptly, and the average length of the graft chains could reach hundreds of monomer units in a relatively shorter time. However, because the initiating groups are randomly distributed along

9 AMPHIPHILIC BLOCK GRAFT COPOLYMERS 1261 Figure 4. Kinetic plot of the solution copolymerization of 9.2ClSEBS with tba ([I] 2.9 mm, COCl/CuCl/bipy 1/1/3, temperature 130 C). the PS block, for such a low average of grafting sites, a considerable amount of the product actually does not contain any graft chains. Furthermore, this observation may suggest that the steric effect plays an important role in this kind of ATRP macromolecular initiator. Both the lower functionality and rather large molecular weight of the initiator make the steric effect significant. We surmised that the steric effect largely contributed to the incomplete initiation just as previously mentioned. Hydrolysis of the Block Graft Copolymer SEBS-g-PtBA The hydrolysis products (SEBS-g-PAA) were prepared according to the methods described in the Experimental section. Figure 7 shows a typical Figure 5. SEC chromatograms of (A) 9.2ClSEBS and (B E) solution copolymerization products (SEBS-g-PtBA) obtained at different times (E1, E2, E4, and E5, respectively, in Table 2).

10 1262 NING ET AL. Figure 6. Dependence of the molecular weight, M n,sec, M n,nmr, M n,cal, and M w /M n (SEC) on the monomer conversion. The experimental conditions are the same as those for Figure 4. FTIR spectrum of the hydrolysis product. The spectrum presents both a strong carboxy carboxyl peak at 1716 cm 1 and a strong and wide peak between 3300 and 2500 cm 1 attributed to the associating hydroxyl of the carboxyl. In addition, complete hydrolysis is demonstrated by the disappearance of the stretching vibration peak of the tert-butyl group at 1390 and 1370 cm 1. Therefore, the amphiphilic copolymers have the same molecular parameters as their parent block graft copolymers, including the graft density and average length of the graft segments. The hydrolysis products were denoted xsebs-g-paa(y), where y designates the average chain length of the graft chain PAA. Aggregation Behavior of the Amphiphilic Copolymer SEBS-g-PAA in Water It is well known that a block copolymer containing both hydrophobic and hydrophilic blocks can Figure 7. FTIR spectra of 9.2SEBS-g-PAA(40). self-assemble into micelles in water, and some such copolymers have been extensively studied. 1 12,57 60 However, until now there have been no systematic studies on the micellization behavior of amphiphilic block graft copolymers. Recently, Eisenberg et al. 6 8 reported the association behavior of amphiphilic diblock copolymers with extremely compositional asymmetry containing a long hydrophobic chain and a short hydrophilic and polyelectrolyte chain. The remarkable feature of such crew-cut aggregates in water is their morphological versatility; that is, with changes in the block copolymer composition or initial concentration in solution, spheres, rods, vesicles, and so forth were observed. Because our block graft amphiphilic copolymers provide a broad spectrum of chain architectures and possess controllable graft densities and average graft-chain lengths, they are obviously very useful for exploring the relationship between the chain architectures and morphologies of the aggregates of the amphiphilic copolymers. TEM and DLS were used to characterize the morphologies, sizes, and size distributions of the aggregates of the block graft copolymers in water. Because this is a preliminary report, we present only a few examples. Figure 8(a) shows the typical morphologies of 9.2SEBS-g-PAA(40) aggregates in water at a concentration of 0.5 mg/ ml. Spherical micelles with diameters of about nm can be observed. A dramatic change in the morphology was observed when the initial concentration increased to 1 mg/ml; the aggre-

11 AMPHIPHILIC BLOCK GRAFT COPOLYMERS 1263 Figure 8. TEM images of the aggregates formed by 9.2SEBS-g-PAA(40) at different concentrations in water: (a) 0.5 and (b) 1 mg/ml. gates possessed an irregular shape [Fig. 8(b)]. Because the aggregates displayed a dark periphery and a brighter center part, we are inclined to think that the aggregates were vesicles. In addition, this morphology is very similar to that of vesicles recently reported for amphoteric diblock copolymers poly[2-(dimethylamino)ethyl methacrylate]-b-poly-(methacrylic acid) by Jerome et al. 61 Figure 9(a,b) shows the typical morphologies of copolymers with much shorter PAA grafts, that is, 9.2SEBS-g-PAA(15), at initial copolymer concentrations of 0.5 and 1 mg/ml, respectively. At the low initial concentration of 0.5 mg/ml, the micrograph still presents spherical morphologies. However, as the initial concentration of the aggregate solution increased to 1 mg/ml, most of the aggregates quite unexpectedly possessed quadrilateral shapes, although such a morphology is associated with high surface energy. To our knowledge, this kind of morphology has not been reported previously. The sizes of the aggregates of 9.2SEBS-g- PAA(15) and 9.2SEBS-g-PAA(40) in water and their distributions were measured with DLS at different concentrations. Although the copolymers had rather narrow molecular weight distributions, their aggregates in water showed broader size distributions. As a representative example, the size distribution of the aggregates of 9.2SEBS-g-PAA(40) is presented in Figure 10. The sizes obtained with DLS generally agreed with those by TEM. CONCLUSIONS We have described the effective synthesis of novel block graft copolymers (SEBS-g-PtBA) by solution ATRP. The triblock macromolecular ATRP initiator was produced with SEBS as the starting material by successive hydrogenation and chloromethylation. With adjustments in the amount of SnCl 4 and the reaction time, the DC, which decided the grafted chain density in the subsequent grafting reaction, could be well controlled. The ATRP mechanism for grafting was supported by the kinetic data of the grafting reaction. Mean-

12 1264 NING ET AL. Figure 9. TEM pictures of the aggregates formed by 9.2SEBS-g-PAA(15) at different concentrations: (a) 0.5 and (b) 1 mg/ml. while, incomplete initiation was observed for special multifunctional initiation, possibly because of the steric effect. The average length of PtBA grafts could be controlled from several to several hundred with changes in either the reaction time or the COC/CuCl/bipy ratio. The copolymeriza- Figure 10. Hydrodynamic diameter distribution of the aggregates of 9.2SEBS-g- PAA(40) in water at a concentration of 1.0 mg/ml and at a scattering angle of 90.

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