Synthesis of Multicompartment Nanoparticles of ABC Miktoarm Star

Letter Cite This: ACS Macro Lett. 2019, 8, 783−788

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Synthesis of Multicompartment Nanoparticles of ABC Miktoarm Star Polymers by Seeded RAFT Dispersion Polymerization Shenzhen Li,†,‡ Huijun Nie,†,‡ Song Gu,‡ Zhongqiang Han,§ Guang Han,*,§ 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, Nankai University, Tianjin 300071, China § State Key Laboratory of Special Functional Waterproof Materials, Beijing Oriental Yuhong Waterproof Technology Co., Ltd, Beijing 100123, China ∥ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: Polymeric multicompartment nanoparticles (MCNs) of μ-ABC miktoarm star polymers composed of poly(N,Ndimethylacrylamide) (PDMA), poly(butyl methacrylate) (PBMA), and polystyrene (PS) were synthesized by Cu(I)-catalyzed click reaction and seeded RAFT dispersion polymerization. The synthesized MCNs have a solvophobic PBMA core with separate segregated PS microdomains and a solvophilic PDMA corona. The size and/or morphology of MCNs are correlative to the length of PDMA, PBMA, and PS segments. Ascribed to the characteristic structure, MCNs of μ-DBS can decrease interfacial tension in n-hexane/water, which is much superior to linear diblock copolymer nanoassemblies.

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considerable challenge of synthetic skill. At this point, research about valid methods to prepare ABC miktoarm star terpolymers and their self-assembled MCNs is needed. Up to now, some MCNs of ABC miktoarm star terpolymers have been prepared via micellization in the selective solvent for the A segment.28−30 However, this strategy has an intrinsical shortcoming that just much diluted MCNs can be prepared. Recently, the strategy of polymerization-induced self-assembly (PISA), especially via RAFT polymerization under heterogeneous conditions, is demonstrated to be a valid method to synthesize block copolymer nanoassemblies.31−35 Herein, MCNs of μ-ABC miktoarm star polymers composed of poly(N,N-dimethylacrylamide) (PDMA), poly(butyl methacrylate) (PBMA), and polystyrene (PS), μ-PDMA-PBMA-PS were synthesized by combining the click reaction of Cu(I)catalyzed Huisgen cycloaddition36−39 and seeded RAFT dispersion polymerization, as illustrated in Scheme 1. For the sake of briefness, D, B, and S stand for PDMA, PBMA, and PS,

olymeric multicompartment nanoparticles (MCNs) consisting of a microphase-segregated core and a solvophilic corona have recently gained much attention1−5 due to their special structure and promising applications. Generally, strong microphase-segregation of constituents in MCNs is strictly required for synthesis of these nanoparticles.6−9 Consequently, linear ABC triblock copolymers, including a fluorinated block,10−15 ABC miktoarm star terpolymers (usually abbreviated as μ-star polymers),16−21 and ABCA or ABCBA multiblock copolymers,22−25 are usually employed, in which A represents the solvophilic block and B and C represent the incompatible solvophobic blocks, respectively. In these polymers, ABC miktoarm star terpolymers, with a character that all the A, B, and C segments are connected at one point, are the most employed,26,27 since this structure is helpful for microphase-segregation between the solvophobic B and C blocks. However, synthesis of miktoarm star polymers, whether in synthetic route or experimental operation, is challenging compared to that of linear polymers. In the synthesis of miktoarm star polymers, choice of synthetic strategy is severely constrained by the structure of the desired star polymers, and therefore, searching for a feasible synthetic protocol is a © 2019 American Chemical Society

Received: May 16, 2019 Accepted: June 10, 2019 Published: June 13, 2019 783

DOI: 10.1021/acsmacrolett.9b00371 ACS Macro Lett. 2019, 8, 783−788

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ACS Macro Letters

drin, ring-opening of epoxide group with sodium azide, and finally esterification of the hydroxyl group with the carboxyl group in S-1-dodecyl-S′-(α,α′-dimethyl-α′′-acetic acid) trithiocarbonate (DDMAT) to yield a bifunctional macroinitiator of PDMA100-TTC-N3 (Mn,GPC = 10.2 kg/mol, Mn,NMR = 10.6 kg/ mol, Đ = 1.12, seeing 1H NMR and GPC in Figure 1 and the calculation in Supporting Information). The complete removal of the dithiobenzoate terminal in PDMA100 was characterized by 1H NMR, as shown in Figure S1. The ring-opening reaction of epoxide group was evidenced by FT-IR analysis (Figure S2), which was confirmed by the characteristic absorption of azido group at 2106 cm−1.40,41 During such transformations, no change of the main chain of the PDMA segment occurred, as shown in 1H NMR spectra (Figure S3A) and the GPC traces (Figure S3B). This successive modification leads to PDMA100TTC-N3, in which the azido moiety is to link PBMA via click reaction to form DB diblock copolymer and the trithiocarbonate moiety is to initiate the PS segment to form μ-DBS, respectively. Then, PBMA with an alkynyl terminal, PBMA100CCH, was synthesized by ATRP polymerization using 3butynyl 2-bromoisobutyrate as initiator (Mn,GPC = 19.2 kg/mol, Mn,NMR = 15.2 kg/mol, Đ = 1.06, Figures 1 and S4). Next, the D100B100 diblock copolymer (Mn,GPC = 28.2 kg/mol, Mn,NMR = 26.7 kg/mol, Đ = 1.07) was synthesized by a CuBr-catalyzed click coupling between PDMA100-TTC-N3 and PBMA100-C CH. The Cu(I)-catalyzed click reaction 42,43 between PDMA100-TTC-N3 and PBMA100-CCH was investigated by 1H NMR spectra (Figure S5A) and FT-IR (Figure S5B). The azide peak for PDMA100-TTC-N3 shows a strong absorption band at 2106 cm−1, which disappears after alkyne−azide coupling, indicating successful synthesis of D100B100. Moreover, the GPC elution of D100B100 shifts to a lower elution time in comparison with those of their precursors of PDMA100-TTC-N3 and PBMA100-CCH, further confirming synthesis of D100B100 (Figure 1B and Table S1), though a very small shoulder peak possibly assigned to the unreacted

Scheme 1. Synthetic Route of PDMA-TTC-N3 (A), PBMACCH (B), and MCNs of μ-DBS (C)

and the PDMA-b-PBMA diblock copolymer and μ-PDMAPBMA-PS miktoarm star terpolymer are denoted as DB and μDBS in the subsequent discussion, respectively. Initially, PDMA containing functional moieties of trithiocarbonate (TTC) and azido group, PDMA100-TTC-N3 for example, was synthesized via solution RAFT polymerization using cumyl dithiobenzoate (CDB) as chain transfer agent and AIBN as initiator in toluene at 60 °C, and then successive modification via removal of the dithiobenzoate terminal by hydrazinolysis, etherification of thiol group with epichlorohy-

Figure 1. (A) 1H NMR spectra of PDMA100-TTC-N3, PBMA100-CCH, D100B100, and μ-D100B100S90; (B) GPC traces of PDMA100-TTC-N3, PBMA100-CCH, D100B100, and μ-D100B100S90; (C) DSC thermograms of PBMA100-CCH, PS100-TTC, PDMA100-TTC-N3, D100B100, and μD100B100S90 under nitrogen at a heating rate of 10 °C/min in the range of 0 to 150 °C. 784


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inactive St monomer and the complex structure of μ-DBS, this is acceptable and further improvement will be performed in the future. From the differential scanning calorimetry (DSC) analysis (Figures 1C and S11) of μ-D100B100S90, as well as its precursors, two glass transition temperature Tg values, 31.2 °C corresponding to PBMA48 and 105.9 °C corresponding to PDMA and PS49 (note: their Tgs are too close to be distinguished) in μ-D100B100S90, are detected. The incompatibility between B and S favors phase separation within the core and therefore the formation of MCNs in the seeded RAFT dispersion polymerization. The μ-D100B100S90 nanoparticles have a Dh centered at 148 ± 3 nm, which is slightly larger than the Dh of D100B100 micelles at 94 ± 8 nm and much larger than the reference D100S200 micelles (Figure 2A). The 1H NMR analysis of μ-D100B100S90 nanoparticles indicates a hydrated D segment in the μD100B100S90 nanoparticles in D2O and B and S segments are dehydrated and undetectable (Figure S12). TEM images indicate spherical morphology of the μ-D100B100S90 nanoparticles with a diameter D of 42 nm (Figure 2B). The particle size D by TEM is usually smaller than Dh by DLS, since the former shows the dry particles and the latter shows the solvated particles.50 The μ-DBS nanoparticles were further stained with uranyl acetate and then checked by TEM. As shown in Figure S13, the dark patches on the particle surface are assigned to PS microdomains, and the light area is assigned to PBMA, respectively. Herein, the hydrated PDMA corona of the stained μ-D100B100S90 nanoparticles is also discerned as undertone surrounding on the periphery of the nanoparticles, although direct imaging of the solvated corona of block copolymer micelles is usually difficult.51 Besides, with the degree of polymerization (DP) of the PS block increasing, the PS microdomains in the μ-DBS nanoparticles become dense. The segregated surface structure of MCNs of μ-D100B100S90 was also confirmed by SEM, and some protrusions on the surface of the nanoparticles can be clearly observed (Figure 2C,D), which demonstrate a different surface morphology with the D100B100 micelles and validated a raspberry-like structure of μ-D100B100S90 MCNs. According to the TEM and SEM analyses above, a schematic structure of MCNs of μD100B100S90 is outlined in Figure 2E. Interfacial tension (IFT) of MCNs of μ-D100B100S90 in the nhexane/water interface at 15 °C was checked. In MCNs of μD100B100S90, D is soluble in water but insoluble in n-hexane, B is insoluble in water but soluble in n-hexane, and S is insoluble both in water and n-hexane, respectively. Distribution of MCNs of μ-D100B100S90 in the n-hexane/water interface can be schematically shown as Figure 3A. The unique amphiphilicity of μ-D100B100S90 MCNs leads them to rearrange at the interface, and during this rearrangement, the n-hexane/water interfacial tension (IFT) is determined by the pendant drop method (Figure S14).52 The variation of IFT with elapsed time reflects the change of the n-hexane/water interface caused by the rearrangement of the μ-D100B100S90 MCNs until it reaches to equilibrium. As shown in Figure 3B, MCNs of μ-D100B100S90 decrease IFT much more than the linear diblock copolymer nanoparticles of D100S200 and D100B100, for example, 50.2 mN m−1 in the case of no nanoparticles, 30.1 mN m−1 at case of 0.10 wt % D100S200, 25.2 mN m−1 in the case of 0.10 wt % D100B100, and 24.0 mN m−1 in the case of 0.10 wt % μD100B100S90 MCNs. Note: see the synthesis and characterized of nanoparticles of D100S200 in Supporting Information (Scheme S1 and Table S1). The interfacial behavior of μ-

homopolymers is discerned. D100B100 micelles were prepared by direct dissolution of D100B100 in an 80/20 ethanol/water mixture, since the PDMA block is solvophilic and the PBMA block is solvophobic. This solvent of the 80/20 ethanol/water mixture, in which the third monomer of styrene is soluble and the synthesized polymer of PS is insoluble, is used, since it meets the requirement to prepare MCNs of μ-DBS under PISA formulation.44,45 These D100B100 micelles have a hydrodynamic diameter (Dh) at 94 ± 8 nm (Figures 2A and S6), and the

Figure 2. (A) Hydrodynamic diameter Dh of D100B100 micelles (blue), the μ-D100B100S90 nanoparticles (black) and D100S200 micelles (red) dispersed in water at 15 °C; (B) TEM image of μ-D100B100S90 nanoparticles (unstrained); the SEM image (C), detailed surface structures (D) and the proposed structure (E) of μ-D100B100S90 nanoparticles.

SEM images indicate spherical morphology of D100B100 micelles with a diameter D of about 30 nm (Figure S7). In this dispersion of D100B100 micelles, the third monomer of styrene and AIBN were added to keep [St]0/[D100B100]0/ [AIBN]0 = 300:3:1 and seeded RAFT dispersion polymerization was performed. Herein, seeded RAFT dispersion polymerization is named, since D100B100 micelles act as macromolecular chain transfer agent (macro-CTA) in the dispersion polymerization.46,47 After 24 h polymerization with 89.6% conversion of styrene, μ-D100B100S90 nanoparticles with 10% concentration were obtained. To make a comparison, the reference D100S200 nanoparticles were also synthesized. Note: see details in Scheme S1 and Figure S8. The μ-D100B100S90 miktoarm star terpolymer was characterized by 1H NMR (Figures 1A and S9) and GPC analysis (Figures 1B and S10). As shown in Table S1, the NMR determined molecular weight of μ-DBS, Mn,NMR, is close to the theoretical value determined by eq S1 and is a little smaller than those of Mn,GPC by GPC analysis via RI detector relative to PS standards. Herein, the Đ of μ-DBS seems a little high. Concerning the dispersed condition for polymerization and the 785


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Figure 4. Average nanoparticle diameter D measured by TEM and Dh by DLS of MCNs of μ-DBS and the interfacial tension (IFT) in nhexane/water: (A) MCNs of μ-DBS with different DP of B and (B) MCNs of μ-DBS with different DP of S.

Figure 3. (A) Schematic representation of different behaviors of the μ-DBS nanoparticles in the n-hexane/water interface at 15 °C; (B) the n-hexane/water interfacial tension in the presence of 0.10 wt % of D100S200, D100B100, and μ-D100B100S90 nanoparticles.

In a short summary, MCNs of μ-DBS were synthesized by combining Cu(I)-catalyzed click reaction and seeded RAFT dispersion polymerization. Although the synthesis of μ-DBS seems laborious, seeded RAFT dispersion polymerization affords MCNs of μ-DBS with well-segregated S microdomains and with polymer concentration competitive to linear ones. The MCNs of μ-DBS can decrease interfacial tension in nhexane/water, which is much superior to linear diblock copolymer nanoassemblies. We hope that our study can afford a promising strategy to synthesize μ-ABC miktoarm star polymers and helps to discover the structure−property relationship for miktoarm star polymers.

D100B100S90 MCNs, such as the shorter time to reach equilibrium IFT and the smaller minimum IFT value than D100S200 and D100B100 micelles, suggests their stable interfacial structure, and indicates that μ-D100B100S90 MCNs can act as a more efficient emulsifier than linear ones. Following the procedures shown in Scheme 1, μ-DBS nanoparticles with different DPs were prepared (Table S1). It is found that the size of μ-DBS nanoparticles, Dh by DLS and D by TEM and/or SEM, decreases with the DP of B and increases with DP of S or D (Figures S15−S18), respectively. Similarity was also found in MCNs of μ-PEE-PEO-PFPO53 and μ-PEG-PS-PCL.54 The size of μ-DBS nanoparticles increasing with DP of the hydrophilic D and decreasing with the DP of hydrophobic B seems a little abnormal, since amphiphilic diblock copolymers tends to form small-sized micelles at case of a long solvophilic block and to form largesized micelles at case of a long solvophobic block, respectively.55 Herein, it is found that the size of the seed DB micelles follows the order of D100B50 > D100B150 > D100B100 (Figure S7). It is thought that the seed DB micelles are highly swollen in the 80/20 ethanol/water mixture, which leads to the abnormal order of the DB micelles and therefore the μ-DBS nanoparticles. IFT of these MCNs in n-hexane/water deceases with DP of B, and the DP of S exerts almost no influence on IFT (Figure 4A and Table S2). This is not surprising, since S is insoluble in n-hexane or water and B in MCNs is soluble in nhexane to decrease IFT of n-hexane/water, as shown in Figure 3A.

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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/acsmacrolett.9b00371. Experimental details and supplementary figures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wangqing Zhang: 0000-0003-2005-6856 Author Contributions †

Equal contribution from S.L. and H.N.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support by the National Science Foundation for Distinguished Young Scholars (No. 21525419) and the Ministry of Science and Technology of the People’s Republic of China (2016YFA0202503) and PCSIRT (IRT1257) is gratefully acknowledged.

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