Redox-Responsive Multicompartment Vesicles of Ferrocene

Dec 23, 2015 - Xiao Wang , C. Adrian Figg , Xiaoqing Lv , Yongqi Yang , Brent S. Sumerlin , and Zesheng .... Sheng-li Chen , Peng-fei Shi , Wang-qing ...
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Redox-Responsive Multicompartment Vesicles of FerroceneContaining Triblock Terpolymer Exhibiting On−Off Switchable Pores Pengfei Shi,† Yaqing Qu,† Chonggao Liu, Habib Khan, Pingchuang Sun,* and Wangqing Zhang* Key Laboratory of Functional Polymer Materials of the Ministry of Education, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Institute of Polymer Chemistry, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: Multicompartment vesicles of ferrocene-containing triblock terpolymer containing on−off switchable pores in the vesicular membrane are prepared by seeded RAFT polymerization. In these multicompartment vesicles, the incompatible solvophobic poly(4-vinylbenzyl ferrocenecarboxylate) (PVFC) and poly(benzyl methacrylate) (PBzMA) blocks form the porous phase-segregated membrane and the solvophilic poly[2-(dimethylamino) ethyl methacrylate] block locates at the inner and outer sides of the membrane. These porous multicompartment vesicles are redox-responsive and the membrane pores can be on−off switched through redox triggering. These porous multicompartment vesicles are deemed to be new nanoassembly of ABC triblock terpolymer and are anticipated to be a smart host to load and release guests.

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triblock terpolymer containing two incompatible solvophobic blocks of B and C have been prepared.45−52 Similarly to the AB diblock copolymer vesicles, in ABC vesicles, the solvophobic B and C blocks form the vesicular membrane and the solvophilic A block locates at the inner and outer sides of the membrane, respectively. It is reasonably expected a microphase separation of the solvophobic B and C blocks within the vesicular membrane to form membrane-segregated vesicles, which herein are called multicompartment vesicles. However, possibly due to the restricted microphase separation of the B and C blocks in the kinetically frozen membrane in the block-selective solvent, just very rare of such multicompartment ABC vesicles have been prepared.50−52 In this study, we introduced the redox-responsive multicompartment vesicles of the poly[2-(dimethylamino) ethyl methacrylate]-block-poly(benzyl methacrylate)-block-poly(4-vinylbenzyl ferrocenecarboxylate) (PDMAEMA-b-PBzMA-bPVFC) triblock terpolymer prepared by seeded RAFT polymerization, as shown in Scheme 1. Note: for the sake of briefness, D, B, and V represent the PDMAEMA, PBzMA, and PVFC blocks, and the PDMAEMA-b-PBzMA diblock copolymer and the PDMAEMA-b-PBzMA-b-PVFC triblock terpolymer are labeled as DB and DBV in the subsequent discussion. This preparation of the multicompartment DBV vesicles includes (1) synthesis of the seed DB vesicles by polymerization-induced self-assembly (PISA) through the macro-RAFT agent mediated dispersion polymerization of benzyl methacrylate (BzMA), and (2) insertion of the ferrocene-containing

ulticompartment block copolymer nanoparticles (MCBNs) have complex inner structure of multiple phase-segregated microdomains,1 and they are deemed to have advantages in drug delivery, nanotechnology, and catalysis. To prepare these MCBNs, linear ABC triblock terpolymers,2−12 for example, fluorinated triblock terpolymer,9−12 and miktoarm star terpolymer,13−18 in which A represents the solvophilic block and B and C represent the two incompatible solvophobic blocks, are usually employed. Either by the micellization strategy,1−18 the blending strategy,19−27 or the two macroRAFT agents comediated dispersion polymerization,28−31 MCBNs with the particle size at nanoscale have been prepared. However, compared with the numerous nanoassemblies of general amphiphilic block copolymers,32−44 MCBNs constructed with fluorinated block copolymers or miktoarm star terpolymers are rather limited, since laborious synthesis of these complex copolymers is generally needed. Besides, compared with the various morphologies of the general amphiphilic block copolymer nanoassemblies, for example, spheres,33−38 worms,35−38 rods,39,40 lamellae and vesicles,41−44 multicompartment nano-objects other than nanospheres of MCBNs are very rare. Therefore, convenient synthesis of multicompartment block copolymer assemblies with new morphology is an interest and challenge in polymer science. Vesicles of amphiphilic block copolymer have enclosed bilayer structure.35−38,42−52 For vesicles of AB diblock copolymer or ABA triblock copolymer,35−38 it is revealed that the solvophobic B block forms the vesicular membrane and the solvophilic A block locates at both the inner and outer sides of the membrane. In recent years, benefiting from the versatile controlled radial polymerization, well-defined ABC triblock terpolymers have been synthesized, and vesicles of ABC © 2015 American Chemical Society

Received: December 18, 2015 Accepted: December 22, 2015 Published: December 23, 2015 88

DOI: 10.1021/acsmacrolett.5b00928 ACS Macro Lett. 2016, 5, 88−93

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ACS Macro Letters

Scheme 1. Synthesis of the Porous Multicompartment Vesicles of the PDMAEMA-b-PBzMA-b-PVFC Triblock Terpolymer by Seeded RAFT Polymerization and the Schematic On−Off Switch of the Membrane Pores of the Multicompartment Vesicles through Oxidation/Reduction

Figure S2), the D35B274 vesicles are collapsed, and therefore, flattened D35B274 vesicles were detected by SEM (Figure 1B). Into the colloidal dispersion of the seed D35B274 vesicles, the VFC monomer and the initiator of 2,2′-azobis(isobutyronitrile) were added, and the seeded RAFT polymerization under [VFC]0/[D35B274]0/[AIBN]0 = 200:1:1/3 were undergone at 70 °C. After 36 h of polymerization with the monomer conversion at 90.1%, the multicompartment vesicles of the D35B274V180 triblock terpolymer (Mn,GPC = 106.5 kg/mol, Đ = 1.29, the weight fraction of the D, B, and V blocks at 4.7, 41.6, and 53.7%) were obtained. The TEM image shown in Figure 1C indicates the D35B274V180 multicompartment vesicles having uniform size centered at 500 nm, which is slightly larger than the D35B274 seed vesicles. From the high-resolution TEM image (Figure 1D), the dark-field TEM image (Figure 1E) and the iron distribution mapping (Figures 1F and S3A,B), two conclusions are made. First, the segregated phases in the vesicle membrane are formed as indicated by the highresolution TEM image, in which the dark area corresponds to the ferrocene-containing PVFC phase and the gray area corresponds to the PBzMA phase, respectively. Second, about 30 nm pores are formed in the vesicular membrane, which is further confirmed by the SEM observation (Figure 1G) and the nitrogen (N2) adsorption analysis (Figure 1H). The formation of the porous phase-segregated membrane is possibly due to the PBzMA block and the PVFC block being highly incompatible, which causes serious phase separation to make pores in the vesicular membrane. The porous multicompartment vesicles are believed to be the new nanoassembly of ABC triblock terpolymer, although the exact reason for the formation of the membrane cores needs further study. To demonstrate the crucial role of the microphase separation of the incompatible PBzMA and PVFC blocks in the formation of the porous multicompartment vesicles, the reference diblock copolymer vesicles of poly(4-vinylpyridine)-block-poly(4-vinylbenzyl ferrocenecarboxylate) (P4VP25-b-PVFC178) were pre-

PVFC block into the seed DB vesicles to form the targeted multicompartment DBV vesicles by seeded RAFT polymerization of 4-vinylbenzyl ferrocenecarboxylate (VFC). The macro-RAFT agent mediated dispersion polymerization is used,36−38 since it affords the convenient synthesis of the seed DB vesicles. The insertion of the third PVFC block into the preprepared DB vesicles by seeded RAFT polymerization can facilitate the microphase separation between the PBzMA and PVFC blocks within the vesicular membrane to form porous phase-segregated membrane and, therefore, helps to form the new nanoassembly of the porous multicompartment vesicles of the DBV triblock terpolymer. The ferrocenecontaining PVFC block being chosen as the third block is due to three reasons. First, PVFC is highly incompatible with PBzMA, and therefore, the microphase separation between the membrane-forming PBzMA and PVFC blocks leads to a porous phase-segregated membrane. Second, PVFC is redox-responsive,53−60 which affords the porous multicompartment vesicles the ability to give response to the redox stimulus and, therefore, to modulate the on−off switch of the membrane pores through the oxidation/reduction of the ferrocene moiety, as shown in Scheme 1. Third, the difference in the electron density between the membrane-forming PBzMA and PVFC blocks helps to conveniently detect the phase-segregated membrane in the multicompartment vesicles by transmission electron microscope (TEM). The seed vesicles of the D35B274 diblock copolymer (Mn,GPC = 60.9 kg/mol, Đ = 1.14) were prepared by PISA through the macro-RAFT agent mediated polymerization of BzMA in the nbutyl alcohol/butanone mixture (90/10 by weight) with the solid content at 10 wt %. The TEM and SEM images shown in Figures 1A,B and S1 indicate that the D35B274 vesicles have an average diameter at 380 nm, which is similar to the intensityweighted hydrodynamic diameter (Dh) determined by dynamic light scattering (DLS) analysis. Due to the low glass transition temperature of the PBzMA block (see the DSC thermograms in 89

DOI: 10.1021/acsmacrolett.5b00928 ACS Macro Lett. 2016, 5, 88−93

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terpolymer nano-objects were checked by TEM or SEM. It was found that the seeded RAFT polymerization underwent a pseudo-first-order kinetics (Figure S5).61 The three cases of the molecular weight of the DBV triblock terpolymer, the theoretical molecular weight Mn,th determined by monomer conversion, Mn,NMR by 1H NMR analysis and Mn,GPC by GPC analysis, were close to each other and Đ was below 1.3 (Figure 2A), suggesting the good control on the triblock terpolymer

Figure 2. Molecular weight, Đ of the synthesized DBV triblock terpolymers (A) and representative TEM, SEM images, and the schematic structure of the DBV nano-objects prepared at the polymerization time of 0 (B), 9 (C), 19 (D), and 36 h (E). Scale bar: 200 nm. Figure 1. TEM (A) and SEM (B) images of the seed D35B274 vesicles, and the TEM images (C, D), dark-field TEM image (E), iron distribution mapping (F), SEM image (G), and N2 adsorption− desorption isotherms (H) of the multicompartment D35B274V180 vesicles. The insets in (A) and (H) show the corresponding hydrodynamic diameter (Dh) distribution of the D35B274 vesicles and the BJH pore size distribution of the multicompartment D35B274V180 vesicles.

molecular weight and the molecular weight distribution in the seeded RAFT polymerization. The seeded RAFT polymerization leads to insertion of the incompatible PVFC block onto the vesicular membrane, which makes the PVFC and PBzMA chains rearranged to minimize the interfacial energy among the PVFC and PBzMA blocks and the interfacial tension between the solvophobic PVFC/PBzMA blocks with the solvent, and therefore to change the membrane structure. The TEM and SEM images of the DBV vesicles shown in Figure 2B−E with different DP of the PVFC block, for example, D35B274 (Figure 2B), D35B274V59 (Figure 2C), D35B274V124 (Figure 2D), and D35B274V180 (Figure 2E) indicate that the membrane structure is dependent on the DP of the PVFC block. For example, as shown by the inset schemes, the seed vesicles of D35B274 have an even membrane (Figure 2B), the D35B274V59 vesicles have a crinkly membrane (Figure 2C), the D35B274V124 (Figure 2D) and D35B274V180 (Figure 2E) vesicles have a lacunose or porous phase-segregated membrane, respectively. Ferrocene-containing polymers are typical redox-responsive materials,53−60 and it is demonstrated that the oxidation of the ferrocene moiety to cationic iron species is totally reversible. This redox-response of the ferrocene moiety has been employed to modulate the self-assembly or disassembly of the ferrocene-containing polymers.57−60 Figure 3 indicates that the D35B274V180 porous multicompartment vesicles convert into general vesicles during the oxidation with FeCl3, and the

pared by PISA (see the detail in Supporting Supporting). Herein, the P4VP25-b-PVFC178 diblock copolymer has the similar polymerization degree of the PVFC block with those in the DBV triblock terpolymer. As shown in Figure S4 (TEM and SEM), the reference P4VP25-b-PVFC178 vesicles have a solid membrane of the PVFC block, which are much different from the porous multicompartment DBV vesicles. This suggests that the microphase separation of the incompatible PBzMA and PVFC blocks leads to the formation of the porous multicompartment vesicles during the seeded RAFT polymerization. The polymerization degree (DP) of the PVFC block affecting the morphology of the DBV triblock terpolymer nano-objects was investigated. To fulfill this investigation, the seeded RAFT polymerization under [VFC]0/[D35B274]0/ [AIBN]0 = 200:1:1/3 was quenched at different polymerization time, the monomer conversion was checked by 1H NMR analysis, the DBV triblock terpolymer was characterized by 1H NMR analysis and GPC analysis, and finally, the DBV triblock 90

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Figure 3. Summary of the redox-response of the D35B274V180 multicompartment vesicles: the D35B274V180 vesicles before oxidation ([R]0, A−C) after oxidation ([O], D−F), the recovered D35B274V180 vesicles after reduction (G−I), and the UV−vis spectra and the inset photograph of the D35B274V180 triblock terpolymer in fresh (black line), oxidized (red line), and reduced (blue line) states (J), and the schematic redox-response of the D35B274V180 multicompartment vesicles (K).

shown in Figure 3K. It is expected that these multicompartment vesicles, on which the membrane pores can be on−off switched through redox triggering, are qualified host to load and release various guests, when the size of the guest is smaller than the pore diameter of about 30 nm. Summarily, a new kind of multicompartment nanoassemblies of the PDMAEMA-b-PBzMA-b-PVFC triblock terpolymer, named porous multicompartment vesicles, are prepared by seeded RAFT polymerization. In this seeded RAFT polymerization, the diblock copolymer vesicles of PDMAEMA-bPBzMA were used as seed, onto which the incompatible ferrocene-containing PVFC block was inserted to induce the phase-separation of the PVFC and PBzMA blocks within the vesicular membrane to form multicompartment vesicles containing a porous and phase-segregated membrane. It is found that the structure of the phase-segregated vesicular membrane is firmly dependent on the DP of the PVFC block. Ascribed to the incompatible ferrocene-containing PVFC block, these porous multicompartment vesicles are redox-responsive and the membrane pores can be reversibly on−off switched through redox triggering. These redox-responsive porous multicompartment vesicles are deemed to be new nanoassembly of ABC triblock copolymer and are deemed to be a smart host to load and release guests.

oxidized vesicles are reversibly recovered into porous multicompartment vesicles by reduction with SnCl2. Note: see the fresh multicompartment vesicles in the [R]0 column for the TEM/SEM images in Figure 3A−C, the oxidized vesicles in the [O] column for the TEM/SEM images in Figure 3D−F, and the recovered porous multicompartment vesicles in the [R]1 column for the TEM/SEM images in Figure 3G−I, respectively. The reversible redox of the D35B274V180 triblock terpolymer was further revealed by UV−vis analysis (Figure 3J), in which the characteristic absorption at 446 nm increased to 636 nm after oxidation and it reversibly backtracked to 446 nm accompanying with the color change of the DBV triblock terpolymer after reduction. The major difference between the reduced and oxidized vesicles of D35B274V180 lies in the membrane structure. That is, the reduced D35B274V180 vesicles have a porous phase-segregated membrane, whereas the oxidized D35B274V180 vesicles have a uniform membrane, in which the oxidized PVFC block and the PBzMA block are blended as indicated by the high-resolution TEM image shown in Figure 3E and by the iron distribution mapping (Figure S3C). The recovered porous multicompartment vesicles shown in [R]1 column can be further oxidized, suggesting that at least two cycles of the morphology transition can be made. It is deemed that the oxidation of the ferrocene moiety changes the solvophobic character of the membrane-forming PVFC block and, therefore, triggers the PVFC and PBzMA blocks being rearranged to form the layered membrane, as schematically 91

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00928. The experimental details and supplementary figures (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel.: 86-22-23509794. Fax: 86-22-23503510. Author Contributions

† These authors contributed equally to this manuscript (P.S. and Y.Q.).

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 (Nos. 21274066 and 21474054), and PCSIRT (IRT125) is gratefully acknowledged.

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