A Facile Approach towards Fluorescent Nanogels with AIE-Active Spacers

A facile and efficient approach for design and synthesis of organic fluorescent nanogels has been developed by using a pre-synthesized polymeric precursor. This strategy is achieved by two key steps: (i) precise synthesis of core–shell star-shaped block copolymers with crosslinkable AIEgen-precursor (AIEgen: aggregation induced emission luminogen) as pending groups on the inner blocks; (ii) gelation of the inner blocks by coupling the AIEgen-precursor moieties to generate AIE-active spacers, and thus, fluorescent nanogel. By using this strategy, a series of star-shaped block copolymers with benzophenone groups pending on the inner blocks were synthesized by grafting from a hexafunctional initiator through atom transfer radical copolymerization (ATRP) of 4-benzoylphenyl methacrylate (BPMA) or 2-(4-benzoylphenoxy)ethyl methacrylate (BPOEMA) with methyl methacrylate (MMA) and tert-butyldimethylsilyl-protected 2-hydroxyethyl methacrylate (ProHEMA) followed by a sequential ATRP to grow PMMA or PProHEMA. The pendent benzophenone groups were coupled by McMurry reaction to generate tetraphenylethylene (TPE) groups which served as AIE-active spacers, affording a fluorescent nanogel. The nanogel showed strong emission not only at aggregated state but also in dilute solution due to the strongly restricted inter- and intramolecular movement of TPE moiety in the crosslinked polymeric network. The nanogel has been used as a fluorescent macromolecular additive to fabricate fluorescent film.

Among the organic nanomaterials, nanogels constructed by covalently crosslinked polymeric network incorporating fluorophore groups are reasonably supposed to have higher chemical and mechanical stability [12,15] than those constructed through van der Waals forces [1]. For nanogel synthesis, several methods have been developed, including crosslinking the pre-synthesized linear block copolymers by coupling their functional side groups [16][17][18][19], initiating difunctional monomer

Synthesis of Hexafunctional Initiator
The hexafunctional initiator was synthesized by esterifying dipentaerythritol with 2-bromo-2-methylpropanoyl bromide. Dipentaerythritol (1.60 g, 6.3 mmol, 1.0 equiv), TEA (11.00 mL, 75.4 mmol, 12.0 equiv) and DCM (100 mL) were added to a flask. After being cooled to 0 • C, 2-bromo-2-methylpropanoyl bromide (7.00 mL, 56.6 mmol, 9.0 equiv) dissolved in DCM (30 mL) was added dropwise in 1 h. The resulting mixture was warmed up to 25 • C and stirred overnight. The reaction solution was washed with saturated NaHCO 3 solution three times, dried over MgSO 4 , and concentrated to give a crude product as dark orange oil. The product was recrystallized from hot ethanol as light yellow crystalline solid, which was further purified by column chromatography to afford white crystalline solid (3.28 g, 45.4%). The successful synthesis of initiator was confirmed by 1 H NMR spectrum ( Figure S1 in supplementary materials).

Synthesis of 4-Benzoylphenyl Methacrylate (BPMA)
BPMA was easily synthesized by esterifying 4-hydroxybenzophenone with methacroyl chloride. 4-Hydroxybenzophenone (10.5 g, 53.1 mmol, 1.0 equiv), TEA (15 mL, 106.2 mmol, 2.0 equiv), and DCM (150 mL) were added to a flask and was cooled to 0 • C by ice-bath followed by dropwise addition of methacroyl chloride (8.1 mL, 79.6 mmol, 1.5 equiv) solution in DCM (20 mL). After being warmed up to 25 • C and stirred for 12 h, the reaction mixture was washed with saturated aqueous NaHCO 3 solution twice, dried over MgSO 4 , and concentrated in vacuum. The crude product was purified by recrystallization in ethanol three times to give pure product (10.2 g, 72.1%). The structure was revealed by 1 H and 13 C NMR spectra ( Figure S2 in supplementary materials).
A solution of the as-synthesized 2-(4-benzoylphenoxy) ethanol (10.0 g, 41.2 mmol, 1.0 equiv) and TEA (12 mL, 82.5 mmol, 2.0 equiv) in DCM (150 mL) was transferred to a flask, and was cooled to 0 • C. After dropwise addition of methacryloyl chloride (6.3 mL, 62.2 mmol, 1.5 equiv) solution in DCM (20 mL), the reaction mixture was warmed up to 25 • C and was stirred for 24 h. The organic phase was washed with saturated aqueous NaHCO 3 solution, dried over MgSO 4 , and was purified by column chromatography to give 8.7 g of BPOEMA as colorless viscous liquid (yield: 69.6%). The structure of BPOEMA was confirmed by 1 H and 13 C NMR spectra ( Figure S4 in supplementary materials).

Protection of Hydroxyl Group of 2-Hydroxyethyl Methacrylate (HEMA)
HEMA (13 g, 1.0 equiv) and imidazole (17 g, 2.5 equiv) were dissolved in 25 mL of DCM and were cooled to 0 • C followed by slow addition of TBDMSCl (18 g, 1.2 equiv) solution in 10 mL of DCM. After warming up to 25 • C and stirring for 5 h, the reaction solution was washed with distilled water for three times, dried over MgSO 4 , concentrated and purified by column chromatography to afford 23.1 g of ProHEMA, which was revealed by 1 H and 13 C NMR (yield 95%) ( Figure S5 in supplementary materials).

Synthesis of Star-Shaped Block Copolymers
The star-shaped block copolymers were synthesized through ATRP by a "grafting from" strategy. Taking the synthesis of P(BPMA-co-ProHEMA)-b-PMMA as an example, hexafunctional initiator (30 mg, 0.026 mmol), BPMA (3.127 g, 11.8 mmol), ProHEMA (5.74 g, 23.6 mmol), CuBr (22.5 mg, 0.157 mmol), and anisole (5.8 mL) were added to a Schlenk flask followed by three freeze-pump-thaw cycles to degas. After charging with Ar and addition of PMDETA (65 µL, 0.314 mmol), the solution was stirred at 70 • C for 40 min. After this reaction, the solution was quenched by putting the flask into an ice-bath and exposing to air. After 10 mL of THF was added, the solution was passed through a small silica gel column to remove the catalyst. The product was precipitated into methanol to get P(BPMA-co-ProHEMA), which was confirmed by 1 H NMR spectrum and GPC traces.
The as-prepared P(BPMA-co-ProHEMA) (290 mg), MMA (800 mg, 8 mmol), CuBr (7.5 mg, 0.052 mmol), anisole (1.2 mL) were added to a Schlenk flask, and the solution was degassed by three freeze-pump-thaw cycles, followed by charging with Ar. After PMDETA (22 µL, 0.104 mmol) was added, the solution was stirred at 50 • C for 70 min. The reaction was processed according to the procedure used for the synthesis of P(BPMA-co-ProHEMA), as mentioned above, to give P(BPMA-co-ProHEMA)-b-PMMA, which was confirmed by 1 H NMR spectrum and GPC traces.
The other two star-shaped block copolymers of P(BPMA-co-MMA)-b-P(ProHEMA) and P(BPOEMA-co-ProHEMA)-b-PMMA were synthesized in a similar way.

Synthesis of Fluorescent Organic Nanogels by McMurry Coupling Reaction
Freshly distilled THF (45 mL), zinc powder (1.5 g), and pyridine (1 mL) were added into a flask. The solution was degassed by bubbling Ar for 30 min and was cooled down to −78 • C by using a mixture of acetone and liquid nitrogen. To the solution, TiCl 4 (1.2 mL) was added dropwise, followed by the addition of 5 mL of P(BPMA-co-ProHEMA)-b-MMA (200 mg) solution in freshly distilled THF. The resultant solution was warmed up to room temperature followed by refluxing in Ar atmosphere at 80 • C for 10 h. Twenty milliliters of K 2 CO 3 aqueous solution (10%, weight percent) was added to quench the reaction. After 100 mL of THF was added and stirred for 30 min, the solution was filtered and concentrated in a vacuum, and precipitated into excess methanol to give the fluorescent nanogel.

Fabrication of Fluorescent Film by Mixing the Nanogel with PMMA
PMMA (100 mg) (M w = 1.6 × 10 5 g/mol, Đ = 1.97) (Đ: molar mass dispersity () and P(BPMA-co-ProHEMA) gel -b-PMMA (0.5 mg) were dissolved in 1 mL of THF. The solution was painted on a clean smooth glass, and was dried naturally to afford a fluorescent film. In addition, a pure PMMA film was prepared in the same way for comparison.

Results and Discussion
The fluorescent organic nanogel is achieved by using a polymeric precursor strategy. In this strategy, precise synthesis of a core-shell star-shaped block copolymer with crosslinkable potential AIEgen groups, pending on the inner blocks, were synthesized through ATRP by using a hexafunctional initiator. The pendent groups were crosslinked to generate AIE-active spacers, leading to a crosslinked network in the core, and thus, a fluorescent nanogel (Scheme 1). The resultant solution was warmed up to room temperature followed by refluxing in Ar atmosphere at 80 °C for 10 h. Twenty milliliters of K2CO3 aqueous solution (10%, weight percent) was added to quench the reaction. After 100 mL of THF was added and stirred for 30 min, the solution was filtered and concentrated in a vacuum, and precipitated into excess methanol to give the fluorescent nanogel.

Fabrication of Fluorescent Film by Mixing the Nanogel with PMMA
PMMA (100 mg) (Mw = 1.6 × 10 5 g/mol, Đ = 1.97) (Đ: molar mass dispersity () and P(BPMA-co-ProHEMA)gel-b-PMMA (0.5 mg) were dissolved in 1 mL of THF. The solution was painted on a clean smooth glass, and was dried naturally to afford a fluorescent film. In addition, a pure PMMA film was prepared in the same way for comparison.

Results and Discussion
The fluorescent organic nanogel is achieved by using a polymeric precursor strategy. In this strategy, precise synthesis of a core-shell star-shaped block copolymer with crosslinkable potential AIEgen groups, pending on the inner blocks, were synthesized through ATRP by using a hexafunctional initiator. The pendent groups were crosslinked to generate AIE-active spacers, leading to a crosslinked network in the core, and thus, a fluorescent nanogel (Scheme 1).

Scheme 1.
Synthesis of fluorescent organic nanogel by using a star-shaped block copolymer as precursor.

Precise Synthesis Star-Shaped Block Copolymers by ATRP with a Hexafunctional Initiator
Starting from a hexafunctional initiator, star-shaped block copolymers were synthesized by using a "grafting from" strategy through ATRP. Taking the synthesis of P(BPMA-co-MMA)-b-P(ProHEMA) and P(BPMA-co-ProHEMA)-b-PMMA as examples (Figure 1), the pre-synthesized BPMA was copolymerized with MMA and ProHEMA, resulting in the responding star-shaped random copolymers of P(BPMA-co-MMA) and P(BPMA-co-ProHEMA), which served as macroinitiators for the followed ATRP of ProHEMA and MMA, leading to core-shell star-shaped block copolymer of P(BPMA-co-MMA)-b-P(ProHEMA) and P(BPOEMA-co-ProHEMA)-b-PMMA, Scheme 1. Synthesis of fluorescent organic nanogel by using a star-shaped block copolymer as precursor.

Precise Synthesis Star-Shaped Block Copolymers by ATRP with a Hexafunctional Initiator
Starting from a hexafunctional initiator, star-shaped block copolymers were synthesized by using a "grafting from" strategy through ATRP. Taking the synthesis of P(BPMA-co-MMA)-b-P(ProHEMA) and P(BPMA-co-ProHEMA)-b-PMMA as examples (Figure 1), the pre-synthesized BPMA was copolymerized with MMA and ProHEMA, resulting in the responding star-shaped random copolymers of P(BPMA-co-MMA) and P(BPMA-co-ProHEMA), which served as macroinitiators for the followed ATRP of ProHEMA and MMA, leading to core-shell star-shaped block copolymer of P(BPMA-co-MMA)-b-P(ProHEMA) and P(BPOEMA-co-ProHEMA)-b-PMMA, respectively. The synthesis of the star-shaped copolymer was monitored by GPC traces (Figure 1). Both of the two star-shaped copolymers of P(BPMA-co-ProHEMA) and P(BPMA-co-MMA) showed narrow mono-distribution in GPC trace (refractive index (RI) signal). The Đs of 1.17 and 1.16 were calculated by using a conventional calibration method built by using PS standards. In the case of P(BPMA-co-ProHEMA), the weight-averaged molecular weight (M w ) of 6.36 × 10 4 g/mol was determined by using static light scattering techniques performed on a MALLS detector equipped on the GPC system with a dn/dc value of 0.153 mL/g pre-determined on a RI detector. The compositions of PBPMA and PProHEMA (50% to 50% in mole) were revealed by the area ratio of protons in -COOCH 2 CH 2 Oof PProHEMA and the aromatic protons belonging to PBPMA in 1 H NMR spectrum ( Figure 2). Although the chemical shifts of aromatic protons in PBPMA and PBOEPMA (δ = 7.2-7.7 ppm) were overlapped with that of solvent (CDCl 3 ) (δ = 7.26 ppm), the composition of the inner block can be calculated with a small deviation (2-3%) by eliminating the peak area of CDCl 3 during integration ( Figure S6, supplementary materials). Both of the two star-shaped copolymers of P(BPMA-co-ProHEMA) and P(BPMA-co-MMA) showed narrow mono-distribution in GPC trace (refractive index (RI) signal). The Đs of 1.17 and 1.16 were calculated by using a conventional calibration method built by using PS standards. In the case of P(BPMA-co-ProHEMA), the weight-averaged molecular weight (Mw) of 6.36 × 10 4 g/mol was determined by using static light scattering techniques performed on a MALLS detector equipped on the GPC system with a dn/dc value of 0.153 mL/g pre-determined on a RI detector. The compositions of PBPMA and PProHEMA (50% to 50% in mole) were revealed by the area ratio of protons in -COOCH2CH2O-of PProHEMA and the aromatic protons belonging to PBPMA in 1 H NMR spectrum ( Figure 2). Although the chemical shifts of aromatic protons in PBPMA and PBOEPMA (δ = 7.2-7.7 ppm) were overlapped with that of solvent (CDCl3) (δ = 7.26 ppm), the composition of the inner block can be calculated with a small deviation (2-3%) by eliminating the peak area of CDCl3 during integration ( Figure S6, supplementary materials). The resultant star-shaped polymer with an initiator moiety in each chain end served as hexafunctional macroinitiator for the following ATRP of MMA to afford the star-shaped block copolymer of P(BPMA-co-ProHEMA)-b-PMMA with an increase in molecular weight and Đ ,which was from 6.36 × 10 4 to 1.70 × 10 5 g/mol and from 1.17 to 1.28, respectively. By using the same method, the other two star-shaped block copolymers were also synthesized ( Table 1). A shoulder that appeared in the P(BPMA-co-ProHEMA)-b-PMMA spectrum was attributed to the non-quantitative initiation of the star-shaped macroinitiator of P(BPMA-co-ProHEMA). 1 Table 1. Molecular characteristics of star-shaped block copolymers and resulting nanogels. Both of the two star-shaped copolymers of P(BPMA-co-ProHEMA) and P(BPMA-co-MMA) showed narrow mono-distribution in GPC trace (refractive index (RI) signal). The Đs of 1.17 and 1.16 were calculated by using a conventional calibration method built by using PS standards. In the case of P(BPMA-co-ProHEMA), the weight-averaged molecular weight (Mw) of 6.36 × 10 4 g/mol was determined by using static light scattering techniques performed on a MALLS detector equipped on the GPC system with a dn/dc value of 0.153 mL/g pre-determined on a RI detector. The compositions of PBPMA and PProHEMA (50% to 50% in mole) were revealed by the area ratio of protons in -COOCH2CH2O-of PProHEMA and the aromatic protons belonging to PBPMA in 1 H NMR spectrum ( Figure 2). Although the chemical shifts of aromatic protons in PBPMA and PBOEPMA (δ = 7.2-7.7 ppm) were overlapped with that of solvent (CDCl3) (δ = 7.26 ppm), the composition of the inner block can be calculated with a small deviation (2-3%) by eliminating the peak area of CDCl3 during integration ( Figure S6, supplementary materials). The resultant star-shaped polymer with an initiator moiety in each chain end served as hexafunctional macroinitiator for the following ATRP of MMA to afford the star-shaped block copolymer of P(BPMA-co-ProHEMA)-b-PMMA with an increase in molecular weight and Đ ,which was from 6.36 × 10 4 to 1.70 × 10 5 g/mol and from 1.17 to 1.28, respectively. By using the same method, the other two star-shaped block copolymers were also synthesized ( Table 1). A shoulder that appeared in the P(BPMA-co-ProHEMA)-b-PMMA spectrum was attributed to the non-quantitative initiation of the star-shaped macroinitiator of P(BPMA-co-ProHEMA). 1 H NMR spectra also indicated the successful synthesis of the products ( Figures S6-S8 in supplementary materials).     The resultant star-shaped polymer with an initiator moiety in each chain end served as hexafunctional macroinitiator for the following ATRP of MMA to afford the star-shaped block copolymer of P(BPMA-co-ProHEMA)-b-PMMA with an increase in molecular weight and Đ ,which was from 6.36 × 10 4 to 1.70 × 10 5 g/mol and from 1.17 to 1.28, respectively. By using the same method, the other two star-shaped block copolymers were also synthesized ( Table 1). A shoulder that appeared in the P(BPMA-co-ProHEMA)-b-PMMA spectrum was attributed to the non-quantitative initiation of the star-shaped macroinitiator of P(BPMA-co-ProHEMA). 1 H NMR spectra also indicated the successful synthesis of the products (Figures S6-S8 in supplementary materials). The M w s were calculated from static light scattering results determined by a GPC-MALLS system; b Đs were determined by GPC (RI signal trace) calibrated by using polystyrene standards; c dn/dc values were determined by using a differential refractometer; d hydrodynamic diamater (D h ) and polydispersity index (PDI) were determined by using dynamic light scattering technique; e the max exciting and emission wavelengths nanogels in THF were determined by using a fluorescence spectrophotometer; f QY sol (1 mg/mL in THF) is determined by using a spectrofluorometer; g not determined.

Synthesis of Fluorescent Nanogels by Crosslinking the Inner Blocks through McMurry Reaction of the Pendent Benzophenone Groups
The pendent benzophenone groups on the inner block of PBPMA or PBPOEMA in the star-shaped block copolymers are ready to be coupled through McMurry reaction in the presence of Zn/TiCl 4 , leading to the formation of TPE groups, and thus, gelation of the core (Scheme 2). In the gelation process, peripheral blocks of PMMA or PProHEMA are used to isolate the inner blocks and to suppress the intermolecular coupling. As a typical AIE-active fluorophore, the TPE groups serving as spacers in the polymeric network are fixed, and are strongly emissive, due to the highly restricted inter-and intramolecular movement by the polymeric network, taking the gelation of P(BPMA-co-ProHEMA) gel -b-PMMA as an example. After McMurry reaction, a strong blue light emission was observed, in contrast to its non-emissive precursor, indicating the formation of the TPE groups, and thus, the nanogel (images shown in Scheme 2). The successful coupling reaction was also proved by comparing FTIR spectra of TPE, P(BPMA-co-ProHEMA) gel -b-PMMA, P(BPMA-co-ProHEMA)-b-PMMA and BPMA ( Figure 3). After McMurry reaction, the intensity of characteristic absorption peak at 1660 cm −1 of the nanogel (blue curve in Figure 3), attributed to the stretch of C=O bond in the benzophenone moiety, was significantly decreased, indicating the transformation from benzophenone groups to TPE groups, which had no absorption at this range. The residue of the peak at 1660 cm −1 indicated non-quantitative consumption of benzophenone groups in the McMurry reaction caused by higher and higher steric hindrance in the gelation process. Although the occurrence of McMurry reaction was qualitatively proved by the FTIR results and the fluorescence of the resultant nanogel, however, it is challenging to quantitatively estimate the efficiency. By using a similar method, another two fluorescent nanogels of P(BPMA-co-MMA) gel -b-P(ProHEMA) and P(BPOEMA-co-ProHEMA) gel -b-PMMA have also been successfully synthesized (Table 1).
at 1660 cm −1 indicated non-quantitative consumption of benzophenone groups in the McMurry reaction caused by higher and higher steric hindrance in the gelation process. Although the occurrence of McMurry reaction was qualitatively proved by the FTIR results and the fluorescence of the resultant nanogel, however, it is challenging to quantitatively estimate the efficiency. By using a similar method, another two fluorescent nanogels of P(BPMA-co-MMA)gel-b-P(ProHEMA) and P(BPOEMA-co-ProHEMA)gel-b-PMMA have also been successfully synthesized (Table 1).

The Fluorescent Behaviors and Sizes of the Organic Nanogels
The regular small AIEgen molecules are not emissive in a dilute solution, but are strongly emissive at solid or aggregated state, due to the restricted intermolecular movement. In comparison, in the nanogel, the TPE moieties as spacers are covalently fixed in the polymeric network, leading to a highly restricted inter-and intramolecular movability, and high local concentration, resulting in strong emission in solute state or even in a high dilute solution, in addition to the solid and aggregated state. A comparison between fluorescent behaviors of the nanogels and TPE molecules were shown in Figure 4 and S9. Both nanogels and TPE were well dissolved in THF, to which water as poor solvent was added to prompt the aggregation. To enhance the contrast, a more diluted nanogel solution (1 mg/mL, cTPE unit < 0.37 μmol/mL) was compared with TPE solution (2 mg/mL, 6.0 μmol/mL). Seen from Figure 4, the TPE was not emissive in THF solution, even with higher concentration than nanogel, until the water volume fractional ratio was up to 50%. By contrast, the nanogels emitted in pure THF and the emission was slightly enhanced by addition of water, which caused collapse of the nanogels, leading to further suppressed movability of TPE spacers in the nanogels, and thus, stronger emission.

The Fluorescent Behaviors and Sizes of the Organic Nanogels
The regular small AIEgen molecules are not emissive in a dilute solution, but are strongly emissive at solid or aggregated state, due to the restricted intermolecular movement. In comparison, in the nanogel, the TPE moieties as spacers are covalently fixed in the polymeric network, leading to a highly restricted inter-and intramolecular movability, and high local concentration, resulting in strong emission in solute state or even in a high dilute solution, in addition to the solid and aggregated state. A comparison between fluorescent behaviors of the nanogels and TPE molecules were shown in Figure 4 and S9. Both nanogels and TPE were well dissolved in THF, to which water as poor solvent was added to prompt the aggregation. To enhance the contrast, a more diluted nanogel solution (1 mg/mL, c TPE unit < 0.37 µmol/mL) was compared with TPE solution (2 mg/mL, 6.0 µmol/mL). Seen from Figure 4, the TPE was not emissive in THF solution, even with higher concentration than nanogel, until the water volume fractional ratio was up to 50%. By contrast, the nanogels emitted in pure THF and the emission was slightly enhanced by addition of water, which caused collapse of the nanogels, leading to further suppressed movability of TPE spacers in the nanogels, and thus, stronger emission. nanogel solution (1 mg/mL, cTPE unit < 0.37 μmol/mL) was compared with TPE solution (2 mg/mL, 6.0 μmol/mL). Seen from Figure 4, the TPE was not emissive in THF solution, even with higher concentration than nanogel, until the water volume fractional ratio was up to 50%. By contrast, the nanogels emitted in pure THF and the emission was slightly enhanced by addition of water, which caused collapse of the nanogels, leading to further suppressed movability of TPE spacers in the nanogels, and thus, stronger emission.  Further fluorescence results revealed that P(BPMA-co-ProHEMA) gel -b-PMMA and P(BPMA-co-MMA) gel -b-P(ProHEMA) emitted at 484 and 472 nm with a shoulder at 510 and 513 nm, respectively ( Figure 5). In the case of P(BPMA-co-ProHEMA) gel -b-PMMA, both of the two emissions have the same max exciting wavelength at 368 nm. The asymmetric emission peaks were found in all the as-prepared nanogels, and were supposed to be a consequence of the cis-trans isomerization of the TPE unit in the formation, which has been observed in the small molecule of TPE derivatives [63]. A quantum yield of P(BPMA-co-ProHEMA) gel -b-PMMA in THF solution (1 mg/mL) of 47.2% was determined (Table 1).
Polymers 2018, 10, x FOR PEER REVIEW 9 of 14 max exciting wavelength at 368 nm. The asymmetric emission peaks were found in all the asprepared nanogels, and were supposed to be a consequence of the cis-trans isomerization of the TPE unit in the formation, which has been observed in the small molecule of TPE derivatives [63]. A quantum yield of P(BPMA-co-ProHEMA)gel-b-PMMA in THF solution (1 mg/mL) of 47.2% was determined (Table 1). The size of nanogel plays an important role in their application. Hydrodynamic radiuses (Rh) of three nanogels in their THF solution were determined by using dynamic light scattering technique (DLS). Taking the nanogels of P(BPMA-co-ProHEMA)gel-b-PMMA for example, the DLS result revealed a mono-distribution with a Dh of 21.4 nm ( Figure 6). The theoretical averaged diameter of these nanogels of 54.6 nm calculated according to the degree of polymerization is larger than and the experimental value of 21.4 nm, which is supposed to be a result of high flexibility of the peripheral arms of the nanogel in solution, and collapse of the inner block caused by gelation. Similar results were also found in the other two samples (Table 1 and Figure S10 in supplementary materials). The size of nanogel plays an important role in their application. Hydrodynamic radiuses (R h ) of three nanogels in their THF solution were determined by using dynamic light scattering technique (DLS). Taking the nanogels of P(BPMA-co-ProHEMA) gel -b-PMMA for example, the DLS result revealed a mono-distribution with a D h of 21.4 nm ( Figure 6). The theoretical averaged diameter of these nanogels of 54.6 nm calculated according to the degree of polymerization is larger than and the experimental value of 21.4 nm, which is supposed to be a result of high flexibility of the peripheral arms of the nanogel in solution, and collapse of the inner block caused by gelation. Similar results were also found in the other two samples (Table 1 and Figure S10 in supplementary materials).
(DLS). Taking the nanogels of P(BPMA-co-ProHEMA)gel-b-PMMA for example, the DLS result revealed a mono-distribution with a Dh of 21.4 nm ( Figure 6). The theoretical averaged diameter of these nanogels of 54.6 nm calculated according to the degree of polymerization is larger than and the experimental value of 21.4 nm, which is supposed to be a result of high flexibility of the peripheral arms of the nanogel in solution, and collapse of the inner block caused by gelation. Similar results were also found in the other two samples (Table 1 and Figure S10 in supplementary materials). In addition, the structures of the nanogels on a silica substrate were visualized by using AFM. Seen from Figure 7, the nanogels of P(BPMA-co-ProHEMA)gel-b-PMMA adopt a spherical structure with a diameter around 30 nm. The larger size than its Dh, revealed by DLS in dilute solution, was attributed to the collapse effect of the soft nanogel on the substrate indicated by a height of ~5 nm. The much smaller particles were attributed to the impurities of silicon wafer as used, which was confirmed by a control experiment. In addition, the structures of the nanogels on a silica substrate were visualized by using AFM. Seen from Figure 7, the nanogels of P(BPMA-co-ProHEMA) gel -b-PMMA adopt a spherical structure with a diameter around 30 nm. The larger size than its D h , revealed by DLS in dilute solution, was attributed to the collapse effect of the soft nanogel on the substrate indicated by a height of~5 nm. The much smaller particles were attributed to the impurities of silicon wafer as used, which was confirmed by a control experiment.

Fabrication of a Fluorescent Film by Mixing the P(BPMA-co-ProHEMA)gel-b-PMMA
Benefiting from the core-shell structure of the fluorescent nanogel, the peripheral block promised tunable compatibility with the other polymeric materials, by which the nanogel could be used as a macromolecular additive to fabricate fluorescent film. Taking the nanogel of P(BPMA-co-ProHEMA)gel-b-PMMA for example, a fluorescent PMMA film was fabricated by mixing the nanogel with PMMA homopolymer (Mw = 1.6 × 10 5 g/mol, Đ = 1.97). Seen from Figure 8, the resultant film is strongly emissive, despite that the weight ratio of nanogel is as low as 0.5% in UV light (365 nm). Due to the good compatibility of the peripheral PMMA block with the PMMA homopolymer, the nanogels could be steadily dispersed in the PMMA, in contrast to the small fluorophore molecules which may move to the surface due to their poor compatibility and high movability in the film. In addition, the nanogels can be reasonably expected as a universal additive to the other polymers by choosing a proper peripheral block.  Benefiting from the core-shell structure of the fluorescent nanogel, the peripheral block promised tunable compatibility with the other polymeric materials, by which the nanogel could be used as a macromolecular additive to fabricate fluorescent film. Taking the nanogel of P(BPMA-co-ProHEMA) gel -b-PMMA for example, a fluorescent PMMA film was fabricated by mixing the nanogel with PMMA homopolymer (M w = 1.6 × 10 5 g/mol, Đ = 1.97). Seen from Figure 8, the resultant film is strongly emissive, despite that the weight ratio of nanogel is as low as 0.5% in UV light (365 nm). Due to the good compatibility of the peripheral PMMA block with the PMMA homopolymer, the nanogels could be steadily dispersed in the PMMA, in contrast to the small fluorophore molecules which may move to the surface due to their poor compatibility and high movability in the film. In addition, the nanogels can be reasonably expected as a universal additive to the other polymers by choosing a proper peripheral block.

Fabrication of a Fluorescent Film by Mixing the P(BPMA-co-ProHEMA)gel-b-PMMA
Benefiting from the core-shell structure of the fluorescent nanogel, the peripheral block promised tunable compatibility with the other polymeric materials, by which the nanogel could be used as a macromolecular additive to fabricate fluorescent film. Taking the nanogel of P(BPMA-co-ProHEMA)gel-b-PMMA for example, a fluorescent PMMA film was fabricated by mixing the nanogel with PMMA homopolymer (Mw = 1.6 × 10 5 g/mol, Đ = 1.97). Seen from Figure 8, the resultant film is strongly emissive, despite that the weight ratio of nanogel is as low as 0.5% in UV light (365 nm). Due to the good compatibility of the peripheral PMMA block with the PMMA homopolymer, the nanogels could be steadily dispersed in the PMMA, in contrast to the small fluorophore molecules which may move to the surface due to their poor compatibility and high movability in the film. In addition, the nanogels can be reasonably expected as a universal additive to the other polymers by choosing a proper peripheral block.

Conclusions
An easy and efficient approach to synthesize fluorescent nanogels has been successfully developed by using a pre-synthesized star-shaped polymer as precursor. The high universal and efficiency of this strategy have been proved by a series of fluorescent nanogels with tunable structures.

Conclusions
An easy and efficient approach to synthesize fluorescent nanogels has been successfully developed by using a pre-synthesized star-shaped polymer as precursor. The high universal and efficiency of this strategy have been proved by a series of fluorescent nanogels with tunable structures. Benefiting from the good controllability of ATRP over molecular weight and distribution, the star-shaped block copolymers can be precisely synthesized, and thus, promises a facile method to control the structures of the nanogels. The AIE-active spacer generated during the crosslinking of the inner blocks for constructing the polymeric network is an ideal fluorophore, due to the restricted structure in the resultant nanogels, and showed strong emission at both solute and aggregated states. The peripheral block is not only used to suppress the intermolecular crosslinking, but also used to improve the solubility and further functionalization. The design and synthesis of red-emissive fluorescent nanogels by using proper AIE-active spacer are ongoing.