Photocontrolled RAFT Polymerization Mediated by a Supramolecular

May 25, 2017 - RAFT polymerization was significantly affected by the subtle interplay of host–guest, electrostatic, and steric interactions among CB...
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Photocontrolled RAFT Polymerization Mediated by a Supramolecular Catalyst Liangliang Shen, Qunzan Lu, Anqi Zhu, Xiaoqing Lv, and Zesheng An* Institute of Nanochemistry and Nanobiology, College of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China S Supporting Information *

ABSTRACT: A photocontrolled reversible addition−fragmentation chain transfer (RAFT) polymerization mediated by a supramolecular photoredox catalyst is reported. Cucurbit[7]uril (CB[7]) was used to form a host−guest complex with Zn(II) meso-tetra(4-naphthalylmethylpyridyl) porphyrin (ZnTPOR) to prevent aggregation of ZnTPOR, which in combination with a chain transfer agent (CTA) initiated efficient and controlled RAFT polymerization in water under visible light. RAFT polymerization was significantly affected by the subtle interplay of host−guest, electrostatic, and steric interactions among CB[7], ZnTPOR, and CTA. Polymerization rate was remarkably improved using CB[7]@ZnTPOR in comparison with that using ZnTPOR. The use of supramolecular interactions to modulate photocontrolled RAFT polymerization provides new opportunities to manipulate controlled radical polymerizations.

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loporphyrins and to improve the efficiency of PET-RAFT polymerization. Two strategies have been reported to improve the efficiency of PET-RAFT. Boyer and co-workers developed an organic electron donor−acceptor photoredox catalyst by covalently connecting a porphyrin with a thiocarbonylthio group to result in an enhanced PET-RAFT process due to the short distance between porphyrin and CTA.25 However, this method demands much effort in organic synthesis and as the polymer chain grows longer the efficiency is expected to decrease due to increasing distance between porphyrin and CTA. The same team also reported that zinc tetraphenylporphine (ZnTPP) allowed effective activation of trithiocarbonate-containing CTAs due to specific recognition between ZnTPP and trithiocarbonate, resulting in an enhanced polymerization efficiency.20 Interestingly, this specific recognition was proposed to be the coordination of the thiocarbonylthio group to the centric zinc of ZnTPP, which represents one of the rare examples of supramolecular chemistry in PET-RAFT that requires a minimal synthesis to achieve enhanced polymerization rates. In the past decades, host−guest supramolecular chemistry has been extensively studied and found tremendous applications in several areas, such as supramolecular polymerization, self-assembly, pharmacy, molecular recognition, biomimetic,

ver the past decades, reversible deactivation radical polymerization (RDRP) techniques have matured into powerful tools for synthesis of precision polymers.1−6 Recently, light-regulated RDRP has attracted considerable interest due to the abundance of cheap light sources, easy operation at low temperatures, environmental friendliness, and importantly, convenient spatiotemporal polymerization adjustment.7−17 For example, Hawker and co-workers have pioneered photocontrolled atom transfer radical polymerization (ATRP) catalyzed by photoredox catalysts.18 More recently, Boyer and co-workers have successfully developed visible light-controlled reversible addition−fragmentation chain transfer (RAFT) polymerization utilizing fac-[Ir(ppy)3] as a photoredox catalyst.19 The photoredox catalyst was exploited to activate chain transfer agents (CTAs) through photoinduced electron transfer (PET) under light irradiation, producing radicals which subsequently trigger RAFT polymerization, namely, PETRAFT polymerization. Afterward, photoredox catalysts were extended to metalloporphyrin families and great advances on PET-RAFT polymerization using a wide range of visible light wavelengths have been achieved.20−23 Although metalloporphyrins exhibit excellent photochemical properties, undesired aggregation in solution due to hydrophobic and π−π interactions can lead to severe self-quenching of the excited state,24 which will undermine their efficiency as photoredox catalysts in PET-RAFT polymerization. Therefore, one of the most important issues to be resolved for PET-RAFT catalyzed by metalloporphyrins is to prevent aggregation of metal© XXXX American Chemical Society

Received: May 8, 2017 Accepted: May 23, 2017

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DOI: 10.1021/acsmacrolett.7b00343 ACS Macro Lett. 2017, 6, 625−631

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ACS Macro Letters

Scheme 1. Schematic Illustration of RAFT Polymerization under Green Light Mediated by Supramolecular Photo-Redox Catalyst CB[7]@ZnTPOR

stimuli-responsive and self-healing materials, surface modification, and protein function regulation.26−31 Cucurbit[n]uril (CB[n]), a family of host molecules, has been widely used to form supramolecular complexes with porphyrins to prevent aggregation of porphyrins in water and improve their spectroscopic properties.32−36 For instance, Zhang and coworkers reported a supramolecular photosensitizer composed of CB[7] and porphyrin, resulting in greatly enhanced antibacterial efficiency.24 As far as we are aware, the use of host−guest chemistry to improve the efficiency of photocontrolled RAFT polymerization in water has yet to be reported. In the present work, an aqueous PET-RAFT mediated by a host−guest supramolecular photoredox catalyst is reported. As depicted in Scheme 1, CB[7], which can include positively charged guest molecules, was used to form a supramolecular photoredox catalyst with Zn(II) meso-tetra(4-naphthalylmethylpyridyl) porphyrin (ZnTPOR), aiming to prevent aggregation and, hence, self-quenching of the excited state of ZnTPOR. Polymerization was conducted in the presence of 4-cyano-4(ethanesulfanylthiocarbonyl)sulfanylpentanoic acid (ETTC) as a CTA under green LED light. Supramolecular interactions, including host−guest complexation between CB[7] and ZnTPOR, electrostatic attraction between the positively charged ZnTPOR and the negatively charged CTA, and the steric repulsion between CB[7]@ZnTPOR and CTA were studied, and their implications on the polymerization kinetics were investigated in detail. The host−guest recognition between cucurbituril and TPOR has been extensively investigated by Zhang and others.24,37 It has been demonstrated that naphthalene moieties can be efficiently encapsulated by cucurbituril, which was confirmed by 1 H NMR spectroscopy. We found that upon the addition of CB[7], the proton signals of ZnTPOR in D2O disappeared (Figure S3), suggesting efficient encapsulation of ZnTPOR by CB[7]. The interactions among CB[7], ZnTPOR, and ETTC were investigated via a combination of UV−vis absorption and fluorescence spectroscopies. Upon addition of CB[7], the fluorescence intensity of a solution containing ZnTPOR was significantly improved (Figure 1A) because CB[7] can effectively prevent aggregation of ZnTPOR and alleviate

Figure 1. (A) Fluorescence spectra of ZnTPOR in water mixed with varying amounts of CB[7]; (B) UV−vis absorption and (C) fluorescence spectra of ZnTPOR in water mixed with varying amounts of ETTC; (D) UV−vis absorption spectra of ZnTPOR in water mixed with 26.6 μM ETTC and varying amounts of CB[7]. The concentration of ZnTPOR in each sample was 5.2 μM.

fluorescence self-quenching. As shown in Figure 1B, when a solution of ZnTPOR was mixed with varying amounts of ETTC, the absorbance of ZnTPOR at 444 nm gradually decreased with a concomitant peak-shift to 458 nm. When the positively charged ZnTPOR was replaced by a negatively charged Zn(II) meso-tetra(4-sulfonatophenyl) porphyrin (ZnTPPS), no change in both absorbance and peak position was observed upon addition of ETTC (Figure S4A). Moreover, mixing the corresponding nonmetal-containing TPOR with ETTC also led to a similar decrease of absorbance and peak shift of the absorption spectrum (Figure S4B). These measurements collectively suggest that electrostatic attraction exists between the positively charged ZnTPOR and the negatively charged ETTC, which brings them in close proximity and thus affects the spectroscopic properties of ZnTPOR. It is worth noting that there is no host−guest binding between ETTC and CB[7] due to the mismatching of both size and charge according to the recognition properties of CB[7].38 626

DOI: 10.1021/acsmacrolett.7b00343 ACS Macro Lett. 2017, 6, 625−631

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ACS Macro Letters Moreover, we found that the UV−vis absorption of ETTC in water remain unchanged when mixed with CB[7] (Figure S5), which suggested no interaction between ETTC and CB[7]. Boyer and co-workers reported that trithiocarbonate compounds can coordinate to ZnTPP in DMSO, which was supported by the appearance of a new absorption peak at 620 nm.20 In our case, the similar new absorption peak was not observed. This is probably because the dominant electrostatic interaction hinders the coordination as coordination requires axially vertical orientation of the trithiocarbonate group with respect to the Zn(II) center while electrostatic interaction is directionless and appears to be much stronger to override the coordination. As shown in Figures 1C and S6, the fluorescence intensity of ZnTPOR at 637 nm gradually decreased upon addition of ETTC, supporting a PET process from ZnTPOR to ETTC. The interplay of host−guest complexation and electrostatic interaction was further investigated for solutions containing CB[7], ZnTPOR, and ETTC. In each sample, the concentration of ZnTPOR and ETTC was fixed at 5.2 and 22.6 μM but the concentration of CB[7] was varied from 0 to 20.8 μM. Interestingly, upon addition of CB[7] with increasing concentrations, the absorption spectrum of a solution containing ZnTPOR and ETTC was gradually restored to finally resemble that for the ZnTPOR solution itself (Figure 1D), indicating that the complexation of CB[7] with the arms of ZnTPOR effectively blocks the approach of ETTC to ZnTPOR and shields the interaction between them. As clearly shown in Figures S7 and S8, fluorescence quenching of ZnTPOR by ETTC was also gradually alleviated as the concentration of CB[7] increased. Combining the studies from UV−vis absorption and fluorescence spectroscopies, we conclude that CB[7] can remarkably improve the fluorescence intensity of ZnTPOR by preventing aggregation of ZnTPOR, CB[7] can also impede the PET process between ZnTPOR and ETTC by shielding the electrostatic interaction between them, and the influence of CB[7] on the PET process and thus PETRAFT should be closely related to the molar ratio of CB[7]/ ZnTPOR. The feasibility of CB[7]@ZnTPOR-catalyzed RAFT polymerization was initially investigated using N,N-dimethylacrylamide (DMA) as a representative monomer. Polymerization of DMA was conducted using either ZnTPOR or CB[7]@ ZnTPOR of different concentrations in water under green LED light (8 w, 520 nm). As shown in Figure 2A, for the polymerization catalyzed by ZnTPOR, the monomer conversion increased from 10% up to 88% as the concentration of ZnTPOR increased from 4.7 to 46.8 μM (Table S1). However, as the concentration of ZnTPOR further increased to 187 μM, a decrease in monomer conversion was observed instead. In principle, higher ZnTPOR concentrations contribute to more effective activation of ETTC, thus leading to higher monomer conversions. However, high concentrations of ZnTPOR in water also inevitably cause aggregation of ZnTPOR and undesired excited state self-quenching (see Figure S9A), thus a decrease in monomer conversion was anticipated. When using CB[7]@ZnTPOR (4:1), monomer conversions were improved relative to those using ZnTPOR especially at high photcatalyst concentrations (Table S2). The UV−vis absorption and fluorescence spectra of ZnTPOR and CB[7]@ZnTPOR of different concentrations (Figures S9 and S10) were acquired and their differences in UV−vis absorbance at 566 nm (ΔA) and fluorescence intensity at 637 nm (ΔI) against [ZnTPOP]

Figure 2. Results of polymerization of DMA (30% w/v) in water catalyzed by different concentrations of ZnTPOR and CB[7]@ ZnTPOR, [ETTC]/[DMA] = 1:200, [CB[7]]/[ZnTPOR] = 4:1, green LED light, 8 w, 520 nm, 20 °C, 4 h. (A) Dependence of monomer conversion on concentration of ZnTPOR; (B) Differences in UV−vis absorbance at 566 nm (ΔA) and fluorescence intensity at 637 nm (ΔI) between CB[7]@ZnTPOR and ZnTPOR versus concentration of ZnTPOR, ΔA = ACB[7]@ZnTPOR − AZnTPOR, ΔI = ICB[7]@ZnTPOR − IZnTPOR; (C) Dependence of molecular weight on DP in the presence of CB[7]; (D) GPC traces of PDMAs corresponding to different CB[7]@ZnTPOR concentrations.

were plotted in Figure 2B. Apparently, both the UV−vis absorbance and fluorescence intensity of CB[7]@ZnTPOR were remarkably higher, especially in the high concentration regime, than those for ZnTPOR, suggesting that the effect of aggregation prevention by host−guest complexation is greater than the effect of electrostatic shielding which lowers polymerization rate. The polymers synthesized under various conditions were characterized by gel permeation chromatography (GPC), which showed good control of molecular weight distributions was achieved (Figures 2C,D and S12). After a thorough investigation of CB[7]@ZnTPOR-catalyzed PETRAFT polymerization of DMA, we next extended this method to other monomer families represented by poly(ethylene glycol) methyl ether acrylate (PEGA, Mn = 480 g/mol), poly(ethylene glycol) methyl ether methacrylate (PEGMA, Mn = 475 g/mol), and 2-hydroxyethyl acrylate (HEA). High monomer conversions (>90%) were achieved within 5 h for the polymerizations of PEGA and PEGMA (Table S3) with relatively good agreements between theoretical and experimental molecular weights (Figure S13). The polymerization of HEA with a conversion of 40% in 1 h showed a much larger molecular weight than the expected value possibly due to the use of PMMA as the reference for molecular weight determination using GPC. Nevertheless, its dispersity remained reasonably low (Mw/Mn = 1.27; Figure S13). As the polymerization efficiency is determined by the interplay of the host−guest and electrostatic interactions for the photopolymerizations catalyzed by CB[7]@ZnTPOR, we next investigated the effect of CB[7]/ZnTPOR molar ratio on the polymerization kinetics. As exhibited in Figure 3A, the apparent polymerization rate constant (kpapp) obtained for the polymerizations conducted with increasing molar ratios of CB[7]/ZnTPOR (from 0 to 3) increased by 3× from 0.54 to 2.12 h−1, consistent with the host−guest interaction alleviating the aggregation of ZnTPOR. Interestingly, when the molar ratio of CB[7]/ZnTPOR further increased to 4, a notable 627

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experimental molecular weights as well as low dispersities (Mw/ Mn < 1.2; Figure S19B). Another advantage of PET-RAFT, the oxygen-tolerant feature, was also checked by polymerization in the presence of oxygen and the result was shown in Figure S20. After polymerization for 20 h, high monomer conversion was achieved (98%) and GPC trace showed good control of molecular weight distribution (Mw/Mn = 1.21). The ability to synthesize well-defined block copolymers using CB[7]@ZnTPOR was also explored by sequential CB[7]@ ZnTPOR-catalyzed PET-RAFT polymerization without intermediate purification and extra-addition of ZnTPOR. First, a PDMA homopolymer was synthesized with a high mononer conversion of 96% using a typical PET-RAFT polymerization protocol. Then a series of multiple block polymers was obtained via seven successive chain extensions (Table S5 and Figure 4A), each with a high monomer conversion. The final Figure 3. Results of polymerizations of DMA (30% w/v) in water utilizing ZnTPOR or CB[7]@ZnTPOR as the photoredox catalyst, [ETTC]/[DMA]/[ZnTPOR] = 1:200:0.01, green LED light, 8 w, 520 nm, 20 °C. (A) Polymerization kinetics corresponding to different molar ratios of CB[7]/ZnTPOR: black (0:1), red (1:1), blue (2:1), green (3:1), and pink (4:1); (B) Dependence of molecular weight and dispersity on conversion, [CB[7]]/[ZnTPOR] = 3:1; (C) GPC traces of PDMAs corresponding to different polymerization times, [CB[7]]/ [ZnTPOR] = 3:1; (D) “ON/OFF” monomer conversion corresponding to different times, [CB[7]]/[ZnTPOR] = 3:1.

decrease in kpapp (1.20 h−1) was observed instead. In line with the spectroscopic results, it is concluded that CB[7] plays two contradictory roles in the polymerization. On one hand, the inclusion of CB[7] prevents aggregation of ZnTPOR and alleviates its exited state self-quenching, which improves the polymerization rate. On the other hand, increasing the molar ratio of CB[7]/ZnTPOR also weakens the PET process between ZnTPOR and CTA due to increased bulkiness as well as more screening of the positive charges of the supramolecular catalyst, which lowers the interaction strength between CTA and the supramolecular catalyst and thus lowers the polymerization rate. Thus, it is very likely that when the molar ratio of CB[7]/ZnTPOR increased up to 4, the approach of ETTC to ZnTPOR was suppressed due to a high steric hindrance exerted by the full complexation of ZnTPOR with 4 CB[7] units and the complete screening of the four positive charges. This can well account for the decrease of polymerization rate when the molar ratio of CB[7]/ZnTPOR increased from 3 to 4. For the polymerizations conducted under the optimal molar ratio of CB[7]/ZnTPOR = 3:1, a good agreement between theoretical and experimental molecular weights and monomodal GPC traces with narrow molecular weight distributions were confirmed by GPC analysis (Figure 3B,C). As a result, in the following investigations, the molar ratio of CB[7]/ZnTPOR was set to 3. For example, PET-RAFT polymerizations of DMA with DPs ranging from 50 to 1000 were then conducted (Table S4 and Figure S18). In all cases, high monomer conversions (>90%) were obtained, good agreements between theoretical and experimental molecular weights were achieved, and narrowdispersity polymers were synthesized. One intriguing advantage of light-regulated polymerization is that the polymerization progress can be conveniently adjusted by simply switching light on and off alternatively. This light regulation for the polymerization catalyzed by CB[7]@ ZnTPOR is shown in Figure 3D, with expected features for controlled RAFT polymerizations: pseudo-first-order kinetics (Figure S19A), good agreements between theoretical and

Figure 4. (A) GPC traces of PDMA multiple block polymers synthesized via successive chain extensions; (B) GPC traces of PDMA150 and PDMA150-PPEGA75 synthesized by CB[7]@ZnTPORcatalyzed RAFT polymerization. [ETTC]//[ZnTPOR]/[CB[7]] = 1:0.01:0.03, green LED light, 8 w, 520 nm, 20 °C.

octablock PDMA homopolymer had a molecular weight of 68.3 kg/mol and low dispersity of 1.22. A diblock copolymer was also successfully synthesized by chain-extension of an isolated polymer (PDMA150, 4 h, 75% conversion) to obtain PDMA150PPEGA75 (Mn = 53 kg/mol, Mw/Mn = 1.21; Figure 4B), which serves to directly verify the high end fidelity of this PET-RAFT catalyzed by CB[7]@ZnTPOR. In this case, the conversion of the PPEGA block in 5 h was 75%, which again highlights the relatively high polymerization rate of this PET-RAFT polymerization using supramolecular catalyst in water. Porphyrin-based assemblies have been widely exploited in several areas, such as phototherapy and biomimetic cataly628

DOI: 10.1021/acsmacrolett.7b00343 ACS Macro Lett. 2017, 6, 625−631

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ACS Macro Letters sis.39−44 Recently, polymerization-induced self-assembly (PISA) via RAFT dispersion polymerization has become a powerful technique for the preparation of nanoparticles with diverse morphologies.45−54 More recently, visible light-mediated RAFT aqueous dispersion polymerization has become an intriguing research area.13,55,56 The combination of PET-RAFT using a supramolecular catalyst with PISA to directly synthesize porphyrin-containing nanoassemblies in water would be interesting for potential applications. There are only few reports on photo-PISA mediated by metalloporpyrins. Boyer et al. reported visible light-mediated PISA using ZnTPP in ethanol.57 Nevertheless, metalloporphyrin-catalyzed photoPISA in water has never been reported. Therefore, PETRAFT catalyzed by CB[7]@ZnTPOR was used for PISA synthesis via aqueous dispersion polymerization (Scheme 2). A

In summary, we have successfully developed a photocontrolled RAFT polymerization mediated by a supramolecular catalyst CB[7]@ZnTPOR, affording well-controlled polymerizations of a wide range of monomers via both homogeneous solution and dispersion polymerization. Supramolecular host− guest, electrostatic and steric interactions among CB[7], ZnTPOR, and CTA affect the spectroscopic properties of the photocatalyst, PET process and thus the PET-RAFT polymerization. The rate of PET-RAFT polymerization is significantly enhanced under optimized conditions whereby the molar ratio of CB[7]/ZnTPOR was found to be 3. Under these conditions, the reduced aggregation of ZnTPOR by host−guest formation and the balanced approach of CTA to ZnTPOR by electrostatic/steric interactions are subtly maintained such that an enhanced polymerization rate was observed. This work provides an interesting example of supramolecular catalysts for regulating RDRP under visible light. The supramolecular catalyst-containing nanoparticles synthesized by PISA directly in water may be useful in phototherapy, catalysis, and antibiosis.

Scheme 2. Schematic Illustration of RAFT Dispersion Polymerization under Green Light Mediated by Supramolecular Photo-Redox Catalyst CB7@ZnTPOR

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications Web site. Experimental details, NMR spectra, fluorescence spectra, UV−vis absorption spectra, and GPC curves (PDF). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acsmacrolett.7b00343.

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hydrophilic PDMA36 was used as a macro-CTA to chain-extend with diacetone acrylamide (DAAM) to afford various PDMA36PDAAMx nanoparticles in water under green light (Table S6). Well-defined PDMA36-PDAAM63 spheres (Dh = 374 nm) were obtained at 20% w/v solids in H2O/CH3CN and PDMA36PDAAM198 vesicles (Dh = 604 nm) were synthesized at 35% w/ v solids (Figure 5). Herein, we introduced a small volume

(PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zesheng An: 0000-0002-2064-4132 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank financial support by National Natural Science Foundation of China (21674050, 21604059) and assistance of Instrumental Analysis and Research Center, Shanghai University.

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Figure 5. TEM images of PDMA36-PDAAMx nanoparticles: (A) PDMA36-PDAAM63, [PDMA36]/[DAAM]/[ZnTPOR]/[CB[7]] = 1:100:0.01:0.03, solids 20% w/v, 20 °C, H2O/CH3CN (4:1, v/v); (B) PDMA36-PDAAM198, [PDMA36]/[DAAM]/[ZnTPOR]/[CB[7]] = 1:200:0.02:0.06, solids 35% w/v, 20 °C, water.

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DOI: 10.1021/acsmacrolett.7b00343 ACS Macro Lett. 2017, 6, 625−631