A Fatty Acid-Inspired Tetherable Initiator for Surface-Initiated Atom

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A Fatty Acid-Inspired Tetherable Initiator for Surface-Initiated Atom Transfer Radical Polymerization Jiajun Yan,† Xiangcheng Pan,† Zongyu Wang,† Zhao Lu,‡ Yi Wang,† Li Liu,§ Jianan Zhang,†,‡,∥ Chien Ho,§ Michael R. Bockstaller,‡ and Krzysztof Matyjaszewski*,† †

Department of Chemistry, ‡Department of Materials Science and Engineering, and §Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ∥ School of Chemistry and Chemical Engineering, Anhui University, Hefei 230601, People’s Republic of China S Supporting Information *

ABSTRACT: A universal tetherable initiator, derived from the structure of fatty acids, for surface-initiated atom transfer radical polymerization (SI-ATRP) from metal oxide surfaces was prepared. A simple amidation between 2-bromoisobutyryl bromide and ω-aminolauric acid allowed preparation of 12-(2bromoisobutyramido)dodecanoic acid (BiBADA). After facile purification, BiBADA was used as a tetherable initiator for a broad range of metal oxide nanoparticles. The modified nanoparticles were grafted with methyl methacrylate or n-butyl acrylate via SI-ATRP with a high grafting density. This is the first report of successful SI-ATRP from a selection of different metal oxide nanoparticles. Sub-10 nm Fe3O4 nanoparticles with an intrinsically tethered initiator were also prepared using BiBADA as a surfactant template. Additional experiments demonstrated successful modification of an aluminum foil surface with polymer brushes using BiBADA as a tetherable initiator.

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INTRODUCTION Polymer hybrid materials consist of polymeric and inorganic components linked on the molecular or nanometer level.1 Interest in polymer hybrid materials is driven by the opportunity to combine the processability and durability of polymers with the mechanical, thermal, optoelectronic, and catalytic properties of inorganics. This has rendered the area of “polymer hybrid materials” as a very active research area.2−6 For example, metal oxide hybrid materials are candidates for applications ranging from medical diagnostics to energy storage.6−9 A key to realizing the opportunities of the “hybrid material approach” is the availability of chemical processes that allow the tethering of polymers to inorganic components. While several methodologies for surface tethering have been reported in the literature, the application of polymer grafting techniques remains a challenge. This is because the coupling of chains typically involves noncovalent coordination that is sensitive to minor differences in surface chemical composition and charge. As a result, only a few reliable and broadly applicable procedures for the preparation of well-defined polymer−metal oxide hybrid materials have been reported. Methods for surface modification with polymers are typically classified as “grafting-onto”,4,10 “grafting-from”,10,11 template,10,12 and “ligand-exchange” methods.13,14 In the “grafting-onto” method, the metal oxide surface reacts directly with a functional end group on the polymer. This method is inexpensive and straightforward, but the grafting density is generally low. On the other hand, the metal oxide surface can © 2017 American Chemical Society

be treated with a small molecule initiating group and subsequently “grafted from” the functionalized surface to form polymer brushes. Polymer brushes with a high density and a well-controlled polymer brush architecture can be achieved. A star-shaped or molecular bottlebrush polymeric template can also be used as a nanoreactor for the synthesis of a metal oxide nanoparticle with the desired size and morphology within the designed template.10 The other method, the “ligandexchange” procedure, uses presynthesized polymers, with a functional end group, to exchange with small molecular ligands covering the metal oxide surface. Both the “grafting-onto” and “grafting-from” methods rely on generating an effective interaction between metal oxide surfaces and the anchoring group on the polymer chain ends or on tetherable initiators.15 Commonly used anchoring groups include phosphonate,9,16,17 carboxylate,16,18,19 halo/alkoxysilane,20 amines,13 catechol,21,22 and poly(ethylene oxide).23 Among these anchoring groups, only phosphonate and alkoxysilane were commonly used as the anchoring group for presynthesized tetherable initiators. The use of presynthesized tetherable initiators avoids the steric effect of the “graftingonto” method as well as the inferior efficiency of two-step heterogeneous modification of the widely used polydopamine chemistry.24 However, the precursors of these two types of Received: March 31, 2017 Revised: May 11, 2017 Published: May 18, 2017 4963

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Chemistry of Materials initiators are expensive and are effective for a narrow selection of metal oxide surfaces. Furthermore, accessibilities of phosphonates are restricted by the Chemical Weapon Convention (Annex on Chemicals, Schedule 2). Therefore, a tetherable initiator that can be prepared at a reasonable cost and is universally suitable for a variety of metal oxide surfaces is lacking. Previous research has demonstrated that carboxylate/ carboxylic acid has the capability to anchor onto some metal oxide surfaces.16 Indeed, oleic acid is one of the most commonly used ligands for stabilization of metal and metal oxide nanoparticles.25−27 The micelle-like structure formed by the amphiphilic layer of oleic acid ensures miscibility in nonpolar solvents while holding firmly to the polar surface of the nanoparticle core. Hence, a novel tetherable initiator for surface-initiated atom transfer radical polymerization (SIATRP) was designed on the basis of the structure of oleic acid. A one-step amidation procedure in which the commonly used monomer for Nylon-12, ω-aminolauric acid, was converted to 12-(2-bromoisobutyramido)dodecanoic acid (BiBADA) was used. The list price for ω-aminolauric acid from major laboratory chemical venders could be as low as $118 per 500 g (TCI). In contrast, the same amounts of dopamine hydrochloride, dimethyl 2-hydroxyethylphosphonate, and triethoxysilane cost at least $1405 (Alfa), $6780 (TCI), and $828 (Alfa), respectively. In addition, a large excess of dopamine is usually required for the formation a stable layer of polydopamine, while two synthetic steps are required for the phosphonate- or silane-based tetherable initiators. Therefore, a considerable amount of labor and money can be saved using the BiBADA initiator. This novel initiator was demonstrated to be suitable for tethering to a large variety of metal oxide nanoparticles allowing initiation of polymerization from the functionalized surfaces (Scheme 1). As the amino group in ω-aminolauric acid is highly accessible to multiple types of functionalization, this method can potentially serve as a proof of concept for a versatile platform for surface modification of a large range of metal oxide nanoparticles. The development of reversible deactivation radical polymerization (RDRP) techniques in the past two decades has allowed the preparation of polymeric materials with well-controlled molecular weights, molecular weight distributions, and various architectures.28−30 Surface-initiated RDRP (SI-RDRP) techniques emerged from these developments as well as the concept of ideal spatial control.31,32 SI-ATRP is the most broadly applied procedure among the SI-RDRP techniques, because of its accessible tetherable initiators, versatile monomer choices, and tolerance to various reaction conditions and impurities.33,34 In recent years, the emergence of an activator regenerated by electron transfer (ARGET) ATRP,35 the use of Cu(0) as supplemental activator and reducing agent (SARA) ATRP,36,37 initiator for continuous activator regeneration (ICAR) ATRP,38 electrochemically mediated ATRP (eATRP), 39 photoATRP,40−44 and metal-free ATRP45−47 techniques provide a broad range of procedures that can be optimally selected for different applications while minimizing the residual metal impurities in the final product. SARA ATRP was employed here because of its facile setup and heterogeneous nature, which mitigated side reactions between the reducing agent and the metal oxide nanoparticles. Other ATRP techniques, including ICAR and photoATRP, may also be used for grafting the

Scheme 1. Preparation of Polymer-Grafted Metal Oxide Nanoparticlesa

a (a) Preparation of BiBADA and surface modification of metal oxide nanoparticles with polymer brushes. (b) ATRP equilibrium of surfaceinitiated polymerization from metal oxide nanoparticles using a parts per million level of the Cu catalyst. (c) Qualitative comparison among anchoring techniques, including two-step polydopamine chemistry (green), two-step aminosilane-based [yellow, e.g., (3-aminopropyl)triethoxysilane, APTES], presynthesized phosphonate initiator (violet), presynthesized silane initiator (blue), and carboxylate-based presynthesized initiator (red, BiBADA, this work) for grafting polymer brushes from metal oxide surfaces.

polymer from modified metal oxide nanoparticles (see Table S1).

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RESULTS AND DISCUSSION Synthesis of BiBADA and Surface Modification. The structure of the tetherable initiator, BiBADA, consists of three parts, a carboxylic acid anchoring group, an aliphatic chain of 11 CH2 units, and a 2-bromoisobutyrate initiating group (Scheme 1a). It was synthesized in a one-step amidation reaction of ωaminolauric acid with 2-bromoisobutyryl bromide. The resulting product was purified simply by repeated washing with a dilute HCl solution. This facile procedure allowed scaled-up preparation of BiBADA. The modification of metal oxide nanoparticles with BiBADA was accomplished by simply dispersing the nanoparticles in tetrahydrofuran (THF) at a concentration of 10 wt %. Five molecules of BiBADA were added per square nanometer of the nanoparticle surface. The modification was conducted in a sonication bath to ensure sufficient separation of nanoparticle aggregates and contact of the exposed surfaces with BiBADA. The stability of the nanoparticle in THF dispersions was significantly improved after the surface modification. Then, the modified nanoparticles were washed three times with THF in a sonication centrifuge (4000g) cycle to remove the excess BiBADA. Surface-Initiated Polymerization. The initiator-modified nanoparticles were then dispersed in a polymerization solution. 4964

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Chemistry of Materials Table 1. Summary of Polymer-Grafted Metal Oxide Nanoparticles Examined in This Studya entry alkaline earth transition metal

post-transition

metalloid metallate

1 2 3 4e 5 6 7 8e 9 10 11 12 13 14 15e 16 17f

particle MgO TiO2 Co3O4 NiO ZnO Y2O3 ZrO2 La2O3 CeO2 WO3 α-Al2O3 α-Al2O3 In2O3 ITO SnO2 Sb2O3 BTO

size (nm) 20 15 10−30 10−20 18 10 40 10−100 10 60 30 30 20−70 20−70 35−55 80−200 200

monomer

Mnb

MMA MMA MMA MMA MMA MMA MMA MMA MMA MMA MMA BA MMA MMA MMA MMA MMA

× × × × × × × × × × × × × × × × ×

1.32 7.24 1.03 7.69 8.77 1.66 5.56 6.35 6.88 2.36 2.37 2.42 1.40 1.23 1.64 3.66 1.85

5

10 104 105 104 104 105 104 104 104 105 105 104 105 105 105 105 105

Mw/Mnb

σ (nm−2)c

1.60 1.25 1.83 1.28 1.33 1.72 1.52 1.23 1.27 1.98 2.10 1.24 1.49 1.92 2.24 1.93 2.38

0.08 0.03 0.14 0.14 0.17 0.24 0.15 0.48 0.13 0.28 0.06 0.06 0.20 0.11 0.22 0.14 0.43

Dh (nm)d 1600 403 4800 236 282 650 236 317 244 762 501 385 377 396 377 870 715

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

200 5 100 3 1 10 1 2 1 5 4 1 9 3 1 20 4

Typical reaction conditions: [MOx-Br, assuming 1 Br/nm2]0/[M]0/[CuBr2]0/[Me6TREN]0 = 1/1000/0.2/0.5, 50 vol % anisole, 1.0 mm × 1 cm copper wire, room temperature. bMeasured via SEC. cCalculated from eq S1. dZ-Averaged hydrodynamic diameter in THF measured by DLS. e Particles modified with neutralized BiBADA. f[BTO-Br, assuming 1 Br/nm2]0/[M]0 = 1/3000. a

BiBADA to functionalize the surfaces. Under these circumstances, the attached BiBADA molecules would not be washed away during the postfunctionalization purification. In addition, coordination bonding and sufficient solvophobicity were essential for the successful surface modification. As a reference, noncoordinating and amphiphilic colloidal silica nanoparticles were treated with BiBADA in a similar procedure. However, no initiating groups were detected on the surface. The molecular weight and dispersity of the detached PMMA brushes ranged from 104 to 105 and from 1.2 to 2.4, respectively. There are three major reasons for the relatively high dispersity, including monomer reactivity, metal cation leaching, and variation in surface curvature. First, SARA ATRP of MMA is challenging because of fast activation,49 and the 2bromoisobutyramide initiating group may also limit the initiation efficiency of MMA because of the penultimate unit effect.50 Substitution of MMA with BA significantly improved the level of control over the polymerization, but the reaction was slower under the same polymerization conditions (Table 1, entry 2, and Table S1) because of the lower rate of activation, i.e., lower overall radical concentration. Second, the metal cations from the surface of nanoparticles may leach into the reaction medium and interact with the copper complexes. This was demonstrated by the fact that no monomer conversion was observed in SI-ATRP from oxidative metal oxide nanoparticles, including CuO, Mn2O3, and MoO3. Third, as the size and morphology distributions of the commercial nanoparticle samples were large (see the Supporting Information), variation in surface curvature might broaden the molecular weight distribution.51,52 The polymer-grafted metal oxide nanoparticles inherited the breadth of size distributions from their inorganic cores (Table 1 and Figure 1); examples include Co3O4, Y2O3, and Sb2O3 (Figures S7, S10, and S17). Large aggregates were observed for MgO-g-PMMA in both DLS and transmission electron microscope (TEM) images (Figure S2), indicating that BiBADA has limited efficiency for the alkali earth metal oxides. Shorter PBA brushes rendered the surface less hindered, which

A use of Cu(0) as supplemental activator and reducing agent (SARA) ATRP reaction was conducted to graft poly(methyl methacrylate) (PMMA) or poly(n-butyl acrylate) (PBA) from the surface. To simplify the calculation, 1000 monomer molecules were added for each square nanometer of the nanoparticle surface. The only exception was the 200 nm barium titanate (BTO) particles, because that ratio was not sufficient to disperse the particles in the monomer/anisole mixture, because of its small specific surface area. The SARA ATRP was chosen as the “grafting-from” procedure because of its facile setup and heterogeneous nature. Both the copper wire and the metal oxide nanoparticles were present as a solid phase; hence, interaction between the reducing agent and particle surfaces was minimized. The reactions were allowed to proceed for 24 h to ensure a sufficient conversion of monomers. The inorganic cores were removed by acid etching to allow measurement of the number-average molecular weight (Mn) and the molecular weight distribution (Mw/Mn) of the grafted polymers by size exclusion chromatography (SEC) (see Table 1). The grafting densities (σ) were calculated from the residual inorganic fractions remaining after thermogravimetric analysis (TGA). The average hydrodynamic diameters (Dh) of the polymer-grafted metal oxide nanoparticles were measured via dynamic light scattering (DLS). SI-ATRP from a large range of metal oxides was demonstrated herein, from alkaline earth metal to metalloid, and from the period 3 to period 6 elements. Barium titanate was also modified as an example of a metallate salt. To the best of our knowledge, this is the first report of successful SI-ATRP from the surfaces of some of the metal oxides, including Co3O4, NiO, Y2O3, ZrO2, La2O3, CeO2, WO3, In2O3, SnO2, and Sb2O3. SI-ATRP from other nanoparticles, such as α-Al2O3, was previously very challenging, and surface grafting was accomplished only after a harsh activation reaction.48 However, with BiBADA as the surface functionalization reagent, no activation was needed to achieve an even higher grafting density. To graft polymers from certain metal oxide nanoparticles with relatively reactive surfaces, including NiO, SnO2, and La2O3, an ammonium salt of BiBADA was used instead of 4965

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Figure 1. Z-Averaged hydrodynamic diameters of polymer-grafted metal oxide nanoparticles in THF measured by DLS.

caused more aggregation in the TEM images. However, DLS indicated that solution dispersibility was maintained. Mechanical Analysis. One particularly intriguing aspect of hybrid materials based on polymer-tethered particles is the possibility of fabricating “one-component hybrid materials” by direct assembly of brush particles (i.e., without the need for a separate matrix polymer).11 To demonstrate this possibility, the mechanical (elastic) properties of films fabricated from ZrO2-gPMMA were evaluated using nanoindentation and compared to those of the respective linear polymer analogues (Figure 2d). Nanoindentation was performed by indentation using a nanoscopic Berkovich tip following analogue procedures as reported previously.53,54 The modulus (E) and hardness (H) of the ZrO2-g-PMMA film were >2 times higher than those of the linear PMMA reference material. Because the elastic modulus is a measure of the bonding strength between material constituents, these results provide direct evidence of strong bonding between inorganic particle and polymeric tethers. The ability to assemble mechanically robust films by simple solution casting of ZrO2 hybrid particles could provide new opportunities for the fabrication of, for example, optical materials with outstanding mechanical strength. One-Pot Synthesis of BiBADA-Modified Fe3O4. In addition to surface modification of metal oxide nanoparticles, BiBADA was also used as a micelle template for one-pot synthesis of magnetite nanoparticles (MNP) with immobilized ATRP initiators (Figure 3a). The synthesis was derived from a known procedure.6 BiBADA was added to an ammonium hydroxide solution to form a mixture of base and surfactant. Poly[oligo(ethylene glycol) methyl ether acrylate-480] (POEGA) was grafted from the surface after the in situ synthesis and surface modification of MNPs. Two reactions, lasting 24 and 12 h, were performed, resulting in preparation of two batches of samples, OEG-MNP-24 and OEG-MNP-12, respectively. The POEGA-grafted MNPs were readily dispersible in water. The magnetic relaxivities of the aqueous dispersion of the POEGA-grafted MNPs were measured at 37.0 °C with a 20 MHz (0.47 T) nuclear magnetic resonance (NMR) spectrometer (Figure 3b)55 and compared to that of Feraheme, the only iron oxide nanoparticle approved by the Food and Drug Administration for human use.56,57 The POEGA-grafted MNPs displayed a similar longitudinal relaxivity (r1) but a >5-fold higher transverse relaxivity (r2). Thus, the relaxivity ratios (r2/

Figure 2. Characterization of PMMA-grafted ZrO2 nanoparticles. (a) Intensity-weighted hydrodynamic size distributions of ZrO2-g-PMMA as an example. The inset shows the correlation function (red square) and its single-exponential fitting (black line) indicating a nearly monodisperse population. (b) Photo of a uniform 2.0 mg/mL dispersion of ZrO2-g-PMMA in THF with slight opalescence. (c) TEM images of ZrO2-g-PMMA. (d) Characteristic load−displacement curves for ZrO2-g-PMMA (blue) and its linear reference (green). The inset shows the corresponding moduli and hardness determined by nanoindentation.

r1) of both of the POEGA-grafted MNPs were significantly higher than that of the reference sample. The relaxivity ratio of iron-based contrast agents increases rapidly with the increase in magnetic field strength,58 and much higher relaxivity ratios can be expected under practical magnetic resonance imaging (MRI) conditions. In addition to the high r2/r1 ratio, OEG-MNP-12 showed an r2 value (400 ± 24 L s−1 mmol−1) higher than those of most of the recent reports.55,59,60 Therefore, the POEGAgrafted MNPs can serve as a potential highly efficient contrast agent for T2*-weighted MRI. BiBADA as an Initiator for Grafting from Metal Surfaces. BiBADA was also demonstrated to be an effective tetherable initiator for large metal or metal oxide surfaces. Because of the presence of a native oxide layer,61 the surface chemistry of aluminum foils is similar to that of metal oxide nanoparticles, which promoted anchoring of BiBADA molecules. A simple and low-cost “paint-on” technique62 was employed to graft dense poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) brushes from the surface of BiBADA4966

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Figure 3. Synthesis of initiator-modified MNPs and magnetic relaxivity measurement of POEGA-grafted MNPs. (a) One-pot synthesis of initiator-modified MNPs. (b) Longitudinal (T1, empty dots) and transverse (T2, filled dots) relaxation time measurements of an aqueous dispersion of Feraheme (gray squares), OEG-MNP-24 (red circles), and OEG-MNP-12 (green triangles) at various Fe concentrations and their corresponding linear fittings (solid and dashed lines). The inset shows the corresponding relaxivity ratios.

Figure 4. Painting PDMAEMA brushes from an aluminum surface using BiBADA as a tetherable initiator. (a) Procedure for surface modification of an aluminum foil with BiBADA and PDMAEMA brushes. Polymerization solution: [DMAEMA]0/[CuCl2]0/[PMDETA]0 = 500/1/1, 50 mmol/L NaCl, 200 mmol/L ascorbic acid, 80 vol % deionized water, 3 h. (b) Average static water contact angles of a pristine aluminum foil (gray), a BiBADA-coated foil (red), and a PDMAEMA-coated foil (green) at 20.0 °C (filled bars) and on a 55.0 °C hot plate (cross-hatched bars). (c) Characteristic photos of 1.0 μL water drops on untreated or treated aluminum foil.

modified aluminum foil (Figure 4a). The reaction was allowed to proceed for 3 h before the surface was rinsed extensively with water to remove detached polymer brushes. While there was no apparent change in surface hydrophilicity after BiBADA modification, the surface became much more hydrophilic after the growth of PDMAEMA brushes (Figure 4b,c). The untreated aluminum foil and treated aluminum foil were placed on a 55.0 °C hot plate. There was a small increase in the water contact angle after heating corresponding to the lower-critical solution temperature (LCST) behavior of PDMAEMA.63 However, the reason for this lack of a steep change in contact angle was likely the insufficient transfer of heat through the glass slide attached to the back of the aluminum foil (Figure S25). A small decrease in water contact angle on the BiBADA-modified surface at an elevated temperature was also observed, which could be expected as hydrogen bonding in water became less dominant.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01338. Experimental procedures, specifications of additional SIATRP from metal oxide nanoparticles, hydrodynamic size distributions of polymer-grafted metal oxide nanoparticles, magnetization of BiBADA-modified MNPs, and an infrared image of aluminum foil on a hot plate (PDF)

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CONCLUSION A universal tetherable initiator, BiBADA, derived from the structure of fatty acids, was designed and synthesized. The simple amidation reaction of 2-bromoisobutyryl bromide with ω-aminolauric acid yielded the tetherable initiator, BiBADA, as a universal platform for surface functionalization of metal oxides and metals. Successful surface modification and surfaceinitiated polymerization from a large range of metal oxide surfaces, including some previously difficult or unreported surfaces, e.g., α-Al2O3 were demonstrated. Facile “paint-on” polymerization from an oxidized metal surface was conducted using the BiBADA initiator. Also, sub-10 nm Fe3O4 nanoparticles with a very high magnetic relaxivity were prepared. Thus, BiBADA can serve as a powerful platform for the preparation of functional polymer nanocomposites and smart metal surfaces.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jiajun Yan: 0000-0003-3286-3268 Xiangcheng Pan: 0000-0003-3344-4639 Michael R. Bockstaller: 0000-0001-9046-9539 Krzysztof Matyjaszewski: 0000-0003-1960-3402 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (DMR 1501324 and CMMI 1663305) for financial support. J.Y. acknowledges the support from the Richard King Mellon Foundation 4967

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Presidential Fellowship. J.Z. acknowledges the support from the CSC Scholarship. L.L. and C.H. acknowledge the support from the National Institutes of Health (P41EB-001977). We acknowledge Nissan Chemical for their generous donation of SiO2 NPs, Dr. Kevin Hitchens of the University of Pittsburgh Cancer Institute for his help with the magnetic relaxivity measurement, Robert Greco of Carnegie Mellon University (CMU) for his help with atomic absorption spectroscopy, Jill Anderson and Prof. Alan Russell of CMU for their help with the contact angle measurement, and J. Andrew Gamble of CMU for his help with the VSM.

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