Topology Affecting Block Copolymer Nanoassemblies: Linear Block

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Topology Affecting Block Copolymer Nanoassemblies: Linear Block Copolymers versus Star Block Copolymers under PISA Conditions Yuan Zhang,† Mengjiao Cao,† Guang Han,*,§ Tianying Guo,† Tengyuan Ying,∥ 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 ∥ Institute of Semiconductor Technology of Tianjin, Tianjin, China S Supporting Information *

ABSTRACT: Linear and star block copolymer (BCP) nanoassemblies of [poly(4-vinylpyridine)-block-polystyrene]n ([P4VP-b-PS]n) with different arm numbers have been synthesized by RAFT dispersion polymerization under formulation of polymerization-induced self-assembly (PISA). All RAFT dispersion polymerizations employing mono- and multifunctional macromolecular chain transfer agents proceed with similar polymerization kinetics. The size and/or morphology of [P4VP-b-PS]n nanoassemblies are firmly correlative to arm number n, and star [P4VP-b-PS]n BCPs have more complex morphology than the linear counterpart. Several interesting morphologies of star BCPs including small-sized vesicles and porous nanospheres have been synthesized, and they are compared with those of the linear counterpart. Our research indicates that topology is a significant parameter to dedicate the size and morphology of star BCP nanoassemblies under PISA conditions.

1. INTRODUCTION Block copolymer (BCP) nanoassemblies dispersed in solvent have attracted great attention for their various morphologies and their potential applications in many fields.1−4 Block copolymer morphology is correlative to several parameters including solvent character,5−7 block copolymer concentration,8−10 and block copolymer architecture.11−21 Of all these parameters, polymer architecture is the most prominent. For examples, star BCPs include multiple arms of linear BCP tethered onto one central core,22−32 and these star BCPs combine special features of both linear BCPs1−4 and star polymers22−26 into one entity. These star BCPs as well as those with complex architectures including miktoarm star copolymers have different morphology from the linear BCPs.27−32 For example, Huh and co-workers revealed that amphiphilic 3miktoarm poly(ethylene glycol)−[poly(ε-caprolactone)]2 with long poly(ε-caprolactone) arms self-assembled to cylindrical micelles in water, whereas the corresponding linear counterparts consistently formed spherical micelles with approximately the same molecular weight and volume fraction.17 Mays et al. discovered that higher-order thin film morphologies were formed for Y-shaped [poly(2-vinylpyridine)]2−polystyrene [(P2VP)2−PS] compared to the linear polystyrene-b-poly(2vinylpyridine) with similar molecular weight.18 Moreover, © XXXX American Chemical Society

Tsukruk and co-workers reported highly branched polymers possess complex and unique assembly behavior in solution at surfaces and interfaces as compared to their linear counterparts.11−13 Recently, polymerization-induced self-assembly (PISA) through RAFT dispersion polymerization has been widely recognized to be a convenient method to synthesize BCP nanoassemblies with controllable morphology.33−38 By tuning the degree of polymerization (DP) of solvophilic/solvophobic blocks, various concentrated BCP nanoassemblies were synthesized.33−38 However, most PISA researches are focused on linear AB diblock copolymers39−45 or ABC46−52 or ABA53−55 triblock copolymers, and copolymers with complex architectures including star BCPs and miktoarm star copolymers are rarely explored.56,57 In this study, 2-, 3-, and 4-arm star and linear BCP nanoassemblies of [poly(4-vinylpyridine)-block-polystyrene]n ([P4VP-b-PS]n, n = 1, 2, 3, and 4) are synthesized by RAFT dispersion polymerization employing mono- and multifunctional macromolecular chain transfer agents (macro-CTAs), Received: May 28, 2018 Revised: June 26, 2018

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DOI: 10.1021/acs.macromol.8b01121 Macromolecules XXXX, XXX, XXX−XXX

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dioxane/ethanol mixture (1/9 by weight, 12.0 g) were weighed into a 50 mL Schlenk flask. The flask content was initially degassed to remove O2, and then polymerization was ran at 70 °C for 9 h and stopped by cooling in ice−water. The monomer conversion of 80.4% was obtained by 1H NMR. The synthesized [P4VP24−TTC]2 was precipitated into diethyl ether and dried under vacuum. By changing the [4VP]0:[2]0:[AIBN]0 molar ratio, the other two [P4VPm-TTC]2 macro-CTAs were also synthesized (Table 1). By employing trithiocarbonates of 1, 3, or 4 as CTAs, P4VP-TTC, (P4VP-TTC)3, and (P4VP-TTC)4 with different DPs of the P4VP arms were synthesized (Table 1). 2.4. RAFT Dispersion Polymerization and Synthesis of (P4VP-b-PS)n Nanoassemblies. RAFT dispersion polymerization of styrene was performed in methanol/water mixture (80/20 by weight) at 70 °C with 20 wt % monomer concentration. Herein is a typical example under [St]0:[(P4VP24-TTC)2]0:[AIBN]0 = 1800:3:2. (P4VP24-TTC)2 (0.044 g, 0.0080 mmol), St (0.500 g, 4.80 mmol), and AIBN (0.870 mg, 0.0053 mmol) dissolved in 80/20 methanol/ water mixture (2.17 g) were weighed into a 25 mL Schlenk flask. The flask content was degassed, then polymerization proceeded at 70 °C under stirring for a given time, and finally it was stopped by rapid cooling. Styrene conversion was detected by UV−vis analysis at 244.9 nm as discussed elsewhere.58,59 The resultant 2-arm star [P4VP-bPS]2 nanoassemblies were dialyzed against the 80/20 methanol/water mixture (molecular weight cutoff: 3500 Da), and then the [P4VP-bPS]2 nanoassemblies were checked by TEM. The [P4VP-b-PS]2 nanoassemblies were collected by centrifugation (10 000 rpm), dissolved in DCM, precipitated into methanol, dried under vacuum, and finally characterized by gel permeation chromatography (GPC) and 1H NMR. 2.5. Characterization. NMR spectra were recorded on a Bruker Avance III 400 MHz NMR spectrometer with CDCl3 as the solvent. A Varian 100 UV−vis spectrophotometer was used for UV−vis analysis. Polymer molecular weight and dispersity (Đ = Mw/Mn) were determined by a Waters 600E GPC system, in which DMF was used as the eluent and samples of narrow-polydispersity polystyrene were used as standards to calibrate apparatus. A Tecnai G2 F20 or a Talos F200C electron microscope performed at 200 kV was used for TEM observation. Dynamic light scattering (DLS) analysis was carried on a NanoBrook Omni (Brookhaven) laser light scattering spectrometer, and the hydrodynamic diameter (Dh) of the synthesized BCP nanoassemblies was measured by intensity following the CONTIN method. The atomic force microscope (AFM) was a NTEGRA Prima (NT-MDT, Russia).

and the effect of block copolymer topology on the star BCP nanoassemblies is explored. It is discovered that (1) RAFT dispersion polymerizations employing mono- and multifunctional macro-CTAs proceed with the similar polymerization kinetics; (2) the morphology of [P4VP-b-PS]n nanoassemblies is firmly dependent on the arm number n, and star BCPs have more complex morphology than the linear counterparts; and (3) the morphology of star [P4VP-b-PS]n nanoassemblies can be modulated by changing the DP of either the P4VP or PS block.

2. EXPERIMENTAL SECTION 2.1. Materials. Styrene (St, >98%, Tianjin Chemical Company) and 4-vinylpyridine (4VP, 96%, Alfa) were distilled under reduced pressure before use. 1-Butanethiol (98%, Alfa Aesar), benzyl bromide and 1,2-bis(bromomethyl)benzene (>98%, Adamas), 1,3,5-tris(bromomethyl)benzene (98%, J&K), and 1,2,4,5-tetrakis(bromomethy)benzene (95%, Tianjin Heowns Chemistry Company) were used without any purification. 2,2′-Azobis(2-methylpropionitrile) (AIBN, >99%, Tianjin Chemical Company, China) was recrystallized from ethanol before use. All other chemical reagents were of analytic grade and were commercially available. Deionized water was used. 2.2. Synthesis of Mono- and Multifunctional Trithiocarbonates. Four trithiocarbonates as listed in Scheme 1 were

Scheme 1. Mono- and Multifunctional Trithiocarbonates

3. RESULTS AND DISCUSSION 3.1. Synthesis of Mono- and Multifunctional Trithiocarbonates. Four trithiocarbonate CTAs of 1, 2, 3, and 4 as listed in Scheme 1 were synthesized. This synthesis includes a nucleophilic addition and a nucleophilic substitution as shown in Scheme S1. All the synthesis affords above 67% yield of trithiocarbonates. High purity of synthesized CTAs was confirmed by NMR (Figure 1). All the four CTAs have very similar/same R and Z groups, except different functional trithiocarbonate numbers. For CTA of 1, just one trithiocarbonate is tethered onto the benzene ring. For CTAs of 2, 3, and 4, two pieces of trithiocarbonate locate at the ortho position on the benzene ring, three pieces of trithiocarbonate locate at the meta position, and four pieces of trithiocarbonate locate at the 1, 2, 4, and 5 positions. When CTAs of 1, 2, 3, and 4 are used in RAFT polymerization, linear polymers and 2-, 3-, and 4-arm star polymers are prepared, and the arms in these star polymers are expected to have similar or same chain length due to the similar/same R and Z groups in the CTAs. 3.2. Synthesis of Linear and 2-, 3-, and 4-Arm MacroCTAs of (P4VP-TTC)n. Linear and star macro-CTAs of (P4VP-TTC)n (Scheme 2) were prepared by solution RAFT

synthesized. Herein, synthesis of butyltrithiomethylbenzene (1) was typically introduced. Potassium phosphate (12.7 g, 0.060 mol), 1butanethiol (6.4 mL, 0.060 mol), and acetone (150 mL) were weighed into a two-neck flask at 0 °C; then carbon disulfide (10.9 mL, 0.18 mol) dissolved in acetone (50 mL) was slowly added. The reaction mixture was kept at 0 °C 1 h and then at 25 °C for 12 h. After that, (bromomethyl)benzene (4.75 mL, 0.040 mol) dissolved in acetone (50 mL) was slowly added. The solution was kept at 25 °C for 24 h and concentrated, and the crude product was purified by column chromatography on silica gel to afford butyltrithiomethylbenzene (1, 8.5 g, 83% yield, 97% end functionality). 2.3. Synthesis of Linear and 2-, 3-, and 4-Arm Star MacroCTAs. The linear and 2-, 3-, and 4-arm star macro-CTAs of (P4VPmTTC)n, in which n and m represent the arm number and the DP of each P4VP arm, with different DPs of the P4VP arms were synthesized by RAFT polymerization employing chain transfer agents (CTAs) of 1, 2, 3, and 4. Here is a typical synthesis of [P4VP24TTC]2 under [4VP]0:[2]0:[AIBN]0 = 240:4:1. 4VP (3.00 g, 28.57 mmol), 2 (0.207 g, 0.48 mmol), AIBN (0.020 g, 0.12 mmol), and 1,4B

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Macromolecules Table 1. Summary of the Synthesized Macro-CTAs Mn (kg/mol) macro-CTA

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

time (h)

conva (%)

Mn,thb

Mn,GPCc

Mn,NMRd

Đe

P4VP17-TTC P4VP26-TTC P4VP66-TTC [P4VP17-TTC]2 [P4VP24-TTC]2 [P4VP66-TTC]2 [P4VP18-TTC]3 [P4VP26-TTC]3 [P4VP68-TTC]3 [P4VP18-TTC]4 [P4VP25-TTC]4 [P4VP70-TTC]4

160:8:1 240:8:1 800:8:1 144:4:1 240:4:1 800:4:1 480:8:3 720:8:3 2400:8:3 160:2:1 320:2:1 800:2:1

9 9 12 9 9 12 9 9 12 9 7 12

83.7 87.0 65.8 94.4 80.4 66.0 91 88.7 68 88.6 61.1 70

2.0 3.0 7.2 4.0 5.5 14.3 6.3 8.8 22.0 8.4 11.3 30.2

3.8 4.5 8.5 5.8 7.0 16.2 7.2 11.2 24.1 9.3 12.7 33.1

2.3 4.5 8.6 4.3 7.1 16.5 6.4 11.3 24.5 8.5 12.8 33.3

1.13 1.14 1.15 1.14 1.15 1.17 1.17 1.20 1.25 1.18 1.22 1.24

a

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

d

Figure 1. 1H NMR spectra (A) and 13C NMR spectra (B) of mono- and multifunctional macro-CTAs of trithiocarbonate. Note: peak e at 220 ppm is out the range of test in the 13C NMR spectra.

of the P4VP arms are synthesized (Table 1). The (P4VPTTC)n macro-CTAs were analyzed by GPC and NMR, and the results for the typical P4VP26-TTC, [P4VP24-TTC]2, [P4VP26-TTC]3, and [P4VP25-TTC]4 with a similar DP of the P4VP arms around 24−26 but different arm numbers are shown in Figures 2 and 3. See the GPC traces of (P4VPmTTC)n (n = 1, 2, 3, 4) with a similar DP of the P4VP arm around 17 in Figure S1. The molecular weight of (P4VPTTC)n by 1H NMR, Mn,NMR, is calculated by comparing the peak areas at 0.88 and 8.33 ppm corresponding to the RAFT terminal and pyridine ring, respectively, and the obtained M n,NMR is close to theoretical molecular weight M n,th determined by monomer conversion following eq S1 (Table 1). Based on Mn,NMR or Mn,th of (P4VP-TTC)n, DP of the each P4VP arms is calculated by assuming all P4VP arms have the same chain length. In Figure 3 and Figure S1 unimodal GPC traces are observed for linear P4VP-TTC, while for star (P4VPm-TTC)n especially for 4-arm star (P4VP-TTC)4 a slight shoulder at the high-molecular-weight side is observed. This usually occurs in

Scheme 2. (P4VPm-TTC)n Macro-CTAs

polymerization using CTAs of 1, 2, 3, and 4. By targeting a given DP of the P4VP arm, (P4VP-TTC)n with a suitable DP C

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Figure 2. 1H NMR spectra of P4VP26-TTC, [P4VP24-TTC]2, [P4VP26-TTC]3, and [P4VP25-TTC]4.

same Z and R groups and similar DP of P4VP arms in (P4VPTTC)n macro-CTAs. Just similar to RAFT dispersion polymerizations discussed elsewhere,63−69 the present RAFT dispersion polymerization employing macro-CTAs of (P4VPTTC)n includes an initial slow homogeneous RAFT polymerization and then a fast heterogeneous one as indicated by twostage plot of ln([M]0/[M]) versus polymerization time (Figure 4B). The synthesized 2-arm star [P4VP24-b-PS]2 are typically analyzed by GPC (Figure 4C) and 1H NMR, in which Mn.NMR is calculated by comparing the chemical shifts at 8.33 ppm assigned to the pyridine ring and 6.20−7.20 ppm assigned to the benzene ring, and the results are summarized in Figure 4D. In GPC traces of [P4VP24-b-PS]2, a slight shoulder is observed at high monomer conversion above 80%, which is reflected by the increasing Đ shown in Figure 4D. Considering the star structure of [P4VP24-b-PS]2 and Đ around 1.3, this RAFT synthesis is qualified. It is also noticed that at low monomer conversion the synthesized star [P4VP24-b-PS]2 has low molecular weight just as expected, and Mn,GPC is close to Mn,NMR. With low monomer conversion increasing above 20%, Mn.NMR of [P4VP24-b-PS]2 becomes larger than Mn,GPC, which is similar to star polymers reported elsewhere.61,62 In the polymerization medium of the 80/20 methanol/water mixture, P4VP is soluble and PS is insoluble, and therefore the resultant [P4VP-b-PS]n BCPs form nanoassemblies in which the solvophobic PS constructs the body of the nanoassemblies and the solvophilic P4VP stabilizes the nanoassemblies in the alcoholic solvent. The typical 2-arm star [P4VP24-b-PS]2 nanoassemblies with different DPs of the PS block were observed by TEM (Figure 5). The morphology of 2-arm star [P4VP24-b-PS]2 nanoassemblies undergoes the transition from 12 ± 2 nm nanospheres of [P4VP24-b-PS30]2, to 56 ± 10 nm vesicles as well as few nanowires of [P4VP24-b-PS95]2, to 73 ± 13 nm vesicles of [P4VP24-b-PS141]2, to the mixture of 98 ± 20 nm vesicles and lacunal nanospheres of [P4VP24-b-PS230]2, to 116 ± 10 nm lacunal nanospheres of [P4VP24-b-PS252]2, and finally to 132 ± 20 nm lacunal nanospheres of [P4VP24-bPS290]2 with increasing DP of the PS block. The 2-arm star

Figure 3. GPC traces of P4VP26-TTC, [P4VP24-TTC]2, [P4VP26TTC]3, and [P4VP25-TTC]4.

synthesis of star polymers, and the possible reason is usually ascribed to the bimolecular termination products from star− star coupling and linear−star coupling,60 since this star structure includes more than two propagating radicals in one molecule. All star (P4VPm-TTC)n macro-CTAs have narrow molecular weight distribution as indicated by Đ centered around 1.2 (Table 1). It is also noticed that molecular weight of star (P4VPm-TTC)n by GPC analysis, Mn,GPC, is close to Mn,NMR, which is different from those reported previously,61,62 in which Mn,GPC < Mn,NMR is usually found. The possible reason is due to the relatively low molecular weight of star polymers, which will be further discussed subsequently. 3.3. RAFT Dispersion Polymerization and Synthesis of Star BCP Nanoassemblies. RAFT dispersion polymerizations employing the synthesized star macro-CTAs of (P4VP-TTC)n with a similar DP of the P4VP arms, e.g., P4VP26-TTC, (P4VP24-TTC)2, (P4VP26-TTC)3, and (P4VP25TTC)4, were investigated. All RAFT dispersion polymerizations were performed under a constant ratio of [St]0: [trithiocarbonate]0:[AIBN]0 = 300:1:1/3. Since the trithiocarbonate number in each star (P4VP-TTC)n macro-CTAs is different, this constant ratio can ensure similar DP of the PS block in each star [P4VP-b-PS]n BCPs at similar monomer conversion. As shown in Figure 4A, all RAFT dispersion polymerizations employing different (P4VP-TTC)n macro-CTAs proceed with similar polymerization kinetics. This is possibly due to the D

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Figure 4. Monomer conversion (A) and ln([M]0/[M])−time plot (B) in RAFT dispersion polymerization using macro-CTAs of linear and 2-, 3-, and 4-arm star [P4VP-TTC]n, GPC traces (C), and summary of Mn,th, Mn,NMR, Mn,GPC, and Đ values of 2-arm star [P4VP24-b-PS]2 prepared through RAFT dispersion polymerization (D).

star [P4VP24-b-PS]2 having different morphologies. Herein, block copolymer topology affecting the [P4VP-b-PS]n nanoassemblies was further investigated. To fulfill this, (P4VP-bPS)n nanoassemblies with a similar P4VP block but different PS block and those with different P4VP block but a similar PS block were synthesized (see the synthesis and characterization of (P4VP-b-PS)n in Table S1), and then they were detected by TEM. Figure 6 summarizes the [P4VP-b-PS]n nanoassemblies with a similar P4VP block but a different PS block, and Dh of the [P4VP-b-PS]n nanoassemblies are summarized in Figure S3. The results indicate that (P4VP-b-PS)n with 1, 2, and 3 arms form discrete nanoassemblies, and 4-arm star (P4VP-b-PS)4 tends to form aggregates even at the case of a short solvophobic PS block. The formation of large-sized aggregates of 4-arm star (P4VP-b-PS)4 is possibly derived from the bridging of single nanoassemblies, which is also found in the micellization of BAB triblock copolymer containing two outside solvophobic B blocks and star block copolymers.70−73 For linear P4VP26-b-PS nanospheres are synthesized by RAFT dispersion polymerization, and their size increases from 18 ± 1 to 91 ± 5 nm (Figure 6A). For 2-arm star (P4VP24-b-PS)2, vesicles and then lacunal nanospheres are synthesized with increasing DP of the PS block (Figure 6B). AFM indicates a smooth surface of the vesicles and lacunal nanospheres (Figure S4), suggesting that pores are located inside of the 2-arm star (P4VP24-b-PS)2 nanoassemblies. For 3-arm star (P4VP26-bPS)3, vesicles, then lacunal nanospheres, and finally porous nanospheres are obtained with increasing DP of the PS block (Figure 6C). It is noticed that vesicles of star (P4VP24-b-PS93)2 and (P4VP26-b-PS94)3 are much different from linear block copolymer vesicles in two points. First, the size of star block

Figure 5. TEM images of 2-arm star [P4VP24-b-PS]2 nanoassemblies prepared through RAFT dispersion polymerization at 2 (A), 3 (B), 4 (C), 7 (D), 9 (E), and 14 h (F). Note: the insets in (B) and (F) show the stained nanoassemblies; in the staining 0.5 wt % dispersion of the nanoassemblies in 80/20 methanol/water was mixed with pH 1 HCl aqueous solution under equal volume for about 10 min, and then the nanoassemblies were checked by TEM.

[P4VP24-b-PS]2 nanoassemblies were also checked by DLS, and it is revealed that Dh of the 2-arm star [P4VP24-b-PS]2 nanoassemblies increases from 21 to 152 nm with increasing DP of the PS block (Figure S2). In comparison, for linear P4VP26-b-PS, just solid nanospheres were formed even at case of high DP of the PS block at 474 (Figure 6), which will be further discussed subsequently. 3.4. Effect of Topology on BCP Nanoassemblies. The aforementioned results demonstrate linear P4VP26-b-PS and E

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Figure 6. TEM images of [P4VP-b-PS]n nanoassemblies including a similar P4VP block but a different PS block. The insets in (B) and (C) show the nanoassemblies stained with pH 1 HCl aqueous solution.

copolymer vesicles is as small as about 65 ± 13 nm, much smaller than general linear BCP vesicles.73−77 Second, all star block copolymer vesicles are formed even at a relatively low DP of the PS block, whereas linear block copolymer vesicles are usually formed at case of a relatively long solvophobic block.74−76 Recently, theoretical simulation indicates formation of porous nanoassemblies of star copolymers under PISA condition,78 which is well-consistent with the present experiment. Herein, formation of small-sized vesicles, lacunal nanospheres, and porous nanospheres of star (P4VP-b-PS)2 and (P4VP-b-PS)3 is possibly due to star P4VP blocks, which are connected to a joint and therefore to increase steric repulsion of the P4VP arms. This steric repulsion of the jointed P4VP arms makes PS chains more stretched in star BCP nanoassemblies, and therefore higher order morphologies such as vesicles are formed even at case of low DP of the PS block. Also, since these star BCP vesicles are formed with a low DP of the PS block, their size is therefore smaller than that of linear BCP vesicles. Figure 7 and Figure S5 summarize the [P4VP-b-PS]n nanoassemblies with a similar DP of the PS block at about 290 while DP of P4VP block increasing from 17 to 70. Similarly as discussed above, discrete nanoassemblies of [P4VP-b-PS]n with n = 1−3 are formed, and 4-arm star (P4VP-b-PS)4 forms bridged aggregates. It is discovered that either linear P4VP-b-PS or star [P4VP-b-PS]2−3 follows the similar rule; that is, the size of [P4VP-b-PS]n nanoassemblies decreases with increasing DP of solvophilic P4VP block, which

is reported previously in synthesis of linear BCP nanoassemblies.71−73 However, star [P4VP-b-PS]2−3 nanoassemblies have much complex morphology, e.g., porous nanospheres and lacunal nanospheres, than linear P4VP-b-PS. Furthermore, with increasing DP of the P4VP block, transitions from porous nanospheres to lacunal nanospheres and finally to the solid nanospheres are observed in both 2-arm star [P4VP-b-PS]2 and 3-arm star [P4VP-b-PS]3. By further comparing three solid nanospheres of [P4VP-b-PS]n with similar DPs of the PS block and the P4VP arm, e.g., 33 ± 4 nm P4VP66-b-PS289 nanospheres, 31 ± 4 nm [P4VP66-b-PS292]2 nanospheres, and 70 ± 16 nm [P4VP68-b-PS291]3 nanospheres, it is discovered that the size of these nanospheres is in the order of [P4VP68-b-PS291]3 > P4VP66-b-PS289 > [P4VP66-bPS292]2. Generally, star BCP nanoassemblies are smaller than those of linear BCPs with similar DP of the solvophobic and solvophilic blocks,15,79,80 and the possible reason is due to star BCPs having better solubility than linear ones (seeing the critical aggregation concentration of [P4VP-b-PS]n in Table S2). However, herein the [P4VP68-b-PS291]3 nanospheres are the largest in all the three nanoassemblies, and the reason is possibly due to increased bridging in the 3-arm star block copolymers, since the block copolymer concentration under the present PISA condition is much higher than the traditional self-assembly of presynthesized BCPs in block-selective solvents.15,79,80 F

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Figure 7. TEM images of [P4VP-b-PS]n nanoassemblies including a similar PS block but a different P4VP block. The insets in (B) and (C) show [P4VP-b-PS]n nanoassemblies stained with pH 1 HCl aqueous solution.

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4. CONCLUSIONS In conclusion, linear and star BCP nanoassemblies of [P4VP-bPS]n with the arm number n at 1, 2, 3, and 4 have been synthesized by RAFT dispersion polymerization using star [P4VP-TTC]n macro-CTAs. These polymerizations employing star [P4VP-TTC]n macro-CTAs with different arm numbers proceed with similar polymerization kinetics, and the reason is due to the similar R and Z groups in the [P4VP-TTC]n macroCTAs. The size and/or morphology of the [P4VP-b-PS]n nanoassemblies are firmly correlative to arm number n, and star [P4VP-b-PS]n has more complex morphology than the linear counterpart. Several interesting morphologies of star BCPs including small-sized vesicles, lacunal nanospheres, and porous nanospheres have been synthesized by changing the arm number n in the [P4VP-TTC]n macro-CTAs and the DP of the P4VP and/or PS blocks. Also, in the RAFT dispersion polymerization, star [P4VP-b-PS]n nanoassemblies undergo transitions from vesicles, to lacunal nanospheres, and finally to porous nanospheres with increasing DP of the PS block. The possible reason that star [P4VP-b-PS]n has more complex morphology than linear counterpart is discussed, and the steric repulsion of star architecture is possibly involved. Our study demonstrates an easy synthesis of star BCP nanoassemblies under PISA conditions and indicates topology is an important parameter to dedicate star BCP nanoassemblies, and these star copolymers may bring great potential to produce porous nanoassemblies.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01121.

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Scheme S1, Figures S1−S5, and Tables S1 and S2 (PDF)

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.

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ACKNOWLEDGMENTS The financial support by the National Science Foundation for Distinguished Young Scholars (no. 21525419), the National Science Foundation of China (no. 21474054), Tianjin Natural Science Foundation (16YFFCZC00130), the National Key Research and Development Program of China (2017YFC1103501) is gratefully acknowledged. G

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