Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Effect of Tertiary Amines on the Photoinduced Electron TransferReversible Addition−Fragmentation Chain Transfer (PET-RAFT) Polymerization Brahim Nomeir, Olivier Fabre, and Khalid Ferji* Université de Lorraine, LCPM, UMR-CNRS 7375, 1 rue Grandville, BP20451, 54000 Nancy, France
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ABSTRACT: Herein, we report a detailed investigation on the influence of a series of tertiary alkylamine on the rate of the photoinduced electron transfer−reversible addition− fragmentation chain transfer (PET-RAFT) polymerization under visible light in the presence and absence of oxygen using Eosin Y as a photo-organocatalyst. Most importantly, we discovered that a simple increase of the alkyl length substituents in the alkylamines leads to an enhanced rate of PET-RAFT polymerization and its tolerance to oxygen. This study contributes to the fundamental comprehension of PETRAFT polymerization process and the role of the tertiary amine as a sacrificial electron donor agent.
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INTRODUCTION Since their discovery more than a quarter of a century ago, controlled radical polymerizations (CRPs) including atom transfer radical polymerization, nitroxide-mediated polymerization, and reversible addition−fragmentation chain transfer (RAFT) polymerization have been inspiring researchers to produce complex (co)polymers architectures with wellcontrolled parameters.1 The technical maturity and the potential of the CRPs have naturally aroused great industrial interests. Unfortunately, there is still a considerable contrast between the huge possibilities offered by CRPs at the laboratory scale and their transposition in the industry.2 It is well-known that CRPs are tolerant to protic solvents but sensitive to oxygen, which is an efficient inhibitor of radical species (Scheme S1). Two methods are currently used to deoxygenate CRPs polymerization medium: (i) purging the reactional mixture with an inert gas (nitrogen or argon) or (ii) displacement of oxygen using freeze/vacuum/thaw method. However, if these techniques are efficient at small scale (few millilitres), their use at a large scale remains difficult and expensive. Recently, several attempts to make CRPs more tolerant to oxygen have been successfully reported in the literature.3 The most promising way is called photoelectron/ energy-transfer reversible addition−fragmentation chain transfer (PET-RAFT) polymerization.4,5 As shown in Scheme 1a, in the PET-RAFT mechanism, the polymerization is conducted under the visible-light radiation in the presence of a photoredox catalyst (PC), which generates an excited species (PC*) either (i) by transferring energy to convert the naturally triplet oxygen (3∑) into an inactive singlet oxygen (1Δ)6 or (ii) by reducing the chain transfer agent (CTA), which will act as an initiator for the RAFT polymerization.7 In the last few years, PET-RAFT has been widely performed using metallic catalysts, including Ir(ppy)3,4 [Ru(bpy)3]Cl2,8,9 and ZnTPP.10 © XXXX American Chemical Society
However, the high cost and the potential toxicity of such catalysts prevent their use in biomaterial synthesis. In contrast, metal-free dyes, including organic molecules11−14 or carbon dot,15,16 are suitable alternatives because of their low cost and nontoxicity. Eosin Y (EY) is a well-known organic dye commonly used as a biological stain in histology and in cancer screening tests. Moreover, EY has been used for over 40 years as a PC to activate reaction in organic chemistry17 due to its facile intersystem crossing that allows the transformation of the singlet photoexcited state to the relatively long-lived triplet photoexcited state under visible light.18 Inspired by the abundant literature19,20 in this area, Boyer’s research group reported the ability of EY to act as a powerful PC with a low reduction potential (E*red = 1.1 V) able to reduce CTA (E*red = −0.4 V) under visible light.11 Recently, Sumerlin and coworkers12 demonstrated that the mechanism of PET-RAFT using EY depends strongly on the irradiation wavelength (UV, blue, or green) employed. In addition, they showed that EY is more efficient under blue light due to its rapid degradation under green light. It was found that adding tertiary alkylamines (TAs) to EY is suitable for initiating free photopolymerization21 and improving the yield of the PET-RAFT polymerization.11,12 Indeed, as described in Scheme 1b, TA (R3N) can act as an electron donor able to transfer electron to the excited triplet state of eosin to produce an amine radical cation (R3N°+) and an eosin radical anion (EY°−),18 which can reduce the CTA by a photoelectron transfer (PET) to generate a propagating radical (P°) by RAFT polymerization. Additionally, since the rate of the photo-RAFT polymerization depends on the photolysis of Received: July 18, 2019 Revised: August 21, 2019
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DOI: 10.1021/acs.macromol.9b01493 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 1. Proposed Mechanisms for PET-RAFT Polymerization under Blue Irradiation Using Eosin Y as PC (a) in the Absence of Tertiary Alkylamine (TA) and (b) in Its Presence
Figure 1. Chemical structures of tertiary amine (TA), RAFT agent, monomer, and Eosin Y as a photocatalyst (PC) used in this study. PMDETA and DMAP are N,N,N′,N″,N″-pentamethyldiethylenetriamine and 4-dimethylaminopyridine, respectively.
scattering (MALLS) detector (mini Dawn Wyatt), differential refractometer detector (RID 10A, Shimadzu), HPLC pump (LC 20AD, Shimadzu), degazer AF (DGU−20A3R, Shimadzu), and three PLgel columns (100 000, 1000, and 100 Å). Refractive index increments (dn/dc) of 0.13 were measured for poly(methyl methacrylate) (PMMA) in THF at 25 °C using a differential refractometer from WYATT Technology (Optilab rEX and HELEOSII). UV Absorbance. The stability of the RAFT agent (TTC) under 365 and 460 nm irradiation was investigated by measuring the absorbance of TTC in DMSO at different times (Figure S2) using the UVikon-XL spectrometer (Bio-Tech instruments). Fluorescence Spectroscopy. Fluorescence spectra were recorded using a FP-8300 JASCO spectrometer. PET-RAFT of MMA in Deoxygenated Medium with and without Addition of TA. In a dried glass vial, MMA (1064 μL, 10 mmol), EY (0.65 mg, 10−3 mmol, 100 ppm), and TTC (14 mg, 0.05 mmol) were dissolved in dried DMSO (7 mL). To evaluate the effect of TA, suitable quantity of the latter was added to the polymerization medium. Then, the homogeneous mixture was purged with nitrogen for 10 min, sealed, and then irradiated with a homemade UV−vis LED (460 nm, 8 W) at room temperature. After 6 h of polymerization, the crude product was analyzed by 1H NMR to determine the conversion (X) in DMSO-d6 (Figure S3) and by SECMALLS in THF at 25 °C. PET-RAFT Polymerization of MMA in the Presence of Oxygen with and without TA. In Case of a Filled Glass Vial (Figure S4a). In a dried glass vial of 8 mL, MMA (1064 μL, 10 mmol), EY (0.65 mg, 10−3 mmol, 100 ppm), and TTC (14 mg, 0.05 mmol) were dissolved in a dried DMSO (7 mL). To evaluate the effect of TA, suitable quantity of the latter was added to the polymerization medium. Then, the homogeneous mixture was sealed and irradiated with a homemade UV−vis LED (460 nm, 8 W) at room temperature. After 6 h of polymerization, the crude product was analyzed by 1H NMR in DMSO-d6 and by SEC-MALLS in THF at 25 °C. In Case of a Partially Filled Glass Vial (Figure S4b). In a dried glass vial of 8 mL, MMA (532 μL, 5 mmol), EY (0.325 mg, 5 × 10−4 mmol, 100 ppm), and TTC (7 mg, 0.025 mmol) were dissolved in dried DMSO (3.5 mL). To evaluate the effect of TA, suitable quantity of this latter was added to the polymerization medium. Then, the homogeneous mixture was sealed and irradiated with a homemade UV−vis LED (460 nm, 8 W) at room temperature. After 6 h of
the carbon−sulfur (C−S) bond to be generated (P°), one can assume that, in the PET-RAFT mechanism, this step may be impacted by the oxidation of R3N to produce EY°−. Thus, we hypothesized that to reduce efficiently the EY*, the amine radical cation (R3N°+) formed should be stable. Thus, varying the chemical structures of TA may change the rate of PETRAFT polymerization. To the best of our knowledge, currently, only two tertiary alkylamines, including triethylamine and 4-(dimethylamino)pyridine (DMAP), have been used in PET-RAFT with EY as PC. However, their influence on the PET-RAFT oxygen tolerance has not been systematically investigated. Herein, we report for the first time a detailed investigation into the influence of various commercial TA, commonly used in polymer synthesis (Figure 1), on the rate of the PET-RAFT polymerization in the absence and presence of oxygen. Most importantly, we discovered that changing the chemical structures of TA enhances greatly the rate of PETRAFT polymerization and its oxygen tolerance.
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EXPERIMENTAL METHODS
Materials. Triethylamine (>99%), tributylamine (>99%), 4dimethylaminopyridine (DMAP, 99%), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, 99%), triethanolamine (99%), Eosin Y (EY, 99%), methyl methacrylate (MMA, 99%), and dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich. 4-(Propylthiocarbono-thioylthio)-4-cyanopentanoic acid (TTC) was synthesized in three steps as previously reported by us.22 Characterization Methods. 1H NMR spectra were recorded on a Bruker Avance 300 apparatus (300, 13 MHz, 25 °C) in DMSO-d6. Size Exclusion Chromatography (SEC). The molecular weights were measured using a SEC in tetrahydrofuran (THF) (flow rate of 1.0 mL min−1) at 25 °C, equipped with a multiangle laser light B
DOI: 10.1021/acs.macromol.9b01493 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules polymerization, the crude product was analyzed by 1H NMR in DMSO-d6 and by SEC-MALLS in THF at 25 °C. Preparation of PMMA-b-PMMA Diblock Copolymer by PETRAFT Polymerization of MMA in the Presence of Oxygen. In a dried glass vial of 8 mL, MMA (1064 μL, 10 mmol), EY (0.65 mg, 10−3 mmol, 100 ppm), tributylamine (12 μL, 0.05 mmol), and TTC (14 mg, 0.05 mmol) were dissolved in dried DMSO (7 mL), sealed, and then irradiated with a homemade UV−vis LED (460 nm, 8 W) at room temperature. After 6 h of polymerization, the crude product was precipitated in methanol to recover the desired PMMA macroRAFT agent (macroCTA) and then analyzed by SEC-MALLS in THF at 25 °C. For the chain extension, MMA (426 μL, 4 mmol), EY (0.26 mg, 0.0004 mmol), tributylamine (4.8 μL, 0.02 mmol), and PMMA macroCTA (340 mg, 0.02 mmol) were dissolved in a dried glass vial of 8 mL containing dried DMSO (7.5 mL), then sealed, and irradiated with a homemade UV−vis LED (460 nm, 8 W) at room temperature. After 4 h of polymerization, the crude product was precipitated in methanol to recover the desired PMMA-b-PMMA and then analyzed by SEC-MALLS in THF at 25 °C.
concentrations. This nonlinearity confirms the presence of a complex fluorescence quenching mechanism by a dynamic and static quenching, which is indicative of an electron transfer between amine and EY*. However, this qualitative test is not able to correlate the fluorescence quenching rate and the degree of electron transfer. The present study began by investigating the PET-RAFT of MMA in DMSO under nitrogen atmosphere using a constant molar ratio [EY]/[MMA] = 100 ppm in the presence of different TA ([TA]/[TTC] = 1) (Table 1, exp: 1, 5, 9, 13, 17, and 22). After 6 h of irradiation, the monomer conversions were estimated by NMR in DMSO-d6 (Figure S3) and both the number molar mass (Mn) and the dispersity (Đ) were obtained by SEC-MALLS chromatography in THF. In comparison to 48% (Table 1, exp: 1) monomer conversion obtained without adding TA, Table 1 shows some exciting results as the monomer conversions were increased depending on the TA used. In fact, a slight increase was observed in the presence of triethylamine (49%, exp: 5), PMDETA (53%, exp: 9), and triethanolamine (53%, exp: 13), while relatively high conversions were obtained with tributylamine (58%, exp: 17) and DMAP (57%, exp: 22). These tests indicate that the PETRAFT rate effectively depends on the chemical structure of TA. In addition, in the presence of all TA, the dispersity (Đ) was lower than 1.2, except for triethanolamine (Đ = 1.3, exp: 22). This high dispersity could be explained by the accumulation of dead chains during photopolymerization, as the mixture of triethanolamine and EY is known to be able to initiate the free radical polymerization,21 which increases the proportion of the side termination reactions. The capacity of the triethanolamine/EY mixture to initiate the photopolymerization of MMA in the absence of TTC is clearly demonstrated, as in this case, the monomer conversion reached 31% (exp: 14) compared to low monomer conversions (
