Iran Polym J (2015) 24:359–365 DOI 10.1007/s13726-015-0328-1

ORIGINAL PAPER

Photo‑induced single‑electron transfer living radical polymerization (SET‑LRP) of MMA in the presence of ZnO Guo‑Xiang Wang1 · Mang Lu2 · Zhao‑Hui Hou1 · En‑Xiang Liang1 · Chang‑An Yang1 · Li‑Chao Liu1 · Hu Wu1 

Received: 14 November 2014 / Accepted: 10 March 2015 / Published online: 26 March 2015 © Iran Polymer and Petrochemical Institute 2015

Abstract  In this study, the photo-induced single-electron transfer living radical polymerization (SET-LRP) of methyl methacrylate (MMA) was successfully performed in N,N-dimethylformamide (DMF) with ethyl α-bromoisobutyrate(EBiB)/Fe(0)/tetramethylethylenedia mine(TMEDA)/[ZnO] as the initiating system. ZnO was used as the inorganic photoinitiator. The living nature of the photo-induced SET-LRP of MMA was confirmed by kinetic studies in the presence or absence of air. The plot of ln([M]0/[M]) versus polymerization reaction time was linear, and the molecular weight distribution (MWD) of the resulting PMMA was narrow. The effects of the amounts of Fe(0)/TMEDA, EBiB, ZnO, and light intensity on the photo-induced SET-LRP of MMA were investigated. The conversion increased with increasing the amounts of Fe(0)/ TMEDA, EBiB, and ZnO. Increasing the light intensity resulted in a higher polymerization reaction rate. The catalyst and ZnO played important roles in the photo-induced SET-LRP of MMA. The polymerization proceeded in Electronic supplementary material  The online version of this article (doi:10.1007/s13726-015-0328-1) contains supplementary material, which is available to authorized users. * Guo‑Xiang Wang [email protected] * Zhao‑Hui Hou [email protected] Li‑Chao Liu [email protected] 1

College of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Yueyang 414006, Hunan, China

2

School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, Jiangxi, China





an uncontrollable fashion when the molar ratio of Fe(0)/ TMEDA was 0:0.1, and no reaction took placed in the absence of ZnO. In comparison with other SET-LRP of MMA in bulk or conventional solvents, the photo-induced SET-LRP of MMA proceeded periodically when the light was turned on or off under the mild experimental conditions. The polymerization did not take place when the light was turned off. However, the polymerization was very fast when the light was turned on. The chemical structure of resulting PMMA was characterized by 1HNMR. The living characteristics were demonstrated by chain extension experiment. Keywords  Irradiation · Photo-induced SET-LRP · Methyl methacrylate · Living polymerization · Kinetics

Introduction As a promising method, single-electron transfer living radical polymerization (SET-LRP) has received much scientific attention in recent years since its emergence in 2006 [1]. Compared with other living radical polymerization methods, SET-LRP has many advantages, for example, lower concentration of catalyst and ease of its removal, mild reaction temperature even in the presence of air, and ultra-fast polymerization rate. In a typical SET-LRP system, Cu(0) was employed as catalyst. Cu(0) was disproportionated to generate Cu(I)X and Cu(II)X2 in aprotic and protic solvents in the presence of an N-containing ligand. By using an activated alkyl halide or dormant polymer chain ends as an initiator, all of the chains can start growing at the same time [1]. A dynamic equilibrium between active propagating radicals and dormant chains is established. Thus, a low concentration of propagating radicals is ensured and the probability of termination is reduced. Iran Polymer and Petrochemical Institute

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SET-LRP can polymerize various vinyl monomers such as methyl acrylate [1–3], styrene [4], acrylate [1, 5] vinyl chloride [1, 6], and acrylonitrile [7]. Furthermore, some water-soluble monomers such as acrylamide [8], N,N-dimethylacrylamide [9], N-isopropylacrylamide [9] N(2-hydroxypropyl)methacrylamide [10], and oligo(ethylene oxide)methyl ether acrylate [11] can be polymerized through SET-LRP technique. Zero valent transition metals, typically based upon Cu [1], Fe [4], Tin [12], Sm [13], La [14], and Yb [15] transition metals, react with an alkyl halide to generate an alkyl radical species and a lower oxidation transition metal (e.g., Cu(I)X/L). Cu(I)X/L is disproportionated to yield zero valent transition metals [e.g., Cu(0)] and the corresponding oxidized transition metals (e.g., Cu(II)X2/L). The alkyl radical species are capable of initiating polymerization. Iron catalysts have been used in the field of polymerization due to their availability, cheapness, non-toxicity, and environment-friendly properties. Our group has made investigations on the Fe(0)-mediated SET-LRP in recent years [8, 16, 17]. Photochemistry has many advantages over heated reactions due to low temperature conditions [18], which prevent the side reaction. Furthermore, the light energy is a cheap and renewable resource. Recently, photo-induced living radical polymerizations have been applied in synthesizing vinyl polymers with predetermined molecular weight, narrow molecular weight distribution, various architectures, and useful end-functionalities [19, 20]. The major advantage of combining photochemistry with SET-LRP is ease of controlling the reaction through turning the light on and off. ZnO is a typical photoinitiator which absorbs at 335 nm and initiate the polymerization of vinyl monomer [21]. Recently, Yagci and co-workers reported the living radical polymerization with ZnO and other inorganic semiconductor as photoinitiator [22, 23]. Motivated by these advantages of SET-LRP and photochemistry, we reported photo-induced SET-LRP of methyl methacrylate (MMA) by using ZnO as the photoinitiator in this study.

Experimental Materials MMA was obtained from Tianjin Fuchen Chemical Reagent Factory, China and purified by distillation procedure prior to use. ZnO particle was purchased from Zhejiang Zhoushan Mingri Nanometer Materials Co. Ltd, China. Fe powder (75 μm, 99 %) was obtained from Shanghai Shenle Iron Wire Co. Ltd, Shanghai and used as received. EBiB Iran Polymer and Petrochemical Institute

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(99 %) was purchased from Tianjin Alfa Aesar Chemical, China and used without further purification. TMEDA was purchased from Aldrich and used without further purification. N,N-dimethylformamide (DMF) was purchased from Tianjin Tianda Chemical Reagents Factory, China and distilled under reduced pressure prior to use. Other reagents were of analytical-reagent grade and used without further purification. Photo‑induced SET‑LRP of MMA in the absence of air MMA (5 mol L−1) solution in DMF, EBiB (2.5 × 10−2  mol L−1), TMEDA (2.5 × 10−3 mol L−1), Fe powder (2.5  × 10−3 mol L−1), and ZnO (2.5 × 10−3 mol L−1) were introduced into a 50-mL three-necked round bottom flask equipped with a magnetic stirrer and degassed at room temperature for 30 min by bubbling with dry nitrogen. The flask was placed in a water bath held by a thermostat at 25 °C. Then, the mixture was irradiated with a 500-W high-pressure mercury lamp. The wavelength was in the range of 300–400 nm, and the light intensity was 150 mW/cm2. After a given time, the reactant was precipitated by large amount of methanol. After precipitation, the poly(methyl methacrylate) (PMMA) was dissolved in CHCl3, precipitated in methanol, and then dried at reduced pressure. Photo‑induced SET‑LRP of MMA in the presence of air For the oxygenated system, the predetermined amounts of Fe(0) powder, EBiB, MMA, DMF, TMEDA, and ZnO were put into a 50-mL three-necked round bottom flask equipped with a magnetic stirrer under air atmosphere. The rest process was in the same way as described above. Characterization Conversion of the monomer was determined gravimetrically. Molecular weights and molecular weight distribution (MWD) of the resulting polymers were determined by gel permeation chromatography (GPC) consisting of a Waters 2414 Differential Refractometer detector, a Waters 1515 HPLC pump, and Waters Styragel columns (HR series 1, 3, 4) with THF as the eluent at a flow rate of 1 mL/min. Polystyrene standards (1.2 × 103–2.73 × 106) with narrow molecular weight distribution were used to calibrate the columns. 1 H nuclear magnetic resonance (NMR) spectra were recorded on a Bruker 400 MHz (Germany) spectrometer in CDCl3. Tetramethylsilane (Me4Si) was used as internal standard. The number average molecular weight (Mn,th) was calculated according to Eq 1.

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Fig. 1  Kinetic plots for the photo-induced SET-LRP of MMA with ZnO as photoinitiator and Fe/TMEDA as catalyst

Mn,th = [M]0 /[EBiB]0 × Mw × Conversion,

(1)

where [M]0 and [EBiB]0 represent the initial concentrations of the monomer and EBiB, respectively, and Mw is the molecular weight of MMA.

Results and discussion Photo‑induced SET‑LRP of MMA in the presence or absence of air

Fig. 2  Evolution of molecular weight of poly(MMA) with conversion (polymerization conditions are the same as Fig. 1)

of conversion in photo-induced SET-LRP of MMA. The Mn,GPC increased in a linear fashion with monomer conversion, and the molecular weights (Mn,GPC) were in good agreement with the theoretical values determined by GPC. However, the values of MWD decreased with monomer conversion and remained low, indicating that control was quickly attained and retained. In view of the above discussion, it is confirmed that photo-induced SET-LRP of MMA was living/controlled radical polymerization in the presence or absence of air. Effect of Fe(0)/TMEDA ratio

The discovery of SET-LRP of vinyl monomers provided a new valuable tool in synthesizing materials with novel architectures [24]. However, the corresponding photoinduced SET-LRP has not been investigated. To examine the living nature of photo-induced SET-LRP of MMA, several experiments on the photo-induced SET-LRP of MMA at room temperature were performed. The molar ratio of [MMA]/[EBiB]/[Fe(0)]/[TMEDA]/[ZnO] was kept at 200/1/0.1/0.1/0.1, and the light intensity was 150 mW/cm2. Figure  1 presents the kinetics of photo-induced SETLRP of MMA conducted at ambient temperature in DMF in the presence or absence of air. Figure 1 shows that the concentration of active species remained constant during the polymerization process, indicating a good living character of the polymerization. It is noted that there was an induction period (about 30 min) in the absence of air and 50 min in the presence of air for all cases. However, the induction periods were less than that our group previously reported [25]. The apparent rate constants of polymerization (Kapp) were 5.09 × 10−5 s−1 in the absence of air and 3.79 × 10−5 s−1 in the presence of air, which were higher than that our group previously reported [25]. Figure 2 depicts the dependence of the number average molecular weight (Mn,GPC) and MWD on the percentage

To further assess the effect of Fe(0) powder on the photoinduced SET-LRP of MMA, varied amount of catalyst was introduced into the mixture. The molar ratios of [MMA]/ [EBiB]/[TMEDA]/[ZnO] were kept at 200/1/0.1/0.1, respectively, and the volume ratio of MMA/DMF was kept at 1/0.5. The light intensity was 150 mW/cm2 and the polymerization time was 8 h. The results are summarized in Table 1. As can be seen from Table 1, a higher monomer conversion was obtained and the molecular weight increased from 6600 to 17,200 with increasing Fe concentration within the same polymerization time. However, the chosen molar ratio of [Fe(0)]/[TMEDA] (0/0.1, Entry 1) did efficiently mediate the SET-LRP of MMA because a higher MWD (MWD = 1.65) was obtained. When the molar ratio of [Fe(0)]/[TMEDA] was changed from 0.02/1 to 0.5/1, the value of MWD decreased from 1.35 to 1.22, indicating that the polymerization proceeded in a controlled/living fashion. Effect of EBiB amount on SET‑LRP of MMA SET-LRP of MMA with EBiB as an efficient initiator has been reported [8, 24]. A series of experiments were Iran Polymer and Petrochemical Institute

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Table 1  Effect of Fe(0)/ TMEDA on the photo-induced SET-LRP of MMA

Table 2  Effect of amount of initiator on the photo-induced SET-LRP of MMA

Table 3  Effects of amount of ZnO on photo-induced SETLRP of MMA

Run

[Fe(0)]/[TMEDA]

Conversion (%)

Mn,th (g/mol)

Mn,GPC (g/mol)

MWD

Kapp × 105 (s−1)

1 2 3 4

0/0.1 0.02/0.1 0.05/0.1 0.1/0.1

26.31 38.44 54.53 75.59

5472 7995 11,342 15,723

6600 8200 12,100 15,800

1.65 1.35 1.28 1.23

1.06 1.68 2.74 5.09

5

0.5/0.1

78.92

16,415

17,200

1.22

5.41

Run

[EBiB]/[Fe(0)]

Conversion (%)

Mn,th (g/mol)

Mn,GPC (g/mol)

MWD

Kapp × 105 (s−1)

1 2 3 4

0.01/0.1 0.1/0.1 0.5/0.1 1/0.1

12.13 33.76 53.35 70.18

2523 7022 11,097 14,597

6500 8600 12,200 15,100

1.18 1.19 1.22 1.23

0.51 1.63 3.03 5.09

5

1.5/0.1

81.44

16,940

17,200

1.31

6.68

Run

[EBiB]/[ZnO]

Conversion (%)

Mn,th (g/mol)

Mn,GPC (g/mol)

MWD

Kapp × 105 (s−1)

1 2 3 4

1/0 1/0.01 1/0.05 1/0.1

0 10.84 36.72 70.31

0 2250 7640 14,620

0 4100 8200 15,100

0 1.20 1.19 1.23

0 0.46 1.82 4.82

5

1/0.2

80.33

16,710

17,300

1.25

6.45

conducted at room temperature in DMF with [MMA]/ [Fe(0)]/[TMEDA]/[ZnO]  = 200/0.1/0.1/0.1. The light intensity was 150 mW/cm2 and the polymerization time was 7 h. The results are summarized in Table 2. As can be seen from Table 2, the rate of polymerization increased significantly with the increase of EBiB. This was a result of more radicals being generated by the decomposition of EBiB under irradiation, which resulted in a faster polymerization rate. The monomer conversion, the polymer molecular weight, and MWD also increased with increasing the amount of EBiB. Effect of ZnO on SET‑LRP of MMA The effect of amount of ZnO on photo-induced SET-LRP of MMA was investigated. The reaction time was 7 h. The molar ratios of [MMA]/[EBiB]/[Fe(0)]/[TMEDA] were kept at 200/1/0.1/0.1, respectively, and the volume ratio of MMA/DMF was kept at 1/0.5. The light intensity was 150 mW/cm2. The results are listed in Table 3. As shown in Table 3, no reaction took place in the absence of ZnO. Increasing the amount of ZnO resulted in a higher reaction rate. However, the MWD values remained

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low. It indicated that the polymerization proceeded under a wide range of concentration conditions. Effect of the light intensity on SET‑LRP of MMA To further investigate the effect of light intensity on photoinduced SET-LRP of MMA, the polymerization was conducted with different light intensities: 100 and 200 W/cm2. The molar ratios of [MMA]/[EBiB]/[Fe(0)]/[TMEDA]/ [ZnO] were kept at 200/1/0.1/0.1/0.1, respectively, and the volume ratio of MMA/DMF was kept at 1/0.5. The results are depicted in Figs. 3 and 4. Figure 3 graphically depicts the effect of light intensity on the polymerization rate of MMA at room temperature. Increasing the intensity from 100 to 200 mW/cm2 accelerated the polymerization rate, as evidenced by the apparent rate constant: 6.34 × 10−5 s−1 for 200 mW/cm2 and 3.06 × 10−5 s−1 for 100 mW/cm2, respectively. Figure 4 is a plot of the number average molecular weight (Mn,GPC) and molecular weight distribution (MWD) versus conversion. The Mn,GPC increased with monomer conversion and raised monotonically and linearly with theoretical molecular weight (Mn,th) (Fig. 4) and close to the theoretical

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Fig. 3  Kinetic plots for the photo-induced SET-LRP of MMA at different light intensities

Fig. 4  Evolution of molecular weight and MWD of poly(MMA) with conversion under different light intensities (polymerization conditions are the same as Fig. 3)

molecular weight. The MWD remained low when the monomer conversion was beyond 20 %, indicating that the photoinduced SET-LRP of MMA proceeded in a controlled/living way. From the above experimental results, it can be concluded that the light intensity has a vital effect on the photo-induced SET-LRP of MMA. Effect of light on SET‑LRP of MMA The molar ratios of [MMA]/[EBiB]/[Fe(0)]/[TMEDA]/ [ZnO] were kept at 200/1/0.1/0.1/0.1, respectively, and the volume ratio of MMA/DMF was kept at 1/0.5. The light intensity was 150 mW/cm2 and the polymerization temperature was 25 °C. The polymerization was performed in the absence of air. The effect of light on photo-induced

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Fig. 5  Conversion of MMA versus irradiation time during periodic light-on–off process

Fig. 6  Evolution of molecular weight and MWD of poly(MMA) with conversion. (polymerization conditions are the same as Fig. 5, except the light was turned off at time intervals of 1.5–2.5, 3.5–4.5, 5.5–6.5, and 7.5–8.5 h)

SET-LRP of MMA was further studied by employing a periodic light-on–off process. The results are plotted in Fig.  5. At the beginning of polymerization, the mixture was irradiated for 1.5 h to obtain 16.9 % conversion. Then, the light was periodically turned off in times elapses of 1.5–2.5, 3.5–4.5, 5.5–6.5, and 7.5–8.5 h. It can be clearly seen from Fig. 5 that the polymerization stopped in the light-off period and no conversion was observed, indicating the negligible concentration of the active radicals. The polymerization proceeded when the light was turned on. It is noteworthy that no chain growth was observed during the repeated light on/off process, indicating that the light played an important role in photoinitiation and the chain growth process. Figure  6 shows the evolution of molecular weight and MWD of poly(MMA) with conversion, except that the light was turned off for 1.5–2.5, 3.5–4.5, 5.5–6.5, and 7.5–8.5 h

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Fig. 7  1H NMR spectrum of PMMA in CDCl3

Fig. 8  GPC trace evolution of the macroinitiator and chain-extended SET-LRP

durations. As shown in Fig. 6, the Mn,GPC increased with conversion and the MWD remained low when the light was turned on (A, C, E, G, I points in Fig. 6). However, the Mn,GPC and the MWD only slight shifted with conversion (B, D, F, H points in Fig. 6) when the light was turned off. It further confirmed that the light significantly activated radicals and thus motivated a rapid process of SET-LRP of MMA. End‑group analysis of obtained PMMA and chain extension with MMA Figure  7 shows the chemical structure of the resulting PMMA with a Mn,GPC value of 7200 g/mol and an MWD value of 1.23 characterized by 1HNMR. The PMMA was obtained with EBiB as the initiator, Fe(0)/TMEDA as the complex catalyst, and ZnO as the photoinitiator, with molar ratios of [MMA]/[EBiB]/[Fe(0)]/[TMEDA]/ [ZnO] = 200/1/0.1/0.1/0.1 and light intensity of 150 mW/ cm2. The signals at 0.75–1.23 ppm (peak “c” in Fig. 7) were attributed to the protons of methyl groups. The signals at 1.32–2.05 ppm (peak “b” in Fig. 7) corresponded to the protons of methylene groups. The signals at 3.44– 3.81 ppm (peaks “d” and “e” in Fig. 7) were attributed to the protons of methoxy groups. Among them, the chemical shift at 3.78 ppm (peak “d” in Fig. 7) contributed to the methyloxy group next to the halogen chain end, as mentioned by Sawamoto [26]. The signals at 4.20 ppm (peak “a” in Fig. 7) contributed to the protons of EBiB residue in the main chain. The obtained PMMA was further used as macroinitiator to perform chain extension experiment in DMF with the molar ratio of [MMA]/[Macroinitiator]/ [Fe(0)]/[TMEDA]/[ZnO]  = 200/1/0.1/0.1/0.1. The GPC curves of PMMA macroinitiator and after chain extension are displayed in Fig. 8. The molecular weight of chainextended PMMA was 22,500 g/mol, and the increase in the

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Scheme  1  Proposed mechanism of photo-induced SET-LRP of MMA with ZnO as photoinitiator

molecular weight was clearly demonstrated, indicating the ‘living’ features of the chain end. Mechanism analysis A proposed mechanism of photo-induced SET-LRP of MMA with ZnO as photoinitiator at 25 °C in DMF is shown in Scheme 1. In this system, ZnO produced a posi− tive hole (h+ VB) and an electron (eCB) under visible-light irradiation, and Fe(III) was reduced by e− CB to Fe(II), which was used as the activator in the SET-LRP. Then, Fe(II) complex was disproportioned in the presence of DMF in situ to produce Fe(III) complex and Fe(0). Meantime, Fe(0) reacts with EBiB to generate Fe(II)X and radical R• which initiated the living polymerization.

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Conclusion As far as we are aware, this is the first successful attempt of a photo-induced SET-LRP of MMA in DMF with ZnO as the photoinitiator, and Fe(0)/TMEDA as the complexes catalyst in the presence or absence of air. The effects of initiator, catalyst, ZnO, and light intensity on the photo-induced SET-LRP of MMA were investigated. Periodic light-on–off process led to an easy control of photo-induced SET-LRP of MMA. The obtained PMMA was used as a macroinitiator for chain extension by photo-induced SET-LRP of MMA in DMF. Acknowledgments  The authors are grateful for the financial support by Scientific Research Fund of Hunan Provincial Education Department (13A031, 12A134, and 13C364), the Science and Technology Planning Project of Hunan Province, China (Nos. 2012FJ4272), and Construct Program of the Key Discipline in Hunan Province.

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