Letter pubs.acs.org/macroletters
Mikto-Brush-Arm Star Polymers via Cross-Linking of Dissimilar Bottlebrushes: Synthesis and Solution Morphologies Yoshiki Shibuya,†,‡ Hung V.-T. Nguyen,‡ and Jeremiah A. Johnson*,‡ †
Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States S Supporting Information *
ABSTRACT: A convergent brush-first ring-opening metathesis polymerization (ROMP) approach for the synthesis of miktobrush-arm star polymers (MBASPs) via cross-linking of dissimilar bottlebrush polymers is reported. Living bottlebrush polymers prepared via ROMP of norbornene-terminated poly(ethylene glycol) (PEG) or polystyrene (PS) macromonomers (MMs) were mixed together in a desired ratio and exposed to a bisnorbornene cross-linker to yield MBASPs with narrow size distributions. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) revealed that the solution morphologies of MBASPs depended on the feed ratio of the PEG and PS bottlebrush polymers at the cross-linking stage. This work provides a robust and modular strategy for the synthesis of a new type of miktoarm star polymer wherein the star arms are bottlebrush polymers.
M
norbornene XLs has proven particularly useful in this regard. BASPs containing a variety of polymer arms, drug molecules, and imaging agents have been prepared using brush-first ROMP,20−26 and this method has been conducted on >100 g scale with excellent control over the final BASP size and composition. In an effort to produce BASPs of varied shell composition, we reported on the cross-linking of Janus bottlebrush copolymers produced via ROMP of a polystyrene (PS)branch-polylactide (PLA) diblock copolymer MM that contained norbornene at the junction between the PS and PLA blocks to produce miktoarm brush-arm star polymers (Miktoarm BASPs, Figure 1a).24 Moreover, copolymerization of a poly(ethylene glycol) (PEG) MM with the aforementioned PS-branch-PLA MM produced miktoarm BASPs with 3 compositionally distinct arms.24 In each of these systems, the bottlebrush polymer precursors possessed side chains of different composition (i.e., they were miktoarm bottlebrush polymers); the resulting miktoarm BASPs presented a uniform distribution of linear arms that were covalently connected together. Thus, though miktoarm BASPs underwent intramolecular phase segregation to produce patchy particles, the extent of phase segregation was limited by the connectivity of the dissimilar bottlebrush polymer side chains.
ikto-arm star polymers, star polymers that possess arms of varied structure, size, or composition,1−3 are highly branched copolymers with tunable shapes and properties that depend on the structural asymmetry of the star arms. These polymers have attracted extensive attention for their unique functions that are distinct from those of linear block copolymers of similar composition.4−15 The synthesis of miktoarm star polymers often requires sequential polymerization from a multifunctional core molecule or “arm-first” cross-linking of preformed macromonomers (MMs).16 Both approaches have been greatly augmented by the development of controlled, living polymerization reactions. For example, Matyjaszewski and co-workers have demonstrated a powerful and versatile strategy for the synthesis of miktoarm star polymers with low dispersity values that utilizes atom transfer radical polymerization to cross-link mixtures of vinyl-based MMs and/or halide-macroinitiators.17−19 In 2012, we reported the “brush-first” method for the synthesis of brush-arm star polymers (BASPs) of tunable size and arm versus core functionality.20 In brush-first synthesis, MMs are first polymerized to produce living bottlebrush polymers. Subsequent addition of a suitable cross-linker (XL) couples these living bottlebrush polymers together to generate BASPs. The arms of BASPs are bottlebrush polymers of variable backbone and side chain length. Due to the steric hindrance of bottlebrush polymer arms, an exceptionally efficient and robust polymerization technique is required for BASP synthesis. Ru-initiated ring-opening metathesis polymerization (ROMP) of norbornene-terminated MMs and bis© XXXX American Chemical Society
Received: July 20, 2017 Accepted: August 15, 2017
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Figure 1. (a) Schematic illustration of our previous work on miktoarm BASP synthesis via brush-first ROMP.24 (b) Schematic illustration of this work on MBASP synthesis via brush-first ROMP. Two different living bottlebrushes are fed to the cross-linking reaction at a ratio of x:(1 − x). (c) Chemical structures of macromonomers (1, 2), cross-linker (XL; 3), and Grubbs 3rd-generation bis-pyridine catalyst (4).
for 20 min to produce PS bottlebrush polymers with a numberaverage degree of polymerization (DP) of 7. Then, aliquots of the reaction solution were added to vials containing 10, 15, 20, 25, and 30 equiv of XL 3. After 6 h, the polymerization reactions were quenched with excess ethyl vinyl ether (EVE). The reaction mixtures were directly analyzed by gel permeation chromatography (GPC). In all cases, the resulting PS-BASPs displayed monomodal refractive index traces (Figure S1a) and narrow size distributions (Figure S1b). These data confirm that 2 is amenable to brush-first ROMP following our established protocols. Next, we turned to MBASP synthesis via cross-linking of a mixture of PEG and PS bottlebrush polymers (Figure 1b). In separate vials, 1 (7 equiv) and 2 (7 equiv) were exposed to 4 (1 equiv for each MM) for 20 min. Equimolar aliquots of these living bottlebrush polymer solutions were then added to vials with N = 10, 15, 20, 25, and 30 equiv of XL 3 relative to the total amount of Ru (Figure 1b; see SI for details). After 6 h, the reactions were quenched with EVE and directly analyzed by GPC (Figure 2a). The traces display a major peak (retention times from ∼16−17 min) that corresponds to the MBASP.
We report here an alternative approach to the synthesis of BASPs with varied shell composition (Figure 1b) whereby two compositionally dissimilar living bottlebrush homopolymers are mixed together in the presence of an XL to generate miktobrush-arm star polymers (MBASPs). Since the bottlebrush precursors of MBASPs are homopolymers, their domains can be much larger than those in our previous miktoarm BASPs.24 Moreover, MBASPs of tunable composition can be easily prepared by varying the ratio of the dissimilar bottlebrush polymers prior to cross-linking. To establish the feasibility of MBASP synthesis via brush-first ROMP, we utilized PEG-MM20 (1, Figure 1c), PS-MM (2, Figure 1c), and photocleavable bis-norbornene XL20 (3, Figure 1c). Grubbs third-generation bis-pyridine catalyst27 (4, Figure 1c) was used as the initiator for all polymerizations. Each of these compounds has been previously reported except for 2, which was prepared via a Cu-catalyzed azide−alkyne cycloaddition reaction between PS-N328 and a norbornene alkyne precursor (see Supporting Information (SI) for details). We first sought to establish that 2 is viable for BASP synthesis. MM 2 (7 equiv) was exposed to 4 (1 equiv) in THF 964
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Figure 2. (a) Refractive index GPC traces at 60 °C for MBASPs synthesized via cross-linking reactions of an equimolar mixture of PEG and PS living bottlebrush polymers, with N = 10, 15, 20, 25, and 30 equiv of XL 3 relative to the total amount of Ru. (b) Refractive index GPC traces at 60 °C for MBASPs synthesized via cross-linking reactions of mixtures of PEG and PS living bottlebrush polymers at ratios of 10:0 (BASP(1)), 8:2 (BASP(0.8)), 6:4 (BASP(0.6)), 5:5 (BASP(0.5)), 4:6 (BASP(0.4)), 2:8 (BASP(0.2)), and 0:10 (BASP(0)). For all syntheses, N was 20. The traces for PEG and PS bottlebrushes are also shown in dotted lines.
When N = 10, the GPC trace (Figure 2a, blue trace) reveals a large amount of uncoupled bottlebrush polymer (retention time ∼19 min), which suggests incomplete cross-linking. When N = 25 or 30 (Figure 2a, light blue or red traces, respectively) the traces display small shoulders at shorter retention times (∼14−16 min) indicative of uncontrolled cross-linking or aggregation. When N = 15 or 20 the GPC traces (Figure 2a, green and orange traces, respectively) show less than 5 wt. % of unreacted bottlebrush polymer and no high molecular weight shoulders; based on these results, we chose N = 20 for all further studies. To investigate the role of bottlebrush polymer feed ratio on MBASP uniformity and properties (vide infra), we prepared a series of MBASPs (hereafter referred to as BASP(x) with N = 20 and molar ratios of MMs 1 and 2 such that the fraction of PEG-MM 1, denoted as x, was varied from 1 (100% PEGBASP) to 0 (100% PS-BASP; see SI for details). Figure 2b shows GPC traces for crude BASP(x) samples taken directly from the brush-first ROMP reaction mixtures following quenching with EVE. The GPC traces showed monomodal molecular weight distributions for all BASP(x), which suggests that PEG and PS BASPs, as well as MBASPs from any desired PEG:PS ratio, could be prepared in parallel with excellent control. These results highlight the modularity of our approach, which is a major advantage over our previous system that would have required new MMs for each new miktoarm BASP.24 Due to the hydrophilic and hydrophobic nature of PEG and PS, respectively, we hypothesized that the solution-state morphologies of MBASPs would depend on the bottlebrush polymer feed ratio (i.e., x). First, we measured the hydrodynamic diameters of BASP(x) samples in various solvents via dynamic light-scattering (DLS) (Figure 3a). In THF, which is a good solvent for both PEG and PS, all BASP(x) samples had hydrodynamic diameters of 28−32 nm (Figure 3a) with narrow
Figure 3. (a) Summary of DLS results at 25 °C for BASP(x) in THF and water. Parentheses are polydispersities (nm). Data for BASP(0) in water is unavailable due to its low solubility. Colors of the bars correspond to the colors in (b) and (c). (b, c) DLS profiles at 25 °C for BASP(x) in THF (b) and water (c). (d−g) Unstained TEM images on a TEM grid of BASP(0.8) drop-casted from its THF solution (d), BASP(0.2) drop-casted from its THF solution (e), BASP(0.8) drop-casted from its aqueous solution (f), and BASP(0.2) drop-casted from its aqueous solution (g). The inset in (g) shows magnified TEM image of a particle of BASP(0.2) from another TEM image. Scale bars: 200 nm and, for an inset in (g), 50 nm.
size-distributions (Figure 3b). This consistency is expected given that both the PEG and PS arms should be solvated. Next, we prepared an aqueous solution of each BASP(x) by first dissolving the sample in DMF and then dialyzing the DMF solution against water (see SI for details). DLS analysis of the aqueous solutions of BASP(x) (Figure 3c) revealed a strong dependence on the PEG content (x); the hydrodynamic radius was much larger for samples with lower x values (i.e., less PEG content), which suggests that increasing the PS content induces MBASP aggregation. Transmission electron microscopy (TEM) was used to visualize BASP(x) samples. Figure 3d−e show representative unstained TEM images of BASP(0.8) and BASP(0.2), respectively, prepared via drop-casting from THF solutions. While Figure 3d shows small particles of BASP(0.8) with relatively uniform sizes (diameters: 20−40 nm), which is consistent with the DLS result (Figure 3a), Figure 3e displays both large particles (diameters over 100 nm) and small particles 965
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ACS Macro Letters (diameters less than 50 nm) for BASP(0.2). These data suggest that THF evaporation induces aggregation of BASP(0.2). TEM images of samples prepared by drop-casting of aqueous solutions of BASP(0.8) and BASP(0.2) are provided in Figure 3f−g. When deposited from water, the TEM image of BASP(0.8) shows well-dispersed particles with diameters of ∼20 nm (Figure 3f). In contrast, the TEM image of BASP(0.2) deposited from water shows large (up to 200 nm) aggregates (Figure 3g). Moreover, these aggregates consist of dark regions with light spots (Figure 3g, inset). Considering the relative weight fraction of PEG and PS in this sample, and the fact the PEG domains should prefer exposure to water, we suggest that the light and the dark domains in the TEM image of BASP(0.2) (Figure 3g) correspond to PEG and PS regions, respectively.29 Collectively, these data provide further evidence that the morphology of MBASPs in water depends on the PEG fraction x. It is possible that the MBASP synthesis strategy described in Figure 1b could yield mixtures of PEG and PS BASPs rather than the desired MBASPs. To rule out this possibility, we obtained TEM images of an equimolar mixture of BASP(1) and BASP(0) prepared following identical procedures to those used for imaging of MBASPs BASP(0.8) and BASP(0.2) (see SI). For the mixed BASP system, TEM images displayed particles with diameters of up to 100 nm and their assemblies (Figure S2); notably, the small light dots seen on the surface of BASP(0.2) (Figure 3g) were not observed. This distinct difference strongly suggests that BASP(0.2) is not a mixture of BASPs bearing homopolymer bottlebrush arms, but instead consists of intramolecularly phase-segregated PEG and PS domains. Based on the results discussed above, we conclude that MBASPs with greater x values (i.e., more PEG) can exist as unimolecular structures in water with the PS bottlebrush arms shielded by the PEG bottlebrush arms (Figure 4a). On the other hand, as the PEG content decreases, the PS domains cannot be shielded by PEG within the same MBASP (Figure 4b); aggregation of many MBASPs occurs to reduce the exposure of PS to water. Finally, we were interested in investigating how UV-induced photocleavage30,31 of the core of MBASPs in the aggregated state could impact the morphologies of these materials.20,21,24,32−34 The size of BASP(0.2) is mostly maintained following irradiation for 4 h with 365 nm light in water (comparing Figure 5a to Figure 3g).20,21,24 After irradiation of BASP(0.2) for 18 h, lighter particles are observed along with a dark background (Figures 5b and S3). Given that photodegradation of the MBASP core should ultimately release the PEG and PS bottlebrush polymer arms, one reasonable explanation for the TEM results provided in Figures 5b and S3 is that the PEG bottlebrush arms form the light-colored spheres while the PS bottlebrush arms interact with the carbon TEM grid to generate the dark background. While DLS analysis of a THF solution of BASP(0.2) after 18 h of irradiation in water (Figure S4) confirms the photocleavage to smaller ∼10 nm bottlebrush polymers,35 the same analysis for the aqueous solution of BASP(0.2) following irradiation (i.e., not transferred to THF) shows uniform particles with an average hydrodynamic diameter of 145 nm (Figure 5c), which is only 21 nm less than the value before irradiation (166 nm: Figure 3a, BASP(0.2) in water). These results can be rationalized by considering that in aqueous solution, the MBASP morphology may not change significantly because of the insolubility of PS in
Figure 4. Schematic illustrations of the difference in possible morphologies of BASP(x) in water between the case with the value of x is large and the case with the value of x is small, by using representative examples of BASP(0.8) (a), and BASP(0.2) (b). The blue strings show PEG and the green strings show PS.
Figure 5. (a, b) Unstained TEM images on a TEM grid of BASP(0.2) drop-casted from its aqueous solution after irradiation with 365 nm light for 4 h (a) and 18 h (b). Scale bars: 200 nm. (c) DLS profile at 25 °C for BASP(0.2) in water after irradiation with 365 nm light for 18 h. Parentheses is polydispersity (nm).
water and its high glass transition temperature; upon drying the photocleaved sample on a carbon TEM grid, the PS bottlebrush polymers can separate from the PEG bottlebrush polymers to produce the morphologies observed in Figures 5b and S3. In conclusion, we have developed a robust and modular strategy for the synthesis of miktoarm star polymers where the individual arms are bottlebrush polymers: Mikto-brush-arm star polymers (MBASPs). We demonstrate the synthesis of MBASPs via ROMP-induced cross-linking of independently synthesized PEG and PS bottlebrush polymers. This approach allows us to synthesize MBASPs with uniform sizes from any feeding ratio of PEG and PS living bottlebrush polymers, which represents a practical advantage compared to previous methods. 966
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DLS and TEM studies confirm that the morphologies of MBASPs depend on the feed ratio of the bottlebrush polymer arms. Future studies on the assembly of MBASPs constructed from bottlebrush polymer arms of various sizes and compositions are underway in our laboratories.
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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.7b00529. Synthetic methods, NMR spectra, and other detailed experimental information (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jeremiah A. Johnson: 0000-0001-9157-6491 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Air Force Office of Scientific Research (FA955014-1-0292) for support of this work. We thank Dr. Mingjun Huang for his valuable discussions. We also thank Dr. Ken Kawamoto for supplying materials for our synthesis of macromonomers. Y.S. thanks Japan Society for the Promotion of Science (JSPS) for a Young Scientist Fellowship and the Program for Leading Graduate Schools (MERIT). H.V.-T.N. thanks National Science Foundation (NSF) for a Graduate Research Fellowship.
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REFERENCES
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