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Visible light-mediated metal-free atom transfer radical polymerization with N-trifluoromethylphenyl phenoxazines
European Polymer Journal 117 (2019) 347–352
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Visible light-mediated metal-free atom transfer radical polymerization with N-trifluoromethylphenyl phenoxazines
T
Gyeong Su Parka, Jisu Backb, Eung Man Choia, Eunsung Leeb, , Kyung-sun Sona, ⁎
a b
⁎
Department of Chemistry, Chungnam National University, Daejeon 34134, Republic of Korea Department of Chemistry, Pohang University of Science and Technology, Pohang 37673, Republic of Korea
ARTICLE INFO
ABSTRACT
Keywords: Polymerization Photocatalysis Organocatalysis Amphiphilic block copolymer Density functional theory
The metal-free atom transfer radical polymerization (ATRP) process mediated by visible light and catalyzed by new organic photocatalysts (PCs) is investigated. The new PCs, designed by diversifying N-trifluoromethylphenyl phenoxazine, are analyzed and evaluated for organocatalyzed ATRP (O-ATRP) experimentally and theoretically. Especially, 3,7-di(4-biphenyl) 4-trifluoromethylphenyl-10-phenoxazine (4) enables successful O-ATRP with good catalytic performance, to produce well-defined polymers with low dispersity (Đ) under irradiation with a CFL bulb, white LEDs, or sunlight. The structure–property–performance relationships of PCs are investigated to determine the key parameters required for a good PC for use in O-ATRP. The scope of this process is explored in terms of monomer substrates and initiators using our new PC 4. Finally, a well-defined amphiphilic block copolymer is prepared using this method to form micelles, providing a basis for future biomedical applications.
1. Introduction Controlled radical polymerization (CRP) is a standardized polymer synthesis method, in which the molecular weight, molecular weight distribution, and structures are effectively controlled [1]. Especially, atom transfer radical polymerization (ATRP), one of the CRP processes, has the advantages of being able to use a wide range of monomers and initiators of various functional groups, making it one of the most commonly used synthetic methods for precise polymer production [2–4]. Concerns about metal residue in the final polymer products limit the application of ATRP for electronics and biomaterials, and there has therefore been a focus on reducing metal catalyst usage and removal of residual metals in this method [5,6]. To overcome these limitations of ATRP, catalytic systems for metal-free ATRP have been developed using various classes of organic photocatalysts (PCs) including those based on perylene, N-aryl phenothiazines, N,N-diaryl dihydrophenazines, and Naryl phenoxazines, all of which have the significant reducing power required to reduce alkyl bromide commonly used for ATRP [7–16]. With the development of organic PCs, interest and accomplishment in organocatalyzed ATRP (O-ATRP) have increased markedly over the past 5 years [17–22]. Previous studies indicated that, to exhibit superior performance in O-ATRP: (i) photoexcitation of a PC should produce a strongly reducing excited state, and (ii) PCs should have spatially separated singly ⁎
occupied molecular orbitals (SOMOs) in their triplet excited state (3PC*) [10,11,20,23–25]. Toward the goal of designing new organic PCs for O-ATRP, we initially performed a density function theory (DFT) calculation based on the hypothesis of the photoredox catalytic cycle of O-ATRP [26], to guide the discovery of strongly reducing PCs for OATRP. One of our new candidates was 4, a phenoxazine derivative bearing an N-aryl substituent with a CF3 group as an electron acceptor. Notably, the calculation predicted the strong excited state reduction potential of 4 (E0* (2PC%+/3PC*) = −2.56 V vs. SCE), which is more reducing than common metal PCs and phenoxazine PCs reported previously (Table S1) [11]. More encouragingly, computational modeling predicted that 4 would possess spatially separated SOMOs in the triplet excited state, with lower- and higher-lying SOMOs localized on the phenoxazine core and on the N-trifluoromethylphenyl group, respectively; this SOMO separation is a characteristic of charge transfer species [27]. These theoretical data prompted us to synthesize 4 and its derivatives, and evaluate them as PCs for O-ATRP experimentally under visible-light irradiation. The experimental and theoretical photoredox properties of our catalysts were investigated to determine which properties play the greatest role in determining each aspect of catalytic performance including low dispersity (Đ), high monomer conversion, and high initiator efficiency (I*) in O-ATRP. Then, the scope of this process using our new PC 4 was expanded in terms of monomer substrates, initiators, and light sources. Finally, the amphiphilic diblock
Corresponding authors. E-mail addresses: [email protected] (E. Lee), [email protected] (K.-s. Son).
https://doi.org/10.1016/j.eurpolymj.2019.05.023 Received 26 February 2019; Received in revised form 13 May 2019; Accepted 13 May 2019 Available online 14 May 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Structures of organic photocatalysts (PCs) examined in this study, with extinction coefficients (εmax) at λmax.
copolymer was prepared using this process and employed for micelle formation as preparation for biomedical applications.
lying SOMOs are localized in the phenoxazine core, while 1, 3, and 4 with aryl groups on phenoxazine core resulted in spatially separated SOMOs by intramolecular charge transfer from the core to the N-trifluoromethylphenyl substituent in the triplet excited state, according to the prediction by DFT calculations. These results indicated that introduction of conjugated groups to the phenoxazine core is necessary to access charge transfer excited states. However, introduction of a trifluoromethylphenyl group into both sides of phenoxazine core in 2 did not afford the charge separation in the 3PC* state observed in 1, 3, and 4, presumably due to competition with the N-aryl substituent with CF3 groups. Instead, the localization of higher-lying SOMO in a half portion of the phenoxazine core was observed. Next, to compare reduction potentials of these PCs obtained by electrochemistry, we carried out DFT calculations at the M06/6-31G** and M06/6-311+G** level in a CPCM-H2O model (Table S1) [31–36]. Although the calculated E0* (2PC%+/3PC*) values for M1 and M2 with our method were off by 0.20 V and 0.26 V to the Miyake group's values, respectively [11], E0* (2PC%+/3PC*) values for 1, 2, and 3 (−1.94 V, −1.70 V, and −2.03 V, respectively) obtained with our method showed a good agreement with experimental values (−1.78 V, −1.59 V, and −1.80 V, respectively) [37]. More importantly, the computationally predicted E0* (2PC%+/3PC*) value of 4 (−2.56 V vs. SCE), as well as its experimental E0* value (−1.98 V), was the largest among the PCs examined. Although the large discrepancy in the theoretical and experimental values of reduction potentials of 4 was observed, the error was further corrected by adopting B3PW91/6-31G** level of theory, which afforded E0* (2PC%+/3PC*) of −1.94 V. Then, we carried out polymerization of methyl methacrylate (MMA) using diethyl 2-bromo-2-methylmalonate (DBMM) as the initiator in dimethylacetamide (DMA) to evaluate the catalytic performance of PCs. Entries 1–7 in Table 1 show the polymerization results carried out under irradiation with a CFL bulb (Fig. S2) for 8 h. M1 showed high Đ (1.78) and poor I* (38%) with the number average molecular weight deviating significantly from the corresponding theoretical value (entry 1), which is likely assiciated with its poor absorprion property in the visible region. Compared to M1, 1 exhibited improved catalytic performance in O-ATRP, in terms of producing lower Đ polymer (1.51) with higher I* (74%, entry 2). Introduction of an electron-withdrawing group (2) onto the phenyl substituent of 1 resulted in even improved I*
2. Results and discussion We developed new organic PCs 1–4 based on a phenoxazine core (Fig. 1), inspired by previous outstanding features of phenoxazinebased PCs [11,24]. Following the precedence for synthetic modification of phenoxazine, PCs 1–4 were prepared via introduction of aromatic groups to the phenoxazine core of M1 (see Supporting Information for details). For direct comparison, we also prepared M1–M3 reported by the Miyake group [10,11]. First, the UV–vis absorbance of each PC was measured to examine the photochemical properties of these PCs (Fig. S19). M1 showed very weak absorption in the visible region. When the conjugation was increased by introducing a phenyl or diphenyl ring onto the phenoxazine core of M1, however, the maximum wavelengths of absorption (λmax) of 1–4 were red-shifted closer to the visible region (Fig. 1) [24]. Among them, 2 (λmax = 386 nm) and 4 (λmax = 383 nm) showed the highest absorption wavelengths, which were comparable to M2 (λmax = 389 nm). In addition, 2 and 4 exhibited the strong absorption in the visible light region, with molar extinction coefficients of εmax = 28,800 and 27,000 M−1 cm−1, respectively. Compared to 1, core modification with an electron-withdrawing group (2) red-shifted λmax by 27 nm toward the visible region [28], while the introduction of an electron-donating group (3) blue-shifted λmax by 3 nm relative to 1. Overall, characterization of the absorption properties of the PCs indicated that (i) the core substituent, rather than the N-aryl substituent, had a greater influence on the photophysical properties of the PCs, and (ii) core substitution with biphenyl rings or phenyl with electronwithdrawing groups was necessary to enhance visible light absorption, in accordance with the previous results in the literature [24]. Previous reports indicated that PCs with spatially separated SOMOs in their 3PC* states show superior performance in O-ATRP with regard to producing well-defined polymers [10,11]. Therefore, we investigated the nature of the 3PC* states of 1–4 and M1–M2 utilizing density functional theory (DFT) calculation [29], to confirm the correlation between charge transfer behavior of PCs in the triplet state and their catalytic abilities (Fig. 2) [30]. In M1, both lower-lying and higher348
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Fig. 2. Computed triplet state singly occupied molecular orbitals (SOMOs) of 3PC* investigated in this study at the the M06/6-31+G** level in the CPCM-H2O model (for M1, 1, and 3) and at the M06/Lanl2dz [38] level in the CPCM-H2O model (for 2, 4, and M2) utilizing Gaussian 09.
(87%) and conversion (82%) but also higher Đ (1.62; entry 3). The increase in monomer conversion means the polymerization proceeds faster. In contrast, introduction of an electron-donating group (3) yielded PMMA with lower Đ (1.38), while I* (77%) and conversion (51%) remained largely unaffected relative to 1 (entry 4). Notably, installation of diphenyl rings on the phenoxazine core, as in 4, improved the catalytic performance in O-ATRP to provide the high degree of control over MMA polymerization, generating PMMA with low Đ (1.27) and good initiation efficiency (92%) at 84% monomer conversion in 8 h (entry 5). The superior results of 4 and M2 (Đ = 1.20; I* = 84%; entry 6) were similar to each other but distinct from those of 1–3, suggesting that the catalytic ability is strongly dependent on the nature of the core substitution. Although PMMA with the lowest Đ (1.19) was obtained with M3 (entry 7), the I* (43%) and monomer conversion (38%) were rather unsatisfactory compared to the results of 4. Plots of Mn and Đ as a function of conversion were obtained by taking time-points aliquots during the course of MMA polymerization catalyzed by selected PCs (Fig. 3, S22–S25). Clearly, a linear increase in
Mn with respect to monomer conversion was observed for the polymerization of MMA mediated by 4, with agreement to the theoretical values of Mn for a well-controlled polymerization (Figs. 3 and S23). In contrast, similar polymerization catalyzed by 2 showed less control evidenced by high Đ over 1.5 throughout the course of polymerization (Fig. S22). The common aspects of PCs 2, 4, and M2, which showed high conversion over 70% in O-ATRP, are their absorption properties; they exhibited strong absorption in the visible region over 400 nm, while the other PCs 1, 3, M1, and M3 showed very weak absorption in this region. On the other hand, the low Đ polymer was obtained with 4, M2, and M3, all of which were predicted to have strong reduction potentials in their excited states [E0* (2PC%+/3PC*) = −2.19 ∼ −2.56 V vs. SCE] and spatially separated SOMOs [10]. Despite desirable absorption in the visible resion, PC 2 failed to produce low Đ polymer due to its relatively weak reducing power [E0* (2PC%+/3PC*) = −1.70 V vs. SCE], as well as to the high-lying SOMO localized on the core substituent (Fig. 2). Despite the strong E0* (2PC%+/3PC*) (−2.23 V vs. SCE), M1 yielded polymers with high Đ as both SOMOs were localized in the phenoxazine 349
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PMMA with good control (Đ of 1.23, I* = 74%; entry 8) in moderate conversion (54%). PMMA prepared using ethyl α-bromoisobutyrate (EBiB) or bromoacetonitrile (BrAN) exhibited higher Đ up to 1.40 (entries S1 and S2). To further explore the benefits of 4, MMA polymerization was performed using other light sources. The use of white LEDs instead of CFL light resulted in PMMA with higher Đ (1.47; entry 9), whereas similar polymerization in natural sunlight (Fig. S3) yielded PMMA with the low Đ (1.28) although initiator efficeintcy was poor (49%; entry 10). Next, to expand the monomer scope of the reaction, polymerization of glycidyl methacrylate (GMA) using 4 was examined. Upon irradiation with a CFL bulb, the system produced poly(glycidyl methacrylate) (PGMA) with poor control (Đ = 1.76, I* = 133; entry S3). Fig. S26 clearly shows that the polymerization of GMA suffered from poor control with high Đ > 1.7 from the early stage of the polymerization. As we suspected that the inevitable heat generated by the light source (polymerization tempearture of approx. 40 °C) may have induced uncontrolled radical polymerization of GMA, we changed the polymerization setup to an ice-water bath environment (Fig. S4) and repeated GMA polymerization at low temperature (4 °C). Although PGMA with higher Đ of 1.82 was obtained (entry S4), the first-order kinetic plot and linear growth of Mn as a function of conversion with close agreement to the theoretical values (Fig. S27) supported that the lower temperature was beneficial to the controlled O-ATRP process. Polymerization of styrene was also attempted with 4, but it showed uncontrolled behavior (Đ = 2.12) and poor initiator efficiency (57%; entry S5). As shown in Fig. S28, styrene polymerization mediated by 4 yieded polymers of unpredictable molecular weights with large deviation from the theoretical Mn values. One of the advantages of photopolymerization is temporal control during polymerization [40,41]. Fig. 4 shows a kinetic plot of Mn, Đ, and conversion over reaction time under white LED irradiation. When the light was switched on/off at 2-hour intervals, Mn, Đ, and conversion increased only when the light was on, while no polymer growth was detected during the dark period. Moreover, the growth of polymer chains in the light-on periods showed a proportional relationship between conversion and reaction time (Fig. 4a), as well as Mn and conversion (Fig. 4b) with multiple light on/off switching cycles, indicating a well-controlled process proceeding only under irradiation. Therefore, it is suggested that the PC excited by light activates the dormant species and the chain begins to grow when the light is switched on; when there is no light, polymerization is deactivated and the chain becomes dormant, thus allowing controlled regulation of the process. Chain extension polymerization was also carried out using isolated PMMA macroinitiator that had been prepared by 4. After the macroinitiator was isolated and purified, it was used to reinitiate the MMA polymerization catalyzed by 4. Chain extension from this
Table 1 Results for the polymerization of MMA using various photocatalysts. Entry
Photocatalyst
Conversion (%)a
Mn,th (g/ mol)a
Mn,NMR (g/ mol)b
Mn,GPC (g/ mol)c
Đc,d
I*
1 2 3 4 5 6 7 8f 9f,g 10f,h
M1 1 2 3 4 M2 M3 4 4 4
49.4 57.4 82.3 50.9 83.6 70.6 38.3 53.5 89.4 54.9
5200 6000 8500 5300 8600 7300 4100 5600 9200 5700
5400 6200 8100 5100 8200 7300 3800 5300 11,600 12,400
13,600 8100 9800 6900 9300 8700 9500 7500 12,900 11,600
1.78 1.51 1.62 1.38 1.27 1.20 1.19 1.23 1.47 1.28
38 74 87 77 92 84 43 74 71 49
e
General conditions: photocatalyst 9.35 μmol (1 eq.), DBMM (initiator) 93.5 μmol (10 eq.), MMA 9.35 mmol (1000 eq.), dimethylacetamide (same volume of monomer), irradiated with a CFL bulb (light source) for 8 h. a Theoretical molecular weight values calculated based on conversion ob1 tained from H NMR (i.e., Mn,th = Minitiator + [Monomer]0/ [Initiator]0 × conversion × Mmonomer). b Number average molecular weight determined by 1H NMR of precipitated polymer by comparing the integration of initiator versus polymer (i.e., Mn,NMR = Minitiator + DP × Mmonomer). c Measured by GPC. d Đ = polydispersity index (Mw/Mn). e Initiation efficiency = (theoretical molecular weight)/(experimental molecular weight determined by GPC) × 100. f Polymerization using ethyl α-bromophenylacetate (EBP) as an initiator. g Polymerization under irradiation with white LEDs. h Polymerization in natural sunlight (see Supporting Information for details).
core. Overall, the results shown in Table 1 and photoredox characteristics of the PCs imply that (i) monomer conversion is closely related to the visible light absorption efficiency, and (ii) good control of polymerization to afford low Đ polymer is associated with both strong excited state reduction potential and spatially separated SOMOs [10,11,22–25]. Fortunately, 4 fulfills all of the requirements for an efficient PC to catalyze controlled polymerization using visible light. A greater understanding of the influence of initiators and light sources on O-ATRP is needed to fully benefit from this new catalyst discovery [39]. Therefore, a range of classical ATRP initiators, monomer substrates and irradiation sources were examined for photoinduced O-ATRP using 4 (Tables 1 and S2). Among the alkyl halide initiators screened, DBMM was the most efficient initiator (I* = 92%) producing PMMA with low Đ (1.27) at high conversion (84%) in 8 h (entry 5). The use of ethyl α-bromophenylacetate (EBP) also generated
Fig. 3. Plots of (a) conversion as a function of reaction time and (b) Mn,NMR (black squares), Đ (red circles), and theoretical Mn (orange dotted line) as a function of MMA conversion in [MMA]:[DBMM]:[4] = [1000]:[10]:[1] equivalent in DMA solvent under continuous CFL irradiation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 350
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Fig. 4. (a) Plot of molecular weight (Mn,NMR, black squares), dispersity (Đ, red circles), and conversion (blue triangles) as a function of reaction time for the polymerization of methyl methacrylate (MMA) mediated by 4 in [MMA]:[EBP]:[4] = [1000]:[10]:[1] equivalent in DMA solvent under white LEDs (irradiation period indicated in white and dark period indicated in gray). (b) Plot of Mn,NMR (black squares) and Đ (red circles) as a function of MMA conversion catalyzed by 4 using a pulsed-irradiation sequence. Filled symbols indicate data after irradiation, while open symbols indicate those after the dark period. (The open symbols at 14% conversion correspond to data obtained during irradiation). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. (a) Synthesis of block copolymer via chain extension polymerization from a poly(ethylene glycol) (PEG)-Br macroinitiator with glycidyl methacrylate (GMA) (n:m = 2:3). (b) Gel permeation chromatography (GPC) traces of PEG-b-poly(glycidyl methacrylate) (PGMA) copolymer (blue trace) and PEG-Br macroinitiator (black trace). (c) Transmission electron microscopy (TEM) images and (d) Dynamic light scattering (DLS) of micelle particle size of PEG-b-PGMA amphiphilic copolymer. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
macroinitiator was successful (Fig. S29), confirming the chain end group fidelity and enabling the block copolymer synthesis. Therefore, we applied the O-ATRP with 4 to the synthesis of amphiphilic diblock copolymer composed of poly(ethylene glycol) (PEG) and PGMA blocks (Fig. 5). Amphiphilic block copolymers have attracted considerable attention, as they can self-assemble into a variety of morphologies, including micelles that can be used as carriers in drug or gene delivery systems [42,43]. In particular, PEG-based micelles provide a useful platform for biomedical applications due to their biocompatibility, and it would even be beneficial if the copolymer contained no metal residue to minimize cytotoxicity. Therefore, PEG-Br macroinitiator (Mn,NMR = 4700; Đ = 1.04) was prepared and used as an initiator in chain extension polymerization of GMA to synthesize PEG-b-PGMA diblock copolymer (Fig. 5a) [44]. The gel permeation chromatography (GPC) results shown in Fig. 5b illustrated the successful formation of the diblock copolymer (Mn,NMR = 27,800 g/mol; Đ = 1.42), as a clear shift toward higher molecular weight was observed in the GPC traces. Both 1H NMR (Fig. S31) and GPC results indicated that the ratio of
repeating units of PEG to those of GMA was approximately 2:3. As the PEG block is hydrophilic and the PGMA block is hydrophobic, the resulting block copolymer has amphiphilic properties, enabling micelle formation. The PEG-b-PGMA micelles formed by direct molecular selfassembly in isopropanol are spherical in shape, with an average diameter of about 800 nm, as determined by transmission electron microscopy (TEM, Fig. 5c) and dynamic light scattering (DLS, Fig. 5d). The micelles were well-defined with hydrophobic PGMA block as the core and hydrophilic PEG block as the shell, as shown in Fig. 5c. 3. Conclusions Successful catalysis for O-ATRP was achieved with new organic PCs using visible light irradiation. Our newly developed PC 4 enabled successful mediation of O-ATRP with good catalytic performance, to produce well-defined polymers of low Đ under a CFL bulb or white LED irradiation, or in sunlight. The propagation process was efficiently controlled by switching the light on/off. From the results of theoretical 351
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and experimental analyses, we gained an understanding of the structure–property–performance relationships of PCs. In addition, the synthesis of amphiphilic diblock copolymer and micelle formation using the diblock copolymer proved to be useful platforms for biomedical applications of the product prepared by this method. With the advantages of visible light catalysis and a metal-free strategy, these organic PCs would further broaden the scope of applications of this process.
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Data availability statement The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Acknowledgements We gratefully acknowledge financial support from the National Research Foundation of Korea (NRF-2015R1D1A1A01057228, NRF2019R1A2C2007825) and Chungnam National University. This work was also supported by Korea Toray Science Foundation. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.eurpolymj.2019.05.023. References [1] W.A. Braunecker, K. Matyjaszewski, Controlled/living radical polymerization: Features, developments, and perspectives, Prog. Polym. Sci. 32 (2007) 93–146. [2] K. Matyjaszewski, J. Xia, Atom transfer radical polymerization, Chem. Rev. 2921–2990 (2001). [3] K. Matyjaszewski, Atom transfer radical polymerization (ATRP): current status and future perspectives, Macromolecules 45 (2012) 4015–4039. [4] K. Matyjaszewski, N.V. Tsarevsky, Macromolecular engineering by atom transfer radical polymerization, J. Am. Chem. Soc. 136 (2014) 6513–6533. [5] S. Dadashi-Silab, M. Atilla Tasdelen, Y. Yagci, Photoinitiated atom transfer radical polymerization: Current status and future perspectives, J. Polym. Sci. Part A: Polym. Chem. 52 (2014) 2878–2888. [6] M. Ding, X. Jiang, L. Zhang, Z. Cheng, X. Zhu, Recent progress on transition metal catalyst separation and recycling in ATRP, Macromol. Rapid Commun. 36 (2015) 1702–1721. [7] N.J. Treat, H. Sprafke, J.W. Kramer, P.G. Clark, B.E. Barton, J. Read de Alaniz, B.P. Fors, C.J. Hawker, Metal-free atom transfer radical polymerization, J. Am. Chem. Soc. 136 (2014) 16096–16101. [8] X. Pan, M. Lamson, J. Yan, K. Matyjaszewski, Photoinduced metal-free atom transfer radical polymerization of acrylonitrile, ACS Macro Lett. 4 (2015) 192–196. [9] G.M. Miyake, J.C. Theriot, Perylene as an organic photocatalyst for the radical polymerization of functionalized vinyl monomers through oxidative quenching with alkyl bromides and visible light, Macromolecules 47 (2014) 8255–8261. [10] J.C. Theriot, C.-H. Lim, H. Yang, M.D. Ryan, C.B. Musgrave, G.M. Miyake, Organocatalyzed atom transfer radical polymerization driven by visible light, Science 352 (2016) 1082–1086. [11] R.M. Pearson, C.-H. Lim, B.G. McCarthy, C.B. Musgrave, G.M. Miyake, Organocatalyzed atom transfer radical polymerization using N-aryl phenoxazines as photoredox catalysts, J. Am. Chem. Soc. 138 (2016) 11399–11407. [12] A. Allushi, S. Jockusch, G. Yilmaz, Y. Yagci, Photoinitiated metal-free controlled/ living radical polymerization using polynuclear aromatic hydrocarbons, Macromolecules 49 (2016) 7785–7792. [13] S. Dadashi-Silab, X. Pan, K. Matyjaszewski, Phenyl Benzo[b]phenothiazine as a visible light photoredox catalyst for metal-free atom transfer radical polymerization, Chem. Eur. J. 23 (2017) 5972–5977. [14] Z. Huang, Y. Gu, X. Liu, L. Zhang, Z. Cheng, X. Zhu, Metal-free atom transfer radical polymerization of methyl methacrylate with ppm level of organic photocatalyst, Macromol. Rapid Commun. 38 (2017) 1600461. [15] L. Wang, R. Li, K.A.I. Zhang, Atom transfer radical polymerization (ATRP) catalyzed by visible light-absorbed small molecule organic semiconductors, Macromol. Rapid Commun. 39 (2018) 1800466. [16] J.P. Cole, C.R. Federico, C.-H. Lim, G.M. Miyake, Photoinduced organocatalyzed atom transfer radical polymerization using low ppm catalyst loading, Macromolecules 52 (2019) 747–754. [17] S. Shanmugam, J. Xu, C. Boyer, Photocontrolled living polymerization systems with reversible deactivations through electron and energy transfer, Macromol. Rapid Commun. 38 (2017) 1700143. [18] S. Shanmugam, C. Boyer, Organic photocatalysts for cleaner polymer synthesis,
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Visible light-mediated metal-free atom transfer radical polymerization with N-trifluoromethylphenyl phenoxazines
T
Gyeong Su Parka, Jisu Backb, Eung Man Choia, Eunsung Leeb, , Kyung-sun Sona, ⁎
a b
⁎
Department of Chemistry, Chungnam National University, Daejeon 34134, Republic of Korea Department of Chemistry, Pohang University of Science and Technology, Pohang 37673, Republic of Korea
ARTICLE INFO
ABSTRACT
Keywords: Polymerization Photocatalysis Organocatalysis Amphiphilic block copolymer Density functional theory
The metal-free atom transfer radical polymerization (ATRP) process mediated by visible light and catalyzed by new organic photocatalysts (PCs) is investigated. The new PCs, designed by diversifying N-trifluoromethylphenyl phenoxazine, are analyzed and evaluated for organocatalyzed ATRP (O-ATRP) experimentally and theoretically. Especially, 3,7-di(4-biphenyl) 4-trifluoromethylphenyl-10-phenoxazine (4) enables successful O-ATRP with good catalytic performance, to produce well-defined polymers with low dispersity (Đ) under irradiation with a CFL bulb, white LEDs, or sunlight. The structure–property–performance relationships of PCs are investigated to determine the key parameters required for a good PC for use in O-ATRP. The scope of this process is explored in terms of monomer substrates and initiators using our new PC 4. Finally, a well-defined amphiphilic block copolymer is prepared using this method to form micelles, providing a basis for future biomedical applications.
1. Introduction Controlled radical polymerization (CRP) is a standardized polymer synthesis method, in which the molecular weight, molecular weight distribution, and structures are effectively controlled [1]. Especially, atom transfer radical polymerization (ATRP), one of the CRP processes, has the advantages of being able to use a wide range of monomers and initiators of various functional groups, making it one of the most commonly used synthetic methods for precise polymer production [2–4]. Concerns about metal residue in the final polymer products limit the application of ATRP for electronics and biomaterials, and there has therefore been a focus on reducing metal catalyst usage and removal of residual metals in this method [5,6]. To overcome these limitations of ATRP, catalytic systems for metal-free ATRP have been developed using various classes of organic photocatalysts (PCs) including those based on perylene, N-aryl phenothiazines, N,N-diaryl dihydrophenazines, and Naryl phenoxazines, all of which have the significant reducing power required to reduce alkyl bromide commonly used for ATRP [7–16]. With the development of organic PCs, interest and accomplishment in organocatalyzed ATRP (O-ATRP) have increased markedly over the past 5 years [17–22]. Previous studies indicated that, to exhibit superior performance in O-ATRP: (i) photoexcitation of a PC should produce a strongly reducing excited state, and (ii) PCs should have spatially separated singly ⁎
occupied molecular orbitals (SOMOs) in their triplet excited state (3PC*) [10,11,20,23–25]. Toward the goal of designing new organic PCs for O-ATRP, we initially performed a density function theory (DFT) calculation based on the hypothesis of the photoredox catalytic cycle of O-ATRP [26], to guide the discovery of strongly reducing PCs for OATRP. One of our new candidates was 4, a phenoxazine derivative bearing an N-aryl substituent with a CF3 group as an electron acceptor. Notably, the calculation predicted the strong excited state reduction potential of 4 (E0* (2PC%+/3PC*) = −2.56 V vs. SCE), which is more reducing than common metal PCs and phenoxazine PCs reported previously (Table S1) [11]. More encouragingly, computational modeling predicted that 4 would possess spatially separated SOMOs in the triplet excited state, with lower- and higher-lying SOMOs localized on the phenoxazine core and on the N-trifluoromethylphenyl group, respectively; this SOMO separation is a characteristic of charge transfer species [27]. These theoretical data prompted us to synthesize 4 and its derivatives, and evaluate them as PCs for O-ATRP experimentally under visible-light irradiation. The experimental and theoretical photoredox properties of our catalysts were investigated to determine which properties play the greatest role in determining each aspect of catalytic performance including low dispersity (Đ), high monomer conversion, and high initiator efficiency (I*) in O-ATRP. Then, the scope of this process using our new PC 4 was expanded in terms of monomer substrates, initiators, and light sources. Finally, the amphiphilic diblock
Corresponding authors. E-mail addresses: [email protected] (E. Lee), [email protected] (K.-s. Son).
https://doi.org/10.1016/j.eurpolymj.2019.05.023 Received 26 February 2019; Received in revised form 13 May 2019; Accepted 13 May 2019 Available online 14 May 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Structures of organic photocatalysts (PCs) examined in this study, with extinction coefficients (εmax) at λmax.
copolymer was prepared using this process and employed for micelle formation as preparation for biomedical applications.
lying SOMOs are localized in the phenoxazine core, while 1, 3, and 4 with aryl groups on phenoxazine core resulted in spatially separated SOMOs by intramolecular charge transfer from the core to the N-trifluoromethylphenyl substituent in the triplet excited state, according to the prediction by DFT calculations. These results indicated that introduction of conjugated groups to the phenoxazine core is necessary to access charge transfer excited states. However, introduction of a trifluoromethylphenyl group into both sides of phenoxazine core in 2 did not afford the charge separation in the 3PC* state observed in 1, 3, and 4, presumably due to competition with the N-aryl substituent with CF3 groups. Instead, the localization of higher-lying SOMO in a half portion of the phenoxazine core was observed. Next, to compare reduction potentials of these PCs obtained by electrochemistry, we carried out DFT calculations at the M06/6-31G** and M06/6-311+G** level in a CPCM-H2O model (Table S1) [31–36]. Although the calculated E0* (2PC%+/3PC*) values for M1 and M2 with our method were off by 0.20 V and 0.26 V to the Miyake group's values, respectively [11], E0* (2PC%+/3PC*) values for 1, 2, and 3 (−1.94 V, −1.70 V, and −2.03 V, respectively) obtained with our method showed a good agreement with experimental values (−1.78 V, −1.59 V, and −1.80 V, respectively) [37]. More importantly, the computationally predicted E0* (2PC%+/3PC*) value of 4 (−2.56 V vs. SCE), as well as its experimental E0* value (−1.98 V), was the largest among the PCs examined. Although the large discrepancy in the theoretical and experimental values of reduction potentials of 4 was observed, the error was further corrected by adopting B3PW91/6-31G** level of theory, which afforded E0* (2PC%+/3PC*) of −1.94 V. Then, we carried out polymerization of methyl methacrylate (MMA) using diethyl 2-bromo-2-methylmalonate (DBMM) as the initiator in dimethylacetamide (DMA) to evaluate the catalytic performance of PCs. Entries 1–7 in Table 1 show the polymerization results carried out under irradiation with a CFL bulb (Fig. S2) for 8 h. M1 showed high Đ (1.78) and poor I* (38%) with the number average molecular weight deviating significantly from the corresponding theoretical value (entry 1), which is likely assiciated with its poor absorprion property in the visible region. Compared to M1, 1 exhibited improved catalytic performance in O-ATRP, in terms of producing lower Đ polymer (1.51) with higher I* (74%, entry 2). Introduction of an electron-withdrawing group (2) onto the phenyl substituent of 1 resulted in even improved I*
2. Results and discussion We developed new organic PCs 1–4 based on a phenoxazine core (Fig. 1), inspired by previous outstanding features of phenoxazinebased PCs [11,24]. Following the precedence for synthetic modification of phenoxazine, PCs 1–4 were prepared via introduction of aromatic groups to the phenoxazine core of M1 (see Supporting Information for details). For direct comparison, we also prepared M1–M3 reported by the Miyake group [10,11]. First, the UV–vis absorbance of each PC was measured to examine the photochemical properties of these PCs (Fig. S19). M1 showed very weak absorption in the visible region. When the conjugation was increased by introducing a phenyl or diphenyl ring onto the phenoxazine core of M1, however, the maximum wavelengths of absorption (λmax) of 1–4 were red-shifted closer to the visible region (Fig. 1) [24]. Among them, 2 (λmax = 386 nm) and 4 (λmax = 383 nm) showed the highest absorption wavelengths, which were comparable to M2 (λmax = 389 nm). In addition, 2 and 4 exhibited the strong absorption in the visible light region, with molar extinction coefficients of εmax = 28,800 and 27,000 M−1 cm−1, respectively. Compared to 1, core modification with an electron-withdrawing group (2) red-shifted λmax by 27 nm toward the visible region [28], while the introduction of an electron-donating group (3) blue-shifted λmax by 3 nm relative to 1. Overall, characterization of the absorption properties of the PCs indicated that (i) the core substituent, rather than the N-aryl substituent, had a greater influence on the photophysical properties of the PCs, and (ii) core substitution with biphenyl rings or phenyl with electronwithdrawing groups was necessary to enhance visible light absorption, in accordance with the previous results in the literature [24]. Previous reports indicated that PCs with spatially separated SOMOs in their 3PC* states show superior performance in O-ATRP with regard to producing well-defined polymers [10,11]. Therefore, we investigated the nature of the 3PC* states of 1–4 and M1–M2 utilizing density functional theory (DFT) calculation [29], to confirm the correlation between charge transfer behavior of PCs in the triplet state and their catalytic abilities (Fig. 2) [30]. In M1, both lower-lying and higher348
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Fig. 2. Computed triplet state singly occupied molecular orbitals (SOMOs) of 3PC* investigated in this study at the the M06/6-31+G** level in the CPCM-H2O model (for M1, 1, and 3) and at the M06/Lanl2dz [38] level in the CPCM-H2O model (for 2, 4, and M2) utilizing Gaussian 09.
(87%) and conversion (82%) but also higher Đ (1.62; entry 3). The increase in monomer conversion means the polymerization proceeds faster. In contrast, introduction of an electron-donating group (3) yielded PMMA with lower Đ (1.38), while I* (77%) and conversion (51%) remained largely unaffected relative to 1 (entry 4). Notably, installation of diphenyl rings on the phenoxazine core, as in 4, improved the catalytic performance in O-ATRP to provide the high degree of control over MMA polymerization, generating PMMA with low Đ (1.27) and good initiation efficiency (92%) at 84% monomer conversion in 8 h (entry 5). The superior results of 4 and M2 (Đ = 1.20; I* = 84%; entry 6) were similar to each other but distinct from those of 1–3, suggesting that the catalytic ability is strongly dependent on the nature of the core substitution. Although PMMA with the lowest Đ (1.19) was obtained with M3 (entry 7), the I* (43%) and monomer conversion (38%) were rather unsatisfactory compared to the results of 4. Plots of Mn and Đ as a function of conversion were obtained by taking time-points aliquots during the course of MMA polymerization catalyzed by selected PCs (Fig. 3, S22–S25). Clearly, a linear increase in
Mn with respect to monomer conversion was observed for the polymerization of MMA mediated by 4, with agreement to the theoretical values of Mn for a well-controlled polymerization (Figs. 3 and S23). In contrast, similar polymerization catalyzed by 2 showed less control evidenced by high Đ over 1.5 throughout the course of polymerization (Fig. S22). The common aspects of PCs 2, 4, and M2, which showed high conversion over 70% in O-ATRP, are their absorption properties; they exhibited strong absorption in the visible region over 400 nm, while the other PCs 1, 3, M1, and M3 showed very weak absorption in this region. On the other hand, the low Đ polymer was obtained with 4, M2, and M3, all of which were predicted to have strong reduction potentials in their excited states [E0* (2PC%+/3PC*) = −2.19 ∼ −2.56 V vs. SCE] and spatially separated SOMOs [10]. Despite desirable absorption in the visible resion, PC 2 failed to produce low Đ polymer due to its relatively weak reducing power [E0* (2PC%+/3PC*) = −1.70 V vs. SCE], as well as to the high-lying SOMO localized on the core substituent (Fig. 2). Despite the strong E0* (2PC%+/3PC*) (−2.23 V vs. SCE), M1 yielded polymers with high Đ as both SOMOs were localized in the phenoxazine 349
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PMMA with good control (Đ of 1.23, I* = 74%; entry 8) in moderate conversion (54%). PMMA prepared using ethyl α-bromoisobutyrate (EBiB) or bromoacetonitrile (BrAN) exhibited higher Đ up to 1.40 (entries S1 and S2). To further explore the benefits of 4, MMA polymerization was performed using other light sources. The use of white LEDs instead of CFL light resulted in PMMA with higher Đ (1.47; entry 9), whereas similar polymerization in natural sunlight (Fig. S3) yielded PMMA with the low Đ (1.28) although initiator efficeintcy was poor (49%; entry 10). Next, to expand the monomer scope of the reaction, polymerization of glycidyl methacrylate (GMA) using 4 was examined. Upon irradiation with a CFL bulb, the system produced poly(glycidyl methacrylate) (PGMA) with poor control (Đ = 1.76, I* = 133; entry S3). Fig. S26 clearly shows that the polymerization of GMA suffered from poor control with high Đ > 1.7 from the early stage of the polymerization. As we suspected that the inevitable heat generated by the light source (polymerization tempearture of approx. 40 °C) may have induced uncontrolled radical polymerization of GMA, we changed the polymerization setup to an ice-water bath environment (Fig. S4) and repeated GMA polymerization at low temperature (4 °C). Although PGMA with higher Đ of 1.82 was obtained (entry S4), the first-order kinetic plot and linear growth of Mn as a function of conversion with close agreement to the theoretical values (Fig. S27) supported that the lower temperature was beneficial to the controlled O-ATRP process. Polymerization of styrene was also attempted with 4, but it showed uncontrolled behavior (Đ = 2.12) and poor initiator efficiency (57%; entry S5). As shown in Fig. S28, styrene polymerization mediated by 4 yieded polymers of unpredictable molecular weights with large deviation from the theoretical Mn values. One of the advantages of photopolymerization is temporal control during polymerization [40,41]. Fig. 4 shows a kinetic plot of Mn, Đ, and conversion over reaction time under white LED irradiation. When the light was switched on/off at 2-hour intervals, Mn, Đ, and conversion increased only when the light was on, while no polymer growth was detected during the dark period. Moreover, the growth of polymer chains in the light-on periods showed a proportional relationship between conversion and reaction time (Fig. 4a), as well as Mn and conversion (Fig. 4b) with multiple light on/off switching cycles, indicating a well-controlled process proceeding only under irradiation. Therefore, it is suggested that the PC excited by light activates the dormant species and the chain begins to grow when the light is switched on; when there is no light, polymerization is deactivated and the chain becomes dormant, thus allowing controlled regulation of the process. Chain extension polymerization was also carried out using isolated PMMA macroinitiator that had been prepared by 4. After the macroinitiator was isolated and purified, it was used to reinitiate the MMA polymerization catalyzed by 4. Chain extension from this
Table 1 Results for the polymerization of MMA using various photocatalysts. Entry
Photocatalyst
Conversion (%)a
Mn,th (g/ mol)a
Mn,NMR (g/ mol)b
Mn,GPC (g/ mol)c
Đc,d
I*
1 2 3 4 5 6 7 8f 9f,g 10f,h
M1 1 2 3 4 M2 M3 4 4 4
49.4 57.4 82.3 50.9 83.6 70.6 38.3 53.5 89.4 54.9
5200 6000 8500 5300 8600 7300 4100 5600 9200 5700
5400 6200 8100 5100 8200 7300 3800 5300 11,600 12,400
13,600 8100 9800 6900 9300 8700 9500 7500 12,900 11,600
1.78 1.51 1.62 1.38 1.27 1.20 1.19 1.23 1.47 1.28
38 74 87 77 92 84 43 74 71 49
e
General conditions: photocatalyst 9.35 μmol (1 eq.), DBMM (initiator) 93.5 μmol (10 eq.), MMA 9.35 mmol (1000 eq.), dimethylacetamide (same volume of monomer), irradiated with a CFL bulb (light source) for 8 h. a Theoretical molecular weight values calculated based on conversion ob1 tained from H NMR (i.e., Mn,th = Minitiator + [Monomer]0/ [Initiator]0 × conversion × Mmonomer). b Number average molecular weight determined by 1H NMR of precipitated polymer by comparing the integration of initiator versus polymer (i.e., Mn,NMR = Minitiator + DP × Mmonomer). c Measured by GPC. d Đ = polydispersity index (Mw/Mn). e Initiation efficiency = (theoretical molecular weight)/(experimental molecular weight determined by GPC) × 100. f Polymerization using ethyl α-bromophenylacetate (EBP) as an initiator. g Polymerization under irradiation with white LEDs. h Polymerization in natural sunlight (see Supporting Information for details).
core. Overall, the results shown in Table 1 and photoredox characteristics of the PCs imply that (i) monomer conversion is closely related to the visible light absorption efficiency, and (ii) good control of polymerization to afford low Đ polymer is associated with both strong excited state reduction potential and spatially separated SOMOs [10,11,22–25]. Fortunately, 4 fulfills all of the requirements for an efficient PC to catalyze controlled polymerization using visible light. A greater understanding of the influence of initiators and light sources on O-ATRP is needed to fully benefit from this new catalyst discovery [39]. Therefore, a range of classical ATRP initiators, monomer substrates and irradiation sources were examined for photoinduced O-ATRP using 4 (Tables 1 and S2). Among the alkyl halide initiators screened, DBMM was the most efficient initiator (I* = 92%) producing PMMA with low Đ (1.27) at high conversion (84%) in 8 h (entry 5). The use of ethyl α-bromophenylacetate (EBP) also generated
Fig. 3. Plots of (a) conversion as a function of reaction time and (b) Mn,NMR (black squares), Đ (red circles), and theoretical Mn (orange dotted line) as a function of MMA conversion in [MMA]:[DBMM]:[4] = [1000]:[10]:[1] equivalent in DMA solvent under continuous CFL irradiation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 350
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Fig. 4. (a) Plot of molecular weight (Mn,NMR, black squares), dispersity (Đ, red circles), and conversion (blue triangles) as a function of reaction time for the polymerization of methyl methacrylate (MMA) mediated by 4 in [MMA]:[EBP]:[4] = [1000]:[10]:[1] equivalent in DMA solvent under white LEDs (irradiation period indicated in white and dark period indicated in gray). (b) Plot of Mn,NMR (black squares) and Đ (red circles) as a function of MMA conversion catalyzed by 4 using a pulsed-irradiation sequence. Filled symbols indicate data after irradiation, while open symbols indicate those after the dark period. (The open symbols at 14% conversion correspond to data obtained during irradiation). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. (a) Synthesis of block copolymer via chain extension polymerization from a poly(ethylene glycol) (PEG)-Br macroinitiator with glycidyl methacrylate (GMA) (n:m = 2:3). (b) Gel permeation chromatography (GPC) traces of PEG-b-poly(glycidyl methacrylate) (PGMA) copolymer (blue trace) and PEG-Br macroinitiator (black trace). (c) Transmission electron microscopy (TEM) images and (d) Dynamic light scattering (DLS) of micelle particle size of PEG-b-PGMA amphiphilic copolymer. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
macroinitiator was successful (Fig. S29), confirming the chain end group fidelity and enabling the block copolymer synthesis. Therefore, we applied the O-ATRP with 4 to the synthesis of amphiphilic diblock copolymer composed of poly(ethylene glycol) (PEG) and PGMA blocks (Fig. 5). Amphiphilic block copolymers have attracted considerable attention, as they can self-assemble into a variety of morphologies, including micelles that can be used as carriers in drug or gene delivery systems [42,43]. In particular, PEG-based micelles provide a useful platform for biomedical applications due to their biocompatibility, and it would even be beneficial if the copolymer contained no metal residue to minimize cytotoxicity. Therefore, PEG-Br macroinitiator (Mn,NMR = 4700; Đ = 1.04) was prepared and used as an initiator in chain extension polymerization of GMA to synthesize PEG-b-PGMA diblock copolymer (Fig. 5a) [44]. The gel permeation chromatography (GPC) results shown in Fig. 5b illustrated the successful formation of the diblock copolymer (Mn,NMR = 27,800 g/mol; Đ = 1.42), as a clear shift toward higher molecular weight was observed in the GPC traces. Both 1H NMR (Fig. S31) and GPC results indicated that the ratio of
repeating units of PEG to those of GMA was approximately 2:3. As the PEG block is hydrophilic and the PGMA block is hydrophobic, the resulting block copolymer has amphiphilic properties, enabling micelle formation. The PEG-b-PGMA micelles formed by direct molecular selfassembly in isopropanol are spherical in shape, with an average diameter of about 800 nm, as determined by transmission electron microscopy (TEM, Fig. 5c) and dynamic light scattering (DLS, Fig. 5d). The micelles were well-defined with hydrophobic PGMA block as the core and hydrophilic PEG block as the shell, as shown in Fig. 5c. 3. Conclusions Successful catalysis for O-ATRP was achieved with new organic PCs using visible light irradiation. Our newly developed PC 4 enabled successful mediation of O-ATRP with good catalytic performance, to produce well-defined polymers of low Đ under a CFL bulb or white LED irradiation, or in sunlight. The propagation process was efficiently controlled by switching the light on/off. From the results of theoretical 351
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and experimental analyses, we gained an understanding of the structure–property–performance relationships of PCs. In addition, the synthesis of amphiphilic diblock copolymer and micelle formation using the diblock copolymer proved to be useful platforms for biomedical applications of the product prepared by this method. With the advantages of visible light catalysis and a metal-free strategy, these organic PCs would further broaden the scope of applications of this process.
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