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Well-Defined Dendritic-Graft Copolymer Grafted Silica Nanoparticle by Consecutive Surface-Initiated Atom Transfer Radical Polymerizations Bin Mu,† Tingmei Wang,‡ and Peng Liu*,† State Key Laboratory of Applied Organic Chemistry, and Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou UniVersity, Lanzhou, Gansu 730000, China, and State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lzhou 730000, China
To obtain well-defined dendritic-graft copolymer grafted silica nanoparticles (SN-HBP-PMMA), the surface halogen atom of the hyperbranched polymer grafted silica nanoparticles (SN-HBP), by surface-initiated selfcondensing vinyl polymerization (SI-SCVP) of p-chloromethyl styrene (CMS) from functionalized silica nanoparticles, was used as initiator for the surface-initiated atom transfer radical polymerization (SI-ATRP) of methyl methacrylate (MMA) for the first time. The well-defined hyperbranched polymer and the dendriticgraft copolymer grafted were validated by 1H NMR analysis. And, the dispersibility of the well-defined polymer grafted silica nanoparticles was investigated with a transmission electron microscope (TEM). Introduction The surface functionalizations of nanomaterials by grafting of polymer are expected to play important roles in the designing of novel organic/inorganic nanocomposite materials. In recent years, much attention has been paid to the use of atom transfer radical polymerization (ATRP) from nanosurfaces via surfaceinitiated techniques,1,2 because this allows better control over the target molecular weight and molecular weight distribution of the target grafted polymers.3 The surface-initiated atom transfer radical polymerization (SI-ATRP) technique had been successfully used for the grafting of well-defined homopolymers,4-9 diblock copolymers,10-17 graft copolymer,18 star polymers,19,20 and hyperbranched polymers21-23 from the nanoparticles, nanotubes, nanowires, and clays. As for the fabrication of the diblock copolymer brushes, the “living” chain ends of the initial homopolymer brushes were used as the macroinitiators for the ATRP of the second monomer.10-17 The self-condensing vinyl polymerization (SCVP) reactions are based on an initiator-monomer (“inimer”) of the general structure AB*, where the double bonds are designated A and B* are groups capable of initiating the polymerization of vinyl groups.24-27 Dendrigraft polystyrene and poly(butyl acrylate) were prepared using poly(p-chloromethyl styrene) as the multicentre initiator by atom transfer radical polymerization (ATRP).28,29 After the surface-initiated SCVP of the inimers from the surfaces of the nanoparticles, well-defined hyperbranched polymers grafted on the nanoparticles should contain a high density of the initiating end groups at the outermost surfaces.21-23 Such surface multifunctionality is ideally independent of the surface curvature of the core nanoparticles and the layer thickness of the polymer shell, which could not be achieved by linear polymers. The well-defined dendritic-graft copolymers could be fabricated from the nanoparticles by using the initiating end groups at the outermost surfaces of the nanoparticles grafted with such hyperbranched polymers as the macroinitiators for the ATRP of the second monomer. Up to the present, we had not found any report about the idea. * To whom correspondence should be addressed. Tel.: 86-9318912516. Fax: 86-931-8912582. E-mail:
[email protected]. † Lanzhou University. ‡ Chinese Academy of Sciences.
In the present work, we prepared the fabrication of the welldefined dendritic-graft copolymer grafted silica nanoparticles (SN-HBP-PMMA) by the surface-initiated atom transfer radical polymerization (SI-ATRP) of methyl methacrylate (MMA) from the hyperbranched polymer grafted silica nanoparticles (SNHBP), prepared by the self-condensing vinyl polymerization (SCVP) of p-chloromethyl styrene (CMS) (as in Scheme 1). Experimental Section Materials and Reagents. The silica nanoparticles with an average particle size of 10 nm used were of type MN1P obtained from Zhoushan Mingri Nano-materials Co. Ltd., Zhejiang, China. It was dried in vacuum at 110 °C for 48 h before use. γ-Aminopropyltriethoxysilane (APTES) (Gaizhou Chemical Industrial Co. Ltd., Liaoning, China) was used as received. Both the bromoacetylbromide and p-chloromethyl styrene (CMS) used were of analytical reagent grade from Acros Organics (Phillipsburg, NJ). Cu(I)Br (Tianjin Chemical Co., Tianjin, China) was analytical reagent grade and was purified by stirring in glacial acetic acid, filtering, washing with ethanol, and drying. 2,2′-Bipyridine (bpy) (A.R., 97.0%) provided by Tianjin Chemical Co., China, was recrystallized twice from acetone. Methyl methacrylate (MMA) was purified by removing inhibitor by filtering through aluminum oxide column, then stirred with CaH2 overnight, and distilled under reduced pressure before use. Triethylamine (TEA) (A.R., 99.0%) was dried by CaH2 overnight, and then distilled under reduced pressure before use. Toluene, tetrahydrofuran (THF), methanol, hydrofluoric acid, and other solvents used were all of analytical reagent grade from Tianjin Chemical Co., China, and were used without further purification. Distilled water was used throughout. Immobilization of Initiator on SN. The preparation procedures of the initiator modified silica nanoparticles (SN), bromoacetyl modified silica nanoparticles (BrA-SN), could be schematically shown as Scheme 2. The aminopropyl-modified silica nanoparticles (AP-SN) were prepared by the self-assembly of APTES from the surfaces of SN with the same procedures as reported previously.30 After 2.0 g AP-SN was dispersed into 30 mL toluene containing 4 mL TEA under electromagnetic stirring in an ice bath. After the above mixture was cooled down to 0 °C, a
10.1021/ie070252+ CCC: $37.00 © 2007 American Chemical Society Published on Web 04/10/2007
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Scheme 1. Fabrication of the Well-Defined Dendritic-Graft Copolymer Grafted Silica Nanoparticles
Scheme 2. Preparation of the Macroinitiator
solution of bromoacethyl bromide (4 mL) and toluene (8 mL) was added drop by drop into the dispersoid and the mixture was stirred with an electromagnetic stirrer for 12 h at room temperature. Then, the BrA-SN was centrifugalized and washed with toluene and ethanol thoroughly and then dried in vacuum for the subsequent polymerization. SI-SCVP of CMS. The surface-initiated self-condensing vinyl polymerization (SI-SCVP) of the inimer p-chloromethyl styrene (CMS) from the BrA-SN macroinitiators was accomplished by the followed procedure (Scheme 1): BrA-SN 0.5 g, the inimer (CMS) 5.0 mL, 215 mg (1.5 mmol) of CuBr, and 470 mg (3 mmol) of bpy were added into a dry round-bottom flask. The mixture was irradiated with ultrasonic vibrations for 30 min, while bubbling with nitrogen (N2). The reaction proceeded at 90 °C for 12 h with electromagnetic stirring. N2 was bubbled throughout the polymerization period. The products, hyperbranched polymer grafted silica nanoparticles (SN-HBP), were separated by centrifugation and subjected to intense washing by toluene. Ultrasonication was used in combination with the above solvents to remove the impurities, and then, the sample was dried in vacuum at 40 °C. Second-Generation Brushes of PMMA. The SI-ATRP of MMA from the SN-HBP nanoparticles was accomplished by immersing the SN-HBP nanoparticles (0.5 g) into the mixture containing MMA (10 mL), 215 mg (1.5 mmol) of CuBr, and 470 mg (3mmol) of bpy. The reaction was preceded at 100 °C for 10 h with electromagnetic stirring after being irradiated ultrasonically for 30 min. N2 was bubbled throughout the polymerization period. The products, dendritic-graft copolymer grafted silica nanoparticles (SN-HBP-PMMA), were treated with the same procedure as the SN-HBP nanoparticles. Polymers for 1H NMR Analysis.31 The hyperbranched polymer grafted silica nanoparticles (SN-HBP) and the dendriticgraft copolymer grafted silica nanoparticles (SN-HBP-PMMA) were dispersed into toluene, and superfluous hydrofluoric acid was added into the dispersion, respectively. Then, the mixture was stirred violently overnight to remove the silica nanoparticle cores. Then, the mixture was distilled, and the residual, the
hyperbranched polymer and the dendritic-graft copolymer, was dried in vacuum overnight for the 1H NMR analysis. Analytical Methods. Elemental analysis (EA) of C, N, and H was performed on an Elementar vario EL instrument. A Bruker IFS 66 v/s infrared spectrometer was used for the Fourier transform infrared (FTIR) spectroscopy analysis. The polymers cleaved from the silica nanoparticles were dissolved in deuterated chloroform and then characterized with 1H NMR using a Varian UNITY INOVA-500 FT-NMR spectrometer. Thermogravimetric analysis (TGA) was performed with a Perkin-Elmer TGA-7 system (Perkin-Elmer Corporation, USA) at a scan rate of 20 °C min-1 to 800 °C in an N2 atmosphere. The morphologies of the silica nanoparticles were characterized with a JEM1200 EX/S transmission electron microscope (TEM). The powders were dispersed in toluene in an ultrasonic bath for 5 min and then deposited on a copper grid covered with a perforated carbon film. Results and Discussion Preparation of the Macroinitiator Based on Silica Nanoparticles. Here, the bromoacetamide group was selected as a group (I*) capable of initiating from the functionalized silica nanoparticles. The group was immobilized by the bromoacetylation of the surface amino groups of the aminopropyl modified silica nanoparticles (AP-SN) with bromoacetylbromide (Scheme 2). After the macroinitiators based on silica nanoparticles (BrASN) were washed with toluene and ethanol thoroughly, the elemental analysis results of C and N indicated that about 0.5 mmol initiator/g silica nanoparticles were immobilized. The amide groups characteristic of BrA-SN at 1628 cm-1 (CdO stretching), 1549 cm-1 (NsH bending), and 1221 cm-1 (Cs Br of bromo-acetamide) were also found in the FTIR spectrum of the bromoacetamide modified silica nanoparticles (BrA-SN). SCVP from the Macroinitiator Based on Silica Nanoparticles. BrA-SN was used as a polyfunctional initiator (I*) in the SCVP via ATRP technique to produce hyperbranched polymer grafted silica nanoparticles (SN-HBP), as illustrated in Scheme 1. During the SCVP of CMS (AB*) via the ATRP process, because both the AB* inimer and the BrA-SN had initiating groups capable of initiating the polymerization of vinyl groups, the chain growth can be started from both I* in the BrA-SN and the B* group in the inimer. The activated B* can add to the double bond, A, to form the ungrafted or grafted dimer with a new propagating center, A*. Further addition of AB* inimer or dimer to A* and B* centers leads to the hyperbranched polymers. From the structures of I* of BrA-SN and B* in AB* inimer, it is clear that I* of BrA-SN has a higher activity of adding to the double bond than that of AB*; thus,
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Figure 1. TGA curves of the silica nanoparticles.
Figure 3. FTIR spectrum of the well-defined dendritic-graft copolymer grafted silica nanoparticles.
Figure 2. 1H NMR spectrum of the hyperbranched polymer cut from the SN-HBP.
most of the B* immobilized on the surface of silica nanoparticles has participated in initiating self-condensing vinyl polymerization via slow addition of AB* inimer, as reported previously.23 Of course, some free ungrafted hyperbranched polymers were still produced during the polymerization. After the polymerization, the reaction mixture was diluted with toluene, and the nanoparticles were extracted thoroughly with toluene and then filtered to remove soluble ungrafted hyperbranched polymers. Washing was done until no polymer was found in the filtrate. And, the percentage of grafting (PG, mass ratio of the grafted polymer to silica nanoparticles) was found to be 80%, according to the elemental analysis. This is consistent with the result of about 83% from the TGA analysis (Figure 1). In order to understand the grrowth characteristics of the surface-initiated SCVP via ATRP technique and the molecular parameters of the grafted polymers, the hyperbranched polymers were cleaved from the surfaces at their points of attachment by treatment with HF solution. Some low molecular weight products like oil were obtained. This showed the low viscosity characteristic of the spherelike molecular architecture.32 The 1H NMR spectrum was used for the structural analysis of the well-defined hyperbranched polymer grafted (Figure 2). It showed each characteristic signal of the monomer unit, chlorobenzyl (b) at 11.4-12.0 ppm, phenyl (a) at 13.4-14.8 ppm, mainchain methylene (c) at 8.4-9.2 ppm, and methine (d) at 9.29.6 ppm. The peak intensity ratio of the phenyl to the benzyl groups was 5.29/2.00, higher than the calculated values (4/2) assuming that the benzyl chloride moiety was kept intact.33 This indicated that the chlorobenzyl groups had acted as the initiating groups in the polymerization as described in Scheme 1. Consecutive SI-ATRP. The surface active site, such as the diethyldithiocarbamoyl end groups of the hyperbranched polymers via controlled/living radical polymerization, was reported to be used as the initiator for the ATRP of other monomers.34 Here, the bromine atom on the outer surfaces of the hyperbranched polymer grafted silica nanoparticles were used to surface-initiate the ATRP of the second monomer MMA.
Figure 4. 1H NMR spectrum of the hyperbranched polymer cut from the SN-HBP-PMMA.
After the SI-ATRP of MMA using the initiating end groups at the outermost surfaces of the nanoparticles grafted with such hyperbranched polymers as the macroinitiators, the characteristic adsorption band of the CdO of poly(methyl methacrylate) at 1728 cm-1 was found in the FTIR spectrum of the product (Figure 3). And, the polystyrene characteristic band at 30203100 cm-1 was also found. This indicated that the well-defined dendritic-graft copolymer had been successfully grafted and the total percentage of grafting (PG, mass ratio of the grafted polymer to silica nanoparticles) was found to be 250%, according to the TGA analysis (Figure 1). The well-defined dendritic-graft copolymer grafted was also cleaved from the silica nanoparticles by the same procedure as the hyperbranched polymer. The polymer obtained was white solid, different from the yellowy oil of the hyperbranched polymer. It had also been characterized with a 1H NMR spectrum (Figure 4). Except that peaks from the hyperbranched polymer were observed, the incoporation of PMMA is evident by the presence of the resonance peak at 3.61 ppm representing the methyl hydrogen of the COOCH3 groups. TEM Analysis. Figure 5 showed the representative TEM images of the hyperbranched polymer grafted silica nanoparticles (SN-HBP) and the well-defined dendritic-graft copolymer grafted silica nanoparticles (SN-HBP-PMMA) obtained from the dilute toluene suspension. The particles are seen to form clusters of micrometer or submicrometer size, although they were dispersed better than the bare silica nanoparticles because of the hydrophobic polymers surface grafted. Such aggregation morphologies were also observed in other branched polymer grafted nanoparticles.21,35 This might be caused by the fact that the primary particles themselves formed large aggregates of 0.1 µm to several micrometers due to van der Vaals interparticle attraction and the aggregation was kept somehow during the
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Figure 5. TEM images.
preparation of the functionalized silica nanoparticles as well as the following polymerization and purification processes. It was interesting that the SN-HBP-PMMA had better dispersibility than the SN-HBP. In the TEM image of the former, the silica nanoparticles in the aggregates seemed to be unhinged because of the thicker polymer shells. Conclusion A facile strategy was developed for the fabrication of welldefined dendritic-graft copolymer on the nanoparticles by the consecutive surface-initiated atom transfer radical polymerizations. The hyperbranched polymer grafted silica nanoparticles were prepared by the surface-initiated self-condensing vinyl polymerization of the inimer p-chloromethyl styrene (CMS). Then, the initiating end groups at the outermost surfaces were used as the initiators for the surface-initiated atom transfer radical polymerization of methyl methacrylate to grow the linear polymer from the surfaces of the hyperbranched polymers. Literature Cited (1) Liu, P. In Polymeric Nanostructures and Their Applications; Nalwa H. S., Eds.; American Scientific Publishers: Stevenson Ranch, CA, 2007. (2) Bontempo, D.; Tirelli, N.; Feldman, K.; Masci, G.; Crescenzi, V.; Hubbell, J. A. Atom transfer radical polymerization as a tool for surface functionalization. AdV. Mater. 2002, 14, 1239. (3) Xia, J. H.; Matyjaszewski, K. Atom transfer radical polymerization. Chem. ReV. 2001, 101, 2921. (4) von Werne, T.; Patten, T. E. Preparation of structurally well-defined polymer-nanoparticle hybrids with controlled/living radical polymerizations. J. Am. Chem. Soc. 1999, 121, 7409. (5) Pyun, J.; Matyjaszewski, K.; Kowalewski, T.; Savin, D.; Patterson, G.; Kickelbick, G.; Huesing, G. Synthesis of well-defined block copolymers tethered to polysilsesquioxane nanoparticles and their nanoscale morphology on surfaces. J. Am. Chem. Soc. 2001, 123, 9445. (6) Qin, S.; Qin, D.; Ford, W. T.; Resasco, D. E.; Herrera, J. E. Polymer Brushes on Single-Walled Carbon Nanotubes by Atom Transfer Radical Polymerization of n-Butyl Methacrylate. J. Am. Chem. Soc. 2004, 126, 170. (7) Kong, H.; Gao, C.; Yan, D. Y. Controlled Functionalization of Multiwalled Carbon Nanotubes by in Situ Atom Transfer Radical Polymerization. J. Am. Chem. Soc. 2004, 126, 412. (8) Liu, P.; Zhang, L. X.; Su, Z. X. Surface-initiated ATRP of HEA from Nano-crystal R-Fe2O3 under Ultrasonic Irradiation. J. Nanosci. Nanotechnol. 2005, 5, 1713. (9) Rupert, B. L.; Mulvihill, M. J.; Arnold, J. Atom-transfer radical polymerization on zinc oxide nanowires. Chem. Mater. 2006, 18, 5045. (10) Watson, K. J.; Zhu, J.; Nguyen, S. T.; Mirkin, C. A. Hybrid nanoparticles with block copolymer shell structures. J. Am. Chem. Soc. 1999, 121, 462. (11) Zhao B.; Brittain, W. J. Synthesis, characterization, and properties of tethered polystyrene-b-polyacrylate brushes on flat silicate substrates. Macromolecules 2000, 33, 8813. (12) Henrik, B.; Manfred, L. H.; Stefan, N.; Hellmuth, W. ATRP grafting from silica surface to create first and second generation of grafts. Polym. Bull. 2000, 44, 223.
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ReceiVed for reView February 16, 2007 ReVised manuscript receiVed March 18, 2007 Accepted March 19, 2007 IE070252+
