Expanding the Scope of Polymerization-Induced Self-Assembly: Z

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Letter Cite This: ACS Macro Lett. 2018, 7, 255−262

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Expanding the Scope of Polymerization-Induced Self-Assembly: Z‑RAFT-Mediated Photoinitiated Dispersion Polymerization Jianbo Tan,*,†,‡ Xueliang Li,†,∥ Ruiming Zeng,†,∥ Dongdong Liu,† Qin Xu,† Jun He,† Yuxuan Zhang,† Xiaocong Dai,† Liangliang Yu,† Zhaohua Zeng,*,§ and Li Zhang*,†,‡ †

Department of Polymeric Materials and Engineering, School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China ‡ Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, Guangzhou 510006, China § School of Materials Science and Engineering, Sun-Yat Sen University, Guangzhou 510275, China S Supporting Information *

ABSTRACT: In this communication, we developed the first well-controlled Z-RAFT (RAFT = reversible addition− fragmentation chain transfer) mediated polymerization-induced self-assembly (PISA) formulation based on photoinitiated RAFT dispersion polymerization of tert-butyl acrylate (tBA) in ethanol/water (60/40, w/w) at room temperature using a Z-type macromolecular chain transfer agent (macroCTA). Polymerizations proceeded rapidly via the exposure of visible-light irradiation (405 nm, 0.45 mW/cm2) with high monomer conversion (>95%) being achieved within 1 h. A variety of polymer nano-objects (spheres, worms, and vesicles) with narrow molar mass distributions were prepared by this ZRAFT mediated PISA formulation. Silver nanoparticles were loaded with the vesicles via in situ reduction, which can be used as a catalyst for the reduction of methylene blue (MB) in the presence of NaBH4. Finally, gel permeation chromatography (GPC) analysis demonstrated that the corona block and the core-forming block could be cleaved by treating with excess initiator. This novel PISA formulation will greatly expand the scope of PISA and provide more mechanistic insights into the PISA research.

Despite great success having been achieved in RAFTmediated PISA, however, almost all polymerizations were mediated by R-type macromolecular chain transfer agents (macro-CTAs). For the R-RAFT-mediated PISA (see Scheme 1a), the RAFT reactive group will always locate at the end of block copolymers and therefore be embedded inside the polymer nanoparticles. In contrast, when using a Z-type macroCTA, the RAFT reactive group will always be in the center of block copolymers. As a result, polymer nano-objects with RAFT reactive groups on the surface can be prepared easily by the Z-RAFT-mediated PISA (see Scheme 1a). These polymer nano-objects can be surface-functionalized via further chain extension. Moreover, the corona block and the core-forming block can be cleaved conveniently by just removing the RAFT reactive group (e.g., treating with excess initiator), which can be utilized to prepare functional nanomaterials (e.g., clean particles, hollow nanoparticles). Therefore, it is highly desirable to develop novel Z-RAFT-mediated PISA formulations for the synthesis of well-defined polymer nano-objects at high solids contents, which will greatly expand the scope of PISA as well as the polymer nano-objects. It should be noted that using Z-

Solution self-assembly of block copolymers is an attractive method for the synthesis of polymer nano-objects with a diverse set of morphologies, including spheres, nanotubes, lamellae, vesicles, large compound vesicles, sunflowers, etc.1,2 The obtained polymer nano-objects have broad applications in the area of catalysis, biomineralization, nanoreactors, imaging, and drug delivery.3−7 However, the concentration of block copolymer is relatively low (typically 2.80). The key for the development of a well-controlled Z-RAFTmediated PISA is to translocate the heterogeneous polymerization from inside the monomer-swollen particles to the particle surface. To suppress the swelling of monomers to the core-forming block, lowering the reaction temperature is an attractive strategy, although most PISA formulations were conducted at a high temperature (typically 70 °C) via thermal initiation. Recently, Boyer, Cai, and our group have developed photoinitiated polymerization-induced self-assembly (photoPISA) formulations for the synthesis of a variety of polymer nano-objects at room temperature or lower.39−55 Quantitative monomer conversions can be achieved within a short irradiation time (e.g., 15 min in one specific case). The development of photo-PISA technique provides a convenient platform for the synthesis of various functional polymer nanoobjects that are difficult to prepare by traditional PISA formulations (e.g., protein−polymer conjugates). Herein, we report the first study of a well-controlled ZRAFT-mediated PISA formulation based on photoinitiated 256

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Figure 1. (a) TEM image of polymer nano-objects prepared via RAFT dispersion polymerization of tBA in ethanol/water (60/40, w/w) at 70 °C using mPEG45-BTPA as the macro-CTA and AIBN as the initiator (target DP of 280, 30% w/w tBA concentration). (b) THF GPC of sample 1a using both RI and UV detectors. (c) TEM image of polymer nano-objects prepared via photoinitiated RAFT dispersion polymerization of tBA in ethanol/water (60/40, w/w) at 70 °C using mPEG45-BTPA as the macro-CTA and SPTP as the initiator (target DP of 280, 30% w/w tBA concentration). (d) THF GPC of sample 1c using both RI and UV detectors. (e) TEM image of polymer nano-objects prepared via photoinitiated RAFT dispersion polymerization of tBA in ethanol/water (60/40, w/w) at 25 °C using mPEG45-BTPA as the macro-CTA and SPTP as the initiator (target DP of 280, 30% w/w tBA concentration). (f) THF GPC of sample 1e using both RI and UV detectors. The light intensity was kept at 0.45 mW/cm2 in all polymerizations.

detectors. A high molar mass impurity was observed in the RI GPC trace, which can be attributed to the presence of uncontrolled polymers at this high temperature. The UV GPC trace further confirmed this hypothesis since no signal was observed under the same elution volume, suggesting the absence of a RAFT reactive group in these uncontrolled polymers. Two small GPC peaks were observed at 21.5 and 22.5 mL elution volumes. The Mp value of the peak at 22.5 mL was 3400 g/mol, suggesting inefficient consumption of mPEG45-BTPA under these conditions. The Mp value of the peak at 21.5 mL was 6400 g/mol, which was approximately twice as large as the peak at 22.5 mL, indicating the occurrence

of dimerization of mPEG45-BTPA in this case. The high intensity of a mPEG45-BTPA signal in the UV GPC trace can be explained by the relatively low molar mass of mPEG45-BTPA compared to the final samples, leading to a relatively high RAFT group concentration. Similar results were also observed for the photoinitiated RAFT dispersion polymerization of tBA that conducted at 70 °C with SPTP as the photoinitiator, as shown in Figure 1c, d. In contrast, conducting the photoinitiated RAFT dispersion polymerization at room temperature with SPTP as the photoinitiator led to the formation of pure vesicles, as shown in Figure 1e. A monomodal and symmetrical RI GPC trace was observed in this case. Moreover, the RI GPC 257

DOI: 10.1021/acsmacrolett.8b00035 ACS Macro Lett. 2018, 7, 255−262

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Figure 2. (a) Polymerization kinetics for photoinitiated RAFT dispersion polymerization of tBA in ethanol/water (60/40, w/w) at 25 °C using mPEG45-BTPA as the macro-CTA (target DP of 200, 20% w/w tBA concentration). (b) Evolution of Mn and Mw/Mn with tBA conversion for photoinitiated RAFT dispersion polymerization of tBA in ethanol/water (60/40, w/w) at 25 °C using mPEG45-BTPA as the macro-CTA (target DP of 200, 20% w/w tBA concentration). (c) RI GPC traces obtained for mPEG45-BTPA and a series of mPEG45-PtBAn diblock copolymer nanoparticles prepared via photoinitiated RAFT dispersion polymerization of tBA in ethanol/water (60/40, w/w) at 25 °C using mPEG45-BTPA as the macro-CTA (30% w/w tBA concentration). (d) Relationship between mean DP of PtBA and Mn for the series of mPEG45-PtBAn diblock copolymer nanoparticles prepared via photoinitiated RAFT dispersion polymerization of tBA in ethanol/water (60/40, w/w) at 25 °C using mPEG45-BTPA as the macro-CTA (30% w/w tBA concentration). The light intensity was kept at 0.45 mW/cm2 in all polymerizations.

trace is almost identical to the UV GPC trace, indicating that good control was maintained at room temperature. It should be noted that the UV GPC signal of mPEG45-BTPA in the final sample was much lower than that of 70 °C, suggesting a much higher blocking efficiency. These results indicate that a wellcontrolled Z-RAFT-mediated PISA formulation can be constructed based on photoinitiated RAFT dispersion polymerization that conducted at room temperature. The kinetics of photoinitiated RAFT dispersion polymerization of tBA conducted in ethanol/water (60/40, w/w) at 20% w/w tBA concentration using mPEG45-BTPA as the macro-CTA (target DP of 200) was studied in detail. Figure 2a shows the evolution of monomer conversion with irradiation time, and high monomer conversion (>95%) was achieved within 60 min of 405 nm visible-light irradiation. The fast polymerization behavior can be attributed to the short half-life of SPTP under 405 nm visible-light irradiation.56 The semilogarithmic plots in the inset of Figure 2a show two distinct regimes. The first regime, which occurs between 0 and 18 min (the red line), corresponds to the homogeneous polymerization of mPEG45-PtBAn. In the second regime (the blue line, from 18 to 60 min), there is a slight increase in the polymerization rate, which corresponds to the onset of aggregation. During this period, the formed nuclei absorb an unreacted tBA monomer, which leads to relatively high local monomer concentration and hence the observed rate enhancement. Unlike typical R-RAFT-mediated PISA, the tBA monomer is more likely to concentrate on the particle surface

rather than inside the particles. Interpolation of the red and blue lines indicates that micellar nucleation occurred at around 18 min, which corresponds to 42.5% tBA conversion for this ZRAFT-mediated photo-PISA formulation. This intermediate monomer conversion corresponds to a mean DP of 85 for the core-forming PtBA block. Samples withdrawn during the kinetic study were also characterized by THF GPC, as shown in Figure 2b. At high monomer conversions (>50%), the evolution of Mn with monomer conversion was linear, suggesting that the polymerization was a pseudoliving polymerization. However, an obvious deviation was observed at low monomer conversions. This can be attributed to the overlap of the mPEG45-BTPA GPC peak and the mPEG 45 -PtBA n GPC peak at low monomer conversions (see Figure S3), leading to relatively low Mn values. A series of mPEG45-PtBAn diblock copolymers were prepared at 30% w/w tBA concentration. The target DP for the PtBA core-forming block was systematically varied from 60 to 280, and high tBA conversions were achieved in all cases. Figure 2c shows RI GPC data of mPEG45-BTPA and mPEG45PtBAn diblock copolymers prepared by photoinitiated RAFT dispersion polymerization of tBA at a concentration of 30% w/ w. Systematic variation of the mean DP of the PtBA block led to a monotonic increase in the GPC traces of the diblock copolymers. Monomodal and symmetrical GPC traces were observed in all cases with narrow molar mass distributions (Mw/Mn < 1.38), regardless of whether the final polymer nanoobjects were worms, mixed morphologies, or vesicles. It is 258

DOI: 10.1021/acsmacrolett.8b00035 ACS Macro Lett. 2018, 7, 255−262

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Figure 3. Phase diagram constructed for mPEG45-PtBAn diblock copolymer nano-objects prepared via photoinitiated RAFT dispersion polymerization in ethanol/water (60/40, w/w) at 25 °C using SPTP as the photoinitiator and mPEG45-BTPA as the macro-CTA by systematic variation of the mean DP of PtBA and the tBA concentration. Phase regions consist of spheres (S), worms (W), mixed morphologies (mixed), and vesicles (V). The light intensity was kept at 0.45 mW/cm2 in all polymerizations.

Figure 4. (a) Schematic representation of the preparation of Ag composites using mPEG45-PtBAn polymer nano-objects as the template and the cleavage of mPEG45 block from the mPEG45−PtBAn polymer nano-objects. (b) TEM image of Ag/PEG45-PtBA278 vesicles prepared via in situ reduction of AgNO3 by PVP at 50 °C. (c) UV−vis spectra of MB reduced by NaBH4 using Ag/PEG45-PtBA278 vesicles as a catalysis. (d) RI GPC traces of mPEG45-BTPA and PEG45-PtBA209 diblock copolymer nano-objects before and after treating with 50-fold SPTP via the exposure of 405 nm visible light for 5 h.

block, monomer concentration, and temperature.56−58 For reproducible synthesis of mPEG45-PtBAn polymer nano-objects via mPEG45-BTPA-mediated photo-PISA, it is highly desirable to construct a detailed phase diagram. Figure 3 shows a phase diagram for a series of mPEG45-PtBAn diblock copolymer nanoobjects prepared by systematic variation of the mean DP of the core-forming PtBA block and the monomer concentration. The final copolymer compositions were determined by 1H NMR

noteworthy that a small GPC peak was observed at the low elution volume, which corresponds to unreacted mPEG45BTPA. A linear increase in Mn with the mean DP of PtBA was observed, which is characteristic of a pseudoliving polymerization. The main advantage of PISA is that tunable morphologies (e.g., spheres, worms, vesicles) can be readily prepared by varying reaction conditions, including DP of the core-forming 259

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same elution volume of mPEG 45-BTPA was observed, suggesting the successful cleavage of mPEG45 from the diblock copolymer. In summary, we have successfully developed a wellcontrolled Z-RAFT-mediated PISA formulation based on photoinitiated RAFT dispersion polymerization of tBA at room temperature. Lowering the reaction temperature to room temperature was critical to maintain good control of the polymerization. A series of mPEG45-PtBAn diblock copolymer nano-objects including spheres, worms, and vesicles were prepared by changing the DP of PtBA and the monomer concentration. A detailed phase diagram was constructed for this Z-RAFT-mediated photo-PISA formulation, which can be used as a roadmap for reproducible synthesis of various mPEG45-PtBAn diblock copolymer nano-objects. Diblock copolymer nano-objects with trithiocarbonate groups on the surface can be used as templates for the synthesis of Ag composite nanomaterials. The corona block can be cleaved from the polymer nano-objects by treating with excess initiator. This novel Z-RAFT-mediated PISA formulation will greatly expand the scope of PISA and provide a facile platform for the synthesis of functional nanomaterials.

spectroscopy. This phase diagram is rather similar to that reported by Tan et al.41 for a R-type macro-CTA (mPEG45CTA) block. In each case, the diblock copolymer morphology is strongly concentration-dependent, with spherical micelles being obtained at low monomer concentrations. This can be attributed to the reduced probability of sphere−sphere fusion at low monomer concentrations. In contrast, higher-order morphologies (worms, vesicles) were only obtained at high monomer concentrations (25% w/w or higher). The pure worm phase only occupies a narrow region, which is similar to that of other PISA formulations.39,59 It should be noted that a transparent physical gel was formed when the morphology was worm-like micelles, which could be attributed to worm−worm entanglements.60 The detailed phase diagram reported in this manuscript can be utilized as a convenient roadmap for reproducible preparation of mPEG45-PtBAn diblock copolymer nano-objects via the Z-RAFT-mediated photo-PISA. This is the first example of a Z-RAFT-mediated PISA formulation that can result in the formation of higher-order morphologies (worms and vesicles). Attaching inorganic nanoparticles onto polymer nano-objects is a well-established method for preparing functional inorganic/ organic composites.61−63 It is well-known that the trithiocarbonate group has strong interaction with heavy metal nanoparticles.64,65 The diblock copolymer nano-objects prepared via Z-RAFT-mediated photo-PISA contained a certain amount of trithiocarbonate groups on the surface, which can be used as the template for the synthesis of silver composites (see Figure 4a). Herein, silver nanoparticles were formed in situ on the mPEG45-PtBA278 vesicles (prepared at 30% tBA concentration) via the reduction of AgNO3 using poly(N-vinylpyrrolidone) (PVP) at 50 °C. Free silver nanoparticles were removed by several centrifugation−redispersion cycles. As shown in Figure 4b, a certain number of black dots was observed on the vesicle surface. The color of the reaction mixture changed from white to gray after the addition of PVP, indicating the formation of silver nanoparticles (Figure S5). The catalytic property of the Ag/mPEG45-PtBA278 vesicles was studied by the reduction of methyl blue (MB) using NaBH4. UV−vis spectroscopy was utilized to track the catalytic behavior of Ag/mPEG45-PtBA278 vesicles as shown in Figure 4c. Characteristic absorption peaks of MB at 615 and 655 nm were decreased with time, indicating the decomposition of MB in the presence of Ag/mPEG45-PtBA278 vesicles. After the addition of Ag/mPEG45-PtBA278 vesicles, the solution became pale gradually and transparent eventually (see the inset in Figure 4c). Another advantage of diblock copolymer nano-objects prepared via the Z-RAFT-mediated photo-PISA is that the corona block and the core-forming block can be cleaved easily by removing the trithiocarbonate group (see Figure 4a). This feature enables the preparation of stabilizer-free nanoparticles and hollow nanomaterials.38,66 Treating RAFT-derived polymers with excess initiator is a common strategy to remove the RAFT reactive group.67 Herein, mPEG45-PtBA209 vesicles (prepared at 30% w/w tBA concentration) were treated with 50-fold SPTP via the exposure of 405 nm visible light for 5 h. Precipitation occurred during this process due to the absence of stabilizer on the particle surface. The sample was dried directly without any purification and then analyzed by THF GPC. Figure 4d shows RI GPC traces of mPEG45-BTPA, mPEG45PtBA209 vesicles, and mPEG45-PtBA209 vesicles treated with 50fold SPTP. After treating with excess initiator, a new peak at the

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00035. Full experimental detail and additional results (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jianbo Tan: 0000-0002-5635-7178 Author Contributions

∥ Xueliang Li and Ruiming Zeng contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the National Natural Science Foundation of China (Grant 21504017, 21774143), Science and Technology Planning Project of Guangdong Province (Grant 2017A010103045), and Science and Technology Program of Guangzhou (Grant 201707010420). J.T. acknowledges the support from Pearl River Young Scholar of Guangdong.

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DOI: 10.1021/acsmacrolett.8b00035 ACS Macro Lett. 2018, 7, 255−262

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DOI: 10.1021/acsmacrolett.8b00035 ACS Macro Lett. 2018, 7, 255−262