Radical crossover reactions of alkoxyamine

Polymer Synthesis and Reactions

Polymer Journal volume 48, pages 147–155 (2016)Cite this article

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The dynamic covalent exchange reactions of side chains in polymer brushes on nanoparticles were successfully demonstrated, and the dispersibility of the nanoparticles after the exchange reactions was investigated in some solvents. A polymer brush with exchangeable alkoxyamine side chains was prepared by surface-initiated atom transfer radical polymerization on silica nanoparticles. Fluorinated and ionic polymers were grafted to the polymer brush via radical exchange reactions of alkoxyamine moieties. The chemical composition of the polymer brushes on nanoparticles before and after the exchange reactions was investigated by X-ray photoelectron spectroscopy (XPS). The XPS measurements indicated that the side chains of the polymer brushes were converted to the corresponding polymers. In addition, the grafted side chains could be detached from the surface of the nanoparticles through a further radical exchange process by treating with alkoxyamines. The XPS results indicate that the structure of the de-grafted polymer brush is nearly the same as the original, thus demonstrating the reversibility of the reactions. Furthermore, the dispersibility of the nanoparticles in solution could be changed by the reversible grafting reactions.

Nanoparticles, nanotubes and nanosheets have recently become the subject of increasing research because of their applications to electronics, photonics, the reinforcement of polymeric materials and biomedical fields.1, 2, 3, 4 For these applications, the dispersibility of the nanomaterials in specific media is important. Accordingly, intense research into the introduction of functional groups on the surface of nanomaterials to modify their dispersion properties has been performed. One practical approach is surface modification with polymer brushes.5, 6 The structure of a polymer brush contains polymer chains directly attached to organic or inorganic materials. Therefore, modified polymer chains can cover the surface of the nanomaterials and prevent their aggregation due to the steric hindrance of perpendicularly stretched polymer chains. Furthermore, polymer brushes with a high graft density7, 8, 9, 10, 11, 12 have been synthesized using the grafting from method with precise polymerization techniques.13, 14, 15, 16 To date, various polymer brushes have been prepared on the surface of nanomaterials.17, 18, 19, 20, 21, 22 In addition, polymer brushes with convertible structures23 have been developed to allow the modification of their properties.24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35 For example, the surface structure and properties of ionic polymers can be altered by changing the solution pH.24, 25, 26 Nanomaterials with ionic polymer brushes can be switched between a dispersed and an aggregated state in an aqueous solution. Another strategy is to change the temperature to control the nanomaterials by exploiting the phase transition of the polymer; when nanoparticles with poly(N-isopropylacrylamide) brushes are heated and cooled, they show changes in their dispersibility and structure.32, 33, 34, 35 However, most of the previously reported systems cannot reversibly alter the structures and compositions of the brushes, limiting their applications.

Recently, the use of reversible bonding for molecular design and synthesis has been studied36 to fabricate materials with new characteristics. In particular, chemistry based on reversible covalent bonds, known as ‘dynamic covalent chemistry’, has attracted much attention because it allows the creation of novel molecules and materials that could not be produced by traditional methods.37, 38, 39 Most dynamic covalent bonds are stable under ambient conditions, while they begin to equilibrate between states when external stimuli, such as heating, chemical additions or light irradiation, are applied. Therefore, materials containing dynamic covalent bonds are stable, but they can react further. In particular, when the concept of dynamic covalent chemistry is applied to polymeric materials,40, 41 polymers that have reorganizable structures, compositions and properties are designed.

One typical example of a dynamic covalent system is the reversible C–ON bond formation of alkoxyamine units derived from stable nitroxide radicals.42, 43, 44, 45 Alkoxyamine derivatives can form carbon and nitroxide radicals on heating due to the homolytic cleavage of the covalent C–ON bond. These are well-known and commonly used as unimolecular initiators or chain end structures in nitroxide-mediated radical polymerization (NMRP).14, 42 Recently, alkoxyamine-based reorganizable structures, such as linear,46, 47, 48 cyclic,49 grafted,50, 51 cross-linked52, 53, 54, 55, 56, 57 and star-shaped58, 59 polymers, have been developed. In these systems, alkoxyamine-based dynamic covalent polymers satisfy the conditions of stability under ambient conditions and radical reactivity. In addition, several dynamic covalent reactions on the surface and at the interface of materials have been reported more recently.60, 61, 62, 63, 64, 65, 66, 67, 68 These reactions are applicable to surface property changes of nanomaterials.69, 70, 71, 72

In this study, we report the reversible control of the chemical composition of the surface of silica nanoparticles (SiNPs) and the effect of the dynamic covalent polymer brushes on the dispersibility of the nanoparticles in solution. The system investigated in this study is based on the dynamic covalent exchange reactions. A schematic representation of the reversible polymer grafting system through dynamic covalent exchange reactions of alkoxyamine units is shown in Figure 1.

Alkoxyamine-based dynamic covalent exchange reactions on nanoparticles. A full color version of this figure is available at Polymer Journal online.

Alkoxyamine derivatives, 4-(methacryloyloxyethylcarbamyl)-1-((1′-phenylethyl)oxy)-2,2,6,6-tetramethylpiperidine (ALMA, 1)58 and 4-methoxy-1-((1′-phenylethyl)oxy)-2,2,6,6-tetramethylpiperidine (5),50 were prepared as previously reported. The surface initiator, (2-bromo-2-methyl)propionyloxyhexyltriethoxysilane (BHE), and the suspension of initiator-modified SiNPs were prepared according to reported procedures.12, 21, 22 Methyl methacrylate (MMA, 98+%) and anisole (99+%) were purchased from Wako Pure Chemical Industries (Osaka, Japan) and were purified by distillation under reduced pressure over calcium hydride. 2,3,4,5,6-Pentafluorostyrene (99%) was purchased from Sigma-Aldrich Japan (Tokyo, Japan) and was purified by distillation under reduced pressure over calcium hydride. 4-Vinylpyridine (95+%) was purchased from Tokyo Chemical Industry (Tokyo, Japan) and was purified by distillation under reduced pressure over calcium hydride. Free initiator, ethyl 2-bromoisobutyrate (98+%) was received from Tokyo Chemical Industry (Osaka, Japan) and was distilled under reduced pressure. Cu(I)Br (99+%) was purchased from Wako Pure Chemical Industries and was purified by repeated stirring in acetic acid (Wako Pure Chemical Industries 99+%), washed with ethanol (Wako Pure Chemical Industries 99+%) and then dried in vacuo. The SiNP suspension (100 nm diameter) was kindly supplied by Nissan Chemical Industries (Chiba, Japan). All other reagents were purchased from commercial sources and were used as received.

1H (400 MHz) nuclear magnetic resonance (NMR) spectroscopic measurements were performed at 25 °C using a Bruker AV-400 spectrometer (Bruker, Silberstrifen, Germany) with tetramethylsilane as an internal standard in chloroform-d (CDCl3).

The relative number- and weight-average molecular weights (Mn and Mw, respectively), as well as the molar-mass dispersity (Mw/Mn) of the polymers were estimated by gel permeation chromatographic (GPC) analysis. GPC measurements were obtained at 40 °C on a PC system equipped with a TSK gel SuperH-L guard column (Tosoh Bioscience, Tokyo, Japan), three columns (α-6000, α-4000 and α-2500) and a 2414 refractive index detector (Waters, Milford, MA, USA), tungsten lamp, wavelength 470–950 nm). Tetrahydrofuran was used as the eluent at a flow rate of 1.0 ml min−1. Seven polystyrene standards (Mn=1060–3 690 000, Mw/Mn=1.02–1.08) were used to calibrate the GPC system.

X-ray photoelectron spectroscopy (XPS) measurements were performed using an XPS-APEX (Physical Electronics, Chanhassen, MN, USA) <10−8 Torr, with a monochromatic Al-Kα X-ray source operating at 150 or 200 W. The X-ray beam was focused on an area with a diameter of ca. 1.2 mm. The take-off angle of the photoelectrons was maintained at 45°, and a low-energy (25 eV) electron flood gun was used to minimize sample charging. The survey spectra were obtained over the range of 0–1000 at 1.0 eV energy steps. The narrow scans were performed with 0.1 eV energy steps.

Thermogravimetric analysis was conducted using a DTG-60 (Shimadzu Corporation, Kyoto, Japan). The heating rate was set at 10 °C min−1.

A radically reactive polymer brush with alkoxyamine side chains was prepared using surface-initiated atom transfer radical polymerization (SI-ATRP, Scheme 1). The surface initiator, BHE, was synthesized by hydrosilylation of 5-hexenyl 2-bromoisobutylate treated with triethoxysilane, as previously reported.12, 21 BHE was immobilized on SiNPs (100 nm in diameter) using an ammonia-assisted method.21 Finally, the nanoparticles modified with surface initiator (BHE-SiNP) were dispersed in trifluoroethanol to obtain a 10 wt% suspension.22 Copolymerization of MMA and the alkoxyamine monomer ALMA(1) (copolymerization feed ratio, MMA/ALMA(1)=10/1) was performed to obtain the corresponding dynamic covalent polymer brush by SI-ATRP. The suspension of BHE-SiNP (2 g with 200 mg of BHE-SiNP) was charged into a glass tube, and then, the trifluoroethanol solution of MMA (2.5 g, 25 mmol) and ALMA(1) (1.08 g, 2.5 mmol) was added slowly to prevent aggregation of SiNP. The mixture was ultrasonicated for 1 min, degassed by seven freeze–pump–thaw cycles, and backfilled with Ar gas. To the mixture, a degassed trifluoroethanol solution of CuBr (19.7 mg, 0.138 mmol) and 4,4′-dinonyl-N,N′-bipyridyl (112.3 mg, 0.275 mmol) was added under Ar flow. The mixture was again degassed by seven freeze–pump–thaw cycles. The free initiator, ethyl 2-bromoisobutyrate (20.1 μl, 0.135 mmol), was added to the mixture, and the glass tube was immersed in a thermostated oil bath maintained at 50 °C under Ar. After 15 h, the mixture was quenched to the liquid N2 temperature and exposed to air. The mixture was then repeatedly centrifuged and dispersed with good solvents to remove the non-grafted free polymers and other low-molecular-weight molecules. Polymer brush-grafted SiNPs (PMMA-co-PALMA-SiNP, 2) were collected and dried under vacuum at 40 °C; 2 was obtained as a white solid. In addition, the supernatant was passed through an alumina column to remove the Cu catalyst and was then poured into excess methanol to collect the free (non-grafted) polymer that had been initiated by ethyl 2-bromoisobutyrate. The precipitate was then collected and dried in vacuo. The purified polymer was obtained as a white powder. In addition, a poly(methyl methacrylate) (PMMA) polymer brush on SiNP (PMMA-SiNP) without dynamic covalent bonds was also prepared as a control sample and was purified using the same procedure. For further examination, the grafted dynamic covalent polymer chains were cleaved from SiNP by HF etching and were characterized by NMR and GPC measurements.

PMMA-co-PALMA-SiNP (2, 20 mg) was charged into a glass tube and was maintained under reduced pressure to remove the air. Then, Ar was purged into the glass tube, and 0.5 ml of an anisole solution of alkoxyamine-terminated poly(2,3,4,5,6-pentafluorostyrene) (PPFS (3), 5 mg, 2.7 μmol ml−1, Mn,GPC=3700) prepared by NMRP14 was charged into the tube. The mixture was ultrasonicated for 1 min, degassed by seven freeze–pump–thaw cycles and sealed off under vacuum. The system was heated at 100 °C for 24 h. After the reaction, the particles were purified by repeating the centrifugation and re-dispersion processes with chloroform, and finally, the particles were dried in vacuo. The corresponding polymer-grafted SiNP (PMMA-co-PALMA-g-PPFS-SiNP, 4) was analyzed by XPS and dispersibility testing.

In addition, as a reference, the above experiment was performed using PMMA-SiNP instead of PMMA-co-PALMA-SiNP (2).

PMMA-co-PALMA-g-PPFS-SiNP (4, 10 mg) was charged into a glass tube and was maintained under reduced pressure. After the glass tube was purged with Ar, an anisole solution (0.25 ml) of alkoxyamine 5 (9.8 mg, 0.13 mmol ml−1, 50 equiv. to PPFS) was added. The mixture was degassed by seven freeze–pump–thaw cycles and sealed off under vacuum. The solution was heated at 100 °C for 24 h. The physically adsorbed polymer was removed from the surface by repeated centrifugation and re-dispersion. The nanoparticles with PMMA-co-PALMA-SiNP (6) were then dried in vacuo and were analyzed by XPS.

A glass tube was charged with PMMA-co-PALMA-SiNP (2, 20 mg) and was maintained under reduced pressure to remove air. After the glass tube was purged with Ar, it was charged with a dimethylformamide solution (0.5 ml) of alkoxyamine-terminated poly(4-vinylpyridine) (P4VP(7), 5 mg, 1.4 μmol ml−1, Mn,NMR=6900) prepared by NMRP.14 The mixture was ultrasonicated for 1 min and degassed by seven freeze–pump–thaw cycles and sealed off under vacuum. The system was heated at 100 °C for 24 h. After the reaction, the particles were purified by repeating the centrifugation and re-dispersion processes with chloroform. Finally, the particles were dried in vacuo. The corresponding PMMA-co-PALMA-g-P4VP-SiNP (8) was analyzed by XPS.

In addition, as a reference, the above experiment was performed using PMMA-SiNP instead of PMMA-PALMA-SiNP.

PMMA-co-PALMA-g-P4VP-SiNP (8) was charged into a sample tube followed by iodomethane in tetrahydrofuran (1 ml, 0.1 M). The solution was maintained at room temperature in the dark for 24 h, and the solution was then removed. The obtained particles were dried under reduced pressure to remove any unreacted iodomethane and remaining solvent. The quaternized poly(4-vinylpyridine) (QP4VP)-grafted SiNP (PMMA-co-PALMA-g-QP4VP-SiNP, 9) was analyzed by XPS and dispersibility testing.

A glass tube was charged with PMMA-co-PALMA-g-QP4VP-SiNP (9, 8 mg) and was maintained under reduced pressure. The tube was purged with Ar, and then alkoxyamine 5 in dimethylformamide (0.4 ml, 16.9 mg, 0.144 mmol ml−1, 100 equiv. to QP4VP) was added. The mixture was degassed by seven freeze–pump–thaw cycles and sealed off under vacuum. The solution was heated at 100 °C for 24 h. The physically adsorbed polymer was removed from the surface by repeated centrifugation and re-dispersion processes. Then, PMMA-co-PALMA-SiNP (10) was dried in vacuo and analyzed by XPS.

PMMA-co-PALMA-SiNP (2) was prepared by SI-ATRP, as shown in Scheme 1. The Mn and Mw/Mn values for the PMMA-co-PALMA initiated by the free initiator were 5700 and 1.34, respectively. Furthermore, the Mn and Mw/Mn values of the polymers cleaved from the silica surface by HF etching showed similar values (Mn=7700 and Mw/Mn=1.39). In addition, no macroscopic side reactions, such as gelation or the appearance of a red color of nitroxides from free polymer, were observed. The structures of the obtained polymers were fully characterized by 1H NMR. The compositions of the free polymer and of the cleaved polymer (PMMA-co-PALMA) were estimated as MMA/ALMA=9.3/1 and 9.4/1, respectively. These results indicate that the copolymerization of ALMA(1) and MMA from the nanoparticles using SI-ATRP was successful. The estimated graft density, calculated from a previous method,22 was 0.19 chains per nm2, indicating that the polymer brush can be classified as the high-density type. In addition, the surface chemical composition for the dynamic covalent polymer brush on SiNP was investigated by XPS. Figure 2 shows the wide-scan XPS spectrum of PMMA-co-PALMA-SiNP (2), which confirms the presence of nitrogen atoms of the alkoxyamine units and urethane bonds on the surface. The copolymerization ratio estimated from nitrogen and carbon peaks was MMA/ALMA=8.8/1, which is slightly lower but is still a similar value compared with the monomer feed ratio.

Wide-scan XPS spectrum of PMMA-co-PALMA-SiNP (2). PMMA, poly(methyl methacrylate); SiNP, silica nanoparticle; XPS, X-ray photoelectron spectroscopy.

The grafting reaction of a fluorinated polymer to PMMA-co-PALMA-SiNP (2) was demonstrated via a dynamic covalent exchange reaction between alkoxyamines, as shown in Scheme 2. Through this reaction, the side chains of the polymer brush can be converted to fluorinated polymer chains, and a grafted copolymer brush can be obtained. Figure 3a shows the wide-scan XPS spectrum of PMMA-co-PALMA-g-PPFS-SiNP (4). After grafting, peaks assignable to fluorine atoms appeared in the spectrum of PMMA-co-PALMA-g-PPFS-SiNP (4) because the fluorinated chains exist on the surface of nanoparticles. To confirm that the appearance of fluorine atoms resulted from the dynamic covalent exchange, a reference XPS measurement of PMMA-SiNP after the same treatment was performed. As shown in Figure 3b, there are no fluorine peaks in the spectrum of the reference nanoparticles. From these results, the peak appearance of fluorine is due to the dynamic covalent exchange of alkoxyamines between the side chains of PMMA-co-PALMA-SiNP (2) and the chain end of PPFS (3). The degree of exchange calculated from the peak ratio of the XPS spectrum was 9.7%. This relatively small value was caused by the steric hindrance of the polymer brush chains. The effective graft density (σeff), which is the graft density of chain ends, was calculated as 0.16 chains per nm2, using a previously reported method.73 This relatively high graft density also indicates that the size exclusion effect had a significant influence on the grafting reactions. These results confirmed that the grafting reaction of PPFS (3) to the nanoparticles occurred via a dynamic covalent exchange process, and the fluorinated chains would be introduced on the outer surface of the brush.69

Wide-scan XPS spectra of (a) PMMA-co-PALMA-g-PPFS-SiNP (4) and (b) PMMA-SiNP after heating with PPFS (3) purified in a similar manner. PMMA, poly(methyl methacrylate); PPFS, poly(2,3,4,5,6-pentafluorostyrene); SiNP, silica nanoparticle; XPS, X-ray photoelectron spectroscopy.

The de-grafting reaction of PPFS chains from PMMA-co-PALMA-g-PPFS-SiNP (4) was performed to confirm the reversibility of the present dynamic covalent system. PMMA-co-PALMA-g-PPFS-SiNP (4) was treated with an excess amount of small molecular alkoxyamine 5 in solution at 100 °C for 24 h. Figure 4 shows the wide-scan XPS spectrum of the de-grafted SiNP, 6, after the radical crossover reaction with alkoxyamine 5. The XPS spectrum shows the peaks indicative of the elements C, N and O; however, the peaks from fluorine completely disappeared from the spectrum after the de-grafting reaction. Furthermore, the elemental ratios of 6 were nearly the same as the initial PMMA-PALMA-SiNP (2). These results indicate that the fluorinated side chains were removed by radical exchange de-grafting, thus converting the SiNP surface structure back to that of the original brush.

Wide-scan XPS spectrum of PMMA-co-PALMA-SiNP (6) after de-grafting of PPFS from PMMA-co-PALMA-g-PPFS-SiNP (4). PMMA, poly(methyl methacrylate); PPFS, poly(2,3,4,5,6-pentafluorostyrene); SiNP, silica nanoparticle; XPS, X-ray photoelectron spectroscopy.

We also grafted alkoxyamine-terminated P4VP to PMMA-co-PALMA-SiNP (2) to demonstrate the flexibility of this system (Scheme 2). Figure 5a shows the wide-scan XPS spectrum of PMMA-co-PALMA-g-P4VP-SiNP (8). The surface chemical composition of carbon and nitrogen was altered after grafting. The ratio of nitrogen to carbon (N/C=0.069) showed a 1.8-fold increase compared with that before grafting. In contrast, the reference PMMA-SiNP showed no increase, as shown in Figure 5b. This indicates that carbon–nitrogen compounds were attached to PMMA-co-PALMA-SiNP (2). The grafting value was 2.2% due to the sterically hindered polymer brush chains. In addition, compared with fluorinated polymer grafting, this value was relatively small because of the difference in the molecular size between PPFS and P4VP used in this study.

Wide-scan XPS spectra of (a) PMMA-co-PALMA-g-P4VP-SiNP (8) and (b) PMMA after heating with P4VP and purified in a similar manner. PMMA, poly(methyl methacrylate); SiNP, silica nanoparticle; XPS, X-ray photoelectron spectroscopy.

To promote the hydrophilicity, quaternization of the P4VP chains was performed. Figure 6 shows the wide-scan XPS spectrum of PMMA-co-PALMA-g-QP4VP-SiNP (9) after the treatment of PMMA-co-PALMA-g-P4VP-SiNP (8) with iodomethane. The XPS spectra show the presence of iodine. Figures 7a–c show the narrow-scan XPS N1s peak for nanoparticles 2, 8 and 9, respectively. Quaternization led to a broadening of the N1s peak, as shown in Figure 7c, because of the appearance of the higher energy quaternary nitrogen peak and the diminishing neutral nitrogen peak. The changes in peak shape and position indicated that a high degree of quaternization occurred. In addition, the ratio of nitrogen to carbon (N/C=0.064) was smaller because of the increment of carbon atoms by quaternization with iodomethane. The grafting ratio of the hydrophilic side chains was estimated as 2.3% from the surface chemical composition, which is nearly equal to the grafting value estimated from the chemical composition before the quaternization reaction.

Wide-scan XPS spectrum of PMMA-co-PALMA-g-QP4VP-SiNP (9) after the quaternization reaction. PMMA, poly(methyl methacrylate); SiNP, silica nanoparticle; XPS, X-ray photoelectron spectroscopy.

Narrow-scan XPS spectra of nitrogen (from 390 to 410 eV for (a) PMMA-co-PALMA-SiNP (2), (b) PMMA-co-PALMA-g-P4VP-SiNP (8), (c) PMMA-co-PALMA-g-QP4VP-SiNP (9) and (d) PMMA-co-PALMA-SiNP (10). PMMA, poly(methyl methacrylate); SiNP, silica nanoparticle; XPS, X-ray photoelectron spectroscopy.

De-grafting of QP4VP (QP4VP) chains was also investigated. PMMA-co-PALMA-g-QP4VP-SiNP (9) was treated with an excess amount of the small molecular alkoxyamine 5 at 100 °C for 24 h. The XPS spectrum of the SiNP after de-grafting is shown in Figure 8. Peaks are present for C, N and O but not for iodine, indicating that it has been completely removed from the surface. Furthermore, the peak for N1s on XPS spectrum became narrow, and the shape of the spectrum changed to that of the original PMMA-co-PALMA-SiNP (2), as shown in Figure 7d. The elemental ratio of the de-grafted SiNP 10 is similar to the original SiNP 2 (N/C=0.042) and was 0.6 times lower than that of PMMA-co-PALMA-g-P4VP-SiNP (8).

Wide-scan XPS spectrum of PMMA-co-PALMA-SiNP (10) after de-grafting of QP4VP from PMMA-co-PALMA-g-QP4VP-SiNP (9). PMMA, poly(methyl methacrylate); SiNP, silica nanoparticle; XPS, X-ray photoelectron spectroscopy.

The grafted polymer chains determine the nanoparticle dispersibility. Figure 9 shows the photographs for the dispersion of PPFS- and QP4VP-modified SiNP (4 and 9, respectively) in water. Due to the difference in affinity of the polymer chains for water, the two polymer-grafted SiNPs showed distinct dispersion behaviors. QP4VP-grafted SiNP 9 was well dispersed because of the size exclusion effects of the hydrated hydrophilic polymer chains. However, the PPFS-grafted SiNP 4 aggregates because the hydrophobic chains shrink in water. This indicates that the dispersibility of the SiNPs was dramatically altered by the grafting reactions. Further insight into the particle dispersion was investigated by the dispersibility tests of the SiNPs for various solvents. Table 1 summarizes the dispersibility of polymer-grafted nanoparticles in four solvents: water, chloroform, methanol and n-hexane. These experiments confirmed that the dispersibility of the nanoparticles depends on the properties of the grafted chains. Good solvents for the polymer brushes lead to good dispersibility. For example, PPFS-grafted SiNP 4 can disperse in chloroform and ethyl acetate, which are good solvents for the PPFS chains. In contrast, PPFS-grafted SiNP 4 aggregates in water and hexane. In all cases, the de-grafting achieves a similar dispersibility to the initial particles.

Photographs of water suspension of polymer brush-modified nanoparticles: (a) PMMA-co-PALMA-SiNP (2), (b) PMMA-co-PALMA-g-PPFS-SiNP (4) and (c) PMMA-co-PALMA-g-QP4VP-SiNP (9). PMMA, poly(methyl methacrylate); PPFS, poly(2,3,4,5,6-pentafluorostyrene); QP4VP, quaternized poly(4-vinylpyridine); SiNP, silica nanoparticle. A full color version of this figure is available at Polymer Journal online.

We demonstrated the reversible grafting of functional polymers to nanoparticles via the dynamic covalent exchange reaction of alkoxyamine-appended polymer brushes. The dynamic covalent polymer brushes with radically exchangeable side chains were prepared on SiNPs using SI-ATRP. Radical crossover reactions were used to reversibly exchange the side chains with fluorinated polymers or ionic polymers. The chemical composition of the altered polymer brushes was characterized by XPS measurements. In addition, the polymer-grafted nanoparticles showed different dispersibility in solutions, depending on the grafted polymer chains. Furthermore, the introduced polymer chains were removed by the radical exchange reactions with an excess amount of small molecular alkoxyamine. These findings indicate that the nanoparticle dispersion was controlled by a combination of using dynamic covalent polymer brushes and the subsequent ‘grafting to’ procedure. The reactions demonstrated in this study can be used for the reversible and desirable surface modification of nanomaterials, and they are appropriate for various applications that require changes in dispersibility by functional group alteration.

Preparation of an alkoxyamine-based dynamic covalent polymer brush on silica nanoparticles. ALMA (1), 2,2,6,6-tetramethylpiperidine; BHE, 2-bromo-2-methyl)propionyloxyhexyltriethoxysilane; MMA, methyl methacrylate; PMMA, poly(methyl methacrylate); SiNP, silica nanoparticle; TFE, trifluoroethanol. A full color version of this scheme is available at Polymer Journal online.

Radical crossover reaction of alkoxyamine moieties on a silica surface. DMF, dimethylformamide; PMMA, poly(methyl methacrylate); PPFS, poly(2,3,4,5,6-pentafluorostyrene); SiNP, silica nanoparticle; THF, tetrahydrofuran. A full color version of this scheme is available at Polymer Journal online.

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This research was supported by JSPS KAKENHI (Grant Number 26288057). We are grateful to Dr M Kikuchi (JST-ERATO), Dr H Yamaguchi (Kyushu University), Dr R Goseki (Tokyo Institute of Technology) and Dr K Imato (Tokyo Institute of Technology) for their helpful discussions.

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo, Japan

Tomoya Sato, Tomoyuki Ohishi & Hideyuki Otsuka

Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Fukuoka, Japan

Tomoya Sato, Atsushi Takahara & Hideyuki Otsuka

Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka, Japan

Yuji Higaki & Atsushi Takahara

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Correspondence to Atsushi Takahara or Hideyuki Otsuka.

The authors declare no conflict of interest.

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Sato, T., Ohishi, T., Higaki, Y. et al. Radical crossover reactions of alkoxyamine-based dynamic covalent polymer brushes on nanoparticles and the effect on their dispersibility. Polym J 48, 147–155 (2016). https://doi.org/10.1038/pj.2015.94

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Received: 11 July 2015

Revised: 17 August 2015

Accepted: 18 August 2015

Published: 14 October 2015

Issue Date: February 2016

DOI: https://doi.org/10.1038/pj.2015.94

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