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Synthesis of polyacrylonitrile using AGET-ATRP in emulsion
Materials Science and Engineering C 33 (2013) 570–574
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Short communication
Synthesis of polyacrylonitrile using AGET-ATRP in emulsion Jing Ma, Hou Chen ⁎, Delong Liu, Naiyi Ji, Guangxi Zong School of Chemistry and Materials Science, Ludong University, Yantai 264025, China
a r t i c l e
i n f o
Article history: Received 28 January 2012 Received in revised form 6 August 2012 Accepted 29 August 2012 Available online 6 September 2012 Keywords: Acrylonitrile Emulsion polymerization AGET-ATRP
a b s t r a c t The technique of activators generated by electron transfer for atom transfer radical polymerization (AGET-ATRP) of acrylonitrile (AN) has been first attempted in emulsion using the procedure of “one-pot”, “two-step” with polyethylene glycol monooleyl ether (Brij 35) as surfactant, cupric chloride (CuCl2) as catalyst, hexamethylenetetramine (HMTA) as ligand, carbon tetrachloride (CCl4) as initiator and ascorbic acid (VC) as reducing agent. The polymerization proceeds in controlled/living manner as indicated by first-order kinetics of the polymerization rate with respect to the monomer concentration, linear increase of the molecular weight of polyacrylonitrile (PAN) with monomer conversion and narrow polydispersity. Monomer conversion increases initially with the increase of ligand HMTA and then decreases. The ratio of [AN1] to [AN2] at 1:3 not only gives better control on the molecular weight and the molecular weight distribution, but also provides a more rapid polymerization rate. The rate of polymerization shows a trend of increase along with CCl4 content. The apparent activation energy of the polymerization is calculated to be 46.6 kJ/mol. Chain extension of PAN with AN was also carried out and the chain extended PAN with 20520 molecular weight and 1.36 polydispersity was successfully obtained. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Atom transfer radical polymerization (ATRP), an especially useful controlled/living radical polymerization, has received tremendous attention for its control on all the aspects of polymers, including controllable polymer molecular weight, narrow molecular weight distribution and desired molecular architectures [1–3]. However, normal ATRP system is sensitive to air and humidity since it employs low valent transition metal as the catalyst. In order to obtain good control on the polymerization, rigorous deoxygenation process is required. As a result, the procedure is challenging to handle [4,5]. A novel procedure was developed for activation of an oxidatively stable catalyst complex added to ATRP. Tin(II) 2-ethylhexanoate was used as a reducing agent of various Cu(II) complexes in ATRP of various monomers initiated by alkyl halides, and this technology is named activator generated by electron transfer for atom transfer radical polymerization (AGET-ATRP) [6]. Various organic reducing agents are used to continuously regenerate CuI activator in activators regenerated by electron transfer for atom transfer radical polymerization. Controlled polymer synthesis is realized with catalyst concentrations as low as 50 ppm. This technique is called activator generated by electron transfer for atom transfer radical polymerization (ARGET-ATRP) [7]. Both the reaction systems have solved the problems arising from normal ATRP.
⁎ Corresponding author. Tel.: +86 535 6697933; fax: +86 535 6695905. E-mail address: [email protected] (H. Chen). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2012.08.051
In recent years, the environment stress and economic pressure have stimulated people to research more environmentally friendly dispersed media for polymerization. Water based polymerizations have attracted most interest for their well known properties and pollution free to society. The advantages of aqueous dispersions for commercializing living/controlled radical polymerization systems provide a powerful incentive for adapting living/controlled radical polymerization to dispersed systems [8]. Emulsion polymerization has been the dominant aqueous dispersed polymerization process due to advantage of easy control and providing direct-use latex products [9]. In a typical emulsion polymerization, radicals are generated by decomposition of free radical initiators distributed in water. The radicals enter the micelles to initiate monomer polymerization. Monomer droplets function as monomer reservoirs to offer monomer for polymerizing particles consumption until monomer is depleted. However, both the polymerization rate and polymer weight are high owing to the compartmentalization of the propagating radicals, and the polymerization will lose control. ATRP in emulsion has been first attempted in 1998 [10]. ATRP starts with oil-soluble initiators and low valent transition metals as catalysts in bulk. It is difficult for monomer and radical activator/deactivator to diffuse from monomer droplets through the aqueous media to micelles. Consequently the polymerization is not well regulated due to the insufficient catalyst concentration in polymerizing particles [11]. Matyjaszewski's group has demonstrated that ATRP could be successfully carried out in a microemulsion system resulting in the preparation of stable translucent microlatexes. The polymerization of methyl methacrylate and styrene
J. Ma et al. / Materials Science and Engineering C 33 (2013) 570–574
was well controlled, resulting in small particles with narrow particle size distribution [12]. The technique of “one-pot”, “two-step” based on microemulsion has been put forward by Matyjaszewski's group to dissolve ATRP complex in emulsion. The first step of the technique is to create a microemulsion polymerization location, in which all the catalyst components are encapsulated in the nuclei before the initiation of polymerization. In the subsequent step, the additional monomer is fed after the nucleation/initiation period is completed [13]. Molecular brushes were successfully synthesized in miniemulsion systems via AGET-ATRP [14]. Polyacrylonitrile (PAN) resins are used in numerous applications because of its outstanding chemical and mechanical properties [15,16]. High molecular weight and narrow polydispersity are essential requirements for synthesis of PAN to satisfy the requirement for high performance PAN fibers. Many controlled/living radical polymerization techniques have been successfully used to prepare PAN with control over molecular weight and narrow molecular weight distribution. The well-defined, linear, high molecular weight PAN was successfully obtained via ARGET-ATRP by Matyjaszewski [17]. Reverse ATRP has been successfully applied in the synthesis of PAN with FeCl3/acetic acid as catalyst in the presence of conventional initiator azobisisobutyronitrile (AIBN) [18]. AGET-ATRP of acrylonitrile (AN) initiated by ethyl 2-bromoisobutyrate was approached using 1, 1, 4, 7, 10, 10-hexamethyl triethylenetetramine and 1, 1, 4, 7, 7-pentamethyl diethylenetriamine as both ligand and reducing agent [19]. Welldefined PAN was grafted onto the surfaces of polystyrene resins via iron (III)-mediated ARGET-ATRP. The cyano group of PAN-gpolystyrene was modified by NH2OH•HCl to yield amidoxime (AO) groups. The AO groups have been demonstrated to be an efficient Hg 2 + absorbent [20]. In this study, we investigated synthesis of PAN via AGET-ATRP using the procedure of “one-pot”, “two-step” with nonionic surfactant polyethylene glycol monooleyl ether (Brij 35) and cupric chloride (CuCl2) as catalyst, hexamethylenetetramine (HMTA) as ligand, carbon tetrachloride (CCl4) as initiator. Kinetic data of polymerization demonstrate the controlled/living character. The appropriate reaction conditions have been discussed for control of the molecular weight and molecular weight distribution by varying the concentration of HMTA, AN and CCl4.
the flask in an ice bath. A flowchart to analyze the situation of polymerization is shown in Scheme 1. An excess of methanol was poured into the resulting mixture to break the emulsion. Finally, the product was washed with hydrochloric acid (HCl), filtrated, and dried to constant weight. 2.3. Characterization The monomer conversion was measured gravimetrically. The theoretical molecular weights were calculated by the formula: Mn,th= conversion × [AN]0/[I]0 × MAN. The number average molecular weight (Mn) and the molecular weight distribution (Mw/Mn) were determined by gel permeation chromatography (GPC). GPC was performed with a Waters 1515 solvent delivery system (Milford, MA) at a flow rate of 1.0 mL/min through a combination of Waters HT2, HT3, and HT4 styragel columns. Linear poly(methyl methacrylate) standards were used to calibrate the columns. The analysis was undertaken at 45 °C with purified high performance liquid chromatography grade DMF as an eluent. A Waters 2414 different refractometer was used as the detector. 3. Results and discussion 3.1. Preparation of PAN via AGET-ATRP in emulsion AGET-ATRPs of AN in emulsion have been conducted under the conditions of [AN]:[CCl4]:[CuCl2]:[HMTA]:[VC] = 100:2:1:2:1.5 at 65 °C in the presence of air with HMTA as ligand, Brij 35 as surfactant, CCl4 as initiator, CuCl2 as catalyst and environmentally friendly VC as reducing agent. The procedure of “one-pot”, “two-step” is applied to transform microemulsion ATRP to an emulsion polymerization. As we know that the prerequisite is the efficient transport of catalyst from monomer droplets to micelles. The first step is aimed to form a stable microemulsion system with all the catalysts encapsulated in
2. Material and methods 2.1. Materials AN (Chengdu Chemical Reagents Co.) was purified under air distillation to get 76–78 °C distillation and stored in the refrigerator before use. Ascorbic acid (VC) (> 99.7%) (Tianjin Chemical Reagents Co.) was used as received. Cupric chloride (CuCl2) (> 98.5%), carbon tetrachloride (CCl4) (> 99.5%) and methanol (> 99.5%) were purchased from Tianjin Chemical Reagents Co. and used without further purification. Hexamethylenetetramine (HMTA) and Brij 35 were used as received. 2.2. Polymerization procedure CuCl2 (0.163 g, 1.216 mmol) and HMTA (0.340 g, 2.432 mmol) were stirred in AN (2.0 ml, 30.4 mmol) to form a complex system. Then CCl4 (0.235 ml, 2.432 mmol) was added to the mixture. The resulting mixture was slowly injected into a flask containing an aqueous solution of Brij 35 (3.224 g, dissolved in 30 ml of deionized water) to form a transparent microemulsion. The mixture system was stirred vigorously for about 30 min before transferring the flask to an oil bath at 65 °C. VC (0.321 g, 1.824 mmol) is dissolved into 20 ml deionized water and was added dropwise. 20 min later, the residual monomer AN (6 ml, 91.2 mmol) was added to the ongoing latex system. The reaction was stopped after 2 h by immersing
571
Scheme 1. Flowchart of AGET-ATRP of AN in emulsion.
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J. Ma et al. / Materials Science and Engineering C 33 (2013) 570–574
1.0
Table 1 Effect of HMTA on AGET-ATRP of AN in emulsion.
0.8
45
0.6 30 0.4 15
ln ([M] o /[M])
Conversion (%)
60
0
3
6
9
0.0 15
12
Time (h) Fig. 1. First-order kinetic plot for AGET-ATRP of AN in emulsion. (T = 65 °C, [AN]: [CCl4]:[CuCl2]:[HMTA]:[VC] = 100:2:1:2:1.5, [AN1]:[AN2] = 1:3).
the micelles [11], avoiding the diffusion of catalyst through the aqueous media. After the catalyst is activated and the polymerization is initiated, the second bath of monomer is added subsequently to an ongoing microemulsion ATRP. In this way, AGET-ATRP in emulsion has been successfully accomplished. The volume ratio of AN1 (monomer added in the first step) to AN2 (monomer added in the second step) is fixed at 1:3. The kinetic plot is illustrated in Fig. 1. As shown in Fig. 1, the monomer conversion increases with the reaction time and a linear increase of ln([M]0/[M]) with reaction time
Mth
Mn
Conversion (%)
Mn
PDI
kpapp × 106(s−1)
1 2 3 4 5
1:1 1:2 1:4 1:5 1:6
27.4 48.6 73.2 57.3 52.8
1100 2300 4200 2900 2600
1.26 1.23 1.29 1.32 1.39
7.4 19.0 30.5 19.7 17.4
[AN]:[CCl4]:[CuCl2]:[VC] = 100:2:1:1.5, the temperature is 65 °C, the concentration of [AN] is 2 mol/L, the reaction time is 12 h.
Δ
1600
Δ
Δ
Δ
Δ
Δ 800
0 0
15
30
45
demonstrates that the propagating radical concentration is almost constant and the termination reactions could be neglected during the whole polymerization process. The apparent rate constant (kpapp) is calculated to be 19.0 × 10 − 6 s − 1 according to the slope of the kinetic plot. Fig. 2(a) and (b) presents the dependence of molecular weight and molecular weight distribution on monomer conversion, respectively. It can be seen from Fig. 2(a) that the value of Mn increases almost linearly with monomer conversion. As shown in Fig. 2(b), the molecular weight distribution displays a higher value at the initial stage of polymerization. The value of PDI decreases along with the polymerization and PDI is ultimately kept at a low value when the conversion is from 25 to 45%. The slow rate of the primary radicals changing into dormant species was responsible for the broader polydispersity during the initial polymerization in the reaction system. 3.2. Effect of HMTA on AGET-ATRP of AN in emulsion Ligand is very important for providing the appropriate solubility and an adjustable redox potential for metal complexes [21]. To further discuss the effect of HMTA on AGET-ATRP of AN in emulsion, a series of experiments have been performed with varied HMTA content, while keeping the amount of other materials constant. The results and reaction conditions have been presented in Table 1. As expected, polymerization rate exhibits an increase with increase of the ratio of [CuCl2]:[HMTA] at first, then a decrease tendency is observed. The ratio of 1:2 gives the narrowest molecular weight distribution with PDI = 1.23. Neither lower nor higher concentration ligand HMTA will lead to better control over the polymerization. This illustrates that a large amount of HMTA not only poisons the metal catalyst, but also has a role in catalyzing the elimination of the initiator.
3200
2400
60 3.3. Effect of the concentration ratio between AN1 and AN2 on AGETATRP of AN in emulsion
Conversion (%)
b
[CuCl2]:[HMTA]
0.2
0
a
Entry
2.0 In order to examine the effect of monomer concentration on polymerization, a train of experiments are carried out with different monomer concentration. Table 2 shows the effect of concentration ratio of AN1/AN2 on AGET-ATRP of AN in emulsion. Resorting to the ratio of [AN1]:[CCl4]:[CuCl2]:[HMTA]:[VC] = 25:2:1:2:1.5 at the microemulsion stage, AGET-ATRP of AN in emulsion is conducted at 65 °C for 12 h with different concentration of the monomer [AN2] added in the second step. According to previous report, the ratio of
Δ
1.8
PDI
1.6
Δ
1.4
Δ
1.2
Δ
Δ
Δ
Table 2 Effect of the ratio of [AN1]/[AN2] on AGET-ATRP of AN in emulsion.
1.0 0
15
30
45
60
Conversion (%) Fig. 2. (a) Dependence of theoretical molecular weight and number-average molecular weight on conversion for AGET-ATRP of AN in emulsion. (T = 65 °C, [AN]:[CCl4]: [CuCl2]:[HMTA]:[VC] = 100:2:1:2:1.5, [AN1]:[AN2] = 1:3). (b) Dependence of molecular weight distribution on conversion for AGET-ATRP of AN in emulsion. (T = 65 °C, [AN]:[CCl4]:[CuCl2]:[HMTA]:[VC] = 100:2:1:2:1.5, [AN1]:[AN2] = 1:3).
Entry
[AN1]:[AN2]
Conversion (%)
Mn
PDI
1 2 3 4 5
1:1 1:2 1:3 1:5 1:7
38.1 42.5 48.6 36.9 33.2
1900 2200 2300 1800 1700
1.31 1.29 1.23 1.33 1.39
[AN1]:[CCl4]:[CuCl2]:[HMTA]:[VC] = 25:2:1:2:1.5, the temperature is 65 °C, the concentration of [AN1] is 0.5 mol/L, the reaction time is 12 h.
J. Ma et al. / Materials Science and Engineering C 33 (2013) 570–574
-10.6
Table 3 Effect of CCl4 on AGET-ATRP of AN in emulsion. [AN]:[CCl4]
Conversion (%)
Mn
PDI
100:1 100:1.5 100:2 100:2.5
34.6 44.1 48.6 53.9
1800 2200 2300 3100
1.39 1.31 1.23 1.42
Δ -10.8
[AN]:[CuCl2]:[HMTA]:[VC] = 100:1:2:1.5, the temperature is 65 °C, the concentration of [AN] is 2 mol/L, the reaction time is 12 h.
surfactant to monomer added in the first stage should be high enough to form a stable microemulsion system [9]. In this article, the mass ratio of Brij 35 to [AN1] is set as 2:1. [AN1] represents the monomer added in the microemulsion stage. As expected, an acceleration of polymerization rate is achieved with the increase of the amount of [AN2]. The monomer droplets function as a monomer reservoir. When the monomer in the micelles is depleted, the monomer stored in the monomer droplets will diffuse through the aqueous phase to the polymerization particles. With an increase of [AN2], the concentration of monomer droplets is increased and further increases the amount of monomer diffusing from droplets to micelles. For [AN1]:[AN2] = 1:3, a highest monomer conversion 48.6% is obtained and the molecular weight distribution is 1.23. When the ratio of [AN1] to [AN2] exceeds 1:3, monomer conversion exhibits a decrease. 3.4. Effect of CCl4 on AGET-ATRP of AN in emulsion To test the effect of initiator on the polymerization, we conduct numerous experiments at 65 °C with [AN]:[CuCl2]:[HMTA]:[VC] = 100:1:2:1.5 for 12 h. Table 3 represents the results of the polymerization with different concentration of CCl4. It is found that the conversion of AN increases with increasing the amount of CCl4. The AN to CCl4 ratio of 100:2 gives better control on polymer's molecular weight and its distribution. According to the ATRP mechanism, increasing the concentration of CCl4 will produce more propagating radicals. Polymerization rate is accelerated and monomer conversion is also increased. On the other hand, excess propagating radicals bring about more termination, as demonstrated by the higher molecular weight and broader molecular weight distribution.
Δ
Δ
app lnk p
Entry 1 2 3 4
573
-11.0
Δ -11.2 2.91
2.94
2.97 3
3.00
-1
1/T∗10 (K ) Fig. 3. Arrhenius plot for lnkpapp of AGET-ATRP of AN.
3.6. Chain extension To verify the controlled/living natures of this polymerization system, chain extension of AN using the above PAN (Mn = 2300, PDI = 1.23) as macroinitiator was carried out at 65 °C with the concentration of [AN] at 2 mol/L, the molar ratio of [AN]:[CCl4]:[CuCl2]: [HMTA]:[VC] = 600:2:1:2:1.5 and the molar ratio of [AN1]:[AN2] at 1:3. Fig. 4 shows GPC curves of PAN before and after a chain extension reaction. The resultant PAN-b-PAN with 20520 molecular weight and 1.36 polydispersity was obtained. 4. Conclusions AGET-ATRP of AN in emulsion has been investigated by using the procedure of “one-pot”, “two-step”. Living/controlled character has been proved by first-order kinetic plot, linear increase of molecular weight increases with the conversion, and narrow molecular weight distribution. The ratio of CuCl2 to HMTA at 1:2 gives the narrowest molecular weight distribution with PDI = 1.23. For [AN1]:[AN2] = 1:3, a highest monomer conversion 48.6% is obtained. Increasing the concentration of CCl4, monomer conversion shows a trend of increase and higher molecular weight and broader molecular weight distribution were observed. Chain extension of PAN with AN also verifies the living nature of the polymerization system.
3.5. Effect of reaction temperature on AGET-ATRP of AN in emulsion Acknowledgment To further investigate the effects of temperature on the polymerization, AGET-ATRP of AN in emulsion was conducted at the different reaction temperature. Results from these experiments are summarized in Table 4. When increasing the temperature from 60 °C to 70 °C, the conversion of AN, the molecular weight of PAN and the apparent rate constant of polymerization increased from 45.8% to 65.7%, 2100 to 3200 and 15.1 × 10 − 6 s − 1 to 24.1 × 10 − 6 s − 1, and the molecular weight distribution of PAN was widened from 1.22 to 1.41. Fig. 3 shows Arrhenius plot of the apparent rate constant versus the reaction temperature. According to the slopes of the kinetic plot, the apparent activation energy of polymerization was calculated to be 46.6 kJ/mol.
The authors are grateful for the financial support by the National Natural Scientific Foundation of China (Nos. 20904018, 51073075), the Program for New Century Excellent Talents in University (No. NCET-11-1028), the Natural Science Foundation for Distinguished
PAN-b-PAN, Mn=20520, PDI=1.36
PAN, Mn=2300, PDI=1.23
Table 4 Effect of Temperature on AGET-ATRP of AN in emulsion. Entry
Temperature(°C)
Conversion (%)
Mn
PDI
kpapp × 106(s−1)
1 2 3 4
60 65 68 70
45.8 48.6 61.2 65.7
2100 2300 2900 3200
1.22 1.23 1.35 1.41
15.1 19.0 21.8 24.1
[AN]:[CCl4]:[CuCl2]:[HMTA]:[VC] = 100:2:1:2:1.5, the concentration of [AN] is 2 mol/L, the reaction time is 12 h.
6
10
14
18
22
26
Elution time (min) Fig. 4. GPC curves of PAN before and after a chain extension.
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J. Ma et al. / Materials Science and Engineering C 33 (2013) 570–574
Young Scholars of Shandong province (No. JQ201203), and the Shandong Provincial Natural Science Foundation of China (No. ZR2010BQ007). References [1] A.W. Braunecker, K. Matyjaszewski, Prog. Polym. Sci. 32 (2007) 93. [2] V. Coessens, T. Pintauer, K. Matyjaszewski, Prog. Polym. Sci. 26 (2001) 337. [3] K. Matyjaszewski, M.J. Ziegler, S.V. Arehart, D. Greszta, T. Pakula, J. Phys. Org. Chem. 13 (2000) 775. [4] S.C. Hong, K. Matyjaszewski, Macromolecules 35 (2002) 7592. [5] Y. Shen, H. Tang, S. Ding, Prog. Polym. Sci. 29 (2004) 1053. [6] W. Jakubowski, K. Matyjaszewski, Macromolecules 38 (2005) 4139. [7] K. Matyjaszewski, W. Jakubowski, K. Min, W. Tang, J. Huang, W.A. Braunecker, N.V. Tsarevsky, Proc. Natl. Acad. Sci. U. S. A. 103 (2006) 15309. [8] M.F. Cunningham, Prog. Polym. Sci. 33 (2008) 365. [9] K. Min, H.F. Gao, K. Matyjaszewski, J. Am. Chem. Soc. 128 (2006) 10521. [10] S.G. Gaynor, J. Qiu, K. Matyjaszewski, Macromolecules 31 (1998) 5951.
[11] L. Tessier, K. Matyjaszewski, B. Charleux, Macromolecules 40 (2007) 8813. [12] K. Min, K. Matyjaszewski, Macromolecules 38 (2005) 8131. [13] K. Min, H.F. Gao, J.A. Yoon, W. Wu, T. Kowalewski, K. Matyjaszewski, Macromolecules 42 (2009) 1597. [14] K. Min, S. Yu, H. Lee, L. Mueller, S.S. Sheiko, K. Matyjaszewski, Macromolecules 40 (2007) 6557. [15] T. Kowalewski, N.V. Tsarevsky, K. Matyjaszewski, J. Am. Chem. Soc. 124 (2002) 10632. [16] J.W. Long, B. Dunn, D.R. Rolison, H.S. White, Chem. Rev. 104 (2004) 4463. [17] J. Ma, H. Chen, G.X. Zong, C.H. Wang, D.L. Liu, J. Macromol. Sci. Part A 47 (2010) 1075. [18] H. Chen, C.H. Wang, D.L. Liu, Y.T. Song, R.J. Qu, C.M. Sun, C.N. Ji, J. Polym. Sci. A: Polym. Chem. 48 (2010) 128. [19] H.C. Dong, W. Tang, K. Matyjaszewski, Macromolecules 40 (2007) 2974. [20] G.X. Zong, H. Chen, R.J. Qu, C.H. Wang, N.Y. Ji, J. Hazard. Mater. 186 (2011) 614. [21] L.J. Bai, L.F. Zhang, J. Zhu, Z.P. Cheng, X.L. Zhu, J. Polym. Sci. A: Polym. Chem. 47 (2009) 2002.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Short communication
Synthesis of polyacrylonitrile using AGET-ATRP in emulsion Jing Ma, Hou Chen ⁎, Delong Liu, Naiyi Ji, Guangxi Zong School of Chemistry and Materials Science, Ludong University, Yantai 264025, China
a r t i c l e
i n f o
Article history: Received 28 January 2012 Received in revised form 6 August 2012 Accepted 29 August 2012 Available online 6 September 2012 Keywords: Acrylonitrile Emulsion polymerization AGET-ATRP
a b s t r a c t The technique of activators generated by electron transfer for atom transfer radical polymerization (AGET-ATRP) of acrylonitrile (AN) has been first attempted in emulsion using the procedure of “one-pot”, “two-step” with polyethylene glycol monooleyl ether (Brij 35) as surfactant, cupric chloride (CuCl2) as catalyst, hexamethylenetetramine (HMTA) as ligand, carbon tetrachloride (CCl4) as initiator and ascorbic acid (VC) as reducing agent. The polymerization proceeds in controlled/living manner as indicated by first-order kinetics of the polymerization rate with respect to the monomer concentration, linear increase of the molecular weight of polyacrylonitrile (PAN) with monomer conversion and narrow polydispersity. Monomer conversion increases initially with the increase of ligand HMTA and then decreases. The ratio of [AN1] to [AN2] at 1:3 not only gives better control on the molecular weight and the molecular weight distribution, but also provides a more rapid polymerization rate. The rate of polymerization shows a trend of increase along with CCl4 content. The apparent activation energy of the polymerization is calculated to be 46.6 kJ/mol. Chain extension of PAN with AN was also carried out and the chain extended PAN with 20520 molecular weight and 1.36 polydispersity was successfully obtained. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Atom transfer radical polymerization (ATRP), an especially useful controlled/living radical polymerization, has received tremendous attention for its control on all the aspects of polymers, including controllable polymer molecular weight, narrow molecular weight distribution and desired molecular architectures [1–3]. However, normal ATRP system is sensitive to air and humidity since it employs low valent transition metal as the catalyst. In order to obtain good control on the polymerization, rigorous deoxygenation process is required. As a result, the procedure is challenging to handle [4,5]. A novel procedure was developed for activation of an oxidatively stable catalyst complex added to ATRP. Tin(II) 2-ethylhexanoate was used as a reducing agent of various Cu(II) complexes in ATRP of various monomers initiated by alkyl halides, and this technology is named activator generated by electron transfer for atom transfer radical polymerization (AGET-ATRP) [6]. Various organic reducing agents are used to continuously regenerate CuI activator in activators regenerated by electron transfer for atom transfer radical polymerization. Controlled polymer synthesis is realized with catalyst concentrations as low as 50 ppm. This technique is called activator generated by electron transfer for atom transfer radical polymerization (ARGET-ATRP) [7]. Both the reaction systems have solved the problems arising from normal ATRP.
⁎ Corresponding author. Tel.: +86 535 6697933; fax: +86 535 6695905. E-mail address: [email protected] (H. Chen). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2012.08.051
In recent years, the environment stress and economic pressure have stimulated people to research more environmentally friendly dispersed media for polymerization. Water based polymerizations have attracted most interest for their well known properties and pollution free to society. The advantages of aqueous dispersions for commercializing living/controlled radical polymerization systems provide a powerful incentive for adapting living/controlled radical polymerization to dispersed systems [8]. Emulsion polymerization has been the dominant aqueous dispersed polymerization process due to advantage of easy control and providing direct-use latex products [9]. In a typical emulsion polymerization, radicals are generated by decomposition of free radical initiators distributed in water. The radicals enter the micelles to initiate monomer polymerization. Monomer droplets function as monomer reservoirs to offer monomer for polymerizing particles consumption until monomer is depleted. However, both the polymerization rate and polymer weight are high owing to the compartmentalization of the propagating radicals, and the polymerization will lose control. ATRP in emulsion has been first attempted in 1998 [10]. ATRP starts with oil-soluble initiators and low valent transition metals as catalysts in bulk. It is difficult for monomer and radical activator/deactivator to diffuse from monomer droplets through the aqueous media to micelles. Consequently the polymerization is not well regulated due to the insufficient catalyst concentration in polymerizing particles [11]. Matyjaszewski's group has demonstrated that ATRP could be successfully carried out in a microemulsion system resulting in the preparation of stable translucent microlatexes. The polymerization of methyl methacrylate and styrene
J. Ma et al. / Materials Science and Engineering C 33 (2013) 570–574
was well controlled, resulting in small particles with narrow particle size distribution [12]. The technique of “one-pot”, “two-step” based on microemulsion has been put forward by Matyjaszewski's group to dissolve ATRP complex in emulsion. The first step of the technique is to create a microemulsion polymerization location, in which all the catalyst components are encapsulated in the nuclei before the initiation of polymerization. In the subsequent step, the additional monomer is fed after the nucleation/initiation period is completed [13]. Molecular brushes were successfully synthesized in miniemulsion systems via AGET-ATRP [14]. Polyacrylonitrile (PAN) resins are used in numerous applications because of its outstanding chemical and mechanical properties [15,16]. High molecular weight and narrow polydispersity are essential requirements for synthesis of PAN to satisfy the requirement for high performance PAN fibers. Many controlled/living radical polymerization techniques have been successfully used to prepare PAN with control over molecular weight and narrow molecular weight distribution. The well-defined, linear, high molecular weight PAN was successfully obtained via ARGET-ATRP by Matyjaszewski [17]. Reverse ATRP has been successfully applied in the synthesis of PAN with FeCl3/acetic acid as catalyst in the presence of conventional initiator azobisisobutyronitrile (AIBN) [18]. AGET-ATRP of acrylonitrile (AN) initiated by ethyl 2-bromoisobutyrate was approached using 1, 1, 4, 7, 10, 10-hexamethyl triethylenetetramine and 1, 1, 4, 7, 7-pentamethyl diethylenetriamine as both ligand and reducing agent [19]. Welldefined PAN was grafted onto the surfaces of polystyrene resins via iron (III)-mediated ARGET-ATRP. The cyano group of PAN-gpolystyrene was modified by NH2OH•HCl to yield amidoxime (AO) groups. The AO groups have been demonstrated to be an efficient Hg 2 + absorbent [20]. In this study, we investigated synthesis of PAN via AGET-ATRP using the procedure of “one-pot”, “two-step” with nonionic surfactant polyethylene glycol monooleyl ether (Brij 35) and cupric chloride (CuCl2) as catalyst, hexamethylenetetramine (HMTA) as ligand, carbon tetrachloride (CCl4) as initiator. Kinetic data of polymerization demonstrate the controlled/living character. The appropriate reaction conditions have been discussed for control of the molecular weight and molecular weight distribution by varying the concentration of HMTA, AN and CCl4.
the flask in an ice bath. A flowchart to analyze the situation of polymerization is shown in Scheme 1. An excess of methanol was poured into the resulting mixture to break the emulsion. Finally, the product was washed with hydrochloric acid (HCl), filtrated, and dried to constant weight. 2.3. Characterization The monomer conversion was measured gravimetrically. The theoretical molecular weights were calculated by the formula: Mn,th= conversion × [AN]0/[I]0 × MAN. The number average molecular weight (Mn) and the molecular weight distribution (Mw/Mn) were determined by gel permeation chromatography (GPC). GPC was performed with a Waters 1515 solvent delivery system (Milford, MA) at a flow rate of 1.0 mL/min through a combination of Waters HT2, HT3, and HT4 styragel columns. Linear poly(methyl methacrylate) standards were used to calibrate the columns. The analysis was undertaken at 45 °C with purified high performance liquid chromatography grade DMF as an eluent. A Waters 2414 different refractometer was used as the detector. 3. Results and discussion 3.1. Preparation of PAN via AGET-ATRP in emulsion AGET-ATRPs of AN in emulsion have been conducted under the conditions of [AN]:[CCl4]:[CuCl2]:[HMTA]:[VC] = 100:2:1:2:1.5 at 65 °C in the presence of air with HMTA as ligand, Brij 35 as surfactant, CCl4 as initiator, CuCl2 as catalyst and environmentally friendly VC as reducing agent. The procedure of “one-pot”, “two-step” is applied to transform microemulsion ATRP to an emulsion polymerization. As we know that the prerequisite is the efficient transport of catalyst from monomer droplets to micelles. The first step is aimed to form a stable microemulsion system with all the catalysts encapsulated in
2. Material and methods 2.1. Materials AN (Chengdu Chemical Reagents Co.) was purified under air distillation to get 76–78 °C distillation and stored in the refrigerator before use. Ascorbic acid (VC) (> 99.7%) (Tianjin Chemical Reagents Co.) was used as received. Cupric chloride (CuCl2) (> 98.5%), carbon tetrachloride (CCl4) (> 99.5%) and methanol (> 99.5%) were purchased from Tianjin Chemical Reagents Co. and used without further purification. Hexamethylenetetramine (HMTA) and Brij 35 were used as received. 2.2. Polymerization procedure CuCl2 (0.163 g, 1.216 mmol) and HMTA (0.340 g, 2.432 mmol) were stirred in AN (2.0 ml, 30.4 mmol) to form a complex system. Then CCl4 (0.235 ml, 2.432 mmol) was added to the mixture. The resulting mixture was slowly injected into a flask containing an aqueous solution of Brij 35 (3.224 g, dissolved in 30 ml of deionized water) to form a transparent microemulsion. The mixture system was stirred vigorously for about 30 min before transferring the flask to an oil bath at 65 °C. VC (0.321 g, 1.824 mmol) is dissolved into 20 ml deionized water and was added dropwise. 20 min later, the residual monomer AN (6 ml, 91.2 mmol) was added to the ongoing latex system. The reaction was stopped after 2 h by immersing
571
Scheme 1. Flowchart of AGET-ATRP of AN in emulsion.
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1.0
Table 1 Effect of HMTA on AGET-ATRP of AN in emulsion.
0.8
45
0.6 30 0.4 15
ln ([M] o /[M])
Conversion (%)
60
0
3
6
9
0.0 15
12
Time (h) Fig. 1. First-order kinetic plot for AGET-ATRP of AN in emulsion. (T = 65 °C, [AN]: [CCl4]:[CuCl2]:[HMTA]:[VC] = 100:2:1:2:1.5, [AN1]:[AN2] = 1:3).
the micelles [11], avoiding the diffusion of catalyst through the aqueous media. After the catalyst is activated and the polymerization is initiated, the second bath of monomer is added subsequently to an ongoing microemulsion ATRP. In this way, AGET-ATRP in emulsion has been successfully accomplished. The volume ratio of AN1 (monomer added in the first step) to AN2 (monomer added in the second step) is fixed at 1:3. The kinetic plot is illustrated in Fig. 1. As shown in Fig. 1, the monomer conversion increases with the reaction time and a linear increase of ln([M]0/[M]) with reaction time
Mth
Mn
Conversion (%)
Mn
PDI
kpapp × 106(s−1)
1 2 3 4 5
1:1 1:2 1:4 1:5 1:6
27.4 48.6 73.2 57.3 52.8
1100 2300 4200 2900 2600
1.26 1.23 1.29 1.32 1.39
7.4 19.0 30.5 19.7 17.4
[AN]:[CCl4]:[CuCl2]:[VC] = 100:2:1:1.5, the temperature is 65 °C, the concentration of [AN] is 2 mol/L, the reaction time is 12 h.
Δ
1600
Δ
Δ
Δ
Δ
Δ 800
0 0
15
30
45
demonstrates that the propagating radical concentration is almost constant and the termination reactions could be neglected during the whole polymerization process. The apparent rate constant (kpapp) is calculated to be 19.0 × 10 − 6 s − 1 according to the slope of the kinetic plot. Fig. 2(a) and (b) presents the dependence of molecular weight and molecular weight distribution on monomer conversion, respectively. It can be seen from Fig. 2(a) that the value of Mn increases almost linearly with monomer conversion. As shown in Fig. 2(b), the molecular weight distribution displays a higher value at the initial stage of polymerization. The value of PDI decreases along with the polymerization and PDI is ultimately kept at a low value when the conversion is from 25 to 45%. The slow rate of the primary radicals changing into dormant species was responsible for the broader polydispersity during the initial polymerization in the reaction system. 3.2. Effect of HMTA on AGET-ATRP of AN in emulsion Ligand is very important for providing the appropriate solubility and an adjustable redox potential for metal complexes [21]. To further discuss the effect of HMTA on AGET-ATRP of AN in emulsion, a series of experiments have been performed with varied HMTA content, while keeping the amount of other materials constant. The results and reaction conditions have been presented in Table 1. As expected, polymerization rate exhibits an increase with increase of the ratio of [CuCl2]:[HMTA] at first, then a decrease tendency is observed. The ratio of 1:2 gives the narrowest molecular weight distribution with PDI = 1.23. Neither lower nor higher concentration ligand HMTA will lead to better control over the polymerization. This illustrates that a large amount of HMTA not only poisons the metal catalyst, but also has a role in catalyzing the elimination of the initiator.
3200
2400
60 3.3. Effect of the concentration ratio between AN1 and AN2 on AGETATRP of AN in emulsion
Conversion (%)
b
[CuCl2]:[HMTA]
0.2
0
a
Entry
2.0 In order to examine the effect of monomer concentration on polymerization, a train of experiments are carried out with different monomer concentration. Table 2 shows the effect of concentration ratio of AN1/AN2 on AGET-ATRP of AN in emulsion. Resorting to the ratio of [AN1]:[CCl4]:[CuCl2]:[HMTA]:[VC] = 25:2:1:2:1.5 at the microemulsion stage, AGET-ATRP of AN in emulsion is conducted at 65 °C for 12 h with different concentration of the monomer [AN2] added in the second step. According to previous report, the ratio of
Δ
1.8
PDI
1.6
Δ
1.4
Δ
1.2
Δ
Δ
Δ
Table 2 Effect of the ratio of [AN1]/[AN2] on AGET-ATRP of AN in emulsion.
1.0 0
15
30
45
60
Conversion (%) Fig. 2. (a) Dependence of theoretical molecular weight and number-average molecular weight on conversion for AGET-ATRP of AN in emulsion. (T = 65 °C, [AN]:[CCl4]: [CuCl2]:[HMTA]:[VC] = 100:2:1:2:1.5, [AN1]:[AN2] = 1:3). (b) Dependence of molecular weight distribution on conversion for AGET-ATRP of AN in emulsion. (T = 65 °C, [AN]:[CCl4]:[CuCl2]:[HMTA]:[VC] = 100:2:1:2:1.5, [AN1]:[AN2] = 1:3).
Entry
[AN1]:[AN2]
Conversion (%)
Mn
PDI
1 2 3 4 5
1:1 1:2 1:3 1:5 1:7
38.1 42.5 48.6 36.9 33.2
1900 2200 2300 1800 1700
1.31 1.29 1.23 1.33 1.39
[AN1]:[CCl4]:[CuCl2]:[HMTA]:[VC] = 25:2:1:2:1.5, the temperature is 65 °C, the concentration of [AN1] is 0.5 mol/L, the reaction time is 12 h.
J. Ma et al. / Materials Science and Engineering C 33 (2013) 570–574
-10.6
Table 3 Effect of CCl4 on AGET-ATRP of AN in emulsion. [AN]:[CCl4]
Conversion (%)
Mn
PDI
100:1 100:1.5 100:2 100:2.5
34.6 44.1 48.6 53.9
1800 2200 2300 3100
1.39 1.31 1.23 1.42
Δ -10.8
[AN]:[CuCl2]:[HMTA]:[VC] = 100:1:2:1.5, the temperature is 65 °C, the concentration of [AN] is 2 mol/L, the reaction time is 12 h.
surfactant to monomer added in the first stage should be high enough to form a stable microemulsion system [9]. In this article, the mass ratio of Brij 35 to [AN1] is set as 2:1. [AN1] represents the monomer added in the microemulsion stage. As expected, an acceleration of polymerization rate is achieved with the increase of the amount of [AN2]. The monomer droplets function as a monomer reservoir. When the monomer in the micelles is depleted, the monomer stored in the monomer droplets will diffuse through the aqueous phase to the polymerization particles. With an increase of [AN2], the concentration of monomer droplets is increased and further increases the amount of monomer diffusing from droplets to micelles. For [AN1]:[AN2] = 1:3, a highest monomer conversion 48.6% is obtained and the molecular weight distribution is 1.23. When the ratio of [AN1] to [AN2] exceeds 1:3, monomer conversion exhibits a decrease. 3.4. Effect of CCl4 on AGET-ATRP of AN in emulsion To test the effect of initiator on the polymerization, we conduct numerous experiments at 65 °C with [AN]:[CuCl2]:[HMTA]:[VC] = 100:1:2:1.5 for 12 h. Table 3 represents the results of the polymerization with different concentration of CCl4. It is found that the conversion of AN increases with increasing the amount of CCl4. The AN to CCl4 ratio of 100:2 gives better control on polymer's molecular weight and its distribution. According to the ATRP mechanism, increasing the concentration of CCl4 will produce more propagating radicals. Polymerization rate is accelerated and monomer conversion is also increased. On the other hand, excess propagating radicals bring about more termination, as demonstrated by the higher molecular weight and broader molecular weight distribution.
Δ
Δ
app lnk p
Entry 1 2 3 4
573
-11.0
Δ -11.2 2.91
2.94
2.97 3
3.00
-1
1/T∗10 (K ) Fig. 3. Arrhenius plot for lnkpapp of AGET-ATRP of AN.
3.6. Chain extension To verify the controlled/living natures of this polymerization system, chain extension of AN using the above PAN (Mn = 2300, PDI = 1.23) as macroinitiator was carried out at 65 °C with the concentration of [AN] at 2 mol/L, the molar ratio of [AN]:[CCl4]:[CuCl2]: [HMTA]:[VC] = 600:2:1:2:1.5 and the molar ratio of [AN1]:[AN2] at 1:3. Fig. 4 shows GPC curves of PAN before and after a chain extension reaction. The resultant PAN-b-PAN with 20520 molecular weight and 1.36 polydispersity was obtained. 4. Conclusions AGET-ATRP of AN in emulsion has been investigated by using the procedure of “one-pot”, “two-step”. Living/controlled character has been proved by first-order kinetic plot, linear increase of molecular weight increases with the conversion, and narrow molecular weight distribution. The ratio of CuCl2 to HMTA at 1:2 gives the narrowest molecular weight distribution with PDI = 1.23. For [AN1]:[AN2] = 1:3, a highest monomer conversion 48.6% is obtained. Increasing the concentration of CCl4, monomer conversion shows a trend of increase and higher molecular weight and broader molecular weight distribution were observed. Chain extension of PAN with AN also verifies the living nature of the polymerization system.
3.5. Effect of reaction temperature on AGET-ATRP of AN in emulsion Acknowledgment To further investigate the effects of temperature on the polymerization, AGET-ATRP of AN in emulsion was conducted at the different reaction temperature. Results from these experiments are summarized in Table 4. When increasing the temperature from 60 °C to 70 °C, the conversion of AN, the molecular weight of PAN and the apparent rate constant of polymerization increased from 45.8% to 65.7%, 2100 to 3200 and 15.1 × 10 − 6 s − 1 to 24.1 × 10 − 6 s − 1, and the molecular weight distribution of PAN was widened from 1.22 to 1.41. Fig. 3 shows Arrhenius plot of the apparent rate constant versus the reaction temperature. According to the slopes of the kinetic plot, the apparent activation energy of polymerization was calculated to be 46.6 kJ/mol.
The authors are grateful for the financial support by the National Natural Scientific Foundation of China (Nos. 20904018, 51073075), the Program for New Century Excellent Talents in University (No. NCET-11-1028), the Natural Science Foundation for Distinguished
PAN-b-PAN, Mn=20520, PDI=1.36
PAN, Mn=2300, PDI=1.23
Table 4 Effect of Temperature on AGET-ATRP of AN in emulsion. Entry
Temperature(°C)
Conversion (%)
Mn
PDI
kpapp × 106(s−1)
1 2 3 4
60 65 68 70
45.8 48.6 61.2 65.7
2100 2300 2900 3200
1.22 1.23 1.35 1.41
15.1 19.0 21.8 24.1
[AN]:[CCl4]:[CuCl2]:[HMTA]:[VC] = 100:2:1:2:1.5, the concentration of [AN] is 2 mol/L, the reaction time is 12 h.
6
10
14
18
22
26
Elution time (min) Fig. 4. GPC curves of PAN before and after a chain extension.
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Young Scholars of Shandong province (No. JQ201203), and the Shandong Provincial Natural Science Foundation of China (No. ZR2010BQ007). References [1] A.W. Braunecker, K. Matyjaszewski, Prog. Polym. Sci. 32 (2007) 93. [2] V. Coessens, T. Pintauer, K. Matyjaszewski, Prog. Polym. Sci. 26 (2001) 337. [3] K. Matyjaszewski, M.J. Ziegler, S.V. Arehart, D. Greszta, T. Pakula, J. Phys. Org. Chem. 13 (2000) 775. [4] S.C. Hong, K. Matyjaszewski, Macromolecules 35 (2002) 7592. [5] Y. Shen, H. Tang, S. Ding, Prog. Polym. Sci. 29 (2004) 1053. [6] W. Jakubowski, K. Matyjaszewski, Macromolecules 38 (2005) 4139. [7] K. Matyjaszewski, W. Jakubowski, K. Min, W. Tang, J. Huang, W.A. Braunecker, N.V. Tsarevsky, Proc. Natl. Acad. Sci. U. S. A. 103 (2006) 15309. [8] M.F. Cunningham, Prog. Polym. Sci. 33 (2008) 365. [9] K. Min, H.F. Gao, K. Matyjaszewski, J. Am. Chem. Soc. 128 (2006) 10521. [10] S.G. Gaynor, J. Qiu, K. Matyjaszewski, Macromolecules 31 (1998) 5951.
[11] L. Tessier, K. Matyjaszewski, B. Charleux, Macromolecules 40 (2007) 8813. [12] K. Min, K. Matyjaszewski, Macromolecules 38 (2005) 8131. [13] K. Min, H.F. Gao, J.A. Yoon, W. Wu, T. Kowalewski, K. Matyjaszewski, Macromolecules 42 (2009) 1597. [14] K. Min, S. Yu, H. Lee, L. Mueller, S.S. Sheiko, K. Matyjaszewski, Macromolecules 40 (2007) 6557. [15] T. Kowalewski, N.V. Tsarevsky, K. Matyjaszewski, J. Am. Chem. Soc. 124 (2002) 10632. [16] J.W. Long, B. Dunn, D.R. Rolison, H.S. White, Chem. Rev. 104 (2004) 4463. [17] J. Ma, H. Chen, G.X. Zong, C.H. Wang, D.L. Liu, J. Macromol. Sci. Part A 47 (2010) 1075. [18] H. Chen, C.H. Wang, D.L. Liu, Y.T. Song, R.J. Qu, C.M. Sun, C.N. Ji, J. Polym. Sci. A: Polym. Chem. 48 (2010) 128. [19] H.C. Dong, W. Tang, K. Matyjaszewski, Macromolecules 40 (2007) 2974. [20] G.X. Zong, H. Chen, R.J. Qu, C.H. Wang, N.Y. Ji, J. Hazard. Mater. 186 (2011) 614. [21] L.J. Bai, L.F. Zhang, J. Zhu, Z.P. Cheng, X.L. Zhu, J. Polym. Sci. A: Polym. Chem. 47 (2009) 2002.