Donut-Shaped Nanoparticles Templated by Cyclic Bottlebrush

Aug 18, 2017 - Cyclic bottlebrush polymers were synthesized and used as single molecular templates to prepare donut-shaped hybrid nanoparticles with ...
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Donut-Shaped Nanoparticles Templated by Cyclic Bottlebrush Polymers Lifen Xiao,†,‡ Lin Qu,†,‡ Wen Zhu,‡ Ying Wu,*,† Zhengping Liu,† and Ke Zhang*,‡ †

Institute of Polymer Chemistry and Physics, Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China ‡ State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: Cyclic bottlebrush polymers were synthesized and used as single molecular templates to prepare donut-shaped hybrid nanoparticles with organo-silica cross-linked and gold nanocluster coordinated internal structures. “Grafting-onto” strategy was adopted to prepare cyclic bottlebrush polymers by combining ring-expansion metathesis polymerization (REMP), reversible addition−fragmentation chain transfer (RAFT) polymerization, and triazolinedione (TAD)-diene click reaction. In this approach, cyclic poly(norbornene imide) backbones with diene side groups (C-PNB-diene) were synthesized based on REMP technique. TAD-terminated diblock copolymer side chains were produced from RAFT polymerization including TAD-terminated poly(3-(triethoxysilyl)propyl methacrylate)-block-poly(oligo(ethylene glycol) methacrylate) (TAD-PTEPM-b-POEGMA) and TAD-terminated poly(glycidyl methacrylate)-block-poly(oligo(ethylene glycol) methacrylate) (TAD-PGMA-b-POEGMA). The cyclic bottlebrush polymers 1 and 2 were then prepared by virtue of TAD-diene click reaction to graft TAD-PTEPM-b-POEGMA and TAD-PGMA-b-POEGMA side chains onto C-PNB-diene backbones, respectively. Furthermore, the donut-shaped hybrid nanoparticles with organo-silica cross-linked internal structures were obtained by in situ cross-linking PTEPM domains of the cyclic bottlebrush polymer 1 templates. For the formation of donut-shaped hybrid nanoparticles coordinating gold nanoclusters inside, the cyclic bottlebrush polymer 2 templates were first postmodified to introduce the functional pyridine groups inside the PGMA domains, which were then used as location to coordinate the gold nanoclusters.

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INTRODUCTION Donut-shaped particles exist widely in nature, such as donutshaped bacterial muramidase,1 syncollin protein,2 and red blood cell.3 Inspired by this, the artificial donut-shaped particles have gained considerable attention due to their unique shape and the resultant anisotropic properties. So far, several effective methods have been established for the formation and functionalization of donut-shaped particles. For example, Okuyama’s group prepared silica microdonuts using a spray drying method, in which the size of the resultant donut-shaped particles was in micrometer range and lack of uniformity.4 Bradley’s group generated narrowly dispersed microdonuts via the dispersion polymerization of styrene in ethanol with 5% dioxane and demonstrated their high selectivity in cellular uptake.5 In the field of functional materials, Guo et al. prepared micrometer-sized Co donuts using a polymer-modified method and investigated the magnetic properties of the donuts.6 Wege et al. reported a surface-formation technique to prepare magnetically functionalized polymeric microdonuts with a permanent magnetic dipole moment of controllable strength and direction.7 To date, the size of the reported donut-shaped particles is all in micrometer scale limited by the current preparation methods. It is well-known that the particle size in © XXXX American Chemical Society

nanometer scale plays an important role in determining their practical properties.8−10 For example, the nanometer-sized electrode materials showed both increased electroactivity and enhanced high power.8 Because of the significantly enhanced light absorption and scattering, gold particles with nanometer size were suitable for the applications in biochemical sensing and imaging.11 In addition, the uptake efficiency of particles with 100 nm size by the intestinal tissue was 15 to 250 times higher compared to that of micrometer-sized particles.12 Therefore, it is highly worthy to explore effective method to prepare donut-shaped particles with nanometer size and investigate their charmingly unknown properties. Bottlebrush polymers are a kind of graft polymers with polymer side chains densely attached onto polymer backbones. The presence of steric repulsion among bulky polymer side chains largely extends the polymer backbones, endowing the bottlebrush polymers with a persistent worm-like molecular morphology. In this case, the size of the bottlebrush polymers can be well controlled in nanometer scale simply by Received: July 17, 2017 Revised: August 4, 2017

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Scheme 1. Preparation of the Cyclic Bottlebrush Polymer Templates and the Corresponding Donut-Shaped Nanoparticles

demonstrated the successful formation of bottlebrush polymers.30,31 The ring-closure techniques, however, hardly produce cyclic polymers with high molecular weight and high production efficiency. This may seriously limit the ring-closure based strategy for the formation of cyclic bottlebrush polymers in practice. In the second strategy, the ring-expansion techniques are used for the formation of cyclic backbones. It has been demonstrated that the ring-expansion metathesis polymerization (REMP) could produce the cyclic polymers with ultrahigh molecular weight in high production efficiency,32 which provided an ideal way to prepare cyclic bottlebrush polymers. Resultantly, the REMP-based preparation methods for cyclic bottlebrush polymers have been successfully developed by combining with “grafting-through”, “graftingonto”, and “grafting-from” techniques from Grubb’s, Tew’s, and our groups.27,33−36 However, the functionalization and application of the resultant cyclic bottlebrush polymers had been seldom explored to date. Herein, we developed a novel preparation method for cyclic bottlebrush polymers with functional diblock copolymer as side chains by combining REMP and “grafting-onto” technique. The functional cyclic bottlebrush polymers were then explored as single molecular templates to prepare donut-shaped hybrid nanoparticles with organo-silica cross-linked and gold nanocluster coordinated internal structures. As shown in Scheme 1, REMP and a following side group modification were used to prepare the cyclic poly(norbornene imide) backbones with diene side groups (C-PNB-diene). Reversible addition− fragmentation chain transfer (RAFT) polymerization and a following end group modification were used to prepare the functional diblock copolymer side chains with triazolinedione (TAD) end groups including TAD-terminated poly(3-

manipulating the degree of polymerization of backbones and side chains. In addition, the chemical composition and functionality of bottlebrush polymers can be easily controlled for both backbones and side chains by virtue of the powerful controlled/living polymerization techniques.13 Resultantly, the bottlebrush polymers supports an ideal molecular template to prepare varied functional particles in nanometer scale.14 For example, Schmidt’s group employed core−shell bottlebrush polymers as templates to prepare hybrid nanowires containing gold nanoclusters.15 By virtue of bottlebrush polymers with block copolymer side chains as templates, Müller’s group prepared a variety of nanoparticles including cadmium sulfide nanowires,16 superparamagnetic hybrid nanocylinders,17 watersoluble organo-silica hybrid nanotubes,18,19 magnetic hybrid cylinders,20 silica nanowires/nanotubes,21 and anatase hybrid nanotubes.22 On the basis of the same concept, Rzayev’s group produced the nanotubes23,24 and nanocapsules25,26 using bottlebrush polymers with degradable/cross-linkable side chains as templates. In contrast, the bottlebrush polymers with cyclic topology have not been used as templates to prepare particles with donut shape, although it can simply control the size of the resultant particles in nanometer range. This may be caused by the difficulty in the synthesis of functional cyclic bottlebrush polymer templates. Cyclic bottlebrush polymers are prepared by densely grafting polymer side chains onto cyclic polymer backbones.27 According to the formation of cyclic backbones,28,29 the current preparation methods for cyclic bottlebrush polymers can be divided into two categories. In the first strategy, the ringclosure techniques are used to prepare the cyclic backbones. By combining with the “grafting-onto” and “grafting-from” approaches, Deffieux’s and Grayson’s groups independently B

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(DCM), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF) were purchased as regent grade from Alfa Aesar, Aldrich, Acros, J&K Chemical, or Beijing Chemical Reagent Co. and used as received unless otherwise noted. GMA, TEPM, and OEGMA were passed through a basic alumina column to remove the inhibitors. 2,2Azobis(isobutyronitrile) (AIBN) was recrystallized from methanol and stored at 4 °C. For making dry solvents: DCM and CHCl3 were refluxed over calcium hydride. Norbornene imide based monomer NB,39 cyclic ruthenium-alkylidene catalyst UC-6,40 DABCO-Br,37,38 functional RAFT agent Urazole-CTA,38 and hexa-2,4-dienyl ester succinic anhydride38 were prepared according to the literature. Spectra/Por 7 dialysis bags were purchased from Spectrum Medical Industries with the molecular weight cut off of 10000 Da. Characterization. 1H NMR spectra were recorded on a Bruker Avance 400 spectrometer at room temperature. Gel permeation chromatography (GPC) in THF was performed using three Agilent mixed columns (PLgel MIXED-C × 2 and PLgel MIXED-D), a Waters 1515 isocratic HPLC pump, and a Waters 2414 RI detector. THF was used as the eluent at a flow rate of 1 mL/min. Polystyrene standards were used for calibration. GPC in DMF was conducted on a system comprised of a Waters 515 HPLC pump, a Waters 2414 RI detector, and three Agilent mixed columns (PLgel MIXED-A and PLgel MIXED-B × 2). DMF with 0.01 mol/L LiBr was used as the eluent at a flow rate of 1 mL/min. Polystyrene standards were used for calibration. Preparative GPC (SHIMADZU LC-20AR) was performed with a LC-201R pump, a RID-20A refractive index detector, and an Agilent Plgel Mixed-B column. DMF was used as the eluent at a flow rate of 10 mL/min. Fourier transform infrared (FT-IR) spectroscopy was performed on a Thermo Nicolet Avatar-330 Spectrometer at room temperature. Ultraviolet (UV) spectra were recorded using a TU-1901

(triethoxysilyl)propyl methacrylate)-block-poly(oligo(ethylene glycol) methacrylate) (TAD-PTEPM-b-POEGMA) and TADterminated poly(glycidyl methacrylate)-block-poly(oligo(ethylene glycol) methacrylate) (TAD-PGMA-b-POEGMA). The efficient TAD-diene click reaction was then used to graft side chains onto backbones and prepare the cyclic bottlebrush polymer templates 1 and 2.37,38 By this preparation approach, the reactive blocks of PTEPM and PGMA were designed as the core domains of the resultant cyclic bottlebrush polymers. The inert POEGMA blocks were used as outside shell domains to keep their stability and solubility. By virtue of the in situ crosslinking PTEPM core domains of cyclic bottlebrush polymers 1, the donut-shaped hybrid nanoparticles with organo-silica crosslinked internal structures were then prepared. In addition, for the formation of donut-shaped hybrid nanoparticles coordinating with gold nanoclusters inside, the cyclic bottlebrush polymers 2 were first postfunctionalized to introduce the functional pyridine groups into the PGMA core domains, which were then used as location to coordinate gold nanoclusters.

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EXPERIMENTAL SECTION

Materials. Glycidyl methacrylate (GMA), 3-(triethoxysilyl)propyl methacrylate (TEPM), oligo(ethylene glycol) methacrylate (OEGMA, Mn = 475), 4-pyridinemethaneamine, 4-dimethylaminopyridine (DMAP), cis-5-norbornene-exo-2,3-dicarboxylic anhydride, 2,3-dimethyl-2-butene, ammonia solution (25%), chloroauric acid (HAuCl4), lithium aluminum hydride (LiAlH4), 1,4-dioxane, toluene, chloroform (CHCl3), methanol, diethyl ether, dichloromethane C

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UV−vis Spectrophotometer. Transmission electron microscopy (TEM) images were obtained using a JEM 2200FS instrument operated at an accelerating voltage of 200 kV. Solution samples were dropped onto carbon-coated copper grids or Formvar stabilized with carbon support films for TEM observation. Preparation of Cyclic Backbone C-PNB-Diene. Monomer NB (232 mg, 1.12 mmol) in 3 mL of DMF and UC-6 (3.3 mg, 4.5 μmol) in 4.5 mL of DMF were put into two 25 mL Schlenk tubes separately. After degassing by three freeze−evacuate−thaw cycles, the two solutions were mixed and stirred at 55 °C for 12 h to produce the cyclic polymer of C-PNB. The quantitative monomer conversion was achieved according to the 1H NMR spectrum. GPC in DMF determined Mn = 748.8 kDa and Mw/Mn = 1.59. Subsequently, hexa-2,4-dienyl ester succinic anhydride (0.85 g, 2.24 mmol) and DMAP (28 mg, 0.23 mmol) in 1 mL DMF were added into above polymerization solution. After being stirred for 12 h at room temperature, the reaction solution was dialyzed against CHCl3 to obtain pure cyclic backbone C-PNB-diene, which was stored in CHCl3. GPC in DMF determined Mn = 832.4 kDa and Mw/Mn = 1.48. Preparation of TAD-Terminated Diblock Copolymer Side Chains. TAD-PTEPM-b-POEGMA. TEPM (8.2 g, 28 mmol), UrazoleCTA (160 mg, 0.35 mmol), AIBN (11.6 mg, 0.07 mmol), and 1,4dioxane (8.2 g) were added into a 25 mL Schlenk tube equipped with a stirrer bar. After degassing through three freeze−evacuate−thaw cycles, the polymerization was carried out at 60 °C for 5.5 h and then quenched by exposing in air. The raw polymerization solution was precipitated in mixed solvents of methanol/H2O (V/V = 7/3) for three times to produce Urazole-PTEPM12. The monomer conversion was calculated as 15% from 1H NMR spectrum. GPC in THF determined Mn = 5020 Da and Mw/Mn = 1.13. Urazole-PTEPM12 (550 mg, 0.14 mmol), OEGMA (6.65 g, 14 mmol), AIBN (4.6 mg, 0.028 mmol), and 1,4-dioxane (6.6 g) were added into a 25 mL Schlenk tube equipped with a stirrer bar. After degassing through three freeze−evacuate−thaw cycles, the polymerization was carried out at 60 °C for 8 h and then quenched by exposing in air. The raw polymerization solution was precipitated in the mixed solvents of diethyl ether/hexane (V/V = 2/1) for three times to produce pure Urazole-PTEPM12 -b-POEGMA15. The monomer conversion was calculated as 15% from 1H NMR spectrum. GPC in THF determined Mn = 6380 Da and Mw/Mn = 1.09. Urazole-PTEPM12-b-POEGMA15 (200 mg, 18 μmol), DABCO-Br (8.5 mg, 5.4 μmol), and DCM (1 mL) were added into a 5 mL roundbottom flask and stirred for 3 h at room temperature to form TADPTEPM12-b-POEGMA15. After filtration, the filtrate of TADPTEPM12-b-POEGMA15 side chains was used directly without further purification. TAD-PGMA-b-POEGMA. GMA (3.1 g, 22 mmol), Urazole-CTA (100 mg, 0.22 mmol), AIBN (7.2 mg, 44 μmol), and 1,4-dioxane (3 g) were added into a 25 mL Schlenk tube equipped with a stirrer bar. After degassing through three freeze−evacuate−thaw cycles, the polymerization was performed at 60 °C for 160 min and then

quenched by exposing in air. The raw polymerization was precipitated in diethyl ether for three times to produce pure Urazole-PGMA10. The monomer conversion was calculated as 10% from 1H NMR spectrum. GPC in THF determined Mn = 2480 Da and Mw/Mn = 1.14. Urazole-PGMA10 (235 mg, 126 μmol), OEGMA (6.0 g, 12.6 mmol), AIBN (4.1 mg, 25 μmol), and 1,4-dioxane (6 g) were added into a 25 mL Schlenk tube equipped with a stirrer bar. After degassing through three freeze−evacuate−thaw cycles, the polymerization was carried out at 60 °C for 4.5 h and then quenched by exposing in air. The raw polymerization solution was precipitated in diethyl ether for three times to produce pure Urazole-PGMA10-b-POEGMA12. The monomer conversion was calculated as 12% from 1H NMR characterization. GPC in THF determined Mn = 5640 Da and Mw/ Mn = 1.08. Urazole-PGMA10-b-POEGMA12 (400 mg, 53 μmol), DABCO-Br (25 mg, 16 μmol), and DCM (2 mL) were added into a 10 mL roundbottom flask and stirred at room temperature for 3 h to form TADPGMA10-b-POEGMA12. After filtration, the filtrate of TAD-PGMA10b-POEGMA12 side chains was used directly without further purification. Preparation of Cyclic Bottlebrush Polymers. Cyclic Bottlebrush Polymer 1. C-PNB-diene (5.8 mg, 15 μmol diene group) in 1.3 mL of CHCl3 was added into the TAD-PTEPM12-b-POEGMA15 (200 mg, 18 μmol TAD group) DCM solution with several drops of toluene as internal standard. After 3 h reaction, 0.2 mL 2,3-dimethyl-2-butene was added as a quencher. The preparative GPC was used to purify the resultant cyclic bottlebrush polymer 1. GPC in DMF determined Mn = 1433.6 kDa and Mw/Mn = 1.15. Cyclic Bottlebrush Polymer 2. C-PNB-diene (17.1 mg, 44 μmol diene group) in 3.7 mL of CHCl3 was added into the TAD-PGMA10b-POEGMA12 (400 mg, 53 μmol TAD group) DCM solution with several drops of toluene as internal standard. After reacting for 2 h, 0.5 mL 2,3-dimethyl-2-butene was added as a quencher. The preparative GPC was used to purify the resultant cyclic bottlebrush polymer 2. GPC in DMF determined Mn = 1089 kDa and Mw/Mn = 1.24. Preparation of Donut-Shaped Nanoparticles. Donut-Shaped Nanoparticles with Organo-Silica Cross-Linked Internal Structure. Cyclic bottlebrush polymer 1 (2 mg, 1 mg/mL in DMF), 1,4-dioxane (18 mL), H2O (2 mL), and ammonia solution (0.4 mL) were added into a 50 mL round-bottom flask and stirred at room temperature for 16 h. The reaction solution was then dialyzed against THF to produce pure donut-shaped organo-silica hybrid nanoparticles dispersed in THF. Donut-Shaped Nanoparticles Containing Gold Clusters. Cyclic bottlebrush polymer 2 (100 mg, 126 μmol epoxy group) and 4pyridinemethaneamine (680 mg, 6.3 mmol) were dissolved in 10 mL DMF and stirred at 35 °C for 24 h. The crude product was dialyzed against THF to obtain cyclic bottlebrush polymer 2-N. GPC in DMF determined Mn = 1086.0 kDa, Mw/Mn = 1.25. Subsequently, cyclic bottlebrush polymer 2-N (5 mg in 10 mL THF) and HAuCl4 (1.3 mg in 1.3 mL of THF) were added into a 25 mL round-bottom flask and D

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Macromolecules stirred for 24 h in dark. After purifying the mixture by dialysis against THF in dark, LiAlH4 (235 μL, 1 mg/mL in THF) was added and stirred for 15 min at room temperature. The reaction solution was again dialyzed against THF to obtain pure donut-shaped nanoparticles coordinating gold clusters.

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RESULTS AND DISCUSSION Preparation of Cyclic Backbone C-PNB-Diene. The cyclic polymer backbone with diene pendants was synthesized by REMP and subsequent side group modification. Using cyclic ruthenium-alkylidene UC-6 as catalyst, the cyclic polymer CPNB was synthesized by REMP with [NB]0/[UC-6]0 = 250/1 and [NB]0 = 0.15 mol/L in DMF solution at 55 °C for 12 h. Figure S1A and B show the 1 H NMR spectra and corresponding peak assignments of NB monomer and the resultant C-PNB raw polymerization solution. The characteristic proton signal Ha at 6.30 ppm from −CH=CH− of NB monomer completely disappeared after REMP, while a new group of peak Ha′ appeared at 5.30−5.70 ppm ascribed to −CH=CH− of the resultant C-PNB. This indicated the quantitative conversion of NB monomer under the polymerization condition. The GPC curve of C-PNB is shown in Figure S2A (black), in which a monomodal peak was observed corresponding to Mn = 748.8 kDa and Mw/Mn = 1.59. Subsequently, C-PNB was postmodified by hexa-2,4-dienyl ester succinic anhydride to prepare the cyclic backbone of CPNB-diene. Figure 1A shows the corresponding 1H NMR spectrum and peak assignments. Compared to that of C-PNB (Figure S1B), a series of new peaks Hj‑m appeared at 5.50−6.30 ppm, ascribed to the diene group (−CH=CH−CH=CH−). Additionally, the complete disappearance of Hx signal (-OH of C-PNB, Figure S1B) and the peak area ratio of 2/3 between He and Hn in Figure 1A strongly indicated the quantitative modification efficiency. Compared to that of C-PNB (Figure S2A, black), the GPC peak of C-PNB-diene (Figure S2A, red) shifted to the high molecular weight direction and peak shape preserved monomodal distribution. The Mn and Mw/Mn were calculated as 832.4 kDa and 1.48, respectively. Preparation of TAD-Terminated Diblock Copolymer Side Chains. The TAD-terminated diblock copolymer side chains were synthesized by the combination of RAFT polymerization and the following end group oxidation. Because of the high reactivity of TAD group, urazole-terminated diblock copolymer side chain precursors were first synthesized by RAFT polymerization with a urazole functionalized RAFT agent. The TAD group was then introduced at the end of the diblock copolymer side chains by oxidizing the urazole end groups.38 For the formation of TAD-PTEPM-b-POEGMA side chains, PTEPM block was first prepared in dioxane solution (mTEPM/ mdioxane = 1) at 60 °C with [TEPM]0/[Urazole-CTA]0/ [AIBN]0 = 80/1/0.2. After 5.5 h of polymerization, UrazolePTEPM12 was obtained with a monomer conversion of 15% from 1H NMR characterization. GPC curve of UrazolePTEPM12 (Figure S2B, black) displayed a monomodal and symmetric peak with Mn and Mw/Mn of 5020 Da and 1.13, respectively. Figure S3A shows the 1H NMR spectrum of Urazole-PTEPM12. Subsequently, Urazole-PTEPM-b-POEGMA was synthesized in dioxane solution (mOEGMA/mdioxane = 1) at 60 °C with [OEGMA]0/[Urazole-PTEPM]0/[AIBN]0 = 100/1/0.2. The monomer conversion reached 15% in 8 h to produce Urazole-PTEPM12-b-POEGMA15 according to 1H NMR characterization. GPC peak (Figure S2B, red) of the

Figure 1. 1H NMR spectra of (A) C-PNB-diene (DMSO-d6), (B) cyclic bottlebrush polymer 1 (CDCl3), and (C) cyclic bottlebrush polymer 2 (DMSO-d6).

resultant Urazole-PTEPM12-b-POEGMA15 demonstrated the same monomodal and symmetric peak shape but higher molecular weight, compared to that of Urazole-PTEPM12 (Figure S2B, black). The corresponding Mn and Mw/Mn were calculated as 6380 Da and 1.09, respectively. 1H NMR spectrum of Urazole-PTEPM12-b-POEGMA15 is shown in Figure S3B, in which the newly formed peaks Hh‑k,m were ascribed to the POEGMA block indicating the successful chain extension and formation of block copolymer. Eventually, the corresponding TAD-PTEPM12-b-POEGMA15 side chains were obtained by oxidizing the urazole group at the end of UrazolePTEPM12-b-POEGMA15 using DABCO-Br as oxidant. TAD-PGMA-b-POEGMA side chains were prepared with the same strategy. PGMA block was first synthesized at 60 °C with [GMA]0/[Urazole-CTA]0/[AIBN]0 = 100/1/0.2. After 2.7 h polymerization, Urazole-PGMA10 was obtained with a monomer conversion of 10% from 1H NMR characterization. The corresponding GPC curve (Figure S2C, black) showed a monomodal peak with Mn and Mw/Mn of 2480 Da and 1.14, E

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Figure 2. (A) GPC characterization for evaluating the grafting density of cyclic bottlebrush polymer 1: C-PNB-diene backbone (black), TADPTEPM12-b-POEGMA15 side chain (red), and raw cyclic bottlebrush polymer 1 (blue). (B) GPC curves of raw cyclic bottlebrush polymer 1 (black) and purified cyclic bottlebrush polymer 1 (red) from preparative GPC. (C) GPC characterization for evaluating the grafting density of cyclic bottlebrush polymer 2: C-PNB-diene backbone (black), TAD-PGMA10-b-POEGMA12 side chain (red), and raw cyclic bottlebrush polymer 2 (blue). (D) GPC curves of raw cyclic bottlebrush polymer 2 (black), purified cyclic bottlebrush polymer 2 (red) from preparative GPC, and functional cyclic bottlebrush polymer 2-N (blue). DMF was used as the eluent, and polystyrene standards were used for calibration.

respectively. 1H NMR spectrum of Urazole-PGMA10 is shown in Figure S4A. Subsequently, chain extension was performed in dioxane with [OEGMA]0/[Urazole-PGMA]0/[AIBN]0 = 100/ 1/0.2. OEGMA monomer conversion reached 12% in 4.5 h from 1H NMR characterization, leading to the formation of Urazole-PGMA10-b-POEGMA12. GPC characterization determined Mn and Mw/Mn as 5640 Da and 1.08, respectively (Figure S2C, red). Figure S4B shows 1H NMR spectrum of Urazole-PGMA10-b-POEGMA12, in which the characteristic proton signals of POEGMA block were clearly observed. Ultimately, the corresponding TAD-PGMA10-b-POEGMA12 was produced by oxidizing Urazole-PGMA10-b-POEGMA12 in the presence of DABCO-Br. Preparation of Cyclic Bottlebrush Polymers. The cyclic bottlebrush polymer 1 and 2 were prepared separately by grafting TAD-PTEPM12-b-POEGMA15 and TAD-PGMA10-bPOEGMA12 side chains onto C-PNB-diene backbones. By virtue of the ultrafast TAD-diene click reaction, the grafting process could be efficiently performed at room temperature without requiring any catalysts or chemical stimuli.38 Cyclic bottlebrush polymer 1 was synthesized in DCM/ CHCl3 (v/v = 2.5/1) with an initial molar concentration of TAD-PTEPM12-b-POEGMA15 as 4 mmol/L and an initial molar ratio between TAD and diene groups as 1.2/1. Figure 2A shows the GPC characterization for evaluating the formation of cyclic bottlebrush polymer 1 from TAD-diene grafting reaction. Toluene was used as internal standard to determine the grafting efficiency of TAD-PTEPM12-b-POEGMA15 side chains, which was preadded into the DCM solution of the side chains. Since the grafting reaction started right away after mixing the backbones and side chains together, the GPC curve at 0 h (Figure 2A, red) was recorded on the DCM solution of side chains with toluene marker. After adding the CHCl3 solution of backbone and performing the grafting reaction 3 h, the excess

2,3-dimethyl-2-butene was added to terminate the unreacted TAD end groups. The raw reaction solution was then characterized by GPC. As shown in Figure 2A, compared to that of backbone (black), the GPC peak of the resultant bottlebrush polymer 1 (blue) shifted completely to the higher molecular weight direction indicating the successful formation of bottlebrush polymers. In addition, the percentage of grafted side chains could be calculated as 41.1% by comparing the peak intensity of the TAD-terminated side chain and toluene marker in the GPC curves. This finally produced the grafting density of 49.3% for the resultant cyclic bottlebrush polymers by multiplying the initial molar ratio of 1.2 between TAD and diene groups. The pure cyclic bottlebrush polymer 1 was then isolated using the preparative GPC to remove the unreacted side chains. From the GPC curve in Figure 2B (red), the side chain residuals were completely removed by this technique and a monomodal and symmetric peak shape was preserved for the resultant pure cyclic bottlebrush polymer 1 with Mn and Mw/ Mn of 1433.6 kDa and 1.15. Figure 1B shows the 1H NMR spectrum of the purified cyclic bottlebrush polymer 1, in which the characteristic peaks of the side chains were clearly observed. This again indicated the successful formation of cyclic bottlebrush polymer 1. Cyclic bottlebrush polymer 2 was synthesized in a similar procedure. The grafting reaction was performed in DCM/ CHCl3 (v/v = 2.5/1) with an initial molar concentration of TAD-PGMA10-b-POEGMA12 as 4 mmol/L and an initial molar ratio between TAD and diene groups as 1.2/1. The grafting reaction was performed at room temperature for 2 h. The corresponding GPC characterization is shown in Figure 2C. Compared to that (black curve) of backbones, the GPC curve (blue) of the resultant cyclic bottlebrush polymer 2 shifted to higher molecular weight direction completely. By virtue of the percentage of grafted side chains of 75.8% from GPC F

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resultant donut-shaped nanoparticles, indicating the successful formation of Si−O−Si bonds from cross-linking Si−O−C bonds.41,42 Because of the formation of organo-silica hybrid structure, TEM could be used to characterize the morphology of the resultant donut-shaped nanoparticles. As shown in Figure 3B, TEM image clearly showed the donut-shaped morphology for the resultant nanoparticles with average outer and inner diameters of 39.8 and 18.4 nm, respectively. Cyclic bottlebrush polymer 2 with reactive PGMA core domain was used to prepare donut-shaped hybrid nanoparticles coordinating with gold nanoclusters inside. First, by virtue of the reaction between epoxy group of PGMA and 4pyridinemethaneamine, the pyridine groups were introduced into the PGMA core domain of cyclic bottlebrush polymer 2 for coordinating metal ions. The postmodification reaction was performed at room temperature in DMF for 24 h with an initial molar ratio of 50/1 between 4-pyridinemethaneamine and epoxy group and an initial molar concentration of 0.63 mol/L for 4-pyridinemethaneamine. After dialysis in THF, the pure pyridine functionalized cyclic bottlebrush polymer 2 (cyclic bottlebrush polymer 2-N) was successfully obtained. Figure 2D shows the corresponding GPC curve (blue), in which a monomodal and symmetric peak was observed, similar to that (red) of cyclic bottlebrush polymer 2 precursor. Mn and Mw/Mn were calculated as 1086.0 kDa and 1.25, respectively. Figure S5 shows the 1H NMR spectrum of cyclic bottlebrush polymer 2N, in which the characteristic protons from pyridine groups was clearly observed indicating the successful introduction of the pyridine groups. Subsequently, the cyclic bottlebrush polymer 2-N was used to coordinate Au+. The coordination process was performed in THF solution for 24 h in dark simply by stirring chloroauric acid and cyclic bottlebrush polymer 2-N with equimolar amounts of Au+ and pyridine group and an initial molar concentration of 0.27 mmol/L for Au+. After that, the coordination system was further purified by dialysis in THF in dark to remove the possibly dissociative Au+. Figure 4A shows the corresponding UV−vis characterization. Compared to that (red curve) of cyclic bottlebrush polymer 2-N, a broad peak at 325 nm was observed for the UV−vis absorption spectrum (blue) of the Au+ coordinated cyclic bottlebrush polymer 2-N, which was similar to that (black curve) of Au+. This clearly indicated the successful coordination of Au+ with cyclic bottlebrush polymer 2-N. Finally, the donut-shaped hybrid nanoparticles containing gold nanoclusters were prepared by virtue of LiAlH4 to in situ reduce the Au+ coordinated in the core domain of cyclic bottlebrush polymer 2-N. After further purification by dialysis in THF, the donut-shaped hybrid nanoparticles coordinating gold nanoclusters were obtained. From Figure 4A, the UV−vis absorption curve (magenta) showed a complete disappearance of the peak at 325 nm, indicating the complete reduction of Au+ in this situation. Because of metal-containing molecular structure, TEM was used to characterize the morphology of the resultant donutshaped nanoparticles coordinating gold nanoclusters. From Figure 4B, TEM image clearly showed the donut-shaped nanoparticles with average outer and inner diameters of 34.6 and 14.8 nm, respectively.

characterization, a grafting density of 91% was calculated for the formation of cyclic bottlebrush polymer 2. Figure 2D (red curve) shows the GPC characterization of the pure cyclic bottlebrush polymer 2 isolated by preparative GPC, in which a monomodal and symmetric peak shape was observed corresponding to Mn = 1089.0 kDa and Mw/Mn = 1.24. Compared to that (Figure 2D, black curve) of the raw cyclic bottlebrush polymer 2 after grafting reaction, the unreacted TAD-PGMA10-b-POEGMA12 side chains were completely removed. Figure 1C shows the 1H NMR spectrum of the purified cyclic bottlebrush polymer 2. The characteristic peaks of PGMA10-b-POEGMA12 side chains were clearly observed, again indicating the successful formation of cyclic bottlebrush polymer 2. Preparation of Donut-Shaped Nanoparticles. Subsequently, the cyclic bottlebrush polymers 1 and 2 were used as single molecular templates to demonstrate the formation of donut-shaped nanoparticles. Since the PTEPM core domain of cyclic bottlebrush polymer 1 has in situ gelation property in acidic or basic solution,41,42 it could be used as molecular templates to prepare the donut-shaped nanoparticles with organo-silica cross-linked internal structure. To achieve this, cyclic bottlebrush polymer 1 was dispersed in a mixture of DMF/dioxane/ammonia solution at room temperature for 16 h to perform the in situ gelation of PETPM core domain. After that, the donut-shaped nanoparticles with organo-silica hybrid internal structure were obtained simply by dialyzing the above reaction solution in THF. FT-IR characterization was used to prove the successful gelation of PTEPM domain. As shown in Figure 3A, the characteristic Si−O−C peaks at 1110 and 1135 cm−1 were clearly observed for the curve (black) of cyclic bottlebrush polymer 1 templates. After gelation, a broad peak at 1050−1170 cm−1 was observed for the curve (red) of the

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CONCLUSIONS Cyclic bottlebrush polymers were synthesized and used as single molecular templates to prepare hybrid donut-shaped nanoparticles for the first time. The combination of REMP and “grafting onto” strategy was used to prepare the cyclic

Figure 3. (A) FT-IR spectra of cyclic bottlebrush polymer 1 (black) and the resultant donut-shaped nanoparticles with organo-silica hybrid internal structure (red). (B) TEM image of the donut-shaped nanoparticles with organo-silica hybrid internal structure. G

DOI: 10.1021/acs.macromol.7b01512 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules *(K.Z.) E-mail: [email protected]. ORCID

Zhengping Liu: 0000-0002-7944-2455 Ke Zhang: 0000-0001-5972-5127 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support was primarily provided by the National Science Foundation of China (21504007 and 21622406) and the Ministry of Science and Technology of China (2014CB932200).

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Figure 4. (A) UV−vis spectra of Au+ (black), cyclic bottlebrush polymer 2-N (red), cyclic bottlebrush polymer 2-N coordinating Au+ (blue), and donut-shaped hybrid nanoparticles coordinating gold nanoclusters (magenta). (B) TEM image of the donut-shaped hybrid nanoparticles coordinating gold nanoclusters.

bottlebrush polymers with functional diblock copolymer side chains, in which the REMP was used to produce cyclic backbones, and the metal-free TAD-diene click reaction was employed to graft diblock copolymer side chains onto the backbones. This approach produced the cyclic bottlebrush polymers with a unique core−shell internal structure inherited from the diblock copolymer side chains, where the functional inner core domain supported the in situ reaction sites for further functionalization and the inert outer shell domain guaranteed their solubility and stability. This facilitated the cyclic bottlebrush polymers as ideal molecular templates to produce various nanoparticles with unique donut shape. Two kinds of hybrid donut-shaped nanoparticles were prepared to demonstrate the concept, in which one had the organo-silica cross-linked internal structures and the other coordinated the gold nanoclusters inside. TEM microscopy clearly demonstrated the donut-shaped morphology for both of them with the outer and inner diameters of 34−40 and 14−19 nm, respectively. Since the synthesis chemistry was designed to prepare cyclic bottlebrush polymers with varied functional block copolymer side chains, this strategy is expected to become a universal method to produce donut-shaped nanoparticles with various functionalities and applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01512. Figure S1−S5, showing NMR spectra and GPC curves, (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(Y.W.) E-mail: [email protected]. H

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DOI: 10.1021/acs.macromol.7b01512 Macromolecules XXXX, XXX, XXX−XXX