Association Behavior of Amphiphilic ABA Triblock Copolymer Composed of Poly(2-methoxyethyl acrylate) (A) and Poly(ethylene oxide) (B) in Aqueous Solution

Poly(2-methoxyethyl acrylate) (PMEA) and poly(ethylene oxide) (PEO) have protein-antifouling properties and blood compatibility. ABA triblock copolymers (PMEAl-PEO11340-PMEAm (MEOMn; n is average value of l and m)) were prepared using single-electron transfer-living radical polymerization (SET-LRP) using a bifunctional PEO macroinitiator. Two types of MEOMn composed of PMEA blocks with degrees of polymerization (DP = n) of 85 and 777 were prepared using the same PEO macroinitiator. MEOMn formed flower micelles with a hydrophobic PMEA (A) core and hydrophilic PEO (B) loop shells in diluted water with a similar appearance to petals. The hydrodynamic radii of MEOM85 and MEOM777 were 151 and 108 nm, respectively. The PMEA block with a large DP formed a tightly packed core. The aggregation number (Nagg) of the PMEA block in a single flower micelle for MEOM85 and MEOM777 was 156 and 164, respectively, which were estimated using a light scattering technique. The critical micelle concentrations (CMCs) for MEOM85 and MEOM777 were 0.01 and 0.002 g/L, respectively, as determined by the light scattering intensity and fluorescence probe techniques. The size, Nagg, and CMC for MEOM85 and MEOM777 were almost the same independent of hydrophobic DP of the PMEA block.


Introduction
Amphipathic block copolymers form interpolymer aggregates because of the hydrophobic interactions of hydrophobic blocks in water [1]. Generally, amphipathic AB diblock copolymers form core-shell spherical polymer micelles in water. ABA triblock copolymers with two hydrophobic A blocks at both ends of the central B block form flower-like micelles caused by interpolymer aggregation in water [2]. The hydrophobic A blocks aggregate to form a core, and the hydrophilic B blocks form loop shape shells with a similar appearance to petals on the surface of the core to form flower micelles. The flower micelles are bridged when the hydrophobic A blocks in the ABA triblock copolymer are incorporated into separate cores in flower micelles. With increasing polymer concentration (C p ), the number of bridges between the flower micelles increases to form a gel [3]. The polymers increase the viscosity of the aqueous solution to form interpolymer aggregates, which can then be applied as associative thickeners with a small amount of addition [4]. For example, flower micelles formed from ABA triblock copolymers have been applied as associative thickeners [5]. Associative thickeners are used in water-based paints, coatings, personal care goods, and adhesive agents [6].
The hydrophilic shells on the surface of polymer micelles formed from amphipathic block copolymers stabilize the micelle structure and maintain their dispersion stability in Polymers 2022, 14, 1678 2 of 11 solution [7]. Polymer micelles formed from high molecular weight polymers generally have a lower critical micelle concentration (CMC) and higher colloidal stability than those formed from low molecular weight surfactants [8]. The CMC of amphiphilic diblock copolymers depends on the ratio of the hydrophobic to hydrophilic block lengths. CMC decreases with increasing hydrophobic block chain length for a constant hydrophilic block chain length in the diblock copolymer [9][10][11]. Zhulina et al. [12] studied thermodynamic properties of block copolymer micelles. With ABA triblock copolymers, the CMC also decreases with increasing hydrophobic block chain length [13]. Borisov and Halperin reported theoretical models of flower micelles [14].
Single-electron transfer-living radical polymerization (SET-LRP) is a method for controlled radical polymerization using a copper catalyst [15]. A copper catalyst is widely used as an inorganic electron donor reagent for organic and polymer syntheses ( Figure  S1). SET-LRP can be performed at low temperatures, e.g., room temperature, because of the low activation energy [16]. Poly(ethylene oxide) (PEO) is often used as a hydrophilic block in amphipathic ABA triblock copolymers because it can form flower micelles easily [17,18]. Furthermore, PEO is widely used in biomedical and biomaterial fields owing to its biocompatibility [19]; 2-Methoxyethyl acrylate (MEA) is an acrylate monomer that can be polymerized by radical polymerization [20]. Poly(2-methoxyethyl acrylate) (PMEA) is highly blood compatible because it has a protein-antifouling effect, and platelets cannot adhere easily to PMEA [21,22]. PMEA forms an intermediate water layer on its surface to suppress protein adsorption [23]. Furthermore, PMEA can be applied as coatings on various substrates because PMEA can be soluble in organic solvents, water insoluble, transparent, and adhesive [24]. Owing to the excellent properties of PMEA, it is also used as a coating material for artificial organs [25]. Haraguchi et al. [26,27] reported protein antifouling and blood compatible coatings using amphiphilic ABA triblock copolymers composed of hydrophobic PMEA (A) and hydrophilic poly(N,N-dimethylacrylamide) (B). The hydrophobic PMEA (A) blocks show good adhesion to both organic and inorganic substrates.
In this study, ABA triblock copolymers (PMEA l -PEO 11340 -PMEA m (MEOM n ; n is average value of l and m)) were prepared by SET-LRP to polymerize MEA using a bifunctional PEO macroinitiator at both chain ends. In particular, we are interested in the association behavior of ABA triblock copolymers with long PEO (B) block in water. MEOM n was composed of a hydrophobic PMEA (A) block and a hydrophilic PEO (B) block. In water, MEOM n formed flower micelles with a hydrophobic PMEA core and PEO loop-shaped shells ( Figure 1). The associative behavior of the flower micelles formed from MEOM n in dilute aqueous solutions was examined using dynamic light scattering (DLS), static light scattering (SLS), transmission electron microscopy (TEM), and fluorescence probe technique. block copolymers stabilize the micelle structure and maintain their dispersion stability in solution [7]. Polymer micelles formed from high molecular weight polymers generally have a lower critical micelle concentration (CMC) and higher colloidal stability than those formed from low molecular weight surfactants [8]. The CMC of amphiphilic diblock copolymers depends on the ratio of the hydrophobic to hydrophilic block lengths. CMC decreases with increasing hydrophobic block chain length for a constant hydrophilic block chain length in the diblock copolymer [9][10][11]. Zhulina et al. [12] studied thermodynamic properties of block copolymer micelles. With ABA triblock copolymers, the CMC also decreases with increasing hydrophobic block chain length [13]. Borisov and Halperin reported theoretical models of flower micelles [14].
Single-electron transfer-living radical polymerization (SET-LRP) is a method for controlled radical polymerization using a copper catalyst [15]. A copper catalyst is widely used as an inorganic electron donor reagent for organic and polymer syntheses ( Figure  S1). SET-LRP can be performed at low temperatures, e.g., room temperature, because of the low activation energy [16]. Poly(ethylene oxide) (PEO) is often used as a hydrophilic block in amphipathic ABA triblock copolymers because it can form flower micelles easily [17,18]. Furthermore, PEO is widely used in biomedical and biomaterial fields owing to its biocompatibility [19]; 2-Methoxyethyl acrylate (MEA) is an acrylate monomer that can be polymerized by radical polymerization [20]. Poly(2-methoxyethyl acrylate) (PMEA) is highly blood compatible because it has a protein-antifouling effect, and platelets cannot adhere easily to PMEA [21,22]. PMEA forms an intermediate water layer on its surface to suppress protein adsorption [23]. Furthermore, PMEA can be applied as coatings on various substrates because PMEA can be soluble in organic solvents, water insoluble, transparent, and adhesive [24]. Owing to the excellent properties of PMEA, it is also used as a coating material for artificial organs [25]. Haraguchi et al. [26,27] reported protein antifouling and blood compatible coatings using amphiphilic ABA triblock copolymers composed of hydrophobic PMEA (A) and hydrophilic poly(N,N-dimethylacrylamide) (B). The hydrophobic PMEA (A) blocks show good adhesion to both organic and inorganic substrates.
In this study, ABA triblock copolymers (PMEAl-PEO11340-PMEAm (MEOMn; n is average value of l and m)) were prepared by SET-LRP to polymerize MEA using a bifunctional PEO macroinitiator at both chain ends. In particular, we are interested in the association behavior of ABA triblock copolymers with long PEO (B) block in water. MEOMn was composed of a hydrophobic PMEA (A) block and a hydrophilic PEO (B) block. In water, MEOMn formed flower micelles with a hydrophobic PMEA core and PEO loop-shaped shells ( Figure 1). The associative behavior of the flower micelles formed from MEOMn in dilute aqueous solutions was examined using dynamic light scattering (DLS), static light scattering (SLS), transmission electron microscopy (TEM), and fluorescence probe technique.

Preparation of MEOM n (n = 85 and 777)
MEOM 85 was prepared via SET-LRP (Scheme S1). Me 6 TREN (2.13 mg, 3.01 µmol) was dissolved in water (1.00 mL) and stirred under an argon atmosphere for 10 min. CuBr (2.96 mg, 20.6 µmol) was then added, and the mixture was stirred for 10 min. PEO-Br (M n (GPC) = 4.64 × 10 5 g/mol, 1.50 g, 3.01 µmol) and MEA (420 mg, 3.22 mmol) were dissolved in water (21.3 mL). The aqueous CuBr/Me 6 TREN solution was added to an aqueous PEO-Br and MEA solution under an argon atmosphere. The reaction solution was stirred for 71 h under an argon atmosphere at room temperature. The conversion of MEA was 16.8%, which was estimated by 1 H nuclear magnetic resonance (NMR) spectroscopy before purification. The polymerization mixture was dialyzed against pure water for three days, and the polymer (MEOM 85 ) was collected by freeze-drying (0.889 g, 46.3%). The DP of the PMEA block was 85, as estimated from the 1 H NMR spectrum. The M n (GPC) and M w /M n estimated from GPC were 5.41 × 10 5 g/mol and 1.17, respectively.
MEOM 777 was prepared using the same procedure (2.42 g, 63.7%). The conversion of MEA before purification was 34.9%, according to 1 H NMR spectroscopy. The DP of the PMEA block was 777, as estimated from the 1 H NMR spectrum. The M n (GPC) and M w /M n were 4.91 × 10 5 g/mol and 1.26, respectively.

Preparation of MEOM 777 Aqueous Solution
MEOM 777 (5.86 mg, 8.35 µmol) was dissolved in THF (6.02 mL), and the C p was adjusted to 1.00 g/L. The THF solution was dialyzed against pure water for two days to remove THF. After dialysis, the aqueous solution was diluted with water to be C p = 0.10 g/L.

Measurements
Using a Bruker (Billerica, MA, USA) DRX-500 and JEOL (Tokyo, Japan) JNM-ECZ400R at 25 • C, 1 H NMR spectroscopy was performed. The water suppression by gradient-tailored excitation (Watergate) with a double pulse field gradient spin echo pulse sequence was used for the D 2 O solutions to suppress the water signal. Water suppression by a gradienttailored excitation (WATERGATE) method was used for the D 2 O sample to reduce the water signal. The GPC measurements were conducted at 40 • C using a Shodex (Tokyo, Japan) DS-4 pump, a Shodex GF-7M column, and a Shodex RI-101 refractive index detector. THF was used as the eluent with a flow rate of 1.0 mL/min. M n (GPC) and M w /M n were determined using standard polystyrene samples. The samples were analyzed by attenuated total reflection-Fourier-transform infrared (ATR-FTIR, FT/IR-4200, Jasco, Tokyo, Japan) spectroscopy. DLS measurements were performed at 25 • C using a Malvern (Malvern, UK) Zetasizer nano ZS at a scattering angle of 173 • . The data were analyzed using a Malvern (Malvern, UK) Zetasizer Software package 7.11 to determine the hydrodynamic radius (R h ), light scattering intensity (LSI), and polydispersity (PDI). SLS measurements were taken at 25 • C using an Otsuka Electronics (Osaka, Japan) DLS-7000. The weightaverage molecular weight (M w (SLS)) was calculated from Debye plots. The refractive index increment (dn/dC p ) was determined using an Otsuka Electronics (Osaka, Japan) DRM-3000 differential refractometer at 25 • C. Transmission electron microscopy (TEM, JEM-2100, Jeol, Tokyo, Japan) was performed at an acceleration voltage of 160 kV. The TEM samples were prepared by placing a drop of the sample solution on a copper grid coated with a form bar, and the samples were stained with a sodium phosphotungstate aqueous solution. The samples were dried for one day under reduced pressure. Fluorescence measurements were taken using a Hitachi High-Tech (Tokyo, Japan) F-2500 fluorescence spectrophotometer. The pyrene aqueous solutions (6.0 ×10 −7 M) were excited at 334 nm; the excitation and emission slit widths were 20 and 2.5 nm, respectively.  (Table 1). Malvern (Malvern, UK) Zetasizer Software package 7.11 to determine the hydrodynamic radius (Rh), light scattering intensity (LSI), and polydispersity (PDI). SLS measurements were taken at 25 °C using an Otsuka Electronics (Osaka, Japan) DLS-7000. The weightaverage molecular weight (Mw(SLS)) was calculated from Debye plots. The refractive index increment (dn/dCp) was determined using an Otsuka Electronics (Osaka, Japan) DRM-3000 differential refractometer at 25 °C. Transmission electron microscopy (TEM, JEM-2100, Jeol, Tokyo, Japan) was performed at an acceleration voltage of 160 kV. The TEM samples were prepared by placing a drop of the sample solution on a copper grid coated with a form bar, and the samples were stained with a sodium phosphotungstate aqueous solution. The samples were dried for one day under reduced pressure. Fluorescence measurements were taken using a Hitachi High-Tech (Tokyo, Japan) F-2500 fluorescence spectrophotometer. The pyrene aqueous solutions (6.0 ×10 −7 M) were excited at 334 nm; the excitation and emission slit widths were 20 and 2.5 nm, respectively.

Characterization
MEOMn was prepared by SET-LRP using MEA and PEO-Br macroinitiator. The 1 H NMR spectra for PEO-Br and MEOMn were measured in CDCl3 ( Figure 2). The terminal groups in PEO-Br could not be observed clearly because of the low signal intensity. The integral intensities of the PMEA pendant methylene protons at 3.3 ppm (e) and the PEO main chain methylene protons at 3.5-4.0 ppm (f) were compared to estimate the DP (NMR = n) of one end of the PMEA block in MEOMn. The DP(NMR) values for MEOM85 and MEOM777 were 85 and 777, respectively (Table 1).   The theoretical DP(theo) and number average molecular weight (M n (theo)) can be calculated from the following equations: GPC was performed for MEOM n using THF as an eluent ( Figure S2). The structure of MEOM n could be controlled because the M w /M n values estimated from GPC were less than 1.3. However, the retention time for the GPC elution curves of MEOM n was similar to that of PEO-Br. Unexpected interactions may have occurred between the MEOM n and GPC column, and polystyrene was used as the standard that may have impeded a correct estimation of the M n (GPC) [29]. ATR-FTIR was performed to characterize the chemical structure of MEOM n ( Figure S3). The C=O vibration stretching peak was observed at 1700 cm −1 for MEOM n , whereas the peak could be observed for PEO-Br. The peak intensity at 1700 cm −1 increased with increasing DP of the PMEA block in MEOM n . These results confirmed that MEOM n had been prepared.

Association Behavior of MEOM n
The R h distributions of the flower micelles formed from MEOM n in water were examined by DLS (Figure 3). MEOM 777 could not dissolve directly in water because of its long hydrophobic PMEA blocks. Therefore, an aqueous solution was prepared to dialyze the THF solution of MEOM 777 against water. In contrast, MEOM 85 could dissolve directly in water. The R h values of flower micelles obtained after directly dissolving them in water and after the dialysis method were compared to confirm the difference between the preparation methods of the MEOM 85 aqueous solutions ( Figure S4). The R h values for MEOM 85 prepared by direct dissolution in water and the dialysis method were 151 and 144 nm, respectively, which are similar. Therefore, flower micelles formed from MEOM 85 regardless of the solution preparation method. The association state of MEOM 85 that was easily soluble in water reaches the lowest association energy independently of the dissolution methods. Unless noted otherwise, the MEOM 85 aqueous solution was prepared using the direct dissolution method. The R h distributions for MEOM n in water were unimodal. The R h values for MEOM 85 and MEOM 777 were 144 and 108 nm, respectively. The DP of the hydrophobic PMEA block in MEOM 777 was larger than that in MEOM 85 , but R h of MEOM 777 was smaller than that of MEOM 85 . As the DP of the PMEA block in MEOM n increased, the hydrophobic interaction became stronger to form a more compact core of the flower micelle. The PDI values for MEOM 85 and MEOM 777 were 0.166 and 0.211, respectively. MEOM 85 formed more uniformly sized flower micelles than MEOM 777 .
The structure of flower micelles formed from MEOM n in water was confirmed by SLS ( Figure S5). The apparent weight-average molecular weight (M w (SLS)) and radius of gyration (R g ) were obtained from the SLS measurements. The refractive index increment (dn/dC p ) required to determine M w (SLS) was obtained using a differential refractometer.  Table 2). The DP of the PMEA block in MEOM 777 was approximately nine times larger than that in MEOM 85 , but both N agg values were close. The interface between the core and shell was sterically crowded with the PEO chains because the DP of the PEO block forms a loop-shaped shell. It was unlikely that the N agg value would increase above a certain number because of the congestion of the PEO shell chains on the core-shell interface. The R g values for MEOM 85 and MEOM 777 were 141 and 164 nm, respectively. From the R h and R g values, the flower micelles formed from MEOM 85 and MEOM 777 have similar size. The R g /R h ratios for MEOM 85 and MEOM 777 were 0.934 and 1.52, respectively. These R g /R h ratios were close to one, suggesting that the shape of flower micelles was spherical [30]. The density (Φ H ) of the micelle can be calculated from Equation (3) [31]: where N A is Avogadro's number. The Φ H values for MEOM 85 and MEOM 777 were 5.47 × 10 −3 and 2.29 × 10 −2 g/mL, respectively. MEOM 777 with a long PMEA chain formed a tightly packed core because the Φ H value of MEOM 777 was larger than that of MEOM 85 ; 1 H NMR spectroscopy of MEOM n was performed in D 2 O ( Figure S6). The PEO signals were observed, but the PMEA signals were not. This observation suggests that the motion of PMEA was restricted due to the formation of the core, but the motion of PEO was not restricted. The structure of flower micelles formed from MEOMn in water was confirmed by ( Figure S5). The apparent weight-average molecular weight (Mw(SLS)) and radius o ration (Rg) were obtained from the SLS measurements. The refractive index incre (dn/dCp) required to determine Mw(SLS) was obtained using a differential refractom The dn/dCp values for MEOM85 and MEOM777 were 0.138 and 0.426 mL/g, respecti The number of PMEA chains (Nagg) forming a single flower micelle was calculated the equation, Nagg = 2Mw(SLS)/(Mn(NMR) × Mw/Mn). The Nagg values for MEOM85 MEOM777 were 156 and 164, respectively ( Table 2). The DP of the PMEA block in MEO was approximately nine times larger than that in MEOM85, but both Nagg values were c The interface between the core and shell was sterically crowded with the PEO chain cause the DP of the PEO block forms a loop-shaped shell. It was unlikely that the value would increase above a certain number because of the congestion of the PEO chains on the core-shell interface. The Rg values for MEOM85 and MEOM777 were 141 164 nm, respectively. From the Rh and Rg values, the flower micelles formed from MEO and MEOM777 have similar size. The Rg/Rh ratios for MEOM85 and MEOM777 were and 1.52, respectively. These Rg/Rh ratios were close to one, suggesting that the sha flower micelles was spherical [30]. The density (ΦH) of the micelle can be calculated Equation (3) [31]: where NA is Avogadro's number. The ΦH values for MEOM85 and MEOM777 were 5  TEM of MEOM 85 and MEOM 777 in water ( Figure 4) revealed spherical aggregates. The radii (R TEM ) of MEOM 85 and MEOM 777 estimated from TEM were 42.4 and 59.2 nm, respectively. These R TEM values were smaller than R h and R g obtained from light scattering measurements. The shells formed from PEO were not observed because PEO cannot be stained by sodium phosphotungstate. The core formed by the association of the PMEA blocks could be stained, as observed by TEM. Therefore, the cores observed by TEM were separated a certain distance due to the unstained PEO loop shells that cannot be observed by TEM. radii (RTEM) of MEOM85 and MEOM777 estimated from TEM were 42.4 and 59.2 nm, respectively. These RTEM values were smaller than Rh and Rg obtained from light scattering measurements. The shells formed from PEO were not observed because PEO cannot be stained by sodium phosphotungstate. The core formed by the association of the PMEA blocks could be stained, as observed by TEM. Therefore, the cores observed by TEM were separated a certain distance due to the unstained PEO loop shells that cannot be observed by TEM.

Critical Micelle Concentration (CMC) of MEOMn
To determine the CMC of flower micelles, the LSI for MEOMn aqueous solutions was measured as a function of Cp ( Figure 5). The ratio (I/I0) of the LSI of the solution (I) to the solvent (I0) was plotted as a function of Cp. The CMC was estimated from the inflection point of the slope [32]. The CMC values for MEOM85 and MEOM777 were calculated to be 0.01 and 0.002 g/L, respectively (Table 3).

Critical Micelle Concentration (CMC) of MEOM n
To determine the CMC of flower micelles, the LSI for MEOM n aqueous solutions was measured as a function of C p ( Figure 5). The ratio (I/I 0 ) of the LSI of the solution (I) to the solvent (I 0 ) was plotted as a function of C p . The CMC was estimated from the inflection point of the slope [32]. The CMC values for MEOM 85 and MEOM 777 were calculated to be 0.01 and 0.002 g/L, respectively (Table 3). rated a certain distance due to the unstained PEO loop shells that cannot be observe TEM.

Critical Micelle Concentration (CMC) of MEOMn
To determine the CMC of flower micelles, the LSI for MEOMn aqueous solution measured as a function of Cp ( Figure 5). The ratio (I/I0) of the LSI of the solution (I) t solvent (I0) was plotted as a function of Cp. The CMC was estimated from the infle point of the slope [32]. The CMC values for MEOM85 and MEOM777 were calculated 0.01 and 0.002 g/L, respectively (Table 3).   The CMC of MEOM n was also estimated using pyrene as a hydrophobic fluorescence probe ( Figure 6). The intensity ratio (I 3 /I 1 ) of the first (I 1 ) to third vibronic peak (I 3 ) of the pyrene fluorescence spectrum depends on the microenvironmental polarity around the pyrene molecule [33]. I 3 /I 1 increased with decreasing microenvironmental polarity. I 3 /I 1 was plotted as a function of C p to determine the CMC (CMC(Em)) ( Figure 6). CMC(Em) was calculated from the intersections of the two tangents of the plots. The CMC(Em) values for MEOM 85 and MEOM 777 were 0.01 and 0.0015 g/L, respectively. The emission maximum wavelength of the 0-0 band in the pyrene excitation spectrum shifts to a longer wavelength when the microenvironment around the pyrene molecule becomes hydrophobic [34]. The 0-0 band maximum wavelengths of the aqueous solutions in the presence and absence of MEOM n were 338 and 335 nm, respectively. The CMC (CMC(Ex)) was determined from a plot of I 338 /I 335 vs. C p , where I 338 and I 335 are the emission intensities at 338 and 335 nm, respectively. The CMC(Ex) values for MEOM 85 and MEOM 777 were 0.01 and 0.002 g/L, respectively. The CMC values estimated from the LSI and fluorescence probe methods were similar. These observations suggest that hydrophobic anticancer drugs can be encapsulated in the core above the CMC.

Sample
(g/L) (g/L) (g/L) MEOM85 0.01 0.01 0.01 MEOM777 0.002 0.0015 0.002 The CMC of MEOMn was also estimated using pyrene as a hydrophobic fluoresc probe ( Figure 6). The intensity ratio (I3/I1) of the first (I1) to third vibronic peak (I3) o pyrene fluorescence spectrum depends on the microenvironmental polarity aroun pyrene molecule [33]. I3/I1 increased with decreasing microenvironmental polarity was plotted as a function of Cp to determine the CMC (CMC(Em)) ( Figure 6). CMC was calculated from the intersections of the two tangents of the plots. The CMC(Em ues for MEOM85 and MEOM777 were 0.01 and 0.0015 g/L, respectively. The emission imum wavelength of the 0-0 band in the pyrene excitation spectrum shifts to a lo wavelength when the microenvironment around the pyrene molecule becomes hy phobic [34]. The 0-0 band maximum wavelengths of the aqueous solutions in the pre and absence of MEOMn were 338 and 335 nm, respectively. The CMC (CMC(Ex)) wa termined from a plot of I338/I335 vs. Cp, where I338 and I335 are the emission intensities a and 335 nm, respectively. The CMC(Ex) values for MEOM85 and MEOM777 were 0.01 0.002 g/L, respectively. The CMC values estimated from the LSI and fluorescence p methods were similar. These observations suggest that hydrophobic anticancer drug be encapsulated in the core above the CMC.

Conclusions
Amphiphilic ABA triblock copolymers, MEOM n , were prepared via SET-LRP using a bifunctional PEO-Br macroinitiator. MEOM 85 and MEOM 777 were prepared with different DP of the hydrophobic PMEA blocks at the central PEO chain ends. The DP of the PMEA block in MEOM 777 was approximately nine times larger than that of MEOM 85 . The R h values for flower micelles formed from MEOM 85 and MEOM 777 in water were 151 and 108 nm, respectively. The hydrophobic PMEA blocks with a large DP in MEOM 777 associated to form a densely packed core due to the strong hydrophobic interactions. The N agg values for flower micelles formed from MEOM 85 and MEOM 777 were similar. The CMC for MEOM 85 and MEOM 777 were 0.01 and 0.002 g/L, respectively. The CMC of MEOM 777 was smaller than that of MEOM 85 because of the strong hydrophobic interactions of MEOM 777 . These results may come from much larger DP of the PEO block than that of the PMEA blocks. The PEO blocks that formed the loop shells of flower micelles and the PMEA blocks that formed the core were both biocompatible. Therefore, the biocompatible flower micelles formed from MEOM n may have applications as novel drug delivery carriers. We believe that the chemical design of MEOM n can be applied for coating on the various biomedical devices.