Cross-Linked CoreâShell Nanoparticles Based on Amphiphilic Block

Article pubs.acs.org/Macromolecules

Cross-Linked Core−Shell Nanoparticles Based on Amphiphilic Block Copolymers by RAFT Polymerization and Palladium-Catalyzed Suzuki Coupling Reaction Kazuhiro Nakabayashi, Hiroshi Oya, and Hideharu Mori* Department of Polymer Science and Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa 992-8510, Japan S Supporting Information *

ABSTRACT: Well-defined amphiphilic block copolymers, poly[poly(ethylene glycol) methyl ether methacrylate]-blockpoly(2,5-dibromo-3-vinylthiophene) poly(PEGMA)-b-poly(DB3VT), were synthesized by reversible addition−fragmentation chain transfer polymerization. A facile bottom-up modification was developed for preparing cross-linked core− shell nanoparticles from the amphiphilic block copolymers, in which an in situ Suzuki coupling reaction was carried out between 2,5-dibromide groups in the block copolymers and various diboronic acid compounds in the presence of a palladium catalyst after micelle formation in THF/H2O solution. The stable and uniform core−shell structure was confirmed by scanning force microscopy and dynamic light scattering observations. Furthermore, the size of the cross-linked core−shell nanoparticles could be controlled by adjusting the compositions of each segment in the block copolymers and Suzuki coupling reaction conditions. The relationship between the optical properties and chemical structures of π-conjugated segments in the particles was clearly observed by changing the coupling agents (diboronic acid compounds). These results demonstrate that stable and uniform cross-linked core−shell nanoparticles were successfully prepared from well-defined amphiphilic block copolymers, and tuning of their optical properties could be accomplished as well.

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INTRODUCTION For several decades, there has been a great demand for the development of π-conjugated polymers for realizing various optoelectrical applications such as organic light-emitting diodes,1 organic field effect transistors,2 and organic photovoltaics.3,4 Considering the applications based on π-conjugated polymers, it is noteworthy that highly ordered structures (i.e., high regioregularity, hierarchical structures derived from selfassembly, etc.) strongly affect their optoelectrical properties and can offer unique behaviors derived from highly ordered structure in some cases.5−8 The synthesis of well-defined πconjugated polymers and block copolymers with self-assembled capability is, thus, an interesting topic from the viewpoint of scientific research and industrial demands.9−12 Among πconjugated polymers, polythiophene and its derivatives are one of the most important classes for promising holetransporting materials and a variety of applications have been also fabricated so far.13,14 In 2005, the synthesis of poly(3hexylthiophene) with the narrow polydispersity (Mw/Mn ≈ 1.10) by the quasi-living Grignard metathesis method based on polycondensation was developed by Yokozawa15 and McCullough.16 Furthermore, the method has been expanded to the synthesis of polythiophene-based block copolymers17,18 and other π-conjugated polymers,19,20 thus contributing to the development of π-conjugated polymeric materials. Controlled radical polymerization of vinyl thiophene derivatives offers a unique opportunity to provide a variety of © 2012 American Chemical Society

well-defined polythiophene derivatives and block copolymers based on polythiophene segments in the field of chain polymerization. Free radical (co)polymerizations of various vinylthiophene derivatives, such as 2-vinylthiophene,21,22 3vinylthiophene,23−25 4-bromo-2-vinylthiophene,26 5-alkyl-2-vinylthiophene,27−29 5-bromo-2-vinylthiophene,30 5-methoxy-2vinylthiophene,31 and 2,5-dimethyl-3-vinylthiophene,32,33 have been established. Cationic polymerization34,35 and electrochemical polymerization36,37 of 2-vinylthiophene were also reported. Furthermore, polymerization of vinyl oligothiophenes (i.e., 3′-vinyl-2,2′:5′,2″-terthiophene,38 5-vinyl-2,2′:5′,2″-terthiophene,39 and N-phenyl-2-(2′-thienyl)-5-(5″-vinyl-2″-thienyl)pyrrole40) has been achieved so far. Controlled radical polymerization of these vinyl thiophene derivatives is, however, still a challenging issue because radicals having electrondonating moieties (e.g., thiophene, carbazole, etc.) are generally unstable (i.e., highly reactive). That is, the high reactivity of the radicals induces undesired chain-transfer and termination reactions, preventing controlled polymerization. In our previous work, we achieved the controlled synthesis of poly(vinylthiophene) derivatives from 2,5-dibromo-3-vinylthiophene (DB3VT) by reversible addition−fragmentation chain transfer (RAFT) polymerization using a dithiobenzoate-type Received: February 2, 2012 Revised: March 12, 2012 Published: March 23, 2012 3197

dx.doi.org/10.1021/ma300239u | Macromolecules 2012, 45, 3197−3204

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Table 1. Synthesis of Poly(PEGMA)-b-poly(DB3VT) by RAFT Polymerization of DB3VT Using Poly(PEGMA) Macro-CTAa Mn run

[M]:[macro-CTA]

time (h)

conv. (%)b

theoryc

NMRb

SECd

Mw/Mnd

1 2 3 4

30:1 60:1 160:1 200:1

12 12 24 24

69 64 65 56

19000 23000 40000 43000

19000 26000 43000 50000

15000 18000 29000 30000

1.24 1.29 1.33 1.26

m:nb 65:35e 48:52e 28:72e 24:76e

(43:23)f (43:47)f (43:111)f (43:136)f

a Polymerization was carried out at [macro-CTA]/[AIBN] = 5/1 at 60 °C in 1,4-dioxane ([M] = 1.00 mol/L, Mn,NMR and Mw/Mn of macro-CTA = 13 000 and 1.16). bCalculated by 1H NMR in CDCl3. cTheoretical molecular weight was calculated from [M]/[macro-CTA] × (Mw of monomer) × conv. + (Mw of macro-CTA). dDetermined by SEC using polystyrene standards in THF. eMolar composition ratios. fPrecise number of repeating units.

chain transfer agent (CTA).41 Well-defined block copolymers composed of poly(methyl methacrylate) and poly(vinylthiophene) segments were successfully synthesized as well. To the best of our knowledge, this is the first report on the controlled synthesis of poly(vinylthiophene) derivatives by the radical polymerization, indicating that even the radical polymerization of vinyl monomers with electron-donating moieties can be controlled by RAFT polymerization using well-chosen CTAs. RAFT polymerization has been employed for the controlled synthesis of various optoelectronic polymers, mainly because of the compatibility with a wide range of functionalities in monomers, solvents, and impurities, and the absence of undesired metal species during the polymerization process, in addition to the feasibility of creating various complex architectures and supermolecular assemblies.42 In this Article, we synthesized novel well-defined amphiphilic block copolymers composed of poly(ethylene glycol) methyl ether methacrylate (PEGMA) and DB3VT segments by RAFT polymerization. The facile bottom-up modification for the preparation of cross-linked core−shell nanoparticles from amphiphilic block copolymers was developed, in which the in situ Suzuki coupling reaction was carried out between 2,5dibromide groups in the block copolymers and various diboronic acid compounds in the presence of a palladium catalyst after the formation of the micelles in the THF/H2O solution. The stable and uniform core−shell structure was confirmed by scanning force microscopy (SFM) and dynamic light scattering (DLS) observations. Furthermore, the size of the cross-linked core−shell nanoparticles could be controlled by adjusting the compositions of each segment in the block copolymers and Suzuki coupling reaction conditions. The relationship between the optical properties and chemical structures of π-conjugated segments in the particles was also investigated in detail.

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with liquid nitrogen. After the reaction mixture was poured into hexane, the precipitate was collected with decantation and dried in vacuo at room temperature to yield pink viscous product (2.1 g, 70%). Mn = 8600 (Mw/Mn = 1.16), Mn,NMR = 13 000, Mn,theory = 9300. 1H NMR (CDCl3, δ): 4.2−3.9 (2H, bs), 3.8−3.3 (19H, bm), 2.0−1.5 (3H, bm), 1.1−0.7 (2H, bm). Synthesis of Amphiphilic Block Copolymer. Typical procedure (Run 3 in Table 1) is as follows: Macro-CTA (0.29 g, 0.023 mmol, Mn,NMR = 13 000, see Supporting Information), DB3VT (1.0 g, 3.7 mmol), AIBN (0.86 mg, 0.005 mmol), and dry 1,4-dioxane (3.7 mL) were placed in a dry glass ampule equipped with a magnetic stir bar, and then the solution was degassed by three freeze−evacuate−thaw cycles. After the ampule was flame-sealed under vacuum, the reaction was carried out at 60 °C for 24 h with stirring. Then, the reaction was stopped by rapid cooling with liquid nitrogen. After the reaction mixture was poured into hexane, the precipitate was collected with decantation and dried in vacuo at room temperature to yield light-red viscous product (65%, 0.84 g). Mn = 29 000 (Mw/Mn = 1.33), Mn,theory = 40 000. 1H NMR (CDCl3, δ): 7.2−6.7 (1H, bm), 4.2−3.9 (2H, bs), 3.8−3.3 (19H, bm), 2.7−2.0 (1H, bm), 2.0−1.1 (5H, bm), 1.1−0.7 (2H, bm). The compositions of each segment (m:n) were estimated by the 1H NMR spectra.

Table 2. Preparation of Cross-Linked Nanoparticles from Poly(PEGMA)-b-poly(DB3VT) by Suzuki Coupling Reactiona run 1 2 3 4 5 6

comonomer composition PEGMA:DB3VT

THF:H2O (vol %)

yield (%)b

Dh (nm)c

80:20

1:9 3:7 5:5 7:3 9:1 3:7

90 87 99 99 95 94

88 88 broad bimodal bimodal 110

68:32

a

Reaction was carried out between the block copolymer and 2,5thiophenediboronic acid at 70 °C for 24 h in THF/H2O in the presence of Pd(dppf)Cl2 (10 mol % for 2,5-thiophenediboronic acid). b After dialysis with water and THF. cDetermined by DLS measurement in THF solution (conc. = 2.0 mg/mL) at 25 °C.

EXPERIMENTAL SECTION

Materials. PEGMA (Aldrich, average Mn ≈ 300) was purified with column chromatography (Aluminium oxide 90 standard). 2,2′Azobisisobutyronitrile (AIBN, Kanto Chemical, 97%) was recrystallized with methanol. Other reagents and solvents were used as received unless otherwise stated. Cumyl dithiobenzoate used as a CTA43,44 and 2,5-dibromo-3-vinyl thiophene (DB3VT)41 were synthesized according to the previous literatures. Synthesis of poly(PEGMA) macro-chain transfer agent (Macro-CTA). Typical procedure is as follows: PEGMA (3.0 g, 10 mmol), cumyl dithiobenzoate (63 mg, 0.25 mmol), AIBN (8.2 mg, 0.05 mmol), and dry 1,4-dioxane (10 mL) were placed in a dry glass ampule equipped with a magnetic stir bar, and then the solution was degassed by three freeze−evacuate−thaw cycles. After the ampule was flame-sealed under vacuum, the reaction was carried out at 60 °C for 12 h with stirring. Then, the reaction was stopped by rapid cooling

Preparation of Cross-Linked Core−Shell Nanoparticles by Suzuki Coupling Reaction. Typical procedure (Run 6 in Table 2) is as follows: The THF solution (15 mL) of poly(PEGMA)68-bpoly(DB3VT)32 (90 mg, 0.1 mmol based on the repeating unit), 2,5-thiophenediboronic acid (86 mg, 0.5 mmol), and NaHCO3 (0.33 g, 4.0 mmol) was stirred at room temperature under an argon atmosphere. After the block copolymer was completely dissolved, Pd(dppf)Cl2 (16.3 mg, 0.02 mmol) and water (35 mL) was added, and the reaction mixture was carried out at 70 °C for 24 h in the dark. After removing some amount of solvents, dialysis of the residue was carried out with water, followed by with THF. Then, the residue was 3198


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Scheme 1. Synthesis of Poly(PEGMA)-b-poly(DB3VT)

evaporated and dried in vacuo at room temperature to yield brown viscous product (94%, 85 mg). Unreacted diboronic acid compounds, NaHCO3, and palladium catalyst were removed from the reaction mixture with dialysis. Therefore, the yield was determined by the amount of the product compared with the sum of the amounts of reacted boronic acid compound and block copolymer that has been cross-linked. 1H NMR (CDCl3, δ): 4.2−3.9 (2H, bs), 3.8−3.3 (19H, bm), 2.7−2.0 (1H, bm), 2.0−1.1 (5H, bm), 1.1−0.7 (2H, bm). Instrumentation. 1H NMR (400 MHz) spectrum was recorded JEOL JNM-ECX400. The UV−vis spectra were recorded with a on a JASCO V-630BIO UV−vis spectrophotometer. Fluorescence spectra were obtained from a JASCO FP-6100 spectrofluorophotometer. Elemental analysis was carried out on a Perkin-Elmer 2400 II CHNS/ O analyzer. Number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) were estimated by GPC using a system consisting of a Tosoh DP-8020 pump and Viscotek. The column set was as follows: a guard column [TSK guard column HXL-H (4.0 cm) and four consecutive columns (Tosoh TSK-GELs (exclusion limited molecular weight): GMHXL (4 × 108), G4000HXL (4 × 105), G3000HXL (6 × 104), and G2500HXL (2 × 104), 30 cm each] eluted with THF at a flow rate of 1.0 mL/min. Polystyrene standards were employed for calibration. DLS measurement was performed by using an Sysmex Zatasizwer Nano with a He−Ne laser (λ0 = 632.8 nm) and a scattering angle of 90°. Prior to the light scattering measurement, the polymer solutions were filtered using Millipore Teflon Filters with a pore size of 0.2 μm into a dust-free cylindrical cuvette. Tapping mode SFM observation was performed with an Agilent AFM 5500, using microfabricated cantilevers with a force constant of ∼34 N/m. The sample was prepared as follows; The chloroform solution of the crosslinked nanoparticles (0.5 mg/L) was casted onto mica substrate and dried at room temperature.

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poly(PEGMA)-b-poly(DB3VT) was synthesized by RAFT polymerization of DB3VT using poly(PEGMA) macro-CTA (Mn,NMR = 13 000 and Mw/Mn = 1.16) under suitable conditions. The polymerization was carried out at 60 °C for 12−24 h with different ratios of [M]:[macro-CTA] (30:1 to 200:1), maintaining the ratio of [macro-CTA]:[AIBN] = 5:1. The results are summarized in Table 1. As expected, DB3VT was successfully polymerized from the dithiobenzoate-terminated macro-CTA to yield amphiphilic block copolymers with narrow polydispersities (Mw/Mn = 1.24 to 1.33). Furthermore, the comonomer compositions (m:n) of each segment in the block copolymers could be precisely controlled by adjusting the [M]:[macro-CTA] ratios. The size exclusion chromatography (SEC) profiles of the poly(PEGMA) macro-CTA and block copolymers show that the molecular weight clearly shifts to a lower elution time, which is consistent with increasing molecular weight (Figure 1). It should be noted that the

RESULTS AND DICUSSION

Synthesis and Characterization of Amphiphilic Block Copolymers. Scheme 1 shows the synthetic route of amphiphilic block copolymers composed of PEGMA and DB3VT, poly(PEGMA)-b-poly(DB3VT). The synthetic approach by RAFT polymerization involves two steps. In the first step, poly(PEGMA) macro-CTAs with narrow polydispersities (1.16−1.26) were prepared from cumyl dithiobenzoate as a CTA and PEGMA. The dithiobenzoate-type CTA having tertiary leaving group, cumyl dithiobenzoate, used in our previous work41 was chosen in consideration of the second step reaction, which was the polymerization of DB3VT from the methacrylate-type macro-CTA. It was reported that cumyl dithiobenzoate was effective to obtain well-defined poly(methyl methacrylate) macro-CTA, and chain extension from the macro-CTA to DB3VT could be well-controlled under suitable conditions to afford block copolymers having cross-linkable poly(DB3VT) segments.41 In this study, the molecular weight of poly(PEGMA) macro-CTA was precisely controlled in the range of 8600 to 38 000. (See Table S1 and Figure S1 of the Supporting Information for details.) In the second step,

Figure 1. SEC profiles of poly(PEGMA)-b-poly(DB3VT) and poly(PEGMA) macro-CTA. See Table 1 for detailed polymerization procedure.

small shoulders at the lower elution time are due to byproduct formed by undesired bimolecular termination. In the 1H NMR spectrum (Figure 2a), the peaks corresponding to both segments are clearly observed, indicating that the desired block copolymers were obtained. The block copolymers showed good solubility in THF, chloroform, and dichloromethane regardless of the comonomer composition. Additionally, the block copolymers with high poly(PEGMA) contents were soluble in methanol. Preparation of Cross-Linked Nanoparticles by Suzuki Coupling Reaction. The preparation of cross-linked nanoparticles was carried out by the Suzuki coupling reaction between poly(PEGMA)-b-poly(DB3VT) and 2,5-thiophenediboronic acids in a THF/H2O solution in the presence of the catalytic amount of Pd(dppf)Cl2 at 70 °C for 24 h (Scheme 2). 3199


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Figure 2. 1H NMR spectra of poly(PEGMA)-b-poly(DB3VT) (top) and cross-linked nanoparticle (bottom) in CDCl3.

In the THF/H2O solution (1:9 and 3:7 vol %), poly(PEGMA)b-poly(DB3VT) can form micelles consisting of a hydrophobic core of poly(DV3VT) covered with a hydrophilic shell of poly(PEGMA) segments. The inter- and intramolecular coupling reaction to form polythiophene segments within micelles (i.e., the formation of cross-linked core−shell nanoparticles) is, thus, likely to prospective under the Suzuki coupling reaction conditions described above. After the coupling reaction, purification was carried out by dialysis with water and then with THF to yield a brown viscous product. The obtained product is soluble in common organic solvents, such as THF, chloroform, and dichloromethane. In the 1H NMR spectra (Figure 2), a broad peak corresponding to the DV3VT unit is clearly observed at 6.7 to 7.2 ppm in poly(PEGMA)-b-poly(DB3VT), whereas no peak corresponding to the poly(DV3VT) segment is detected in the product obtained after the Suzuki coupling reaction, implying the formation of the cross-linked core−shell nanoparticles with the polythiophene core. The solvent (i.e., volumes of THF and H2O) is a dominant factor in the preparation of well-defined nanoparticles (Table 2). Except for THF/H2O = 1:9 and 3:7 vol %, broad or bimodal hydrodynamic diameters are observed in DLS analysis (Figure 3), which is probably due to the fact

Figure 3. DLS profiles of cross-linked nanoparticles in THF solution (conc. = 2.0 mg/mL): (a) Runs 1, 2, and 6 and (b) Runs 3, 4, and 5 in Table 2.

that the micelles are not stable before the coupling reaction under such conditions. Comparison to the DLS profiles of the block copolymer before the cross-linking reaction (Figure S2 in the Supporting Information) suggested that the signal for small diameter (∼30 nm) observed in Figure 3b (Runs 3−5) can be corresponding to that of free block copolymers. Sharp and unimodal DLS profiles are observed from nanoparticles prepared in THF/H2O = 1:9 and 3:7 vol % (Runs 1, 2, and 6 in Table 2), and the size of nanoparticles could be precisely controlled by adjusting the compositions of each segment. The aforementioned conditions are particularly effective for preparing well-defined nanoparticles. Note that other factors, such as nature of the palladium catalyst, reaction temperature, and reaction procedure, were investigated in addition to the solvent conditions (see Tables S2−S4 in the Supporting

Scheme 2. Preparation of Cross-Linked Core-Shell Nanoparticle by Suzuki Coupling Reaction

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of the cross-linked dense polythiophene core is expected. (See the next section.) SFM Observation of Cross-Linked Nanoparticles. The tapping mode phase and height images of the cross-linked nanoparticles were recorded under ambient conditions on 2 × 2 μm2 size scales (Figure 5). The sample was prepared from a

Information for details), and these parameters had no significant effects on the size and distribution of the crosslinked nanoparticles. Optical Properties of Cross-Linked Nanoparticles. The optical properties of the cross-linked nanoparticles in the solutions were investigated by absorption and fluorescence spectra (Figure 4). The absorption maximum peak correspond-

Figure 5. SFM images of the cross-linked nanoparticles (Run 2 in Table 2): (a) height image (z-range: 20 nm), (b) phase image (zrange: 10°), (c) cross-sectional height information in the square in panel a, and (d) cross-sectional phase information in the square in panel b.

diluted chloroform solution (0.5 mg/L) that would fully dissolve the block copolymer prior to cross-linking. Uniform nanoparticles are clearly observed in the phase and height images, indicating that the resulting nanoparticles are stable due to the cross-linking structure. The height and diameter of the nanoparticles on the mica substrate were determined to be about 10 and 150 nm, respectively. Furthermore, in the information of cross-sectional phase image, the difference of the hardness (i.e., positive and negative in the z-axis) corresponds to the cross-linked hard core derived from poly(DV3VT) and soft shell of the poly(PEGMA) segments, demonstrating the formation of the desired core−shell nanoparticles. These SFM data are consistent with the aforementioned 1H NMR characterization and optical properties. The formation of the cross-linked nanoparticles is also supported by the DLS profiles of the block copolymer and cross-linked product in a good solvent for both components of the block copolymer (Figure S4, Supporting Information). The size of the block copolymer before cross-linking is apparently different from that of the cross-linked nanoparticles in the good solvent (e.g., THF and chloroform), suggesting that the block copolymers do not form the micelles under such conditions. In other words, this result indicates that the products visualized by SFM measurement correspond to the cross-linked nanoparticles, whereas the block copolymers do not provide similar SFM images in the good solvent. In general, cross-linked micelles stay in their spherical shape with a height similar to the diameter (usually less because the shell is soft and flattens). In this study, the height of the products determined by SFM measurement is only 10 nm, which may be due to the fact that the poly(PEGMA) shell is very flexible owing to poly(ethylene glycol) side chain. Note

Figure 4. (a) Absorption and (b) fluorescence spectra of block copolymer and cross-linked nanoparticle in THF solution (conc. = 1.0 × 10−4 thiophene unit mol/L). See Table 2 for detailed sample information.

ing to a thiophene ring is observed at around 250 nm in the absorption spectrum of the block copolymer before the coupling reaction. The cross-linked nanoparticles obtained after the coupling reaction show the red-shifted absorption maximum peaks at ∼330 nm. These results indicate that the Suzuki coupling reaction was successfully achieved in the block copolymer, resulting in the red-shifted absorption derived from the expansion of π-conjugation lengths. In the fluorescence spectra, no emission peak is observed in the block copolymer, whereas the emission peaks derived from the polythiophene core are clearly observed at ∼530 nm in the cross-linked nanoparticles. According to Rentsch and coworkers,45 the absorption maximum peak at ∼330 nm corresponded to that of terthiophene, and its Stokes shift was observed as ∼70 nm (i.e., the emission maximum peak was observed at ∼400 nm). Compared with these data, the emission peaks of the crosslinked nanoparticles are further red-shifted, which should be attributed to the strong interchain π−π interaction. Given the 1 H NMR characterization and optical properties, the formation 3201


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Scheme 3. Preparation of Cross-Linked Core-Shell Nanoparticles with Various Coupling Agents

obtained in high yields (>90%). The obtained nanoparticles were soluble in common organic solvents such as THF, chloroform, and dichloromethane. In the absorption spectra, the Ph-, Flu-, and dTh-based nanoparticles show the absorption maximum peaks derived from the expansion of π-conjugated lengths at 332, 346, and 370 nm, respectively. In the fluorescence spectra, the emission maximum peaks of Ph-, Flu-, and dTh-based nanoparticles are observed at 467, 446, and 524 nm, respectively (Figure 6). Flu-based nanoparticles show much stronger emissions than Ph- and dTh-based nanoparticles due to the efficient fluorescence quantum yield of fluorine moieties. Furthermore, a smaller Stokes shift is

that the block copolymers having long poly(PEGMA) segments were employed for the preparation of the cross-linked nanoparticles, resulting in the formation of the core−shell nanoparticles having relatively thick poly(PEGMA) shell. Nevertheless, SFM measurements indicate that the size of all nanoparticles is uniform, in which the cross-linking structure may contribute to the uniformity of the nanoparticle. The advantages of cross-linked core−shell nanoparticles46−50 are the uniformity and stability compared with stabilized nanoparticles (non-cross-linked nanoparticles).51,52 In our system, crosslinked core−shell nanoparticles based on amphiphilic block copolymers were prepared by facile bottom-up modification (in situ cross-linking reaction after micellization). In contrast, the preparation of cross-linked core−shell nanoparticles is usually carried out by two-step modification (micellization, followed by the cross-linking reaction).46,47 These results clearly demonstrate that our process is the efficient approach (i.e., one-step cross-linking) for preparing stable and uniform core−shell nanoparticles. Preparation of Cross-Linked Nanoparticles with Various Coupling Agents. The tuning of optical properties in nanoparticles is an interesting topic.53−55 The relationship between the optical properties and chemical structures of πconjugated components in nanoparticles was thus investigated by changing coupling agents in the Suzuki coupling reaction. Using the method of preparing cross-linked core−shell nanoparticles with 2,5-thiophenediboronic acid, three nanoparticles, phenylene- (Ph-), fluorine- (Flu-), and dithiophene(dTh-) based nanoparticles, were prepared with 1,4-benzenediboronic acid bis(pinacol) ester, 9,9-dihexylfluorene-2,7diboronic acid, and 2,2-bithiophene-5,5-diboronic acid bis(pinacol) ester, respectively (Scheme 3). As can be seen in Table 3, cross-linked core−shell nanoparticles were successfully Table 3. Preparation of Cross-Linked Core-Shell Nanoparticles Using Various Coupling Agentsa sample

coupling agentb

yield (%)c

λmaxabs (nm)d

λmaxem (nm)d

Ph-based particle Flu-based particle dTh-based particle

Ph Flu dTh

93 91 91

332 346 370

467 446 524

a

Reaction was carried out between poly(PEGMA)85-b-poly(DB3VT)15 and coupling agent at 70 °C for 24 h in THF/H2O (3:7 vol %) in the presence of Pd(dppf)Cl2 (10 mol % for the coupling agent). bPh:1,4benzenediboronic acid bis(pinacol) ester, Flu:9,9-dihexylfluorene-2,7diboronic acid, and dTh:2,2-bithiophene-5,5-diboronic acid bis(pinacol) ester. cAfter dialysis with water and THF. dMeasured in THF solution.

Figure 6. (a) Absorption and (b) fluorescence spectra of Ph-, Flu-, and dTh-based nanoparticles in THF solution (conc. = 1.0 × 10−4 thiophene unit mol/L). 3202


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observed in Flu-based nanoparticles compared with Ph- and dTh-based nanoparticles, resulting from the weaker interchain π−π interaction due to alkyl side chains in the fluorine moieties. These results demonstrate that cross-linked nanoparticles with various π-conjugated segments can be readily prepared by our facile bottom-up modification and the optical properties are tunable by adjusting the structures of the πconjugated segments.

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CONCLUSIONS A well-defined amphiphilic block copolymer, poly(PEGMA)-bpoly(DB3VT), was successfully synthesized by RAFT polymerization. Cross-linked nanoparticles were then prepared by facile bottom-up modification, which was the formation of micelles in THF/H2O solution, followed by the in situ Suzuki coupling reaction between 2,5-dibromide groups in the block copolymer and 2,5-thiophenediboronic acids in the presence of a palladium catalyst. The size of the cross-linked nanoparticles could be readily controlled by adjusting the composition of each segment in the block copolymers. Red-shifted absorption maximum peaks and appearance of emission peaks were observed after the cross-linking reaction, indicating the formation of a polythiophene core by the Suzuki coupling reaction. SFM observation revealed the formation of stable and uniform cross-linked nanoparticles. Three nanoparticles (Ph-, Flu-, and dTh-based nanoparticles) were also prepared with various coupling agents using the procedure described above, and the optical properties could be controlled by adjusting the structure of π-conjugated segments. Consequently, cross-linked core−shell nanoparticles were successfully prepared by the facile bottom-up modification using well-defined amphiphilic block copolymers having cross-linkable sites, and the tuning of their optical properties was also accomplished by adjusting the structure of the π-conjugated components.

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ASSOCIATED CONTENT

* Supporting Information S

Figure showing SEC profiles of poly(PEGMA) macro-CTAs, DLS profiles of the block copolymer in different solvents, comparison of DLS profiles of the block copolymer before and after the cross-linking reaction, and tables summarizing results of synthesis of poly(PEGMA) macro-CTAs and preparation of cross-linked nanoparticles under various coupling reaction conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Macromolecules

Article

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Cross-Linked CoreâShell Nanoparticles Based on Amphiphilic Block

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