Published: August 25, 2011

r 2011 American Chemical Society 12575 dx.doi.org/10.1021/la2029774 | Langmuir 2011, 27, 12575–12584

ARTICLE

pubs.acs.org/Langmuir

Miniemulsion Synthesis of Metal�Oxo Cluster ContainingCopolymer NanobeadsMichele H. Pablico,† Julie E. Mertzman,† Emily A. Japp,† William L. Boncher,† Maki Nishida,‡

Edward Van Keuren,‡ Samuel E. Lofland,§ Norman Dollahon,|| Judith F. Rubinson,† K. Travis Holman,†

and Sarah L. Stoll*,†

†Department of Chemistry, Georgetown University, 37th and O Streets NW, Washington, D.C. 20057, United States‡Department of Physics, Georgetown University, Washington, D.C. 20057, United States§Department of Physics, Rowan University, Glassboro, New Jersey 08020, United States

)Department of Biology, Villanova University, Villanova, Pennsylvania 19085, United States

bS Supporting Information

’ INTRODUCTION

The miniemulsion technique allows for the formation of mono-dispersed polymeric nanobeads in the range of 50�500 nm. Mini-emulsion involves extreme shearing conditions (ultrasonication) ona system of a monomer with an osmotic pressure agent (orhydrophobe), and a continuous phase containing a surfactant. Eachnanodroplet is like a nanoreactor, because mass transport andsecondary nucleation are restricted,1 so the droplet diameter deter-mines the polymer bead dimensions. The versatility of themethod ismanifest in the range of polymerization types including radical,anionic, cationic, polyaddition, and polycondensation.2 The methodcan be adapted for direct or inverse miniemulsions to preparehydrophobic or hydrophilic polymer beads. It is also possible toincorporate soluble compounds, such as dyes, luminescent com-plexes, and hydrophobic metal salts, to prepare hybrid nanobeads orinsoluble compounds (e.g., solid photoinitiator or luminescentquantum dots) to form composite nanoparticles.3

Metal containing hybrid polymer nanobeads can be synthe-sized using soluble metal complexes, in both direct and indirectminiemulsion polymerization conditions. For example, simpleneutral complexes with metals such as Pt, In, Zn, Fe, Cr, and Cohave been dissolved in monomers of styrene or methyl metha-crylate to form metal containing polymer latexes.4 Metal salts havealso been used, which are thought to form a complex in situ with themonomer or macroradicals formed during polymerization.5 Thesemetal containing polymer nanobeads can form a monolayer super-lattice on a surface and then be converted to metal particles usingplasma etching. This method provides a route to form a patterned

surface where the distance between metal particles is determined bythe diameter of the metal�polymer bead.4b

Composite inorganic�organic polymer nanobeads can also beprepared by the dispersion of a hydrophobically capped nano-particle in the monomer phase. Metal nanoparticles that havebeen investigated include silica, titania, CaCO3, and magnetite.

6

In some cases, a single colloid can be encapsulated in the latexbead, but more commonly many small inorganic nanoparticlesare incorporated within each polymer nanobead. Generally, if thenanoparticle is more hydrophobic than the monomer, it is easilyencapsulated within the nanobead; however, the distribution ofthe nanoparticles is often heterogeneous.7 When the polarity ofthe nanoparticle (or the coating of the nanoparticle) lies some-where between the oil and water, the nanoparticles becomeenriched at the oil�water interface. The segregation of inorganicnanoparticles at the interface (Pickering stabilizers)8 results in asurface bound shell of inorganic particles described as “armoredlatexes” or “raspberry hybrids”.9

Since the pioneering work of Ugelstad10 to prepare hydro-phobic, monosized-polystyrene magnetic nanoparticles, therehas been significant interest in the preparation of magneticpolymer nanobeads. Miniemulsion has been used to preparemagnetic nanobeads of polystyrene with particles of magnetite(Fe3O4, Fe2O3) embedded within the polymer matrix.11 Mag-netite has limitations in that the iron oxide nanoparticles in the

Received: December 16, 2010

ABSTRACT:Hybrid nanobeads containing either amanganese�oxo or manganese�iron�oxo cluster have been preparedvia the miniemulsion polymerization technique. Two newligand substituted oxo clusters, Mn12O12(VBA)16(H2O)4 andMn8Fe4O12(VBA)16(H2O)4 (where VBA = 4-vinylbenzoate),have been prepared and characterized. Polymerization of thefunctionalized metal�oxo clusters with styrene under mini-emulsion conditions produced monodispersed polymer nano-particles as small as ∼60 nm in diameter. The metal�oxo polymer nanobeads were fully characterized in terms of syntheticparameters, composition, structure, and magnetic properties.

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appropriate size regime are polydispersed in size,12 and theoxidation of ferromagnetic Fe3O4 to α-Fe2O3 (an antiferro-magnet) is quite difficult to prevent.13 Both problems can com-promise the magnetic properties.14 Size has an important effecton themagnetic properties because below a critical diameter theybecome superparamagnetic;15 moreover, a linear correlationbetween decreasing particle size and decreasing saturation mag-netization has been reported.16 The relative ease of oxidation ofhigh surface area Fe3O4 nanoparticles to the far weaker magneticmoment of α-Fe2O3 is one reasonable explanation for thereduced saturation magnetization. Coated magnetic metal-oxidenanoparticles can be dispersed in the monomer, and the mixtureemulsified and polymerized to produce magnetite-containingpolymer nanobeads. Although this method produces biocompa-tible magnetic materials, the distribution of magnetic particleswithin the bead is often heterogeneous, and the fraction of metal-oxide from bead to bead is inconsistent.11 Frequently, thesynthesis results in high and low metal content beads.17 In addi-tion to homogeneity, challenges for this system include chemicalstability and particle agglomeration.11b,18

Here, we describe a new adaptation of the miniemulsiontechnique to prepare manganese and iron oxide containing poly-styrene nanobeads. Bulk copolymers of methacrylate-substitutedMn12 clusters with styrene have previously been prepared andfound to formmagnetic polymeric materials.19We demonstratethe first example of nanobeads composed of copolymers fromanalogous compounds. Two new hydrophobic metal�oxoclusters, Mn12O12(L)16(H2O)4 and the iron substituted ver-sion, Mn8Fe4O12(L)16(H2O)4, have been prepared and char-acterized with the ligand, L = 4-vinylbenzoate. These clusterswere dissolved in styrene and polymerized under miniemulsionconditions to form highly monodispersed homogeneous co-polymer nanobeads with diameters between 60 and 80 nm.We have refined the synthesis and characterized the struc-ture, morphology, and magnetic properties of the resultingnanobeads.

The system described here has several distinctions. One is thatbecause the solubility of the inorganic cluster is determined bychoice of ligands substituted around the cluster, it should bepossible to use this nanometer-sized, paramagnetic molecule ineither direct or inverse miniemulsion. For example, the nonpolarmolecules reported here can be homogeneously dissolved in themonomer phase in direct miniemulsion, but the approach caneasily be adapted for inverse miniemulsion. Second, the ligandnot only confers solubility for the cluster but also contains anolefin, thus enabling the cluster ligands to participate in thepolymerization reaction, resulting in a copolymer of substitutedmetal�oxo clusters with styrene. This allows the metal�oxocluster to be firmly grafted into the polymer latex renderingenhanced chemical stability. Previously, it has been reported thatluminescent semiconductor nanoparticles having surfaces mod-ified with nonpolymerizable ligands tend to leach out of the latexbecause of poor interactions between the nanoparticles andpolymer matrix.20 However, polymerizable ligand capped semi-conductor nanoparticles were found to be firmly attached intothe latex.20 In addition, luminescent polyoxometalates (POMs)incorporated into polystyrene latexes using polymerizable sur-factants under miniemulsion conditions proved to be successfulin enhancing stability.21 We have successfully applied thisapproach for the first time to a magnetic system using olefin-functionalized metal�oxo clusters integrated within polystyrenelatexes via miniemulsion polymerization. Finally, the metal�oxo

clusters are magnetic, and as previously reported with bulkcopolymers19 the properties of the individual clusters are wellpreserved. We are able to demonstrate that these copolymers canbe prepared as novel magnetic nanobeads. Because the proper-ties are due to the individual clusters, unlike other magneticnanoparticles, the magnetic properties of this system should notbe dependent on nanobead size. Further, the miniemulsiontechnique can be tailored to produce a wide range of nanobeaddiameters (50�500 nm), and thus, this method opens newavenues of research for the production of magnetic polymernanobeads which havemany potential applications particularly inthe area of drug delivery and diagnosis.

’EXPERIMENTAL SECTION

Materials. Mn(II) acetate tetrahydrate, Fe(II) acetate, potassiumpermanganate, 4-vinylbenzoic acid, methacrylic acid, dichloromethane,heptane, ethanol, hexanes, acetone, glacial acetic acid, sodium dodecylsulfate (SDS), hexadecane, 2,20-azobis(isobutyronitrile) (AIBN), to-luene, hydrochloric acid (HCl), and tetrahydrofuran (THF) were allpurchased from Aldrich and used as received. Styrene and divinylben-zene (DVB) were also purchased from Aldrich, and their inhibitors wereremoved by filtering through a column of Al2O3. Iron and manganesestandards for AAS were obtained from Fluka.

Mn12O12(O2CCH3)16(H2O)4 andMn8Fe4O12(O2CCH3)16(H2O)4were synthesized according to the literature.22 Briefly, to prepareMn12O12(O2CCH3)16(H2O)4, Mn(II) acetate was dissolved in a solu-tion of 60% acetic acid (40 mL) at room temperature. Freshly groundKMnO4 was added to the solution with stirring for approximately 2 min.The reaction mixture was stored in the dark for 36 h, and small, black,rodlike crystals were collected via filtration and washed with copiousamounts of acetone. On the other hand, Mn8Fe4O12(O2CCH3)16-(H2O)4 was synthesized similarly by using Fe(II) acetate instead ofMn(II) acetate and heating to 60 �C. The reaction mixture was cooledand layered with an equal volume of acetone. After storing in the dark for5 days, black, rodlike single crystals appeared.Synthesis ofMn12O12(O2CC6H4CHdCH2)16(H2O)4orMn12O12-

(VBA)16.The ligand exchange reactionswere completed according to theliterature23 with slight variations. Mn12O12(O2CCH3)16(H2O)4 (2 mmol)and 4-vinylbenzoic acid (0.0625 mmol) were dissolved in CH2Cl2(25 mL) and stirred for 4 h. The solution was subsequently filtered intoa beaker, and heptane (50 mL) was added. After 36�48 h, a brownpowder was collected via filtration andwashed with approximately 50mLof heptane. The above procedure was then repeated with the productbeing used as the starting material with the same reagent/solvent ratios.The resulting brown powder was collected by filtration and washed withheptane. The product was determined to be Mn12O12(O2CC6H4-CHdCH2)16(H2O)4 3CH2dCHC6H4CO2H 3H2O. Percent yield ∼ 33%.Calcd (found) for C153H130Mn12O51: C, 53.36 (53.36); H, 3.81 (3.85). IRdata (KBr, cm�1): 3401 (m, br), 3082 (w), 3066 (w), 3006 (w), 2923 (w),1929 (w), 1835 (w), 1700 (m), 1624 (w), 1606 (s), 1578 (s), 1507 (s), 1410(vs), 1347 (s), 1289 (m), 1183 (s), 1110 (m), 1016 (s), 988 (m), 914 (m),862 (s), 790 (s), 771 (m), 718 (s), 663 (m), 625 (s), 552 (m), 518 (m).Synthesis of Mn8Fe4O12(O2CC6H4CHdCH2)16(H2O)4 or

Mn8Fe4O12(VBA)16. The ligand 4-vinylbenzoic acid (2 mmol) wasadded to a slurry of Mn8Fe4O12(O2CCH3)16(H2O)4 (0.125 mmol) in50mLCH2Cl2 and stirred for 4 h. The reactionmixture was then filteredto remove any unreacted starting material. The resulting filtrate waslayered with 50 mL of ethanol and 50 mL of hexanes. After about 3 days,brown solids emerged which were then subjected to a second round ofligand exchange to ensure complete ligand substitution. This was doneby redissolving the brown powder in 50 mL of CH2Cl2, 4-vinylbenzoicacid was added, and themixture stirred again for 4 h, filtered, and layeredwith 50 of mL ethanol and 50 mL of hexanes. After another 3 days, black

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microcrystals appeared, which were washed with copious amounts ofethanol and hexanes, and isolated via vacuum filtration. The product wasfound to be Mn8Fe4O12(O2CC6H4CHdCH2)16(H2O)4 32CH2dCHC6H4-CO2H 3CH2Cl2. Percent yield ∼ 17%. Calcd (found) for C163H138Mn8-Fe4O52Cl2:C, 53.45 (53.20);H, 3.80 (3.81). IR data (KBr, cm

�1): 3428 (m,br), 3085 (w), 3007 (w), 2926 (w), 1933 (w), 1833 (w), 1694 (w), 1670(w), 1582 (s), 1530 (s), 1411 (vs), 1183 (s), 1142 (w), 1111 (w), 1016 (w),990 (w), 916 (w), 864 (w), 791 (s), 718 (s), 615 (s), 534 (m), 516 (m).A small amount of the product was put in a vial, dissolved in toluene, andcapped. The solution was left undisturbed for several months. Thisallowed for very slow evaporation of the solvent, which resulted in theproduction of reddish brown single crystals suitable for single crystalX-ray analysis.Synthesis of Mn8Fe4O12(O2CC(CH3)=CH2)16(H2O)4 or

Mn8Fe4O12(MA)16. Methacrylic acid (2 mmol) was added to a slurryof Mn8Fe4O12(O2CCH3)16(H2O)4 (0.125 mmol) in 50 mL of CH2Cl2and stirred for 4 h. The reaction mixture was then filtered to remove anyunreacted startingmaterial. The resulting filtrate was layered with 50mL ofhexanes. After about 3 days, brown solids formed which were thensubjected to a second round of ligand exchange to ensure complete ligandsubstitution. This was done by redissolving the brown powder in CH2Cl2(50mL) and addingmethacrylic acid, and themixture was stirred again for4 h, filtered, and layered with 50 mL of hexanes. After another 3 days, largeblack crystals appeared which were suitable for single crystal X-ray analysis.The product was found to be Mn8Fe4O12(O2C(CH3)CdCH2)16-(H2O)4 3 4CH2dC(CH3)CO2H 3 2H2O. Percent yield ∼ 29%. Calcd(found) for C80H116Mn8Fe4O58: C, 36.00 (35.79); H, 4.38 (4.16). IRdata (KBr, cm�1): 3415 (m, br), 2925 (m), 2858 (w), 1703 (w), 1641 (w),1532 (s), 1456 (m), 1421 (vs), 1246 (s), 1131 (w), 1045 (w), 1010 (w),942 (w), 826 (w), 614 (s), 548 (m), 444 (w).Miniemulsion Polymerization Synthesis of Mn12O12(VBA)16-

co-Polystyrene and Mn8Fe4O12(VBA)16-co-Polystyrene Nano-beads. Miniemulsion polymerizations were completed as reported byLandfester et al.2 Mn12O12(VBA)16 (0.045 mmol) was dissolved instyrene (8.7 mmol) and DVB (0.087 mmol) in a small vial. In a separatelarger vial, SDS (0.7 mmol) was dissolved in deionized water with stirringat ambient temperature. To the Mn12O12(VBA)16/styrene/DVB mix-ture were added hexadecane (0.13 mmol) and AIBN (0.24 mmol). Thetwo solutions were subsequently combined, and the whole reactionmixture was homogenized with an ultrasound sonicator set at an outputof 10 W (speed 5) for 120 s. The reaction vessel was then placed in awater bath set at 60 �C for 6 h. A small aliquot of solutionwas removed fordynamic light scattering (DLS)measurements, and the solid polymerizedparticles were separated from the solution by centrifugation and washedwith dry ethanol to remove any unpolymerized styrene. A similarprocedure (same reactant ratios and reaction conditions) was used forthe miniemulsion polymerization of Mn8Fe4O12(VBA)16 with styrene.Calcd (found) for Mn12O12(VBA)16-co-polystyrene: Mn, 2.72% (Mn,2.50%). Calcd (found) for Mn8Fe4O12(VBA)16-co-polystyrene: Mn,1.8%; Fe 0.91% (Mn, 1.67%; Fe, 0.97%). IR data (KBr, cm�1) forMn12O12(VBA)16-co-polystyrene: 3410 (s, br), 3050 (w), 3020 (w),2920 (s), 2850 (s), 1920 (w), 1840 (w), 1780 (w), 1670 (w), 1590(m), 1530 (w), 1460 (w), 1400 (s), 1260 (m), 1210 (m), 1080 (m), 1020(w), 903 (w), 798 (s), 750 (w), 696 (s), 498 (s, br). IR data (KBr, cm�1)for Mn8Fe4O12(VBA)16-co-polystyrene: 3420 (s, br), 3050 (w), 3060(w), 3030 (s), 2920 (s), 2850 (s), 1940 (w), 1860 (w), 1780 (w), 1600(s), 1580 (w), 1540 (w), 1490 (s), 1450 (s), 1410 (s), 1210 (m, br), 1180(w), 1060 (w), 1030 (w), 904 (w), 754 (s), 698 (vs), 536 (m, br).Characterization. Fourier transform infrared (FTIR) spectrosco-

py measurements were recorded in the range 4000�400 cm�1, frompressed pellets in KBr on a Nicolet 380 FTIR instrument. PowderX-ray diffraction patterns were obtained from a Rigaku RAPIDcurved IP X-ray powder diffractometer with Cu Kα radiation and animage plate detector. Simultaneous thermogravimetric analysis and

differential thermal analysis (TGA-DTA) data were studied fromsamples in an aluminum pan from 20 to 1000 �C with a heating rateof 20 �C/min.

The ac magnetic measurements were collected on a QuantumDesignPhysical Measurement system, in zero dc field for temperatures rangingfrom 1.8 to 50 K. The ac frequency range was 10�10 000 Hz, and theamplitude was 10 Oe. Experimental data were corrected for the sampleholder and for diamagnetic contributions calculated from Pascalconstants. The dc magnetic measurements were carried out on a QDSQUID instrument from 50 to 250 K in a field of 10 000 Oe. Powdersamples containing ca. 10�20 mg of material were loaded into a gelatincapsule and packed with quartz wool. The sample was positionedwithin a plastic straw for analysis. The diamagnetic correction ofthe sample holder was measured and subtracted from the data. Forthe cluster Mn8Fe4O12(VBA)16(H2O)4, the diamagnetic correctionfor the sample was calculated using cd =(�3662.646/2)10�6 emu/mol.Whereas the diamagnetic correction for the miniemulsion nanobeadswas done experimentally by subtracting the χ versus T plot ofpure polystyrene nanobeads from the plot of the metal containingnanobeads. From 1/χ versus T plots, the slope (1/C, where C = Curieconstant) was determined from a linear regression. The R2 valuefor the fit of the cluster was 0.9946, and that of the Mn8Fe4O12-(VBA)16-co-polystyrene nanobeads was 0.9934. The Curiec constantswere calculated, resulting in Ccluster = 5� 10�8 m3

3K/g, and Cnanobead =2 � 10�8 m3

3K/g.Samples were prepared for gel permeation chromatography (GPC)

and atomic absorption spectroscopy (AAS) by adding the solidcopolymer nanobeads (10 mg) to toluene (5 mL) and mixing withan aqueous solution of HCl (37%, 5 mL). This solution was stirred for4 h at room temperature, after which the mixture was transferred to aseparatory funnel and allowed to stand overnight. During this process,the brown color of the copolymer nanobeads leached from the polymerwhich settled at the meniscus of the HCl layer. The acid turned deepyellow when the copolymer had Mn8Fe4O12(VBA)16 and pale pink tocolorless for the copolymer with Mn12O12(VBA)16. The two layerswere then separated. The toluene layer containing the residue waswashed with toluene and methanol, leaving a white powder. The FTIRspectrum of the white powder matches polystyrene stretching fre-quencies, but also has a weak stretch at 1680 cm�1 indicative of a smallamount of the 4-vinylbenzoate which copolymerized with styrene. Thepolymer component was only sparingly soluble in THF. For GPCanalysis, the white solids were immersed in THF (1 mg/mL),sonicated for about 2 h, and filtered through 0.45 μm nylon membranesyringe filters prior to data collection. GPC data were obtained at a flowrate of 1.0 mL/min with a Bioanalytical Systems (BAS) PM-80 solventdelivery system, a CC-5e liquid chromatograph compartment, and aUV-116A UV�vis detector. The column used was PLgel 5 μmMIXED-C from Agilent Technologies. EasiCal PS-1 polystyrenestandards (580�7 500 000 g/mol) from Agilent Technologies wereused for calibration.

The inorganic layer (aqueous HCl) was used for AAS measurements.Atomic absorption was measured with a BUCK Scientific model 200Aatomic absorption spectrophotometer. Instrument detection limits forAA are calculated to be 0.099 ppm by calculating 3 times the 95%confidence level. A calibration curve was prepared from iron andmanganese standards (Fluka). The metal content was found to be2.50% for Mn12O12(VBA)16-co-polystyrene nanobeads and 1.67% Mn,0.97% Fe for the Mn8Fe4O12(VBA)16-co-polystyrene nanobeads.

To determine the conversion of the monomer during the miniemul-sion process, an aliquot (0.25 mL) of the reaction latex was taken out atcertain time intervals for a period of 6 h. The samples were poured intoEppendorf tubes and immediately immersed in liquid nitrogen toquench the progression of polymerization. The samples were then driedvia the use of a speed vacuum pump for 2 h. The percent conversion was

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calculated using the formula given below:

% conversion ¼ ðMDML �MMMWÞMSMW

� 100%

whereMD andMW are themasses of the dried andwet latex, respectively,ML is themass of the total latex,MM is the mass of the metal�oxo clusterused, andMS is the mass of monomer. The mass of surfactant used wasalso taken into account.

A Hitachi H-7600 transmission electron microscope, equipped withan AMT XR 40B camera and operated at 100 kV, was used to evaluateparticle size and morphology. HR-TEM images were obtained using aJEOL JEM 2100 LaB6 transmission electron microscope equipped withan Oxford energy dispersive X-ray (EDS) spectrometer. The sizedistribution of the particles (polydispersity index, PDI) was determinedby using the equations below:

DV ¼∑inidi

4

∑inidi

3, DN ¼∑inidi

∑ini

, and PDI ¼ DV

DN

where ni is the number of particles with diameter di, DV is the volumeaverage diameter, and DN is the number average diameter.

Dynamic light scattering (DLS), was also used to determine particlesize. The DLS apparatus used in this study utilizes light from a HeNelaser that illuminates dilute suspensions of particles. Light scattered at afixed angle (usually 90�) is coupled though a narrow band pass opticalfilter into a single-mode optical fiber, which leads to a high-sensitivityavalanche photodiode photon counting module (EG&G SPCM-15).The count rates from this detector are analyzed by a hardwareautocorrelator (ALV-5000, ALV GmbH, Germany). With standardassumptions, it can be shown that the decay rate of the count rateautocorrelation function is inversely proportional to the particle diffu-sion coefficient, from which information on the particle size is obtained.Initial calculations of the particle sizes were determined from a singleexponential fit to the autocorrelation functions. The following nonlinearfit model was used:

g2ðtÞ � 1 ¼Z Γmax

Γmin

e�Γt GðΓÞ dΓ !2

Single Crystal X-ray Diffraction. Single crystal X-ray diffractiondata were collected at 100(2) K on a Siemens SMART three-circle X-raydiffractometer equipped with an APEX II CCD detector (Bruker-AXS)and an Oxford Cryosystems 700 Cryostream, using Mo Kα radiation(Æλæ = 0.71073 Å). The crystal structure was solved by direct methodsusing SHELXS, and all structural refinements were conducted usingSHELXL-97-2.24 With the exception of included solvents and disor-dered moieties as described below, all non-hydrogen atoms weremodeled with anisotropic displacement parameters. All hydrogen atomswere placed in calculated positions andwere refined using a ridingmodelwith coordinates and isotropic displacement parameters being depen-dent upon the atom to which they are attached. The program X-Seed25

was used as a graphical interface for the SHELX software suite and forthe generation of figures. The SQUEEZE subroutine of PLATON wasused to estimate solvent accessible volumes and residual electrondensity, but was not implemented in the final refinement model.26

CCDC-834515 and CCDC-834516 (refinement of Mn8Fe4O12-(O2CC6H4CHdCH2)16 (H2O)4 3 2.5(C6H5CH3) and Mn8Fe4O12-(O2CC(CH3)=CH2)16(H2O)4 3 4(C4H6O2) 3 2H2O, respectively) con-tain supplementary crystallographic data for this paper. These datacan be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing data_request@ccdc.cam.ac.uk, or by contacting TheCambridge Crystallographic Data Centre, 12, Union Road, Cambridge

CB2 1EZ, U.K.; fax, +44-1223-336033. A summary of crystallographicdata for both structures is provided in the Supporting Information(Table S1).Refinement of [Mn8Fe4O12(O2CC6H4CHdCH2)16(H2O)4] 3

2.5(C6H5CH3) or Mn8Fe4O12(VBA)16. The structure was solvedin the space group P4n2. The title molecule is found to reside on a four-bar symmetry position in the crystal, and as such the asymmetric unitcontains one-quarter of the title molecule and 5/8 molecules of toluene;so, Z = 2 with respect to the title compound. Two of the four symmetry-unique 4-vinylbenzoate ligands are found to be disordered. One of thetwo is found disordered over two positions in a symmetry-imposed50:50 ratio. The styrene portion of the other disordered ligand wasmodeled over three separate positions (50:26:19 ratio), and the carbonatoms of the disordered sites were modeled with isotropic displacementparameters. The toluene solvent molecules are also disordered, beingfound in two symmetry-unique positions. One of the two toluene species(50% of a toluene molecule) is disordered about a 2-fold symmetry axis.The other (presumably) toluene species lies highly disordered about acrystallographic 222 symmetry position and is nestled in a positionbetween the highly disordered 4-vinylbenzoate ligands. A reasonablerefinement model of the highly disordered species could not beobtained, and so this species is not included in the refinement but isnonetheless included in the formula (estimated to be 1/8 of a tolueneaccording to the multiplicity of the 222 position). Accordingly, thestructure exhibits peaks of somewhat large residual electron density thatcan be attributed to the disordered, and not-modeled, 1/8 toluenespecies. The top four peaks in the residual electron density differenceFourier map (1.34, 1.26, 0.86, and 0.77 e�/Å3) correspond to these sites.Hydrogen atoms of the terminal water ligands could not be located butwere included in the molecular formula. A thermal ellipsoid plot of thedodecanuclear metal�ligand cluster is given in Figure 3.Refinement of [Mn8Fe4O12(O2CC(CH3)=CH2)16(H2O)4] 3

4(C4H6O2) 3 2H2O or Mn8Fe4O12(MA)16(H2O)4. The structurewas solved in the space group C2/c. In the refinement model, thedCH2 and —CH3 groups of the methacrylate ligands were found tobe disordered over two orientations. Attempts to model the disorderwere found to be unsatisfactory due to insufficient data resolution. Asolvate species modeled as a water molecule was also found to bedisordered in three positions. Hydrogen atoms of the terminal waterligands could not be located but were likewise included in the molecularformula. Figure 2 shows a thermal ellipsoid plot of this cluster.

’RESULTS AND DISCUSSION

The metal�oxo cluster, Mn12O12(O2CCH3)16(H2O)4, orMn12, is perhaps the most famous of the single molecule magnets(SMMs), due to the high spin state, anisotropy, and unusualmagnetic behavior.19,27 One of the notable attributes of Mn12 isthat, within the class of SMMs, this is one of the rare clusters thatmaintains its core structure and properties upon ligand exchange.27b

The cluster has a distinct geometry with four Mn(IV) atoms atthe center with eight Mn(III) atoms around the periphery. Thisprovides the opportunity to use ligand exchange not only toprobe the electronic states of the cluster but also to form largerarchitectures by grafting the cluster to surfaces,28 beads,29 orwithin porous materials.19b Here, we demonstrate the advantagesof this cluster for miniemulsion, as a result of the ability to tailorthe hydrophobicity of the cluster (ensuring monomer solubility).Mn12 with ligands containing olefins results in a polymerizablemetal�oxo cluster that can effectively cross-link polymers suchas methyl methacrylate19a or ethyl acrylate.19b However, the4-vinylbenzoate substituted cluster has not yet been reported,Mn12O12(VBA)16(H2O)4. The iron substituted version ofMn12, Mn8Fe4O12(O2CCH3)16(H2O)4, orMn8Fe4, was originally

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studied to compare the effect of changing the electron count of thecluster core, by increasing the number of d electrons by 4.22b,30Wealso report the synthesis and characterization of the first substitutedMn8Fe4 cluster, or Mn8Fe4O12(VBA)16(H2O)4, here. The effectsof ligand substitution are striking for the Mn8Fe4O12L16(H2O)4cluster, which is water-soluble when L is acetate but soluble inhexanes when L is 4-vinylbenzoate. We have used both substitutedclusters, Mn12O12(VBA)16(H2O)4 and Mn8Fe4O12(VBA)16-(H2O)4, to prepare copolymers with styrene under miniemul-sion conditions.Synthesis and Characterization of Substituted Complexes:

Mn12O12(VBA)16, Mn8Fe4O12(VBA)16, and Mn8Fe4O12(MA)16.The Mn12O12(O2CC6H4CHdCH2)16(H2O)4 or Mn12O12-(VBA)16 derivative was prepared by a ligand exchange reactionstarting fromMn12O12(O2CCH3)16(H2O)4. Ligand exchange hasbeen commonly used to substituteMn12with a variety of carboxylicacids.23,31 Although we were unable to prepare single crystals ofthe substituted complex, the elemental analysis is consistent withthe formation of [Mn12O12(O2CC6H4CHdCH2)16(H2O)4] 3CH2dCHC6H4CO2H 3 4H2O, and FTIR shows characteristicpeaks of both the ligand and the core cluster (breathing modeobserved between 400 and 800 cm�1; see the SupportingInformation, S3). The ac susceptibility measurements are alsoconsistent with ligand substituted Mn12 as described below.The magnetic properties of Mn12O12(VBA)16 have not been

previously reported, so we include those studies here. We havedetermined the ground state of the cluster using the in-phasemolar ac susceptibility χ0 measurements as a function of tem-perature T. From the plateau in the χ0T versus T plot (Figure 1),we found an effective moment μeff for Mn12O12(VBA)16 to be19.1μB with spin S = 9.1. For comparison, Mn12 has beenreported to have S = 10, though a ground spin state with S = 9has been previously reported for Mn12O12(O2CC6H5)16(H2O)4at zero applied field.23 The two plateaus in the χ0T versus T plotsuggests two relaxation mechanisms (vide infra). One of thesignatures of SMMs is a maximum in temperature dependence ofthe out-of-phase ac susceptibility signal (χ00). The peak in χ00 attemperature Tp corresponds to the temperature at which the rateof the flip of the molecular moment is equal to the ac excitationfrequency, υ. We observe two maxima in χ00 at 2.2 and 4.4 K atυ = 10 Hz (see the Supporting Information, S2), which alsosuggests two relaxation mechanisms. Several complexes withcomposition [Mn12O12(O2CR)16(H2O)4] exhibit two out-of-phase ac magnetic susceptibility signals, typically in the 4�7 and

2�3 K region. The presence of two relaxation mechanisms hasbeen attributed to the presence of two isomers of the complex,either in the placement of the four H2O ligands or Jahn�Tellerisomerism.32 Likewise, two plateaus that shift to lower tempera-ture as the frequency decreases in the χ00 as a function of T istypically seen in Mn12 clusters.31 The Arrhenius plots forMn12O12(VBA)16,

16 where ln(1/υ) is plotted against 1/Tp,yields an effective activation energy U = 65 K and relaxationtime t0 = 8.7� 10�9 s�1 for the high-temperature peak, and U =35.7 K and t0 = 1.9 � 10�9 s�1 for the low-temperature peak(Table 1).By contrast to Mn12, ligand substitution of the Mn8Fe4

complex has not been reported. We were able to synthesize[Mn8Fe4O12(O2CC6H4CHdCH2)16(H2O)4] 3 2CH2dCHC6H4-CCO2H 3CH2Cl2 or Mn8Fe4O12(VBA)16, based on the elementalanalysis and FTIR. Initially single crystals were elusive, so toconfirm that ligand substitution ofMn8Fe4 does not alter the corestructure, as is well established forMn12, we usedmethacrylic acid(MA) for ligand exchange. Our experience with Mn12O12-(MA)16(H2O)4, whose structure was recently reported,

19b sug-gested that this compound crystallized particularly easily. Indeed,using this ligand, we were able to isolate single crystals of the fullysubstituted cluster, Mn8Fe4O12(O2CC(CH3)dCH2)16(H2O)4orMn8Fe4O12(MA)16 (R factor of 4.19%). Mn8Fe4O12(MA)16 isisostructural with Mn12O12(MA)16(H2O)4, with both crystal-lizing in the C2/c space group (see Figure 2). The cell parametersare essentially unchanged for Mn12O12(MA)16(H2O)4 (a =30.445(6)Å, b = 14.098(3)Å, c = 29.300(6)Å, β = 116.28(3))and for Mn8Fe4O12(MA)16 (a = 30.164(3)Å, b = 14.0209(16)Å,c = 29.550(5)Å,β = 116.8930(10)). Subsequently, wemanaged toalso isolate single crystals of Mn8Fe4O12(VBA)16 upon slow evap-oration in toluene, with this structure crystallizing in the tetragonalspace group, P4n2 (Figure 3). As was observed for the methacry-late substituted cluster, there was complete substitution of theacetate ligands by 4-vinylbenzoate. The crystallographic informa-tion is summarized in the Supporting Information (Table S1).Primarily, our interest in the structures was to unequivocally

confirm full ligand substitution. Because the manganese and ironhave such similar scattering factors, analysis of the metal sub-stitution by X-ray diffraction is limited. However, as observedpreviously,22b there is clear crystallographic evidence that ironsubstitutes preferentially in symmetry-related positions (asshown in Figure 2), based on the loss of Jahn�Teller distortionsfor the Fe�O bonds. Comparing the same atom sites withMn12O12(MA)16(H2O)4, it can be seen that all the Mn(III)�Obonds show clear Jahn�Teller axial elongation, with the axialMn�Omethacrylate bond lengths (average 2.171 Å) about 10%longer than the equatorial values (average 1.949 Å). The fouroxygen atoms of the terminal water ligands are also involved inthe elongation of the Mn�O bonds.19b For Mn8Fe4O12(MA)16,

Figure 1. χ0T as a function of temperature for Mn12O12(VBA)16.

Table 1. Effective Energy Barriers and Relaxation Times

low temperature high temperature

U (K) t0 (s) U (K) t0 (s)

Mn12a 37.1 8.0� 10�10 69 1.2� 10�8

Mn12O12(VBA)16 35.7 1.9� 10�9 65 8.7� 10�9

Mn12O12(VBA)16-co-

polystyrene nanobeads

nab nab 60.4 2.0� 10�8

aData previously reported.40 b na: Not applicable.

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it is evident that the axial Mn(III)�Omethacrylate bond lengths(average 2.187 Å) were also found to be longer than equatorialbonds (average 1.943 Å). On the other hand, the Fe(III)�Omethacrylate bond lengths are all similar (average 2.013 Å),indicating loss of Jahn�Teller distortion due to the high-spin d5

configuration of Fe(III). Likewise, for the Mn8Fe4O12(VBA)16,the axial Mn(III)�O vinylbenzoate bond lengths (average2.209 Å) were found to be longer than equatorial bonds(average 1.935 Å). Again, the Fe(III)�O vinylbenzoate bondlengths are all similar with an average of 1.996 Å. Table 2 showsselected Mn�O and Fe�O bond distances for these structures.

The elemental analysis of our product is similar to previousreports, where the Mn/Fe ratio is slightly off the ideal stoichi-ometry of 8:4, with something closer to 7.64:4.36, suggestingslightly higher Fe substitution.22b Our data are consistent withprevious reports22b that suggest that small amounts of Mn(III)have been replaced by Fe(III), although there is no crystal-lographic evidence for this small deviation.Synthesis and Characterization of Miniemulsion Polymer

Nanobeads with Mn12O12(VBA)16(H2O)4 or Mn8Fe4O12-(VBA)16(H2O)4 and Styrene. Previously, molecular clusters withthe formula Mn12O12(O2CR)16 for O2CR = methacrylic acidand acrylic acid have been used as cross-linkers in copolymeri-zation with the organic polymers methyl methacrylate and ethylacrylate, respectively.19 This work found that thermal stability ofthe copolymer is improved while preserving its magnetic char-acter. The functionalized cluster acts as a cross-linker for thepolymer chains, which is evidenced in shifts in the thermaldecomposition temperatures shown in TGA data. For example,Mn12O12(MA)16(H2O)4 starts to decompose at 160 �C, whilethe onset of thermal decomposition of the copolymer with the

Figure 2. Thermal ellipsoid plot of the refinement model of [Mn8Fe4O12(O2CC(CH3)dCH2)16(H2O)4] 3 4(C4H6O2) 3 2H2O or Mn8Fe4O12-(MA)16(H2O)4 at 70% probability (left) and representation of the crystal structure of Mn12O12(MA)16(H2O)4

19b (right). Co-crystallized solvents andhydrogen atoms were omitted for clarity. Code for atoms: aqua, Mn; yellow, Fe; red, O; gray, C.

Figure 3. Thermal ellipsoid plot of the refinement model of[Mn8Fe4O12(CH2CHC6H4CO2)16(H2O)4] 32.5(C6H5CH3) orMn8Fe4O12-(VBA)16 at 70% probability. Only major occupancies are shown. Co-crystallized solvents and hydrogen atoms were excluded for clarity. Codefor atoms: aqua, Mn; yellow, Fe; red, O; gray, C.

Table 2. Selected Mn(III)�O and Fe(III)�O Bond Dis-tances (Å)a for [Mn8Fe4O12(O2CC(CH3)dCH2)16(H2O)4] 34(C4H6O2) 3 2H2O and [Mn8Fe4O12(CH2CHC6H4CO2)16-(H2O)4] 3 2.5(C6H5CH3)

[Mn8Fe4O12(O2CC(CH3)=CH2)16-

(H2O)4] 3 4(C4H6O2) 3 2H2O

[Mn8Fe4O12(CH2CHC6H4CO2)16-

(H2O)4] 3 2.5(C6H5CH3)

Mn(1)�O(1) 1.938(2)eq Mn(2)�O(5) 1.940(3)eq

Mn(1)�O(2) 1.946(2)eq Mn(2)�O(7) 1.930(3)eq

Mn(1)�O(3) 2.183(2)ax Mn(2)�O(2) 2.198(3)ax

Mn(1)�O(6) 2.193(2)ax Mn(2)�O(4) 2.219(3)ax

Fe(1)�O(17) 2.007(2) Fe(3)�O(3) 1.983(3)

Fe(1)�O(19) 2.005(3) Fe(3)�O(6) 2.025(3)

Fe(1)�O(20) 2.016(2) Fe(3)�O(8) 1.980(6)aMn�O bonds of the Mn3+ ions are divided into two groups,equatorial (eq) and axial (ax) bonds (∼10% longer). Fe�O bondshave similar lengths.

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Mn12O12(MA)16(H2O)4 cluster was shifted to higher tempera-tures by ∼40 �C.19bHere, we adapted the method used to form polystyrene

nanobeads previously published.2 Polystyrene was chosen forits well-established conditions for miniemulsion synthesis ofhighly monodispersed nanobeads,1,2 as well as the fact that it ischeap and easily functionalized. The metal�oxo cluster withvinylbenzoate ligands polymerized with styrene easily under bulkconditions. By contrast, we found the methacrylic acid substi-tuted clusters took significantly longer to polymerize with styreneunder bulk conditions, so we did not investigate them underminiemulsion conditions. During miniemulsion, a hydrophobe(hexadecane) was introduced to the cluster/styrene/DVB mix-ture to prevent Ostwald ripening. Finally, the initiator, AIBN, wasadded to themonomer phase. AIBNwas chosen as the initiator inorder to maintain relatively low temperatures during polymeri-zation (60 �C), thereby preserving the cluster integrity.Separately, the aqueous phase was formed by the addition of

the surfactant, SDS. The surfactant concentration is one of thevariables that has previously been determined to control polymernanobead size.2 In these experiments, a surfactant-to-monomerratio of 0.08 was used, which has been reported to produceparticles of approximately 60 nm in diameter.6c The monomerphase was subsequently mixed with the aqueous phase and pre-emulsified for approximately 10 min. The miniemulsion wasformed by ultrasonication of themixture, and polymerization wasthen initiated by placing the reaction vessel in a hot water bath.When the reaction was complete, the solution particle diameterswere measured by dynamic light scattering (DLS) as the con-centration of reagents was optimized. The nanobeads were thenseparated from solution and washed with ethanol for furthercharacterization. During the polymerization period, the particlesize was likewise monitored via DLS every hour (see the Sup-porting Information, S12).The percent monomer conversion of styrene versus time of

polymerization in the absence and presence of the metal�oxoclusters is shown in Figure 4. Based on this data, the polymer-ization rate is fastest for pure polystyrene, and it also has thehighest final percent conversion. Our pure polystyrene data isclose to previous reports of the miniemulsion polymerization ofpure polystyrene in the absence of inorganic particles.33 The data

shows that, for the miniemulsion system containing the metal-oxo clusters, there is a slower rate of polymerization, and thepercent monomer conversion was reduced. There are severalvariables that determine the polymerization rate, including theamount of initiator, ultrasonication energy, surfactant concentra-tion, and concentration of the costabilizer,34 which have beenkept constant for these experiments. One important differencebetween these systems (pure polystyrene, Mn12O12(VBA)16-co-polystyrene, and Mn8Fe4O12(VBA)16-co-polystyrene) is that theinitial droplet size was smallest for pure polystyrene (∼50 nm)and larger for the metal�oxo copolymers (∼110 nm). Thechange in size is consistent with the literature, which shows thatthe efficiency of droplet fission during ultrasonification is re-duced when the metal-oxide is present, resulting in larger initialdroplets.35 The polymerization rate is generally faster for smallerdroplets, so this is clearly one factor in the difference in observedrates. However, the metal content of the cluster clearly makes adifference to the polymerization rate, as the system containingthe Mn12 cluster has a slower rate than the one containingMn8Fe4. Changes to the polymerization rate are also observed inthe miniemulsion preparation of Fe3O4 in polystyrene. Thedecrease in polymerization34,36 is thought to be due to the radicalquencher Fe3+.37 In addition to the reduction in the polymerizationrate, the data show a decrease inmonomer conversion, which is alsoobserved when iron oxide nanoparticles (Fe3O4) were incorpo-rated into polymer matrices.33,34 We have not found an analogousstudy of the effect of Mn on polystyrene polymerization, but it isclearly worse than Fe, resulting in the lowest polymerization rateand percent conversion for the Mn12 copolymer.We also used a combination of DLS and transmission electron

microscopy (TEM) to determine the particle size of the cluster�polystyrene nanobeads prepared by miniemulsion, as well as toobtain information about the homogeneity of the resultingparticles. Figure 5 shows a comparison between the TEM ofMn12O12(VBA)16-co-polystyrene and Mn8Fe4O12(VBA)16-co-polystyrene nanobeads. A histogram (Supporting Information,S10) of 200 well-defined Mn12O12(VBA)16-co-polystyrene nano-beads suggests a range in size from 35 to 90 nm with an averagesize of 63.2( 10.1 nm and a particle size distribution of 1.11. TheDLS measurements (Supporting Information, S13) of a solutionof the nanobeads were found to be unimodal with an averageparticle diameter of 72 nm, slightly higher than that found byTEM. From TEM, Mn8Fe4O12(VBA)16-co-polystyrene nano-beads were found to be extremely uniform, having an averageparticle size of 70.9( 9.4 nmwith a particle size distribution 1.05.TheDLSmeasurements showed an average particle size of 84 nm,again slightly higher than by TEM as expected. This is in contrastto studies of polystyrene latex nanobeads prepared in the presenceof Fe3O4 nanoparticles, which exhibits the presence of twopopulations of latex particles as shown in both DLS and TEM.Based on TEM, one population appears to be large compositenanobeads with a high content of Fe3O4 nanoparticles withPickering stabilization (i.e., the nanoparticles have migrated tothe polymer/aqueous surface), and the second population containsmuch smaller pure polystyrene nanobeads formed from secondarynucleation. In our system, however, it is important to note that theTEM results exhibit no evidence of phase separation or segregationof clusters within the nanobeads. High resolution TEM of a singlenanobead (inset, Figure 5) further shows that the material ishomogeneous. This is in distinct contrast to polystyrene nanobeadsof iron oxide nanoparticles, which show random dots of nanopar-ticles within the polymer matrix.11a

Figure 4. Percent monomer conversion as a function of time forthe miniemulsion polymerization synthesis of (A) pure polystyrene,(B) Mn8Fe4O12(VBA)16-co-polystyrene, and (C)Mn12O12(VBA)16-co-polystyrene nanobeads.

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The previous reports of Mn12 copolymers indicated a higherthermal stability of the copolymers in comparison to theindividual cluster or the homopolymer. The TGA curve for theMn12O12(VBA)16-co-polystyrene nanobeads indicates that theystart to decompose around 230 �C (with an inflection point at385 �C), a higher temperature than that of individual cluster(159 �C), which is in agreement with the literature.19b Analysis ofthe remaining material by powder X-ray diffraction indicates theformation of Mn3O4.

16 Similarly, TGA data shows that theMn8Fe4O12(VBA)16-co-polystyrene nanobeads start to decom-pose at 240 �C (inflection point at 400 �C), with final productsmainly consisting of Mn3O4 and MnFe2O4 as confirmed bypowder X-ray diffraction data. In both cases, the metal contentbased on the weight loss is on the order of 3%, as anticipatedbased on the cluster/monomer ratio used. In addition, whencompared to pure polystyrene nanobeads, the onset of decom-position for the two metal�oxo copolymer nanobeads was alsoslightly higher (∼225 �C for pure polystyrene), indicatingenhanced thermal stability. We also separated the inorganic corefrom the organic component using a toluene�HCl mixture.Using atomic absorption spectroscopy to determine the metalcontent of the acid layer, we found that the percent metal of thenanobeads was very close to the percent metal of the reactants.Previous work suggests that the percent metal content inminiemulsion polymer beads is partly influenced by the solubilityof the metal complex. In addition, some metals appear toinfluence the interface stability of the miniemulsion, resultingin a broadening of the size distribution, perhaps due to interac-tion between the SDS and metal ions.4b However, our sizedistribution is quite narrow.

To determine the average molecular weight of the polymercomponent of theMn12O12(VBA)16-co-polystyrene andMn8Fe4O12-(VBA)16-co-polystyrene nanobeads, first the inorganic clusters wereremoved by acid and the organic component separated andwashed. The molecular weight of the soluble portion of theorganic fraction was determined byGPC and compared with purepolystyrene nanobeads formed under the same miniemulsionconditions (Table 3). Previously, Landfester and Ramirez11a,38

reported molecular weights for both pure polystyrene and PS/Fe3O4miniemulsion nanobeads on the order of 200� 103 g/mol.We note that the reported polymerization conditions were atmuch higher temperatures (>80 �C) than reported here. Highertemperatures should cause an increase in the polymerization rateand decrease in molecular weight. By contrast, Qiu et al.17 found amolecular weight for pure polystyrene similar to our observedvalues (∼106 g/mol), with polymerization temperatures close toours. However, for the composite PS/Fe3O4 nanobeads, Qiuet al.17 found a strong effect of the iron oxide nanoparticles with asignificant decrease in molecular weight (200 � 103 g/mol). Wedid not observe such a marked effect on the molecular weight, inthe presence or absence of cluster. Both the DLS and PDIsconfirm the distribution of molecular weight is much narrowerfor pure polystyrene, which is also consistent with the literaturereports of PS and PS/Fe3O4 system.17

To show that the magnetic character of the Mn12O12(VBA)16cluster was preserved after copolymerization, ac magnetic sus-ceptibility measurements were performed. The temperaturedependence of the χ0 and χ00 for the Mn12O12(VBA)16-co-polystyrene nanobeads has been obtained at frequencies rangingfrom 100 to 10 000 Hz. For the magnetic polymer nanoparticles,

Figure 5. TEM images of Mn12O12(VBA)16-co-polystyrene (left) and Mn8Fe4O12(VBA)16-co-polystyrene (right) nanobeads. Inset: HR-TEM.

Table 3. Number, Weight, and z-Average Molecular Weights (Mn,Mw, andMz, Respectively) and Molecular Weight Distribution(PDI) of Nanobeads

Mn (�106) Mw (�106) Mz (�106) PDI = Mw/Mn

Mn12O12(VBA)16-co-polystyrene nanobeadsa 0.669( 0.013 1.50( 0.03 2.07( 0.06 2.24

Mn8Fe4O12(VBA)16-co-polystyrene nanobeadsb 0.611( 0.098 1.26( 0.14 1.73( 0.29 2.06

polystyrene nanobeads 0.718( 0.058 1.39( 0.14 1.81 ( 0.29 1.94

polystyrene/Fe3O4c 0.194 0.541 0.973 2.79

a Particles are 2.5% Mn by weight as determined by atomic absorption. b Particles are 1.67% Mn and 0.97% Fe as determined by atomic absorption.c Previously reported.17

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a high-temperature peak is present in χ00 versusT, that shifts from5.5 to 8.5 K as the frequency is increased. From the Arrheniusplot (see the Supporting Information, S9), it was found that U =60.4 K and τ0 = 2.0� 10�8 s. These values are similar to what wasobtained for the high-temperature peak of the starting cluster,Mn12O12(VBA)16, as Table 1 shows. As found previously, copoly-merization does not substantially alter the magnetic properties of thecluster.Similarly, to prove that the Mn8Fe4 cluster remains intact within

the polymer nanobead, infrared spectroscopy (Supporting Infor-mation, S6) was carried out. The low frequency bands situatedbetween 450 and 600 cm�1 (449, 536, 590 cm�1) correspondingto theMn�Oand Fe�Ostretches, as well as the carboxylate groupstretches in the 1410�1600 cm�1 region, clearly indicate that thecluster is well-preserved inside the polymeric matrix. Moreover, theCdC band, originally found at 1670 cm�1 in the Mn8Fe4O12-(VBA)16 spectrum, was strongly diminished in the IR spectrum ofthe Mn8Fe4O12(VBA)16-co-polystyrene nanobeads. This supportsthe fact that the 4-vinylbenzoate ligands participated in thepolymerization process, making the cluster covalently bonded tothe polymer.19

’CONCLUSIONS

Polymerizable magnetic clusters, Mn12O12(VBA)16 andMn8Fe4O12(VBA)16, have been synthesized and characterized.By employing the miniemulsion polymerization technique, themagnetic clusters were encapsulated, resulting in homogeneous,monodisperse, magnetic polymer nanobeads with an averagediameter of ∼60�80 nm. Magnetic susceptibility studies confirmthat theMn12 andMn8Fe4 cores remain intact after polymerization,retaining the integrity of the magnetic cluster. These magneticcopolymer beads have the potential for functionalization forbiomedical applications.39 Studies on the use of the clusters, aswell as the magnetic polymer nanobeads, as potential contrastagents for magnetic resonance imaging are currently underway.

’ASSOCIATED CONTENT

bS Supporting Information. Additional table and figures.This material is available free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Fax: 202-687-6209. Telephone: 202-687-5839. E-mail: sls55@georgetown.edu.

’ACKNOWLEDGMENT

This work was supported by National Science Foundation(NER, 0304273; CAREER, 0449829). We extend our gratitudeto Nick Deifel and Clare Rowland of George WashingtonUniversity for additional crystallographic input. We likewiseacknowledge the support of the Maryland NanoCenter and itsNispLab. The NispLab is supported in part by the NSF as aMRSEC Shared Experimental Facility. SEL acknowledges sup-port by NSF grants DMR 0908779 and DMR 0520471.

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