Controlled Radical Polymerization of

Sep 4, 2014 - energy, simple operation process, and mild polymerization conditions. ... Co(acac)2 mediated living radical polymerization,16 have been ...
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Visible Light Induced Living/Controlled Radical Polymerization of Acrylates Catalyzed by Cobalt Porphyrins Yaguang Zhao, Mengmeng Yu, Shuailin Zhang, Yuchu Liu, and Xuefeng Fu* Beijing National Laboratory for Molecular Sciences, State Key Lab of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: Visible light induced living radical polymerization of a wide scope of acrylates mediated by organocobalt porphyrins was developed. The photocleavage of the Co−C bond of organocobalt porphyrin produced carbon centered radicals, which initiated polymerization, and porphyrin cobalt(II), a persistent metal-centered radical. The organocobalt porphyrins were highly sensitive to external visible light irradiation so that photostimulus was used to control the initiation steps and regulate chain growth by reversibly activating the Co−C bond. Polymerization occurred spontaneously under irradiation and stopped promptly once shutting down light source. The scope of monomers was successfully extended from acrylamides to various hydrophobic and hydrophilic acrylates via the control of the light intensity. The structure of polyacrylate obtained was confirmed by 2D NMR, 13C NMR, GPC, and MALDI-TOF-MS. One of the unique features of this neat visible light induced polymerization process is that organocobalt porphyrins played dual roles of photoinitiators and mediators without addition of any dyes, photosensitizers, or sacrificial reagents.

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Co(acac)2 mediated living radical polymerization,16 have been reported. These systems often require high-energy UV irradiation, which increases both the cost and inconvenience compared with the utilization of abundant visible light. Photocleavage of the metal−carbon bond in organometallic compounds to produce organic radicals, which initiate polymerization, and metalloradicals, which reversely bind propagating radicals, provides a straightforward strategy in design of novel organometallic catalysts requiring highly efficient reversible photolysis of the metal−carbon bond. Thus, for this type of catalysis, the number of propagating radicals and polymerization kinetics can be controlled through modulation of light intensity, and photochemical stimuli provide opportunities for both spatial and temporal control of the living radical polymerization process. Recently, our research group has developed the visible light induced LRP of acrylamides mediated by organocobalt porphyrins, and a series of acrylamide based functional diblock and triblock copolymers have been synthesized by adopting this strategy.17 Herein we report the sunlight/visible light induced LRP of acrylates, including both hydrophobic and hydrophilic monomers methyl acrylate (MA), n-butyl acrylate (nBA), tertbutyl acrylate (tBA), and 2-hydroxyethyl acrylate (HEA), as well as the synthesis of a series of novel multiblock copolymers at ambient temperature mediated by organocobalt porphyrins with no demand for photoinitiators or sacrifice reagents. The effect of visible light on the initiation and kinetics of

INTRODUCTION Controlled/living radical polymerization (LRP) has been wellknown as a powerful tool for synthesis of well-defined polymeric materials with predetermined molecular weight, narrow polydispersity, and diverse architecture.1 The development of different strategies to regulate the equilibrium between dormant and activate species has led to the advent of several LRP methods, such as nitroxide mediated radical polymerization (NMP),2 atom transfer radical polymerization (ATRP),3 reversible addition−fragmentation chain transfer polymerization (RAFT),4 organoiodine mediated radical polymerization (IRP),5 and organometallic mediated radical polymerization (OMRP).6 In general, various physical and chemical stimuli, such as heat, metal catalyst(s), electrochemical,7 and photochemical stimuli,8 were employed to efficiently control the reversible activation of dormant species. Photoinduced LRPs are receiving growing attention recently due to its inherent advantages including environmentally benign reagent, low activation energy, simple operation process, and mild polymerization conditions. Photopolymerization represents one of the most effective and advantageous technique for production of polymer materials in coatings, adhesives, inks, and so forth.9 Efficient photoinduced LRP remains a challenging topic although promising progress in this area has been made.10 Since the development of photoinduced polymerization using dithiocarbamate as photoiniferter (initiator−transfer−terminator) under UV irradiation,11 various strategies have been employed in typical LRP systems including ATRP,12 RAFT,13 and NMP14 by taking the advantages of photoirradiation. However, only a few photoinduced OMRP, such as organotellurium15 and © XXXX American Chemical Society

Received: July 12, 2014 Revised: August 20, 2014

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synthesized by similar conditions simply by replacing CH3OH with CD3OD and CO with 13CO. 1H NMR (CDCl3, 500 MHz, 303 K, δ): 1.66 (s, 12 H), 2.02 (s, 12 H), 2.60 (s, 12 H), 7.20 (s, 4H), 7.27 (s, 4H), 8.65 (s, 8 H). ESI-HRMS calcd for C57H52CoN4O213CD3 [M]+: 902.38734; found: 902.388 84. Typical Procedure for Photopolymerization. A typical procedure for polymerization of acrylate mediated by organocobalt porphyrins is as follows: (1) a certain amount of porphyrin cobalt complexes and monomers were mixed in C6D6 in a J. Young valve NMR tube; (2) after being thoroughly mixed and three freeze− pump−thaw cycles, the sample was refilled with nitrogen; (3) the sample was placed in a room temperature water bath and irradiated for a period of time. The light intensity was adjusted by employing neutral density filters. The progress of polymerization was followed by 1H NMR measurement. When desired conversion was reached, the reaction was stopped by exposure to air. The solvent and excess monomers were removed under vacuum. The resulting product without further purification was dissolved in DMF for GPC analysis. Photopolymerization of HEA was performed in methanol, and all the other monomers were in benzene. Synthesis of Multiblock Copolymer. The light intensity was maintained at 3 mW/cm2 throughout the polymerization process. Well-defined block copolymers were synthesized by sequential polymerization of acrylates and acrylamide. After a desired conversion of a certain monomer was obtained, the resulting polymer was used as the macroinitiator for the next monomer. The polymerization was conducted at room temperature for MA, nBA, and DMA but at 0 °C for tBA. Characterization. Conversions of monomers were determined by 1 H NMR spectrometry on a Bruker 400 MHz FT spectrometer with benzaldehyde as external reference in C6D6 or CD3OD. 13C NMR and 2 D NMR (D2O as external reference) spectra were recorded on a Bruker 500 MHz FT spectrometer. IR (film) was recorded with a Nicolet Avatar 330 FT-IR infrared spectrometer. The UV−vis spectra were acquired using a Shimadazu UV3100 spectrometer. Matrixassisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF-MS) measurement was performed on a TOF/TOF 5800 system (AB SCIEX) with α-cyano-4-hydroxycinnamic acid (CHCA) as a matrix in positive reflection mode. Gel permeation chromatography (GPC) was performed in an Agilent 1200 series system, equipped with one or two VARIAN PolarGel-M columns (300 × 7.5 mm), an Iso Pump (G1310A), a UV detector at 420 nm, and a differential refractive index detector (RID). The number-average molecular weight (Mn), weight-average molecular weight (Mw), and the polydispersity (PDI) were measured in DMF at 50 °C with a flow rate of 1.0 mL/min. A series of poly(methyl methacrylate)s (molecular weight range of 2710−679 000 g/mol, from Polymer Laboratories) were used as standards for calibration.

polymerization was also investigated by the radical trapping reaction and intermittent irradiation method. The structure of synthesized polyacrylate was confirmed by 2D NMR, 13C NMR, and MALDI-TOF-MS characterization.

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EXPERIMENTAL SECTION

Materials. Methyl acrylate (MA, Alfa Aesar, 99%), n-butyl acrylate (nBA, Alfa Aesar, 98+%), and tert-butyl acrylate (tBA, Alfa Aesar, 99%) were purified by passing through a neutral alumina column and distilled under reduced pressure to remove the inhibitor. 2Hydroxyethyl acrylate (HEA, Acros, 97%) was purified by passing through a basic alumina column and stored in the refrigerator. N,NDimethylacrylamide (DMA, Alfa, 99.5%), N,N-diethylacrylamide (DEA, TCI, >98.0%), and N-acryloylmorpholine (AMO, TCI, >98.0%) were distilled under reduced pressure and stored in the refrigerator before use. Cobalt porphyrins (TMP)CoII (tetramesitylporphyrin cobalt) and (TMP-OH)CoII ((5-(4-(10-hydroxyl-1decyloxy)phenyl)-10,15,20-tris(2,4,6-trimethylphenyl)porphyrin)cobalt) were prepared according to a previously reported method.18 All other reagents and solvents were used as received if otherwise mentioned. Light Source. A 500 W xenon lamp (CEL-S500, Aulight, Beijing, China) was used as the light source with a 420−780 nm filter to give visible light (Figure S1). The intensity of the irradiation is modulated by employing neutral density filters with different transmittance. The intensity of visible light irradiation was measured by a FZ-A radiometer (Photoelectric Instrument Factory of Beijing Normal University) equipped with a 400−1000 nm sensor. A household CFL (compact fluorescent lamp, 27 W) was used as the light source; the sample was placed at an approximate distance of 5 cm to the lamp, and the light intensity was measured to be 3−5 mW/cm2. Synthesis of Complexes I−III. (TMP)Co-PMA and (TMPOH)Co-PDMA are the abbreviations for organocobalt porphyrins with different polymer chains as the substituent group. PMA: poly(methyl acrylate); PDMA: poly(N,N-dimethylacrylamide). (TMP)Co-PMA I (Mn = 17 800, Mw/Mn = 1.24), (TMP-OH)CoPDMA II (Mn = 28 700, Mw/Mn = 1.18), and (TMP)Co-PMA III (Mn = 1300, Mw/Mn = 1.17) (Figure 1) were synthesized and purified according to procedures previously reported.17,18

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RESULTS AND DISCUSSION Photo-LRP of Acrylates with Macroinitiators. Recently we reported the visible light induced LRP of acrylamides mediated by organocobalt porphyrins.17 Here we synthesized four different organocobalt porphyrins ((TMP)Co-PMA I and III, (TMP-OH)Co-PDMA II, (TMP)Co-CO2CH3 IV) to investigate whether they can control the polymerization of acrylates under visible light irradiation. UV−vis spectra of I, III, and IV complexes (Figures S2−S6) are identical with those of the corresponding (TMP)CoII as previously reported.19 However, when we conducted the polymerization of MA under the same reaction conditions as we previously reported photo LRP of DMA,17 the molecular weight distribution of resulting PMA was quite broad, and the molar mass was much smaller than the theoretical value (Table 1, entry 1). On the other hand, without the presence of complex I, no polymer was observed upon irradiation of the benzene solution of MA for 14 h (Table 1, entry 2). Without irradiation, mixture of I and MA

Figure 1. Structure of organocobalt porphyrins used in this research. Synthesis of Complex (TMP)Co-CO2CH3 (IV) and (TMP)Co-13CO2CD3. Methanol (300 μL) was added into 3 mL of a toluene solution containing (TMP)CoII (1.17 × 10−2 mmol, 9.8 mg), AgOTf (1.17 × 10−1 mmol, 30 mg), and Na2HPO4 (1.17 × 10−1 mmol, 16.6 mg). The mixture was subsequently degassed by three freeze−pump− thaw cycles and refilled with carbon monoxide (CO, 1 atm). The sample was shielded from light and left stirring overnight at room temperature. After the solvent was removed under reduced pressure, the crude product was purified by column chromatography (basic alumina, CH2Cl2 as eluent). (TMP)Co-CO2CH3 (IV) (9.7 mg, 92% yield) was obtained as a red solid. 1H NMR (CDCl3, 500 MHz, 303 K, δ): 1.14 (s, 3 H), 1.66 (s, 12 H), 2.02 (s, 12 H), 2.60 (s, 12 H), 7.20 (s, 4H), 7.27 (s, 4H), 8.65 (s, 8 H). ESI-HRMS calcd for C58H55CoN4O2 [M]+: 898.365 15, found: 898.363 39. (TMP)Co-13CO2CD3 was B

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Table 1. Visible Light Induced LRP of Acrylates Mediated by Organocobalt Porphyrins at Room Temperaturea entry

catalyst

M

I (mW/cm2)

t (h)

convb (%)

Mn,thc

Mn,exd

Mw/Mnd

1 2 3 4 5 6 7 8 9 10e 11 12 13e 14 15e 16f

I

MA MA MA MA MA MA MA nBA nBA nBA nBA nBA nBA tBA tBA HEA

80 80 0 50 16 7 3 7 3 3 household CFL sunlight sunlight 3 3 3

12 14 15 12 13 24 21 17 24 23 11 8 8 4 14 14

77 0 4 48 50 41 36 56 60 50 46 54 49 54 69 35

57 570

29 000

1.87

19 870 42 590 43 630 38 980 36 400 60 860 63 940 56 250 53 170 59 330 55 480 59 330 70 860 53 080

18 200 26 100 37 800 38 200 34 600 54 600 58 000 50 200 46 400 57 500 50 200 61 100 67 500 63 600

1.23 1.50 1.28 1.23 1.21 1.28 1.22 1.20 1.24 1.27 1.20 1.37 1.25 1.26

I I I I I I I I I I I I I II

a

[M]0/[catalyst]0 = 600/1, [M]0 = 1.0 M, (TMP)Co-PMA I (Mn = 17 800, Mw/Mn = 1.24), (TMP-OH)Co-PDMA II (Mn = 28 700, Mw/Mn = 1.18). bThe monomer conversion was determined based on 1H NMR spectra. cMn,th = MW(Initiator) + MW(Monomer) × 600 × conv (%). dDetermined using gel permeation chromatography in DMF calibrated against poly(methyl methacrylate) standards. ePolymerization at 0 °C. fIn methanol-d4.

Figure 2. (a) Kinetic plots for the visible light induced LRP of MA mediated by I in benzene at room temperature with different light intensity. (b) Changes in number-average molecular weight and Mw/Mn with MA conversion during visible light induced LRP of MA ([MA]0 = 1.0 M; [MA]0/[I]0 = 600/1; irradiated with a 420−780 nm filter).

(Figure 2a). The apparent rate constants (kobs, rate = kobs × [monomer], kobs = kp × [radical]) at different light intensities were 8.8 × 10−2 h−1 (80 mW/cm2), 5.9 × 10−2 h−1(50 mW/ cm2), 2.6 × 10−2 h−1 (7.0 mW/cm2), and 1.8 × 10−2 h−1 (3.0 mW/cm2) (Figure S7). These observed rate constants were consistent with the fact that more radicals were generated under higher light intensity due to the homolysis of the cobalt− carbon bond. The visible light intensity determined the concentrations of the dormant species and the active propagating radicals. The evolution of the molar mass and polydispersity (Mw/ Mn) of PMAs is shown in Figure 2b. The number-average molecular weight (Mn) increased linearly with monomer conversion. The molecular weight distributions of the obtained polymers were around 1.25, and the experimental molar masses are close to the theoretical value, indicating that almost all polymer chains were initiated from organocobalt initiator. The GPC traces for the photopolymerization of MA with intensity of 3 mW/cm2 were monomodal and reasonably symmetrical throughout the whole process (Figure S8). A larger monomer conversion led to a clear shift of GPC traces to the range of higher molecular weight without any noticeable dead polymers. Photopolymerization of nBA was also conducted under the same conditions mediated by organocobalt porphyrin I. When light intensity used was 7 mW/cm2, PnBA with controlled

solution at room temperature for 15 h gave only 4% conversion (Table 1, entry 3). But it is worth noting when the intensity of the irradiation was reduced, the photopolymerization of acrylates gradually became a well-controlled process. The molecular weight distribution of the formed PMA decreased gradually with reducing the light intensity (Table 1, entries 4− 7), such as the resulting PMA with polydispersity of 1.50 for light intensity of 50 mW/cm2 (Table 1, entry 4) and polydispersity of 1.28 with intensity of 16 mW/cm2 (Table 1, entry 5). Therefore, it is obvious that organocobalt porphyrin complexes are necessary for photopolymerization of MA, and more importantly, the light intensity plays a critical role in the visible light induced LRP of acrylates. According to the data collected in Table 1, the weaker the light intensity (7 or 3 mW/ cm2), the better the LRP of MA can be controlled (Table 1, entries 6 and 7, with the polydispersity around 1.2). The poor control under stronger light intensity might result from the formation of excess of organic radicals due to overirradiation of the relatively weak cobalt−carbon bond. To evaluate the living character of these polymerization processes, the kinetic rate plots for the photopolymerization of MA were studied (Figure 2). The polymerization of MA, without induction period, was observed to proceed with linear first-order kinetics, indicating the generation of a constant concentration of growing radicals during polymerization C

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Figure 3. (a) Kinetic plots for the visible light induced LRP of nBA in benzene at room temperature with the light intensity of 3 mW/cm2. (b) Changes in number-average molecular weight and Mw/Mn with conversion of nBA. Experimental conditions: [nBA]0 = 1.0 M; [nBA]0/[I]0 = 600/1; irradiated with 420−780 nm filter.

molar mass and narrow polydispersity was obtained after 17 h irradiation with the conversion of 56% (Table 1, entry 8). Reducing the light intensity to 3 mW/cm2 led to improved control over polymerization of nBA, 24 h giving well-controlled PnBA with 60% conversion (Table 1, entry 9). As shown in Figure 3a, the linear kinetic plots indicated a constant concentration of propagation radicals during photoirradiation. The rate of photopolymerization of nBA with an apparent polymerization rate of 3.8 × 10−2 h−1 was obtained at 3 mW/cm2. Evolution of the molar mass and polydispersity (Figure 3b) showed that polymerization was well controlled with a narrow polydispersity (PDI close to 1.20). The experimental values of the molar masses of the polymers were very close to their theoretical values. GPC traces gave clear shift to higher molar mass without any observable dead macroinitiator (Figure S9). All of these results demonstrated a high initiator efficiency for the visible light induced LRP of nBA mediated by organocobalt porphyrins. Polymerization of tBA has never been controlled by (TMP)CoII due to too weak Co−C bonds in the corresponding dormant species. When we conducted the photopolymerization of tBA with I at room temperature, monomer conversion reached 54% after 4 h of irradiation (Table 1, entry 14). Although the light intensity was as low as 3 mW/cm2, PtBA with relatively broad polydispersity (Mw/Mn = 1.37) was obtained. In fact, when the reaction mixture was protected from light at room temperature, 47% conversion was detected after 18 h due to lability of the Co−C bond at room temperature. However, the obtained PtBA showed a broad double-peak distribution in GPC trace (Figure 4, red line). The smaller molecular weight peak corresponded to remaining macroinitiator (Figure 4, black line), while the larger one indicated thermal polymerization under room temperature. So the poor control for photoinduced polymerization of tBA at room temperature was due to coexistence of thermal cleavage and photocleavage of the cobalt−carbon bond, which resulted in high concentration of active propagation radicals. One strategy to improve control over photoinduced polymerization of tBA is to minimize thermal cleavage. For this purpose, the photopolymerization of tBA was conducted at 0 °C. Polymerization was much slower compared with that at room temperature, and monomer conversion reached 69% after 14 h of irradiation. Thus, well-controlled PtBA with relatively narrow polydispersity (Mw/Mn = 1.25) was obtained at 0 °C, and the molecular weight was very close to theoretical value (Table 1, entry 15, and Figure 4). The scope of this versatile photopolymerization method could be further extended to water-soluble acrylate HEA simply

Figure 4. GPC traces for visible light induced polymerization of tBA under different conditions in benzene. [tBA]0 = 1.0 M, [tBA]0/[I]0 = 600/1, I = 3 mW/cm2. Black line: Mn =17 800, Mw/Mn = 1.24; red line: Mn = 54 300, Mw/Mn = 2.13; blue line: Mn = 61 100, Mw/Mn = 1.37; pink line: Mn = 67 500, Mw/Mn = 1.25.

by replacing (TMP)CoII with (TMP-OH)CoII, which is more soluble in polar solvents. (TMP-OH)Co-PDMA complex II was prepared following the reported method.17 The visible light induced polymerization of HEA was performed in methanol with II at room temperature (Table 1, entry 16). After 14 h of irradiation, the formed PHEA showed increased molar mass with relatively narrow polydispersity (Mw/Mn = 1.26). To test the capability of this system in synthesis of polymers with higher molecular weight, nBA was polymerized with different nBA/I molar ratio ranging from 300/1 to 1200/1 (Figure 5 and Figures S10−S12). Varying the molar ratio of nBA/I while keeping the amount of complex I constant, the rates of nBA consumption remained the same because a nearly identical concentration of active propagation radical was produced. Consequently, linear evolutions of the molecular weight of PnBA with monomer conversions were observed. The experimental molar masses were very close to the theoretical values, and relatively narrow polydispersity values were achieved up to about 600 DP (degrees of polymerization) (Figure 5b). These results showed that the visible light induced LRP of nBA can be controlled over a wide range of nBA/I molar ratio. The unique features of photopolymerization system include low activation energy and high initiation efficiency at or below room temperature.9,13a,17,20 The photopolymerization of nBA at 0 °C was well controlled, and the resulting PnBA with narrow polydispersity (Mw/Mn = 1.20) was obtained after 23 h of irradiation (Table 1, entry 10). D

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[macroinitiator] ratio of 600, polymerizations became uncontrolled when monomer conversion exceeded 60%, which might be resulted from the gel effect. In order to polymerize acrylates to high conversion and also investigate the efficiency of lower molecular weight macroinitiator, we synthesized low molecular weight macroinitiator (TMP)Co-PMA III (Mn = 1300, Mw/Mn = 1.17) to minimize the gel effect. Under these conditions, polyacrylates with high monomer conversion, controlled molecular weight, and narrow polydispersity were obtained (Table 2, entries 1−3). Furthermore, although we did this photo-LRP in sealed NMR tubes, it could also be conducted in a Schlenk tube and over 500 mg of PtBA with controlled molar mass could be obtained (Table 2, entry 4). Photo-LRP with a Molecular Organocobalt Porphyrin. The macroinitiators (complexes I, II, and III) were prepared from thermal polymerization with polydispersity around 1.2. The structure of the well-defined molecular organocobalt porphyrin complex seemed more attractive for photoinduced polymerization. However, porphyrin cobalt alkyl complexes are light and air sensitive, and extreme care should be paid in the synthesis and subsequent purification process. An air stable organocobalt porphyrin complex (TMP)Co-CO2CH3 IV was successfully prepared and isolated. When stored in solid state at room temperature in dark, only 4% decomposition of IV was observed after 3 months. This complex exhibited a strong CO stretching absorption at 1695 cm−1, which was similar to analogous (TPP)RhCO2C2H5 (1700 cm−1, TPP referred to tetraphenylporphyrin).22 The visible light induced polymerizations of different acrylates mediated by complex IV were performed at room temperature (Table 3, entries 1−3). PMA,

Figure 5. (a) Kinetic plots of ln([M]0/[M]t) versus irradiation time and (b) evolution of the molar mass and polydispersity with conversion for the visible light induced LRP of nBA at room temperature in benzene with different molar ratios: [I]0 = 1.67 mM; [nBA]0 = 0.5 M (for [nBA]0/[I]0 = 300/1), [nBA]0 = 1.0 M (for [nBA]0/[I]0 = 600/1), [nBA]0 = 1.5 M (for [nBA]0/[I]0 = 900/1), [nBA]0 = 2.0 M (for [nBA]0/[I]0 = 1200/1); light intensity = 3 mW/ cm2; wavelength = 420−780 nm.

Household Light, Sunlight Induced LRP. The polymerization of nBA was also performed using a commercial household compact fluorescent lamp at room temperature. PnBA with controlled molar mass and narrow polydispersity could be obtained after 11 h (Table 1, entry 11). Sunlight is a natural and clean energy. Direct use of sunlight for photoinduced LRP has only been reported for limited examples.12b,i,13h,21 In our photoinduced LRP system, sunlight could be directly used as light source without any optical filters (Table 1, entries 12 and 13). The intensity of solar irradiation during the experimental period varied between 0.8 and 10 mW/cm2 within the wavelength range of 400−1000 nm. The samples irradiated at room temperature and 0 °C produced PnBA with narrow polydispersity and the molecular weight close to theoretical value. GPC traces showed clear shift to higher molecular weight without any residual macroinitiator (Figure S13). Photo-LRP of Acrylates with High Conversion Using Lower Molecular Weight Macroinitiator. When macroinitiators (complexes I and II) were used with a [M]0/

Table 3. Visible Light Induced Polymerization of Acrylates with IV as a Mediator at Room Temperaturea entry

M

t (h)

convb (%)

Mn,thc

Mn,exd

Mw/Mnd

1 2 3 4 5 6

MA nBA tBA DMA DEA AMO

120 120 9 42 6 5

70 85 74 84 72 82

12 900 22 700 19 900 17 600 19 200 24 000

12 700 18 100 21 800 17 600 20 400 24 200

1.22 1.29 1.18 1.09 1.20 1.36

a Polymerizations were performed in benzene with a [M]0/[IV]0 ratio of 200/1. [M]0 = 1.0 M. I = 3 mW/cm2 at 420−780 nm wavelength range. bThe monomer conversion was determined based on 1H NMR spectra. cMn,th = Mw(IV) + Mw(monomer) × 200 × conv (%). dDetermined using gel permeation chromatography in DMF calibrated against poly(methyl methacrylate) standards.

PnBA, and PtBA with narrow polydispersity (Mw/Mn < 1.3) were obtained although the polymerization rate was slow. The

Table 2. Visible Light Induced LRP of Acrylates toward High Conversion Mediated by Organocobalt Porphyrins at Room Temperaturea entry

catalyst

M

I (mW/cm2)

t (h)

convb (%)

Mn,thc

Mn,exd

Mw/Mnd

1 2 3 4e

III III III III

MA nBA tBA tBA

3 3 3 3

114 65 16 12

81 77 88 80

15 200 21 000 23 800 21 800

15 500 21 500 26 200 23 100

1.19 1.15 1.20 1.22

a Polymerizations were performed in benzene with a [M]0/[III]0 ratio of 200/1. [M]0 = 1.0 M, (TMP)Co-PMA III (Mn = 1300, Mw/Mn = 1.17), I = 3 mW/cm2 at 420−780 nm wavelength range. bThe monomer conversion was determined based on 1H NMR spectra. cMn,th = Mw(III) + Mw(monomer) × 200 × conv (%). dDetermined using gel permeation chromatography in DMF calibrated against poly(methyl methacrylate) standards. e Polymerization at a Schlenk tube; 563 mg of PtBA was obtained after drying under reduced pressure.

E

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experimental molecular weight was close to theoretical value, indicating high initiator efficiency. The photo-LRP was also suitable for acrylamides including DMA, DEA, and AMO, and high monomer conversion and well controlled molecular weight were obtained (Table 3, entries 4−6). Polymer Structure Analysis. As reported previously,17 the photo-LRP mediated by organocobalt porphyrins might undergo reversible termination mechanism. The propagation process is through monomer insertion into the cobalt−carbon bond in organocobalt porphyrins. Thus, α ends of obtained polymers should be organo groups of starting cobalt complexes, and ω ends should be cobalt porphyrins. In order to investigate the polymer structure, (TMP)Co-13CO2CD3 with analogous structure to IV was synthesized under similar conditions. It was characterized by ESI-MS, 1H NMR, and 2D NMR. Before photopolymerization, the 2D NMR spectrum gave a clear signal at −3.667 ppm, corresponding to the deuterium atoms of methyl group in (TMP)Co-13CO2CD3 (Figure 6a). After

Figure 7. 13C NMR spectrum of PnBA synthesized by photo-LRP mediated by (TMP)Co-CO2CH3 (Mn,GPC = 14 700, Mw/Mn = 1.08) and (TMP)Co-13CO2CD3 (Mn,GPC = 12 500, Mw/Mn = 1.09). CDCl3 was used as the solvent.

Figure 8. Gel permeation chromatography (GPC) traces of the PnBA produced by photo-LRP mediated by (TMP)Co-13CO2CD3 (Mn,th = 11 700, Mn,GPC = 12 500, Mw/Mn = 1.09). Black line indicated the refractive index detection trace, and blue line indicated the visible light (420 nm) detection trace.

Figure 6. 2D NMR spectra of benzene solutions of (TMP)Co-13CO2CD3 and PnBA synthesized by photo-LRP mediated by (TMP)Co-13CO2CD3 (Mn = 12 500, Mw/Mn = 1.09).

detection due to series connection of equipment. The elution curves both showed a symmetrical monomodal peak, indicating that each polymer chain contained one cobalt porphyrin molecular at ω end. MALDI-TOF-MS was also used to further investigate the structure of polyacrylate produced by photo-LRP mediated by organocobalt porphyrins. Figure 9 shows the MALDI-TOF-MS spectrum of the PMA sample (Mn,GPC = 4200, Mw/Mn = 1.11) obtained with the ratio of [MA]0/[(TMP)Co-CO2CH3]0 = 200 under visible light irradiation at room temperature. The results indicated that there were two main series of peaks with regular interval of 86.04, corresponding to the monoisotopic mass of MA repeating unit. The experimental isotopic mass values of the main peak series agreed well with the theoretical values, as shown in the upper part of Figure 9. The (TMP)Co terminal group might undergo chain transfer reaction24 under the MALDI conditions due to lack of free monomers so that fragmented polymer chain was observed. Effect of Visible Light. To confirm that photolysis of the cobalt−carbon bond in organocobalt porphyrins took place under visible light irradiation, we carried out a typical radical trapping experiment to illustrate the products (Figure 10). After photolysis of the deuterated benzene solution of (TMP)Co-CO2CH3 IV and 2,2,6,6-tetramethylpiperidinyl-1oxy (TEMPO), as a radical trap, with 3 mW/cm2 visible light for 1 h at room temperature, the cleavage of the Co−C bond occurred. The resulting CH3CO2• radical was trapped by TEMPO to form TEMPO-CO2CH3 with a yield of 98% analyzed by 1H NMR spectra and ESI-MS (Figure 10, Figure

polymerization of nBA and washed with methanol, the 2D NMR spectrum of PnBA clearly showed a peak at −1.446 ppm, which corresponded to the 2D in deuterated methyl group (Figure 6b). This result indicated that the deuterated methyl group from (TMP)Co−13CO2CD3 was attached to the polymer chain ends. The structure of PnBA synthesized from photo-LRP mediated by (TMP)Co-CO2CH3 and (TMP)Co-13CO2CD3 was further investigated by 13C NMR (Figure 7). The PnBA polymers formed were highly linear as evidenced by the inability to detect the 13C NMR resonances in the range (δ(13C) = 37−40 and 47−49 ppm) which were characteristic of n-butyl acrylate branching.23 The spectra were nearly the same except for the signal at 172 ppm (Figure 7). This peak was assigned to the carbon atom in the ester group from the starting (TMP)Co-13CO2CD3. The 13C NMR and 2D NMR analyses both demonstrated that α ends of polyacrylates synthesized by photo-LRP mediated by organocobalt porphyrins were organo groups deriving from starting cobalt complexes. In order to identify the cobalt porphyrin groups in ω ends of polymers, GPC traces for PnBA produced by (TMP)Co-13CO2CD3 mediated polymerization were detected by both refractive index and visible electronic spectra (420 nm) (Figure 8). GPC analysis with the UV−vis detector set to 420 nm allowed the selective detection of polymers that had the cobalt porphyrin chromophore. GPC traces by RID and UV detector were nearly superimposable except for delay in RID F

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Figure 11. Effect of visible light during the intermittent photopolymerization of nBA in benzene mediated by organocobalt porphyrin at room temperature: [nBA]0 = 1.0 M, [nBA]0/[I]0 = 600/1.

LRP of acrylates was further demonstrated by chain extension and synthesis of novel multiblock copolymers. Sequential addition of monomers after desired conversion of certain monomer was reached gave various block copolymers. The detailed polymerization procedure is described in the Supporting Information. For example, a well-defined acrylate−acrylamide hybrid tetrablock copolymer PMA-b-PnBA-bPDMA-b-PtBA was prepared. GPC traces showed a clear shift to higher molecular weight as block copolymerization proceeded (Figure 12). The remarkably narrow, clearly

Figure 9. Matrix-assisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF-MS) of PMA (Mn,GPC = 4200, Mw/ Mn = 1.11) obtained from photo-LRP mediated by (TMP)CoCO2CH3 at room temperature.

Figure 10. 1H NMR spectra (in the range of 1.0−3.5 ppm) of benzene-d6 solution of (TMP)Co-CO2CH3 IV (1.67 mM) and TEMPO (8.35 mM) under visible light irradiation (I = 3 mW/cm2 at 420−780 nm wavelength) at room temperature for 1 h.

Figure 12. GPC traces of the macroinitiator I, chain extended PMA, diblock copolymer PMA-b-PnBA, triblock copolymer PMA-b-PnBA-bPDMA, and tetrablock copolymer PMA-b-PnBA-b-PDMA-b-PtBA. Both the chain extension and block copolymerizations were performed by visible light induced polymerization. Experimental conditions: [M]0 = 1.0 M, [M]0/[macroinitiator]0 = 600/1, I = 3 mW/cm2 at 420−780 nm wavelength range, in benzene, room temperature for polymerization of MA, nBA, and DMA, 0 °C for polymerization of tBA.

S15). Simultaneously, the complex IV disappeared with the formation of paramagnetic (TMP)CoII detected by 1H NMR spectra (Figure S14). The effect of visible light on photoinduced LRP of nBA was further examined by employing a periodic light on−off process (Figure 11). When the lamp was turned on, the polymerization “woke up” and polymer chain grew smoothly. When the lamp was turned off, polymerization stopped immediately. No monomer conversion was observed during the light-off period, and all polymer chains were reserved as organocobalt dormant species in dark. Moreover, polymerization proceeded with the same kinetic character as that observed in the former light-on process. Thus, the visible light not only controlled the initiation steps but also efficiently controlled the chain growth process by reversely activating Co−C bonds. Chain Extension and Synthesis of Novel Multiblock Copolymers. The living character of the visible light induced

monomodal, and reasonably symmetrical GPC traces of the block copolymers demonstrated the living character of this photopolymerization. A variety of multiblock copolymers could be prepared following the same procedure, such as triblock copolymer PMA-b-PnBA-b-PtBA with controlled molecular weight and narrow polydispersity (Mn = 61 500, Mw/Mn = 1.29) (Figure S16).

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CONCLUSION In summary, visible light induced LRP of hydrophobic and hydrophilic acrylates including MA, nBA, tBA, and HEA mediated by organocobalt porphyrins was reported. Various well-controlled novel multiblock copolymers containing both G

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acrylate and acrylamide segments were obtained. Visible light with 16 mW/cm2 or lower, household light, and sunlight all gave well-controlled polymerization of acrylates. The organocobalt porphyrin complexes I−IV were highly responsive to visible light which provided temporal control of polymerization just by periodic switch of light on/off. Polymerization kinetics and activation/deactivation processes of the polymerization were regulated by tuning the light intensity. Development of neat photosensitive organometallic materials, which could undergo efficiently reversible homocleavage of the metal− carbon bond, providing a unique approach to obtain photopolymerization of a wide range of monomers, is the central objective in our future research.

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ASSOCIATED CONTENT

S Supporting Information *

Emission spectra of light source and absorption spectra of organocobalt porphyrins, figure of kinetic plots with different light intensities, GPC traces of synthesized homopolymers and block copolymers, 1H NMR and ESI-MS analysis of radical trapping experiment. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (21171012 and 21322108). REFERENCES

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