In situ synthesis of the Fe3O4@poly(4-vinylpyridine)-block-polystyrene magnetic polymer nanocomposites via dispersion RAFT polymerization

Abstract This work presents a facile strategy for the generation of Fe3O4@poly(4-vinylpyridine)-block-polystyrene (Fe3O4@P4VP-b-PS) magnetic polymer nanocomposites. P4VP-b-PS nanoparticles prepared by dispersion RAFT polymerization of styrene in methanol/water in the presence of macro-P4VP chain transfer agent was employed as templates to grow Fe3O4 on polymeric network templates. With the following addition of FeCl3 into the above polymer colloids, pyridyl moieties in P4VP block of nanoparticles could further react with Fe3+ through a coordination reaction to generate Fe3+@P4VP-b-PS nanoparticles. After the subsequent addition of FeCl2 and NH3⋅H2O, the co-precipitation reaction occurred on the surface of the polymeric nanoparticle to form Fe3O4@P4VP-b-PS magnetic polymer nanocomposites. Herein, FT-IR, XRD, TGA, XPS, TEM and VSM were applied to characterize the morphology, structure and magnetism performance of the synthesized magnetic polymeric nanocomposites. Graphical abstract

Among them, Fe 3 O 4 polymer nanocomposites are highly focused due to their unique and tunable magnetic properties. It has broad application prospects in water pollution [13][14][15][16][17], drug deliver [18][19][20], antibacterial fields [21], energy storage [22], shape memory materials [23], etc. Fe 3 O 4 polymer nanocomposites are formed by combining Fe 3 O 4 particles and organic polymers through appropriate methods. The first method would be desirable to encapsulate the core magnetic NPs with a polymer as the shell [18,24,25]. For instance, Miao et al. reported a core-shell-shell structured Fe 3 O 4 @P4VP@MIL-100(Fe) catalyst, which was assembled by coating MIL-100(Fe) (MIL, Mat erial Institut Lavoisier) metal-organic framework shell on the surface of Fe 3 O 4 @P4VP microspheres [26]. They also reported the preparation of Fe 3 O 4 @ P4VP(poly(4-vinylpyridine))@ZIF-8(Zeolitic Imidazolate Framework-8) catalyst [27]. The zinc derived ZIF-8 was grafted onto the surface of the Fe 3 O 4 @P4VP microspheres through layer-by-layer assembly. Guo et al. fabricated Fe 3 O 4 @poly(4-vinylpyridine-co-divinylbenzene)@Au (Fe 3 O 4 @P(4VP-DVB)@Au) nanostructures. 4VP was polymerized on the surface of Fe 3 O 4 nanoparticles by poly(acrylic acid)-modified Fe 3 O 4 to get the core-shell structure [28]. More recently, Wan et al. fabricated Fe 3 O 4 @ZIF-8@polymer core-shell-shell magnetic nanospheres by sequential in situ self-assembly and precipitation polymerization [29]. However, the polymer does not play a dominant role in this type of structure. Its role is to modify the nuclear surface of the iron oxide to improve their interfacial properties. In contrast, the second type of magnetic polymer is in-situ formation of Fe 3 O 4 particles in a polymer matrix, which places more emphasis on the polymer. For example, Royale et al. described the preparation of polyisoprene-block-poly(2-cinnamoylethyl methacrylate)-block-poly(tert-butyl acrylate) spherical micelles in tetrahydrofuran (THF)/hexanes (HX). After photo-crosslinking of the shell, hydroxylating of the double bonds of the corona, followed by removing the tert-butyl groups of PtBA, the obtained nanospheres were used as templates for incorporating iron oxide magnetic particles into the cores [30]. Pastukhov et al. successfully prepared magnetic polymer composites as magnetic sorbents by the immobilization of iron oxides into the pores of hypercrosslinked polystyrene sorbents via chemical precipitation [16]. More recently, Li and coworkers synthesized poly(methyl methacrylate)(PMMA) latexes armored with modified Fe 3 O 4 using citric acid-modified Fe 3 O 4 as stabilizers by Pickering emulsion polymerization [31]. It is worth noting that the morphology of magnetic polymer composites is particularly important for fundamental studies and applications.
With the development of polymerization-induced self-assembly (PISA) technique, the functionalization of nanoobjects generated using a PISA procedure is becoming a recent focus [32,33]. PISA provides an efficient method in the preparation of polymer-inorganic composited nanoparticles. The advent of RAFT dispersion polymerization as one type of PISA technique has made it possible to prepare polymer-inorganic composited nanoparticles with designed nanostructures in terms of their size, morphology and chemical compounds.
Herein, aiming to further enrich the versatility of the magnetic polymer nanocomposites, we adopted the advantage of the dispersion RAFT polymerization to prepare the polymeric nanoparticles used as template in the preparation of Fe 3 O 4 containing magnetic polymer nanocomposites. The dispersion RAFT polymerization was carried out in a methanol/water solution with styrene as monomer and P4VP macro-RAFT agent as stabilizer. After polymerization, the sequential addition of FeCl 3 into dispersion, make the pyridyl moieties of P4VP in template to react with Fe 3þ via a metal coordination reaction. Upon the addition of FeCl 2 methanol/water solution followed by using NH 3 ÁH 2 O as reducing agent, the chemical co-precipitation occurred between Fe 3þ and Fe 2þ on the surface of polymer nanoparticles under the basic conditions, leading to the generation of Fe 3 O 4 coated polymeric nanoparticles.

Characterization
The number-average molecular weight (M n ), weight-average molecular weight (M w ), and polydispersity (M w /M n ) of the polymers were determined by gel permeation chromatograph (GPC), which was recorded on a Waters Breeze 1525 GPC analysis system with two PL mix-D columns and a Waters 2414 RI detector, using DMF with 0.5 M LiBr as eluent and PMMA as a calibration sample at the flow rate of 1.0 mL min À1 at 60 C. Transmission electron microscopy (TEM) was obtained by using an FEI Tecnai G2 F20. The elemental compositions of the samples were characterized by energy-dispersive Xray spectroscopy (EDS), Oxford instruments X-Max 80 T. The thermogravimetric analysis (TGA) curves were recorded using a TGA instrument (TGA-2900 model, TA instrument) thermo-balanced in a temperature range of 25-800 C at a heating rate of 10 K min À1 under nitrogen. 1 H NMR spectra were recorded on an AVANCE 500 (500 MHz, Bruker) spectrometer in CDCl 3 . The infrared spectra were recorded from 400 to 4000 cm À1 on an iS50FT-IR recorded on an UV-2501PC spectrometer. X-ray diffraction (XRD) patterns were recorded by an Ultima IV X-ray powder diffractometer using Cu Ka radiation (k ¼ 1.5406 Å). X-ray photoelectron spectroscopy (XPS) was recorded on an ESCALAB-250Xi spectrometer with an unmonochromated Al Ka X-ray source for excitation. The magnetic properties of the samples were tested on a Lakeshore 7404 vibrating sample magnetometer (VSM).

Synthesis of P4VPtrithiocarbonate(P4VP-TTC)
Poly(4-vinylpyridine) trithiocarbonate was synthesized by solution polymerization using DDMAT as RAFT agent. In a typical run, DDMAT (0.462 g, 1.27 mmol), AIBN (0.052 g, 0.32 mmol), and 4VP (4.0 g, 0.038 mol) were dissolved in 10 mL of ethanol. The solution was transferred to a round-bottom flask. After sealing, the solution was purged with nitrogen for at least 30 min at 0 C to remove oxygen, and then the flask was immersed into the preheated oil bath at 70 C for 24 h. The polymerization was quenched by rapid cooling upon immersing the flask into ice-water bath. The synthesized P4VP-TTC was precipitated in cold diethyl ether and collected by three precipitation/filtration cycles. The product was then dried under vacuum at room temperature overnight. The pink product of 3.04 g (yield: 68.2%) was obtained. The molecular weight of 2200 gmol À1 and molecular weight distribution of 1.32 of the products were determined by GPC with PMMA standards.

Synthesis of P4VP-b-PS nano-objects by dispersion polymerization
In a typical experiment of the dispersion polymerization, styrene (1.0 g, 9.60 mmol), P4VP-TTC (0.1 g, 3.89 Â 10 À2 mmol) and AIBN (2.2 mg, 0.04 mmol) in 10 g of methanol and 2 g of DI water mixture was introduced into a round bottom flask with a magnetic bar. The flask content was initially degassed with nitrogen at 0 C to remove oxygen, and then the polymerization was started by immersing the flask in a preheated oil bath at 70 C under vigorous stirring. After 24 h, the polymerization was quenched by rapid cooling upon immersion of the flask in an ice-water bath, without further purification in the following step of the synthesis of FeCl 3 @P4VP-PS nano-objects. To check the resultant P4VP-b-PS diblock copolymer nano-objects, a small drop of the colloidal dispersion was diluted and then deposited onto a piece of copper grid, dried at room temperature under vacuum, and then observed by TEM. To collect the P4VP-b-PS diblock copolymer for GPC analysis and 1 H NMR analysis, the P4VP-b-PS nano-assemblies were separated by centrifugation of the colloidal dispersion (12,500 rpm, 5 min), washed three times with cold diethyl ether, and finally dried at room temperature at vacuum to afford the P4VP-b-PS diblock copolymer. The molecular weight of 12300 gmol À1 and the molecular weight distribution of 1.26 of the product were determined by GPC with PMMA standards.

Synthesis of the Fe 3þ @P4VP-b-PS nanoobjects
0.1 M FeCl 3 was prepared using MeOH/H 2 O with a weight ratio of 5:1 as solvent. In each run of experiment, FeCl 3 solution was introduced into the P4VPb-PS nanoparticle suspension at room temperature. The mole ratio between Fe 3þ and pyridyl group of P4VP-b-PS was 1:10. The mixed liquid was sealed and stirred for 24 h. The unreacted FeCl 3 was removed by dialysis against MeOH/H 2 O (5:1 wt/wt; 2 Â 200 mL).

Synthesis of the Fe 3 O 4 @P4VP-b-PS nanoobjects
0.1 M FeCl 2 Á4H 2 O solution was prepared using MeOH/H 2 O with weight ratio of 5:1 as solvent. Typically, the above FeCl 3 @P4VP-PS nano-objects were diluted 3 times and then pumping with nitrogen for 30 min followed by the addition of Fe 2þ with the molar ratio of Fe 3þ :Fe 2þ ¼ 1:1. The mixture was allowed to stir for 1.0 h. Then NH 3 H 2 O (0.5 mL) was added into the system to adjust the pH of system to 9-10, the reaction was carried out at 50 C for 1 h, and then aged at 70 C for 2 h. The purification process was carried out by centrifugation at 12,000 rpm for 5 min to remove the supernatant. The precipitate was collected and redisperse into MeOH/H 2 O (5:1 wt/wt).

Synthesis and characterization of P4VP-b-PS nanoparticles
P4VP-trithiocarbonate was synthesized by using S-1-Dodecyl-S 0 -(a,a 0 -dimethyl-a 00 -acetic acid) trithiocarbonate (DDMAT) as chain transfer agent in a solution polymerization at 70 C for 24 h. P4VP-trithiocarbonate was considered both as a stabilizer and chain transfer agent to synthesize P4VP-b-PS nano-objects via dispersion polymerization of styrene in methanol/water solution. As shown in Scheme 1, the dispersion polymerization was prepared from a mixture of P4VP-TTC, styrene monomer, water and methanol with ratio 1：10：20：100 by weight. All ingredients are soluble in the initial reaction mixture and the system is homogeneous. The clean solution turned to cloudy gradually due to the formation of polystyrene which is not soluble in methanol/water (5:1 weight ratio) solution. However, the polymers do not start to precipitated, this is because that P4VP-TTC can act as a stabilizer in the dispersion polymerization of styrene in a methanol/water mixture to stabilize the formed poly(4-vinylpyridine)-block-polystyrene. With continuous polymerization, the polymer solution was turned to milk-white. The white colloid was stable and no sedimentation was observed within one week of standing. The molecular weight, polydispersity and repeat units of P4VP-TTC and P4VP-b-PS were characterized by GPC and 1 H NMR, respectively. The results are summarized in Table 1. The chain extension is evidenced by the molecular weight increase from homopolymer to block copolymer with narrow polydispersity (1.26), which suggests well-defined block copolymers were obtained as designed. Comparison of the apparent molecular weight of the polymer with the molecular weight obtained from NMR determination, showed that the M n , GPC is larger than M n , NMR. GPC analysis provided underestimated molecular weight values. This is because PMMA standards were used for the molecular weight calculation. Such use might not be truly appropriate.

Synthesis of Fe 3 O 4 @P4VP-b-PS nanoparticle
The addition of Fe 3þ to the P4VP-b-PS nanoparticles was carried out by stirring the polymer nanoparticle suspension in a FeCl 3 methanol/water solution at room temperature for 1 day, as shown in Scheme 1. The molar ratio of pyridyl group of the P4VP block to Fe 3þ was varied from 10:1 to 2:1. Fe 3þ ion could coordinate with pyridyl group of P4VP-b-PS, as displayed in Scheme 2. We also notice that further increases in the amount of FeCl 3 in the system resulted in significant aggregates. This may be caused by the crosslinking reaction between the particles. The unreacted iron ions can be removed by dialysis against a large amount of water-methanol (5:1 wt/wt) solution.
The TEM images for the P4VP-b-PS nanoparticles and Fe 3þ @P4VP-b-PS nanoparticles are shown in Figure 1. As shown, spherical particles of relatively uniform size are observed as expected. The diameter of particles estimated from the images is in the range of ca. 35-50 nm for neat P4VP-b-PS nanoparticles in Figure 1(A) by counting particles from the image. The addition of Fe 3þ did not significantly increase the size of spherical particle, as shown in Figure 1(B) from the counting particles. However, the boundaries of the spheres in Figure  1(C) are not as clear as in Figure 1(A), suggesting that the surface of the particles is not as smooth as before, which provides evidence supporting the reaction between FeCl 3 and pyridyl group of P4VPb-PS.
The P4VP-b-PS nanoparticle and Fe 3þ @P4VP-b-PS nanoparticles were also examined by TGA in the temperature range of 50-800 C under the N 2 atmosphere (Figure 2). The decomposition of the neat P4VP-b-PS spheres started at 330 C and went to its completion at 430 C. The thermal decomposition trend of Fe 3þ @P4VP-b-PS is basically the same as that of pure polymers due to the low content of iron. This phenomenon matches the residual iron content of 1.14%.
Since Fe 3þ @P4VP-b-PS nanoparticle contains Fe 3þ , it can be further reacted with FeCl 2 in alkaline solution for the generation of Fe 3 O 4 [34,35]. Since the coordination between FeCl 3 and pyridyl groups is located on the surface of the nanospheres, the resulting Fe 3 O 4 is also located on the surface of the nanospheres. It should be noted that the suspension  containing Fe 3þ @P4VP-b-PS solution needs to be diluted to three times the original volume with water/methanol (5:1 wt/wt) before the reaction during the experimental operation. This is because the original concentration will cause the accumulation and precipitation of a large number of particles during the addition of ferrous iron. After the addition of FeCl 2 methanol/water solution, followed by using NH 3 ÁH 2 O as reducing agent, the suspension turned to dark brown immediately, and no precipitate was observed at this stage. The molar ratio of Fe 2þ to Fe 3þ is 1:1 and the pH of system is ca. 9-10. The dark brown appearance indicated the generation of Fe 3 O 4 . As mentioned in the experimental procedure,   after one hour of reaction, the suspension needs to be further aged at 70 C for two hours. The suspension became a flocculent state which can be redispersed under shaking. The Fe 3 O 4 @P4VP-b-PS nanoparticles were then purified and collected for further testing.
XPS spectrum of Fe 3 O 4 @P4VP-b-PS nanoparticle and neat P4VP-b-PS polymer was recorded and shown in Figure 3(A) and (B). The XPS spectrum of Fe 2p shows that the binding energies of Fe 2p3/2 and Fe 2p1/2 ( Figure 3B) are located at 710.9 and 724.8 eV. Among them, the peak of Fe 2p3/2 can be divided into two peaks, at 710.8 and 713.2 eV, corresponding to Fe 2þ and Fe 3þ . This observation is characteristic of Fe 3þ and Fe 2þ in Fe 3 O 4 [36,37]. It suggested that Fe 3 O 4 has been anchored on the surface of the P4VP-b-PS nanoparticle and there is no Fe2p signal peak from the sample of neat P4VP-b-PS as shown in Figure 3 [38,39]. The average particle size of the Fe 3 O 4 estimated using the Debye-Scherrer formula is 2.44 nm. The most intense (311) peak with full width at half maximum (FWHM) 3.57 degree was used in calculation. Figure 4 shows the FT-IR spectra of neat P4VPb-PS and Fe 3 O 4 @P4VP-b-PS nanoparticles. In the spectrum of P4VP-b-PS, the characteristic adsorptions at 1600, 1560, and 1417 cm À1 are attributed to the vibration of the pyridine ring from the P4VP block. The band at 1600 cm À1 corresponds to the stretching vibration absorption of the C-N bond, and the bands at around 1560 and 1417 cm À1 are attributed to the stretching vibration absorption of the C ¼ C bond. The peaks at 3000 to 3100 cm À1 and 700 cm À1 can be assigned to the aromatic rings, which indicate the presence of styrene block. In the spectrum of Fe 3 O 4 @P4VP-b-PS, a new peak at 565 cm À1 was assigned to the characteristic absorption peak of Fe-O vibrations [20,40]. As discussed above, it was easy to conclude that the Fe 3 O 4 was successfully coated onto the P4VP-b-PS nanoparticles.
The morphology of Fe 3 O 4 @P4VP-b-PS particles was characterized using TEM as shown in Figure 5. Compared with P4VP-b-PS nanoparticles in Figure  5(A), the diameter of Fe 3 O 4 @P4VP-b-PS particles as shown in Figure 5(B). No significant size change was observed. The diameter is in the range of ca. 38-55 nm by counting particles from image. It is worth noting that the nanoparticles obtained from different batches of experiments will have an error of about ±3 nanometers. However, a large number of aggregates appeared, which is consistent with the precipitation that occurred during aging in the experiment. Moreover, dark dot-like material of 2-5 nm appeared on the surface of the nanoparticles, indicating the formation of Fe 3 O 4 , which has a higher electron density compared to pure polymer nanoparticles as shown in Figure 5(E). In addition, the background of the image is clean, which also indicates that the Fe 3 O 4 is generated in situ on the surface of the polymer particles. We also prepared Fe 3 O 4 @P4VP-b-PS with an increased amount of Fe 3 O 4 . The ratio of Fe 3þ to pyridyl group of P4VP block has been increased from 1:10 to 1:2, while the molar ratio of ferric and ferrous iron was maintained at 1:1. As we expected, it is clear from Figure 5(C) and (D) that the content of Fe 3 O 4 punctate particles has increased. We also noted that precipitation became more pronounced with increasing iron content during the aging process. The sample from Figure 5(D) was selected for STEM-HAADF image and EDX mapping of carbon, iron and nitrogen (see Figure 6). In the HAADF image, the brightness in the image reflects the intensity of scattered electrons from different substances and is proportional to the atomic number (Z) [41,42]. In Figure 6(A), the Fe 3 O 4 particles, which have the higher Z compared to the P4VP-b-PS polymeric nanoparticles, are imaged as brighter dots. In addition, the TEM (see Figure 5(D)) and HAADF image (see Figure 6(A)) together suggest that the Fe 3 O 4 nanoparticle was generated uniformly on the surface of polymeric nanoparticle. Figure 6(C) further evidences that iron is uniformly distributed on the surface of P4VP-b-PS polymer particles. We further performed thermogravimetric analysis on the three samples used in Figure 5. The results are shown in Figure 7(C), and the remaining weight of the samples rose from 3.65% to 5.74% with the increase of iron content in the samples.   magnetization is also affected by the size of Fe 3 O 4 particles, and usually the increase of particle size will lead to the increase of saturation magnetization [34,44]. In the experiment, with the increase of Fe 3þ and Fe 2þ concentration in the system, not only the amount of Fe 3 O 4 generated increases, but also the Fe 3 O 4 has a greater chance to increase the particle size. It can be seen in Figure  5(D) that the particle size of a small portion of Fe 3 O 4 reaches ca. 5 nm, which is slightly larger than the particles in Figure 5(B). Magnetic sensitivity tests of the Fe 3 O 4 @P4VP-b-PS nanoparticles were conducted as well. As shown in Figure 7(D), the left bottle holds the brown suspension of Fe 3 O 4 @P4VPb-PS nanoparticles dissolved in methanol/water solution. When the glass vial is connected to an external magnetic field, the particles in the solution are attracted by the magnet during 15 mins, as shown on the right of Figure 7(D). Once the external magnetic field is removed, the nanoparticles can be easily redispersed in solution without aggregation by slightly shaking or sonication. It can therefore be concluded that the Fe 3 O 4 @P4VP-b-PS nanoparticles exhibit an excellent magnetic response.

Conclusion
In summary, a robust method for the synthesis of Fe 3 O 4 @P4VP-b-PS magnetic nanoparticles via RAFT dispersion polymerization as template was achieved. In this work, the chelate polymer of macro-CTA P4VP was employed as stabilizer in a dispersion polymerization of styrene in a methanol/water solution. The sequential addition of FeCl 3 into dispersion makes the pyridyl moieties of P4VP-b-PS particles to react with Fe 3þ through a coordination reaction. Upon the addition of FeCl 2 and NH 3 ÁH 2 O in to above dispersion, a co-precipitation occurred in situ on the surface of the nanoparticles to form the Fe 3 O 4 @P4VP-b-PS nanocomposites. The products were characterized using GPC, FT-IR, TGA, XPS, XRD, SEM and VSM. Results suggested that P4VP-b-PS spherical nanoparticles with controlled molecular weight were obtained. In addition, 2-5 nm crystals of Fe 3 O 4 can be prepared according to the designed route, and these crystals can be uniformly distributed on the surface of the polymer nanospheres to form Fe 3 O 4 @P4VP-b-PS. Moreover, VSM analysis of Fe 3 O 4 @P4VP-b-PS nanoparticles revealed an excellent magnetic response.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This research was funded by the National Natural Science Foundation of China (No. 22005004, 21902002), the Natural Science Foundation of Anhui Province Education Department (KJ2019A0559) and Anhui Natural Science Foundation (1808085MB42).

Notes on contributors
Dr. Lin Wu received her PhD degree in Colour Science from University of Leeds and joined in Anqing Normal University in 2013. Her research focuses on RAFT polymerization and the synthesis and applications of polymer colloids.
Dr. Tao Pang is now an Associated Professor in the School of Chemistry and Chemical Engineering, Anqing Normal University. She has obtained her PhD degree from Central China Normal University (2013). Her research interests include macromolecular self-assembly, selective catalysis.  interests are in the application of nano-metal electrode materials and organic electrosynthesis.
Dr. Yebin Guan, currently working as a professor in the School of Chemistry and Chemical Engineering, Anqing Normal University. His research interests include metallopolymers and organic electrosynthesis.
Dr. Liwei Yin is an associate professor in College of Life Science, Anqing Normal University, and her current research focuses on the synthesis of polymers and biomedical applications.