How the Polymerization Procedures Affect the Morphology of the Block

Oct 14, 2016 - Polymerization-induced self-assembly (PISA) is proven to be a powerful approach of in situ synthesis of block copolymer (BCP) ...
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How the Polymerization Procedures Affect the Morphology of the Block Copolymer Nanoassemblies: Comparison between Dispersion RAFT Polymerization and Seeded RAFT Polymerization Heng Zhou,† Chonggao Liu,† Yaqing Qu,† Chengqiang Gao,† Keyu Shi,*,† and Wangqing Zhang*,†,‡ †

Key Laboratory of Functional Polymer Materials of the Ministry of Education, Institute of Polymer Chemistry and ‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: Polymerization-induced self-assembly (PISA) is proven to be a powerful approach of in situ synthesis of block copolymer (BCP) nanoassemblies, and polymerization conditions are found to be correlative to the block copolymer morphology. In this study, three PISA formulations, e.g., the poly(ethylene glycol) macro-RAFT agent mediated dispersion RAFT polymerization, seeded dispersion RAFT polymerization, and seeded emulsion RAFT polymerization, are comparatively investigated. Our results reveal that dispersion RAFT polymerization undergoes much slower than other two PISA formulations of seeded dispersion RAFT polymerization and seeded emulsion RAFT polymerization. Besides, the results reveal that the BCP morphology of poly(ethylene glycol)-block-polystyrene (PEG45-b-PS) produced via three PISA cases is much different. That is, dispersion RAFT polymerization affords vesicles, seeded dispersion RAFT polymerization affords the mixture of vesicles and porous nanospheres, and seeded emulsion RAFT polymerization affords porous nanospheres of PEG45-b-PS. The reason for formation of porous nanospheres by seeded RAFT polymerization is discussed, and the fed styrene monomer swelling the seeded vesicles is ascribed. Our study clarifies how the PISA procedures affect the morphology of BCP nanoassemblies, and it is expected to be effective to prepare BCP nanoassemblies with interesting morphology. performed under aqueous emulsion condition.18−24 Aqueous emulsion polymerization is one of the most adopted approaches for preparation of polymeric colloids. The reaction starts from a monomer-in-water (O/W) emulsion containing a surfactant and a water-soluble radical initiator, and this emulsion polymerization results in formation of colloidal particles. Hawkett et al. pioneered this field of PISA by employing a poly(acrylic acid) macro-RAFT agent to obtain poly(acrylic acid)-b-poly(n-butyl acrylate) nanospheres under good control.18 On this PISA through emulsion RAFT polymerization, Charleux,19,20 Monteiro,21,22 Cunningham,23 and Davis24 made notable contributions. This preparation of BCP nanoassemblies affords the benefit that the environmentally friendly solvent of water instead of organic solvent is used as the polymerization medium, whereas this PISA under emulsion condition generally affords spherical BCP morphology,18−21,23,24 and the reason is ascribed to BCP nanoassemblies being kinetically frozen in water. The second PISA formulation is performed under dispersion RAFT polymerization,25−60 and this is widely investigated by Pan,25−27 Armes,28−32 Lowe,33−35 An,36,37 Boyer,38−40 Davis,39,40 Char-

1. INTRODUCTION In the past few decades, block copolymer (BCP) nanoassemblies attracted close attention due to their applications in many fields.1−3 Generally, two strategies are proposed to prepare and/or synthesize these BCP nanoassemblies. The first strategy is via micellization of amphiphilic BCPs in block selective solvents.4−13 Following the micellization strategy, amphiphilic BCP is first dissolved in a common solvent; subsequently, its self-assembly can be triggered usually by dialysis against the block selective solvent,4−6 temperature switch,7,8 pH switch,8,9 thin film rehydration,10 and other methods11−13 to afford BCP nanoassemblies. This approach usually encounters from two drawbacks of dilute copolymer concentration (98%), methanol (MeOH, >99%) and 2,2′-azobis(isobutyronitrile) (AIBN, >99%) were purchased from Tianjin Chemical Company (China). AIBN was purified via recrystallization from ethanol. Styrene was distilled in vacuo prior to use. Deionized water was employed in this study. 2.2. Dispersion RAFT Polymerization and Synthesis of the PEG45-b-PS Nanoassemblies. Dispersion RAFT polymerization was performed under [St]0:[PEG45-TTC]0:[AIBN]0 = 350:1:1/2 with 15 wt % weight ratio of styrene to the MeOH/H2O mixture (80/20, w/ w). A typical example is introduced. PEG45-TTC (0.0379 g, 0.0165 mmol), St (0.600 g, 5.77 mmol), AIBN (0.00135 g, 0.0082 mmol), and the 80/20 MeOH/H2O mixture (4.00 g) were weighed into a 25 mL Schlenk flask with a magnetic bar. After dissolved O2 being removed by degassing with N2 at 0 °C, polymerization was conducted at 70 °C. At a predetermined time, polymerization was quenched and styrene conversion was analyzed by UV−vis as discussed previously.74 The PEG45-b-PS nanoassemblies were checked by TEM/SEM. The PEG45-b-PS nanoassemblies were isolated via centrifugation (12 500 rpm, 30 min), washed with MeOH, and finally dried at room temperature in vacuo to obtain pale yellow powder of PEG45-b-PS for GPC and 1H NMR analysis. 2.3. Seeded Dispersion RAFT Polymerization and Synthesis of the PEG45-b-PS Nanoassemblies. Synthesis of the PEG45-bPS151-b-PS nanoassemblies includes (1) preparation of the PEG45-bPS151-TTC seed vesicles through dispersion RAFT polymerization and (2) preparation of the PEG45-b-PS151-b-PS nanoassemblies via seeded dispersion RAFT polymerization, which are introduced as below. Synthesis of the PEG45-b-PS151-TTC Seed Vesicles. Typically, PEG45-TTC (0.8848 g, 0.385 mmol), St (6.2087 g, 59.7 mmol), AIBN (0.0210 g, 0.128 mmol), and the 80/20 MeOH/H2O mixture (40.00 g) were weighed into a 100 mL Schlenk flask with a magnetic bar. After the flask content being degassed with N2 at 0 °C, polymerization was run at 70 °C for 24 h. Styrene conversion at 97.6% was analyzed by UV−vis as discussed above. The freshly prepared colloidal dispersion was dialyzed against the 80/20 MeOH/H2O mixture to remove the residual monomer, and the resultant 15.0 wt % dispersion of the PEG45-b-PS151-TTC seed vesicles was kept at room temperature for the next use. Synthesis of the PEG45-b-PS151-b-PS Nanoassemblies. Typically, 15.0 wt % dispersion of the PEG45-b-PS151-TTC vesicles (1.661 g, containing 0.0139 mmol of PEG45-b-PS151-TTC), St (0.2889 g, 2.78 mmol), and the initiator of AIBN (1.13 mg, 0.006 89 mmol) dissolved in the 80/20 MeOH/H2O mixture (1.93 g) were weighed into a 25 mL Schlenk flask with a magnetic bar. The flask content was vigorously stirred for 10 min, degassed with N2 at 0 °C, and finally polymerization was conducted at 70 °C. After a predetermined time, polymerization was quenched, styrene conversion was analyzed by UV−vis, and the resultant BCP nanoassemblies of PEG45-b-PS151-b-PS were checked by TEM/SEM. Note: herein PEG45-b-PS prepared by seeded RAFT polymerization was labeled as PEG45-b-PS151-b-PS to clearly show the newly extended PS chains. To obtain the polymers for GPC or 1H NMR analysis, the BCP nanoassemblies were isolated via centrifugation (12 500 rpm, 10 min), washed with MeOH, and last dried at room temperature in vacuo. 2.4. Seeded Emulsion RAFT Polymerization and Synthesis of the PEG45-b-PS Nanoassemblies. Seeded emulsion RAFT polymerization was conducted similarly with seeded dispersion RAFT polymerization except different polymerization medium being employed. Typically, 15.0 wt % dispersion of the PEG45-b-PS151TTC vesicles (1.661 g, containing 0.0139 mmol of PEG45-b-PS151TTC), St (0.2889 g, 2.78 mmol), and the initiator of AIBN (1.13 mg, B

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Macromolecules Scheme 1. Summary of Three PISA Formulations: (A) PEG45-TTC Macro-RAFT Agent Mediated Dispersion RAFT Polymerization, (B) Seeded Dispersion RAFT Polymerization, and (C) Seeded Emulsion RAFT Polymerization

0.006 89 mmol) dissolved in the MeOH/H2O mixture (1.93 g) with water fraction at 30−70 wt % were weighed into a 25 mL Schlenk flask with a magnetic bar. The flask content was stirred for 10 min and degassed with N2 at 0 °C, and finally polymerization was conducted at 70 °C. The monomer conversion and the resultant polymers were checked and characterized similarly with those discussed in seeded dispersion RAFT polymerization. 2.5. Characterization. 1H NMR analysis was conducted on a Bruker Avance III 400 MHz NMR spectrometer. GPC analysis was conducted on a Waters 600E GPC system equipped with three TSKGEL columns and a Waters 2414 refractive index detector, in which near-monodispersed polystyrene standards were used and THF at 0.6 mL min−1 at 40.0 °C was acted as fluent. TEM observation was carried on a JEOL 100CX-II electron microscope at 100 kV or a Tecnai G2 F20 electron microscope at 200 kV. SEM observation was carried on a JSM-7500F electron microscope. UV−vis analysis was conducted on a Varian 100 UV−vis spectrophotometer. Dynamic light scattering (DLS) analysis was carried on a NanoBrook Omni (Brookhaven) laser light scattering spectrometer at the wavelength of 659 nm at 90° angle, and number-average hydrodynamic diameter (Dh) of the synthesized BCP nanoassemblies at 25 °C was measured by intensity following the CONTIN method.

ization takes place predominantly in these nanoreactors to afford the PEG45-b-PS nanoassemblies. Seeded dispersion RAFT polymerization of styrene is performed under similar condition with dispersion RAFT polymerization, except that the seeded nanoparticles of the PEG45-b-PS151-TTC vesicles but not the soluble PEG45-TTC are added to act as RAFT agent. In the 80/20 MeOH/H2O mixture, the PEG45-b-PS151-TTC vesicles cannot be molecularly dissolved and therefore act as seeds for the growth of PEG45-bPS nanoassemblies. This selection of the PEG45-b-PS151-TTC vesicles containing a PS151 block is made to ensure the targeted PEG45-b-PS nanoassemblies synthesized through different PISA formulations having the same chemical composition. To clearly show the extended PS chains, the PEG45-b-PS diblock copolymer was labeled as PEG45-b-PS151-b-PSx, in which the x represents the DP of the newly extended PS block. Seeded emulsion RAFT polymerization is run using the PEG45-b-PS151-TTC seed vesicles as RAFT agent, which is similar to seeded dispersion RAFT polymerization. Their difference of two PISA formulations lies in the polymerization solvent. As discussed previously in seeded dispersion RAFT polymerization, styrene is soluble in the 80/20 MeOH/H2O mixture. However, when the MeOH/H2O mixture contains >30 wt % water,75 styrene is insoluble in this solvent. Therefore, seeded emulsion RAFT polymerization is conducted in the MeOH/H2O mixture with water fraction at 35−60 wt %. In the MeOH/H2O mixture with water fraction above 70 wt %, the PEG45-b-PS151-TTC seed vesicles and/or the final PEG45-bPS151-b-PS nanoassemblies cannot keep suspending in the water-rich MeOH/H2 O mixture, and therefore seeded emulsion RAFT polymerization in a water-rich MeOH/H2O mixture with water fraction above 70 wt % is out of the present study. The three PISA formulations are schematically summarized in Scheme 1. The difference lies in the format of the RAFT agents and the solubility of styrene in the polymerization solvent. That is, in dispersion RAFT polymerization, the RAFT agent of PEG45-TTC and styrene are soluble in the polymerization solvent. In seeded dispersion RAFT polymerization, the PEG45-b-PS151-TTC seed vesicles acting as RAFT agent are dispersed in the polymerization solvent, styrene is soluble in the polymerization solvent, styrene is partly enriched within the cavity, and the membrane of the PEG45-b-PS151-TTC seed vesicles since styrene is compatible with the vesicular membrane of the solvophobic PS block. In seeded emulsion

3. RESULTS AND DISCUSSION 3.1. General Introduction of Three PISA Formulations. Three RAFT polymerizations under dispersed conditions, e.g., a soluble PEG45-TTC mediated dispersion polymerization which herein is named dispersion RAFT polymerization, dispersion RAFT polymerization based on the PEG45-b-PS151TTC vesicles which is named seeded dispersion RAFT polymerization, and emulsion RAFT polymerization based on the PEG45-b-PS151-TTC vesicles which is called seeded emulsion RAFT polymerization, are run and in situ formed PEG45-b-PS nanoassemblies are checked. In this section, these three PISA formulations are briefly introduced. Dispersion RAFT polymerization is extensively employed to prepare BCP nanoassemblies.25−60 In this dispersion RAFT polymerization, three reactants including PEG45-TTC, AIBN, and styrene are fed into the 80/20 MeOH/H2O mixture. In the 80/20 MeOH/H2O mixture, all the reactants are molecularly soluble at the beginning of polymerization. With the monomer consumption, the BCP of PEG45-b-PS forms. When DP of the solvophobic PS block, DPps, increases above critical value, PEG45-b-PS becomes insoluble and self-assembles into micelles in the 80/20 MeOH/H2O mixture. After that the in situ synthesized micelles act as nanoreactors for the incoming monomers, and the subsequent heterogeneous RAFT polymerC

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Figure 1. Monomer conversion vs time plot (A) and ln([M]0/[M]) vs time plot (B) for dispersion RAFT polymerization in the 80/20 MeOH/H2O mixture. GPC traces (C) and evolution of the molecular weight and Đ (D) of PEG45-b-PS.

Figure 2. TEM images of the PEG45-b-PS nanoassemblies formed via dispersion RAFT polymerization in the 80/20 MeOH/H2O mixture at 5 h (A), 7 h (B), 8 h (C), 9 h (D), and 12 h (E) and the average membrane thickness of the PEG45-b-PS vesicles (F).

the 50/50 MeOH/H2O mixture as introduced in the Experimental Section, the concentration of styrene within the PEG45-b-PS151-TTC vesicles calculated following eq S2 is higher than those in the solvent under dispersion RAFT polymerization (26 wt % vs 15 wt %). These three PISA formulations lead to different RAFT polymerization kinetics and different BCP morphology, which will be discussed subsequently. 3.2. Dispersion RAFT Polymerization and Synthesis of the PEG45-b-PS Nanoassemblies. Dispersion RAFT polymerization was run under [St]0:[PEG45-TTC]0:[AIBN]0 = 350:1:1/2 with styrene concentration at 15 wt %. This dispersion RAFT polymerization underwent an initial stage of slow polymerization in the initial 5 h at about 21% monomer

RAFT polymerization, the PEG45-b-PS151-TTC seed vesicles acting as RAFT agent are dispersed in the polymerization solvent; however, styrene is insoluble. Under this seeded emulsion RAFT polymerization, 54−96% of styrene is encapsulated within the cavity of the PEG45-b-PS151-TTC seed vesicles, which leads to a much higher monomer concentration than that under dispersion RAFT polymerization. Note: the encapsulated styrene within cavity of the PEG45-b-PS151-TTC seed vesicles can be approximately calculated by the totally fed monomer and the monomer molecularly dissolved in the polymerization solvent, and 54− 96% of styrene, which is calculated following eq S1, is dependent on water fraction in the MeOH/H2O mixture. Under the present seeded emulsion RAFT polymerization in D

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Macromolecules conversion and then a subsequent stage of fast polymerization occurred until to 94% monomer conversion in 16 h (Figure 1A), which is reflected by the two-stage semilogarithmic kinetic plot (Figure 1B). In the initial stage, the BCP of PEG45-b-PS prepared at low monomer conversion has a relatively short solvophobic PS block and is dissolved or highly dispersed in the alcoholic solvent at 70 °C, and therefore RAFT polymerization undergoes generally under homogeneous condition. In the second stage, with further extension of the PS block, PEG45-bPS became molecularly insoluble to form micelles, and subsequently heterogeneous polymerization conducted mainly in these micelles, and finally PEG45-b-PS nanoassemblies are formed at end of dispersion RAFT polymerization. The apparent polymerization rate constants (kpapp) at the two stages,76 which can be determined by slope of the semilogarithmic kinetic plot in the linear part, are 0.046 and 0.42 h−1, respectively, and the kpapp results indicate the accelerated RAFT polymerization in the heterogeneous stage. The accelerated RAFT polymerization can be attributed to radical segregation or compartmentalization effect under heterogeneous conditions.77 The synthesized BCPs of PEG45-b-PS are analyzed by GPC (Figure 1C) and 1H NMR, and results summarized in (Figure 1D) indicate the good control on the molecular weight and its distribution of PEG45-b-PS with Đ (Đ = Mw/Mn) below 1.2. Figure 2 summarizes the PEG45-b-PS nanoassemblies synthesized via dispersion RAFT polymerization at different polymerization time. Vesicles of PEG45-b-PS are formed when DPps is above 74. As depicted in Figures 2A−E, DPps seems to exert no or just slight influence on average size of the PEG45-bPS vesicles, which located at 160 ± 40 nm, whereas membrane thickness of the PEG45-b-PS vesicles increases with DPps as summarized in Figure 2F. For vesicles of poly(acrylic acid)block-polystyrene (PAA47-b-PS), their membrane thickness d was approximately determined by the equation of d = 0.10DPS − 1.92 (red line in Figure 2F),4 in which DPS is the polymerization degree of the PS block. Herein, our results show a linear fit of d = 0.112DPS + 6.738 for the present PEG45b-PS vesicles (black line in Figure 2F), and it indicates that the PEG45-b-PS vesicles prepared via dispersion RAFT polymerization have a thicker membrane. The thicker membrane of the PEG45-b-PS vesicles than the PAA47-b-PS vesicles is possibly due to the PAA block being more hydrophilic than the PEG block or the stronger static repulsion among the PAA chains than among the PEG chains. Nevertheless, the present linear fit suggests that the PEG45-b-PS vesicles may be close to thermodynamically equilibrium. 3.3. Seeded Dispersion RAFT Polymerization and Synthesis of the PEG45-b-PS Nanoassemblies. Prior to seeded dispersion RAFT polymerization, the PEG45-b-PS151TTC seed vesicles were prepared via dispersion RAFT polymerization similarly with those discussed above, except that polymerization was performed under a low value of [St]0: [PEG45-TTC]0:[AIBN]0 = 155:1:1/3. These vesicles of PEG45b-PS151-TTC were prepared at high monomer conversion of 97.6%, which was useful to decrease or eliminate the unreacted St monomer. After dialysis against the 80/20 MeOH/H2O mixture, the PEG45-b-PS151-TTC seed vesicles were obtained. Figure 3A indicates that the seeded vesicles can keep suspending in the 80/20 MeOH/H2O mixture, which is essential for next RAFT polymerization to afford stable PEG45b-PS151-b-PSx nanoassemblies. Figures 3B,C show TEM and SEM images of the seeded vesicles, from which mean size of the

Figure 3. Optical images of 15 wt % colloidal dispersion (A), TEM (B), and SEM (C) images and schematic structure (D) of the PEG45b-PS151-TTC seed vesicles.

vesicles at 260 nm and membrane thickness at 23 nm are confirmed. As schematically shown in Figure 3D, the solvophobic PS block constructs the bilayer membrane of vesicles and the solvated PEG45 chains locate at the inner and outer sides of the membrane, which keep the vesicles suspending in the 80/20 MeOH/H2O mixture. Besides, since Z-group of the functional RAFT locates at the PS side, the Zgroup is deemed to be mingled in the PS membrane as shown in Figure 3D. Into colloidal dispersion of the PEG45-b-PS151-TTC vesicles, styrene and AIBN were added under [St]0:[PEG45-b-PS151TTC]0:[AIBN]0 = 200:1:1/2 with the content of styrene plus the PEG45-b-PS151-TTC vesicles at 15 wt %. In this polymerization solvent, the fed styrene monomer swelled the PS membrane of the PEG45-b-PS151-TTC seed vesicles, and the PS block was extended with proceeding of RAFT polymerization. As depicted in Figures 1A,B, RAFT polymerization runs quickly, and almost full conversion of styrene is obtained in 6 h, which is highly faster than dispersion RAFT polymerization under similar conditions. The probable reason is deduced. As shown above, dispersion RAFT polymerization undergoes a slow homogeneous stage (0−5 h) and a fast heterogeneous stage (>5 h). Whereas, seeded dispersion RAFT polymerization undergoes under fully heterogonous condition from the beginning to end and therefore runs faster than dispersion RAFT polymerization. The synthesized PEG45-b-PS151-b-PSx prepared at different monomer conversions is analyzed by 1H NMR and GPC (Figure S3), and the results indicate a wellcontrolled seeded dispersion RAFT polymerization. Figure 4 summarizes the PEG45-b-PS151-b-PSx nanoassemblies formed via seeded dispersion RAFT polymerization at different time. From the TEM images depicted in Figure 4, two specials are remarkable. The first is formation of few porous nanospheres of PEG45-b-PS151-b-PSx. The size of these porous nanospheres locates at 190 ± 60 nm, and the pore size is about 20 nm. Porous nanoparticles of random copolymers have been prepared by emulsion polymerization.78 As far as we know, very rare porous nanospheres of BCPs have been prepared by emulsion RAFT polymerization, by dispersion RAFT polymerization, by self-assembly in block selective solvent, or by simulation.26,60 It is believed that the present formation of the PEG45-b-PS151-b-PSx porous nanospheres is due to the encapsulated St monomer within the PEG45-b-PS151-TTC seed vesicles, which has higher concentration than those in the solvent. This encapsulated St monomer swells the PEG45-bPS151-TTC seeded vesicles and therefore leads to formation of porous nanospheres, which will be further discussed subsequently. The second remark is the increase of the membrane E

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Figure 4. TEM images of the PEG45-b-PS151-b-PSx nanoassemblies formed via seeded dispersion RAFT polymerization in the 80/20 MeOH/H2O mixture at 0 h (A), 0.5 h (B), 1 h (C, D), and 6 h (E, F).

thickness of the PEG45-b-PS151-b-PSx vesicles (herein, the porous nanospheres are not included) with the increasing x, which is somewhat similar to those in dispersion RAFT polymerization discussed above. 3.4. Seeded Emulsion RAFT Polymerization and Synthesis of the PEG45-b-PS Nanoassemblies. Seeded emulsion RAFT polymerization was conducted similarly with seeded dispersion RAFT polymerization except the difference in solvent. In seeded emulsion RAFT polymerization, the polymerization solvent of the MeOH/H2O mixture contains more than 30 wt % of water, and therefore styrene is insoluble or partly soluble in the solvent, which is different from those in seeded dispersion RAFT polymerization in which styrene is molecularly soluble in the solvent. Herein, a typical seeded emulsion RAFT polymerization in the 65/35 MeOH/H2O mixture under [St]0:[PEG45-b-PS151TTC]0:[AIBN]0 = 200:1:1/ 2 was conducted, and polymerization kinetics and the PEG45-bPS151-b-PSx nanoassemblies were checked. As indicated in Figures 1A and 1B, the seeded emulsion RAFT polymerization runs at a similar rate to seeded dispersion RAFT polymerization, and 6 h of polymerization leads to almost full monomer conversion of styrene. The synthesized PEG45-b-PS151-b-PSx diblock copolymers were analyzed by 1H NMR and GPC (Figure S4), and results indicate a well-controlled seeded emulsion RAFT polymerization. Figure 5 summarizes the PEG45-b-PS151-b-PSx nanoassemblies prepared via seeded emulsion RAFT polymerization. Note: Figures 5A,C,D show unstained PEG45-b-PS151-b-PSx nanoassemblies formed at different times, and Figure 5B shows the typical stained nanoassemblies synthesized at 0.5 h. These results clearly indicate formation of the PEG45-b-PS151-bPSx porous nanospheres in seeded emulsion RAFT polymerization. Compared with those formed via seeded dispersion RAFT polymerization, the PEG45-b-PS151-b-PSx nanoassemblies by seeded emulsion RAFT polymerization are slightly different, and almost all the BCP nanoassemblies are porous.

Figure 5. TEM images of the PEG45-b-PS151-b-PSx nanoassemblies produced via seeded emulsion RAFT polymerization in the 65/35 MeOH/H2O mixture at 0.5 h (A and B, unstained and stained nanoparticles), 1 h (C), and 6 h (D).

The formation of porous nanospheres of PEG45-b-PS151-bPSx by seeded dispersion/emulsion RAFT polymerization makes the present study different from any study reported previously. This attracts our great curiosity to clarify how these porous nanospheres of PEG45-b-PS151-b-PSx are formed. As discussed above, the seeded RAFT polymerization under dispersion or/and emulsion condition affords porous nanospheres of PEG45-b-PS151-b-PSx, and the reason is ascribed to the PEG45-b-PS151-TTC seeded vesicles being swelled by the encapsulated St monomer. If this is the exact reason, the F

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Macromolecules PEG45-b-PS151-TTC seeded vesicles should be converted into porous nanospheres just by swelling with the styrene monomer or a suitable organic solvent such as toluene. To verify this hypothesis, into the PEG45-b-PS151-TTC seeded vesicles dispersed in the 65/35 MeOH/H2O mixture, the styrene monomer was added to keep [St]0:[PEG45-b-PS151-TTC]0 = 200:1, and the flask was sealed; then the mixture was heated at 70 °C for about 30 min with magnetic stirring. Clearly, these manipulations are very close to the seeded emulsion RAFT polymerization except that no initiator was added into the mixture. Under this condition, the polymerization of styrene is not initiated due to the absence of the initiator. Note: GPC analysis shows that the collected polymer after the manipulations has almost the same molecular weight with PEG45-bPS151-TTC (Figure S5), and therefore very slight or no monomer conversion occurs at this handling. Figure 6 depicts

Figure 7. TEM images of the PEG45-b-PS151-b-PSx nanoassemblies formed via seeded RAFT polymerization in the MeOH/H2O mixture with the weight fraction of water at 20% (A), 30% (B), 35% (C), 40% (D), 50% (E), and 60% (F).

nanoassemblies are checked by TEM (Figure 7), SEM (Figure S7), and DLS (Figure S8), and the synthesized BCPs of PEG45b-PS151-b-PSx are analyzed by GPC and 1H NMR (Table S1 and Figure S9). As shown by Figure 7, all the BCPs of PEG45-bPS151-b-PSx have a similar DPps with x ≈ 190, but the BCP morphology is firmly dependent on water content in the MeOH/H2O mixture. In case of 20−30 wt % water content, styrene is soluble in the MeOH/H2O mixture; therefore, polymerization adopts the formulation of seeded dispersion RAFT polymerization, and mixture of vesicles and porous nanospheres of PEG45-b-PS151-b-PSx are formed (Figures 7A,B). In the case of water content above 30 wt %, styrene is insoluble, polymerization adopts seeded emulsion RAFT polymerization, and porous nanospheres of PEG45-b-PS151-bPSx are formed (Figures 7C−F). Interestingly, the size and/or number of the pores in the PEG45-b-PS151-b-PSx porous nanospheres decrease with water content increasing from 30 to 60 wt %. This decrease in size and/or number of the pores in the PEG45-b-PS151-b-PSx porous nanospheres is possibly due to the increased concentration of styrene encapsulated within the PEG45-b-PS151-TTC seeded vesicles, which swells the seeded vesicles and finally leads to the PEG45-b-PS151-TTC seeded vesicles being reorganized to form porous nanospheres. The pore formation is ascribed to the solvophilic PEG block, which helps to form W/O/W multiple emulsions of PEG45-b-PS151TTC/St/H2O as discussed elsewhere.78

Figure 6. TEM images of the PEG45-b-PS151-TTC nanoassemblies before (A) and after (B) swelled by styrene under emulsion conditions.

the PEG45-b-PS151-TTC seeded vesicles before and after swelling with styrene under emulsion conditions. It clearly indicates that the PEG45-b-PS151-TTC vesicles convert into porous nanospheres, even though the BCPs before and after the manipulations have the same chemical composition or polymer molecular weight (Figure S5). Formation of porous nanospheres by swelling the PEG45-b-PS151-TTC vesicles with toluene under similar conditions was also found (Figure S6). These results as well as those discussed above demonstrate that formation of porous nanospheres is ascribed to the seeded vesicles being swelled by the styrene monomer. By comparing the PEG45-b-PS151-b-PSx nanoassemblies prepared by seeded dispersion RAFT polymerization with those prepared by seeded emulsion RAFT polymerization, the solvent character or the styrene solubility in the polymerization solvent is found to be the key parameter to determine the morphology of PEG45-b-PS151-b-PSx. Herein, to check how solvent affects morphology of the PEG45-b-PS151-b-PSx nanoassemblies, several cases of seeded RAFT polymerization in different MeOH/H2O mixtures with 20−60 wt % water content are performed under [St]0:[ PEG45-b-PS151-TTC]0: [AIBN]0 = 200:1:1/2. In case of below 30 wt % water content (Figures 7A,B), seeded dispersion RAFT polymerization runs; in the case of water content at 35−60 wt % (Figures 7C−F), seeded emulsion RAFT polymerization runs; and in the case of water content above 70 wt %, polymeric colloids become unstable and precipitate is found. When similar monomer conversion as high as 94−98% is obtained, all cases of RAFT polymerization are quenched, and the PEG45-b-PS151-b-PSx

4. CONCLUSIONS In this study, three PISA formulations, e.g., the PEG45-TTC macro-RAFT agent mediated dispersion polymerization, seeded dispersion RAFT polymerization, and seeded emulsion RAFT polymerization, are comparatively studied. The first PISA formulation of dispersion RAFT polymerization employs a soluble PEG45-TTC as macro-RAFT agent. In the other two PISA formulations, the presynthesized vesicles of PEG45-bPS151-TTC are used as macro-RAFT agent. Our results revealed that the latter two PISA formulations run at a similar rate, and both of them run much faster than the first PISA formulation of dispersion RAFT polymerization; the reason for accelerated polymerization is ascribed to the fully heterogeneous condition in the latter two PISA formulations. Besides, morphology of the PEG-b-PS nanoassemblies formed via the three PISA formulations is different even though DPps are very similar to each other. That is, dispersion RAFT polymerization affords vesicles, seeded dispersion RAFT polymerization results in mixture of vesicles and porous nanospheres, and seeded emulsion RAFT polymerization leads to porous nanospheres of PEG-b-PS with DPps at about 330. The PEG-b-PS porous nanospheres are expected to be a new BCP morphology, and G

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formation of porous nanospheres is ascribed to the seeded PEG45-b-PS151-TTC vesicles being swelled by the monomer under heterogeneous conditions. Our study clarifies how the PISA procedure affects morphology of BCP nanoassemblies, and seeded RAFT polymerization is believed to an effective approach to prepare BCP nanoassemblies with new and interesting morphology.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01756. Supplementary characterization and experimental details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.S). *E-mail: [email protected] (W.Z.). 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. 21274066 and 21474054), and the National Key Research and Development Program of China (2016YFA0202503) is gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b01756 Macromolecules XXXX, XXX, XXX−XXX