RAFT Dispersion Polymerization in the Presence of Block

Jul 2, 2019 - By changing the molar ratio of [P4VP49-b-PS197]0/[PEG45-TTC]0, block copolymer nanoassemblies with different molar ratios of...
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RAFT Dispersion Polymerization in the Presence of Block Copolymer Nanoparticles and Synthesis of Multicomponent Block Copolymer Nanoassemblies Shuwen Qu,† Rui Liu,† Wenfeng Duan,*,# and Wangqing Zhang*,†,‡

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Key Laboratory of Functional Polymer Materials of the Ministry of Education, Institute of Polymer Chemistry, College of Chemistry and ‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China # State Key Laboratory of Special Functional Waterproof Materials, Beijing Oriental Yuhong Waterproof Technology Co., Ltd., Beijing 100123, China S Supporting Information *

ABSTRACT: Multicomponent block copolymer nanoassemblies are constructed by two or more than two block copolymers. Herein, multicomponent block copolymer nanoparticles constructed with poly(4-vinylpyridine)-block-polystyrene (P4VP-b-PS) and poly(ethylene glycol)-block-polystyrene (PEG-b-PS) are synthesized by RAFT dispersion polymerization of styrene employing poly(ethylene glycol) trithiocarbonate (PEG-TTC) as the macromolecular chain transfer agent (macro-CTA) in the presence of P4VP-b-PS nanoparticles, which are presynthesized by ATRP dispersion polymerization. It is found that multicomponent block copolymer nanoassemblies instead of the mixture of the PEG-b-PS nanoassemblies and the P4VP-b-PS nanoparticles are formed under suitable conditions. The possibility how multicomponent block copolymer nanoassemblies are synthesized is discussed, and the evolution of the size and/or morphology of the multicomponent block copolymer nanoassemblies is investigated. copolymer nanoassemblies is obtained,9−37 searching for new and convenient synthesis is still needed. Block copolymer nanoassemblies are not kinetically frozen in the polymerization medium during RAFT dispersion polymerization.38−40 These block copolymer nanoparticles can be fused8,41−51 and reassembled,38−40,52−54 and the aggregation number of block copolymer nanoassemblies is not kept constant in the RAFT dispersion polymerization. Based on this, it is expected that, if the RAFT dispersion polymerization is performed in the presence of presynthesized block copolymer nanoassemblies, the newly formed block copolymer may emerge in the presynthesized block copolymer nanoassemblies to form multicomponent block copolymer nanoassemblies. Following this concern, RAFT dispersion polymerization in the presence of presynthesized block copolymer nanoparticles is performed, and its possibility to synthesize multicomponent nanoassemblies constructed with two block copolymers is discussed. Initially, AB diblock copolymer nanoparticles of poly(4-vinylpyridine)-block-polystyrene (P4VP-b-PS) were prepared by ATRP dispersion polymerization. Then RAFT dispersion polymerization of styrene employing the macroCTA of poly(ethylene glycol) trithiocarbonate (PEG-TTC) in the presence of P4VP-b-PS nanoparticles was performed. It is

1. INTRODUCTION Block copolymer nanoassemblies have attracted wide attention due to their academic interesting and promising applications.1,2 Block copolymer nanoassemblies are usually composed of one block copolymer.3−8 In recent years, block copolymer nanoassemblies composed of two or more than two block copolymers, named multicomponent block copolymer nanoassemblies, have been prepared.9−37 Since they are constructed by two block copolymers, their characteristics combining advantages of two block copolymers are expected. There exist two general strategies to prepare multicomponent block copolymer nanoassemblies.10−37 The first strategy is through comicellization of two or more different block copolymers, such as AB and CB diblock copolymers, in which A and C are solvophilic blocks and B is a solvophobic block, in a selective solvent for A and C blocks.10−31 Following this strategy, besides the targeted multicomponent block copolymer nanoassemblies, nanoassemblies constructed with individual block copolymers are unavoidably formed.18 The second strategy to prepare multicomponent block copolymer nanoassemblies is via RAFT dispersion polymerization simultaneously employing two macromolecular chain transfer agents (macro-CTAs) under polymerization-induced self-assembly (PISA).32−37 This strategy has to meet the requirement that the nucleation of the two block copolymers should take place almost at the same time, and therefore, the degrees of polymerization (DPs) of the two macro-CTAs should be close to each other. Although successful synthesis of multicomponent block © XXXX American Chemical Society

Received: April 29, 2019 Revised: June 1, 2019

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DOI: 10.1021/acs.macromol.9b00879 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

evaporation, dried under reduced pressure at room temperature overnight, and then characterized. 2.3. Synthesis of Multicomponent Block Copolymer Nanoassemblies Constructed with P4VP-b-PS and PEG-b-PS. Herein, a typical synthesis of multicomponent block copolymer nanoassemblies constructed with P4VP49-b-PS197 and PEG45-b-PS (1:1 by molar ratio) was introduced. In a 10 mL Schlenk flask, 0.880 g of the 21.0 wt % dispersion of the P4VP49-b-PS197 nanoassemblies (0.00720 mmol of P4VP49-b-PS197), PEG45-TTC (17.0 mg, 0.00720 mmol), St (0.300 g, 2.88 mmol), and AIBN (0.394 mg, 0.00240 mmol) were dissolved in the 80/20 ethanol/H2O mixture (1.31 g) under [St] 0 /[PEG 45 -TTC] 0 /[AIBN] 0 /[P4VP 49 -b-PS 197 ] 0 = 400:1:0.333:1. After three freeze−pump−thaw cycles, polymerization was performed at 70 °C. At a given time of polymerization, the polymerization was quenched by putting the flask in ice water, and the monomer conversion of styrene was analyzed by UV−vis analysis.56 To detect the nanoparticle morphology, the raw colloidal dispersion was diluted with the 80/20 ethanol/H2O mixture, and a small drop of the diluted colloidal dispersion was deposited onto a piece of copper grid, dried at room temperature, subsequently stained with phosphotungstic acid (PTA) (if necessary), and finally checked by TEM. The average size of block copolymer nanoassemblies was obtained by analyzing more than 100 nanoparticles with the ImageJ software. The resultant dispersion of block copolymer nanoassemblies was dialyzed against ethanol for three days to obtain the block copolymer nanoassemblies of P4VP49-b-PS197/PEG45-b-PS. By changing the molar ratio of [P4VP49-b-PS197]0/[PEG45-TTC]0, block copolymer nanoassemblies with different molar ratios of P4VP49-b-PS197/PEG45-b-PS were obtained. By employing the macro-CTA PEG113-TTC in RAFT dispersion polymerization in the presence of P4VP49-b-PS197 nanoparticles, P4VP49-b-PS197/PEG113-bPS nanoassemblies were also obtained. To make a comparison, the PEG45-b-PS or PEG113-b-PS nanoassemblies were also synthesized via RAFT dispersion polymerization under similar conditions as mentioned above but in the absence of the P4VP49-b-PS197 nanoassemblies.55,57 To collect the block copolymers of P4VP49-b-PS197/PEG45-b-PS for GPC and 1H NMR analysis, the nanoassemblies of P4VP49-b-PS197/ PEG45-b-PS dispersed in ethanol were concentrated with rotary evaporation and dried under reduced pressure at room temperature overnight. To check the synthesized block copolymer nanoassemblies of P4VP49-b-PS197/PEG45-b-PS by 1H NMR, the dispersion of the P4VPb-PS/PEG-b-PS nanoassemblies was concentrated by rotary evaporation under reduced pressure at room temperature, centrifuged (10000 rpm, 15 min), washed with deuterated methanol, and then dispersed in deuterated methanol. After three cycles of dispersion/ centrifugation, the nanoassemblies were monitored by 1H NMR analysis. To make a comparison, block copolymer nanoassemblies of P4VP50-b-PS287/PEG45-b-PS287 were also prepared by RAFT dispersion polymerization employing two macro-CTAs, namely, PEG45TTC and P4VP50-TTC. Details can be found in the Supporting Information. 2.4. Characterization. 1H NMR analysis was conducted on a Bruker Avance III 400 MHz NMR spectrometer. A Waters 600E GPC system was applied for gel permeation chromatography (GPC) analysis in which a series of poly(methyl methacrylate) (PMMA) samples with narrow molar mass distribution were used as the calibration standard and DMF was used as the eluent. TEM observation was carried out on a JEOL 100CX-II electron microscope at 100 kV or a Tecnai G2F20 electron microscope at 200 kV. Dynamic light scattering (DLS) analysis was performed on a NanoZS90 (Malvern) laser light scattering spectrometer using a He−Ne laser at a wavelength of 633 nm at a 90° angle.

found that multicomponent block copolymer nanoassemblies constructed with P4VP-b-PS and poly(ethylene glycol)-blockpolystyrene (PEG-b-PS) instead of the mixture of PEG-b-PS nanoassemblies and P4VP-b-PS nanoparticles are formed under suitable conditions. This present synthesis of multicomponent block copolymer nanoassemblies is different from those reported previously.9−37 It is believed that this method may be a new and valid way to prepare multicomponent block copolymer nanoassemblies.

2. EXPERIMENTAL SECTION 2.1. Materials. (1-Chloroethyl) benzene (PECl, 97%, Heowns, China), 4-vinylpyridine (4-VP, 99%, Heowns, China), and styrene (St, >98%) were distilled under reduced pressure before use. Cuprous chloride (CuCl) was synthesized by cupric chloride and sodium sulfite and used as newly synthesized. Tris(pyridin-2-ylmethyl)amine (TPMA, 99%, Annaiji, China) and tris(2-dimethylaminoethyl)amine (Me6TREN, 99%, Annaiji, China) were used as received. Two macroCTAs of poly(ethylene glycol) trithiocarbonate (PEG-TTC in which TTC represents the RAFT terminal of trithiocarbonate) with different molecular weights, namely, PEG45-TTC and PEG113-TTC, were synthesized through esterification between S-1-dodecyl-S′-(α,α′dimethyl-α″-acetic acid)trithiocarbonate (DDMAT) and poly(ethylene glycol) monomethyl ether (PEG-OH),55 and macro-CTA of poly(4-vinylpyridine) trithiocarbonate (P4VP50-TTC) was synthesized through RAFT polymerization. The synthesis details are shown in Figure S1. 2,2′-Azo-bis-isobutyronitrile (AIBN, >99%, Tianjin Chemical Company, China) was recrystallized from ethanol prior to use. Isopropanol (>99%, Tianjin Chemical Company, China) and ethanol (>99%, Tianjin Chemical Company, China) were used as received, and deionized water was used. All other chemical reagents were used as received unless otherwise specified. 2.2. Synthesis of P4VP-b-PS Nanoassemblies. Initially, the chloric-terminated poly(4-vinylpyridine) macroinitiator for ATRP, that is, P4VP-Cl, was synthesized under [4-VP]0/[PECl]0/[CuCl]0/ [Me6TREN]0 = 80:1:1:1.3. In a 50 mL Schlenk flask, PECl (0.167 g, 1.18 mmol), Me6TREN (0.356 g, 1.55 mmol), 4-VP (10.0 g, 9.51 mmol), the internal standard 1,3,5-trioxane (0.125 g, 1.39 mmol), and isopropanol (10.0 mL) were added. After purging with nitrogen at cool temperature, CuCl (117 mg, 1.18 mmol) was added quickly to the mixture under nitrogen. The mixture was degassed by three freeze−pump−thaw cycles and then placed in an oil bath at 40 °C. After polymerization for 8 h with 61% monomer conversion, which is determined by 1H NMR, the mixture was diluted by dichloromethane and passed through a column of neutral alumina, and the polymer was precipitated in diethyl ether and lastly dried under reduced pressure with 95.8% yield of P4VP49-Cl. Then the P4VP-b-PS nanoassemblies were synthesized by ATRP dispersion polymerization. Herein, a typical ATRP dispersion polymerization under [St]0/[P4VP49-Cl]0/[CuCl]0/[TPMA]0 = 250:1:1:1.2 with 20 wt % solid content was described. In a 500 mL three-neck flask, P4VP49-Cl (6.07 g, 1.15 mmol), St (30.0 g, 288 mmol), and the 80/20 ethanol/H2O mixture (132.5 g) were weighed, and the mixture was purged with nitrogen. In a 50 mL Schlenk flask with the 80/20 ethanol/H2O mixture (15.0 g), TPMA (402 mg, 1.38 mmol) was added, and the mixture was purged with nitrogen, then CuCl (105 mg, 1.15 mmol) was added quickly, and the mixture was stirred until CuCl was totally dissolved. The catalytic solution was added into the three-neck flask via a syringe, the mixture was degassed by three freeze−pump−thaw cycles, and then the flask was placed in an oil bath at 45 °C. The polymerization was quenched after 25 h, and 79% monomer conversion was evaluated by UV−vis as discussed elsewhere.56 The resultant P4VP49-b-PS197 nanoassemblies were dialyzed against the 80/20 ethanol/H2O mixture for three days and were kept at room temperature with polymer concentration at 21.0 wt % for next use. To collect P4VP49-b-PS197 for GPC and NMR analysis, a portion of the colloidal dispersion was concentrated with rotary

3. RESULTS AND DISCUSSION 3.1. Synthesis of P4VP-b-PS Nanoassemblies. The P4VP-b-PS nanoassemblies were synthesized by ATRP B

DOI: 10.1021/acs.macromol.9b00879 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of P4VP49-Cl and P4VP49-b-PS197 by ATRP Polymerization

polymerization.60 Hence, herein, the P4VP49-b-PS197 nanoassemblies are synthesized by ATRP and used in the subsequent RAFT polymerization. 3.2. RAFT Dispersion Polymerization in the Presence of P4VP-b-PS Nanoassemblies. RAFT dispersion polymerization employing the macro-CTA PEG45-TTC in the 80/20 ethanol/H2O mixture in the presence of P4VP49-b-PS197 nanospheres was investigated. Herein, by balancing the evolution of the block copolymer morphology and polymerization, this polymerization medium consisting of the 80/20 ethanol/H2O mixture containing a suitable amount of water was used since the solvent is found to be a key parameter to determine polymerization rate and block copolymer morphology.61,62 Under this condition, four possibilities, such as (1) the mixture of block copolymer nanoassembles constructed with individual block copolymers of P4VP49-b-PS197 and PEG45-b-PS, (2) the mixture of P4VP49-b-PS197 nanospheres and core−shell nanoassemblies constructed with P4VP49-bPS197 and PEG45-b-PS, (3) the mixture of PEG45-b-PS nanoassemblies and core−shell nanoassemblies constructed with P4VP49-b-PS197 and PEG45-b-PS, and (4) multicomponent nanoassemblies constructed with P4VP49-b-PS197 and PEG45-b-PS (Scheme 2), are expected. For the sake of

dispersion polymerization as shown in Scheme 1. Initially, the poly(4-vinylpyridine) macroinitiator P4VP-Cl was synthesized by ATRP polymerization of 4-VP employing PECl as the initiator and Me6TREN/CuCl as the catalyst. P4VP-Cl was synthesized by targeting the degree of polymerization (DP) at 80. After 8 h of polymerization at 40 °C with 61% monomer conversion, P4VP-Cl with a theoretical DP of 49 was obtained. The synthesized P4VP49-Cl was characterized by 1H NMR and GPC analysis (Figure S2). From the 1H NMR spectra, the molecular weight Mn,NMR (4.41 kg/mol) can be evaluated via comparing the area ratio at δ = 3.49 (a) and δ = 8.06−8.65 ppm (c), which is close to the theoretical molecular weight Mn,th of 5.27 kg/mol calculated following eq S1. GPC analysis indicates a much larger molecular weight Mn,GPC of 16.7 kg/ mol with a dispersion Đ of 1.15. The overestimated Mn,GPC is ascribed to the residue of the Cu catalyst and the PMMA standards employed in the GPC analysis.58 Employing P4VP49-Cl as the macroinitiator in ATRP dispersion polymerization, the P4VP49-b-PS197 nanoassemblies were synthesized by targeting DP of the PS block at 250 with 79% monomer conversion in 25 h. The synthesized P4VP49-bPS197 was characterized by NMR (Mn,NMR = 25.2 kg/mol, estimated by comparing the area ratio at δ =3.49 and δ = 6.21−7.25 ppm) and GPC (Mn,GPC = 173 kg/mol, Đ = 1.33) as shown in Figure S2. It is found that Mn,NMR is very close to the theoretical molecular weight (Mn,th = 25.7 kg/mol). The overestimation of Mn,GPC is just as similar as discussed above. Figure 1 shows the TEM images of the P4VP49-b-PS197

Scheme 2. Schematic Structure of Four Possible Block Copolymer Nanoassemblies Synthesized via RAFT Dispersion Polymerization in the Presence of P4VP49-bPS197 Nanospheres

Figure 1. TEM images of P4VP49-b-PS197 nanoassemblies: unstained (A) and stained by PTA (B), and their schematic structures (C).

simplicity, RAFT dispersion polymerization in the 80/20 ethanol/H2O mixture in the presence of P4VP49-b-PS197 nanoparticles with a molar ratio of PEG45-b-PS/P4VP49-bPS197 of 1:1 by targeting the DP of the PS block in PEG45-b-PS at 400 was first investigated. After 16 h of polymerization with 89% monomer conversion, the synthesized block copolymer nanoassemblies were checked. Figure 2A,B shows the TEM images of the synthesized P4VP49-b-PS197/PEG45-b-PS354 nanoassemblies before and after staining with PTA, indicating formation of 70 nm nanospheres. These nanospheres have uniform diameter distribution, larger diameter (70 vs 40 nm), and larger hydrodynamic diameter Dh (130 vs 70 nm, Figure S3) than the P4VP49-b-PS197 nanospheres (Figure 1). (See the characterization of block copolymers in Figure S4.) The stained nanospheres have a black periphery, which is assigned to P4VP,63 so it is concluded that multicomponent nanoassemblies constructed with P4VP49-b-PS197 and PEG45-b-PS354 as shown in Scheme 2 are formed. Furthermore, a control

nanoassemblies before (Figure 1A) and after staining with PTA (Figure 1B). P4VP49-b-PS197 nanospheres with a diameter of 40 nm are formed, and the TEM images of the stained nanospheres show a black periphery assigned to P4VP and a white-gray inner assigned to PS, confirming a core−shell structure of the P4VP49-b-PS197 nanospheres as schematized in Figure 1C. Herein, it is noted that P4VP-b-PS nanoassemblies were synthesized by RAFT dispersion polymerization previously.59 However, these P4VP-b-PS nanoassemblies synthesized by RAFT polymerization are not used in the present study since the PS block can be further extended in the subsequent RAFT polymerization and therefore makes things complex. The removal of RAFT terminals of trithiocarbonate in the P4VP-bPS nanoassemblies with hydrazine is also tried. However, these P4VP-b-PS nanoassemblies after converting the RAFT terminal into thiol can decelerate the subsequent RAFT C

DOI: 10.1021/acs.macromol.9b00879 Macromolecules XXXX, XXX, XXX−XXX

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multicomponent block copolymer nanoassemblies as shown in Scheme 2 are formed in the present RAFT dispersion polymerization. To further study the process of RAFT dispersion polymerization in the presence of the P4VP49-b-PS197 nanospheres, the polymerization kinetics and the synthesized nanoassemblies were checked. To make a comparison, RAFT dispersion polymerization in the absence of the P4VP49-b-PS197 nanospheres was also studied (Figures S5 and S6). As shown in Figure 3A, the polymerization in the presence of P4VP49-bPS197 nanospheres was very slow in the initial 6 h, which is even slower than that in the absence of the P4VP49-b-PS197 nanospheres (Figure S5A). It is thought that this latent period is helpful for synthesis of multicomponent block copolymer nanoassemblies since this latent period is essential for the reassembly of the PEG45-TTC macro-CTA and the P4VP49-bPS197 nanospheres. Control experiments of heating the mixture of the P4VP49-b-PS197 nanospheres in 80/20 ethanol/H2O in the presence of toluene (which is somewhat similar to styrene) and in the presence of toluene and PEG45-TTC at 70 °C for 6 h indicate a Dh change in the presence of PEG45-TTC (Figure S7), which suggests a possible reassembly of PEG45-TTC and the P4VP49-b-PS197 nanospheres. After this latent period, monomer conversion increases quickly and the RAFT polymerization follows a pseudo-first-order kinetics as indicated by the linear ln([M]0/[M])−time plot (Figure 3A). The P4VP49-b-PS197/PEG45-b-PS mixture at different polymerization times was analyzed by GPC (Figure 3B) from which the peak assigned to P4VP49-b-PS197 at a constant eluent time of 20.5 min and a peak assigned to PEG45-b-PS with a decreasing eluent time with monomer conversion increasing are observed. Since the two peaks are not overlapped, the peak

Figure 2. TEM images of P4VP49-b-PS197/PEG45-b-PS354 nanoassemblies with a molar ratio of P4VP49-b-PS197/PEG45-b-PS354 of 1:1 before (A) and after staining by PTA (B), and TEM images (C) of PEG45-b-PS349 nanoassemblies. Note that PEG-b-PS is briefly named ES, in which E represents PEG and S represents PS, and P4VP-b-PS is briefly named VS, in which V represents P4VP and S represents PS.

experiment of RAFT dispersion polymerization in the absence of P4VP49-b-PS197 nanospheres under other similar conditions was performed (Figures S5 and S6), and the PEG45-b-PS349 vesicles as shown in Figure 2C are formed. This confirms the formation of P4VP49-b-PS197/PEG45-b-PS354 multicomponent nanoassemblies, and the other three possibilities to form nonergodic block copolymer nanoassemblies are excluded. The formation of P4VP49-b-PS197/PEG45-b-PS354 multicomponent nanoassemblies is also confirmed by 1H NMR analysis of the nanoassemblies dispersed in deuterated methanol. By comparing the chemical shifts at 3.65 ppm assigned to the PEG45 block and at 6.40−7.20 ppm assigned to the P4VP49 block in Figure S4C, a molar ratio of 1.04:1, which is very close to the values in the fed constituents (1:1), is obtained. As known,64 if a soluble polymer is shielded by a solvophobic one, the signals become feeble. This suggests that

Figure 3. Polymerization kinetics of the RAFT dispersion polymerization in the presence of P4VP-b-PS nanoassemblies (A), GPC traces (B), 1H NMR spectrums (C), molecular weight and Đ (D) of the P4VP49-b-PS197/PEG45-b-PS mixture. Polymerization conditions: [St]0/[PEG45-TTC]0/ [AIBN]0/[V49S197] = 400:1:0.333:1, 80/20 ethanol/H2O mixture, 20 wt % solid content, 70 °C. D

DOI: 10.1021/acs.macromol.9b00879 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. TEM images of P4VP49-b-PS197/PEG45-b-PS nanoassemblies synthesized via RAFT dispersion polymerization at times of 0 (A), 3 (B), 8 (C), 11 (D), 15 (E), and 20 h (F). Average diameters of P4VP49-b-PS197/PEG45-b-PS nanoassemblies at different polymerization times (G) and the chain aggregation numbers Nagg of the P4VP49-b-PS197/PEG45-b-PS nanoassemblies.

top molecular weight of PEG45-b-PS (Mp,GPC) and its Đ can be obtained, and the results are summarized in Figure 3D. Note that Đ may be somewhat underestimated due to overlapping in the GPC traces. The P4VP49-b-PS197/PEG45-b-PS mixture was further characterized by NMR (Figure 3C) from which the molecular weight Mn,NMR is calculated by comparing the chemical shifts at 3.65 and 6.21−7.21 ppm. It is found that the three cases of molecular weights of PEG45-b-PS, namely, Mp,GPC, Mn,NMR, and Mn,th, are very close to each other and increase linearly with monomer conversion, suggesting a controllable synthesis of PEG45-b-PS in the RAFT dispersion polymerization. Figure 4A−F shows the TEM images of the P4VP49-b-PS197/ PEG45-b-PS nanoassemblies stained by PTA in which the DP of the PS block in PEG45-b-PS is also pointed out as insets. It is found that uniform nanospheres of P4VP49-b-PS197/PEG45-bPS are formed and the diameter gradually increases from 40 to 70 nm with the increasing DP of the PS block in PEG45-b-PS

(Figure 4G). Also, all PTA-stained nanospheres of P4VP49-bPS197/PEG45-b-PS have a black periphery, indicating P4VP located in the nanosphere surface and therefore formation of P4VP49-b-PS197/PEG45-b-PS multicomponent nanoassemblies. Besides, to confirm formation of multicomponent nanoassemblies, RAFT dispersion polymerization in the absence of P4VP49-b-PS197 nanospheres was performed under other similar conditions and the PEG45-b-PS vesicles were formed (Figure S6). All these results confirm the formation of P4VP49b-PS197/PEG45-b-PS multicomponent nanospheres. Nagg =

ρπD3NA 3MSt(197 + DP)

(1)

The aggregation number of polymer chains including P4VP49-b-PS197 and PEG45-b-PS (Nagg) in single multicomponent block copolymer nanospheres is calculated by eq 1,65 in which ρ is the density of PS, D is the diameter of nanospheres, NA is Avogadro’s constant, MSt is the molecular E

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Macromolecules weight of styrene, and DP represents DP of the PS block in PEG45-b-PS. As shown in Figure 4H, Nagg slightly increases to about 2200 in the initial 6 h, then it undergoes a 2-fold increase in the subsequent 2 h, and it almost remains constant at about 4000 until the end of the RAFT dispersion polymerization with 93% monomer conversion in 20 h. It is thought that, in the initial stage, the PS block in PEG45-b-PS has a relatively short chain length and is soluble in the solvent and the synthesized PEG45-b-PS and the residual monomer can act as plasticizers for the P4VP49-b-PS197 nanospheres; under this condition, the P4VP49-b-PS197 nanospheres are not kinetically frozen in the polymerization solvent, and therefore, chain exchange between the soluble PEG45-b-PS and P4VP49-bPS197 nanospheres can take place.66−69 Since the PS block in PEG45-b-PS is much shorter than those in the P4VP49-b-PS197 nanospheres, the chain exchange can make the PS block less stretched in nanospheres, which therefore can make the nanospheres to host more polymer chains and leads to a slight increase in Nagg. With monomer conversion in the subsequent 2 h, Nagg is almost doubled, indicating nanosphere−nanosphere fusion in this stage. In synthesis of block copolymer nanoassemblies via RAFT dispersion polymerization under PISA conditions, nanoparticle−nanoparticle fusion usually occurs, and this fusion usually leads to the micelle-to-vesicle transition.42−44,47−50 In the present study, this fusion as well as the increasing DP of the PS block in PEG45-b-PS just leads to an increasing size of the P4VP49-b-PS197/PEG45-b-PS multicomponent nanoassemblies but no morphology change. After 8 h of polymerization with 28% monomer conversion, Nagg almost remains constant, although the size of the P4VP49-bPS197/PEG45-b-PS multicomponent nanoassemblies further increases. This suggests that, at this stage of high DP of the PS block at high monomer conversion, the P4VP49-b-PS197/ PEG45-b-PS multicomponent nanoassemblies are kinetically frozen in the RAFT dispersion polymerization. To make a comparison, 40 nm P4VP50-b-PS287/PEG45-bPS287 multicomponent nanoassemblies (Figure S8) were also prepared by RAFT dispersion polymerization employing two macro-CTAs.32−38 The prepared P4VP50-b-PS287/PEG45-bPS287 nanoassemblies are smaller than the P4VP49-b-PS197/ PEG45-b-PS112 (57 nm) and P4VP49-b-PS197/PEG45-b-PS276 (66 nm) nanospheres prepared by RAFT dispersion polymerization in the presence of P4VP49-b-PS7 nanospheres (Figure 4C,D), and even the DP of the PS block in the former is larger. This possibly indicates that P4VP49-b-PS197/PEG45-b-PS multicomponent nanoassemblies in the latter case are kinetically frozen as discussed above. 3.3. Synthesis of Multicomponent Nanoassemblies with Different Chemical Compositions. The P4VP49-bPS197/PEG45-b-PS multicomponent nanoassemblies with different molar ratios of P4VP49-b-PS197/PEG45-b-PS were also synthesized by RAFT dispersion polymerization in the presence of the P4VP49-b-PS197 nanospheres employing the PEG 45 -TTC macro-CTA under [St] 0 /[PEG 45 -TTC] 0 / [AIBN]0/[P4VP49-b-PS197]0 = 400:1:0.333:x. By quenching the RAFT dispersion polymerization at high monomer conversions of 92−99% (Table S1), the P4VP49-b-PS197/ PEG45-b-PS multicomponent nanoassemblies as shown in Figure 5 and Figures S9 and S10 were synthesized. The block copolymers were characterized by GPC and NMR, and the results are summarized in Figure S11. Figure 5 summarizes the P4VP49-b-PS197/PEG45-b-PS multicomponent nanoassemblies with different PEG45-b-PS frac-

Figure 5. Effect of the PEG45-b-PS fraction on the size of P4VP49-bPS197/PEG45-b-PS nanoassemblies. Insets: the TEM images and schematic structures of P4VP49-b-PS197/PEG45-b-PS nanoassemblies. Scale bar: 50 nm.

tions. Note that the DP of the PS block in PEG45-b-PS is slightly different. It is found that the size of P4VP49-b-PS197/ PEG45-b-PS multicomponent nanospheres slightly increases with the PEG45-b-PS fraction until formation of the PEG45-bPS375 vesicles. It is also noticed that uniform P4VP49-b-PS197/ PEG45-b-PS multicomponent nanospheres are formed in the case of the low PEG45-b-PS fraction, and with the PEG45-b-PS fraction increasing, some small-sized nanospheres even smaller than the P4VP49-b-PS197 nanospheres are formed and the size distribution of the multicomponent nanospheres becomes broad (Figure S10). Besides, at a high PEG45-b-PS fraction, a small amount of small-sized vesicles are formed (Figure S9E). This suggests that, in the case of the low PEG45-b-PS fraction, the morphology of P4VP49-b-PS197/PEG45-b-PS multicomponent nanospheres is inherited more from the P4VP49-b-PS197 nanospheres than the PEG45-b-PS nanoassemblies, and in the case of the high PEG45-b-PS fraction, PEG45-b-PS contributes more to the morphology of the P4VP49-b-PS197/PEG45-b-PS multicomponent nanoassemblies. In the P4VP49-b-PS197/PEG45-b-PS multicomponent nanoassemblies, the DPs of the solvophilic P4VP49 and PEG45 blocks are very close to each other. The P4VP49-b-PS197/ PEG113-b-PS370 multicomponent nanoassemblies with different solvophilic blocks of P4VP49 and PEG113 and different solvophobic blocks of PS197 and PS370 were also prepared by RAFT dispersion polymerization employing the PEG113-TTC macro-CTA by targeting the DP of the PS block in PEG113-bPS at 400 (Figure 6A). (See the characterization of the block copolymers in Figure S12.) It is found that these P4VP49-bPS197/PEG113-b-PS370 multicomponent nanoassemblies are nanospheres with a uniform diameter of 75 nm, and they are bigger than the PEG113-b-PS375 nanospheres (25 nm, Figure 6B) and the P4VP49-b-PS197 nanospheres (40 nm, Figure 6C) constructed with single block copolymers. It is thought that the great DP difference between the P4VP49 and PEG113 blocks and between the PS197 and PS370 blocks in the P4VP49-b-PS197/ PEG113-b-PS370 multicomponent nanoassemblies can decrease the steric repulsion, and these multicomponent nanoassemblies can host more polymer chains and therefore have larger sizes than those constructed with single block copolymers, although the exact reason needs further study. F

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Macromolecules

of China (2016YFA0202503), and the Science and Technology Commission Foundation of Tianjin (No. 15JCZDJC40800) is gratefully acknowledged.

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Figure 6. TEM images of P4VP49-b-PS197/PEG113-b-PS370 nanoassemblies (A) prepared under [St]0/[PEG113-TTC]0/[AIBN]0/ [V49S197] = 400:1:0.333:1, 70 °C, and 20 wt % in 80/20 ethanol/ H2O, PEG113-b-PS375 nanoassemblies (B), and P4VP49-b-PS197 nanoassemblies (C). Note: unstained nanoassemblies (A1, B1, C1), the schematic structures of nanoassemblies (A2, B2, C2), and P4VP49b-PS197/PEG113-b-PS370 nanospheres stained by PTA (A3).

4. CONCLUSIONS Multicomponent block copolymer nanoparticles constructed with P4VP-b-PS and PEG-b-PS are synthesized by RAFT dispersion polymerization of styrene employing PEG-TTC macro-CTA in the presence of presynthesized P4VP-b-PS nanoassemblies. It is found that multicomponent block copolymer nanoassemblies instead of the mixture of the PEG-b-PS nanoassemblies and the P4VP-b-PS nanoparticles were formed under suitable conditions. The RAFT dispersion polymerization undergoes an initial latent period, and then it is accelerated until above 90% monomer conversion in about 20 h; the size of multicomponent block copolymer nanoassemblies slightly increases initially, and then nanoparticle− nanoparticle fusion occurs to form large-sized multicomponent block copolymer nanoassemblies. By tuning the ratio of PEGTTC macro-CTA to P4VP-b-PS nanoparticles, the chain length of the PEG-TTC macro-CTA, and/or the DP of the PS block in PEG-b-PS, P4VP-b-PS/PEG-b-PS multicomponent nanoassemblies with different chemical compositions can be obtained. It is believed that this method may be a new and valid way to prepare multicomponent block copolymer nanoassemblies.

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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.9b00879. Experimental details and characterization data (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.D.). *E-mail: [email protected]. Tel: 0086-22-23509794 (W.Z.). ORCID

Wangqing Zhang: 0000-0003-2005-6856 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the National Science Foundation for Distinguished Young Scholars (No. 21525419), the Ministry of Science and Technology of the People’s Republic G

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