J. Phys. Chem. C 2009, 113, 6009–6013
6009
Graftlike Block Copolymer Bearing Biodegradable ε-Caprolactone Branches: A Facile Route to Hollow Nanocages Youhua Tao,†,‡ Xiaoya Liu,*,†,§ Dongjian Shi,† Mingqing Chen,†,§ Cheng Yang,†,§ and Zhongbin Ni†,§ School of Chemical and Material Engineering, Jiangnan UniVersity, Wuxi, 214122, P. R. China, Department of Applied Chemistry, Graduate School of Engineering, Nagoya UniVersity, Japan, State Key Laboratory of Food Science and Technology, Jiangnan UniVersity, Wuxi, 214122, P. R. China ReceiVed: December 5, 2008; ReVised Manuscript ReceiVed: February 23, 2009
A graftlike block copolymer strategy was used to construct hollow nanocages with changeable cavity dimensions, where a graftlike block copolymer bearing biodegradable poly(ε-caprolactone) branches (PMAAb-PFM) was prepared by the selective hydrolysis of block copolymer of tert-butyl methacrylate and macromonomer FM [CH2dC(CH3)OOCH2CH2(OCOCH2CH2CH2CH2CH2)5OH] (PtBMA-b-PFM) precursor, which was synthesized by sequential controlled/living radical polymerizations of tBMA and FM. Selfassembling of PMAA-b-PFM into polymer micelles in water and cross-linking of the hydrophilic shell via condensation reactions between the carboxylic acid groups and the amino groups, followed by lipase degradation of the poly(ε-caprolactone) core, ultimately produces hollow nanocages. Transmission electron microscopy and laser light-scattering studies showed that the resultant hollow nanocages were nanoscale with narrow size distribution. Moreover, relative lengths of the hydrophobic and hydrophilic segments strongly influenced the hydrodynamic radius of the spherical micelles (52-101 nm) as well as the cavity size of the resultant hollow nanocages (62-85 nm in radius). The obtained polymeric nanocages should be useful in various fields, such as catalysis, drug delivery and structural material. 1. Introduction Because of the great range of potential applications, there has been great progress in the past decade1-9,34-36 in the development of polymeric nanostructures. Among the target materials, nanometer-sized hollow polymer spheres, or so-called hollow nanocages, are especially important for a wide range of applications, including drug delivery, encapsulation, and imaging.10-19,37-42 The most studied procedures for preparing polymeric hollow nanospheres from block copolymer precursors involves many steps: preparing micelles in the selective solvents, cross-linking of the micellar shell, and removing the core by chemical degradation.20-24 Much work has been done to prepare the hollow nanocages, even though the key challenge comes from the complicated and tedious synthesis of block copolymers containing both crosslinkable and degradable blocks. Wooley employed diblock copolymers of polyisoprene-block-poly(acrylic acid) for the construction of hollow nanocages. The composition of the nanocages from the outer surface, through the shell, and including the inner surface was well-controlled.21 Following Wooley’s discoveries, Liu developed a kind of nanocage using triblock copolymers of polyisoprene-block-poly(2-cinnamoylethyl methacrylate)-block-poly(tert-butyl acrylate).23 However, the block copolymers mentioned above were synthesized by sequential anionic polymerization, which was limited by its strict conditions, and the core removal was through ozonolysis of core material instead of by biodegradation, which restricted its * Corresponding author. Fax: +86-510-89880780. E-mail: lxy@ jiangnan.edu.cn. † School of Chemical and Material Engineering, Jiangnan University. ‡ Nagoya University. § State Key Laboratory of Food Science and Technology, Jiangnan University.
practical application. Subsequently, core-biodegraded nanocages were obtained employing poly(ε-caprolactone)-b-poly(acrylic acid) (PCL-b-PAA), which was prepared from the selective hydrolysis of a poly(ε-caprolactone)-b-poly(tert-butyl acrylate) (PCL-b-PtBA) precursor, which was synthesized by ringopening polymerization of ε-CL, followed by controlled/living radical polymerizations of tBA.20 Recently, using block copolymer of poly(N-isopropylacrylamide-co-N-hydroxymethylacrylamide-co-3-(trimethoxysilyl) propyl methacrylate)-b-poly(εcaprolactone), Zhuo achieved thermoresponsive hollow polymer spheres with an organic/inorganic hybrid shell layer; however, the synthesis of this block copolymer was extremely tedious, including free radical copolymerization, ring-opening polymerization, and the cross-linking reaction of PCL block and P(NIPAAm-co-HMAAm-co-MPMA) block.24 In previous studies, Jiang and Liu have reported the preparation of hollow nanospheres by a “block-copolymer-free strategy”, employing PCL and the random copolymer MAF from methylacrylic acid (MAA),macromonomerFA[CH2dCHOOCH2CH2(OCOCH2CH2CH2CH2CH2)3OH], and methyl methacrylate (MMA).25 The polymers mentioned above were easy to synthesize; however, the structure and molecular weight of the polymers were difficult to control due to the intrinsic defect of free radical polymerization. Therefore, the composition and morphologies of the resultant nanocages were hard to control. Macromonomer FM [CH2dC(CH3)OOCH2CH2(OCOCH2CH2CH2CH2CH2)5OH], bearing long biodegradable hydrophobic ε-caprolactone branches, is commercially available and novel in both structure and properties.26 In the present report, as shown in Scheme 1, an amphiphilic graftlike block copolymer of MAA and macromonomer FM (PMAA-b-PFM) was prepared by the selective hydrolysis of a block copolymer of tert-butyl methacrylate and macromonomer FM (PtBMA-b-PFM), which was
10.1021/jp8107073 CCC: $40.75 2009 American Chemical Society Published on Web 03/25/2009
6010 J. Phys. Chem. C, Vol. 113, No. 15, 2009 SCHEME 1: Synthesis Route of Graftlike PMAA-b-PFM Block Copolymer
Tao et al. TABLE 1: Characterization Data of Parent Copolymers PtBMA-b-PFMa PtBMAn-b-PFMm
nb
mb
Mnc × 10-4 (g/mol)
Mw/Mnc
PtBMA50-b-PFM25 PtBMA64-b-PFM16 PtBMA108-b-PFM11 PtBMA201-b-PFM10
50 64 108 201
25 16 11 10
2.5 2.1 2.3 3.5
1.16 1.14 1.13 1.16
a Solution polymerization in toluene at 90°C; The molar ratio of CuCl to PMDETA to macroinitiator was 1.2 to 2.6 to 1. b The numbers of tBMA repeating units (n) and FM repeating units (m) were determined by comparison of the intensity of the unique PCL protons resonating at 4.2 ppm to the intensity of the resonance at 1.19 ppm of side methyl protons in the 1H NMR spectra. c Determined by GPC.
TABLE 2: Characterization Data of PMAA-b-PFM Copolymers PMAAn-b-PFMm
na
PCL wt % in Mnb × 10-4 ma copolymers (%) (g/mol) Mw/Mnb
PMAA50-b-PFM25 50 25 PMAA64-b-PFM16 64 16 PMAA108-b-PFM11 108 11 PMAA201-b-PFM10 201 10
SCHEME 2: Schematic Illustration of the Preparation Route to the Hollow Nanocage
70.5 58.9 39.8 25.3
2.2 1.7 1.8 2.4
1.18 1.15 1.14 1.18
a The numbers of MAA repeating units (n) and FM repeating units (m) were determined by comparison of the intensity of the unique PCL protons resonating at 4.2 ppm to the intensity of the resonance at 1.19 ppm of side methyl protons in the 1H NMR spectra. b Determined by GPC.
copolymer MAF,25 the composition and cavity size of the nanocages presented here is easy to control, which is also very encouraging, since a changeable dimension in cavity of the nanocages may be suitable to encapsulate guest molecules with varying molecular size. 2. Results and Discussion
synthesized by the sequential controlled/living radical polymerizations of tBMA and FM, where the PCL branches in PMAA backbone serves as the biodegradable core. Self-assembly of PMAA-b-PFM into polymer micelles in water and cross-linking of the hydrophilic shell via condensation reactions between the carboxylic acid groups and the amino groups, followed by lipase degradation of the PCL core, ultimately produces hollow nanocages (Scheme 2). Unlike hollow nanocages from PCLb-PAA or P(NIPAAm-co-HMAAm-co-MPMA)-b-PCL,20,24 the synthesis of PMAA-b-PFM, involves only controlled/living radical polymerizations, and the selective cleavage of the tertbutyl esters was relatively simple and easy to deal with, which is extremely important for its future applications. Moreover, different from hollow nanospheres using PCL and the random
Block copolymer of MAA and macromonomer FM (PMAAb-PFM) was synthesized by controlled/living radical polymerization (CRP).28 To avoid damage to the catalytic system in CRP by the carboxyl groups in MAA, first of all, diblock copolymers of FM and tert-butyl methacrylate (tBMA) were prepared via CRP. The compositions and molecular weight distributions of PtBMA-b-PFM diblock copolymers were characterized by 1H NMR and gel permeation chromatography (GPC) (Table 1). The relative and overall lengths of the PtBMA and PFM blocks were varied to investigate the effect of copolymer composition on the properties of the shell crosslinked nanospheres (SCKs) as well as hollow nanocages. After the selective cleavage of the tert-butyl esters of the PtBMA block, amphiphilic block copolymers of FM and MAA were obtained (Scheme 1). The removal of the tert-butyl ester groups was confirmed by the complete disappearance of the tert-butyl 1H resonance at 1.42 ppm (see the Experimental Section). By controlling the hydrolysis time, tert-butyl ester side groups were removed selectively while leaving the PCL branches essentially intact (no change in the numbers of FM repeating units according to 1H NMR results shown in Tables 1 and 2). Furthermore, the molecular weight difference between the parent PtBMA-b-PFM diblock copolymers and PMMA-bPFM copolymers was close to that of lost tert-butyl side groups. For example, PMAA50-b-PFM25 corresponded to a loss of ∼50 tert-butyl units of PtBMA50-b-PFM25 (Tables 1 and 2), also indicating the PCL branches were intact, though GPC measurement was not as accurate as that of 1H NMR.
Graftlike Block Copolymer
J. Phys. Chem. C, Vol. 113, No. 15, 2009 6011
Figure 1. TEM images of polymeric micelles from PMAA50-b-PFM25.
Figure 2. Rh of the nanoparticles from PMAA50-b-PFM25 before and after cross-linking in aqueous solution: 9, un-cross-linked nanoparticles; b, cross-linked nanoparticles.
The shell cross-linked nanospheres (SCKs) were then prepared by the self-assembly and cross-linking procedure, employing the PMAA-b-PFM materials as the amphiphilic diblock copolymers (Scheme 2). Polymer micelles were formed by first dissolving PMAA-b-PFM in DMF and then gradually adding water as a nonsolvent for PCL branches. The micelle solution was stirred for 24 h and then dialyzed in water using a cellulose dialyzer tube. Micelle morphologies were directly observed using a transmission electron microscope (TEM). A typical TEM image of the micelles is shown in Figure 1. As shown in the Figure, spherical micelles with a narrow size distribution were obtained from PMAA-b-PFM. The number-average radius (Rn), weight-average diameter (RW), and the polydispersity index (PDI) of micelles can be estimated from TEM micrographs using the following equations:29 N
Rn )
∑ Ri/N
(1)
Figure 3. Effect of the hydrodynamic radius of micelles on the content of PCL branches in copolymer in aqueous solution (C ) 5.0 × 10-4 g/mL).
i)1
N
Rw )
∑ i)1
N
Ri4 /
∑
Ri3
(2)
i)1
PDI ) Rw /Rn
(3)
According to the calculation results from TEM, the Rn of the micelles was ∼49nm, and the PDI value was 1.007, much lower than 1.05. If the PDI value was less than 1.05, the resulting micelles could be considered to be monodisperse.30 The average size and size distrubution of micelles could also be determined from dynamic laser light-scattering (DLS) measurement. The Rh was 52 nm from the DLS method, which was consistent with the TEM results. The hydrodynamic radius distribution of the micelles was unimodal (Figure 2), also indicating formation of the expected monodispersed nanosphere micelles. PMAA-bPFM copolymers with different composition and relative lengths of the hydrophobic and hydrophilic block segments were used to prepare micelles of different dimensions. The dependence of micelle size on the composition of PCL branches was investigated by DLS. In all cases, size polydispersity (µ2/2) was found to be in the range 0.08-0.16, indicating the particles showed relatively narrow size distributions. The particle size varied considerably, depending on the weight fraction of the PCL branches, and ranged from 52 to 101 nm (Figure 3). This observation was reasonable because an increase in the PCL branch content implied there was a relative increase in the hydrophobic interaction of copolymers, which would certainly make more tight the aggregate of the hydrophobic PCL branches. Stabilization of the polymer micelles was then
achieved through formation of shell cross-linked nanoparticles by cross-linking the hydrophilic shell layer via condensation reactions between the carboxylic acid functionalities of PMAA and the amine functional groups of 2,2′-(ethylenedioxy)bis(ethylamine) in the presence of 1-(3-dimethylaminopropyl)-3ethylcarbodiimide methiodide as the cross-linking reagent (Scheme 2).20,25 DLS measurements showed that after crosslinking, the SCKs displayed less swelling than the un-crosslinked shell. For example, for nanoparticles from PMAA50-bPFM25, the average hydrodynamic radius (Rh) varied from 52 to 39 nm as a result of cross-linking (Figure 2). A similar phenomenon was also observed in the other cross-linked micelle system.25,27 The shell cross-linked nanospheres, composed of a PCL core domain that was surrounded by a cross-linked PMAA shell, serve as the precursors to the hollow nanocages. The enzyme lipolase was a type of lipase that can selectively degrade polyesters and fats; hence, it was chosen to hydrolyze the polyester core domain without affecting the cross-linked PMAA main chains. The morphology of the particles after core degradation was studied directly by TEM. As shown in Figure 4, all of the particles display a hollow structure and regular globular shape, indicating the successful construction of hollow nanocages. Moreover, different from the SCKs shown in Figure 1, the hollow nanocages have expanded significantly to about 110 nm in radius (Figure 4A), which is indicative of the swelling of the nanocages after removing the PCL core. This expansion was caused by releasing the restriction imposed by the insoluble PCL branches on the solvated PMAA backbone in the shell, similartothepreviouslyreportedmicellesfromblockcopolymers.21,31 PMAA-b-PFM copolymers with different compositions and
6012 J. Phys. Chem. C, Vol. 113, No. 15, 2009
Tao et al. controlled/living radical polymerization after the removel of the tert-butyl group by hydrolysis, and a graftlike block copolymer bearing biodegradable branches (PMAA-b-PFM) was obtained, which provided a practical and facile route to construct polymeric hollow nanocages. The resultant hollow nanocages bear a regular spherical shape as well as narrow size distribution. The relative lengths of the hydrophobic and hydrophilic segments strongly influenced the cavity size of the resultant hollow nanocages (62-85 nm in radius), which greatly facilitated the application of nanocages. 4. Experimental Section
Figure 4. TEM images of nanocages from PMAA-b-PFM with different PCL branches contents: A, PCL% ) 70.5%; B, PCL% ) 25.3%.
Figure 5. The effects of pH value on the Rh of the nanocages from PMAA50-b-PFM25.
relative lengths of the hydrophobic and hydrophilic block segments were used to prepare nanocages with different cavity dimensions. The number average radius of nanocages’ cavity could also be estimated from TEM micrographs. As shown in Figure 4, nanocages from copolymer with higher PCL branche contents (PMAA50-b-PFM25) had a much larger cavity size (85 nm in radius), whereas the lower PCL branch content samples (PMAA201-b-PFM10) had much a smaller cavity dimension (62 nm in radius). This is very encouraging, since a changeable dimension in the cavity of the nanocages may be suitable to encapsulate guest molecules with varying molecular sizes. Nanocages from block copolymer PMAA-b-PFM displayed pH sensitivity in aqueous media since it contained the pHsensitive PMAA block.43 Figure 5 shows the dependence of the Rh of the micelles on the solution pH value. It was found that the average radius changed drastically between pH 6 and 7. Below pH 6, deprotonation of the PMAA segments was inhibited, so nanocages could be thought of as more tightly integrated under these conditions. Above pH 7, however, electrostatic repulsion between the PMAA segments became a crucial factor, directly affecting the average size of the polymeric nanocages. The application of these pH-sensitive nanocages as a drug carrier will be subsequently reported. 3. Conclusions Block copolymers of tert-butyl methacrylate and macromonomer FM with well-defined structure were synthesized via
Materials. Macromonomer FM [CH2dC(CH3)OOCH2CH2(OCOCH2CH2CH2CH2CH2)5OH] was purchased from Daicel Chem. Co. Ltd. and used as received. tert-Butyl methacrylate (t-BMA) was purchased from Wako, dried over CaH2, and then distilled under reduced pressure before polymerization. EthylR-bromopropionate (EPN-Br), PMDETA (99.5%) purchased from Aldrich were used as received. All other reagents from Shanghai Chemical Reagent Factory were purified following standard procedures prior to use. Measurements. 1H NMR spectra were acquired on a Bruker Avance Digital 400 MHz NMR spectrometer in CDCl3. The molecular weight and polydispersity of the polymer were determined using gel permeation chromatography (GPC, Agilent 1100) and DMF as the eluent (1.0 mL/min) at 35 °C. The calibration curve was established using a set of narrowly distributed polystyrene as the standard. The hydrodynamic radius, Rh, of the nanoparticles was measured by laser light scattering (ALV500E, ALV Co., Germany). All DLS measurements were performed at θ ) 80° and 25 °C, and all samples with a total polymer concentratin of 5.0 × 10-4 g/mL were clarified through a 0.45 µm Teflon membrane filter (Chromatographlic Specialties Inc.). The pH of the solution was monitored using a pH meter (Horiba 50-H). TEM images were obtained with a Hitachi 7000A microscope operated at an acceleration voltage of 150 kV at a magnification of 100 000 times. Specimens were prepared by slow evaporation of a drop of approximately diluted solution deposited onto a collodioncoated copper mesh grid, followed by carbon spattering or by OsO4 staining. General Procedure for the Preparation of of PtBMA-bPFM Block Copolymer. The diblock copolymers were prepared by sequential controlled/living radical polymerizations of tBMA and FM. PtBMA macroinitiator was synthesized on a double manifold connected to a high-vacuum line and nitrogen following a procedure reported elsewhere. PtBMA-b-PFM block copolymer was then prepared from the PtBMA macroinitiator by the polymerization of FM. In a typical CRP experiment, the PtBMA macroinitiator, catalyst CuCl (1.2 equiv in comparison to macroinitiator), and PMDETA (2.2 equiv in comparison to CuCl) were introduced into a previously dried Schlenck flask filled with nitrogen. A cycle of evacuation under vacuum and backfilling with nitrogen was repeated three times to remove oxygen. To the Schlenck flask were added toluene and tBMA via a syringe, and the mixture was degassed three times by freeze-pump-thaw cycles. The polymerization was performed under nitrogen flow at 90 °C for 24 h. The polymers formed were then dissolved in THF and passed through a short alumina column to remove the catalysts. The diblock copolymers were recovered by precipitation into an excess of 1:1 (v/v) methanol/ water and dried in vacuo at room temperature. The molecular weight distribution (Mw/Mn) of PtBMA-b-PFM was determined by GPC, and the numbers of tBMA repeating units (n) vs FM
Graftlike Block Copolymer repeating units (m) were determined by 1H NMR analysis. Four PtBMA-b-PFM samples with different block lengths were prepared. The relative lengths of PtBMA block and PFM block were controlled primarily by the concentration of the polymerization mixture as well as the polymerization time. 1 H NMR (CDCl3): δ 1.19 [s, CH2C(CH3)(COOtBu)], 1.2-1.9 [br, CH2C(CH3)(COOtBu)], 1.31 [m, CO(CH2)2CH2(CH2)2O], 1.42 [s, CH2C(CH3)COOC(CH3)3], 1.61 [m, COCH2CH2CH2CH2CH2O], 2.28 [t, COCH2(CH2)4O], 3.85 [t, CO(CH2)4CH2O)], 4.2 [t, COOCH2CH2OCO] ppm. General Procedure for the Preparation of of PMAA-bPFM Block Copolymer. In a typical experiment, PtBMA-bPFM was dissolved in dry dichloromethane in a previously flame-dried round-bottom flask under nitrogen. To the solution was then added TMSI (1.3 equiv in comparison to tBMA groups), and the reaction mixture was stirred under nitrogen for 80 min until dichloromethane was evaporated out. The resulting polymer was dissolved in THF. To the polymer solution was added dropwise 0.1 N HCl solution containing ∼1 wt % Na2S2O5. The mixture was stirred for 2 h and then dialyzed against deionized water for 2 days to remove the salt and other byproducts. The numbers of MAA repeating units (n) and FM repeating units (m) were determined by 1H NMR analysis. 1H NMR (CDCl3): δ1.19 [s, CH2C(CH3)(COOH)], 1.2-1.9 [br, CH2C(CH3)(COOH)], 1.31 [m, CO(CH2)2CH2(CH2)2O], 1.61 [m, COCH2CH2CH2CH2CH2O], 2.28 [t, COCH2(CH2)4O], 3.85 [t, CO(CH2)4CH2O)], 4.2 [t, COOCH2CH2OCO], 12 [s, CH2C(CH3)(COOH)] ppm. Formation of Micelles. Block copolymer PMAA-b-PFM (0.05 g) was dissolved in 100 mL of DMF, followed by filtration of the solution through a 0.45 mm Teflon membrane filter (Chromatographic Specialties, Inc.). The micelles were formed by the addition of the resulting DMF solution into water at a rate of 1 dropper per 60 s with continuous stirring. The micelle solution was stirred for 24 h and then dialyzed in water using a cellulose dialyzer tube. General Procedure for Cross-Linking the PMAA Block To Form Shell Cross-Linked Nanospheres. The micellar structure was locked by cross-linking the hydrophilic shell layer through condensation reactions between the carboxylic acid groups of PMAA block and the amino groups of hexamethylenediamine in the presence of 1-(3-dimethyl aminopropyl)-3ethylcarbdiimide methiodide, which activates the carboxylic acid at room temperature.20 The cross-linked product was then dialyzed with distilled water for 3 days to remove the byproduct of the reaction. The success of the cross-linking was confirmed by the fact that the resultant nanoparticles maintained their integrity upon switching the medium from water to a solvent mixture containing a large proportion of DMF. DLS studies on the cross-linked particles found that no particle aggregation had taken place, which meant that there had been almost zero interparticle cross-linking. Hydrolysis of the PCL Core of the Shell Cross-Linked Nanospheres. For conducting biodegradation of the core, an appropriate amount of dust-free lipolase solution in water (Novozymes Co.) and dilute aqueous NaOH solution was added into the shell cross-linked nanospheres dispersion.32,33 Typical reaction conditions: SCKs, C ) 5.0 × 10-4 g/mL, pH ) 8-11at 25 °C. Acknowledgment. This work was supported by National Natural Science Foundation of China (Grant no. 50673038). Helpful discussion with Prof. Ming Jiang in Fudan University, is gratefully acknowledged.
J. Phys. Chem. C, Vol. 113, No. 15, 2009 6013 References and Notes (1) Moffitt, M.; Khougaz, K.; Eisenberg, A. Acc. Chem. Res. 1996, 29, 95. (2) Stupp, S. I.; LeBonheur, V.; Walker, K.; Li, L. S.; Huggins, K. E.; Keser, M.; Amstutz, A. Science 1997, 276, 384. (3) Feng, Y. K.; Klee, D.; Hocker, H. Macromol. Chem. Phys. 1999, 200, 2276. (4) Wei, H.; Cheng, C.; Chang, C.; Chen, W. Q.; Cheng, S.; Zhang, X. Z.; Zhuo, R. X. Langmuir 2008, 24, 4564. (5) Zhang, Y.; Pan, M. G.; Liu, C.; Huang, J. L. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 2624. (6) Zhang, L.; Bernard, J.; Davis, T. P.; Barner-Kowollik, C.; Stenzel, M. H. Macromol. Rapid Commun. 2008, 29, 123. (7) Yin, H. Q.; Lee, E. S.; Kim, D.; Lee, K. H.; Oh, K. T.; Bae, Y. H. J. Controlled Release 2008, 126, 130. (8) Wei, H.; Yu, C. Y.; Chang, C.; Quan, C. Y.; Mo, S. B.; Cheng, S. X.; Zhang, X. Z.; Zhuo, R. X. Chem. Commun. 2008, 4598. (9) Xiao, N.-Y.; Li, A.-L.; Liang, H.; Lu, J. Macromolecules 2008, 41, 2374. (10) Ding, J.; Liu, G. J. Phys. Chem. B 1998, 102, 6107. (11) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903. (12) Cheng, C.; Qi, K.; Khoshdel, E.; Wooley, K. L. J. Am. Chem. Soc. 2006, 128, 6808. (13) Ding, Y.; Hu, Y.; Zhang, L. Y.; Chen, Y.; Jiang, X. Q. Biomacromolecules 2006, 7, 1766. (14) Guo, L.; Liang, F.; Wen, X. G.; Yang, S. H.; He, L.; Zheng, W. Z.; Chen, C. P.; Zhong, Q. P. AdV. Funct. Mater. 2007, 17, 425. (15) Miao, S. D.; Zhang, C. L.; Liu, Z. M.; Han, B. X.; Xie, Y.; Ding, S. F.; Yang, Z. Z. J. Phys. Chem. C 2008, 112, 774. (16) Moughton, A. O.; Reilly, R. K. J. Am. Chem. Soc. 2008, 130, 8714. (17) Nagai, A.; Hirabayashi, T.; Kudo, H.; Nishikubo, T. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 4879. (18) Ras, R. H. A.; Kemell, M.; de Wit, J.; Ritala, M.; ten Brinke, G.; Leskela, M.; Ikkala, O. AdV. Mater. 2007, 19, 102. (19) Turner, J. L.; Wooley, K. L. Nano Lett. 2004, 4, 683. (20) Zhang, Q.; Remsen, E. E.; Wooley, K. L. J. Am. Chem. Soc. 2000, 122, 3642. (21) Huang, H.; Remsen, E. E.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1999, 121, 3805. (22) Sanji, T.; Nakatsuka, Y.; Ohnishi, S.; Sakurai, H. Macromolecules 2000, 33, 8524. (23) Stewart, S.; Liu, G. Chem. Mater. 1999, 11, 1048. (24) Wei, H.; Wu, D. Q.; Li, Q.; Chang, C.; Zhou, J. P.; Zhang, X. Z.; Zhuo, R. X. J. Phys. Chem. C 2008, 112, 15329. (25) Liu, X. Y.; Jiang, M.; Yang, S.; Chen, M. Q.; Chen, D. Y.; Yang, C.; Wu, K. Angew. Chem., Int. Ed. 2002, 41, 2950. (26) Tao, Y. H.; Liu, R.; Liu, X. Y.; Chen, M. Q.; Yang, C.; Ni, Z. B. Chem. Lett. 2008, 37, 1162. (27) Shi, D. J.; Liu, X. Y.; Chen, M. Q.; Yang, C.; Gu, Z. B.; Xiang, F. Acta Polym. Sin. 2004, 600. (28) Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1995, 28, 1721. (29) Chen, M. Q.; Kaneko, T.; Chen, C. H.; Akashi, M. Chem. Lett. 2001, 30, 1306. (30) Chen, M.; Kaneko, T.; Zhang, M.; Liu, X.; Wu, K.; Akashi, M. Chem. Lett. 2003, 32, 1138. (31) Zhu, H.; Yuan, X.; Zhao, H.; Liu, S.; Jiang, M. Chin. J. Appl. Chem. 2001, 18, 336. (32) Gan, Z.; Jim, T. F.; Li, M.; Yuer, Z.; Wang, S.; Wu, C. Macromolecules 1999, 32, 590. (33) Gan, Z.; Fung, J. T.; Jing, X.; Wu, C.; Kuliche, W. K. Polymer 1999, 40, 1961. (34) Rao, J. Y.; Xu, J.; Luo, S. Z.; Liu, S. Y. Langmuir 2007, 23, 11857. (35) Wang, X. F.; Zhang, Y. F.; Zhu, Z. Y.; Liu, S. Y. Macromol. Rapid Commun. 2008, 29, 340. (36) Xu, Y.; Shi, L. Q.; Ma, R.; Zhang, W.; An, Y.; Zhu, X. Polymer 2007, 48, 1711. (37) Miao, S. D.; Zhang, C. L.; Liu, Z. M.; Han, B. X.; Xie, Y.; Ding, S. J.; Yang, Z. Z. J. Phys. Chem. C 2008, 112, 774. (38) Liu, J.; Fan, F. T.; Feng, Z. C.; Zhang, L.; Bai, S. Y.; Yang, Q. H.; Li, C. J. Phys. Chem. C 2008, 112, 16445. (39) Xu, Y. Y.; Chen, D. R.; Jiao, X. L.; Xue, K. Y. J. Phys. Chem. C 2007, 111, 16284. (40) Li, G. L.; Yang, X. L. J. Phys. Chem. B 2007, 111, 12781. (41) Jofre, A.; Hutchison, J. B.; Kishore, R.; Locascio, L. E.; Helmerson, K. J. Phys. Chem. B 2007, 111, 5162. (42) Du, H. B.; Zhu, J. T.; Jiang, W. J. Phys. Chem. B 2007, 111, 12781. (43) Burkhardt, M.; Martinez-Castro, N.; Tea, S.; Drechsler, M.; Babin, I.; Grishagin, I.; Schweins, R.; Pergushov, D. V.; Gradzielski, M.; Zezin, A. B.; Muller, A. H. E. Langmuir 2007, 23, 12864.
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