Photoinduced Electron TransferâReversible AdditionâFragmentation

Article pubs.acs.org/Macromolecules

Photoinduced Electron Transfer−Reversible Addition− Fragmentation Chain Transfer (PET-RAFT) Polymerization of Vinyl Acetate and N‑Vinylpyrrolidinone: Kinetic and Oxygen Tolerance Study Sivaprakash Shanmugam,† Jiangtao Xu,*,†,‡ and Cyrille Boyer*,†,‡ †

Centre for Advanced Macromolecular Design (CAMD), School of Chemical Engineering, and ‡Australian Centre for NanoMedicine, School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia S Supporting Information *

ABSTRACT: Photoinduced electron transfer−reversible addition−fragmentation chain transfer (PET-RAFT) polymerization was employed for the polymerization of unconjugated monomers, including vinyl acetate, vinyl pivalate, N-vinylpyrrolidinone, dimethyl vinylphosphonate, vinyl benzoate, and N-vinylcarbazole, in the presence of low concentration (ppm range) of photoredox catalyst, fac-[Ir(ppy)3], under low energy visible light irradiation. Kinetic studies of vinyl acetate indicated excellent control of molecular weights and molecular weight distributions (Mw/Mn = 1.09−1.2), even with high monomer conversion (>75%), in different catalyst concentrations. High molecular weights of poly(vinyl acetate) (Mn > 100 000 g/mol) and poly(N-vinylpyrrolidinone) (Mn > 40 000 g/mol) with low dispersities (Mw/Mn < 1.25) were obtained in bulk polymerizations. Moreover, the online kinetic study using Fourier transform near-infrared (FTNIR) showed comparable kinetic rates for the polymerizations in the absence and presence of relatively large amount of air, which demonstrates that the PET-RAFT technique possesses the ability of tolerance toward oxygen. Successful chain extensions of homopolymers of poly(vinyl acetate) and poly(N-vinylpyrrolidinone) to vinyl acetate and vinyl pivalate confirmed their integrities of end-group S−(SZ)−O.

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INTRODUCTION Vinyl monomers can be roughly divided into two categories: conjugated (or “more-activated”) monomers, such as styrene, methyl acrylate, and methyl methacrylate, and unconjugated (or “less-activated”) monomers, such as vinyl acetate (VAc), Nvinylpyrrolidinone, and N-vinylcarbazole.1−7 In comparison to styrene, the double bond of unconjugated monomer, e.g. vinyl acetate, is slightly electron rich and has little resonance interaction with the acetate group.6,7 Thus, its activity is close to ethylene or propylene than styrene or butadiene. Consequently, unconjugated monomers tend to be polymerized by free radical polymerization than anionic or cationic polymerization. Living radical polymerization of conjugated monomers has been extensively investigated in the past several decades, whereas that of unconjugated monomers is still a challenge due to poor control of molecular weight and dispersity arising from less stabilized propagating radical chains.4,8,9 Nevertheless, the living radical polymerization of unconjugated monomers is desirable due to its diversity of industrial applications.10−14 For instance, poly(vinyl acetate) (PVAc) has a wide range of applications in materials science, and PVAc is the precursor for poly(vinyl alcohol) which is a water-soluble biocompatible polymer.15,16 Polymerization of © XXXX American Chemical Society

PVAc has been reported by controlled/“living” radical polymerization (CLRP) techniques including nitroxide-mediated polymerization (NMP),17 atom transfer radical polymerization (ATRP),18−22 reversible addition−fragmentation chain transfer polymerization (RAFT)/macromolecular design via the interchange of xanthate (MADIX),3,4,14,23−26 organostibinemediated living radical polymerizations (SBRP),27 iodine transfer polymerization,9 and cobalt-mediated radical polymerization (CMRP).11,28−31 In our previous studies,32 inspired by the seminal work of Hawker and co-workers on controlled polymerization under light,33−36 we developed a novel photoinduced living polymerization technique, photoinduced electron transfer−reversible addition−fragmentation (PET-RAFT) polymerization, that is able to utilize visible light in the presence of ppm range concentration of fac-[Ir(ppy)3] or Ru(bpy)3Cl2 as a photoredox catalyst to polymerize a large range of conjugated monomers with narrow dispersities. For conjugated monomers, such as MMA or MA, PET-RAFT requires a low concentration of Received: April 22, 2014 Revised: July 2, 2014

A

dx.doi.org/10.1021/ma500842u | Macromolecules XXXX, XXX, XXX−XXX

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

radical polymerization of unconjugated monomers (VAc and NVP) under air, to our best knowledge, is the first reported. Scheme 1B shows the mechanism of PET-RAFT polymerization. The generation of an electron by fac-[Ir(ppy)3] through emission from the lowest triplet excited state to singlet ground state happens through spin−orbit coupling (SOC) which allows for metal-to-ligand charge transfer (MLCT) state in the presence of visible light. Essentially, MLCT state permits for an electron to be transferred from the heavy Ir(III) metal center to a ligand-centered π* orbital.48−55 In Scheme 1B, Ir(III) ( fac[Ir(ppy)3]) is irradiated under visible light to generate an excited Ir(III)* species through photoinduced electron transfer (PET)56 which is capable of reducing thiocarbonylthio compounds and produce a radical (P•). The generated radical (P•) has the capacity of initiating RAFT process or to react with Ir(IV) to regenerate the chain transfer agent (also dormant species) and Ir(III) that restarts the cycle.32 Finally, the polymerization can be performed at room temperature in the presence of ultralow concentration of photoredox catalyst using low energy blue LED as light source (4.8 W, λmax = 435 nm).

photoredox catalyst (typically ppm level) and commercially available low-energy visible light (blue LED with λ = 435 nm), typically between 1 and 4.8 W. In this article, we investigated in detail the PET-RAFT polymerization of unconjugated monomers, more specifically VAc and N-vinylpyrrolidinone (NVP), and other unconjugated monomers, including vinyl pivalate (VP), dimethylvinylphosphonate (DVP), vinyl benzoate (VBz), and vinylcarbazole (VCB) (Scheme 1A) in solution or bulk conditions. In the literature, there are only few photopolymerization systems which allow for the control of polymerization of unconjugated monomers, such as VAc and NVP, under visible or UV light.6,37−39 In a second part, we explored the strong reductive properties of photocatalyst fac[Ir(ppy)3] in activating polymerizations of VAc in the presence of oxygen to form well-defined polymers with narrow molecular weight distributions. Furthermore, we provide evidence of high end-group fidelity in the presence and absence of oxygen through the synthesis of diblock copolymers. Although conjugated monomers (styrene, methacrylate, or acrylate) were successfully polymerized in the presence of air by previous light-mediated techniques,40−47 controlled/living B


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General Procedures for PET-RAFT Polymerization of VAc in the Presence of Large Volume of Air. Polymerization of vinyl acetate (VAc) under air with 20 ppm (relative to monomer) concentration of catalyst was carried out in a 25 mL Cospak bottle sealed with a rubber septum. The reaction mixture consisted of DMSO (1.44 mL), VAc (1.24 g, 14.42 mmol), xanthate (15 mg, 0.072 mmol), and fac-[Ir(ppy)3] (0.192 mg, 2.93 × 10−4 mmol). The reaction mixture was then irradiated under blue LED light (4.8 W, λmax = 435 nm) at room temperature for 20 h. The reaction mixture was then analyzed by 1H NMR (CDCl3) and GPC (DMAc) to measure the conversions, number-average molecular weights (Mn), and dispersities (Mw/Mn). Online Fourier Transform Near-Infrared (FTNIR) Measurements. A reaction stock solution consisted of DMSO (0.5 mL), VAc (0.413 g, 4.81 mmol), xanthate (5 mg, 0.024 mmol), and fac[Ir(ppy)3] (0.064 mg, 9.77 × 10−5 mmol). The 0.5 mL 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 while degassing for 30 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 then transferred to the sample holder manually for FTNIR measurement every 20 min. After 15 s scanning, the cuvette was moved back into the light source. The monomer conversions were calculated by taking the ratio of integrations of the wavenumber area 6250−6150 cm−1 for all curves at different reaction times to that at 0 min. Another 0.5 mL stock solution was taken for kinetic study under identical conditions except degassing step was eliminated. General Procedures for the Preparation of PVAc and NVP by Bulk PET-RAFT Polymerization. PET-RAFT polymerization of vinyl acetate with 20 ppm (relative to monomer) concentration of catalyst was carried out in a 5 mL glass vial with a rubber septum. The reaction mixture consisted of VAc (1.24 g, 14.42 mmol), xanthate (3.0 mg, 0.014 mmol), and fac-[Ir(ppy)3] (0.189 mg, 2.88 × 10−4 mmol). The reaction vial was sealed with a rubber septum and covered with aluminum foil while degassing for 30 min with N2. The reaction vial was then irradiated under blue LED light (4.8 W, λmax = 435 nm) at room temperature. These final reaction mixtures were then analyzed by 1H NMR (CDCl3) and GPC (DMAc) to measure the conversions, number-average molecular weights (Mn), and dispersities (Mw/Mn). General Procedures for the Preparation of Diblock Copolymers by PET-RAFT Polymerization in the Absence or Presence of Air. In the synthesis of the diblock copolymer poly(vinyl acetate)-b-poly(vinyl acetate) (PVAc-b-PVAc), a 5 mL glass vial was equipped with a rubber septum and charged with DMSO (1.8 mL), VAc (1.24 g, 14.42 mmol), xanthate (15 mg, 0.072 mmol), and fac[Ir(ppy)3] (0.192 mg, 2.93 × 10−4 mmol). The reaction vial was covered with aluminum foil while degassing for 30 min with N2. The reaction vial was then irradiated under blue LED light (4.8 W, λmax = 435 nm) at room temperature for 6 h. The final solution was precipitated in mixture of diethyl ether/petroleum spirit (1/1, v/v) with vigorous stirring. The precipitate was then collected and precipitated for a second and third time using mixture of diethyl ether/petroleum spirit (1/1, v/v). The sample was then analyzed in GPC to measure the number-average molecular weight and dispersity (Mw/Mn): Mn = 6740 g/mol, Mw/Mn = 1.19. Chain extension was carried out in an 8 mL glass vial with the reaction mixture composed of DMSO (0.5 mL), VAc (2.39 g, 27.76 mmol), PVAc macroinitiator (0.075 g, Mn = 6740 g/mol, 0.0111 mmol), and fac-[Ir(ppy)3] (0.364 mg, 5.55 × 10−4 mmol). The reaction mixture was sealed with a rubber septum and covered with aluminum while degassing for 30 min. The reaction mixture was then irradiated by a blue LED strip (4.8 W, λmax = 435 nm) at room temperature for 21 h. The final solution was precipitated in mixture of diethyl ether/petroleum spirit (1/1, v/v) with vigorous stirring. The precipitate was then collected and precipitated for a second and third time using mixture of diethyl ether/petroleum spirit (1/1, v/v). The sample was then analyzed in GPC to measure the number-average molecular weight and dispersity (Mw/Mn): Mn = 18 650 g/mol, Mw/ Mn = 1.39.

EXPERIMENTAL SECTION

Materials. Vinyl acetate (VAc, 99%), N-vinylpyrrolidinone (NVP, 97%), vinyl pivalate (VP, 99%), dimethyl vinylphosphonate (DVP, 95%), vinyl benzoate (VBz, 99%), vinylcarbazole (VCB, 98%), and tris[2-phenylpyridinato-C2,N]iridium(III) ( fac-[Ir(ppy)3], 99%) were all purchased from Aldrich. Deinhibition of monomers was done 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: cyanomethyl methyl(phenyl)carbamodithioate (dithiocarbamate) was purchased from Aldrich; methyl 2-[(ethoxycarbonothioyl)sulfanyl]propanoate (xanthate) was synthesized according to literature procedures.5 Instrumentation. Gel permeation chromatography (GPC) was performed using tetrahydrofuran (THF) and dimethylacetamide (DMAC) as the eluent. The GPC system consists of 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) for the DMAC system, two MIX C columns provided by Polymer Lab for the THF system, and a differential refractive index detector and a UV detector (λ = 290 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. Nuclear magnetic resonance (NMR) spectroscopy was carried out on a Bruker Avance III with SampleXpress operating at 300 MHz for 1H 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. Online Fourier transform near-inf rared (FTNIR) spectroscopy was used to determine the monomer conversions by following the decrease 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 FT-NIR quartz cuvette (1 cm × 2 mm) and polymerized under blue LED light irradiation. Every 5, or 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. UV−vis spectroscopy spectra were recorded using a CARY 300 spectrophotometer (Varian) equipped with a temperature controller. Photopolymerization reactions were carried out in the same reaction vessel used in our previous work,32 where the reaction mixtures were irradiated by one meter of blue LED strip (4.8 W, λmax = 435 nm). General Procedures for Kinetic Studies of PET-RAFT Polymerization of Vinyl Acetate. Kinetic study of PET-RAFT polymerization of vinyl acetate with 5 ppm (relative to monomer) concentration of catalyst was carried out in a 5 mL glass vial with a rubber septum. The reaction mixture consisted of DMSO (1.5 mL), VAc (1.24 g, 14.42 mmol), xanthate (15 mg, 0.072 mmol), and fac[Ir(ppy)3] (0.048 mg, 7.33 × 10−5 mmol). The reaction vial was sealed with a rubber septum and covered with aluminum foil while degassing for 30 min with N2. The reaction vial was then irradiated under blue LED light (4.8 W, λmax = 435 nm) at room temperature. Aliquots of the reaction mixture were then removed at predetermined time intervals using nitrogen purged needle. These samples were then analyzed by 1H NMR (CDCl3) and GPC (DMAc) to measure the conversions, number-average molecular weights (Mn) and dispersities (Mw/Mn). Kinetic studies of PET-RAFT polymerization of vinyl acetate with 1 and 20 ppm (relative to monomer) concentrations of catalyst were performed under identical conditions. Kinetic study of PET-RAFT polymerization of vinyl acetate in the presence of oxygen with 20 ppm (relative to monomer) concentration of catalyst was carried out under identical conditions except that the degassing step was eliminated. C


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Table 1. Examples of Poly(vinyl acetate) Synthesized by PET-RAFT Polymerization in This Study no. 1 2 3 4 5 6 7 8 9 10e 11e 12e 13e 14e 15e

exp conda [M]:[thiocar.]:[Ir] −4

200:1:2 × 10 200:1:10 × 10−4 200:1:20 × 10−4 200:1:40 × 10−4 1000:1:50 × 10−4 1000:1:200 × 10−4 2000:1:400 × 10−4 80:1:16 × 10−4 200:1:40 × 10−4 200:1:40 × 10−4 1000:1:200 × 10−4 1000:1:200 × 10−4 2000:1:400 × 10−4 2000:1:400 × 10−4 5000:1:1000 × 10−4

monomer

thiocar

[Ir]/[M] (ppm)

time (h)

αb (%)

VAc VAc VAc VAc VAc VAc VAc VAc VAc VAc VAc VAc VAc VAc VAc

xanthate xanthate xanthate xanthate xanthate xanthate xanthate xanthate dithiocarbamate xanthate xanthate xanthate xanthate xanthate xanthate

1 5 10 20 5 20 20 20 20 20 20 20 20 20 20

20 22 20 16 24 24 24 20 21 13 3 14 3 14 30

56 79 87 76 57 76 39 90 18 84 36 49 15 37 23

Mn,thc (g/mol) 9 13 15 13 49 65 69 6 3 14 30 42 26 63 99

840 780 180 280 230 630 200 400 300 670 640 760 200 300 900

Mn,GPCd (g/mol) 10 14 15 13 47 61 63 8 3 14 28 35 29 54 101

510 140 270 170 430 330 940 740 120 760 660 020 020 110 400

Mw/Mn 1.18 1.20 1.19 1.15 1.51 1.36 1.42 1.09 1.49 1.17 1.13 1.24 1.17 1.17 1.24

a

The reactions were performed at room temperature under 4.8 W blue LED light (λmax = 435 nm) in the absence of oxygen. bMonomer conversion determined by 1H NMR spectroscopy. cTheoretical molecular weight calculated using the following equation: Mn,th = [VAc]0/[xanthate]0 × MWM × α + MWxanthate, where [VAc]0, [xanthate]0, MWM, α, and MWxanthate correspond to VAc and xanthate concentration, molar mass of VAc, monomer conversion, and molar mass of xanthate. dMolecular weight and dispersity determined by GPC analysis (DMAc used as eluent), calibrated by Mark− Houwink constants. eBulk polymerization.

Figure 1. PET-RAFT polymerization of vinyl acetate (VAc) using various concentrations of photoredox catalyst Ir(ppy)3 with blue LED light in the presence of xanthate at room temperature, using a molar ratio of [VAc]:[xanthate] = 200:1 in DMSO: (A) ln([M]0/[M]t) vs time of exposure; (B) Mn vs conversion; (C) Mw/Mn vs conversion; (D) molecular weight distributions of PVAc at 20 ppm catalyst concentration.

monomer that acts as a poor leaving group. 59 The fragmentation of the RAFT-adduct radical through destabilization of adduct radical can be enhanced by increasing the electron density at the radical center by the use of RAFT agents, such as xanthates and N,N-diakyl dithiocarbamates.59,60 Both these RAFT agents are effective in their roles to mediate the polymerization of vinyl acetate.59 In our study of vinyl acetate polymerization (Table 1) using the PET-RAFT technique, we discovered that xanthate is a more effective chain transfer agent as compared to dithiocarbamate due to higher conversion and lower dispersity (Table 1, no. 4 and 9), which possibly induced by the different activating rates of

The polymerization in the presence of oxygen was carried out under identical conditions, except for the elimination of deoxygenation step.

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RESULTS AND DISCUSSION PET-RAFT Polymerization of VAc at Different Concentration of Photocatalyst in the Absence of Oxygen. RAFT/MADIX polymerization is one of the few living polymerization techniques that are able to control the polymerization of unconjugated monomers, such as vinyl acetate (VAc),4 N-vinylpyrrolidinone,57 and other unconjugated monomers.14,58 The lack of steric bulk and poor stabilization of propagating radical affords VAc highly reactive D


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rate. It is worthy to note that the slopes, apparent propagation rate constants from linear fitting of the data in Figure 1 (kpapp = 0.090 h−1 for 20 ppm, 0.074 h−1 for 5 ppm, and 0.058 h−1 for 1 ppm), equal to kp[R•] (kp and [R•] correspond to the propagation rate constant of VAc and the concentration of propagating chain radicals during polymerization, respectively) are relatively close to each other, which suggests that [R•] are comparable for the different systems. These values from PETRAFT technique are comparable to those from other living radical polymerization techniques, such as cobalt-mediated polymerization,62 but lower than those from bulk polymerization using the conventional thermally initiated RAFT/ MADIX technique.26 However, our technique gave much shorter induction periods and lower dispersities (Mw/Mn < 1.2, Figure 1C), which suggests less termination or side reactions than the other methods. Gel permeation chromatography (GPC) analysis revealed a linear increase of molecular weights measured by GPC (Mn,GPC) with respect to conversion of vinyl acetate (Figure 1B), low dispersities (Figure 1C), and narrow molecular weight distributions (Figure 1D and Supporting Information Figure S1) for all catalyst concentration systems (1, 5, and 20 ppm). The theoretical molecular weights (Mn,th) were close to those measured by GPC. The catalyst concentration was fixed at 20 ppm for all further experiments, since lower dispersities were achieved using a [catalyst]:[monomer] ratio of 20 ppm. PET-RAFT Polymerization of VAc in Different Solvents. To further investigate the catalytic efficiency of fac[Ir(ppy)3] in different solvents, we carried out polymerizations of VAc in other common solvents, including acetonitrile, toluene, and methanol (Table 2), with xanthate as chain transfer agent using [catalyst]:[monomer] ratio of 20 or 50 ppm. PVAc were synthesized with good control of molecular

Table 2. Results for PVAc Synthesized by PET-RAFT Polymerization in Other Common Organic Solventsa solvent

[Ir]/[M] (ppm)

convb α (%)

Mn,thc (g/mol)

Mn,GPC (g/mol)

Mw/Mn

DMSO acetonitrile acetonitrile toluene toluene methanol methanol

20 20 50 20 50 20 50

91 45 74 28 37 0 20

15880 7750 12740 4820 6370

16100 7610 12640 5210 6820

1.15 1.11 1.24 1.09 1.11

3440

4090

1.06

a

The reactions were carried out at room temperature for 22 h using 4.8 W blue LED lamp as light source in the absence of oxygen; molar ratio [VAc]:[xanthate]:[ fac-[Ir(ppy)3]] = 200:1:40 × 10−4. bMonomer conversion determined by 1H NMR spectroscopy was calculated by the following equation: α = (I4.4 ppm/I4.9 ppm) × 100. c Theoretical molecular weight is calculated by the following equation: Mn,th = [VAc]0/[xanthate]0 × MWVAc × α + MWxanthate, where [VAc]0, [xanthate]0, MWVAc, α, and MWxanthate correspond to VAc and xanthate concentration, molar mass of VAc monomer, conversion, and molar mass of xanthate.

xanthate and dithiocarbamate by the photoredox catalyst during PET process. In addition, varied concentrations of photoredox catalyst (1, 5, 10, and 20 ppm relative to monomer) were investigated for PET-RAFT polymerization of VAc in DMSO as solvent (Table 1, no. 1−9). Polymerization of vinyl acetate showed first-order kinetic plots as ln([M]0/[M]t) increased linearly with the exposure time to light (Figure 1A) for the three concentrations of photoredox catalyst (1, 5, and 20 ppm).61 Interestingly, no inhibition period was observed for catalyst concentrations of 5 and 20 ppm. However, 6 h inhibition period was observed for 1 ppm catalyst concentration, suggesting much slower initiating

Figure 2. “ON/OFF” experiments for PET-RAFT polymerization of VAc in the absence and presence of oxygen: (A) ln([M]0/[M]t) versus time in the presence (“ON”) or absence (“OFF”) of light; (B) Mn and Mw/Mn versus conversion; (C) molecular weight distributions for PVAc at different exposure time in the absence of oxygen; (D) molecular weight distributions for PVAc at different exposure time in the presence of oxygen. E


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Figure 3. Online Fourier transform near-infrared (FTNIR) measurement for kinetic study of PET-RAFT polymerization of vinyl acetate (VAc) with Ir(ppy)3 as photoredox catalyst, blue LED light as light source, and xanthate as chain transfer agent at room temperature, using a molar ratio of [VAc]:[xanthate] = 200:1 in DMSO: (A) FTNIR full spectra (bottom) and narrower spectra (top) windows for the reaction solution at different time points in the absence of air; (B) ln([M]0/[M]t) vs time of exposure in the absence (black) and presence (red) of air; (C) molecular weight distributions for PVAc after 555 min (absence of air, black line) and 600 min (presence of air, red line) light irradiation. Polymer in the absence of air (555 min): Mn = 10 550 g/mol, Mw/Mn = 1.09, conversion (NMR) = 43.5%, conversion (NIR) = 38.4%. Polymer in the presence of air (600 min): Mn = 10 150, Mw/Mn = 1.11, conversion (NMR) = 42.3%, conversion (NIR) = 36.1%.

(polymerization mixture) to volume of free space (air). In this work, we decided to investigate the oxygen tolerance of PETRAFT polymerization of VAc. Temporal control of PET-RAFT polymerization of VAc in the absence and presence of oxygen was demonstrated by “ON/OFF” experiments (Figure 2A). The system remained dormant with no polymerization taking place in the absence of light, while when the light was back “ON”, the system was activated and resumed polymerizing. These “activation” and “deactivation” processes were easily manipulated by controlling “ON” and “OFF” periods. Most importantly, the evolution of molecular weights to monomer conversion showed a linear plot with good agreement to theoretical molecular weights (Figure 2B). Molecular weight distributions indicated clear shift with exposure time (Figure 2C,D) and narrow dispersities (Mw/Mn) lower than 1.2 in the absence of oxygen and 1.27 in the presence of oxygen. An interesting fact to note is that there are no obvious differences for the polymerization rates in the absence and presence of oxygen (Figure 2A). In order to investigate the kinetics of PET-RAFT polymerization of VAc in the absence and presence of oxygen, with fixed amount of catalyst and with the volume of free space unchanged during measurement, a convenient and reliable method, namely online Fourier transform near-infrared (FTNIR) spectroscopy, was employed. This technique avoids frequent sampling, which normally caused some experimental errors and has been successfully employed by Haddleton and others for monitoring conversion.69−71 Figure 3A depicted full spectra (bottom) and a

weights and narrow molecular weight distributions in these solvents. Interestingly, no polymer was detected in methanol at 20 ppm after 22 h of irradiation. High concentration of catalyst, 50 ppm, was required to reach 20% monomer conversion. We attributed this result to the polarity of solvent, which could affect the reactivity of the catalyst. Lower monomer conversions were obtained in toluene than those in acetonitrile and DMSO, which can be attributed to the retardation commonly observed for the free radical polymerization of VAc in aromatic solvents due to degradative chain transfer at 60 °C.63,64 Consistent with our previous investigations,32 faster polymerization kinetics and lower Mw/Mns in DMSO were observed as compared to other solvents due to possible stabilization of propagating radicals in DMSO as compared to other solvents65 or a difference of catalyst activity in these solvents.66,67 PET-RAFT Polymerization of VAc in the Presence of Oxygen. Oxygen tolerance is one of the unique properties of PET-RAFT polymerization,32 which could eliminate the traditional deoxygenation or freeze−pump−thaw steps for free radical polymerization. This remarkable property originates from the excellent reducing ability of photoredox catalyst, fac[Ir(ppy)3], which could reduce molecular oxygen into inactive species, superoxide, via single-electron reduction.32,68 In a closed and sealed reaction vessel, the polymerization of conjugated monomers, including acrylates and methacrylates, starts after an inhibition period, when all the oxygen in the vessel is consumed or converted, and proceeds as a conventional polymerization. This inhibition period depends on the amount of photoredox catalyst and the ratio of volume of liquid F


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Figure 4. 1H NMR spectra for purified PVAc prepared by PET-RAFT polymerization in the absence (A) and presence (B) of oxygen. The endgroup fidelity was calculated by NMR using the following equation: f xanthate = I5/(I1/3). For both polymerizations, end-group fidelity was greater than 95%.

narrower spectral window (top, 6250−6130 nm) as a function of time during the controlled radical polymerization. The exceptional absorbance (6250−6130 nm) of vinyl protons is not overlapped with any other peaks, which is excellent for quantitative measurement of monomer conversion. These peaks gradually decreased as the polymerization proceeded over time. The monomer conversions were calculated by the ratios of integrations of the area 6250−6150 nm for all curves at different reaction times to that at 0 min. Moreover, the values of monomer conversion for the final products at 555 min determined by FTNIR (38.4%) and NMR (43.5%) were

comparable, demonstrating the applicability and reliability of this method for our system. The oxygen tolerance experiment was carried out in a sealed quartz cell with a total volume of 0.9 mL (0.5 mL reaction solution and 0.4 mL free space filled with air). For the polymerization system in the absence of oxygen (degassed system), the reaction mixture was sealed with rubber septum and degassed for 30 min with nitrogen. For the polymerization system in the presence of oxygen (air system), the reaction mixture was sealed for direct light irradiation. The FTNIR spectra were recorded every 15 min. For both systems, the plots of ln([M]0/[M]t) as a function of reaction time gave the firstG


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Figure 5. Kinetics and “ON/OFF” test for PET-RAFT polymerization of NVP in DMSO in the absence and presence of oxygen. (A) ln([M]0/[M]t) versus time in the presence (“ON”) or absence (“OFF”) of light; (B) Mn and Mw/Mn versus conversion; (C) molecular weight distributions for PNVP at different exposure time in the absence of oxygen; (D) molecular weight distributions for PNVP at different exposure time in the presence of oxygen.

Table 3. Polymerization of Unconjugated Monomers Using PET-RAFT Polymerization in This Study no. 1 2f 3 4 5 6 7 8

exp conda [M]:[thiocar.]:[Ir] −4

1000:1:200 × 10 1000:1: 200 × 10−4 200:1:40 × 10−4 200:1:40 × 10−4 200:1:40 × 10−4 200:1:80 × 10−4 200:1:40 × 10−4 200:1:40 × 10−4

monomer NVP NVP VP VP DVP DVP VBz VCB

thiocar

[Ir]/[M] (ppm)

xanthate xanthate xanthate xanthate xanthate xanthate xanthate xanthate

20 20 20 20 20 20 20 20

time (h) 4 2 3 24 6 24 24 24

αb (%) g

18 49g 32 85 33 69 55 45

Mn,thc (g/mol) 20 55 6 22 4 10 16 17

210 000 500 100 800 800 800 700

Mn,GPCd (g/mol) 14 38 4 23 5 11 14 21

330 900 500 800e 400 300 300 300

Mw/Mn 1.42 1.29 1.16 1.12e 1.28 1.23 1.26 1.33

a The reactions were performed at room temperature under 4.8 W blue LED light (λmax = 435 nm). bMonomer conversion determined by 1H NMR spectroscopy. cTheoretical molecular weight calculated using the following equation: Mn,th = [M]0/[xanthate]0 × MWM × α + MWxanthate, where [M]0, [xanthate]0, MWM, α, and MWxanthate correspond to monomer and xanthate concentration, molar mass of monomer, monomer conversion, and molar mass of xanthate. dMolecular weight and dispersity determined by GPC analysis (DMAc used as eluent). eMolecular weight and dispersity determined by GPC analysis (THF used as eluent). fBulk polymerization. gMonomer conversions were determined by gravimetric analysis.

In order to further test the versatility of fac-[Ir(ppy)3] in polymerizing VAc especially in the presence of large volume of air, the polymerization was carried out in a glass vial with 90% of the total volume was air while the remaining 10% of the total volume of the glass vial was filled by reaction mixture ([VAc]: [xanthate]:[Ir(ppy)3] = 200:1:40 × 10−4). The polymerization was carried out at room temperature for 20 h in DMSO under blue LED light irradiation. GPC analysis (Supporting Information Figure S3) revealed a molecular weight of 18 970 g/mol (monomer conversion 80%) and a PDI of 1.47. In comparison to the degassed reaction mixture (Table 1, no. 4, monomer conversion 76%, Mn = 14 880 g/mol, and PDI = 1.15), the polymerization of VAc seems to be not affected by the presence of air in terms of monomer conversion, although a broader molecular weight distribution was obtained in the presence of air. These findings can be extremely important for industrial applications, especially in polymerizations that are highly sensitive to trace amount of oxygen, particularly free radical polymerization.

order kinetic in Figure 3B, without inhibition periods. The comparable apparent propagation rate constants, kpapp, for both systems suggested that the oxygen effect was negligible. Moreover, the molecular weight distributions for the final products were in good agreement with one another (Figure 3C). 1 H NMR measurement (Figure 4) and UV−vis spectroscopy (Supporting Information Figure S2) clearly indicated the attachment of xanthate end-group to the PVAc synthesized. End-group fidelity of MADIX agent was calculated using NMR after purification, using the following equation: f xanthate = I5/(I1/3), with I5 and I1 corresponding to integrals of peak signal 5 and 1 in Figure 4. Interestingly, the end-group fidelities of PVAc synthesized in the presence or absence of oxygen were both greater than 95%. Therefore, these present results demonstrate that PVAc can be polymerized in a sealed vessel in the presence of oxygen when 20 ppm (relative to monomer) photoredox catalyst is used; consequently, the need for deoxygenation step could be eliminated. H


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Figure 6. Molecular weight distributions for PVAc and PNVP macroinitiators and their diblock copolymers prepared by PET-RAFT polymerization in the absence (A, B, C, and D) and presence (E, F, G, and H) of air with Ir(ppy)3 as photoredox catalyst, blue LED light as light source, and xanthate as chain transfer agent at room temperature in DMSO: (A and E) PVAc-b-PVAc; (B and F) PVAc-b-PVP; (C and G) PNVP-b-PNVP; (D and H) PNVP-b-PVAc.

PET-RAFT Polymerization of NVP in the Absence and Presence of Oxygen. PNVP is attracting continuous attention from both academia and industry due to its remarkable physical and chemical properties, which include good film-forming and adhesive characteristics, pH stability, strong resistance to thermal decomposition, very low toxicity, and biocompatibility. The physical properties are highly dependent on the polymer structures that comprise of molecular weights, dispersities, and architectures. Nevertheless, polymerization of NVP has only been reported by free radical process, as most controlled radical polymerization techniques

(including, NMP, ATRP, and ITP) do not work well in NVP polymerization. Only RAFT/MADIX polymerization,2,38,72−79 cobalt-mediated radical polymerization,80 and organostibinemediated polymerization27,81,82 were found to be suitable techniques to deactivate PNVP chain growth in a reversible manner. Our PET-RAFT approach is the first reported technique that could control NVP polymerization with light in the absence and presence of oxygen, which truly is a significant contribution to this research area. The kinetics of NVP in DMSO in the presence or absence of oxygen were investigated by online FTNIR spectroscopy. By I


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of oxygen (Figure S5), which suggests the excellent retention of xanthate end-group. In addition, UV−vis spectroscopy confirms the presence of signal 290 nm (Supporting Information Figure S6). The end-group fidelity was calculated to be greater than 95% for both polymerizations performed in the presence and absence of oxygen using a calibration curve (Supporting Information Figure S7). Other unconjugated monomers, including vinyl pivalate (VP), dimethyl vinylphosphonate (DVP), vinyl benzoate (VBz), and vinyl carbazole (VCB), were successfully polymerized by the PET-RAFT technique with a good control of molecular weights and molecular weight distributions (Table 3). The kinetic study for PET-RAFT polymerization of VCB by online FTNIR showed excellent “ON/OFF” light control (Supporting Information Figure S8A) and growth of molecular weights (Supporting Information Figure S8B,C), although high dispersities were obtained. Diblock Copolymers by PET-RAFT Polymerization in the Absence and Presence of Oxygen. Chain extensions of PVAc and PNVP were performed to further prove the livingness of the resultant polymer chains prepared by PETRAFT polymerization in the absence and presence of oxygen (Figure 6). PVAcs were used as the macroinitiators for chain extension in the presence of VAc (Figure 6A,E) and VP (Figure 6B,F), respectively. PNVPs were employed for chain extension in the presence of NVP (Figure 6C,G) and VAc (Figure 6D,H), respectively. All the diblock copolymers were synthesized at room temperature in DMSO under blue LED light irradiation. The ratios of monomers to macroinitiators ([monomer]: [macroinitiator]) were 200:1 for the experiments in Figure 6A,E, 4500:1 for those in Figure 6B,F, and 200:1 for those in Figure 6C,D,G,H. GPC analysis indicated a shift of macroinitiators to high molecular weights. Using a previous method

Figure 7. Molecular weight distributions (RI signals) of unpurified PVAc synthesized by PET-RAFT bulk polymerization under 4.8 W blue LED light irradiation at 20 ppm catalyst concentration using a molar ratio of [VAc]:[xanthate] = 200:1, 1000:1, 2000:1, and 5000:1 (Table 1, no. 10, 12, 14, and 15).

plotting ln([M]0/[M]t) versus time including light “ON” and “OFF” periods (Figure 5A), a nearly linear relationship at low monomer conversion was obtained, despite the slight deviation after the “OFF” period. The molecular weights of the homopolymers in the presence and absence of oxygen obtained from the GPC agreed closely with theoretical values (Figure 5B) and those measured by 1H NMR (Supporting Information Figure S4). The dispersities remained lower than 1.16 during the polymerization, which can be confirmed by clear evolution of the molecular weight distributions with exposure time measured by GPC in Figure 5C,D. The purified PNVPs measured by GPC with dual RI and UV detectors (λ = 290 nm) showed a good agreement between molecular weight distributions in RI and UV signals in the absence and presence

Figure 8. Kinetics and “ON/OFF” test for PET-RAFT polymerization of NVP in bulk condition in the absence of oxygen. (A) ln([M]0/[M]t) versus time with light “ON” and “OFF”; (B) ln([M]0/[M]t) versus time; (C) Mn and Mw/Mn versus conversion; (D) molecular weight distributions for PNVP at different exposure time (50, 80, and 120 min); (E) molecular weight distributions for PNVP collected by RI and UV detectors at 80 and 120 min of exposure time. J


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reported in the literature,32,83,84 we were able to calculate the amount of macroinitiators successfully chain extended (Supporting Information Table S1). In our condition, we found that more of 80% macroinitiator was chain extended. Interestingly, we observed the similar livingness (or end-group fidelity) for both polymerizations performed in the presence or absence of oxygen. Therefore, it can be concluded that photoredox catalyst fac-[Ir(ppy)3] can catalyze the PETRAFT polymerization of VAc in the presence of oxygen without altering the molecular weight distributions, livingness, and polymerization rates. PET-RAFT Bulk Polymerization of VAc and NVP in the Absence of Oxygen. Although successful polymerizations of VAc were achieved in DMSO using high molar ratio of vinyl acetate to xanthate, the molecular weights of the homopolymers were often less than 70 000 g/mol, and a significant increase of dispersity was observed for high molecular weights (Table 1, no. 5 and 6). Inspired by Peng and co-workers, who were able to synthesize PVAc in bulk condition with molecular weights exceeding 60 000 g/mol with relatively narrow molecular weight distributions (Mw/Mn = 1.24−1.34),62 we attempted to polymerize VAc by PET-RAFT polymerization in bulk condition in the absence of oxygen. By varying the ratios of monomer to xanthate, we were able to synthesize PVAc with high molecular weights in the range of 16 000−102 000 g/mol (Figure 7 for GPC RI signals, Supporting Information Figure S9 for GPC UV signals, and Table 1, no. 10, 12, 14, and 15). In these conditions, we obtained a relative narrow molecular weight distribution (Mw/Mn < 1.25) for molecular weight greater than 90 000 g/mol. Conventional controlled/living polymerization technique, such as RAFT/MADIX and cobaltmediated radical polymerization, results typically by the synthesis of PVAc with Mw/Mn > 1.35.6,14,29,31,62 The lower dispersity observed in PET-RAFT in comparison to the conventional controlled/living radical polymerization was attributed to the low temperature of the polymerization. Indeed, the proportion of side reactions, including chain transfer reactions and head-to-head addition, increases with the temperature.14,85 Additionally, no inhibition periods were observed for all bulk polymerizations. In comparison to the polymerizations in the presence and absence of solvent (DMSO) (Table 1, no. 7 and 14), a narrower molecular weight distribution was observed in bulk polymerization due to the absence of chain transfer to solvent.14 Polymerizations in DMSO performed for periods greater than 24 h led to broad molecular weight distributions (Table 1, no. 5 and no. 6). In the case of bulk polymerization, the molecular weight distributions remained lower than 1.25 even in the preparation of polymers with molecular weights higher than 100 000 g/mol after 30 h light irradiation (Table 1, no. 15). Similar findings were observed in the polymerization of NVP. Lower dispersity and higher conversion were obtained in bulk polymerization compared to polymerization in solution (Table 3, no. 1 and 2). The kinetic study and “ON/OFF” test (Figure 8) for NVP bulk polymerization were investigated by using similar procedures to that of solution polymerization. Linear plotting of ln([M]0/[M]t) with respect to exposure time (Figure 8B) and monomodal molecular weight distributions recorded by both UV and RI detectors (Figure 8D,E) demonstrated the controlled polymerization of NVP in bulk through the PETRAFT technique.

Article

CONCLUSION Photoinduced electron transfer−reversible addition−fragmentation chain transfer (PET-RAFT) polymerization is a versatile technique for the control of unconjugated monomers, including vinyl acetate, vinyl pivalate, N-vinylpyrrolidinone, dimethyl vinylphosphonate, vinyl benzoate, and N-vinylcarbazole, in the presence of ultralow concentration (ppm range) of photoredox catalyst, fac-[Ir(ppy)3]. The kinetic study of vinyl acetate indicated excellent control of molecular weights and molecular weight distributions (Mw/Mn = 1.09−1.2), even at high monomer conversions (>75%). In addition, the online Fourier transform near-infrared (FTNIR) was utilized to monitor monomer conversions of the polymerization of vinyl acetate in the presence and absence of oxygen. Interestingly, the polymerization rates are not affected by the presence of relatively large amount of oxygen (or air). The polymerizations of vinyl acetate performed in the presence of air yielded welldefined polymers with a narrow molecular weight distribution (Mw/Mn ∼ 1.20). Successful chain extensions of homopolymers of poly(vinyl acetate) to other monomers confirmed the integrity of xanthate end-group.

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ASSOCIATED CONTENT

* Supporting Information S

UV−vis spectra, NMR spectra, GPC traces (Figures S1−S11), and 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 C.B. acknowledges the Australian Research Council (ARC) for his Future Fellowship (FT120100096).

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