Synthesis of Polyacrylonitrile by ARGET Atom Transfer Radical

Jan 8, 2014 - Synthesis of Polyacrylonitrile by ARGET Atom Transfer Radical. Polymerization in the Presence of Zinc Powder and Its Adsorption. Propert...
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Synthesis of Polyacrylonitrile by ARGET Atom Transfer Radical Polymerization in the Presence of Zinc Powder and Its Adsorption Properties after Modification Dongju Wang, Hou Chen,* Fujun Cao, and Jinming Sun School of Chemistry and Materials Science, Ludong University, Yantai 264025, China ABSTRACT: Zinc powder was used as both reducing agent and supplemental activator for atom transfer radical polymerization via activators regenerated by electron transfer (ARGET ATRP) to prepare polyacrylonitrile (PAN). Zinc powder was found to be an excellent additional agent with better controlled behavior, higher monomer conversion, and faster polymerization rate in comparison with ascorbic acid (VC). The effects of catalyst, ligand, and initiator on polymerization rate and molecular weight of PAN were investigated in detail. The resulting PAN was modified and employed as an adsorbent for removal of metal ions. Compared with Cu(II) and Pb(II), the modified PAN showed an excellent adsorption performance toward Hg(II).

1. INTRODUCTION Polyacrylonitrile (PAN) has attracted a significant attention in recent years as a precursor for high performance material.1 In general, commercially available PAN is prepared by conventional radical polymerization containing high molecular weight and broad molecular weight distribution. High molecular weight, narrow molecular weight distribution, and wellcontrolled architecture are recognized as the essential requirements for high performance PAN. These requirements could be achieved by reversible deactivated radical polymerization (RDRP).2−4 Atom transfer radical polymerization (ATRP) demonstrates its potential as one of the most versatile and robust RDRP methods for preparation of well-defined polymers ranging from blocks, stars, combs, and cyclic structures to hybrids and bioconjugates.5−10 Typically, ATRP is initiated by alkyl halides in the presence of CuI complexes with N-based ligands as catalysts and its controllability is achieved by a dynamic equilibrium between a dormant species and active propagating radicals.11,12 The presence of toxic alkyl halides, the large amounts of catalysts, and specialized equipment limited the development of normal ATRP. Further to this point, novel ATRP initiating systems such as activators generated by electron transfer ATRP (AGET ATRP),13−19 initiators for continuous activator regeneration ATRP (ICAR ATRP),20 and activators regenerated by electron transfer ATRP (ARGET ATRP)21−29 are introduced to overcome the intrinsic drawbacks. ARGET ATRP as an environmentally friendly process was developed on the base of AGET ATRP which introduced the concept of reducing air-stable deactivators (i.e., CuIIX2) to their corresponding activators.30,31 In ARGET process, the amounts of copper catalysts could be decreased by 103 times from prior levels and it can tolerate a large excess of reducing agents, such as ascorbic acid (VC) and tin(II) 2-ethylhexanoate (Sn(EH)2) which form a steady regeneration of Cu(I) from Cu(II) continuously. In order to reduce the absolute amounts of catalyst complex in ARGET ATRP, the strong inorganic reducing agent zinc (Zn) powder was examined for ARGET ATRP of acrylonitrile © 2014 American Chemical Society

(AN) in this study. Effects of reaction parameters on polymerization control and the resulting PAN were discussed. Use of Zn powder allowed starting ATRP with high oxidation state Fe(III) species and Zn powder could also be used as a supplemental activator compensating for lack of catalyst. The organic reducing agent VC was also used as a reducing agent and discussed. More detailed reduction mechanism and polymerization kinetics of the process were investigated. In recent years, the removal and recovery of metal ions from industrial wastewater have been a significant concern in most industrial branches. Consequently, effective adsorbents with strong affinities and high loading capacity for heavy metal ions were subsequently prepared by functionalizing the surface of various substrates, such as activated carbon,32 clay,33 resin,34 and so on. The existence of cyano groups on PAN made it a good promising substrate for preparation of adsorbent.

2. EXPERIMENT SECTION Materials. Analytical reagent grade acrylonitrile (AN) (98%, Tianjin Fuchen Chemical Reagents, Tianjin, China) was distilled under normal pressure and stored at 5 °C. N,N,N′,N′-Tetramethylethylenediamine (TEMED) was supplied by Aladdin Chemistry. FeCl3 was purchased from Sinopharm Chemical Reagent Co. and used without further purification. Ascorbic acid (VC) was purchased from Tianjin Ruijinte Chemical Reagents Corporation (Tianjin, China). N,N-Dimethylformamide (DMF, >99.5%,), carbon tetrachloride (CCl4, >99.5%), Zn powder, and methanol (CH3OH, >99.5%) were purchased from Tianjin Chemical Reagents and used as received. Polymerization. A representative ARGET ATRP procedure was as follows: FeCl3, AN, TEMED, Zn powder, CCl4, and DMF were added into the dry two neck flask under magnetic stirring. The reaction mixture was evacuated and Received: Revised: Accepted: Published: 1632

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backfilled with nitrogen three times. The flask was placed in a water bath at the desired reaction temperature. After a definite time, the polymerization was terminated by cooling in ice water. The resultant polymer was precipitated in a large excess of methanol−water solution (V:V = 1:1), filtered and dried at 60 °C in vacuum. Adsorbent Preparation. The resulting PAN was used as substrate to prepare the adsorbent by modification with hydroxylamine hydrochloride (NH2OH·HCl).35 A 3.0 g portion of PAN and 4.5 g NH2OH·HCl were added into a reactor loaded with 21 mL methanol and equipped with the reflux condensator. About 5 mL sodium hydroxide (NaOH, 3.4 g) aqueous solution was added dropwise to adjust the pH of the mixture to be 9.0. The reaction was stirred at 70 °C for 20 h. The modified PAN was extracted for 12 h in ethanol and dried at 60 °C. Adsorption Properties of Modified PAN for Metal Ions. Adsorption experiments were performed to determine adsorption properties of modified PAN for metal ions using batch method. A 0.01 g portion of modified PAN was shaken with 20 mL solution containing 1 mL 0.1 mol/L metal ions and 19 mL acetate buffer solution at different pH values for 24 h at 35 °C. Thereafter, a certain amount solution was taken out and put in 25 mL colorimetry-used tube, and the distilled water was put in until the whole volume was 25 mL. The concentrations of metal ions solution were determined on a GBC-932 atomic adsorption spectrophotometer. The adsorption capacity was calculated according to the following equation.

Figure 1. Monomer conversion and kinetic plots versus reaction time. The polymerization was conducted at 65 °C in DMF with [AN]0: [CCl4]0:[FeCl3]0:[TEMED]0:[Zn]0 = 200:1:0.02:0.08:0.1 and [AN]0 = 7.6 M.

plots to be 10.5 × 10−6 s−1. The linear kinetic plot (Figure 1) indicated that the number of radical was constant during the reaction. Figure 2 illustrated the plots of Mn and Mw/Mn of

Q = (C 0 − C )V / W

where Q is the adsorption amount (mmol/g); C0 and C, the initial concentration and concentration of metal ion at contact time (mmol/mL), respectively; V, the volume (mL); and W, the weight of the sample modified PAN (g). Characterization. The monomer conversion was determined gravimetrically. The number average molecular weight (Mn) and molecular weight distribution (Mw/Mn) of PAN were measured on a gel permeation chromatography (GPC) which was conducted with a Waters 1515 solvent delivery system (Milford, MA) at a flow rate of 1.0 mL/min through a combination with Waters HT3, HT4, and HT5 styragel columns. A Waters 2414 differential refractometer was used as the detector. PMMA standards were used to calibrate the columns. The analysis was undertaken at 35 °C with purified high performance liquid chromatography grade DMF as an eluent. IR spectrum (KBr pellets) was recorded on a Magna-IR 550(series II) Fourier Transform Spectrometer, Nicolet Co., USA. The concentrations of metal ions before and after adsorption were determined on a 932B model atomic adsorption spectrophotometer (AAS, GBC, Australia) equipped with air acetylene flame.

Figure 2. Mn and Mw/Mn of PAN versus monomer conversion. Polymerization was conducted at 65 °C in DMF with [AN]0:[CCl4]0: [FeCl3]0:[TEMED]0:[Zn]0 = 200:1:0.02:0.08:0.1 and [AN] = 7.6 M.

PAN versus monomer conversion. Though use of a PMMA standard may caused the departure between the determined Mn values and the absolute Mn values of PAN due to the difference of the hydrodynamic volume between PAN and PMMA, the determined Mn values could relatively illustrate the absolute Mn values to a certain extent according to Matyjaszewski3 and Liu.36 As shown in Figure 2, Mn increased linearly with monomer conversion and Mw/Mn was narrow in the range of 1.09−1.19. The appearance of an apparent nonzero intercept of Mn values may be attributed to the existence of an induction period, which results in the low concentration radical at the beginning of polymerization and needs time to establish a dynamic equilibrium between the concentration of Fe(II) and Fe(III) species.37 In order to explore the difference between Zn powder and VC, similar polymerizations were conducted with the molar ratio [AN] 0 :[CCl 4 ] 0 :[FeCl 3 ] 0 :[TEMED] 0 :[VC] 0 at 200:1:0.02:0.08:0.1 at 65 °C in DMF. As shown in Figures 3 and 4, the polymerization existed an induction period similar to the description illustrated in Figure 1. Obviously, polymerization using Zn powder as a reducing agent had a higher level of control (Mw/Mn 1.09−1.19) than that employing VC as reducing agent (Mw/Mn 1.13−1.29). It was also observed that the utilization of Zn powder resulted in a higher polymerization −6 −1 −6 −1 rate (kapp s ) than VC (kapp s ). p , 10.5 × 10 p , 9.1 × 10

3. RESULTS Polymerization of AN. ARGET ATRP of AN initiated by CCl4 and catalyzed by FeCl3/TEMED was investigated in the presence of Zn powder. The polymerization was conducted with the molar ratio [AN]0:[CCl4]0:[FeCl3]0:[TEMED]0:[Zn]0 at 200:1:0.02:0.08:0.1 at 65 °C in DMF. The concentration of monomer AN was kept at 7.6 M. The monomer conversion and ln([M]0/[M]) versus reaction time were shown in Figure 1. The monomer conversion increased along with reaction time and reached 60.7% conversion after 24 h. The apparent rate constant (kapp p ) was calculated from the slope of the kinetic 1633

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Scheme 2. Proposed Mechanism for ARGET ATRP of AN with Zinc Powder as Reducing Agent and Supplemental Activator

Figure 3. Monomer conversion and kinetic plots versus reaction time. The polymerization was conducted at 65 °C in DMF with [AN]0: [CCl4]0:[FeCl3]0:[TEMED]0:[VC]0 = 200:1:0.02:0.08:0.1 and [AN]0 = 7.6 M.

supplemental activator of Zn powder is the difference in comparison with VC as reducing agent.39,40 Effect of Catalyst on ARGET ATRP of AN. The catalyst determines the equilibrium state of atom transfer and exchange between the dormant and active species. As shown in Table 1, Table 1. Effect of Catalyst Concentration on ARGET ATRP of AN

Figure 4. Mn and Mw/Mn of PAN versus monomer conversion. The polymerization was conducted at 65 °C in DMF with [AN]:[CCl4]: [FeCl3]:[TEMED]:[VC] = 200:1:0.02:0.08:0.1 and [AN]0 = 7.6 M.

no.

[AN]0:[CCl4]0:[FeCl3]0: [TEMED]0:[Zn]0

conv (%)

Mn

Mw/ Mn

kapp p × 106s−1

1 2 3 4

200:1:0.02:0.06:0.1 200:1:0.02:0.08:0.1 200:1:0.02:0.10:0.1 200:1:0.02:0.12:0.1

17.4 60.7 50.9 39.2

13900 18100 16800 15500

1.08 1.18 1.14 1.13

2.1 10.5 8.3 5.7

entries 1−4 were designed to increase the concentration of catalyst to evaluate the appropriate amount of FeCl3 and TEMED. In order to eliminate the effect of TEMED in FeCl3 in the disproportionation process, the ratio of FeCl3/TEMED was maintained at 1:4. The monomer conversion and the apparent rate constant of polymerization (kapp p ) reached the maximal value of 60.7% and 10.5 × 10−6 s−1 with the molar ratio of [AN]0/[FeCl3]0 at 200:0.02. Effect of Ligand on ARGET ATRP of AN. Ligand is the main component of the catalyst system and plays the role of increasing solubility of catalyst and adjusting redox potential of metals. The effect of different TEMED concentrations on ARGET ATRP was investigated in DMF at 65 °C for 24 h at a ratio of [AN]0:[CCl4]0:[FeCl3]0:[Zn]0 = 200:1:0.02:0.1 and V(AN)/V(DMF) = 1:1. Table 2 presented the polymerization results of AN under different TEMED concentrations. Increasing the molar ratio of [TEMED]0/[FeCl3]0 from 3 to 6 led to a decrease in the monomer conversion and an obvious increase in Mn and Mw/Mn. These results may be attributed to excess TEMED which had more preferential binding to higher oxidation state and caused the unavoidable termination events.

The results were in good agreement with the structure and properties of Zn powder and VC. Zn powder is an effective inorganic reducing agent and increases its accessibility by combining with ligand in organic solvent DMF. In contrast, VC is a strong reducing agent, and it very quickly converts Fe(III) to Fe(II) species,38 as described in Scheme 1. The fast reduction process of Fe(III) complexes by VC diminishes their concentration to a very low level, increases concentration of radicals, and decreases polymerization control. Therefore, the limited availability of Zn powder in DMF made it achieve a significantly higher level of control in polymerization rate and molecular weight distribution. The possible mechanism of ARGET ATRP with Zn powder was described in Scheme 2. The reducing agent Zn powder was used to react with the higher oxidation state catalyst Fe(III) complex and to generate the lower oxidation state catalyst Fe(II) complex in situ. Then the polymerization proceeded via a conventional ATRP process. Zn powder also can directly react with alkyl halides to yield the propagating species in the presence of ligand. Such a dual role of reducing agent and

Scheme 1. Proposed Reduction Mechanism for FeCl3/TEMED with Zinc Powder and VC

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stable chelating compounds with many metal ions. The experiments were designed to elucidate the appropriate adsorption pH for the metal ions of Hg(II), Pb(II), and Cu(II). The value of pH not only affects the surface structure of adsorbent and the formation of metal ions but also influences the interaction between adsorbent and metal ions. As shown in Figure 6, the optimal pH values of Hg(II), Pb(II), and Cu(II)

Table 2. Effect of Ligand Concentration on ARGET ATRP of AN no.

[AN]0:[CCl4]0:[FeCl3]0: [TEMED]0:[Zn]0

conv (%)

Mn

Mw/ Mn

kapp p × 106s−1

1 2 3 4

200:1:0.02:0.06:0.1 200:1:0.02:0.08:0.1 200:1:0.02:0.10:0.1 200:1:0.02:0.12:0.1

61.2 60.7 35.4 25.8

15600 18100 19100 23700

1.09 1.18 1.36 1.40

10.9 10.5 5.0 3.5

Effect of Initiator on ARGET ATRP of AN. To explore the role of initiator in ARGET ATRP of AN, ARGET ATRP of AN with different concentrations of CCl4 was studied at 65 °C for 24 h in DMF with [AN]0:[FeCl3]0: [TEMED]0:[Zn]0 = 200:0.02:0.08:0.1 and V(AN)/V(DMF) = 1:1. Experiments (entries 1−4) were conducted by varying the molar ratio of [AN]0/[CCl4]0 from 200:0.8 to 200:1.4, and the results were presented in Table 3. As illustrated in Table 3, with increasing Table 3. Effect of Initaitor Concentration on ARGET ATRP of AN no.

[AN]0:[CCl4]0:[FeCl3]0: [TEMED]0:[Zn]0

conv (%)

1 2 3 4

200:0.8:0.02:0.08:0.1 200:1.0:0.02:0.08:0.1 200:1.2:0.02:0.08:0.1 200:1.4:0.02:0.08:0.1

44.5 60.7 63.7 68.3

Mn

Mw/ Mn

kapp p × 106s−1

18200 18100 14800 13000

1.26 1.18 1.11 1.05

6.7 10.5 11.8 13.2

Figure 6. Effect of pH on adsorption of modified PAN for Hg(II), Cu(II), and Pb(II) (35 °C; adsorbent dosage 0.01 g).

are 2, 3, and 3, respectively. Obviously, the adsorption capacity of modified PAN for Hg(II) is the highest and the adsorption capacity could reach 3.94 mmol/g. This could be attributed to high affinity of the Hg(II) ion to the amidoxime groups. According to the hard−soft acid−base (HSAB) theory,41 Hg(II) is a kind of soft ion which can form very strong bonds with groups containing nitrogen and oxygen atoms.42 Therefore, the modified PAN with amino and cyano groups showed a very high adsorption capacity for Hg(II) ions.

the concentration of CCl4, monomer conversion and the apparent rate constant of polymerization increased from 44.5% to 68.3% and 6.7 × 10−6 to 13.2 × 10−6 s−1, respectively, while the Mn and Mw/Mn had a corresponding decrease. The experimental phenomena could be attributed to the increase of propagating radical concentration with increasing in concentration of CCl4. Characterization of Modified PAN. Infrared spectra of PAN and modified PAN (AO PAN) were shown in Figure 5. The disappearance of CN groups peaks at 2244 cm−1 and the emergence of new peaks at 1642 and 929 cm−1 corresponding to the stretching vibration of C−N and N−O bonds of AO groups demonstrated that the modification had accomplished successfully. Adsorption of Metal Ions. Through transforming cyano groups to amidoxime groups, the modified PAN could form the

4. CONCLUSIONS ARGET ATRP with Zn powder as reducing agent and supplemental activator was successfully applied for the preparation of well-defined PAN with CCl4 as initiator and FeCl3/TEMED as catalyst system. Compared with VC, Zn powder can not only serve as reducing agent to constantly regenerate ATRP activator but also act as supplemental activator compensating for lack of catalyst. The resulting PAN was used as adsorbent for removal of metal ions after modification and exhibited high adsorption capabilities toward Hg(II).

Figure 5. FTIR spectrum of PAN (a) and modified PAN (b). 1635

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support by the National Natural Scientific Foundation of China (No. 20904018), the Program for New Century Excellent Talents in University (No. NCET-11-1028), the Natural Science Foundation for Distinguished Young Scholars of Shandong Province (No. JQ201203), and the Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province.

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