Fabrication of Spaced Concentric Vesicles and Polymerizations in

Feb 19, 2014 - successfully fabricate the spaced concentric vesicles (SCVs) via RAFT dispersion polymerization, and continuous prop- agation of the re...
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Fabrication of Spaced Concentric Vesicles and Polymerizations in RAFT Dispersion Polymerization Wen-Jian Zhang, Chun-Yan Hong,* and Cai-Yuan Pan* CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *

ABSTRACT: Different from concentric vesicles without spacing between the walls, the concentric vesicles with uniform spacing between the walls were rarely fabricated. We successfully fabricate the spaced concentric vesicles (SCVs) via RAFT dispersion polymerization, and continuous propagation of the residual polymer chains inside the large vesicles induces self-assembling to form SCVs. Concentration of the residual polymer chains in the solution of the nascent-formed vesicles is the determining factor for formation of SCVs, and continuous propagation of less or too more residual polymer chains will not form SCVs but form other morphologies. Generally, the concentration of the residual polymer chains after formation of vesicles is too low to self-assemble, so formation of SCVs is impossible. By adjusting the ratio of St/methanol or macro-RAFT agent P4VP-b-PS/P4VP, the concentration of residual polymer chains can be controlled, and further control of the morphologies is achieved. Formation of the inner vesicles by self-assembling inside the large vesicles requires high molecular weight of the polymer chains due to their very low concentration. The polymers of inner vesicles possess very high molecular weight (×106 g/mol) in comparison with the polymer of outmost vesicles (×105 g/mol). Polymerization kinetic study reveals very high increasing rate of the molecular weight inside the vesicles probably owing to long duration of the chain radicals, and the polymerization rate (Rp) inside the vesicles is faster than the Rp in the outmost vesicles, but both rates are in the same order.

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INTRODUCTION As one of the efficient strategies for fabrication of nanostructural materials, the polymerization-induced self-assembly and reorganization (PISR) has been used to create a broad range of intricate polymeric nanomaterials.1,2 The polymerizations used for this purpose include living anionic and living radical polymerizations, especially reversible addition−fragmentation transfer (RAFT) dispersion polymerization.3 The factors influencing the polymer aggregates involve recipe, nature of starting materials, and reaction conditions. When the precipitator of one block chain is used as media of the RAFT dispersion polymerization, only spherical micelles are formed because a significant decrease of the polymerization rate in the spheres formed by microphase separation leads to transition of the spherical micelles to other morphologies impossible.4,5 One feasible method for solving this problem is to minimize decrease of the polymerization rate after phase separation because one determining factor of morphologies formed via self-assembling strategy is chain length ratio of two blocks in the diblock copolymers,6 which can be achieved by increasing concentration of the monomers in the cores of micelles. Consider monomer distribution between two phases formed by phase separation, two methods, enhancing initial monomer concentration and lowering solubility of the monomer in the reaction media, can be used for this purpose. Polymerization © 2014 American Chemical Society

with high ratio of styrene (St)/methanol resulted in slight decrease of the polymerization rate after phase separation, and continuous increase of the chain length ratio of the polystyrene/poly(4-vinylpyridine) (PS/P4VP) brought about the transition of spherical micelles to other morphologies.7 Dispersion polymerization of 2-hydroxypropyl methacrylate with low solubility in water displayed enhanced polymerization rate upon formation of particle seeds due to favorable partition of the monomer in the particle core, and morphology transition occurred upon further polymerization.8 Almost all factors influencing the polymerization affect the morphology of polymer aggregates,1,9 the polymerization in the aggregates of different morphologies behaves differently. One of the interesting behaviors is formation of high molecular weight (MW) polymers during the morphology transition; for example, the block copolymer P4VP-b-PS with Mn = 3.9 × 105 g/mol was formed in the transition of spherical micelles to vesicles, which is never obtained in the homogeneous RAFT polymerization even in the RAFT dispersion polymerization without transition of morphology.1,10 Up to now, all studies on the polymerization in the nanomaterials are limited to single Received: December 5, 2013 Revised: January 24, 2014 Published: February 19, 2014 1664

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anhydrous magnesium sulfate, St was distilled under reduced pressure. All other reagents were of analytical grade and used as received. Characterization. Nuclear Magnetic Resonance (NMR). The 1H NMR (300 MHz) measurements were performed on Bruker DMX300 spectrometer in CDCl3 using tetramethylsilane as an internal reference. Gel Permeation Chromatography (GPC) Measurements. The molecular weight and molecular weight distribution of the P4VP were determined by a gel permeation chromatography (GPC) equipped with four Styragel columns (HR1, HR3, HR4, and HR5) at 60 °C and a Waters 2414 differential refractive index detector at 40 °C; dimethylformamide (DMF) was utilized as eluent at a flow rate of 1 mL/min. The monodispersed polystyrene standards were used in the calibration of Mn, Mw, and Mw/Mn. The molecular weight and molecular weight distribution of the P4VP-PS were determined on a Waters 150C gel permeation chromatography (GPC) equipped with three Ultrastyragel columns in series and RI 2414 detector at 30 °C, and THF was used as eluent at a flow rate of 1.0 mL/min. Monodispersed polystyrene standards were used in the calibration of molecular weight and molecular weight distribution. Transmission Electron Microscope (TEM). TEM observations were performed on a Hitachi H-800 TEM at an accelerating voltage of 100 kV. The samples for TEM observations were prepared by depositing a drop of the polymer solution in methanol on copper grids. Field-Emission Scanning Electron Microscope (FESEM). FESEM images were measured on a JEOL JSM-6700F. Laser Light Scattering (LLS) Spectrometer. A commercial LLS spectrometer (Zetasizer Nano ZS90, Malven Instruments Ltd., Malvern, UK) equipped with a He−Ne Laser (4.0 mW, 633 nm) was used. All the dynamic light scattering measurements were carried out at 25 °C at fixed angle or at 90°. Synthesis of CPDB-Terminated Poly(4-vinylpyridine) (P4VPCPDB). The 4-cyanopentanoic acid dithiobenzoate (CPDB) used in this study was synthesized according to the previous report.18 The general synthetic procedure of P4VP-CPDB is as follows: 4VP (15.75 g, 0.15 mol), CPDB (418 mg, 1.5 mmol), AIBN (25 mg, 0.15 mmol), and THF (12 mL) were added into a 50 mL polymerization tube with a magnetic bar. After three freeze−evacuate−thaw cycles, the tube was sealed under high vacuum, and then the sealed tube was placed in an oil bath at 80 °C. After 18 h of polymerization, the polymer solution was diluted with CH2Cl2 and then poured into excess petroleum ether while stirring. The precipitate was collected by filtration and then dried in a vacuum oven at room temperature. The red solid was dissolved in CH2Cl2, and the dissoving−precipitation procedure was repeated three times. The resulting product was dried under vacuum at room temperature overnight. The product (10.7 g) was obtained in yield of 68%. Its GPC trace and 1H NMR spectrum are shown in Figures S1 and S2, respectively. RAFT Dispersion Polymerization of Styrene in Methanol. The P4VP-CPDB (21 mg, 2.66 × 10−6 mol), St (2.60 g, 2.50 × 10−2 mol), AIBN (0.044 mg, 2.66 × 10−7 mol), and methanol (0.88 g) were successively added into a 5 mL glass tube with a magnetic bar, and then the system was degassed by three freeze−pump−thaw cycles. The tube was sealed under vacuum, and then the sealed tube was placed in an oil bath at 80 °C while stirring. After polymerization was carried out for 84 h, the reaction mixture was cooled to room temperature and then diluted with methanol. Most of the residual styrene was removed by three centrifugation−redispersion cycles. The mixture was dried naturally, and 0.56 g of product was obtained. To prepare the samples for TEM observation, a portion of the solid was dispersed in methanol. For measuring the NMR and GPC of the PS-b-P4VP, a part of the dried polymer nanomaterial was dissolved in CH2Cl2 and precipitated by pouring the solution into excess petroleum ether while stirring. The resultant powder was filtrated and dissolved in CH2Cl2 and then was precipitated in petroleum ether again. The polymer obtained by filtration was dried in a vacuum oven at room temperature overnight, and the dried polymer was used for NMR and GPC measurements. For studying formation mechanism of the SCVs, the P4VP-CPDB, St, AIBN, and methanol with the same feed molar ratio mentioned above were added into four 5 mL glass tubes. The polymerizations

structure-like spherical micelles, nanorods, and vesicles, etc., and respective studies on the polymerization behaviors or kinetics in the solution and in the aggregates for a dispersion polymerization system have not been reported based on our knowledge. Among the polymeric nanomaterials created by self-assembly strategy and PISR, the spaced concentric vesicle is a special type of vesicle, which cannot be obtained from phospholipids or small molecular surfactants owing to their high mobility11 but can be fabricated from block copolymers due to the polymer chain dynamics, whereas a very few articles studied this type of vesicle.12,13 Comparatively, investigation on fabrication of this morphology from inorganic materials is relatively extensive.14 Here, we emphasize that in this article the spaced concentric vesicles (SCVs) refer to the concentric vesicles with uniform spacing between the walls, which is different from another type of concentric vesicle in which there is no spacing between the walls. The latter is relatively easy to fabricate and has been successfully prepared via self-assembling strategy,15 also directly from polymerization.16 However, fabrication of the SCVs via PISR strategy has not been reported. Formation mechanism of the SCVs through self-assembly strategy, which was first proposed by Eisenberg, is progressive formation of smaller vesicles inside the larger vesicles by self-assembly of the entrapped PS-b-poly(acrylic acid) chains with diffusion of the water into the large vesicles.12 Since both solutions inside and outside the vesicles have the same polymer concentration, why is the self-assembly not taken place in the solution outside the vesicles? This question was not answered but is very important for understanding the formation of SCVs because self-assembly in the solution outside the vesicles will produce different morphology. During studying PISR behaviors, we observed the SCVs fabricated in the RAFT dispersion polymerization by TEM observations. Preliminary results reveal that continuous propagation of the residual block copolymer chains inside the large vesicles induces their self-assembly, forming the SCVs. To validate this formation mechanism, the following questions should be clarified. According to the concept of living polymerization, the polymer chain lengths at any polymerization time are slightly different; that is, the molecular weight distributions of the resulting polymers are narrow.17 Therefore, concentration of the residual polymer chains in solution after formation of the vesicles should be very low; can the selfassembly occur in so diluted solution? If it really occurs, why is the aggregation taking place only inside the large vesicles? What is the difference of polymerizations inside and outside the vesicles as well as in the outmost vesicles? To answer these questions, we study changes of the formed vesicles with polymerization time, the formation conditions of the SCVs, polymerization behaviors and kinetics in both solutions inside and outside the vesicles as well as in the wall of vesicles, and influence of the residual polymer concentration on the resulting morphologies.

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EXPERIMENTAL SECTION

Materials. 4-Vinylpyridine (4VP, Acros, 96%) was dried over CaH2 and then was distilled under reduced pressure prior to use. N,N′Azobis(isobutyronitrile) (AIBN) was purified by recrystallization from ethanol. Styrene (St, Shanghai Chem. Co., >99%) was washed with an aqueous solution of sodium hydroxide (5 wt %) three times and then washed with distilled water until neutralization. After being dried with 1665

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were carried out at 80 °C and then stopped at 24, 48, 72, and 84 h, and the polymerization solution was treated with the same procedure mentioned above. For constructing the phase diagram to predict the precise conditions of the SCVs, the P4VP-CPDB (21 mg, 2.66 × 10−6 mol), St (2.60 g, 2.50× 10−2 mol), AIBN (0.044 mg, 2.66 × 10−7 mol), and varying amounts of methanol (0.8, 0.85, 0.9, 1.0, 1.1, and 1.2 g) were added into 6 mL glass tubes. The polymerizations were carried out at 80 °C and then stopped at 24, 48, 72, and 84 h. After the polymerization, the solution was treated with the same procedure mentioned above, and then TEM, GPC, and 1H NMR of all the obtained samples were measured. RAFT Dispersion Polymerization of St in Methanol with Various Macro-RAFT Agents. Macro-RAFT agents [P4VP73−PS654/ P4VP73 (9/1, or 6/1, w/w) or P4VP73−PS654] with a total 2.66 × 10−6 mol, St (2.60 g, 2.50 × 10−2 mol), AIBN (0.044 mg, 2.66 × 10−7 mol), and methanol (0.88 g) were added into 5 mL glass tubes with a magnetic bar. After being degassed by three freeze−pump−thaw cycles, the tube was sealed under vacuum, and then the sealed tube was placed in an oil bath at 80 °C while stirring. After polymerization was carried out for 36 h, the reaction mixture was cooled to room temperature and then diluted with methanol. Most of the residual St was removed by three centrifugation−redispersion cycles. The product was obtained after naturally dried. To prepare the samples for TEM observation, a portion of the solid was dispersed in methanol. For the measurements of NMR and GPC, a part of the dried polymer nanomaterial was dissolved in CH2Cl2 and precipitated by pouring the solution into excess petroleum ether while stirring. The resultant powder was filtrated and dissolved in CH2Cl2 and then was precipitated in petroleum ether again. The polymer obtained by filtration was dried in a vacuum oven at room temperature overnight, and the dried polymer was used for NMR and GPC measurements. Concentration Measurements of St in the Vesicles and in Solution Outside the Vesicles. Five mixtures of P4VP-CPDB (21 mg, 2.66 × 10−6 mol), St (2.60 g, 2.50× 10−2 mol), AIBN (0.044 mg, 2.66 × 10−7 mol), and methanol (0.88 g) were added into five glass tubes, and then every tube was degassed by three freeze−pump−thaw cycles and was sealed under vacuum. The sealed tube was placed in an oil bath at 80 °C while stirring. After polymerization was carried out for 24, 36, 48, 72, or 84 h, the reaction mixture was cooled to room temperature and then centrifugated quickly. The supernatant and the precipitates were respectively taken for 1H NMR measurement, and then the concentrations of St in both places were calculated based on integral values of vinyl protons signals. Measurements of Polymerization Rates Inside Vesicles and in Outer Wall. Seven mixtures of P4VP-CPDB (21 mg, 2.66 × 10−6 mol), St (2.60 g, 2.50 × 10−2 mol), AIBN (0.044 mg, 2.66 × 10−7 mol), and methanol (0.88 g) were added into seven 5 mL glass tube with a magnetic bar; the polymerization procedure was the same as that mentioned above except the polymerizations were carried out for 24, 36, 48, 62, 67, 72, and 84 h. The aggregates were obtained by centrifugation of the resulted polymer dispersion and dried naturally, a portion of the dried aggregates were redispersed in methanol for TEM observation; other portion of the dried aggregates were dissolved in THF or CDCl3 for measurements of GPC and 1H NMR. Since the concentrations of monomer at different time inside the vesicles and in the outer wall are difficult to measure, assume that the RAFT polymerizations of St inside the vesicles and in outer wall are of living nature, so, the [M]0 − [M]t can be estimated by eq 1. [M]0 − [M]t = [RAFT agent]DPPS

polymerization rates, Rps, at different times were obtained by ([M]2 − [M]1)/(t2 − t1).

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RESULTS AND DISCUSSION During studying fabrication of various morphologies via PISR, we observed the SCVs formed in RAFT dispersion polymerization of St using P4VP-CPDB as macro-RAFT agent (Figure 1A). For further understanding the formation mechanism of

Figure 1. Formation of the SCVs in RAFT dispersion polymerization. (A) Fabrication of SCVs by RAFT dispersion copolymerization. (B) TEM and FESEM images of the SCVs: (a) TEM image and (b) FESEM image of the SCVs obtained by RAFT dispersion polymerization of St with a feed molar ratio of P4VP73/St/AIBN = 10/94000/ 1 in methanol (St/methanol = 2.95, w/w) at 80 °C for 84 h. (c) Cross-section FESEM image of the vesicles obtained from grinding the sample after frozen in liquid nitrogen. The sample used in (c) is the same as that in (a).

this morphology, the first step is preparation of P4VP-CPDB, which was synthesized by RAFT polymerization in THF at 80 °C for 18 h using 4-cyanopentanoic acid dithiobenzoate (CPDB) as RAFT agent. The GPC trace (Figure S1 in Supporting Information) is unimodal, and the number-average MW (Mn(GPC)) and the molecular weight distribution (Mw/ Mn) are 7200 g/mol and 1.17, respectively. Its structure is verified by characteristic proton signals of the phenyl in CPDB at δ = 7.8 (e), 7.5 (g), and 7.3 ppm (f) and the pyridyl in 4VP unit at 6.2−6.8 (a) and 8.1−8.6 ppm (b) in the 1H NMR spectrum of Figure S2. The Mn(NMR) of P4VP was calculated based on the integral values of the signals at 8.1−8.6 ppm (b) and 7.8 ppm (e), and the P4VP-CPDB with degree of polymerization (DP) = 73 and Mn(NMR) = 7900 g/mol was used in the following study. A typical polymerization system for fabrication of SCVs is the RAFT dispersion polymerization of St with a feed molar ratio of P4VP73/St/AIBN = 10/94000/1 in methanol (St/CH3OH = 2.95, w/w) at 80 °C (this recipe and temperature were used in the following study except where specially mentioned) for 84 h. A transmission electron microscopy (TEM) image of the resulting aggregates (Figure 1Ba) displays concentric vesicles with spacing between the walls. They consist of up to five walls, and their overall diameters can exceed 1 μm. The internal walls are fairly smooth, but the outer wall is more rugged in comparison with those in the interior, which is probably caused

(1)

Here, [M]0 and [M]t represent the monomer concentrations at the beginning and the reaction time, t; DPPS is degree of polymerization of the PS block in the P4VP-PS formed inside the vesicles or in the outer wall; [RAFT agent] is the RAFT agent concentration inside the vesicles or in the outer wall, which are calculated based on the area ratio of the peaks at the high and low molecular weight positions, their molecular weights, and initial concentration of the RAFT agent. The [M]0 − [M]t was plotted against t to afford the kinetic curves. The 1666

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formation of SCVs for the polymerization with high ratio of St/ methanol is due to use of fewer methanols, which results in the concentration increase of the residual copolymer chains inside the vesicles. According to the formation mechanism of vesicles in our previous reports,1,7,10 the hydrophobic PS block grows with evolution of the RAFT dispersion polymerization of St using P4VP-CPDB as macro-RAFT agent and stabilizer; the spherical micelles are formed at first. Continuous polymerization in the spheres induces morphologies transition from spheres to other morphologies, but formation of the SCVs is different. To understand the mechanism and also to further clarify the formation conditions of the SCVs as shown in Figure 2, the TEM, 1H NMR, and GPC were used to trace the polymerization. The 1H NMR spectra of the resultant polymers indicate formation of the block copolymers; the characteristic signals of pyridyl protons in 4VP unit appear at δ = 8.36 ppm (b) and phenyl protons in St unit appear at δ = 7.05 ppm (e, g) (Figure 3). The DPs of PS blocks were calculated based on the integral

by rapidly drying during the sample preparation for TEM observation. From the shape of vesicles observed by the fieldemission scanning electron microscope (FESEM) (Figure 1Bb), all the vesicles are spherical, but their surface are not smooth; pits on the surface can be clearly seen, which indicates the existence of inner space in the SCVs. In order to see the inner structure of the vesicles, the sample frozen by liquid nitrogen was ground; the SCVs displayed a space between the two adjacent walls as shown in Figure 1Bc, but compared to the wall thickness, such space is much smaller than that in inorganic SCVs.19 The TEM image in Figure 1Ba reveals that the space between the two adjacent walls (40 nm based on 25 measurements) is smaller than the wall thickness (75 nm), which is quite different from the SCVs prepared by the selfassembling method,12 and the difference may be ascribed to different formation mechanisms. Fabrication of the SCVs via self-assembly strategy requires highly asymmetric amphiphilic block copolymer and operation in high polymer concentration.12 Therefore, polymerization rate and time as well as residual polymer concentration are important factors for formation of the SCVs via PISR. To ascertain the formation conditions of the SCVs, a series of RAFT polymerizations with different weight ratios of St/ methanol for different polymerization time were studied; the results are listed in Table S1, and their phase diagram is shown in Figure 2. The TEM images and GPC traces are respectively

Figure 3. 1H NMR spectra in CDCl3 of the block copolymer, P4VPPS obtained from RAFT polymerization of St with a feed molar ratio of P4VP73/St/AIBN = 10/94000/1 in methanol (St/CH3OH = 2.95, w/w) at 80 °C for 84 h.

values of these two signals and DP of P4VP. The relationship of the DP ratios of PS to P4VP with the morphologies formed at different polymerization times is shown in Figure 4g, which demonstrates progressive growth of the PS chains and transition of the single-wall vesicles to the SCVs with evolution of the polymerization. When the ratio of DPPS/DPP4VP increases to 11.4 after 24 h polymerization, the single-wall vesicles are fabricated because increasing the PS chain length will lead to curvature decrease of the molecular assemblies.20 TEM image of the aggregates formed at 24 h polymerization shows that all vesicles have relatively thin walls (Figure 4a), and larger vesicles coexist with smaller vesicles, so the size distribution is broad. After 48 h polymerization, average wall thickness of the vesicles increases from 40 to 64 nm, but most are single wall vesicles; only a few double-shelled vesicles appear in the TEM image of Figure 4b. Most of the vesicles fabricated at 72 h polymerization are double-shelled vesicles, and a very few small single-wall vesicles can be seen in Figure 4c. The SCVs are fabricated after 84 h polymerization (Figure 4d). Since the SCVs are formed through single-walled vesicles (24 and 48 h polymerization), then double-shelled vesicles (72 h polymerization) and finally the SCVs as shown in Figure 4g, which indicates that formation of the SCVs is induced by

Figure 2. Detailed phase diagram constructed for P4VP73−PSx by systematic variation of the number-average molecular weight of the block copolymers in the outmost vesicles and the weight ratio of St/ methanol used for each synthesis. All the RAFT dispersion polymerizations were conducted with a feed molar ratio of P4VP/ St/AIBN = 10/94000/1 at 80 °C for different polymerization time. Abbreviation: v = vesicles (solid square), dv = double-shelled vesicles (solid round), scv = spaced concentric vesicles (solid triangle).

shown in Figures S3 and S4. Figure 2 reveals influences of the weight ratio of St/methanol and molecular weight of the resultant block copolymers on morphologies of the formed aggregates. When the weight ratios are less than 2.8, only single-wall vesicles are produced even after 84 h polymerization, but when this ratio is raised to 2.89, the double-shelled and the SCVs are formed respectively after 72 and 84 h polymerization. With increase of this ratio from 2.89 to 3.25, the double-shelled vesicles are formed at 60 h polymerization, and the SCVs start to form at 72 h polymerization. Compared with the lower weight ratio of St/methanol, an advance on the 1667

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two polymerization systems can be estimated based on their GPC curves in Figure 5d and their yields. Figure 5d reveals two

Figure 4. Formation mechanism of the SCVs in RAFT dispersion polymerization. TEM images of the aggregates obtained from RAFT dispersion polymerization of St with a feed molar ratio of P4VP73/St/ AIBN = 10/94000/1 in methanol (St/CH3OH = 2.95, w/w) at 80 °C for different polymerization times: (a) 24, (b) 48, (c) 72, and (d) 84 h. (e) SCVs obtained after purification via standing the sample of (d) for 72 h and collecting the precipitates. (f) Magnified image of the SCV in red square in (e). All the scale bars are 400 nm. (g) Variation of the aggregates’ morphologies and the chain length ratio of PS to P4VP blocks with polymerization time.

Figure 5. Effect of the residual copolymer concentration on formation of the SCVs. (a−c) TEM images of the aggregates obtained from RAFT dispersion polymerization of St with a feed molar ratio of Macro-RAFT agent/St/AIBN = 10/94000/1 in methanol (St/ CH3OH = 2.95, w/w) using (a) P4VP73−PS654, (b) P4VP73−PS654/ P4VP73 (9/1, w/w), and (c) P4VP73−PS654/P4VP73 (6/1, w/w) as macro-RAFT agent at 80 °C for 36 h. (d) GPC curves of the P4VP73− PSm obtained from the same RAFT dispersion polymerization with (b) and (c) using (1) P4VP73−PS654/P4VP73 (9/1, w/w) and (2) P4VP73−PS654 as macro-RAFT agent.

polymerization; more accurately, continuous propagation of the PS-b-P4VP chains entrapped in the large vesicles induces progressive formation of the inner vesicles under conditions similar to those which led to the formation of the outmost vesicle, and the SCVs are fabricated finally because the polymerizations in the wall of vesicles and in solution outside the vesicles cannot fabricate the inner vesicles. Different from the self-assembling strategy, the polymerization in the walls of inner vesicles continues after the formation of SCVs, leading to the wall thickness bigger than the spacing between the two adjacent walls as shown in Figure 4f. Based on the above formation mechanism, enough concentration of the PS-b-P4VP chains remained in the solution at the formation of nascent vesicles which is crucial for formation of the inner vesicles in the larger vesicles. Generally, the amount of the residual copolymer chains in the smaller vesicles is not enough to form inner vesicles via PISR, which will be discussed later, so the resultant SCVs always coexist with the smaller single-wall vesicles (Figure 4d). Since it is difficult accurately to measure concentration of the block copolymers inside the vesicles before the formation of inner vesicles, we did comparative experiments for understanding influence of the residual copolymer concentration on the formation of SCVs. The P4VP73−PS654 and the P4VP73−PS654/ P4VP73 with weight ratios of 9/1 and 6/1 were respectively used as macro-RAFT agent in the RAFT dispersion polymerization of St because the P4VP73−PS654−CPDB chains have much higher molecular weight than the P4VP73−CPDB chains, and during the polymerization, the P4VP73−PS654−CPDB chains form the vesicles before the vesicles are formed from P4VP73, so, the latter will remain in the reaction media. Thus, the P4VP73−PS654/P4VP73 polymerization system has more residual copolymer chains inside the vesicles than the P4VP73− PS654 system. In addition, their residual concentrations for the

peaks: one at lower MW position is ascribed to the block copolymer of the outmost vesicles; the other one is attributed to the polymers of inner vesicles, which will be further discussed later. The residual polymer concentrations for the P4VP73−PS654 and P4VP73−PS654/P4VP73 systems are 3.1 × 10−9 and 4.83 × 10−8 mol/mL, respectively, which is consistent with prediction based on MWs of the P4VP73−PS654 and the P4VP73. Similar to high asymmetry and relatively high concentration of the diblock copolymers required by selfassembling strategy for formation of the SCVs,12 higher concentration of the residual copolymer chains is needed for fabrication of the smaller vesicles inside the larger vesicles via continuous propagation of the P4VP73−PS chains. TEM image of the aggregates obtained from the P4VP73−PS654/P4VP73 polymerization system in Figure 5b demonstrates the formation of SCVs, but for the P4VP73−PS654 polymerization system, the SCV is not fabricated (Figure 5a) owing to relatively lower concentration of the residual copolymer chains. When more P4VP73−CPDB (the weight ratio of P4VP73−PS654/P4VP73 = 6/1) is used, the compound vesicles are formed as shown in Figure 5c. This is because too more residual copolymer chains in the solution at the formation of nascent vesicles will lead not only to self-assembling of the polymer chains inside the vesicles but also to aggregation of the copolymer chains in solution outside the vesicles during the polymerization. Various vesicles formed outside the vesicles coagulate and are further fused to fabricate the compound vesicles with a number of holes. Thus, the amount of residual block copolymers at the formation of nascent vesicles is very important for formation of the SCVs; propagation of the residual block copolymers with too low and too high concentrations cannot produce the SCVs. Also, the 1668

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much larger than that (240 and 138 nm) formed at 48 h polymerization, that is, the latter volume is approximately a tenth of the former, which is due to conformational change from loose aggregates to compact vesicle structure when comparing the TEM image in Figure 4a with that in Figure 4b. This phenomenon was also observed in the polymerizationinduced micellization.5 Volume contraction of the vesicles results in concentration increases of the monomer and the residual macro-RAFT agent inside the vesicles, which will lead to enhancement of the polymerization rate. Since we are not able to measure the concentrations of St in the outer wall and in the solution inside the vesicles, increase of the St concentration inside the vesicles cannot be further confirmed, whereas the concentrations of St in the vesicles and in the solution outside the vesicles can be measured respectively by measuring 1H NMR spectra of the supernatant liquid and the precipitates which were obtained through centrifugation of the vesicles dispersions. The 1H NMR with N,N-dimethylformamide as internal standard was applied to follow the polymerization, and the results are shown in Figure 7A. The concentration of St in the solution outside the vesicles

small vesicles contain too few residual polymer chains to selfassemble, which is approximately a 15th of that in the large vesicles estimated based on their sizes in Figure 4d; the SCVs cannot be formed. In addition, influence of the residual polymer concentration on the fabrication time of the SCVs is observed; for the P4VP73 polymerization system with lower residual polymer concentration, the fabrication time is 84 h, but for the P4VP73−PS654/P4VP73 with weight ratio of 9:1, only 36 h is required, which is similar to that micellar formation rate is dependent on concentration of macro-RAFT agent in the RAFT dispersion polymerization.5 To understand why the polymerization inside the vesicles can, but the polymerization outside the vesicles cannot induce the self-assembling of the PS-b-P4VP chains, studying the difference of polymerizations inside and outside the vesicles is necessary. For this purpose, the GPC was used to trace the polymerization. The GPC curves of all the block copolymers forming double-shelled vesicles or/and SCVs display double peaks as shown in Figure 6A, Figure 5d, and Figure S4. To

Figure 6. RAFT polymerization in the SCVs. (A) GPC curves of the block copolymers, P4VP73−PSm prepared by RAFT dispersion polymerizations for different polymerization times: (a) 12, (b) 24, (c) 48, (d) 72, and (e) 84 h; (f) the same sample as (e) but after removal of the small single-wall vesicles. (B) The relationship between Rh of the SCVs and the polymerization time. All are the RAFT dispersion polymerization of St with a feed molar ratio of P4VP73/St/ AIBN = 10/94000/1 in methanol (St/CH3OH = 2.95, w/w) at 80 °C.

Figure 7. (A) Difference between molar ratios of St/methanol in the vesicles (a) and in the solution outside the vesicles (b) in the dispersion polymerization for different polymerization time. Short dash is the molar ratio of St/methanol in feed. (B) The polymerization rates in the outer wall (a) and in the inner vesicles (b); increased rates of molecular weights (RMW) in the outer wall (c) and in the inner vesicles (d). All are the RAFT dispersion copolymerization of St with a feed molar ratio of P4VP73/St/AIBN = 10/94000/1 in methanol (St/ CH3OH = 2.95, w/w) at 80 °C.

identify which peak corresponds to the copolymers of outmost vesicle or the inner vesicles, we measured the GPC curve of the block copolymers obtained after removal of the smaller singlewall vesicles (Figure 6Af), which was achieved by standing the resultant SCVs dispersion for 72 h and collecting the precipitates. Compare with GPC curve of the precursor copolymers in Figure 6Ae, the GPC curve in Figure 6Af reveals that the peak at the higher MW position is relatively strengthened owing to removal of the smaller single-wall vesicles, indicating that the copolymer of outmost vesicle has much lower MW than the inner vesicles copolymer. Therefore, we reasonably deduce that there are two chain growth rates in the same polymerization system: the faster rate inside the vesicles and the slower rate in the outer wall. This interesting phenomenon seems unreasonable because both polymerizations were carried out in the same reaction media and the same conditions. To clarify this phenomenon, the laser light scattering (LLS) was used to follow the polymerization, and the resulted LLS curves exhibit two broad size distribution peaks (Figure S5). Their average Rhs alter with the polymerization time as shown in Figure 6B; Rhs of the bigger (560 nm) and the smaller vesicles (309 nm) formed at 24 h polymerization are

decreases with evolution of the polymerization (Figure 7Ab), and the low concentration of St results in low propagation rate of the polymer chains, and self-assembling in low concentration of the residual P4VP-PS requires their high MW; all these lead to the self-assembly being impossible. Thus, the consumption of St in the solution outside the vesicles is mainly due to the polymerization in the vesicles. The big difference of MWs between the outmost and the inner vesicles implies their different polymerization behaviors, so we studied the polymerization kinetics. Assuming that the RAFT polymerizations inside the vesicles and in the outer walls are of “living” nature, the consumption amount of St ([M]0 − [M]t) inside the vesicles and in the outer wall at different polymerization time can be calculated based on the recipe and the GPC curves; then the polymerization rates (Rps) at different polymerization times were calculated, and the results are shown in Figure 7Ba,b. The Rps inside the vesicles before 62 h polymerization were not obtained because the polymers formed inside the vesicles and in the outer walls cannot be identified by GPC, whereas the Rps before 62 h polymerization might be similar to the Rp at 62 h 1669

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whereas the Rp outside the vesicles is too low to be detected by GPC and NMR. The RMW inside the vesicles is much larger than that in outer wall. The concentrations of residual polymer chains in solution at the phase separation and morphology transition are generally very small; however, their amount can be adjusted through variation of recipe and use of appropriate macro-RAFT agents. The polymerization in larger vesicles forms inner vesicles and produces very high molecular weight copolymers (above 1 × 106 g/mol), but smaller vesicles are difficult to transform into SCVs. Appropriately increasing the residual copolymer chains in solution will promote formation of the SCVs and can increase the possibility of smaller vesicles to form SCVs.

polymerization because TEM image of the product obtained at 62 h polymerization does not display double-shelled or SCVs. We can see that although the concentration of copolymer chains inside the vesicles is much lower than that in outer walls, the Rp inside the vesicles is higher than the Rp in the outer walls, but both Rps are in the same order (Figure 7Ba,b), which is probably attributed to the concentration increase of the chain radicals and the monomer due to volume contraction of the vesicles during the polymerization. Compared with the singlewall vesicles, the monomer concentration in the outer wall of SCVs is lower, which is supported by molecular weight decrease of the polymers in the outer wall obtained after removal of the small single-wall vesicles as shown in Figures 6Ae and 6Af. In addition to consumption of St by the polymerization, the monomer in the outer wall has to be supplied to the polymerization inside the vesicles through diffusion, which is also the reason that the Rp and the increase rate of molecular weight (RMW) in the outer wall decrease before 62 h polymerization, while with decrease of the Rp and RMW inside the vesicles after formation of the SCVs, the Rp and RMW in the outer wall increase. At the last stage of polymerization, relative reduction of the monomer concentration causes decrease of both rates in both places (Figure 7B). Different from the Rps, the RMWs inside the vesicles are much faster than that in the outer wall (Figure 7Bc,d); the only reason is long duration of the chain radicals inside the vesicles, which is probably reasonable when we consider that the polymerization in very viscous media would result in relatively high steady concentration and long duration of the chain radicals because addition reaction of the chain radicals with the CPDB-terminated polymer chains (P-CPDB) is concentration dependent, but the fragmentation reaction is concentration independent of the polymer chains, and the polymer chains inside the vesicles is approximately a 90th of the P-CPDB in the outer wall calculated based on the recipe and GPC data. From the above discussion, one conclusion is that the amount of residual P-CPDB in the solution at the formation of nascent vesicles is a crucial factor influencing the polymerization. To further support this conclusion, the P4VP73−PS654/ P4VP73 was used as macro-RAFT agent in the RAFT dispersion polymerization instead of the P4VP73−PS654 alone. The GPC data in Figure 5d demonstrate that MWs of the polymer chains obtained from the P4VP73−PS654/P4VP73 system is lower than that from P4VP73−PS654 system, and the residual copolymer chains inside the vesicles are 16 times that for the P4VP73− PS654 system, which was calculated based on the GPC data and the yield. High concentration of the residual P-CPDB leads to increase of the Rp but decrease of the RMW inside the vesicles when comparing with the polymerization using P4VP73−PS654 alone as macro-RAFT agent.

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ASSOCIATED CONTENT

S Supporting Information *

Characterizations of the polymers and the aggregates obtained (Figures S1−S5). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.-Y.H). *E-mail: [email protected] (C.-Y.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under Contracts 21074121, 21090354, 21374107, and the Fundamental Research Funds for the Central Universities (WK 2060200012).

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REFERENCES

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CONCLUSION The spaced concentric vesicles have been successfully fabricated in the RAFT dispersion polymerization of St in methanol using P4VP-CPDB as macro-RAFT agent. Different from the formation mechanism of SCVs by self-assembling strategy, continuous polymerization inside the vesicles induces selfassembly of the entrapped polymer chains to form the SCVs. After formation of the single-wall vesicles in the RAFT dispersion polymerization, the polymerization behaviors inside the vesicles, in the outer wall, and in the solution outside the vesicles are different: the Rp inside the vesicles is faster than the Rp in the outer wall, but both Rps are in the same order, 1670

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