Synthesis, Self-Assembly, and Photoresponsive Behavior of Tadpole

Letter pubs.acs.org/macroletters

Synthesis, Self-Assembly, and Photoresponsive Behavior of TadpoleShaped Azobenzene Polymers Xing Wang,† Yanyu Yang,† Peiyuan Gao, Fei Yang, Hong Shen, Hongxia Guo, and Decheng Wu* Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics & Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: Herein, we report a feasible method to prepare a tadpole-shaped PEG-POSS-(Azo)7 polymer. The polymer selfassembled into a large vesicle in aqueous solution, undergoing reversible smooth-curling transformation responsive to UV and dark conditions. Incorporating POSS units into the azopolymer furnished quick trans−cis isomerization along a cubic orientation. The orientational isomerization formed some pores on the vesicular membrane and endowed the highly sensitive photoresponsive property. Encapsulation of various fluorescent dyes affected the hydrophilic/hydrophobic ratio of self-assemblies, causing their morphological transition from vesicles to micelles. Response to UV irradiation, the quick trans−cis isomerization resulted in rapid release of the encapsulated dyes. The intriguing photoresponsive property renders this kind of tadpole-shaped POSS hybrid azopolymer a potential for application in controlled release of drug.

A

properties compared with linear, star, randomly branched, and comb-shaped polymers.18,19 Incorporation of a polyhedral oligomeric silsesquioxanes (POSS) unit into polymeric matrices is an effective method to construct tadpole-shaped polymers because of its uniform cage-shaped structure and eight corner organic groups.20−22 Meanwhile, some amphiphilic tadpoleshaped POSS hybrid polymers have been investigated in selfassembly behaviors from vesicles to wormlike cylinders to spherical micelles by controlling their molecular structures and hydrophilic/hydrophobic ratios.23 However, synthesis and selfassembly of these amphiphiles are mostly adjusted by the hydrophilic moieties such as solvophilic species, topologies, molecular weights, and functional groups. That is because the adopted POSS compounds usually only possess one reactive group to be linked into the hydrophilic parts for construction of the tadpole-shaped polymers, and the residual seven inert terminal groups of POSS are very difficult to be further modified for tuning hydrophobic segments. Herein, we use a heteroreactive AB7 POSS compound with one hydroxyl and seven vinyl groups to fabricate a well-defined tadpole-shaped polymer.24 Seven vinyl end groups are further sealed with a thiol-modified azobenzene to yield the amphiphilic azopolymer. The obtained azopolymer with novel tadpole-shaped topology not only gives rise to self-assembly morphological transition in aqueous solution, but also endows a photoresponsive property under UV irradiation.

zobenzene-containing polymers, namely, azopolymers, have attracted significant attention in recent years due to their properties such as photochemically induced phase transition,1 optical nonlinearities,2 photocontrolled reversible property changes,3 and photoinduced birefringence and dichroism.4 Specially, the unique characteristic of photoinduced motion and deformation, based on the readily reversible trans− cis isomerization of azobenzene moieties, renders azopolymers potentially useful applications in optical data storage,5 NLO devices,6 sensors and actuators,7 as well as materials suitable for photofabrication and processing.8 To date, azopolymers have been prepared with various topological architectures, such as side-chain polymers,9 main-chain polymers,10 cross-linked networks,11 block copolymers,12 hyperbranched polymers and dendrimers.13 Furthermore, many efforts have been directed toward constructing multifarious amphiphilic azopolymers, which indeed presented wealthy self-assembly morphologies and photoresponsive behaviors.14−16 Except for adjusting polymeric hydrophilic/hydrophobic balance, topology of the amphiphilic azopolymers was recognized as a significant factor to fabricate the self-assemblies, producing spherical micelles, rod-like micelles, hollow nanotubes, and vesicles accompanying with a series of unique photoresponsive properties. So, to create novel molecular architectures of the amphiphilic azopolymers is greatly desired to enrich their self-assemblies and benefit a systematic study of the structure−property relationship. Tadpole-shaped polymers (e. g., semitelechelic polymers) are metaphorically referred to as a solid bead and long tadpole tail by Marques in 1997.17 In general, these tadpole-like polymers, consisting of one reactive group at the end of polymer chain, possessed unique architectural characteristics and self-assembly © XXXX American Chemical Society

Received: September 29, 2015 Accepted: November 9, 2015

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ACS Macro Letters The synthetic approach of the tadpole-shaped hybrid polymer is summarized in Figure 1A. First, a septvinyl

Figure 1. (A) Synthetic route of a tadpole-shaped PEG-POSS-(Azo)7 polymer. (B) 1H and 13C NMR spectra of the PEG-POSS-(Azo)7 polymer. Figure 2. (A) TEM images, (B) SEM images, and (C) DLS profiles of the morphologies self-assembled from PEG-POSS-(vinyl)7 (a), and PEG-POSS-(Azo)7 (b) before and (c) after UV irradiation for 2 min. (D) Schematic illustration of orientation and arrangement of azobenzene groups in the vesicles under UV and dark conditions.

monohydroxyl POSS ((vinyl)7-POSS-OH) was prepared according to our previous work.24 Then (vinyl)7-POSS-OH was esterified with monocarboxyl PEG (Figure S1) to produce PEG-POSS-(vinyl)7 (Figure S2). The seven terminal vinyl groups were further modified via thiol−ene click reaction with Azo-SH (Figure S3) to obtain PEG-POSS-(Azo)7 polymer, which possessed the characteristics of reversible trans−cis isomerization under UV and dark conditions (Figure S4A). Figure 1B clearly presented the assignment of the tadpoleshaped framework and the integration ratio (Ia/Iq) of methyl to azobenzene was extremely close to 1:21, indicating the welldefined PEG-POSS-(Azo)7 polymer was successfully prepared with high purity. To further testify the structural integrity, GPC coupled with a triple detector array was used to determine the accurate molecular weights of PEG-POSS-(vinyl)7 and PEGPOSS-(Azo)7 polymers. GPC traces in Figure S4B exhibited the symmetrical and unimodal peaks of these two polymers. The absolute molecular weights of PEG-POSS-(vinyl)7 and PEG-POSS-(Azo)7 were about 980 and 3000, with narrow PDIs (1.04 and 1.05, Table S2), which were in accordance with the calculated values of 1082 and 2886. The NMR and GPC results manifested the well-defined azopolymer with tadpoleshaped topology was successfully obtained via postmodification of PEG-POSS-(vinyl)7 through a simple and effective thiol−ene click reaction. It is reported that amphiphilic hemitelechelic POSScontaining polymers were prone to self-assemble into ellipsoidal micelles, regardless of the hydrophilic/hydrophobic volume ratio because of the capability of POSS aggregates to form layered crystals.20 Indeed, the hemitelechelic PEG-POSS(vinyl)7 exactly generated the large random ellipsoids, as indicated in Figure 2A-a,B-a. Thanks to the reactive end vinyl

groups, the PEG-POSS-(vinyl)7 polymer can be further functionalized to obtain various self-assembles. After modification with rigid azobenzene groups, the tadpole-shaped PEG-POSS-(Azo)7 polymer formed large vesicles with a thickness of about 50 nm in aqueous solution, as shown in Figure 2A-b,B-b. The cryo-TEM image in Figure S5A further confirmed formation of the stable large vesicles. The morphology was changed from ellipsoidal-like micelles to vesicles with the diameter slightly increasing from 610 to 680 nm from the DLS results (Figure 2C and Table S2). The formation of vesicular morphology should mainly ascribe to rigid azobenzene groups effectively obstructed direct strong aggregation of POSS as well as the hydrophilic/hydrophobic ratio of PEG-POSS-(Azo)7 exactly in good agreement with the condition to form vesicles.16 Dissipative particle dynamics (DPD) simulation is a mesoscopic simulation technique and a promising tool to study the self-assembly behavior of amphiphilic polymers. We performed DPD simulations (Scheme S1 and Table S1) to provide intuitionistic conformation to better understand self-assembly of the PEGPOSS-(vinyl)7. Figure S6A clearly disclosed strong aggregation of POSS units to obtain ellipsoidal-like micellar morphologies. Figure S6B,C intuitively revealed that synergic aggregation of POSS and azobenzene groups tended to yield the stable vesicular structure when the peripheral short PEG shells cannot stabilize the assembled ellipsoidal micelles. 1322

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the assembled vesicles in water showed different photoresponsive behaviors with a broader π−π* transition and a little red-shift (∼3 nm; Figure S8), which originated from the aggregation and arrangement of azobenzene chromophores in an orderly fashion. Figure 3 also indicated that PEG-POSS(Azo)7 polymers exhibited high sensitivity to UV irradiation. Azobenzene molecules, oriented along the cubic direction in the process of trans−cis isomerization, can produce an ordered torque, which promoted rapid conformational conversion and accelerated the azobenzene chromophores to reach the photostationary state within an extremely short time. After UV illumination for less than 8 s, a remarkable sharp decrease in absorbance and a hypsochromic shift of the π−π* transition both in CHCl3 solution (from 345 to 307 nm) and in aqueous solution (from 348 to 321 nm) was observed in Figure 3A,B. No further evolution of the UV−vis spectra was detected after irradiation for a longer time, manifesting the completion of isomerization within 8 s. Compared to the previous azopolymers that need a minimum of several minutes to achieve the photostationary state,14−16 the tadpole-shaped polymer with azo groups located in POSS yielded very quick trans−cis isomerization along a cubic orientation. Figure 3 further disclosed the assembled vesicles expressed higher sensitive photoresponsive property compared with a pure polymer solution in CHCl3. For a dilute polymer solution, individual trans−cis isomerization of azobenzene molecules resulted in the gradually decreased absorption intensity as reflected in Figure 3A. As a comparison, the accumulational trans-isomers in assembled vesicles were thermodynamically unstable and apt to quickly form cis-isomers in response to UV irradiation, as shown in Figure 3B. It is noted that, although the vesicular morphology rebounded via cis−trans back isomerization after 24 h in the dark, UV absorption still did not totally recover the initial value, especially for the wrinkle vesicles. The probable reason was because cis-azobenzene chromophores in highly restricted domains were very difficult to be completely rearranged and reorganized into original trans-states within 1 day. On account of a unique self-assembly mechanism and highly sensitive light-responsive behavior, PEG-POSS-(Azo)7 vesicles were doomed to have great potential as novel carriers in encapsulation and controlled release fields. Here, we adopt three model fluorescent dyes, TPE (hydrophobicity, 330 nm/ 487 nm), FITC (moderate hydrophilicity, 490 nm/525 nm), and RhB (high hydrophilicity, 550 nm/610 nm), to systematically estimate their photoinduced release behaviors. As shown in Figure 4A, it is amazingly found that when different hydrophilic dyes were encapsulated into the vesicles, the dyeloaded assemblies unexpectedly generated a morphological transition from vesicles (TPE-loaded) to spindle-like micelles (FITC-loaded) to sphere micelles (RhB-loaded). The morphological transition may ascribe to that the encapsulated dyes with distinct hydrophobicity significantly altered hydrophilic/hydrophobic balance of the complexes. Hydrophobic TPE molecules were loaded in the wall of vesicles to increase the hydrophobicity accompanying with the size slightly increased from 580 to 600 nm (Figure 2A-b, Figure 4A-a, and Tables S2 and S3). Hydrophilic FITC and RhB dyes were encapsulated at the internal cavity and/or tangled with external PEG chains to improve the hydrophilicity. The enhanced hydrophilicity greatly changed the hydrophilic/hydrophobic ratio and caused variation of self-assembled morphologies from vesicles to micelles. Figure 4A-b and 4A-c showed the sizes decreased to

It is well-known that azo compounds can exhibit a typical trans−cis isomerization under UV irradiation of light and an adverse cis−trans isomerization forced by irradiation of visible light or in the dark. As shown in Figure S7A, upon UV irradiation for 2 min, the vesicular membranes transformed from smoothness to curling and returned to smoothness after 24 h in the dark. The DLS results in Figure S7B reflected reversible membrane transformation, accompanied by a change of polydispersities from 0.217 to 0.407 to 0.287. The deformed vesicles may turn bigger or smaller, leading to the broader polydispersity. The membrane transformation should be mainly attributed to the change in net dipole moment associated with reversible trans−cis isomerization. However, due to high rigid POSS moieties locating in the wall of vesicles, the polymersomes can fix basically skeletal shapes but yield some obvious crumples and wrinkles (Figures 2A-c,B-c and S5B). Specially, we can also observe some small pores around the vesicular membrane. The formation of the pores resulted from the special isomerization behavior and individual assembled mechanism. Once the trans−cis isomerization was triggered upon UV irradiation, the trans-azobenzene groups restricted by rigid POSS units caused their orientation and arrangement along the cubic direction. As a consequence, the intertwined trans-isomers converted to mutual separated cis-isomers, thus, producing a few pores among the adjacent cis-isomers (a schematic illustration is shown in Figure 2D). For a better understanding of the local conformation of PEG-POSS-(Azo)7 before and after the UV irradiation, we further performed a molecular mechanic (MM) simulation to clearly illustrate the molecular structural change. The polymer consistent force field (PCFF) was employed and PEG−POSS-(Azo)7 molecules with trans or cis conformation were simulated in Figure 2D. The results showed the cis-azobenzene branches were more curved to curl up into balls, weakening the extramolecular aggregation of POSS-(Azo)7 segments to generate some small pores. After 24 h in the dark, reversible cis−trans transition induced morphological reorganization of the vesicles to recover their initial intact shapes. Consequently, the interpenetrating transisomers refilled the formed pores in cis-isomers, causing the pores to completely disappear. The specific conformational conversion of the tadpoleshaped POSS hybrid azopolymer can yield unique photoresponsive properties. For the trans−cis isomerization of azo chromophores, UV irradiation caused a decrease in the absorbance around 350 nm attributed to the π−π* transition and a weak increased signals close to 440 nm corresponding to the n−π* transition with extended irradiation time (Figure 3). Distinct from the homogeneous polymer solution in CHCl3,

Figure 3. Photochemical processes of (A) PEG-POSS-(Azo)7 (5 × 10−6 mol L−1 in CHCl3) and (B) PEG-POSS-(Azo)7 vesicles (1 mg mL−1 in water) under UV irradiation of 365 nm light for different times. 1323

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and increase permeability of the assemblies so as to allow the rapid leaking of loaded fluorescent probes. Emission spectra of the dye-loaded assemblies were also recorded to investigate their release behaviors. Figure 5A

Figure 5. Emission spectra of the (A) TPE, (B) FITC, and (C) RhBencapsulated assemblies after UV illumination for different times. (D) Schematic illustration of encapsulation and controlled release of various fluorescent dyes (cyan dot is TPE molecule, green dot is FITC molecule, and red dot is RhB molecule). Figure 4. TEM images (A), DLS profiles (B), and CLSM images (C) before and (D) after UV irradiation for 2 min of the (a) TPE, (b) FITC, and (c) RhB-encapsulated PEG-POSS-(Azo)7 assemblies.

showed a sharp decline of the TPE fluorescence intensity after UV irradiation for 2 min. After 24 h in the dark, the intensity recovered 80% of its initial value, indicating the most TPE molecules can be reloaded in the vesicles. In comparison, release of the hydrophilic FITC and RhB probes into aqueous solution increased their fluorescence intensities after UV illumination for 2 min (Figure 5B,C). Since FITC and RhB dyes were well soluble in water, it was very difficult for these two dyes to be re-encapsulated in the micelles as indicated in nearly unchanged intensities even after 24 h in the dark. The slight shift emission in Figure 5B,C also provided a power evidence to testify the varied environments of the dyes from the micelles to the water. Figure 5D gave a vivid cartoon picture to clearly illustrate encapsulation and controlled release behaviors of the dyes. These results proved that the PEG-POSS-(Azo)7 polymer possessed excellent encapsulation and controlled release ability for both hydrophilic and hydrophobic fluorescent dyes. In summary, we demonstrated a facile approach to constructing an amphiphilic tadpole-shaped hybrid azopolymer, PEG-POSS-(Azo)7. The azopolymer self-assembled into a stable vesicle in aqueous solution that can reversibly transform from smoothness to curling accompanying with formation and vanishment of pores in response to UV and dark conditions. Azo groups located in POSS were arranged along the cubic orientation, expressing highly sensitive UV-responsive trans−cis isomerization within 8 s. The assembled vesicles served as nanocontainers to load both hydrophobic and hydrophilic dyes. Encapsulation of the dyes altered the hydrophilic/hydrophobic ratio of self-assemblies, causing their morphological transition from vesicles to micelles. Under UV irradiation, the trans−cis isomerization significantly improved permeability of carriers, resulting in rapid release of the encapsulated dyes. This kind of POSS hybrid azopolymer with a unique light-responsive

130 and 110 nm for the FITC-encapsulated and RhBencapsulated micelles (Table S3). DLS characterization disclosed the “saturated state” of the fully swollen assemblies in water (Figure 4B). The TPE-loaded vesicles with highly hydrophobic walls only slightly swell, as indicated in the size, which varied from 600 to 650 nm, while the FITC-encapsulated and RhB-encapsulated micelles with highly hydrophilic shells significantly enlarged from 130 and 110 nm to 400 and 680 nm (Table S3). Confocal laser scanning microscope (CLSM) images in Figure 4C intuitively visualized the TPE-, FITC-, and RhB-loaded assembled structures were cyan vesicles, green spindle-like micelles, and red spherical micelles, demonstrating successful encapsulation of various dyes. Figure 4D further provided a visual sense to in situ monitor release of TPE, FITC and RhB molecules in the interior or exterior of assemblies after UV irradiation. Since TPE is an aggregation-induced emission (AIE) molecule, the decrease of TPE concentrations in the vesicles led to a decline of the fluorescence intensity. Compared to original florescent intensity (Figure 4C-a), Figure 4D-a displayed the weaker florescent intensity under UV illumination, which should ascribe to some hydrophobic TPE dyes slipping into the water. Distinct from TPE, hydrophilic FITC and RhB dyes have aggregation-caused quenching (ACQ) effects that can emit higher fluorescence intensity in a dilute aqueous solution. Figure 4C-b and Figure 4C-c displayed that no dyes were observed in original aqueous solutions. After UV irradiation, the background turned green and red and initial fluorescent dots were still visible, proving some FITC and RhB dyes released from the interior of the micelles to the aqueous media, as shown in Figure 4D-b,D-c. The results visually verified that UV illumination can change net dipole moment 1324

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J.; Liu, H. J.; Yuan, Y.; Jiang, S. Z.; Yao, Y. F.; Chen, Y. ACS Macro Lett. 2013, 2, 67−71. (d) Marchi, E.; Baroncini, M.; Bergamini, G.; Van Heyst, J.; Vogtle, F.; Ceroni, P. J. Am. Chem. Soc. 2012, 134, 15277− 15280. (14) (a) Zheng, Y. B.; Pathem, B. K.; Hohman, J. N.; Thomas, J. C.; Kim, M.; Weiss, P. S. Adv. Mater. 2013, 25, 302−312. (b) Wang, J.; Wang, X.; Yang, F.; Shen, H.; You, Y. Z.; Wu, D. C. Langmuir 2014, 30, 13014−13020. (c) Liu, Y.; Yu, C. Y.; Jin, H. B.; Jiang, B. B.; Zhu, X. Y.; Zhou, Y. F.; Lu, Z. Y.; Yan, D. Y. J. Am. Chem. Soc. 2013, 135, 4765−4770. (15) (a) Sun, W. Q.; He, X. H.; Gao, C. Y.; Liao, X. J.; Xie, M. R.; Lin, S. L.; Yan, D. Y. Polym. Chem. 2013, 4, 1939−1949. (b) Koskela, J. E.; Liljestrom, V.; Lim, J.; Simanek, E. E.; Ras, R. N. A.; Priimagi, A.; Kostiainen, M. A. J. Am. Chem. Soc. 2014, 136, 6850−6853. (c) Blasco, E.; Schmidt, B. V. K. J.; Barner-Kowollik, C.; Pinol, M.; Oriol, O. Polym. Chem. 2013, 4, 4506−4514. (d) Zhao, W. R.; Feng, L.; Xu, L.; Xu, W. L.; Sun, X.; Hao, J. C. Langmuir 2015, 31, 5748−5757. (16) (a) Fihey, A.; Perrier, A.; Browne, W. R.; Jacquemin, D. Chem. Soc. Rev. 2015, 44, 3719−3759. (b) Puntoriero, F.; Ceroni, P.; Balzani, V.; Bergamini, G.; Vogtle, F. J. Am. Chem. Soc. 2007, 129, 10714− 10719. (c) Blasco, E.; Schmidt, B. V. K. J.; Barner-Kowollik, C.; Pinol, M.; Oriol, L. Macromolecules 2014, 47, 3693−3700. (d) Yang, P. C.; Chen, H. C.; Wen, H. W.; Wu, P. I. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3107−3117. (17) Marques, C. M. Langmuir 1997, 13, 1430−1433. (18) (a) Tezuka, Y. Prog. Polym. Sci. 1992, 17, 471−514. (b) Wan, X. J.; Liu, T.; Liu, S. Y. Biomacromolecules 2011, 12, 1146−1154. (c) Hatakeyama, F.; Yamamoto, T.; Tezuka, Y. ACS Macro Lett. 2013, 2, 427−431. (d) Doi, Y.; Ohta, Y.; Nakamura, M.; Takano, A.; Takahashi, Y.; Matsushita, Y. Macromolecules 2013, 46, 1075−1081. (e) Xu, F. G.; Fang, Z. H.; Yang, D. G.; Gao, Y.; Li, H. M.; Chen, D. Y. ACS Appl. Mater. Interfaces 2014, 6, 6717−6723. (19) (a) Schmidt, B. V. K. J.; Fechler, N.; Falkenhagen, J.; Lutz, J. F. Nat. Chem. 2011, 3, 236−240. (b) Liu, L. M.; Liu, M.; Su, Y. J.; Dong, Y. G.; Zhou, W.; Zhang, L. N.; Zhang, H.; Dong, B.; Chi, L. F. Nanoscale 2015, 7, 2276−2280. (c) Zhang, W. B.; Sun, F.; Tirrell, D. A.; Arnold, F. H. J. Am. Chem. Soc. 2013, 135, 13988−13997. (d) Josse, T.; Altintas, O.; Oehlenschlaeger, K. K.; Dubois, P.; Gerbaux, P.; Coulembier, O.; Barner-Kowollik, C. Chem. Commun. 2014, 50, 2024−2026. (20) (a) Zhang, W. A.; Muller, A. H. E. Prog. Polym. Sci. 2013, 38, 1121−1162. (b) Zhou, F.; Xie, M. X.; Chen, D. Y. Macromolecules 2014, 47, 365−372. (c) Zhang, W. A.; Fang, B.; Walther, A.; Muller, A. H. E. Macromolecules 2009, 42, 2563−2569. (d) Zhang, W. A.; Yuan, J. Y.; Weiss, S.; Ye, X. D.; Li, C. L.; Muller, A. H. E. Macromolecules 2011, 44, 6891−6898. (e) Zhang, W. A.; Muller, A. H. E. Macromolecules 2010, 43, 3148−3152. (21) (a) Huang, M. J.; Hsu, C. H.; Wang, J.; Mei, S.; Dong, X. H.; Li, Y. W.; Li, M. X.; Liu, H.; Zhang, W.; Aida, T.; Zhang, W. B.; Yue, K.; Cheng, S. Z. D. Science 2015, 348, 424−428. (b) Li, Y. W.; Zhang, W. B.; Hsieh, I. F.; Zhang, G. L.; Cao, Y.; Li, X. P.; Wesdemiotis, C.; Lotz, B.; Xiong, H. M.; Cheng, S. Z. D. J. Am. Chem. Soc. 2011, 133, 10712− 10715. (c) Li, Y.; Guo, K.; Su, H.; Li, X.; Feng, X.; Wang, Z.; Zhang, W.; Zhu, S.; Wesdemiotis, C.; Cheng, S. Z. D.; Zhang, W. B. Chem. Sci. 2014, 5, 1046−1053. (d) Ni, B.; Huang, M. J.; Chen, Z. R.; Chen, Y. C.; Hsu, C. H.; Li, Y. W.; Pochan, D.; Zhang, W. B.; Cheng, S. Z. D.; Dong, X. H. J. Am. Chem. Soc. 2015, 137, 1392−1395. (e) Su, H.; Zheng, J. K.; Wang, Z.; Lin, F.; Feng, X. Y.; Dong, X. H.; Becker, M. L.; Cheng, S. Z. D.; Zhang, W. B.; Li, Y. W. ACS Macro Lett. 2013, 2, 645−650. (22) (a) Yang, Y. Y.; Wang, X.; Hu, Y.; Hu, H.; Wu, D. C.; Xu, F. J. ACS Appl. Mater. Interfaces 2014, 6, 1044−1052. (b) Wang, X.; Li, D.; Yang, F.; Shen, H.; Li, Z. B.; Wu, D. C. Polym. Chem. 2013, 4, 4596− 4600. (c) Jiang, B. B.; Tao, W.; Lu, X.; Liu, Y.; Jin, H. B.; Pang, Y.; Sun, X. Y.; Yan, D. Y.; Zhou, Y. F. Macromol. Rapid Commun. 2012, 33, 767−772. (d) Li, D. W.; Niu, Y. G.; Yang, Y. Y.; Wang, X.; Yang, F.; Shen, H.; Wu, D. C. Chem. Commun. 2015, 51, 8296−8299. (23) (a) Wen, J. G.; Yuan, L.; Yang, Y. F.; Liu, L.; Zhao, H. Y. ACS Macro Lett. 2013, 2, 100−106. (b) Igari, M.; Heguri, H.; Yamamoto,

property is promising to be further explored as novel carriers for controlled release of drug.

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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.5b00698. Experimental details and supporting figures and tables (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

These authors contributed equally (X.W. and Y.Y.).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge MOST (2014CB932200), “Young Thousand Talents Program”, and NSFC (21504096, 21174147, and 21474115) for financial support.

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REFERENCES

(1) (a) Park, J. W.; Nagano, S.; Yoon, S. J.; Dohi, T.; Seo, J.; Seki, T.; Park, S. Y. Adv. Mater. 2014, 26, 1354−1359. (b) Ishiba, K.; Morikawa, M.; Chikara, C.; Yamada, T.; Iwase, K.; Kawakita, M.; Kimizuka, N. Angew. Chem., Int. Ed. 2015, 54, 1532−1536. (c) Hayata, Y.; Nagano, S.; Takeoka, Y.; Seki, T. ACS Macro Lett. 2012, 1, 1357−1361. (2) Li, Z.; Qin, A. J.; Lam, J. W. Y.; Dong, Y. P.; Dong, Y. Q.; Ye, C.; Williams, I. D.; Tang, B. Z. Macromolecules 2006, 39, 1436−1442. (3) (a) Wang, Y. P.; Ma, N.; Wang, Z. Q.; Zhang, X. Angew. Chem., Int. Ed. 2007, 46, 2823−2826. (b) Yu, H. F. Prog. Polym. Sci. 2014, 39, 781−815. (4) (a) Koskela, J. E.; Vapaavuori, J.; Ras, R. H. A.; Priimagi, A. ACS Macro Lett. 2014, 3, 1196−1200. (b) del Barrio, J.; Blasco, E.; Toprakcioglu, C.; Koutsioubas, A.; Scherman, O. A.; Oriol, L.; Sanchez-Somolinos, C. Macromolecules 2014, 47, 897−906. (5) (a) Faul, C. F. J. Acc. Chem. Res. 2014, 47, 3428−3438. (b) Fang, G. J.; Maclennan, J. E.; Yi, Y.; Glaser, M. A.; Farrow, M.; Korblova, E.; Walba, D. M.; Furtak, T. E.; Clark, N. A. Nat. Commun. 2013, 4, 1521. (6) Priimagi, A.; Ogawa, K.; Virkki, M.; Mamiya, J.; Kauranen, M.; Shishido, A. Adv. Mater. 2012, 24, 6410−6415. (7) (a) Fasano, V.; Baroncini, M.; Moffa, M.; Iandolo, D.; Camposeo, A.; Credi, A.; Pisignano, D. J. Am. Chem. Soc. 2014, 136, 14245− 14254. (b) Chi, X. D.; Ji, X. F.; Xia, D. Y.; Huang, F. H. J. Am. Chem. Soc. 2015, 137, 1440−1443. (8) Natansohn, A.; Rochon, P. Chem. Rev. 2002, 102, 4139−4175. (9) (a) Komura, M.; Komiyama, H.; Nagai, K.; Iyoda, T. Macromolecules 2013, 46, 9013−9020. (b) Srichan, S.; Mutlu, H.; Badi, N.; Lutz, J. F. Angew. Chem., Int. Ed. 2014, 53, 9231−9235. (10) (a) Wang, L. B.; Pan, X. Q.; Zhao, Y.; Chen, Y.; Zhang, W.; Tu, Y. F.; Zhang, Z. B.; Zhu, J.; Zhou, N. C.; Zhu, X. L. Macromolecules 2015, 48, 1289−1295. (b) Weber, C.; Liebig, T.; Gensler, M.; Pithan, L.; Bommel, S.; Bleger, D.; Rabe, J. P.; Hecht, S.; Kowarik, S. Macromolecules 2015, 48, 1531−1537. (11) (a) Szymanski, W.; Beierle, J. M.; Kistemaker, H. A. V.; Velema, W. A.; Feringa, B. L. Chem. Rev. 2013, 113, 6114−6178. (12) (a) Rao, J. Y.; Hottinger, C.; Khan, A. J. Am. Chem. Soc. 2014, 136, 5872−5875. (b) Fukuhara, K.; Nagano, S.; Hara, M.; Seki, T. Nat. Commun. 2014, 5, 3320. (13) (a) Jiang, D. L.; Aida, T. Nature 1997, 388, 454−456. (b) Ueki, T.; Nakamura, Y.; Usui, R.; Kitazawa, Y.; So, S.; Lodge, T. P.; Watanabe, M. Angew. Chem., Int. Ed. 2015, 54, 3018−3022. (c) Zhang, 1325

DOI: 10.1021/acsmacrolett.5b00698 ACS Macro Lett. 2015, 4, 1321−1326

Letter

ACS Macro Letters T.; Tezuka, Y. Macromolecules 2013, 46, 7303−7315. (c) Dong, Y. Q.; Tong, Y. Y.; Dong, B. T.; Du, F. S.; Li, Z. C. Macromolecules 2009, 42, 2940−2948. (d) Wen, J. G.; Zhang, J.; Zhang, Y.; Yang, Y. F.; Zhao, H. Y. Polym. Chem. 2014, 5, 4032−4038. (24) Wang, X.; Yang, Y. Y.; Gao, P. Y.; Li, D.; Yang, F.; Guo, H. X.; Xu, F. J.; Wu, D. C. Chem. Commun. 2014, 50, 6126−6129.

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DOI: 10.1021/acsmacrolett.5b00698 ACS Macro Lett. 2015, 4, 1321−1326