Oxygen Tolerance Study of Photoinduced Electron Transfer

Article pubs.acs.org/Macromolecules

Oxygen Tolerance Study of Photoinduced Electron Transfer− Reversible Addition−Fragmentation Chain Transfer (PET-RAFT) Polymerization Mediated by Ru(bpy)3Cl2 Jiangtao Xu,* Kenward Jung, and Cyrille Boyer* Centre for Advanced Macromolecular Design (CAMD) and Australian Centre for NanoMedicine (ACN), School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia S Supporting Information *

ABSTRACT: This study reports a highly efficient photoredox catalyst, Ru(bpy)3Cl2, capable of controlling the polymerization of methacrylates, acrylates, and acrylamides in the presence of thiocarbonylthio compounds via a photoinduced electron transfer−reversible addition−fragmentation chain (PET-RAFT) process. This polymerization technique was performed in a closed vessel in the presence or absence of air. Online Fourier transform near-infrared spectroscopy (FTNIR) was employed to monitor the monomer conversions of methyl methacrylate, methyl acrylate, and N,N′-dimethylacrylamide in the presence or absence of air. Interestingly, after an induction period, the polymerization proceeded in the presence of air to yield well-defined polymers (PDI < 1.20). The polymers were characterized by 1H NMR, UV−vis spectroscopy, and gel permeation chromatography. Excellent end-group retention was also demonstrated by NMR, UV−vis, and successive chain extensions of the resulting homopolymers to yield diblock and multiblock copolymers (decablock copolymers).

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atmosphere28−41 and an excellent control in the postfunctionalization of polymer.42−44 Inspired by the seminal work of Hawker and Fors,45−49 we developed a novel photoinduced living polymerization technique named photoinduced electron transfer−reversible addition−fragmentation chain transfer (PET-RAFT) polymerization.50 The technique offers the facile preparation of well-defined polymers using ultralow catalyst concentrations (ppm range) irradiated by low energy visible light (4.8 W, λ = 435 nm). This robust and versatile polymerization technique has the unique property of performing the polymerization in the presence of oxygen due to the ability of the catalyst to reduce molecular oxygen into inactive superoxide via single-electron reduction (Scheme 1).51 More importantly, the polymerization rates were not affected, except for the presence of an induction period. This induction period was attributed to the consumption of oxygen by the photoredox catalyst, after which the polymerization proceeded in the same manner as degassed samples. In this paper, we decided to expand this polymerization technique to a new photoredox catalyst, Ru(bpy)3Cl2 (Scheme 2), which is 10 times less expensive and presents a greater solubility than fac-[Ir(ppy)3] (originally employed in our first system). Ru(bpy)3Cl2 has been largely employed as photoredox catalysts in organic synthesis by MacMillan,51 Stephenson,52,53 Yoon,54,55 and others.56−58 Although previous reports by

INTRODUCTION Free radical polymerization is known to be highly sensitive to oxygen, as oxygen is an excellent radical scavenger. Controlled/ “living” radical polymerization techniques, which include nitroxide-mediated polymerization (NMP),1 metal-catalyzed living radical polymerization,2−4 atom transfer radical polymerization (ATRP),5−7 and reversible addition−fragmentation chain transfer (RAFT) polymerization,8−11 typically require stringent deoxygenation procedures, such as nitrogen purging or freeze−pump−thaw cycles, which limits their application in industrial processes. Introduction of external additives, such as reducing agents, tin(II) 2-ethylhexanoate (Sn(EH)2), or glucose for ARGET ATRP,12−14 metallic zinc, magnesium, and iron for SARA ATRP,15−17 and hydrazine hydrate for SETLRP,18,19 have been employed to regenerate the activators while maintaining chain end functionalities. Nevertheless, these techniques can only polymerize a narrow range of monomer families (i.e., usually acrylate). Therefore, new techniques with higher tolerance toward oxygen, compatibility with a broad range of monomers, and simple manipulation for industrial setup are desirable. Photopolymerization techniques can be performed in the presence of air and are widely employed in industry for a large range of applications, including coating, surface modification, etc.20−27 However, the polymers prepared via these techniques present a poor control of the molecular weight and broad molecular weight distribution (typically, Mw/Mn > 2.0). More recently, new photopolymerization techniques have emerged which allow a good control of molecular weight under inert © XXXX American Chemical Society

Received: April 28, 2014 Revised: June 7, 2014

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dx.doi.org/10.1021/ma500883y | Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. (A) Chemical Structures of Monomers Investigated in This Study and (B) Proposed Mechanism for PET-RAFT Polymerization and Its Oxygen Tolerance

1

H NMR, UV−vis spectroscopy, and gel permeation chromatography (GPC) to determine the end-group fidelity and the polymer structure. Finally, this method has been successfully exploited for the first time for the synthesis of complex polymer structures, such as multiblock copolymers by successive chain extensions in the presence of air.

Scheme 2. Chemical Structure of Commercially Available Water-Soluble Photoredox Catalyst Ru(bpy)3Cl2 (bpy = 2,2′Bipyridyl)−Tris(2,2′-bipyridyl)ruthenium(II) Chloride Hexahydrate

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EXPERIMENTAL SECTION

Materials. Methyl methacrylate (MMA, 99%), methyl acrylate (MA, 99%), styrene (St, 99%) N,N′-dimethylacrylamide (DMA, 99%), tris[2-phenylpyridinato-C2,N]iridium(III) ( fac-[Ir(ppy)3], 99%), and tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate (Ru(bpy)3Cl2, 99.95%) were all purchased from Aldrich. Monomers were deinhibited by percolating over a column of basic alumina (Ajax Chemical, AR). Dimethyl sulfoxide (DMSO, Ajax Chemical), diethyl ether (Ajax Chemical), and petroleum spirit (Ajax Chemical) were used as received. Thiocarbonylthiol compounds 4-cyanopentanoic acid dithiobenzoate (CPADB) and 2-(n-butyltrithiocarbonate)propionic acid (BTPA) were synthesized according to literature procedures.69−71 Instrumentation. Gel permeation chromatography (GPC) was performed using dimethylacetamide (DMAC) as the eluent. The GPC system consists of a Shimadzu modular system with an autoinjector, a Phenomenex 5.0 μm bead-size guard column (50 × 7.5 mm) followed by four Phenomenex 5.0 μm bead-size columns (105, 104, 103, and 102 Å), and a differential refractive-index detector and a UV detector (λ = 305 nm). The calibration of the system was based on narrow molecular weight distribution of polystyrene standards with molecular weights of 200−106 g mol−1. The molecular weights were calculated using the Mark−Houwink constants for the different polymers employed in this study. Absolute molecular weights were also measured for multiblock copolymers using light scattering. GPC-LS analysis was carried out using two PLgel columns, 5 μm, 1000 Å, 300 × 7.5 mm (Polymer Laboratories), using DMF as solvent, 1.0 mL/min

Lalevee and others59−65 have utilized this photoredox catalyst to perform free radical polymerizations, this paper demonstrates for the first time its applicability to perform a living polymerization in the presence of air to afford the synthesis of well-defined polymethacrylates, polyacrylates, and polyacrylamides. In the first part of this article, we have compared the performance of Ru(bpy)3Cl2 to fac-[Ir(ppy)3] in dimethyl sulfoxide (DMSO) for the polymerization of methacrylates, acrylates, styrene, and acrylamides. Subsequently, the polymerization kinetics of methyl methacrylate (MMA), methyl acrylate (MA), and N,N′-dimethylacrylamide (DMA) were investigated in the absence and presence of air. Inspired by early works of Haddleton and others,66−68 the polymerization rates in the absence and presence of air were investigated using an online Fourier transform near-infrared (FTNIR) spectroscopy, which allows real time monitoring of the monomer conversion and the exact determination of the induction period. At the end of the polymerization kinetics, the final polymers were analyzed by B


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at 40 °C. The sample was detected using a refractive index detector (Viscostar-II viscometer, available from Wyatt Technology Co.), and the molar mass was measured using a multiple-angle light scattering detector (miniDAWN available from Wyatt Technology Co.). This technology determines accurate molecular weight distributions without using calibration standards. The processing parameters that were used for the molar mass calculation included an exponential firstorder molar mass fit and a zero detector fit degree. The calculation method included using a measured AUX calibration constant for the refractive index detector and assumed 100% mass recovery. Nuclear magnetic resonance (NMR) spectroscopy was carried out on a Bruker Advance III with SampleXpress operating at 300 MHz for 1 H using CDCl3 as solvent and tetramethylsilane (TMS) as a reference. The data obtained were reported as chemical shift (δ) measured in ppm downfield from TMS. UV−vis spectroscopy spectra were recorded using a CARY 300 spectrophotometer (Varian) equipped with a temperature controller. Online Fourier transform near-infrared (FTNIR) spectroscopy was used to measure the monomer conversions by following the decrement of the vinylic C−H stretching overtone of the monomer at ∼6200 cm−1. A Bruker IFS 66/S Fourier transform spectrometer equipped with a tungsten halogen lamp, a CaF2 beam splitter, and liquid nitrogen cooled InSb detector was used. The sample was placed in a FTNIR quartz cuvette (1 cm × 2 mm) and polymerized under blue LED light irradiation (λmax = 435 nm). Every 5, 10, or 30 min, the sample was put into holder manually, and each spectrum in the spectral region of 7000−5000 cm−1 was constructed from 32 scans with a resolution of 4 cm−1. The total collection time per spectrum was about 15 s. Spectra were analyzed with OPUS software. Photopolymerization reactions were carried out in the same reaction vessel used in our previous work,50 where the reaction mixtures were irradiated by one meter of blue LED strip (4.8 W, λmax = 435 nm). General Procedure for the Kinetic Studies of PET-RAFT Polymerization of N,N′-Dimethylacrylamide (DMA) Mediated by Ir(ppy)3 or Ru(bpy)3Cl2. A 6 mL glass vial equipped with a rubber septum was charged with DMA (1.68 g, 16.95 mmol), BTPA (20 mg, 0.084 mmol), Ir(ppy)3 (0.011 mg, 1.74 × 10−5 mmol, 260 μL of 0.05 mg/mL DMSO solution), and DMSO (1460 μL, total solvent = 1720 μL) at a molar ratio of [monomer]:[CTA]:[Ir(ppy)3] = 200:1:2 × 10−4 (leading to a catalyst concentration of 1 ppm with respect to the monomer) and a molar concentration of 10 M of the monomer with respect to the solution. The reaction mixture was covered with aluminum foil and degassed with N2 in a cold water bath for 30 min. After purging, the reaction vessel was sealed and irradiated with blue LED light (LED strip, 4.8 W, λmax = 435 nm) at room temperature. Aliquots were withdrawn using nitrogen-purged syringes at predetermined time points and subsequently analyzed via 1H NMR (CDCl3) and GPC to measure the conversion, number-average molecular weight (Mn) and polydispersity (PDI), respectively. The polymerization of DMA mediated by Ru(bpy)3Cl2 was carried out under identical conditions using Ru(bpy)3Cl2 instead of Ir(ppy)3 (additional details are reported in the Supporting Information). General Procedure for the Kinetic Studies of PET-RAFT Polymerization of Methyl Methacrylate (MMA) by Online Fourier Transform Near-Infrared (FTNIR) Spectroscopy in the Absence and Presence of Oxygen. A reaction stock solution consisting of DMSO (1 mL), MMA (0.86 g, 8.6 mmol), CPADB (9.6 mg, 0.034 mmol), and Ru(bpy)3Cl2 (0.129 mg, 1.72 × 10−4 mmol) was prepared in a 6 mL glass vial. 0.7 mL of this stock solution was transferred into a 0.9 mL FTNIR quartz cuvette (1 cm × 2 mm). To examine the polymerization in the absence of oxygen, the cuvette was sealed with a rubber septum and covered with aluminum foil while degassing for 20 min with N2. The quartz cuvette was then irradiated under blue LED light (4.8 W, λmax = 435 nm) at room temperature. The cuvette was transferred to the sample holder manually for FTNIR measurement every 30 min. After 15 s scanning, the cuvette was moved back into light. The monomer conversions were calculated by the ratio of the integral of the wavenumber area 6220−6120 cm−1 at different time points to that at 0 min. Aliquots of the final reaction

mixtures were analyzed by 1H NMR (CDCl3) and GPC (RI and UV detectors) to measure the conversions, number-average molecular weights (Mn, GPC), and polydispersities (Mn/Mw). The remainder was purified via precipitation in methanol/petroleum ether (1/1, v/v). The final pink powder was collected and submitted for UV−vis spectroscopy and 1H NMR measurements to confirm chain endgroup fidelities and calculate absolute molecular weights, Mn,NMR

M n,NMR = (I 3.6 ppm/3)/(I 7.8 ppm/2) × MW MMA + MW CPADB where I3.5 ppm and I7.8 ppm correspond to integrals of peak signal at δ 3.5 ppm and δ 7.8 ppm attributed to OCH3 of MMA and phenyl group of CPADB, respectively. To examine the polymerization in the presence of oxygen, another 0.7 mL aliquot of the stock solution was polymerized and analyzed using an identical procedure except for the elimination of the degassing step. General Procedure for the Kinetic Studies of PET-RAFT Polymerization of Methyl Acrylate (MA) by Online Fourier Transform Near-Infrared (FTNIR) Spectroscopy in the Absence and Presence of Oxygen. The polymerization kinetics of methyl acrylate (MA) and N,N-dimethylacrylamide (DMA) by online Fourier transfer near-infrared spectroscopy were performed using the similar procedure as that of MMA at different concentrations of catalyst (additional details are reported in the Supporting Information). The molecular weight (Mn,NMR) measured by NMR was calculated by following equations: for PMA:

M n,NMR = (I 3.6 ppm/3)/(I 0.9 ppm /3) × MW MA + MW BTPA where I3.6 ppm and I0.9 ppm correspond to integrals of peak signal at δ 3.6 ppm and δ 0.9 ppm attributed to OCH3 of MA and end CH3 group of BTPA, respectively. for PDMA:

M n,NMR = (I1.0 − 2.0 ppm/2)/(I 0.9 ppm/3) × MW DMA + MW BTPA where I1.0−2.0 ppm and I0.9 ppm correspond to integrals of peak signal at δ 1.0−2.0 ppm and δ 0.9 ppm attributed to CH2 of DMA and end CH3 group of BTPA, respectively. Kinetic Studies for the Preparation of Diblock Copolymer by PET-RAFT Polymerization of MA Using PDMA as Macroinitiator by Online Fourier Transform Near-Infrared (FTNIR) Spectroscopy in the Presence of Oxygen. PDMA macroinitiator was prepared by PET-RAFT polymerization of DMA using 10 ppm catalyst Ru(bpy)3Cl2 in the presence of oxygen under blue light irradiation, as described above. (Mn = 9670 g/mol, Mw/Mn = 1.08). A reaction stock solution consisting of DMSO (0.8 mL), MA (0.173 g, 2.02 mmol), PDMA (0.078 g, Mn = 9670 g/mol, 8.07 × 10−3 mmol), and Ru(bpy)3Cl2 (0.015 mg, 2.02 × 10−5 mmol, 10 ppm relative to monomer) was prepared in a 4 mL glass vial. A 0.7 mL aliquot of this stock solution was transferred into a 0.9 mL FTNIR quartz cuvette (1 cm × 2 mm). The cuvette was sealed with rubber septum and covered with aluminum foil and then directly irradiated under blue LED light (4.8 W, λmax = 435 nm) at room temperature. The cuvette was transferred to the sample holder manually for FTNIR measurement every 10 or 15 min. After 15 s scanning, the cuvette was moved back into light. The monomer conversions were calculated by the ratios of the integral of the wavenumber area 6220−6120 cm−1 at different time points to that at 0 min. Aliquots of the final reaction mixtures were analyzed by 1H NMR (CDCl3) and GPC (RI and UV detectors) to measure the conversions, number-average molecular weights (Mn, GPC), and polydispersities (Mn/Mw). The remainder was purified through precipitation in diethyl ether. The final light yellow powder was collected and submitted for 1H NMR measurement to calculate absolute molecular weight, Mn,NMR. M n,NMR = (I 3.6 ppm/3)/(I 0.9 ppm/3) × MW DMA + MW PDMA where I3.6 ppm and I0.9 ppm correspond to integrals of the peak signal at δ 3.6 ppm and δ 0.9 ppm attributed to OCH2 of MA and end CH3 C


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Table 1. Comparison of the Polymerizations Mediated by Ru(bpy)3Cl2 and Ir(ppy)3 as Photoredox Catalysta monomer

initiating system

catalyst conc (ppm)

time (h)

convb (%)

Mn,theoc (g/mol)

Mn,GPCd (g/mol)

Mw/Mn

MMA MMA MMA MMA MA MA styrene styrene DMA DMA

Ru(bpy)3Cl2/CPADB Ir(ppy)3/ CPADB Ru(bpy)3Cl2/CPADB Ir(ppy)3/ CPADB Ru(bpy)3Cl2/BTPA Ir(ppy)3/ BTPA Ru(bpy)3Cl2/BTPA Ir(ppy)3/ BTPA Ru(bpy)3Cl2/BTPA Ir(ppy)3/ BTPA

5 5 1 1 1 1 20 20 1 1

36 36 36 36 3 3 24 24 3 3

73 85 45 65 83 99 18 38 76 87

14900 17300 9100 13200 14570 17800 3900 7600 19400 22200

13000 17000 8500 12500 16200 17400 3400 7200 16600 19800

1.14 1.12 1.14 1.09 1.11 1.05 1.13 1.13 1.08 1.07

a The reactions were performed in DMSO at room temperature under 4.8 W blue LED light (λmax = 435 nm) using [monomer]:[thiocarbonylthio] = 200:1 in the absence of air. bMonomer conversion determined by 1H NMR spectroscopy. cTheoretical molecular weight calculated using the equation Mn,th = [M]0/[thiocar]0 × MWM × α(NMR) + MWthiocar, where [M]0, [thiocar]0, MWM, α(NMR), and MWthiocar correspond to monomer and thiocarbonylthio compound concentration, molar mass of monomer, conversion measured by NMR, and molar mass of thiocarbonylthio compound respectively. dMolecular weight and polydispersity determined by GPC.

group of BTPA, respectively, and MWPDMA corresponds to the molecular weight of PDMA macroinitiator (Mn = 9670 g/mol). Synthesis of Multiblock Homopolymer by Successive Chain Extensions in the Presence of Air. Synthesis of Undecablock Poly(methyl acrylate) Homopolymers via 10 Successive Chain Extensions. A 21 mL glass vial equipped with a rubber septum was charged with MA (0.86 g, 10 mmol), BTPA (34 mg, 0.143 mmol), Ru(bpy)3Cl2 (0.154 mg, 2 × 10−4 mmol, dissolved in DMSO), and DMSO (930 μL, total solvent = 1000 μL) at a molar ratio of [monomer]:[CTA]:[Ru(bpy)3Cl2] = 70:1:14 × 10−4 (leading to a catalyst concentration of 20 ppm with respect to the monomer) and a molar concentration of 10 M of the monomer with respect to the solvent. The reaction was sealed with a rubber septa in the presence of air and irradiated with blue LED light (LED strip, 4.8 W, λmax = 435 nm) at room temperature. Aliquots were withdrawn using nitrogenpurged syringes at predetermined time points and subsequently analyzed via 1H NMR (CDCl3) and GPC to measure the conversion, number-average molecular weight (Mn), and polydispersity (PDI). When the conversion reached above 95% conversion (4 h later), a second aliquot of non-degassed MA (0.86 g, 10 mmol) and DMSO (1 mL) was added to the reaction vessel. The monomer conversion was determined by NMR, and the reaction was again stopped at 95% and a third aliquot was added. The process was repeated until to obtain an undecablock copolymer. At the sixth chain extension, half the reaction mixture was withdrawn using a syringe, and MA (0.43 g) and DMSO (2 mL) were added to the reaction mixture. The molecular weight distribution and the monomer conversion were monitored at each chain extension using GPC and NMR. Synthesis of Hexablock Poly(N,N-dimethylacrylamide) Homopolymers via Five Successive Chain Extensions. A 21 mL glass vial equipped with a rubber septum was charged with DMA (0.99 g, 10 mmol), BTPA (39.5 mg, 0.166 mmol), Ru(bpy)3Cl2 (0.077 mg, 1 × 10−4 mmol, dissolved in DMSO), and DMSO (930 μL, total solvent = 1000 μL) at a molar ratio of [monomer]:[CTA]:[Ru(bpy)3Cl2] = 60:1:6 × 10−4 (leading to a catalyst concentration of 10 ppm with respect to the monomer) and a molar concentration of 10 M of the monomer with respect to the solvent. The reaction was sealed with a rubber septa in the presence of air and irradiated with blue LED light (LED strip, 4.8 W, λmax = 435 nm) at room temperature. Aliquots were withdrawn using nitrogen-purged syringes at predetermined time points and subsequently analyzed via 1H NMR (CDCl3) and GPC to measure the conversion, number-average molecular weight (Mn), and polydispersity (PDI). When the conversion reached above 95% conversion (3 h later), a second aliquot of non-degassed DMA (0.99 g, 10 mmol) and DMSO (1 mL) was added to the reaction vessel. The reaction was again placed under blue LED light and stopped at 95%, and a third aliquot was added. The process was repeated until the production of hexablock copolymer. At the sixth chain extension, half of the reaction solution was withdrawn, and DMA (3 g) and DMSO (6 mL) were added to the reaction mixture. The polymerization was

restarted and terminated at high conversion (85%). The molecular weight distribution and the monomer conversion were monitored at each chain extension using GPC and NMR.

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RESULTS AND DISCUSSION Photoinduced electron transfer−reversible addition−fragmentation chain transfer (PET-RAFT) polymerization50 was performed in the presence of Ru(bpy)3Cl2 catalyst with different monomers, including methyl methacrylate (MMA), methyl acrylate (MA), styrene (St), and N,N-dimethylacrylamide (DMA), and with two thiocarbonylthio compounds (4cyanopentanoic acid dithiobenzoate (CPADB) and 2-(nbutyltrithiocarbonate)propionic acid (BTPA)) as initiator and chain transfer agent (Scheme 1A). Scheme 1B describes the proposed mechanism for PET-RAFT with Ru(bpy)3Cl2. The photoredox catalyst (Ru(bpy)3Cl2, Ru(II)) generated an excited species (Ru(II)*) under visible light irradiation, which was then able to reduce thiocarbonylthio compounds via photoinduced electron transfer (PET).51,72 In previous works, Matyjaszewski and co-workers have also demonstrated that thiocarbonylthio compounds could be reduced in the presence of copper(I).73−75 This results in the generation of radicals (Pn•) and Ru(III) species (Scheme 1) via an oxidative quenching mechanism. The generated radical (P•) is able to initiate polymerization of monomers and participate in the reversible addition−fragmentation chain transfer (RAFT) process,9,10,76 or it can be deactivated by Ru(III) which results in regeneration of the initial Ru(II). The regeneration of the initial Ru(II) species then restarts the catalytic cycle. Comparison of Ir(ppy)3 and Ru(bpy)3Cl2 for PET-RAFT Polymerization in the Absence of Oxygen. In our first attempts, we compared the catalytic performance of Ru(bpy)3Cl2 to activate PET-RAFT polymerization for MMA, MA, St, and DMA (Table 1) with our previous photoredox system (Ir(ppy)3) in DMSO. MMA was polymerized in the presence of 4-cyanopentanoic acid dithiobenzoate (CPADB) as thiocarbonylthio compound, and 2-(n-butyltrithiocarbonate)propionic acid (BTPA) was employed for the polymerization of MA, St, and DMA (Scheme 1). For these initial experiments, the solutions were purged with nitrogen for 30 min and then placed under blue LED light (λ = 435 nm and 4.8 W). Welldefined polymers with a good accord between theoretical and experimental molecular weights (with PDIs < 1.20) were synthesized in the presence of Ru(bpy)3Cl2 as catalyst, which D


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Figure 1. PET-RAFT polymerization of DMA mediated by Ir(ppy)3 (■) and Ru(bpy)3Cl2 (●) in DMSO using BTPA as thiocarbonylthio compound: (A) ln([M]0/[M]t) versus time; (B) molecular weights and Mw/Mn versus conversion; (C) GPC traces of PDMA synthesized using Ir(ppy)3; (D) GPC traces of PDMA synthesized using Ru(bpy)3Cl2. Note: the reactions were performed in DMSO at room temperature under 4.8 W blue LED light (λmax = 435 nm) using [monomer]:[thiocarbonylthio]:[catalyst] = 200:1:2 × 10−4 in the absence of air.

(1100 ns).78 A longer lifetime of the excited species is beneficial as the catalyst can engage in bimolecular electron transfer reactions in competition with deactivation pathways.79 For both catalysts, the evolutions of the experimental molecular weights (determined by GPC) versus monomer conversion increase gradually with monomer conversion and are in good agreement with the theoretical ones (Mn,th = [M]0/[thiocar]0 × MWM × α(NMR) + MWthiocar, where [M]0, [thiocar]0, MWM, α(NMR), and MWthiocar correspond to monomer and thiocarbonylthio compound concentration, molar mass of monomer, conversion measured by NMR, and molar mass of thiocarbonylthio compound, respectively). In addition, comparable Mw/Mn values were obtained for both catalysts. Figure 1C,D shows the GPC traces of PDMA synthesized using Ir(ppy)3 and Ru(bpy)3Cl2 at different monomer conversions. The shift of the polymer peaks to lower retention time confirms the living character of this polymerization technique. PET-RAFT Polymerization of Methyl Methacrylate (MMA) Using Ru(bpy)3Cl2 as Catalyst in the Absence of Oxygen. Similar to the polymerization regulated by Ir(ppy)3, the polymerization of MMA regulated by Ru(bpy)3Cl2 can be activated and deactivated by the light. The polymerization was exposed to an alternating sequence of light “on” and “off” environment using a molar ratio of [MMA]: [CPADB]:[Ru(bpy)3Cl2] = 200:1:40 × 10−4. No polymerization was observed in the absence of light (“off”), while the polymerization could be switched “on” by light (Figure 2A). Ln([M]0/[M]t) increased linearly versus exposure time (Figure 2B), which demonstrates a constant propagating radical concentration during the polymerization. The molecular weights determined by GPC gradually increased with exposure time and were in good agreement with the theoretical values

demonstrates that Ru(bpy)3Cl2 can effectively regulate PETRAFT polymerization. In comparison with Ir(ppy)3, Ru(bpy)3Cl2 gave lower monomer conversions under identical reaction conditions (Table 1), suggesting that Ir(ppy)3 is a more efficient catalyst than Ru(bpy)3Cl2. Control experiments were performed without light and without addition of catalyst under light. In both experiments, no polymerization was observed after 24 h for MMA, MA, and styrene (data not shown). These results support our proposed mechanism described in Scheme 1B. Subsequently, we decided to monitor the monomer conversion for the polymerization of DMA in DMSO catalyzed by Ir(ppy)3 and Ru(bpy)3Cl2 using a molar ratio [DMA]: [BTPA]:[catalyst] of 200:1:2 × 10−4. The plot of ln([M]0/ [M]t) versus exposure time gave a linear relationship for both catalysts, indicating that the concentration of radicals is constant, in agreement with a living radical polymerization (Figure 1). However, the polymerization employing Ir(ppy)3 as photoredox catalyst presented a faster polymerization rate than Ru(bpy)3Cl2. As the experiments have been performed using identical conditions, i.e., concentration and light source, we can compare the effect of the catalyst on the apparent propagation constant (kpapp). This difference in kpapp can be attributed to a difference of radical concentrations in the solution. As shown in Scheme 1, the concentration of radical in solution is affected by the efficiency of the catalyst. The catalyst efficiency can be correlated to the redox potential of the catalyst and/or to the excited lifetime. In the first case, Ir(IV)/Ir(III)* presents a greater redox potential than Ru(III)/Ru(II)*, with a redox potential of −1.73 and −0.81 V versus saturated calomel electrode (SCE) for Ir(IV)/Ir(III)* and Ru(III)/Ru(II)*, respectively.51 In addition, Ir* (1900 ns)77 presents a longer excited lifetime than Ru* E


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Figure 2. PET-RAFT polymerization of methyl methacrylate (MMA) at room temperature in the absence (left column) and presence (right column) of oxygen, using a molar ratio of [MMA]:[CPADB]:[Ru(bpy)3Cl2] = 200:1:40 × 10−4 in DMSO: (A, E) conversion vs time; (B, F) ln([M]0/[M]t) vs time of exposure; (C, G) molecular weights and Mw/Mn vs conversion; (D, H) molecular weight distributions of PMMA at different time points.

(Figure 2C). Mw/Mn values decreased during the polymerization as expected for a living radical polymerization. Molecular weight distributions assessed by GPC were monomodal and shifted from low to high molecular weights (Figure 2D). PET-RAFT Polymerization of MMA Using Ru(bpy)3Cl2 as Catalyst under Air. After demonstrating that the polymerization can be performed in the absence of oxygen, we decided to investigate the oxygen tolerance property of Ru(bpy)3Cl2. This catalyst has previously been successfully employed as a photoredox catalyst in organic synthesis to perform various organic transformations under air.80−85 Based

on the proposed mechanism of the PET-RAFT technique, it supposed to be as highly tolerant to oxygen as Ir(ppy)3.50 Our attempt was first carried out using MMA as monomer and a molar ratio of [MMA]:[CPADB]:[Ru(bpy)3Cl2] = 200:1:40 × 10−4. After 24 h, PMMA with a molecular weight of 12 000 g/ mol and Mw/Mn of 1.16 was detected by GPC, demonstrating successful polymerization under air. A polymerization kinetics using an alternate “on” and “off” was then performed to confirm that the polymerization can be regulated by the light (Figure 2E). When the light was “off”, no conversion was detected by NMR. While the reaction was exposed to light, the polymerization proceeded as expected. The linear increase of F


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molecular weight and slow decrease of Mw/Mn with monomer conversion demonstrated a living polymerization to high conversion (>90%) even with multiple “on”/“off” cycles (Figure 2G). In addition, a linear relationship for ln([M]0/ [M]t) versus time of light exposure indicated a constant radical concentration in the system (Figure 2F), although an inhibition period was observed for 4 h. Figure 2H shows monomodal molecular weight distributions at different time points. Interestingly, the slope of ln([M]0/[M]t), which is proportional to kpapp, is lower than that of the polymerization performed in the absence of oxygen (Figure 2B,F). In order to investigate the detailed effect of oxygen in PET-RAFT polymerization mediated by Ru(bpy)3Cl2, as well as to keep the amount of catalyst and the volume of free space unchanged during measurement, a convenient and reliable method, online Fourier transform near-Infrared (FTNIR) spectroscopy, was employed to study the polymerization kinetics in the presence or absence of oxygen. This technique avoids frequent sampling, which is more likely to cause some experimental errors and was successfully implemented by Haddleton and co-workers. In addition, it will provide important information in the polymerization kinetics, such as the determination of induction period. The polymerizations were carried out in a sealed quartz cell with a total volume of 0.9 mL, containing 0.7 mL of reaction solution and 0.2 mL of free space. For the polymerizations performed in the absence of oxygen (degassed system), the reaction mixture was sealed with rubber septum and degassed for 30 min with nitrogen bubbling, while the reaction mixture was sealed without degassing for the polymerization in the presence of air. The kinetics of PET-RAFT polymerization of MMA in the absence and presence of oxygen was carried out using a molar ratio of [MMA]:[PDMA]:[Ru(bpy)3Cl2] = 250:1:50 × 10−4 in DMSO. Monomer conversions were measured by online FTNIR using the vinylic C−H stretching of the monomer at 6100−6200 cm−1 (Supporting Information, Figure S1A) every 30 min. Plotting of ln([M]0/[M]t) with respect to exposure time (Supporting Information, Figure S1B) gave a linear relationship with 100 min induction period for MMA polymerization in the absence of oxygen (under nitrogen). This induction period was attributed to the trace amount of oxygen in the vessel or the slow fragmentation of the dithiobenzoate (CPADB) typically observed in RAFT polymerization when using dithiobenzoates.86,87 As expected, the polymerization in the presence of oxygen showed a longer induction period (∼200 min). However, after this induction period, the polymerization proceeded as expected. The induction period is attributed to the consumption of oxygen by the photoredox catalyst. The apparent propagation rate in the presence of oxygen appeared slower than the polymerizations performed under nitrogen, which suggested that the concentration of propagating radicals in the presence of oxygen was lower than that in the absence of oxygen. Initially, the slow polymerization rate was hypothesized to be caused by two most likely reasons: partial degradation of the photoredox catalyst during the induction period and/or sluggish consumption of oxygen during the polymerization. To confirm the former hypothesis, two experiments were carried out. First, we monitored the absorbance of the catalyst at 460 nm (the maximum wavelength absorption of Ru(bpy)3Cl2) before and after polymerization using a UV−vis spectrometer (Supporting Information, Figure S2). We did not observe any change in the absorbance, which suggested that the catalyst was unlikely

degraded during the polymerization. In the second experiment, the catalyst in DMSO was preirradiated for 16 h under blue LED light in an unsealed reaction vessel without MMA and CPADB in the presence of air. Subsequently, the catalyst was employed for the polymerization of MMA in the absence of air (degassed by nitrogen). A parallel control experiment was performed using a fresh catalyst under identical condition. The monomer conversions of both polymerizations were monitored by online FTNIR (Figure 3). From the kinetic plots of these

Figure 3. Degradation test of Ru(bpy)3Cl2 for PET-RAFT polymerization of methyl methacrylate (MMA) by online Fourier transform near-Infrared (FTNIR) and GPC measurements. (A) ln([M]0/[M]t) vs time of exposure; (B) comparison of molecular weight distributions recorded by dual RI and UV (λ = 305 nm) detectors for unpurified PMMA polymers at different reaction times.

two polymerizations (Figure 3A), we did not observe obvious difference, suggesting the absence of significant degradation under air. Therefore, the partial degradation of catalyst could be excluded for the explanation of retardation of PET-RAFT polymerization under air. We hypothesized that it could be attributed to sluggish consumption of oxygen, which caused the decrease of the radical concentration during the polymerization. Nevertheless, narrow molecular weight distributions (1.09 and 1.10) and comparable GPC traces recorded using RI and UV (λ = 305 nm) detectors (Figure 3B) of the final polymers prepared in the absence and presence of oxygen were observed. Both molecular weights assessed by 1H NMR (Mn,NMR) (Figure 4 and Supporting Information, Figure S3) and by GPC (Mn,GPC) were in good accord with the theoretical ones (Mn,th) G


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of dithioester with a peak absorption at 305 nm (Supporting Information, Figure S5). The end-group fidelity (f UV (%)) was close to 100%, using the following equation f UV (%) = (Abs305 nm /ε)/[polymer], where Abs305 nm, ε, and [polymer] correspond to absorbance at 305 nm, extension coefficient of dithioester, and polymer concentration.88 PET-RAFT Polymerization of Methyl Acrylate (MA) and N,N′-Dimethylacrylamide (DMA) Using Ru(bpy)3Cl2 as Catalyst under Air. Subsequently, we decided to investigate the polymerization of acrylates. First, methyl acrylate (MA) was chosen as a model monomer. The polymerization was carried out in the presence of BTPA as chain transfer agent and initiator in DMSO. The oxygen tolerance study was investigated at two different catalyst concentrations, 5 and 10 ppm (relative to monomer), as shown in Figure 5. The vinylic C−H stretching of MA appeared in the similar region of MMA in the FTNIR spectrum (Figure 5A). The monomer conversion was calculated by the ratios of the integral of peak area at the wavenumber of 6220−6120 cm−1. A short induction was observed at 5 and 10 min and 40 and 140 min for 10 and 5 ppm catalyst concentrations in the absence and presence of oxygen, respectively. The linear kinetic plots (Figure 5B) indicated constant radical concentration during the polymerizations. It is interesting to note that the apparent propagating rates of polymerizations with different molar ratios of [catalyst]:[MA] of 5 and 10 ppm were comparable. However, the comparison at the same catalyst concentrations (5 or 10 ppm) in the absence or presence of oxygen showed a lower propagating rate in the presence of oxygen. This result was attributed by the slow consumption of oxygen in the system by the catalyst, which results by a retardation in the polymerization kinetic. Nevertheless, excellent control of molecular weight distributions was observed for the polymerizations performed in the presence or absence of oxygen, with narrow PDIs (Figure 5C,D). Moreover, the 1H NMR spectra of purified polymers prepared at 5 ppm catalyst concentration showed the characteristic signals of BTPA. Using the characteristic signal of CH in adjacent position of trithiocarbonate at 4.8−4.9 ppm and methyl group of R group at 1.2 ppm, the end-group fidelity (f (%)) was calculated to be greater

Figure 4. 1H NMR spectrum of purified PMMA polymer synthesized by PET-RAFT polymerization for 780 min in the presence of oxygen using CPADB as chain transfer agent, 20 ppm Ru(bpy)3Cl2 as catalyst and 4.8 W blue LED lamp as light source. Mn,GPC = 11090 g/mol, Mn,th = 10180 g/mol, Mn,NMR = 9780 g/mol, Mw/Mn = 1.10, conversion (NMR) = 39.6%, conversion (FTNIR) = 35.9%; Mn,NMR = (I3.6 ppm/ 3)/(I7.8 ppm/2) × MWMMA + MWCPADB, where I3.5 ppm and I7.8 ppm correspond to integrals of peak signals at δ 3.5 ppm and δ 7.8 ppm attributed to OCH3 of MMA and phenyl group of CPADB, respectively. Table 2, no. 2. The end-group fidelity was 100%, according to the equation f NMR (%) = [(I7.8 ppm/2)/(I2.4−2.5 ppm/4)] × 100.

(Table 2). After purification by several precipitations in methanol, 1 H NMR analysis confirmed the presence dithiobenzoate group by the signal at 7.2, 7.4, and 7.8−7.9 ppm for both polymers. The end-group fidelity was calculated to be 100% using the signal of dithiobenzoate group and the signal at 2.5 ppm attributed to cyanovaleric group (Figure 4). The integrity of dithioester group was also calculated by measuring the absorbance intensity at 305 nm of UV−vis spectra of the polymerization mixture before and after polymerization (Supporting Information, Figure S4). No significant changes were observed, which confirmed that dithioester end group was not degraded. Additionally, the purified polymers prepared in the absence and presence of air were analyzed by UV−vis spectrometer to confirm the presence

Table 2. List of Polymers Synthesized by PET-RAFT Polymerization in This Study no. 1 2g 3 4g 5 6g 7g 8g 9g 10g,h 11g

exp conda [M]:[thiocar]:[Ru] −3

250:1:5 × 10 250:1:5 × 10−3 200:1:2 × 10−3 200:1:2 × 10−3 200:1:1 × 10−3 200:1:1 × 10−3 200:1:4 × 10−3 200:1:2 × 10−3 200:1:1 × 10−3 200:1:4 × 10−3 250:1:2.5 × 10−3

monomer

thiocar

[Ru]/[M] (ppm)

time (min)

αb (%, NMR)

αc (%, FTNIR)

Mn,thd (g/mol)

Mn,GPCe (g/mol)

Mn,NMRf (g/mol)

Mw/Mne

MMA MMA MA MA MA MA DMA DMA DMA DMA MA

CPADB CPADB BTPA BTPA BTPA BTPA BTPA BTPA BTPA BTPA PDMA

20 20 10 10 5 5 20 10 5 20 10

600 780 30 160 50 240 110 85 140 180 330

47.3 39.6 84.2 67.9 81.5 67.6 75.1 41.6 38.6 59.1 76.1

46.8 35.9 78.4 62.7 76.4 61.8 75.5 37.2 33.3 58.5 72.8

12110 10180 14720 11920 14260 11870 15130 8490 7890 11960 26030

13100 11090 16700 13030 15040 12620 15880 9670 9110 13180 28930

12050 9780 NDi NDi 14490 10300 ND 9020 7610 ND 26540

1.09 1.10 1.08 1.05 1.06 1.05 1.09 1.08 1.08 1.08 1.11

a

The reactions were performed in DMSO at room temperature under 4.8 W blue LED light (λmax = 435 nm) in the absence or presence of air. Monomer conversion determined by 1H NMR spectroscopy. cMonomer conversion determined by online FTNIR spectroscopy. dTheoretical molecular weight calculated using the equation Mn,th = [M]0/[thiocar]0 × MWM × α (NMR) + MWthiocar, where [M]0, [thiocar]0, MWM, α(NMR), and MWthiocar correspond to monomer and thiocarbonylthio compound concentration, molar mass of monomer, conversion by NMR, and molar mass of thiocarbonylthio compound, respectively. eMolecular weight and polydispersity determined by GPC analysis (DMAc used as eluent). f Molecular weight calculated by 1H NMR followed the equation provided in the Experimental Section and Figures S3, S6, S7, and S10−S13 of the Supporting Information. gPolymerization was performed in the presence of air. hPolymerization was performed in water. iNot determined. b

H


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Figure 5. Online FTNIR measurement for kinetic study of PET-RAFT polymerization of methyl acrylate (MA) with Ru(bpy)3Cl2 as photoredox catalyst, blue LED light as light source, and BTPA as chain transfer agent and initiator at room temperature in the absence and presence of oxygen, using a molar ratio of [MA]:[BTPA] = 200:1 in DMSO: (A) FTNIR full (bottom) and enlarged (top) spectra windows for the reaction solution at different time points in the absence of oxygen; (B) ln([M]0/[M]t) vs time of exposure in the absence (■, ●) and presence (□, ○) of oxygen; (C, D) comparison of molecular weight distributions recorded by dual RI (black curves) and UV (λ = 305 nm) (red or blue curves) detectors for final unpurified PMA polymers in the absence (top) and presence (bottom) of air. For conditions and results refer to Table 2, no. 3−6.

Figure 6. Online FTNIR measurement for kinetic study of PET-RAFT polymerization of N,N′-dimethylacrylamide (DMA) with Ru(bpy)3Cl2 as photoredox catalyst, blue LED light as light source, and BTPA as chain transfer agent at room temperature in the presence of oxygen, using a molar ratio of [DMA]:[BTPA] = 200:1 in DMSO (A, B) and water (C, D): (A, C) ln([M]0/[M]t) vs time of exposure for DMA polymerization at different concentrations of photocatalyst; (B, D) comparison of molecular weight distributions recorded by dual RI and UV (λ = 305 nm) detectors for final unpurified PDMA polymers. For conditions and results refer to Table 2, no. 7−10.

than 90% by NMR using the following equation: f NMR (%) = (I4.8−4.9 ppm/I1.2 ppm) × 100 (Supporting Information, Figures S6 and S7). The end-group fidelity measured by UV−vis ( f UV =

95%) was in good accord with NMR results for both polymers (Supporting Information, Figures S8 and S9). Finally, other acrylic monomers, including tert-butyl acrylate, n-butyl acrylate, I


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Figure 7. Kinetic study of PET-RAFT polymerization of methyl acrylate (MA) with poly(N,N′-dimethylacrylamide) (PDMA, Mn = 9670 g/mol, Mw/Mn = 1.08) as macroinitiator at room temperature in the presence of oxygen, using a molar ratio of [MA]:[PDMA]:[Ru(bpy)3Cl2] = 250:1:2.5 × 10−3 in DMSO: (A) ln([M]0/[M]t) vs time of exposure; (B) molecular weights and Mw/Mn versus conversion; (C) molecular weight distributions of PDMA and PDMA-b-PMA at different time points; (D) comparison of molecular weight distributions recorded by RI detectors for macroinitiator PDMA (black curve) and dual RI and UV (λ = 305 nm) detectors for final diblock copolymer PDMA-b-PMA (blue curve for RI detector and red curve for UV detector). For conditions and results refer to Table 2, no. 11.

ethyl acrylate, hydroxyl ethyl acrylate, etc., were successfully polymerized by PET-RAFT with an excellent control of molecular weight and molecular weight distribution under air (Supporting Information, Table S1). High end-group fidelity was confirmed using NMR and UV−vis spectroscopy. Finally, PET-RAFT polymerization of acrylamide, using N,N′-dimethylacrylamide (DMA) as a model monomer, was also investigated in DMSO and water using online FTNIR in the presence of oxygen (Figure 6). The reactions were performed using the same setup employed for MMA and MA. In DMSO, the amount of Ru(bpy)3Cl2 varied from 5 to 20 ppm. As expected by our proposed mechanism, longer induction periods were observed for lower catalyst concentrations in the presence of oxygen in DMSO (Figure 6A). Interestingly, the apparent propagating rates were not significantly affected by the catalyst concentrations, in good agreement with our previous observations for MA. Figure 6B shows the GPC traces recorded by RI and UV using different catalyst concentrations. The excellent agreement between RI and UV traces confirm the full retention of the trithiocarbonate. 1 H NMR spectra for purified PDMA (Supporting Information, Figures S10 and S11) also confirms the presence of the trithiocarbonate group with the signal at 5.2 ppm. Both UV and NMR analyses show that the end group was preserved (∼100%) during the polymerization in the presence and absence of air (Supporting Information, Figure S12). As DMA is a water-soluble monomer and Ru(bpy)3Cl2 presents a good solubility in different solvents, we decided to investigate the tolerance of PET-RAFT in water using a ratio of catalyst to monomer of 20 ppm. At the end of a long induction period of 100 min, the polymerizations proceeded as expected (Figure 6C,D). In water, high catalyst concentration was essential to perform polymerization within reasonable time as the induction periods when using catalyst concentrations of 10

and 5 ppm relative to monomer were greater than 10 h (data not shown). Preparation of Diblock Copolymers by PET-RAFT Polymerization Using Ru(bpy)3Cl2 as Catalyst under Air. To confirm the livingness of PDMA homopolymer synthesized by the PET-RAFT technique in the presence of oxygen in DMSO, we decided to prepare a diblock PDMA-bPMA copolymer (Figure 7) using 10 ppm catalyst concentration relative to monomer, again in the presence of air. We performed a polymerization kinetic study using the online FTNIR system. After an induction period of ∼70 min, the polymerization proceeded as expected (Figure 7A). GPC analysis at different time points revealed that the molecular weights increases with monomer conversion with a good control of Mw/Mn (Figure 7B and C). Narrow molecular weight distributions were recorded using RI and UV detector on GPC (Figure 7D), which show a perfect chain extension. 1H NMR analysis after purification confirmed the synthesis of PDMA-b-PMA copolymers by the presence of characteristic signals of PMA and PDMA segments. Finally, the presence of the end group was also confirmed by the signal at 4.8−4.9 ppm attributed to CH in adjacent position of trithiocarbonate (Supporting Information, Figure S13). Encouraged by these results, we decided to exploit the temporal influence over the polymerization process for block copolymer synthesis. PMA (Mn,GPC = 12620 g/mol, Table 2, no. 6) and PDMA (Mn,GPC = 9110 g/mol, Table 2, no. 9) were prepared under air using [monomer]:[BTPA]:[Ru(bpy)3Cl2] = 200:1:1 × 10−3. The polymerization mixtures were stored at room temperature in the dark without further treatment. After one month, the polymers were reanalyzed by GPC. Interestingly, the molecular weights and Mw/Mns did not change during the storage, which confirms the excellent temporal control and suggests a great industrial attraction for long-term storage of custom designed products. Subsequently, J


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Figure 8. Multiblock homopolymers PMA (A−C) and PDMA (D−F) obtained by PET-RAFT polymerization at room temperature in the presence of oxygen: (A, D) molecular weight distributions for successive block chain extensions of the multiblock homopolymers; (B, E) evolution of Mns and Mw/Mns with the number of chain extensions; (C, F) evolution of end-group fidelity vs number of chain extensions.

to show a clear shift of the molecular weight distribution. The polymerization reaction was performed using a blue LED light (4.8 W) at room temperature for 6 h for the first block, while the polymerization time was reduced to 4 h for the subsequent blocks. The monomer conversion was monitored by NMR to be greater than 95% for each chain extension. GPC analysis of the molecular weight distributions (Figure 8A,D) confirmed successful chain extensions as evidenced by clear shifts to higher molecular weights in each step. No obvious tailing or coupling observed for both polymers were observed. More importantly, we observed a good agreement (Figure 8B,E) between the theoretical molecular weights (Mn,theo) and number-average molecular weights measured by GPC (Mn,GPC) suggested the great polymer livingness during all chain extensions. Remarkably, the polydispersities were remaining less than 1.2 even after 10 chain extensions. Semiquantitative analysis using GPC traces was also employed to estimate end-group fidelity after each chain extension using a previous published method (Supporting Information, Figures S16 and S17).89,90,97,102,103 Figure 8C,F showsthe plots of endgroup fidelity versus number of chain extensions. It is worth to note the end-group fidelity for both copolymers was higher than 86% after multiple chain extensions.

chain extensions of these polymers were investigated using MA and DMA as monomers to afford PMA-b-PMA and PDMA-bPDMA diblock copolymers, respectively. After addition of monomers, the solution mixtures were sealed and irradiated using a blue LED light (4.8 W) for 4 h without degassing and addition of catalyst. GPC confirmed complete shift of the molecular weights distributions to higher ones (Supporting Information, Figures S14 and S15). Such control will not be achievable using conventional radical initiator system, such as azo-initiator (e.g., azobis(isobutyronitrile)). As we know, thermal initiator will result in a slow polymerization during the storage of the samples at room temperature due to its slow decomposition. Preparation of Multiblock Copolymers by PET-RAFT Polymerization Using Ru(bpy)3Cl2 as Catalyst under Air. Having demonstrated the versatility of preparation of diblock copolymers in the presence of oxygen, we decided to investigate the synthesis of high-order multiblock homopolymers using PET-RAFT in the presence of oxygen (or without predeoxygenation). Recently, multiblock homopolymers have generated lot of interests due to the development of new macromolecular tools.89−101 To illustrate the potential of our PET-RAFT technique, successive chain extensions of PMA and PDMA, without deoxygenation, intermediate purification steps, and supplementary addition of catalyst, were investigated. To our best knowledge, it is the first report to prepare multiblock homopolymers with high chain end livingness and low polydispersities in the presence of oxygen using light as an external stimulus. Two model multiblock copolymers, (PMA)11 and (PDMA)6 obtained after ten and five successive chain extensions, respectively, were prepared using an initial 20 ppm for PMA and 10 ppm for PDMA of Ru(bpy)3Cl2 photcatalyst (relative to monomer). Each block was composed of 70 and 90 repeating monomer units for PMA (block 1 to 10 and 10 to 11, respectively) and 60 repeating units for PDMA for block 1 to 5. We added 300 units for the synthesis of the last block of PDMA

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CONCLUSION In conclusion, we have demonstrated that Ru(bpy)3Cl2 is an efficient catalyst for the activation of photoredox electron transfer−reversible addition−fragmentation chain (PETRAFT) using low wattage blue LED light (4.8 W) and low doses of catalyst (typically in the ppm range). In addition, the oxygen tolerance of PET-RAFT polymerization mediated by Ru(bpy)3Cl2 was demonstrated by online Fourier transform near-infrared (FTNIR) spectroscopy in organic and aqueous solvents. The photocatalyst can play two roles: activating the PET-RAFT process and reducing the oxygen into inactive species. Successful polymerizations were achieved using nonK


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deoxygenated reaction media. However, under this condition, the polymerization exhibited an induction period and a lower apparent propagation rate compared to degassed samples. More importantly, after this inhibition period, polymerization proceeded and afforded the synthesis of well-defined polymers with excellent retention of end-group fidelity (>95%). In addition, very high chain end functionalities (thiocarbonylthio groups) for the final polymers synthesized via the PET-RAFT technique in the presence of oxygen were confirmed by 1H NMR, UV−vis spectroscopy, and GPC. The high end-group fidelity was utilized for the synthesis of complex polymer by successive chain extensions.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, fluorescence spectrum, UV−vis spectra, NMR spectra, and GPC traces (Figures S1−S17, Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (C.B.). *E-mail [email protected] (J.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank UNSW for funding. C.B. acknowledges Australian Research Council (ARC) for his Future Fellowship. REFERENCES

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