A versatile method for the preparation of end-functional polymersonto SiO2 nanoparticles by a combination of surface-initiatedATRP and Huisgen [3 + 2] cycloaddition

J.C. Chen a,*, W.Q. Luo a, H.D. Wang a, J.M. Xiang a, H.F. Jin a, F. Chen b, Z.W. Cai b

a Department of Chemistry and Chemical Engineering, Ankang University, 92 Yucai Road, Ankang 725000, Chinab Department of Agriculture and Life Sciences, Ankang University, Ankang 725000, China

Applied Surface Science 256 (2010) 2490–2495

A R T I C L E I N F O

Article history:

Received 30 July 2009

Received in revised form 26 October 2009

Accepted 26 October 2009

Available online 3 November 2009

Keywords:

Surface modification

Atom-transfer radical polymerization

(ATRP)

Cu-catalyzed Huisgen [3 + 2] cycloaddition

Chain-end functionalization

A B S T R A C T

A versatile method was developed for the chain-end functionalization of the grafted polymer chains for

surface modification of nanoparticles with functionalized groups through a combination of surface-

initiated atom-transfer radical polymerization (ATRP) and Huisgen [3 + 2] cycloaddition. First, the

surface of SiO2 nanoparticles was modified with poly(methyl methacrylate) (PMMA) brushes via the

‘‘grafting from’’ approach. The terminal bromides of PMMA-grafted SiO2 nanoparticles were then

transformed into an azide function by nucleophilic substitution. These azido-terminated PMMA brushes

on the nanoparticle surface were reacted with alkyne-terminated functional end group via Huisgen

[3 + 2] cycloaddition. FTIR and 1H NMR spectra indicated quantitative transformation of the chain ends

of PMMA brushes onto SiO2 nanoparticles into the desired functional group. And, the dispersibility of the

end-functional polymer-grafted SiO2 nanoparticles was investigated with a transmission electron

microscope (TEM).

� 2009 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Applied Surface Science

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1. Introduction

‘‘Click’’ reactions have recently emerged as a powerful andversatile class of chemical transformations. The popularity of thesereactions can be attributed to their mild conditions, quantitativeyields, absence of by-product formation, and regioselectivity [1].Click reactions include the Cu-catalyzed azide/alkyne cycloaddi-tion [2], usually known as Huisgen [3 + 2] cycloaddition, which hasfound wide applications in various research areas includingchemical synthesis [3,4], bioconjugation [5], drug discovery [6],combinatorial chemistry [7], and materials science [8,9].

The surface functionalizations of nanomaterials by grafting ofpolymer are expected to play important roles in the designing ofnovel organic/inorganic nanocomposite materials. In recent years,much attention has been paid to the use of atom-transfer radicalpolymerization (ATRP) from nanosurfaces via surface-initiatedtechniques [10–14], because this allows better control over thetarget molecular weight and molecular weight distribution of thetarget grafted polymers [15]. The surface-initiated ATRP techniquehad been successfully used for the grafting of well-definedhomopolymers [16,10], diblock copolymers [17], graft copolymer

* Corresponding author. Tel.: +86 915 3261415; fax: +86 915 3261415.

E-mail address: chenjiucun@163.com (J.C. Chen).

0169-4332/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2009.10.093

[18], star polymers [19], and hyperbranched polymers [20] fromthe nanoparticles, nanotubes, nanowires, and clays.

Previous reports on click chemistry have indicated that theHuisgen [3 + 2] cycloaddition can be used to efficiently functio-nalize polymers prepared using ATRP [21–23]. Nevertheless, therange of possibilities of ATRP can be further broadened by clickchemistry. Combining the chain-end functionality control of ATRPand the efficiency of click chemistry is an interesting pathway forthe synthesis of end-functional polymers. This combinationrepresents a particularly powerful approach because the chainends of polymers prepared using ATRP can be easily transformedinto azides, and a diverse range of functional alkynes iscommercially available. Furthermore, the chain-end strategiesthrough a combination of surface-initiated ATRP and click reactionmay be further exploited for the synthesis of defined macro-molecular architectures such as block copolymers [24], graftcopolymers [25], macromolecular brushes [26], stars [27],miktoarm stars [28], macrocycles [29], or networks [30].

There have been few reports on the modification of nano-particles by click chemistry. Meldal et al. found that click couplingworked well on solid supports [2]. Ranjan and Brittain havereported that a combination of click chemistry and RAFT can beused for surface modification with the ‘‘grafting to’’ approach [31].Zhen et al. synthesized novel b-cyclodextrin covalently modifiedsingle-walled carbon nanotubes via a click coupling reaction [32].

Scheme 1. Synthetic routes to modification of SiO2 nanoparticles through surface-

initiated ATRP and click reaction.

J.C. Chen et al. / Applied Surface Science 256 (2010) 2490–2495 2491

Zhao et al. reported two b-cyclodextrin (b-CD) bonded stationaryphases for reversed-phase HPLC, including 1,12-dodecyldiollinked b-CD stationary phase and olio (ethylene glycol) (OEG)linked b-CD stationary phase, had been synthesized viaclick chemistry [33]. Nierengarten et al. prepared a stable C60

derivative bearing an azide functional group and used as abuilding block under the copper-mediated Huisgen 1,3-dipolarcycloaddition conditions for the preparation of a fullerene–porphyrin conjugate [34]. Here we report that the Cu-catalyzedHuisgen [3 + 2] cycloaddition reaction between azide end-functional polymers on the nanoparticle surfaces and functionalalkynes is an easy, versatile method for the preparation of variousend-functional polymers on nanoparticle surfaces. We furtherreport the transformation of the chain ends of these polymers intothe essential functionalities (v-carboxyl) through a combined useof surface-initiated ATRP and Huisgen [3 + 2] cycloaddition. Thismethod provides opportunities for grafting a dense layer ofhydrophilic or hydrophobic polymer brushes on the nanoparticlesurfaces. The bromine-terminated polymers isolated after ATRPwere transformed to azido end group and were subsequentlyreacted in situ with alkyne-functionalized groups, which canserve as reactive sites for further functionalization.

2. Experimental

2.1. Chemicals and reagents

Methyl methacrylate (MMA, 99%, Shanghai Chemical Reagent)was passed through basic alumina column to remove inhibitor andthen distilled from CaH2 in vacuum prior to use. Copper(I) bromide(A.R. grade, Shanghai Chemical Reagent) was washed with glacialacetic acid in order to remove any soluble oxidized species, filtered,washed with ethanol, and dried. SiO2 nanoparticles (Zhou ShanMingri Nanometer Material Co., Ltd., China) used in this study has aspecific surface area of 180 m2/g and a mean particle diameter of25 nm. g-Aminopropyltriethoxysilane (APTES) (Gaizhou ChemicalIndustrial Co.) was used as received. 4-oxo-4-(prop-2-ynyloxy)bu-tanoicacid was synthesized according to the literature procedures[35]. Dimethylformamide (DMF) was dried with magnesium sulfateand distilled under reduced pressure. Triethylamine (A.R., 99%) wasdried over barium oxide for 48 h at r.t. and distilled under reducedpressure. Toluene (Tianjin Chemical Co., 99%) was dried by refluxingover CaH2 and distilled just before use. Propargyl alcohol, 2-bromoisobutyryl bromide (BIBB, 98%), copric sulfate, sodiumascorbate, N,N,N0,N00,N00-pentamethyldiethylenetriamine (PMDETA),sodium azide and N,N-dimethylamino-4-pyridine (DMAP) werepurchased fromAladdinReagentCo. withthe highestpurityandwereused as received without further purification. All other solvents andchemicals were used as supplied without further purification.

2.2. Analytical methods

Elemental analysis (EA) of C, N, Br and H was performed on anElementar vario EL. FTIR spectroscopy was performed on a Diglab FTS 3000. Thermogravimetric analysis (TGA) was performedin nitrogen at a heating rate of 10 8C/min on a PerkinElmer TG/DTA 6300. The microstructure of these hybrid nanocompositeswas imaged using a Hitachi H-600 transmission electronmicroscope (TEM). TEM samples of nanoparticles were preparedby casting one drop of a dilute colloid solution onto a carbon-coated copper grid. Zeta potential of the SiO2 particles wasmeasured with a Delsa 440SX Zeta Potential Analyzer (USA) andthe measurement was repeated three times, and the averagefrom them was reported as the final result. The polymers cleavedfrom the SiO2 nanoparticles were dissolved in deuteratedchloroform and then characterized with 1H NMR using a Varian

UNITY INOVA-500 FT-NMR spectrometer. The average molecularweights and polydispersity indices of all the samples weremeasured with a GPC system equipped with a Waters 515 high-performance liquid chromatography pump, three Waters Styr-agel columns (HT2, HT3, and HT4), a Rheodyne 7725i sampler,and a Waters 2414 refractive-index (RI) detector. Narrow PSstandards were used to calibrate the GPC system. THF was usedas the eluent at a flow rate of 1 mL/min at 35 8C.

The polymer grafting (%) (PG, mass ratio of the grafted polymerto the SiO2 nanoparticles) was determined by TGA and wascalculated by Eq. (1) [36]:

Polymer grafting ð%Þ ¼ Organic component=g

Bare SiO2=g� 100 (1)

where the mass of the organic component was calculated from theTGA weight loss between 150 and 700 8C. Weight loss in thisregime corresponds to the decomposition of PMMA. The mass ofbare SiO2 was assumed to be the retained weight after thisdecomposition, as measured by TGA.

2.3. Immobilization of atom-transfer radical polymerization (ATRP)

initiators on the SiO2 nanoparticle surfaces

As shown in Scheme 1, immobilization of the ATRP initiatorson the SiO2 nanoparticle surfaces was carried out in two steps:(1) the aminopropyl-modified SiO2 nanoparticles (SiO2–NH2)were prepared by the self-assembly of APTES from the surfacesof SiO2 and (2) reaction of the amide groups of the SiO2 surfaceswith 2-bromoisobutyryl bromide (BIBB) to produce the 2-bromoisobutyryl-immobilized SiO2 nanoparticle (SiO2–Br) forthe subsequent surface-initiated ATRP. The procedure foraminopropyl modification of SiO2 nanoparticles was adaptedfrom literature [37]. Briefly, SiO2 nanoparticles (4.0 g) wererefluxed with 10 mL 3-aminopropyltrimethoxysilane in 25 mLdry toluene for 24 h. The solids were separated by centrifugationand washed with 2� 200 mL of dry toluene, and then 2� 200 mLof dry acetone. The resulting product (SiO2–NH2) was driedunder vacuum for 12 h. The nitrogen content was determined byelemental analysis to be 0.58 wt%. For the reaction of amidegroups of the SiO2 nanoparticle surfaces with BIBB. SiO2–NH2

Fig. 1. FTIR spectra of (a) bare SiO2, (b) SiO2–PMMA–Br, (c) SiO2–PMMA–N3, and (d)

v-carboxyl functionalized SiO2–PMMA.

J.C. Chen et al. / Applied Surface Science 256 (2010) 2490–24952492

(1.5 g) was dispersed in 20 mL of chloroform and 3 mL oftriethylamine. This mixture was cooled to 0 8C in an ice waterbath. A solution of 3 mL 2-bromoisobutyryl bromide and 6 mLchloroform was added dropwise over 30 min and the mixturewas stirred at room temperature for 24 h. This mixture wassuccessively washed with 3� 200 mL of chloroform and 3�200 mL of ethanol. The resulting product was collected and driedovernight under vacuum at 40 8C, affording 1.44 g of SiO2-supported ATRP initiator (SiO2–Br). Elemental analysis of C andN indicated that about 0.508 mmol initiator groups wereimmobilized per gram of SiO2–Br.

2.4. Surface modification of SiO2 nanoparticles with PMMA by

surface-initiated ATRP

SiO2–Br (1.0 g) and dry toluene (30 mL) were charged into a100 mL dry flask equipped with a magnetic stirrer. The mixturewas ultrasonicated while stirring for 30 min. Then CuBr (0.081 g,0.565 mmol) and PMDETA (0.118 mL, 0.565 mmol) were addedinto this flask. The flask was subsequently evacuated and flushedwith nitrogen three times. Then degassed MMA (8 mL, 113 mmol)was injected and the solution was further degassed by threefreeze-pump thaw cycles. The mixture was stirred rapidly undernitrogen and placed in a thermostatically controlled oil bath at thereaction temperature 100 8C. After 30 min, the viscous mixturewas diluted with THF and the polymer-grafted SiO2 nanocompo-site (SiO2–PMMA–Br) was separated by centrifugation. SiO2–PMMA–Br was Soxhlet extracted for 24 h, after which thephysically adsorbed polymer could be removed from the surfaceof the SiO2. SiO2–PMMA–Br samples were obtained and driedovernight under vacuum. The conversion of MMA was 24%. ThePMMA was degrafted from the SiO2 by cleavage in HF and analyzedby GPC: Mn = 9690 g/mol, Mw/Mn = 1.18.

2.5. General procedure for the synthesis of azide end-functionalized

SiO2 nanoparticles

SiO2–PMMA–Br (0.60 g) were dissolved in 25 mL DMF. Subse-quently, NaN3 (0.20 g) was added and the reaction mixtures werestirred for 20 h at 80 8C. Afterwards, the particles were recoveredby centrifugation at 3000 rpm for 30 min. These particles wereredispersed in water and centrifuged. This cycle was repeatedthree times to yield modified SiO2 nanoparticles (SiO2–PMMA–N3).The polymer was degrafted and analyzed by GPC: Mn = 9680 g/mol,Mw/Mn = 1.18.

Caution!: Although we had never experienced any adverseevent when working with these products, organic azides arepotentially explosive compounds and they should be handled withutmost care!!

2.6. Cycloaddition reactions between azide end-functional SiO2

nanoparticles and functional alkynes

In a typical reaction, 0.20 g of SiO2–PMMA–N3 was dispersedin 20 mL of a 1:1 ethanol–water mixture along with CuSO4

(9.4 mg) and sodium ascorbate (7.4 mg). This was followed bythe addition of 0.2 g (1.282 mmol) of 4-oxo-4-(prop-2-ynylox-y)butanoicacid, dissolved in 1 mL of water, to the above reactionmixture with vigorous stirring under N2 conditions at 50 8Cfor 24 h. The particles were recovered by centrifugation at3000 rpm for 30 min. These particles were redispersed in solventand centrifuged; this cycle was repeated four times in toluene,one time in water, and then one time in toluene. The modifiedSiO2 particles were obtained after drying at reduced pressure.The polymer was degrafted and analyzed by GPC: Mn = 9730 g/mol, Mw/Mn = 1.19.

2.7. Cleavage of grafted polymer from modified SiO2 nanoparticles

To detach the grafted polymer, some of the compositenanoparticle was dissolved in 5 mL of toluene, and 75 mg ofAliquat 336 was added as a phase transfer catalyst. A 49% aqueousHF solution (5 mL) was added, and the mixture was stirred for 2 h.The organic layer was removed, and the polymer was isolated byprecipitation from CH3OH, filtration, and removal of volatilematerials under vacuum.

3. Results and discussion

The process of end-functional polymers onto SiO2 nanoparticlesvia controlled radical polymerization and click chemistry wasshown in Scheme 1: (i) surface-initiated polymerization from SiO2

nanoparticles surface via ATRP, (ii) synthesis of azide-modifiedSiO2 nanoparticles, and (iii) cycloaddition reactions between azideend-functional SiO2 nanoparticles and functional alkynes. Detailsof the surface functionalization process are discussed below.

3.1. FTIR analysis

FTIR spectra of bare SiO2 nanoparticles and PMMA-grafted SiO2

nanoparticles were showed in Fig. 1. In Fig. 1a, the characteristicabsorption band of SiO2 appeared at 1100 cm�1. The broad bandcentered around 3452 cm�1 was assigned to the hydroxyl group,which was attributed to residual water on the surface of SiO2

nanoparticles. The band at 1630 cm�1 was due to water. While inFig. 1b, the peaks at 2990, 2850, and 1446 cm�1 were the absorptionbands of methyl and methylene. At the same time, a new band at1737 cm�1 appears, which is a characteristic peak of the C55Opresent of PMMA in SiO2–PMMA–Br. These results suggested thatPMMA has grafted from the surface of SiO2 nanoparticles. Thisbromide was substituted with azide by reaction with sodium azide.An azide group was verified by an FTIR absorption at 2093 cm�1

(Fig. 1c). The obtained SiO2–PMMA–N3 was subsequently involvedin click reactions with functional alkyne (4-oxo-4-(prop-2-ynylox-y)butanoicacid) to prepare functional SiO2 nanoparticles. Thetransformation of the chain ends of SiO2–PMMA–N3 into the desiredfunctionalities was studied by FTIR spectroscopy. From Fig. 1d, weobserved the complete disappearance of characteristic azideabsorbance peak at 2093 cm�1 for v-carboxyl functionalized PMMAonto SiO2 nanoparticles, as compared to that of SiO2–PMMA–N3.

Fig. 2. 1H NMR spectra and peak labels for (a) PMMA, and (b) v-carboxyl functionalized PMMA. All measurements were performed in CDCl3 solution, at room temperature,

using tetramethylsilane (TMS) as the standard.

Table 1GPC data of polymers by ATRP and click reaction.

Polymera Mnb (g mol�1) (Mw/Mn)b

PMMA–Br 9690 1.18

PMMA–N3 9680 1.18

v-Carboxyl functionalized PMMA 9730 1.19

a PMMA chains attached to silica were cleaved using HF.b Determined by GPC in THF with calibrated PS standards.

Fig. 3. TGA analysis of (a) bare SiO2, (b) SiO2–Br, (c) SiO2–PMMA, and (d) v-acryloyl

functionalized SiO2–PMMA.

J.C. Chen et al. / Applied Surface Science 256 (2010) 2490–2495 2493

The hydroxyl stretch of silanols coincided with the hydroxyl stretchof carboxylic acid groups, evidenced as a broad peak at 3300–3600 cm�1. Moreover, the relative intensity of the absorbance peakat 1737 cm�1 characteristic of the C55O from the ester group and thecarboxylic acid group also increased considerably. These resultsdemonstrated the success of the v-carboxyl functionalized PMMAon SiO2 nanoparticles.

3.2. 1H NMR analysis

In order to understand the growth characteristics of thepolymer on SiO2 nanoparticles via surface-initiated ATRP techni-que and click reaction, the grafted polymers were cleaved from thesurfaces at their points of attachment by treatment with HFsolution. The 1H NMR spectrum was used for the structuralanalysis of the well-defined polymer grafted (Fig. 2). Thecharacteristic peaks of grafted PMMA chains at 3.60, 1.80 and0.70 ppm for –OCH3, –CH2– and –C(CH3)– protons, respectively,were detected by 1H NMR (Fig. 2a). Next, using click chemistrystrategy, v-carboxyl functionalized PMMA polymer was obtainedfrom an azide end-functionalized nanoparticle and an alkyne-terminal compound (4-oxo-4-(prop-2-ynyloxy)butanoicacid) inthe presence of CuSO4/Na ascorbate in ethanol–water mixture at50 8C for 24 h. In Fig. 2b, except that peaks from the PMMApolymer were observed, the characteristic peaks of triazole ring at7.75 and 5.15 ppm for C55C–H of triazole and OCH2 linked totriazole, respectively, were detected by 1H NMR.

3.3. GPC analysis

The PMMA was degrafted from the silica by cleavage in HF andGPC was used tocalculate molecular weight (Mn = 9690 g mol�1) andpolydispersity (1.18). Then the bromide end-functionality of PMMAwas quantitatively converted to azide end group. By GPC analysis, thesynthesis of the SiO2–PMMA–N3 precursors involved no molecularweight reduction because narrow symmetrical signals wereobserved at the same position as the starting SiO2–PMMA–Brprecursors. Finally, using click chemistry strategy, an azide end-functionalized SiO2–PMMA–N3 and low molecular weight functionalalkynes (4-oxo-4-(prop-2-ynyloxy)butanoicacid) were reacted to

give corresponding end-functionalized silica nanoparticles in thepresence of CuSO4/Na ascorbate in ethanol–water mixture at 40 8Cfor 48 h. However, click reactions have little effect on the molecularweight and the molecular weight distribution of the producedpolymers, as shown in Table 1.

3.4. Thermogravimetric analysis

As shown in Fig. 3, we carried out TGA studies on thenanoparticles modified with ATRP and click reaction. The SiO2

nanoparticles gave distinctive TGA curves after each stage of

J.C. Chen et al. / Applied Surface Science 256 (2010) 2490–24952494

modification, which provided an indication of the amount ofPMMA and of the extent of click reaction on the SiO2 nanoparticles.The weight loss below 200 8C, due to the physisorbed water andresidual organic solvent, was 4.2% for bare SiO2 (Fig. 3a). Weobserved a weight loss of 5.5% for the bromide-modifiednanoparticles in the TGA curve (Fig. 3b). The weight fractionsof organic moieties of the modified SiO2 nanoparticles weremeasured with TGA, and the obtained values are 15.4 wt% (Fig. 3c)and 16.3 wt% (Fig. 3d) for modified SiO2–PMMA–N3 and the click-modified SiO2 nanoparticles, respectively. And, the percentage ofgrafting (PG) was found to be 20.3% calculated from Eq. (1),according to the TGA analysis. This is in good agreement with theresults of the elemental analysis.

3.5. TEM analysis

The morphologies of bare SiO2 particles and functional SiO2

particles modified by ATRP and click reaction were investigatedusing TEM (Fig. 4). Samples were prepared by drop-casting diluteethanol solutions of these nanoparticles onto carbon-coatedcopper grids, followed by evaporation of the solvent in air. Thebare SiO2 exhibited an average external diameter about 10–20 nmand showed large aggregates (Fig. 4a). The functionalized SiO2

particles are seen to form the clusters, although SiO2 particles arecompletely encapsulated with polymer and the size of functionalSiO2 particles was obviously larger than the bare SiO2 particlesand then free SiO2 can nearly not be observed (Fig. 4b). Suchaggregation morphologies were also observed in other polymer-grafted nanoparticles [38–40]. This can be investigated using zetapotential measurement [41–43]. In general, the zeta potential isan important factor controlling the dispersion stability of SiO2

Fig. 4. TEM images of (a) bare SiO2, and (b) v-carboxyl functionalized SiO2–PMMA.

suspensions, which results in the existence of an energy barrierpreventing the proximity of particles. Zeta potential measure-ments were performed in water as a function of pH in order todetermine the electrokinetic charge of the nanospheres. However,since TEM casting was performed from ethanol, we can’t be certainof the colloidal stability of the particles just before casting.

4. Conclusion

Combining the chain-end functionality control of surface-initiated ATRP and the efficiency of click chemistry is an interestingpathway for the synthesis of end-functional polymers ontonanoparticle surfaces, because chain ends of polymers preparedusing ATRP can be easily transformed into azides, and a plethora offunctional alkynes is commercially available. Thus, the target of thepresent work was to illustrate that Huisgen [3 + 2] cycloaddition ofazide end-functional polymers to functional alkynes is an easyversatile method for the preparation of various end-functionalpolymers onto nanoparticle surfaces. The efficiency of the couplingreaction was determined by the chain-end functionalities of bothan azide end-functionalized nanoparticle and an alkyne-terminalcompound. Various characterization techniques, such as FTIR, 1HNMR, TGA and TEM were employed to evaluate the chain-endfunctionality and the quality of the obtained v-carboxyl functio-nalized PMMA onto SiO2 nanoparticles.

Acknowledgements

This work was financially supported by the Ankang UniversityScience Foundation (grant no. 2007AKXY019).

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