## 3 Results and Discussion
### Preparation of PNAMAm Microspheres and MSP@PNAMAm Core-Shell Microspheres  
To satisfy different potential applications, we studied the copolymerization of the monomer NAMAM-\(p\) with crosslinker and other monomers to fabricate different kinds of alkoxyamine functional microspheres. The detailed design was shown in Scheme 2. First, we investigated the effect of crosslinker species and relative amounts on the microsphere properties. The three often used crosslinkers, divinylbenzene (DVB), ethylene glycol dimethylacrylate (EGDMA) and \(N\),\(N^{\prime}\)-methylene bisacrylamide (MBA), were utilized for the preparation of PNAMAm-\(p\) microspheres. The results showed that only MBA could afford the corresponding microspheres, the possible reason is related to the higher reactivity of MBA. TEM images in Figure 2 display that the particle size is greatly influenced by the feed amount of crosslinker MBA. When 20 wt % MBA was used, the particle size was about 300 nm. With the increase of MBA content to 30 wt % and 50 wt %, the particle sizes accordingly decreased to 200 and 150 nm. Meanwhile, the amount of alkoxyamine groups in the microspheres were reduced (Figure S2, Supplementary Material). When the MBA content was less than 20 wt %, no microspheres were formed. Figure 2: TEM images of PNAMAm with different amount of MBA as crosslinker: (**a**) 20%; (**b**) 30%; (**c**) 40%; (**d**) 50%. The scale bar is 200 nm. On the basis of the above results, the core-shell magnetic microsphere consisting of a crosslinking PNAMAm-\(p\) in shell (20% MBA) and a magnetite supraparticle (MSP) in core was successfully prepared (Figure 3). The characteristic peaks at 1790 and 1735 cm\({}^{-1}\) were found in the FT-IR spectrum for the MSP@PNAMAm-\(p\), proving the formation of PNAMAm-\(p\) component (Figure S3, Supplementary Material). After the deprotection, the MSP@PNAMAm was obtained, and the core-shell structure was preserved, without any change in morphology (Figure 3c). Meanwhile, TGA results were applied to analyze the compositions, and the density of alkoxyamine group (_d_) could be calculated by the formula below:  
\[d\ =\ (W_{1}-W_{2})/(W_{2}\times M_{\rm pt}) \tag{1}\]  
where, \(W_{1}\) and \(W_{2}\) represent the final residual weight percentages of the core-shell composite microspheres after and before the deprotection at 600 \({}^{\circ}\)C in air; \(M_{\rm pt}\) is 132 g/mol, which is the differ molecular weight caused by deprotection. The microspheres remained 65.4% mass and 78.3% mass at 600 \({}^{\circ}\)C before and after deprotection, respectively (Figure 4a). According to the formula above, the alkoxyamine group density was calculated to be 1.49 mmol/g (More details about the calculation are provided in Supplementary Material). The VSM results also could give the mangetic contents of the resultant microspheres, which coincided with that obtained from the TGA results. Besides, the magnetic hysteresis curves proved that the supraparamagnetic characteristic was unchanged during the deprotection process (Figure 4b). In our experiment, we found that the solid content played a key role in adjusting the thickness of the polymer shell (Figure 5). When the solid content was 0.25%, only \(\sim\)5 nm polymer shell could be obtained. When the solid content was 0.375%, the shell thickness increased to 30 nm. With continuous increase of the solid content to 0.675%, the shell thickness could be changed to almost 100 nm. This increment trend was also confirmed by DLS (Figure 5e). Figure 4: (a) TGA and; (b) VSM curves of (i) MSP; (ii) MSP@PNAMAm-\(p\); (iii) MSP@PNAMAm. Figure 3: TEM images of (a) MSP (magnetic supraparticle); (b) MSP@PNAMAm-\(p\); (c) MSP@PNAMAm. The scale bar is 100 nm. In order to quantitatively measure the density of alkoxyamine group in the shell of magnetic composite microspheres, a series of samples with different crosslinking densities were tested by TGA (Table 1).