Star Block Copolymer Nanoassemblies: Block Sequence is All

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Star Block Copolymer Nanoassemblies: Block Sequence is AllImportant Yuan Zhang,† Tianyun Guan,† Guang Han,*,‡ Tianying Guo,† and Wangqing Zhang*,†,§ †

Downloaded via LUND UNIV on January 1, 2019 at 13:29:38 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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: Star and linear block copolymers of [poly(4vinylpyridine)-block-polystyrene]n [(P4VP-b-PS)n] and [polystyrene-block-poly(4-vinylpyridine)]n [(PS-b-P4VP)n] (n = 1−4) with similar chemical composition but different block sequence were synthesized by RAFT polymerization. Star (P4VP-b-PS)n is composed of a jointed P4VP core and several outer PS arms, and (PS-b-P4VP)n has just the opposite block sequence and different conformation. The effect of block sequence on the block copolymer nanoassemblies is explored. It is discovered that star (P4VP-b-PS)n tends to form complicated nanoassemblies in the block selective solvent for P4VP due to the PS arms bridging, and star (PS-b-P4VP)n acts somewhat like linear amphiphilic block copolymers. Our study demonstrates the crucial role of block sequence in star block copolymer nanoassemblies.

1. INTRODUCTION Self-assembly of amphiphilic block copolymers (BCPs) in a block selective solvent attracts continuous and great attention for their rich ordered morphologies and potential application in drug delivery, fabrication of smart polymer films, and other fields.1−4 The nanoassembly morphologies of amphiphilic BCPs are dependent on the degree of polymerization (DP) of solvophilic/solvophobic blocks,5−11 solvent character,12−16 and molecular architecture.17−24 Up to now, self-assembly of linear amphiphilic BCPs has been widely reported.1−4 In comparison, self-assembly of nonlinear amphiphilic BCPs, for example, dendrimers, hyperbranched polymers, star polymers, and graft polymers, is not as popular as that of linear ones,25−30 although they exhibit special performances and interesting chemical and physical properties in solution or bulk.25−30 Star block copolymers including a three-dimensional branched architecture, in which chemically different building blocks are linked to a single junction point, show interesting characteristics.31,32 For example, Huang and co-workers revealed that miktoarm star polymers of (methoxy-poly(ethylene glycol))2-(ε-caprolactone)2 showed lower crystallization and melting temperature and better phase separation compared with the linear block copolymers.33 Chen et al. reported that star-shaped [poly(n-butyl acrylate)-b-oligosaccharide]4 not only showed superior memory performances but also formed physical networks that imparted mechanical resilience to the thin films in contrast to their linear counterparts.34 Just recently, we prepared the star block copolymer nanoassemblies of [poly(4-vinylpyridine)-block© XXXX American Chemical Society

polystyrene]n [(P4VP-b-PS)n] with different arm number n by RAFT dispersion polymerization and it was found that the morphology and/or size of the star (P4VP-b-PS)n nanoassemblies were firmly dependent on the arm number n.35 For star block copolymers with different block sequence, e.g., 3arm [poly(4-vinylpyridine)-block-polystyrene]3 [(P4VP-bPS)3] and [polystyrene-block-poly(4-vinylpyridine)]3 [(PS-bP4VP)3] (Scheme 1), they clearly have different conformations, although their chemical compositions may be very similar or same. Up to now, considerable synthetic efforts to star block copolymers have been made over the past few decades.31−40 However, the effect of block sequence on star Scheme 1. Scheme Structure of [P4VP-b-PS]3 (A) and [PSb-P4VP]3 (B) with Similar Chemical Composition but Different Block Sequence

Received: November 13, 2018 Revised: December 16, 2018

A

DOI: 10.1021/acs.macromol.8b02427 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 1. Summary of the Synthesized Macro-CTAs of (P4VP-TTC)n and (PS-TTC)n Mn (kg/mol) macro-CTA

[M]0:[CTA]0:[I]0

t (h)

conv (%)a

Mn,thb

Mn,GPCc

Mn,NMRd

Đe

P4VP26-TTC [P4VP24-TTC]2 [P4VP14-TTC]3 [P4VP26-TTC]3 [P4VP36-TTC]3 [P4VP46-TTC]3 [P4VP93-TTC]3 [P4VP25-TTC]4 PS188-TTC [PS187-TTC]2 [PS25-TTC]3 [PS50-TTC]3 [PS95-TTC]3 [PS149-TTC]3 [PS186-TTC]3 [PS187-TTC]4

240:8:1 240:4:1 360:8:3 720:8:3 1200:8:3 1680:8:3 4800:8:3 320:2:1 2000:5:1 4000:5:2 450:5:3 1200:5:3 2850:5:3 4500:5:3 6000:5:3 8000:5:4

9 9 9 9 10.5 10.5 15 7 45 45 32 32 35 42 45 45

87.0 80.4 90.0 88.7 71.7 64.8 46.5 61.1 47.0 46.8 83.3 62.6 50.1 49.9 46.5 46.8

2.99 5.47 5.02 8.80 11.95 15.10 29.91 11.29 19.81 39.33 8.41 16.21 30.25 47.10 58.64 78.58

3.4 5.6 5.1 8.2 10.1 11.8 25.9 9.7 18.7 27.1 7.9 12.8 26.5 33.4 36.7 48.2

3.2 5.5 5.3 9.3 10.8 14.7 26.1 12.8 19.2 36.5 8.7 15.3 30.0 46.9 56.8 76.3

1.14 1.15 1.20 1.20 1.22 1.23 1.24 1.22 1.18 1.26 1.22 1.20 1.21 1.23 1.26 1.27

a

Monomer conversion determined by 1H NMR. bTheoretical molecular weight according to eq S1. cMolecular weight determined by gel permeation chromatography (GPC). dMolecular weight determined by 1H NMR. eĐ (Mw/Mn) determined by GPC.

Table 2. Experimental Details and Summary of the Synthesized Linear and Star Block Copolymers Mn (kg/mol) polymers

[M]0:[CTA]0:[I]0

t (h)

conv (%)a

Mn,thb

Mn,GPCc

Mn,NMRd

Đe

P4VP26-b-PS93 P4VP26-b-PS188 P4VP36-b-PS187 P4VP46-b-PS188 [P4VP24-b-PS192]2 [P4VP26-b-PS48]3 [P4VP26-b-PS97]3 [P4VP26-b-PS141]3 [P4VP26-b-PS190]3 [P4VP36-b-PS190]3 [P4VP46-b-PS188]3 [P4VP14-b-PS190]3 [P4VP93-b-PS23]3 [P4VP25-b-PS194]4 PS188-b-P4VP25 [PS187−P4VP22]2 [PS50-b-P4VP26]3 [PS95-b-P4VP26]3 [PS149-b-P4VP26]3 [PS186-b-P4VP24]3 [PS186-b-P4VP13]3 [PS186-b-P4VP36]3 [PS186-b-P4VP46]3 [PS25-b-P4VP95]3 [PS187−P4VP24]4

400:4:1 800:4:1 800:4:1 800:4:1 800:2:1 750:5:3 1200:4:3 1800:4:3 2400:4:3 2400:4:3 2400:4:3 2400:4:3 720:4:3 800:1:1 120:4:1 120:2:1 360:4:3 360:4:3 360:4:3 360:4:3 261:4:3 660:4:3 840:4:3 1200:4:3 120:1:1

12 16 16 16 16 10 12 13 15 15 15 15 7 16 29 28 24 24 29 30 25 30 40 24 28

93.0 94.0 93.5 93.4 96.0 95.1 96.8 93.9 95.0 95.0 94.0 95.0 38.3 97.0 83.3 73.3 86.6 86.6 88.1 78.9 72.2 65.5 65.7 95.0 80.0

12.66 22.54 23.48 24.64 45.41 23.78 39.07 52.79 69.95 71.23 73.76 64.30 37.08 91.99 22.22 43.95 24.40 38.44 55.29 66.20 62.74 68.98 73.13 38.33 88.66

11.5 20.1 20.8 21.0 35.0 15.6 30.9 36.0 42.1 45.3 46.9 39.4 28.3 64.2 20.5 35.3 16.1 31.1 36.5 41.7 38.8 44.0 47.2 28.1 62.5

13.0 22.7 24.1 24.4 45.6 23.5 39.0 52.8 69.8 70.2 72.5 61.3 36.2 92.0 21.9 44.1 23.8 39.5 54.1 68.7 61.5 70.0 72.1 38.0 90.5

1.13 1.17 1.17 1.18 1.21 1.26 1.25 1.26 1.25 1.27 1.28 1.25 1.28 1.25 1.20 1.28 1.27 1.27 1.29 1.30 1.30 1.32 1.33 1.27 1.32

a

Styrene conversion determined by UV−vis and 4-vinylpyridine conversion determined by 1H NMR. bTheoretical molecular weight according to eq S2. cMolecular weight determined by GPC dMolecular weight determined by 1H NMR. eĐ (Mw/Mn) determined by GPC.

block copolymer nanoassemblies is rarely explored,41 possibly due to laborious synthesis of such star block copolymers. In this study, star and linear block copolymers with different block sequence, e.g., (P4VP-b-PS)n and (PS-b-P4VP)n (n = 1− 4), were initially prepared by RAFT polymerization. Then, their nanoassemblies in the P4VP block selective solvent of the methanol/water mixture (80/20 by weight) were prepared via solution self-assembly. Herein, star (P4VP-b-PS)n and (PS-b-

P4VP)n are chosen, since self-assembly of the linear (PS-bP4VP) has been widely studied as typical examples.42−44 Note: in the present study, the strategy of solution self-assembly is used to prepare the star (P4VP-b-PS)n and (PS-b-P4VP)n nanoassemblies, which is different from polymerizationinduced self-assembly (PISA)45−47 to synthesize star block copolymer nanoassemblies as discussed in ref 35. This solution self-assembly but not PISA used in the present study is due to B


Article

Macromolecules

(P4VP-b-PS)n were prepared using RAFT dispersion polymerization employing (P4VP-TTC)n as macro-CTA (Table 2) as discussed elsewhere.35 Here is a typical synthesis of 3-arm [P4VP26b-PS190]3 under [St]:[P4VP26-TTC]3:[AIBN] = 2400:4:3. St (0.500 g, 4.80 mmol), (P4VP26-TTC)3 (0.071 g, 0.0080 mmol), and AIBN (0.986 mg, 0.0060 mmol) dissolved in the 80/20 methanol/water mixture (2.28 g) were weighed into a 25 mL Schlenk flask. The mixture was degassed and then polymerization ran at 70 °C for 15 h. Styrene conversion was detected by UV−vis analysis at 244.9 nm as discussed elsewhere.52 The synthesized [P4VP26-b-PS190]3 was collected by centrifugation (10000 rpm), dissolved in dichloromethane (DCM), precipitated into methanol, dried under vacuum, and finally characterized by GPC and 1H NMR. 2.4. Preparation of (P4VP-b-PS)n and (PS-b-P4VP)n Nanoassemblies. The (P4VP-b-PS)n and (PS-b-P4VP)n nanoassemblies were prepared via solution self-assembly with very similar procedures. (P4VP-b-PS)n or (PS-b-P4VP)n was dissolved in DMF to make a 0.50 mg/mL solution at 25 °C, into which a given volume of 80/20 methanol/water mixture was added at a rate of 1 drop (1 drop is about 7 μL) every 10 s with stirring. With addition of the alcoholic solvent, nanoassemblies were formed as indicated by the polymer solution becoming turbid. Addition of the alcoholic solvent was continued until polymer concentration at about 0.25 mg/mL. Finally, the resultant nanoassemblies were dialyzed against the alcoholic solvent at 25 °C for 3 days to remove DMF. The obtained (P4VP-bPS)n and (PS-b-P4VP)n nanoassemblies with 0.2 mg/mL concentration were kept at room temperature for next characterization. Similarly, the (P4VP-b-PS)n and (PS-b-P4VP)n nanoassemblies dispersed in toluene were also prepared, and the details are shown in the Supporting Information. 2.5. Characterization. Polymer molecular weight and dispersity (Đ, Đ = Mw/Mn) were determined by GPC using a Waters 600E GPC system, in which DMF was used as eluent and narrow-polydispersity polystyrene samples were used as standards to calibrate the apparatus. A Varian 100 UV−vis spectrophotometer was used for UV−vis analysis. The NMR spectra were recorded on a Bruker Avance III 400 MHz NMR spectrometer using CDCl3 as the solvent. A Tecnai G2 F20 electron microscope performed at 200 kV was used for transmission electron microscopy (TEM) observation. In the TEM sampling of unstained block copolymer nanoassemblies dispersed in the 80/20 methanol/water mixture, a small drop of 0.2 mg/mL colloidal dispersion was dripped onto a piece of copper grid, dried at room temperature until the solvent was evaporated; for the stained block copolymer nanoassemblies, 0.2 mg/mL dispersion of the block copolymer nanoassemblies was mixed with pH 1 HCl aqueous solution under equal volume for about 10 min, and then a small drop of the colloids was deposited onto a piece of copper grid, dried at room temperature, and finally observed by TEM. Note: P4VP can be acidified by HCl, and this acidification of P4VP makes the (P4VP-bPS)n and (PS-b-P4VP)n nanoassemblies to be stained as discussed in ref 53. A JSM-7500F electron microscope was used for field-emission scanning electron microscopy (SEM) observation. In the SEM sampling, a small drop of diluted block copolymer nanoassemblies was dropped on a piece of silica wafer, dried at room temperature, and the sample was sprayed with a gold layer about 3 nm thick and then checked by SEM. Differential scanning calorimetry (DSC) analysis was performed on a NETZSCH DSC 204 differential scanning calorimeter under nitrogen atmosphere with a cooling and heating rate of 10 °C min−1 and the second heating cure was used to determinate the glass transition temperature (Tg) at the middle of the step transition.

two reasons. First, the star (P4VP-b-PS)n and (PS-b-P4VP)n nanoassemblies were prepared in the same block selective solvent of the methanol/water mixture, therefore the solvent effect on the star block copolymer nanoassemblies were excluded. Second, in the PISA synthesis of block copolymer nanoassemblies, it has been demonstrated that the polymerization solvent exerted somewhat influence on the block copolymer nanoassemblies under PISA conditions.48−50 However, the star (P4VP-b-PS)n and (PS-b-P4VP)n nanoassemblies cannot be synthesized in the same solvent via PISA. For example, the (P4VP-b-PS)n nanoassemblies can be synthesized in the selective solvent for the P4VP block, e.g., alcohol, via PISA, and whereas the (PS-b-P4VP)n nanoassemblies can be prepared in the selective solvent for the PS block, e.g., toluene. By comparing the star (P4VP-b-PS)n and (PS-b-P4VP)n nanoassemblies prepared via solution selfassembly in the same solvent of the methanol/water mixture, it is discovered that, (1) the morphology of star block copolymer nanoassemblies is firmly correlative to the arm number n, which is valid for both (P4VP-b-PS)n and (PS-bP4VP)n, and (2) star block copolymer nanoassemblies of (P4VP-b-PS)n have more complex morphologies than (PS-bP4VP)n, demonstrating that block sequence is a crucial parameter to determine star block copolymer nanoassemblies.

2. EXPERIMENTAL SECTION 2.1. Materials. Styrene (St, 99.5%, Annaiji, China) and 4vinylpyridine (4VP, 96%, Alfa) were distilled under reduced pressure before use. The chain transfer agents (CTAs) of 1−4 for RAFT polymerizations were synthesized as discussed elsewhere,51 and their chemical structures are shown in Scheme S1. 2,2′-Azobis(2methylpropionitrile) (AIBN, >99%, Tianjin Chemical Company, China) was recrystallized from ethanol prior to use. Other chemical reagents were used without any further purification. Deionized water was used. 2.2. Synthesis of Linear and Star Macro-CTAs. Linear and star macromolecular chain transfer agents (macro-CTAs) of (P4VPTTC)n or (PS-TTC)n, in which n represents the arm number of the P4VP or PS arms and TTC represents the RAFT terminal of trithiocarbonate, were synthesized by RAFT polymerization employing chain transfer agents (CTAs) of 1−4. Here is a typical synthesis of 3-arm [PS186-TTC]3 under [St]:[3]:[AIBN] = 6000:5:3. St (5.00 g, 0.048 mol), 3 (0.024 g, 0.040 mmol) and AIBN (3.95 mg, 0.0240 mmol) dissolved in 1,4-dioxane (5.0 g) were weighted into a 50 mL Schlenk flask. The mixture was degassed and ran at 70 °C for 45 h. The styrene conversion was determined by 1H NMR employing 1,3,5trioxane as internal standard. The synthesized [PS186-TTC]3 was precipitated into methanol and then dried under vacuum. By changing the [St]0:[CTA]0:[AIBN]0 molar ratio, other [PS-TTC]n macroCTAs with different arm chain length and arm number n were also synthesized (Table 1). By employing trithiocarbonates of 1, 2, 3, or 4 as CTAs, (P4VPTTC)n with different arm chain length and arm number n (Table 1) were synthesized as discussed elsewhere.35 2.3. Synthesis of Linear and Star Block Copolymers of (P4VP-b-PS)n and (PS-b-P4VP)n. (PS-b-P4VP)n with different DP of the PS and P4VP blocks and with different arm number n were synthesized by RAFT dispersion polymerization employing (PSTTC)n as macro-CTA (Table 2). Here is a typical synthesis of 3-arm [PS186-b-P4VP24]3 under [4VP]:[PS186-TTC]3:[AIBN] = 360:4:3. Into a 50 mL Schlenk flask, 4VP (0.200 g, 1.90 mmol), [PS186-TTC]3 (1.241 g, 0.0211 mmol) and AIBN (2.60 mg, 0.0158 mmol) dissolved in toluene (2.16 g) were weighed. The mixture was degassed and then polymerization ran at 70 °C for 30 h. The 4-vinylpyridine conversion was determined by 1H NMR. The synthesized [PS186-b-P4VP24]3 was diluted with dichloromethane and precipitated into methanol, dried under vacuum, and finally characterized by GPC and 1H NMR.

3. RESULTS AND DISCUSSION 3.1. Synthesis of Linear and Star Macro-CTAs of (P4VP-TTC)n and (PS-TTC)n. The linear and star macroCTAs of (P4VP-TTC)n and (PS-TTC)n (n = 1, 2, 3, 4) with suitable DPs of the P4VP or PS arms were synthesized by RAFT solution polymerization using CTAs of 1−4 (Table 1). C


Article

Macromolecules

Figure 1. GPC traces of [P4VP-TTC]n with similar DPs of the P4VP arms at around 25 but different arm number n (A), and GPC traces of [P4VP-TTC]3 with different DPs of the P4VP arms (B).

two typical 3-arm samples of [P4VP26-TTC]3 and [PS50TTC]3. Their molecular weight by 1H NMR, Mn,NMR, is calculated by comparing the peak areas at 0.88 and 8.33 ppm or 6.20−7.20 ppm corresponding to the RAFT terminal and pyridine ring or benzene ring, respectively. As summarized in Table 1, for star macro-CTAs of (P4VP-TTC)n and (PSTTC)n with low molecular weight, Mn,GPC is close to Mn,NMR, both of which are well-consistent with the theoretical molecular weight Mn,th determined following eqn S1; whereas for star polymers with high molecular weight, Mn,GPC is smaller than Mn,NMR, since star polymers have much compacted conformation.56−60 3.2. Synthesis of Star Block Copolymers of (P4VP-bPS)n and (PS-b-P4VP)n. Initially, we tried to synthesize the star block copolymers, e.g., [P4VP-b-PS]3, by solution RAFT polymerization in the solvent of DMF. However, it was found that the solution RAFT polymerization employing the (P4VPTTC)3 macro-CTAs ran relatively slow even at high monomer concentration of 50%, and just about 55% monomer conversion was obtained in 47 h (Table S1). Besides, the synthesized (P4VP-b-PS)n star block copolymers had a much broad distribution of molecular weight due to bimolecular termination from star−star coupling and linear-star coupling (Figure S2), which generally occurred in star polymer synthesis.54,55 To enhance RAFT polymerization and minimize bimolecular termination, the (P4VP-b-PS)n star block copolymers were synthesized via RAFT dispersion polymerization of styrene in the 80/20 methanol/water mixture by employing the macro-CTAs of (P4VP-TTC)n. This heterogeneous RAFT polymerization was used, since it afforded synthesis of macromolecules with a fast high monomer conversion.61−63 By quenching the polymerization at 10−16 h (depending on

Figure 1A and B summarizes the GPC traces for the typical [P4VP-TTC]n samples with similar DP of the P4VP arm but different arm number n and the typical [P4VP-TTC]3 samples with different DP of the P4VP arm, respectively. The monomodal GPC traces are observed in the [P4VP-TTC]n samples at case of low DP of the P4VP arm as indicated by the low Đ around 1.1−1.2. At high DP of the P4VP arm, e.g., [P4VP46-TTC]3, Đ slightly increases close to 1.3. The possible reason is ascribed to the bimolecular termination via star−star coupling and linear-star coupling, since this star structure includes more than two propagating radicals in one molecule.54,55 Similarity is observed in the GPC traces of [PS-TTC]n (Figure S1). The (P4VP-TTC)n and (PS-TTC)n macro-CTAs were analyzed by NMR. Figure 2 shows the 1H NMR spectra of

Figure 2. 1H NMR spectra of [P4VP26-TTC]3 (A) and [PS50-TTC]3 (B).

Figure 3. GPC traces of several typical [P4VP-b-PS]n with different arm number n but very similar DPs of the P4VP and PS blocks (A) and GPC traces of (P4VP26-b-PS)3 with a constant arm number n of 3 but different DP of the PS block (B). D


Article

Macromolecules

Figure 4. DSC thermograms of 3-arm [P4VP46-b-PS188]3 and precursor (A) and 3-arm [PS186-b-P4VP46]3 as well as its precursor (B).

Figure 5. TEM images of [P4VP-b-PS]n and [PS-b-P4VP]n (n = 1−4) with similar chemical composition but different block sequence. Note: the insets show the TEM images of nanoassemblies stained with pH 1 HCl aqueous solution or SEM images of nanoassemblies.

copolymers. By comparing the chemical shifts at 8.33 ppm corresponding to the P4VP block and at 6.20−7.20 ppm assigned to the PS block, Mn,NMR of (P4VP-b-PS)n was obtained. As summarized in Table 2, whether for linear or star block copolymers of (P4VP-b-PS)n, Mn,NMR is very close to Mn,th. This confirms synthesis of qualified (P4VP-b-PS)n samples via RAFT dispersion polymerization, although somewhat star−star coupling and linear-star coupling occurred. The (PS-b-P4VP)n samples as summarized in Table 2 were also synthesized via RAFT dispersion polymerization. Different from the synthesis of (P4VP-b-PS)n in the alcoholic solvent, the synthesis of (PS-b-P4VP)n was performed in toluene, which is a solvent for the PS block but a nonsolvent for the P4VP block. The (PS-b-P4VP)n samples were characterized by NMR (Figure S5) and GPC (Figure S6), from which synthesis of qualified (PS-b-P4VP)n samples is concluded. DSC analysis of the linear block copolymers and the typical 3-arm block copolymers as well as their precursors was performed. As shown in Figure 4, it is found that the core-

the targeted DP of the PS block) with above 90% monomer conversion, star (P4VP-b-PS)n with a targeted DP of the PS block can be obtained. The (P4VP-b-PS)n samples were characterized by GPC (Figures 3 and S3) and NMR (Figure S4). Figure 3A and B shows the GPC traces of typical (P4VP-b-PS)n samples with different arm number n but very similar DPs of the P4VP and PS blocks and those (P4VP26-b-PS)3 with a constant arm number of 3 but different DP of the PS block, respectively. The molecular weight Mn,GPC of linear block copolymer of P4VP-bPS has a narrow molecular distribution (Đ = 1.13−1.18), and Mn,GPC is very close to Mn,th. Whereas, for star (P4VP-b-PS)n, especially for those with n = 3 or 4, Mn,GPC is smaller than Mn,th just as expected.56−60 In comparison with the synthesis of star [P4VP-b-PS]n by solution RAFT polymerization, the synthesis of [P4VP-b-PS]n via RAFT dispersion polymerization is highly improved, although slight shoulder at the high molecular side corresponding to the bimolecular termination products is observed and Đ becomes a little larger than that of linear block E


Article

Macromolecules forming blocks, e.g., P4VP in [P4VP-b-PS]3 or PS in [PS-bP4VP]3, having a much higher glass transition temperature (Tg) than that of the linear counterparts. Meanwhile, the Tg of the arm-forming block, e.g., PS in [P4VP-b-PS]3 or P4VP in [PS-b-P4VP]3, is also slightly higher than that of the linear counterparts. Similarity was also reported in the star poly(Llactide) (sPLLA)64 and star polystyrene.65 Furthermore, by comparing the Tg of [P4VP46-b-PS188]3 and [PS186-b-P4VP46]3 with very similar DPs, a higher Tg of the core-forming block than that of the arm-forming block is found, although they have the almost same DP. It is thought that the core-forming blocks in [P4VP-b-PS]3 and [PS-b-P4VP]3 are jointed together, which is somewhat like cross-linking, and this makes the core-forming blocks become more rigid than free counterparts and therefore leads to a higher Tg. 3.3. Nanoassemblies of [P4VP-b-PS]n and [PS-bP4VP]n. The nanoassemblies of [P4VP-b-PS]n and [PS-bP4VP]n were initially prepared via self-assembly in the 80/20 methanol/water mixture, which is a solvent for the P4VP block but a nonsolvent for the PS block. As summarized in Table 2, many star block copolymers were synthesized. In this section, the nanoassemblies of [P4VP-b-PS]n and [PS-b-P4VP]n with similar DPs of the P4VP and PS blocks but with different block sequence are initially prepared in the 80/20 methanol/water mixture and checked by TEM and SEM, demonstrating the key role of block sequence on the block copolymer morphology. Then, the nanoassemblies of 3-arm [P4VP-b-PS]n and [PS-bP4VP]n are chosen as typical examples to show how block sequence affecting the block copolymer morphology. Figure 5 summarizes the TEM/SEM images of [P4VP-bPS]n and [PS-b-P4VP]n with different arm number n. In these nanoassemblies, the DPs of the P4VP and PS blocks are kept very similar, and the difference is just in the block sequence and the arm number n. As expected, the two linear block copolymers of P4VP26-b-PS188 and PS188-b-P4VP25, the former was synthesized employing P4VP26-TTC as macro-CTA and the latter was synthesized employing PS188-TTC as macroCTA, formed the similar vesicle-like nanoassemblies, since these two block copolymers are just same although they were synthesized by different procedures. At the case of arm number n = 2, [P4VP24-b-PS192]2 and [PS187-b-P4VP22]2 are somewhat like BAB66−71 and ABA72−75 triblock copolymers, in which B represents a solvophobic block and A represents a solvophilic block, respectively, and flower-like particles of [P4VP24-bPS192]2 and compartmentalized vesicles of [PS187-b-P4VP22]2 are formed; at the case of arm number n = 3, bicontinuous nanospheres of [P4VP26-b-PS190]3 and vesicles of [PS186-bP4VP24]3 are formed. Similar bicontinuous nanospheres of poly(ethylene oxide)-b-poly(octadecyl methacrylate) containing a long side octadecyl chain76 and amphiphilic polynorbornene block copolymer77 and the mixture of poly(ethylene glycol)-b-polystyrene/polystyrene-b-poly(ethylene glycol)-b-polystyrene78 were also prepared; and in the case of arm number n = 4, large compound micelles of [P4VP25-bPS194]4 and vesicles of [PS187-b-P4VP24]4 are formed, respectively. As shown in Figure 5, some of the star [P4VPb-PS]n and [PS-b-P4VP]n block copolymers have new morphologies much different from linear ones. Scheme 2 illuminates the primary difference of the self-assembly of star [P4VP-b-PS]n and [PS-b-P4VP]n as well as the linear P4VP-bPS, although the exact reason that how and why these new morphologies of star [P4VP-b-PS]n and [PS-b-P4VP]n are formed needs to be further studied. Despite this, the results

Scheme 2. Schematic Self-Assembly of P4VP-b-PS (A), [P4VP-b-PS]3 (B), and [PS-b-P4VP]3 (C) in the Block Selective Solvent for the P4VP Block

clearly indicate that the block sequence is a very important parameter to dedicate the star block copolymer morphologies. As shown in Scheme 1, star [P4VP-b-PS]n and [PS-bP4VP]n block copolymers have much different conformation. In [P4VP-b-PS]n, the P4VP block forms the core and the PS block acts as the arm, and [PS-b-P4VP]n has the just opposite conformation. In the block selective solvent for the P4VP block, [P4VP-b-PS]n has more possibility to form complex nanoassemblies than [PS-b-P4VP]n through the arm bridging of the PS block as similar as those BAB triblock copolymers (Scheme 3).66−71 Besides, the jointed P4VP core in star [P4VP-b-PS]n makes the nanoassemblies more complex than linear BAB samples.35,68 Scheme 3. Star [P4VP-b-PS]3 Bridging in the Block Selective Solvent for the P4VP Block

In the subsequent section, the typical 3-arm [P4VP-b-PS]3 and [PS-b-P4VP]3 nanoassemblies with an increasing DP of the P4VP block (Figure 6) or an increasing DP of the PS block (Figure 7) were prepared and compared, and the block sequence effect on the star block copolymer nanoassemblies was demonstrated. Figure 6 summarizes the TEM images of 3-arm [P4VP-bPS]3 and [PS-b-P4VP]3 nanoassemblies with a similar DP of PS block at about 190 but a different P4VP block. The results indicate that the morphology of [P4VP-b-PS]3 changes from large compound micelles to bicontinuous nanospheres and finally to multilayered vesicles with the DP of the P4VP block increases from 14 to 46. While [PS-b-P4VP]3 forms large F


Article

Macromolecules

Figure 6. TEM images of [P4VP-b-PS]3 (A) and [PS-b-P4VP]3 (B) with a similar DP of PS but an increasing DP of P4VP. Note: the insets show TEM images of nanoassemblies stained with pH 1 HCl aqueous solution or SEM images of nanoassemblies.

Figure 7. TEM images of [P4VP-b-PS]3 (A) and [PS-b-P4VP]3 (B) with a constant DP of P4VP but an increasing DP of PS. Note: the insets show the TEM images of nanoassemblies stained with pH 1 HCl aqueous solution or SEM images of nanoassemblies.

self-assembly of [P4VP-b-PS]3 and [PS-b-P4VP]3. The selfassembly of [PS-b-P4VP]3 with a jointed PS core is somewhat like general linear amphiphilic BCPs, which follows the general rules, e.g., the morphology evolution of vesicles-to-nanorodsto-nanospheres occurs with the increasing DP of the solvophilic block or with the decreasing DP of the solvophobic block vesicles change into nanospheres, and/or the size of nanoassemblies decreases with the increasing DP of the solvophilic block.79−81 Note: this rule is also valid for the

compound micelles, and with the increasing DP of the P4VP block from 13 to 46, the nanoassemblies change to vesicles and finally to nanospheres. Figure 7 indicates the [P4VP-b-PS]3 forms nanoparticle clusters at the low DP of the PS block, and the nanoassemblies change to bicontinuous nanospheres with the increasing DP of the PS block from 48 to 190. For [PS-bP4VP]3, it forms discrete nanoparticles, and then they change to vesicles with the increasing DP of the PS block. These confirm that the block sequence is crucially important in the G


Article

Macromolecules present linear P4VP-b-PS as demonstrated in Figure S7. In comparison, the nanoassemblies of [P4VP-b-PS]3 are much complex due to the bridging of the PS arms in the micellization. This bridging of the outer PS arms improves to particle−particle collision and/or fusion, which is observed in self-assembly of linear amphiphilic block copolymer at case of high fraction of the hydrophobic block,82−84 and leads to complicated nanoassemblies including nanoparticle clusters and bicontinuous nanospheres. Lastly, the self-assembly of two typical star block copolymers, e.g., [P4VP93-b-PS23]3 and [PS25-b-P4VP95]3, with similar chemical composition but different block sequence, in toluene, which is block selective solvent for the PS block, is checked (see the preparation of the micelles in the Supporting Information). In toluene, the insoluble P4VP block forms the core of the nanoassemblies and the soluble PS block keeps the nanoassemblies suspended in the solvent. As shown in Figure S8, 29 nm discrete nanospheres of [P4VP93-b-PS23]3 and 200−400 nm large compound micelles of [PS25-bP4VP95]3 are formed, respectively, further verifying the importance of block sequence in the self-assembly of star block copolymers.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G. H.). *E-mail: [email protected] (W. Z.). ORCID

Tianying Guo: 0000-0001-6587-6466 Wangqing Zhang: 0000-0003-2005-6856 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support by the National Science Foundation for Distinguished Young Scholars (no. 21525419), the National Science Foundation of China (no. 21474054), and the National Key Research and Development Program of China (2017YFC1103501) is gratefully acknowledged.

■

REFERENCES

(1) Mai, Y.; Eisenberg, A. Self-assembly of block copolymers. Chem. Soc. Rev. 2012, 41, 5969−5985. (2) Blanazs, A.; Armes, S. P.; Ryan, A. J. Self-Assembled Block Copolymer Aggregates: From Micelles to Vesicles and their Biological Applications. Macromol. Rapid Commun. 2009, 30, 267−277. (3) Holder, S. J.; Sommerdijk, N. A. J. M. New micellar morphologies from amphiphilic block copolymers: disks, toroids and bicontinuous micelles. Polym. Chem. 2011, 2, 1018−1028. (4) Rodríguez-Hernán dez, J.; Chéc ot, F.; Gnanou, Y.; Lecommandoux, S. Toward ‘smart’ nano-objects by self-assembly of block copolymers in solution. Prog. Polym. Sci. 2005, 30, 691−724. (5) Chernyy, S.; Kirkensgaard, J. J. K.; Mahalik, J. P.; Kim, H.; Arras, M. M. L.; Kumar, R.; Sumpter, B. G.; Smith, G. S.; Mortensen, K.; Russell, T. P.; Almdal, K. Bulk and Surface Morphologies of ABC Miktoarm Star Terpolymers Composed of PDMS, PI, and PMMA Arms. Macromolecules 2018, 51, 1041−1051. (6) Herfurth, C.; Laschewsky, A.; Noirez, L.; von Lospichl, B.; Gradzielski, M. Thermoresponsive (star) block copolymers from onepot sequential RAFT polymerizations and their self-assembly in aqueous solution. Polymer 2016, 107, 422−433. (7) Khanna, K.; Varshney, S.; Kakkar, A. Designing Miktoarm Polymers Using a Combination of “Click” Reactions in Sequence with Ring-Opening Polymerization. Macromolecules 2010, 43, 5688−5698. (8) Roth, P. J.; Davis, T. P.; Lowe, A. B. Comparison between the LCST and UCST Transitions of Double Thermoresponsive Diblock Copolymers: Insights into the Behavior of POEGMA in Alcohols. Macromolecules 2012, 45, 3221−3230. (9) Rho, Y.; Kim, C.; Higashihara, T.; Jin, S.; Jung, J.; Shin, T. J.; Hirao, A.; Ree, M. Complex Self-Assembled Morphologies of Thin Films of an Asymmetric A3B3C3 Star Polymer. ACS Macro Lett. 2013, 2, 849−855. (10) Sun, Z.; Chen, Z.; Zhang, W.; Choi, J.; Huang, C.; Jeong, G.; Coughlin, E. B.; Hsu, Y.; Yang, X.; Lee, K. Y.; Kuo, D. S.; Xiao, S.; Russell, T. P. Directed Self-Assembly of Poly(2-vinylpyridine)-bpolystyrene-b-poly(2-vinylpyridine) Triblock Copolymer with Sub-15 nm Spacing Line Patterns Using a Nanoimprinted Photoresist Template. Adv. Mater. 2015, 27, 4364−4370. (11) Quek, J. Y.; Roth, P. J.; Evans, R. A.; Davis, T. P.; Lowe, A. B. Reversible Addition-Fragmentation Chain Transfer Synthesis of Amidine-Based, CO2-Responsive Homo and AB Diblock (Co)Polymers Comprised of Histamine and their Gas-Triggered SelfAssembly in Water. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 394−404. (12) Liao, Y.; Liu, N.; Zhang, Q.; Bu, W. Self-Assembly of Polyoxometalate-Based Starlike Polymers in Solvents of Variable

4. CONCLUSIONS Star and linear block copolymers of [P4VP-b-PS]n and [PS-bP4VP]n (n = 1, 2, 3, 4) with similar chemical composition but different block sequence were prepared by RAFT dispersion polymerization. The effect of block sequence on block copolymer nanoassemblies is explored. It is discovered that the linear block copolymers of P4VP-b-PS and PS-b-P4VP form very similar nanoassemblies in the block selective solvent for the P4VP block, although these two are synthesized via different procedures. Whereas for star (P4VP-b-PS)n and (PSb-P4VP)n (n = 2−4) just with different block sequence, their nanoassemblies are much different even they have almost same chemical composition. The star (P4VP-b-PS)n tends to form complicated nanoassemblies including nanoparticle clusters, bicontinuous nanospheres and multilayered vesicles, and the self-assembly of star (PS-b-P4VP)n is somehow similar to linear block copolymers, respectively. The possible reason leading to the different nanoassemblies of (P4VP-b-PS)n and (PS-bP4VP)n is discussed. It is thought that (P4VP-b-PS)n and (PSb-P4VP)n with different block sequence have different conformation. For star (P4VP-b-PS)n composing of a jointed P4VP core and several outer PS arms, the PS arms bridging can occur in the self-assembly in the block selective solvent for the P4VP core and therefore leads to complicated nanoassemblies. For star [PS-b-P4VP]n composed of a jointed PS core and several outer P4VP arms, the jointed PS core is shielded by the hydrophilic and soluble outer P4VP arms, which leads to its self-assembly somewhat like general linear amphiphilic BCPs. Our results reveal the crucial role of block sequence in star block copolymers, which should be helpful to clarify the formation of high ordered block copolymer nanoassemblies.

■

star block copolymers in DMF; GPC traces; 1H NMR spectra; TEM images (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02427. Structures of mono- and multifunctional trithiocarbonates; experimental details and summary of synthesized H


Article

Macromolecules Quality: From Free-Standing Sheet to Vesicle. Macromolecules 2014, 47, 7158−7168. (13) Kepola, E. J.; Loizou, E.; Patrickios, C. S.; Leontidis, E.; Voutouri, C.; Stylianopoulos, T.; Schweins, R.; Gradzielski, M.; Krumm, C.; Tiller, J. C.; Kushnir, M.; Wesdemiotis, C. Amphiphilic Polymer Conetworks Based on End-Linked “Core-First” Star Block Copolymers: Structure Formation with Long-Range Order. ACS Macro Lett. 2015, 4, 1163−1168. (14) Bhargava, P.; Zheng, J. X.; Li, P.; Quirk, R. P.; Harris, F. W.; Cheng, S. Z. D. Self-Assembled Polystyrene-block-poly(ethylene oxide) Micelle Morphologies in Solution. Macromolecules 2006, 39, 4880−4888. (15) Kamps, A. C.; Cativo, M. H. M.; Fryd, M.; Park, S.-J. SelfAssembly of Amphiphilic Conjugated Diblock Copolymers into OneDimensional Nanoribbons. Macromolecules 2014, 47, 161−164. (16) Raffa, P.; Brandenburg, P.; Wever, D. A. Z.; Broekhuis, A. A.; Picchioni, F. Polystyrene-Poly(sodium methacrylate) Amphiphilic Block Copolymers by ATRP: Effect of Structure, pH, and Ionic Strength on Rheology of Aqueous Solutions. Macromolecules 2013, 46, 7106−7111. (17) Yoon, K.; Kang, H. C.; Li, L.; Cho, H.; Park, M.-K.; Lee, E.; Bae, Y. H.; Huh, K. M. Amphiphilic poly(ethylene glycol)-poly(εcaprolactone) AB2 miktoarm copolymers for self-assembled nanocarrier systems: synthesis, characterization, and effects of morphology on antitumor activity. Polym. Chem. 2015, 6, 531−542. (18) Kim, H.; Kang, B.-G.; Choi, J.; Sun, Z.; Yu, D. M.; Mays, J.; Russell, T. P. Morphological Behavior of A2B Block Copolymers in Thin Films. Macromolecules 2018, 51, 1181−1188. (19) Strandman, S.; Zarembo, A.; Darinskii, A. A.; Laurinmäki, P.; Butcher, S. J.; Vuorimaa, E.; Lemmetyinen, H.; Tenhu, H. Effect of the Number of Arms on the Association of Amphiphilic Star Block Copolymers. Macromolecules 2008, 41, 8855−8864. (20) Minehara, H.; Pitet, L. M.; Kim, S.; Zha, R. H.; Meijer, E. W.; Hawker, C. J. Branched Block Copolymers for Tuning of Morphology and Feature Size in Thin Film Nanolithography. Macromolecules 2016, 49, 2318−2326. (21) Georgopanos, P.; Lo, T.-Y.; Ho, R.-M.; Avgeropoulos, A. Synthesis, molecular characterization and self-assembly of (PS-bPDMS)n type linear (n = 1, 2) and star (n = 3, 4) block copolymers. Polym. Chem. 2017, 8, 843−850. (22) Yin, H.; Kang, S.-W.; Bae, Y. H. Polymersome Formation from AB2 Type 3-Miktoarm Star Copolymers. Macromolecules 2009, 42, 7456−7464. (23) Zhulina, E. B.; Borisov, O. V. Effect of Block Copolymer Architecture on Morphology of Self-Assembled Aggregates in Solution. ACS Macro Lett. 2013, 2, 292−295. (24) Deng, H.; Li, W.; Qiu, F.; Shi, A.-C. Self-Assembled Morphologies of Linear and Miktoarm Star Triblock Copolymer Monolayers. J. Phys. Chem. B 2017, 121, 4642−4649. (25) Xu, W.; Ledin, P. A.; Shevchenko, V. V.; Tsukruk, V. V. Architecture, Assembly, and Emerging Applications of Branched Functional Polyelectrolytes and Poly(ionic liquid)s. ACS Appl. Mater. Interfaces 2015, 7, 12570−12596. (26) Peleshanko, S.; Tsukruk, V. V. The architectures and surface behavior of highly branched molecules. Prog. Polym. Sci. 2008, 33, 523−580. (27) Zhou, Y.; Huang, W.; Liu, J.; Zhu, X.; Yan, D. Self-Assembly of Hyperbranched Polymers and Its Biomedical Applications. Adv. Mater. 2010, 22, 4567−4590. (28) Zhang, S.; Sun, H.-J.; Hughes, A. D.; Draghici, B.; Lejnieks, J.; Leowanawat, P.; Bertin, A.; Leon, L. O. D.; Kulikov, O. V.; Chen, Y.; Pochan, D. J.; Heiney, P. A.; Percec, V. Single-Single” Amphiphilic Janus Dendrimers Self-Assemble into Uniform Dendrimersomes with Predictable Size. ACS Nano 2014, 8, 1554−1565. (29) Hu, H.; Liu, G. Miktoarm Star Copolymer Capsules Bearing pH-Responsive Nanochannels. Macromolecules 2014, 47, 5096−5103. (30) Li, Z.; Ma, J.; Cheng, C.; Zhang, K.; Wooley, K. L. Synthesis of Hetero-Grafted Amphiphilic Diblock Molecular Brushes and their

Self-Assembly in Aqueous Medium. Macromolecules 2010, 43, 1182− 1184. (31) Wu, W.; Wang, W.; Li, J. Star polymers: Advances in biomedical applications. Prog. Polym. Sci. 2015, 46, 55−85. (32) Ren, J. M.; McKenzie, T. G.; Fu, Q.; Wong, E. H. H.; Xu, J.; An, Z.; Shanmugam, S.; Davis, T. P.; Boyer, C.; Qiao, G. G. Star Polymers. Chem. Rev. 2016, 116, 6743−6836. (33) Zhang, J.; Zhang, Q.; Zhou, S.; Liu, Y.; Huang, W. Synthesis and characterization of amphiphilic miktoarm star polymers based on sydnone-maleimide double cycloaddition. Polym. Chem. 2018, 9, 203−212. (34) Hung, C.-C.; Nakahira, S.; Chiu, Y.-C.; Isono, T.; Wu, H.-C.; Watanabe, K.; Chiang, Y.-C.; Takashima, S.; Borsali, R.; Tung, S.-H.; Satoh, T.; Chen, W.-C. Control over Molecular Architectures of Carbohydrate-Based Block Copolymers for Stretchable Electrical Memory Devices. Macromolecules 2018, 51, 4966−4975. (35) Zhang, Y.; Cao, M.; Han, G.; Guo, T.; Ying, T.; Zhang, W. Topology Affecting Block Copolymer Nanoassemblies: Linear Block Copolymers versus Star Block Copolymers under PISA Conditions. Macromolecules 2018, 51, 5440−5449. (36) Higashihara, T.; Hayashi, M.; Hirao, A. Synthesis of welldefined star-branched polymers by stepwise iterative methodology using living anionic polymerization. Prog. Polym. Sci. 2011, 36, 323− 375. (37) Zayas, H. A.; Truong, N. P.; Valade, D.; Jia, Z.; Monteiro, M. J. Narrow molecular weight and particle size distributions of polystyrene 4-arm stars synthesized by RAFT-mediated miniemulsions. Polym. Chem. 2013, 4, 592−599. (38) Whittaker, M. R.; Urbani, C. N.; Monteiro, M. J. Synthesis of 3Miktoarm Stars and 1st Generation Mikto Dendritic Copolymers by “Living” Radical Polymerization and “Click” Chemistry. J. Am. Chem. Soc. 2006, 128, 11360−11361. (39) Gao, H.; Ohno, S.; Matyjaszewski, K. Low Polydispersity Star Polymers via Cross-Linking Macromonomers by ATRP. J. Am. Chem. Soc. 2006, 128, 15111−15113. (40) Chan, J. W.; Yu, B.; Hoyle, C. E.; Lowe, A. B. Convergent synthesis of 3-arm star polymers from RAFT-prepared poly(N,Ndiethylacrylamide) via a thiol-ene click reaction. Chem. Commun. 2008, 4959−4961. (41) Sheng, Y.-J.; Nung, C.-H.; Tsao, H.-K. Morphologies of StarBlock Copolymers in Dilute Solutions. J. Phys. Chem. B 2006, 110, 21643−21650. (42) Wu, Y.; Wang, K.; Tan, H.; Xu, J.; Zhu, J. Emulsion Solvent Evaporation-Induced Self-Assembly of Block Copolymers Containing pH-Sensitive Block. Langmuir 2017, 33, 9889−9896. (43) Wu, Y.; Tan, H.; Yang, Y.; Li, Y.; Xu, J.; Zhang, L.; Zhu, J. Regulating Block Copolymer Assembly Structures in Emulsion Droplets through Metal Ion Coordination. Langmuir 2018, 34, 11495−11502. (44) Rahikkala, A.; Soininen, A. J.; Ruokolainen, J.; Mezzenga, R.; Raula, J.; Kauppinen, E. I. Self-assembly of PS-b-P4VP block copolymers of varying architectures in aerosol nanospheres. Soft Matter 2013, 9, 1492−1499. (45) Dan, M.; Huo, F.; Zhang, X.; Wang, X.; Zhang, W. Dispersion RAFT Polymerization of 4-Vinylpyridine in Toluene Mediated with the Macro-RAFT Agent of Polystyrene Dithiobenzoate: Effect of the Macro-RAFT Agent Chain Length and Growth of the Block Copolymer Nano-Objects. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1573−1584. (46) Khan, H.; Cao, M.; Duan, W.; Ying, T.; Zhang, W. Synthesis of diblock copolymer nano-assemblies: Comparison between PISA and micellization. Polymer 2018, 150, 204−213. (47) Zhao, W.; Gody, G.; Dong, S.; Zetterlund, P. B.; Perrier, S. Optimization of the RAFT polymerization conditions for thein situ formation of nano-objects via dispersion polymerization in alcoholic medium. Polym. Chem. 2014, 5, 6990−7003. (48) Jones, E. R.; Semsarilar, M.; Wyman, P.; Boerakker, M.; Armes, S. P. Addition of water to an alcoholic RAFT PISA formulation leads I


Article

Macromolecules to faster kinetics but limits the evolution of copolymer morphology. Polym. Chem. 2016, 7, 851−859. (49) Kang, Y.; Pitto-Barry, A.; Maitland, A.; O’Reilly, R. K. RAFT dispersion polymerization: a method to tune the morphology of thymine-containing self-assemblies. Polym. Chem. 2015, 6, 4984− 4992. (50) Zhang, X.; Rieger, J.; Charleux, B. Effect of the solvent composition on the morphology of nano-objects synthesized via RAFT polymerization of benzyl methacrylate in dispersed systems. Polym. Chem. 2012, 3, 1502−1509. (51) Cao, M.; Han, G.; Duan, W.; Zhang, W. Synthesis of multi-arm star thermo-responsive polymers and topology effects on phase transition. Polym. Chem. 2018, 9, 2625−2633. (52) Ding, Z.; Ding, M.; Gao, C.; Boyer, C.; Zhang, W. In Situ Synthesis of Coil-Coil Diblock Copolymer Nanotubes and Tubular Ag/Polymer Nanocomposites by RAFT Dispersion Polymerization in Poly(ethylene glycol). Macromolecules 2017, 50, 7593−7602. (53) Ma, L.; Kang, H.; Liu, R.; Huang, Y. Smart Assembly Behaviors of Hydroxypropylcellulose-graft-poly(4-vinyl pyridine) Copolymers in Aqueous Solution by Thermo and pH Stimuli. Langmuir 2010, 26, 18519−18525. (54) Stenzel-Rosenbaum, M.; Davis, T.; Chen, V.; Fane, A. StarPolymer Synthesis via Radical Reversible Addition-Fragmentation Chain-Transfer Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2777−2783. (55) Qu, Y.; Chang, X.; Chen, S.; Zhang, W. In situ synthesis of thermoresponsive 4-arm star block copolymer nano-assemblies by RAFT dispersion polymerization. Polym. Chem. 2017, 8, 3485−3496. (56) Whittaker, M. R.; Monteiro, M. J. Synthesis and Aggregation Behavior of Four-Arm Star Amphiphilic Block Copolymers in Water. Langmuir 2006, 22, 9746−9752. (57) Pang, X.; Zhao, L.; Akinc, M.; Kim, J. K.; Lin, Z. Novel Amphiphilic Multi-Arm, Star-Like Block Copolymers as Unimolecular Micelles. Macromolecules 2011, 44, 3746−3752. (58) Wang, Q.; Chu, B.-F.; Chu, J.-H.; Liu, N.; Wu, Z.-Q. Facile Synthesis of Optically Active and Thermoresponsive Star Block Copolymers Carrying Helical Polyisocyanide Arms and Their Thermo-Triggered Chiral Resolution Ability. ACS Macro Lett. 2018, 7, 127−131. (59) Li, C.; Lavigueur, C.; Zhu, X. X. Aggregation and Thermoresponsive Properties of New Star Block Copolymers with a Cholic Acid Core. Langmuir 2011, 27, 11174−11179. (60) Yuan, W.; Chen, X. Star-shaped and star-block polymers with a porphyrin core: from LCST-UCST thermoresponsive transition to tunable self-assembly behaviour and fluorescence performance. RSC Adv. 2016, 6, 6802−6810. (61) Wang, X.; Shen, L.; An, Z. Dispersion polymerization in environmentally benign solvents via reversible deactivation radical polymerization. Prog. Polym. Sci. 2018, 83, 1−27. (62) Derry, M. J.; Fielding, L. A.; Armes, S. P. Polymerizationinduced self-assembly of block copolymer nanoparticles via RAFT non-aqueous dispersion polymerization. Prog. Polym. Sci. 2016, 52, 1− 18. (63) Couturaud, B.; Georgiou, P. G.; Varlas, S.; Jones, J. R.; Arno, M. C.; Foster, J. C.; O’Reilly, R. K. Poly(Pentafluorophenyl Methacrylate)-Based Nano-Objects Developed by Photo-PISA as Scaffolds for Post-Polymerization Functionalization. Macromol. Rapid Commun. 2018, 1800460. (64) Zhang, X.; Wang, C.; Fang, S.; Sun, J.; Li, C.; Hu, Y. Synthesis and characterization of well-defined star PLLA with a POSS core and their microspheres for controlled release. Colloid Polym. Sci. 2013, 291, 789−803. (65) Roovers, J. E. L.; Toporowski, P. M. Glass transition temperature of star-shaped polystyrenes,. J. Appl. Polym. Sci. 1974, 18, 1685−1691. (66) Skrabania, K.; Li, W.; Laschewsky, A. Synthesis of DoubleHydrophilic BAB Triblock Copolymers via RAFT Polymerization and their Thermoresponsive Self-Assembly in Water. Macromol. Chem. Phys. 2008, 209, 1389−1403.

(67) Papagiannopoulos, A.; Zhao, J.; Zhang, G.; Pispas, S.; Radulescu, A. Thermoresponsive aggregation of PS-PNIPAM-PS triblock copolymer: A combined study of light scattering and small angle neutron scattering. Eur. Polym. J. 2014, 56, 59−68. (68) Qu, Y.; Wang, S.; Khan, H.; Gao, C.; Zhou, H.; Zhang, W. One-pot preparation of BAB triblock copolymer nano-objects through bifunctional macromolecular RAFT agent mediated dispersion polymerization. Polym. Chem. 2016, 7, 1953−1962. (69) Wang, W.; Wang, X.; Jiang, F.; Wang, Z. Synthesis, order-todisorder transition, microphase morphology and mechanical properties of BAB triblock copolymer elastomers with hard middle block and soft outer blocks. Polym. Chem. 2018, 9, 3067−3079. (70) Rodríguez-Hidalgo, M. d. R.; Soto-Figueroa, C.; Vicente, L. Study of structural morphologies of thermoresponsive diblock AB and triblock BAB copolymers (A = poly(N-isopropylacrylamide), B = polystyrene). Chem. Phys. Lett. 2018, 695, 170−175. (71) Zahoranová, A.; Mrlík, M.; Tomanová, K.; Kronek, J.; Luxenhofer, R. ABA and BAB Triblock Copolymers Based on 2Methyl-2-oxazoline and 2-n-Propyl-2-oxazoline: Synthesis and Thermoresponsive Behavior in Water. Macromol. Chem. Phys. 2017, 218, 1700031. (72) He, Y.; Lodge, T. P. Thermoreversible Ion Gels with Tunable Melting Temperatures from Triblock and Pentablock Copolymers. Macromolecules 2008, 41, 167−174. (73) Ding, A.; Lu, G.; Guo, H.; Huang, X. PDMAEMA-b-PPOA-bPDMAEMA double-bond-containing amphiphilic triblock copolymer: synthesis, characterization, and pH-responsive self-assembly. Polym. Chem. 2017, 8, 6628−6635. (74) Yang, L.; Han, Q.; Song, Q.; Li, H.; Zhao, Q.; Shen, Y.; Luo, Y. Control over ABA-type triblock copolymer latex morphology in RAFT miniemulsion polymerization and mechanical properties of the latex films. Colloid Polym. Sci. 2017, 295, 891−902. (75) Wu, J.; Sun, X.; Zhang, R.; Yuan, S.; Wu, Z.; Lu, Q.; Yu, Y. RAFT preparation and self-assembly behavior of thermosensitive triblock PNIPAAm-b-PODA-b-PNIPAAm copolymers. Colloid Polym. Sci. 2016, 294, 1989−1995. (76) McKenzie, B. E.; Friedrich, H.; Wirix, M. J. M.; de Visser, J. F.; Monaghan, O. R.; Bomans, P. H. H.; Nudelman, F.; Holder, S. J.; Sommerdijk, N. A. J. M. Controlling Internal Pore Sizes in Bicontinuous Polymeric Nanospheres. Angew. Chem., Int. Ed. 2015, 54, 2457−2461. (77) Barnhill, S. A.; Bell, N. C.; Patterson, J. P.; Olds, D. P.; Gianneschi, N. C. Phase Diagrams of Polynorbornene Amphiphilic Block Copolymers in Solution. Macromolecules 2015, 48, 1152−1161. (78) Gao, C.; Wu, J.; Zhou, H.; Qu, Y.; Li, B.; Zhang, W. SelfAssembled Blends of AB/BAB Block Copolymers Prepared through Dispersion RAFT Polymerization. Macromolecules 2016, 49, 4490− 4500. (79) Qi, W.; Ghoroghchian, P. P.; Li, G.; Hammer, D. A.; Therien, M. J. Aqueous self-assembly of poly(ethylene oxide)-block-poly(εcaprolactone) (PEO-b-PCL) copolymers: disparate diblock copolymer compositions give rise to nano- and meso-scale bilayered vesicles. Nanoscale 2013, 5, 10908. (80) Azzam, T.; Eisenberg, A. Control of Vesicular Morphologies through Hydrophobic Block Length. Angew. Chem., Int. Ed. 2006, 45, 7443−7447. (81) He, W.-D.; Sun, X.-L.; Wan, W.-M.; Pan, C.-Y. Multiple Morphologies of PAA-b-PSt Assemblies throughout RAFT Dispersion Polymerization of Styrene with PAA Macro-CTA. Macromolecules 2011, 44, 3358−3365. (82) Huang, J.; Bonduelle, C.; Thévenot, J.; Lecommandoux, S.; Heise, A. Biologically Active Polymersomes from Amphiphilic Glycopeptides. J. Am. Chem. Soc. 2012, 134, 119−122. (83) Zhang, W.; D’Agosto, F.; Boyron, O.; Rieger, J.; Charleux, B. Toward a Better Understanding of the Parameters that Lead to the Formation of Nonspherical Polystyrene Particles via RAFT-Mediated One-Pot Aqueous Emulsion Polymerization. Macromolecules 2012, 45, 4075−4084. J


Article

Macromolecules (84) He, W.-D.; Sun, X.-L.; Wan, W.-M.; Pan, C.-Y. Multiple Morphologies of PAA-b-PSt Assemblies throughout RAFT Dispersion Polymerization of Styrene with PAA Macro-CTA. Macromolecules 2011, 44, 3358−3365.

K


Star Block Copolymer Nanoassemblies: Block Sequence is All

Recommend Documents