ARGET ATRP of copolymerization of styrene and acrylonitrile with environmentally friendly catalyst and ligand

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1 e-polymers 2011, no ISSN ARGET ATRP of copolymerization of styrene and acrylonitrile with environmentally friendly catalyst and ligand Guoxiang Wang, 1* Mang Lu 2 1* College of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Yueyang , Hunan Province, China; wanggxwzl@163.com 2 School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen , Jiangxi Province, China. (Received: 25 April, 2011; published: 10 July, 2012) Introduction Abstract: The atom transfer radical polymerization of styrene and acrylonitrile using activators regenerated electron transfer (ARGET) has been carried out in N,N-dimethylformamide (DMF) at 90 C in the presence of air with a catalyst system of FeCl 3 6H 2 O/succinic acid using 2-bromoisobutyrate as the initiator and ascorbic acid as the reducing agent. The ARGET ATRP of styrene and acrylonitrile was proved a living /controlled polymerization such as a linear increase in molecular weights of polymers with monomer conversion and relatively narrow polydispersities (<1.25) when the conversion was beyond 20%. The chain extension experiment was carried out to confirm the controlled manner of the polymerization system. The resultant SAN was characterized by nuclear magnetic resonance and gel permeation chromatography. Styrene-co-acrylonitrile (SAN) copolymers are widely used due to their superior optical transparency, thermoplasticity and easy processability [1]. The SAN copolymers can be further enhanced through an additional control of molecular weights and polydispersities. There are numerous literatures about radical copolymerization of styrene and acrylonitrile, such as emulsion [2], solution [3], bulk [4], etc. The SAN copolymers were obtained with broad molecular weight distribution. Controlled/living radical polymerizations can be used to synthesize polymers with narrow molecular weight distribution and have been applied to copolymerization of styrene and acrylonitrile, such as nitroxide-mediated radical polymerization [5], atom transfer radical polymerization (ATRP) [6] and reversible addition-fragmentation chain transfer polymerization [7]. Atom transfer radical polymerization (ATRP) is one of the most effective techniques in all of the controlled/living radical polymerization. It is based on a dynamic equilibrium between the active propagating radicals and the dormant species. The equilibrium relies on the reversible transition metal catalyzed homolytic cleavage of a covalent carbon-halogen in the dormant species and an accompanying one electron oxidation of the transition metal. However, there are some drawbacks in the ATRP process, such as large amounts of catalyst and the toxicity of alkyl halide. In order to overcome these drawbacks, more and more attention has been paid to reducing the concentration of catalyst. Therefore, some improved ATRP techniques have been 1

2 developed, such as initiators for continuous activator regeneration (ICAR) ATRP [8-11] and activators regenerated by electron transfer (ARGET) ATRP [12-15]. ATRP is a metal complex-mediated reaction, and catalyst removal is of primary importance. In a typical ARGET ATRP system, a tiny amount of catalyst is used with an excess of reducing agent. Therefore, the removal of residual catalyst may not be necessary for some application. Transition metal copper is one of the most commonly used transition metals for an ATRP catalyst. Matyjaszewski et al. reported on the high molecular weights SAN copolymers using copper-based ARGET ATRP. The obtained copolymers displayed controlled high molecular weights (up to ) with low polydispersiity (<1.3) [16]. To the best knowledge of authors, only copper-based ARGET ATRP has been reported. In view of the known toxicity of copper compounds, a more environmental friendly iron-based catalyst was reported by Sawamoto [17] and Matyjaszewski [18]. Organic acids, which are inexpensive and environmentally friendly, can easily complex with iron. The complexes of iron and organic acids have been widely applied to controlled/living radical polymerization [19-25]. Until now, there is no report on the iron-mediated ARGET ATRP of styrene and acrylonitrile. In this work, the controlled/living radical polymerization of styrene and acrylonitrile was performed, catalyzed by FeCl 3 6H 2 O/succinic acid in N,N-dimethylformamide (DMF) with 2- bromoisobutyrate (EBriB) as initiator and ascorbic acid (VC) as reducing agent. The living characteristics were confirmed by chain extension. Results and discussion ARGET ATRP of styrene and acrylonitrile in the presence of air In this work, the copolymerization of styrene and acrylonitrile was investigated in the presence of air. The polymerization was performed at 90 C in DMF, and the molar ratio of [M] 0 /[EBriB]/[FeCl 3 6H 2 O]/[SA]/[VC] was fixed to 1000:1:0.05:0.1:0.1. The semilogarithmic plot of ln([m] 0 /[M]) versus the polymerization time is shown in Figure 1. Fig. 1. Kinetic plots of the solution copolymerization of St/AN in DMF at 90 C. [St+AN] 0 /[EBriB] 0 /[FeCl 3 6H 2 O]/[SA]/[VC]=1000:1:0.05:0.1:0.1. 2

3 As shown in Figure 1, the semilogarithmic plot of ln([m] 0 /[M]) verse reaction time is nearly linear, suggesting that the chain growth from the macroinitiator was almost consistent with a controlled or living process. The radical polymerization rate can be expressed by the following equation: -d[m]/dt=k p [P ][M] (1) By integration of eq. (1), the kinetic equation was obtained as: ln([ M ] /[ M ]) k t (2) 0 app p The value of slope is equal to the value of the apparent rate constant (k p app ). Therefore, the apparent rate constant of polymerization was obtained as s - 1. Fig. 2. Dependence of the molecular weights and molecular weight distributions on monomer conversion for St/AN copolymerization in DMF at 90 C. [St+AN] 0 /[EBriB] 0 /[FeCl 3 6H 2 O]/[SA]/[VC]=1000:1:0.05:0.1:0.1. Figure 2 shows the dependence of the number-average molecular weights on monomer conversion for St/AN copolymerization in DMF at 90 C. It can be seen From Figure 2 that the values of M n,gpc increased linearly with conversion, and the number-average molecular weight increased from to when the conversion of monomer increased from 10% to 53%. The M n,gpc s were slightly higher than their corresponding theoretical molecular weights, especially in higher monomer conversions, indicating the relatively low initiator efficiency. The polydispersity index (PDI) as a function of monomer conversion of all copolymers of SAN is depicted in Figure 2. The values of PDI of SAN decreased from 1.33 to The values of PDI were less than 1.25 when the conversion was beyond 20%, indicating a well-controlled polymerization process. The effect of the concentration of FeCl 3 6H 2 O on ARGET ATRP of St and AN The effect of FeCl 3 6H 2 O on the ARGET ATRP of St/AN was further investigated, and the results are shown in Figure 3. As shown in Figure 3, the linearity of ln([m] 0 /[M]) versus reaction time was observed at different concentrations of Fe. It indicated that the concentrations of growing radical remained constant during the 3

4 polymerization process. The conversion increases with increasing the concentration of Fe for a given polymerization time. Fig. 3. Kinetic plots of the solution copolymerization of St/AN in DMF at 90 C. [St+AN] 0 /[EBriB] 0 /[SA]/[VC]=1000:1:0.1:0.1. The concentration of Fe is 20 ppm, 50 ppm and 100 ppm, respectively. Based on Figure 3, the apparent rate constants of copolymerization of app k p were s -1, s -1, s -1, corresponding to the concentration of Fe of 20 ppm, 50 ppm and 100 ppm, respectively. This suggested that the concentration of Fe has an important role in the polymerization. Fig. 4. Dependence of the molecular weights and polydispersities on monomer conversion for St/AN copolymerization in DMF at 90 C. The concentrations of Fe=20 ppm, 50 ppm, 100 ppm, respectively. [St+AN] 0 /[EBriB] 0 /[SA]/[VC]=1000:1:0.1:0.1. The dependence of the molecular weights and molecular weight distributions on monomer conversion for St/AN copolymerization are shown in Figure 4. As shown, the number-average molecular weight linearly increased with monomer conversion and is somewhat higher than the theoretical values. The values of PDI were a little broad at the beginning and decreased with increasing conversion. The polymerization reactions were carried out with 50 ppm and 100 ppm of Fe, respectively. The values of PDI were less than 1.25 when the conversion was 4

5 beyond 20%. However, with 20 ppm of Fe, the values of PDI were less than 1.25 when the conversion was beyond 30%. The advantage of low amounts of catalyst lies in the reduced side reactions, leading to well-defined SAN. On the other hand, the metal residue was reduced in the products. The effect of the concentration of VC on ARGET ATRP of St and AN In a typical ARGET ATRP system, the Fe (III) complexes were constantly reduced by the reducing agent VC to produce the active Fe (II) complexes. Therefore, the reducing agent VC plays an important role in the ARGET ATRP process [26]. The reactions were performed at different concentrations of VC at 90 C in DMF with 50 ppm of Fe. The results are listed in Table 1. The conversion increased from 32% to 52% and the number-average molecular weight increased from to as the amount of VC increased. After 50-h reaction, The conversion was 32% with the ratio of [FeCl 3 6H 2 O]/[VC] at 0.05/0.04 and 52% with the ratio of [FeCl 3 6H 2 O]/[VC] at 0.05/0.5. However, the polymerization remained the characteristics of living/controlled. The number-average molecular weights of the obtained polymers were higher to theoretical values. However, the M w /M n values were also low (PDI<1.2). It indicated that this was a controlled/living radical polymerization. Tab. 1. Effects of the concentration of VC on ARGET ATRP of styrene and acrylonitrile in the presence of air at 90 C in DMF. a [M] Entry 0 /[EBriB]/[FeCl 3 6H 2O]/[SA]/[VC] :1:0.05:0.1:0.04 [a][m] 0 =5.0 M. Chain Extension of SAN Fe ppm Time (min) Conversion /% :1:0.05:0.1: :1:0.05:0.1: To confirm the existence of living chain ends, chain extensions of the obtained chlorine-terminated SAN were performed. The obtained SAN (M n =66800, PDI=1.19) was used macroinitiator; the concentration of monomer was 5.0 mol L -1 ; the molar ratio was [St+AN] 0 /[EBriB] 0 /[FeCl 3 6H 2 O]/[SA]/[VC]=1000:1:0.05:0.1:0.1. In brief, 6.65 g SAN sample was firstly dissolved in 20 ml DMF, and then styrene (10.4 g, 0.1 mol), FeCl 3 ( mol), SA ( mol), and VC ( mol) were added. After 48-h polymerization, the polymerization was terminated. The obtained polymer was further purified by washing with water and methanol. The dried polymer was characterized by 1 H-NMR and analyzed with GPC. The 1 H NMR spectrum of SAN is shown in Figure 5. As shown, the chemical shift δ= ppm corresponded to the phenyl (-C 6 H 5 ) protons of styrene. The chemical shift δ= were attributed to the CH 2 and CH, the chemical shift δ=5.3 ppm was CHCl proton. The 1 H NMR spectra of SAN verified the chloro atom was at the end of polymer chain. Figure 6 shows the GPC result. As can be seen, the M n increased from to Meanwhile, the PDI also increased from 1.19 to 1.34.The increase of M n suggested the dormant sites M th M n PDI :1:0.05:0.1:

6 at the SAN chain ends have allowed reactivation during the subsequent polymerization process. Fig H NMR spectrum of SAN. Fig. 6. GPC curves of before and after a chain extension reaction. Conclusions In this study, ARGET ATRP of St/AN was successfully performed at 90 C using VC as reducing agent, EBriB as initiator and FeCl 3 6H 2 O/SA as catalyst. The kinetics experimental results showed that copolymerization of St/AN was a living /controlled process. The values of PDI were broader at low conversion and almost kept constant at high conversion. The prepared SAN copolymer possessed a chlorine-terminated atom, which could be reactivated during the chain extension reaction process. Experimental Materials Styrene (St, Shanghai Chemical Reagents Co., AR grade), acrylonitrile (AN, Shanghai Chemical Reagents Co., AR grade) were distilled under reduced pressure prior to use. 2-Bromoisobutyrate (EBriB, Tianjin Alfa Aesar chemical Co., Ltd., 99%) was used without further purification. N,N-Dimethylformamide (DMF) was distilled at reduced pressure before use. Ferric chloride hexahydrate (FeCl 3 6H 2 O), succinic acid (SA), ascorbic acid (VC), and other regents were used as received. 6

7 Polymerization In a typical experiment, g of AN (0.0375mol), 6.5 g of St ( mol), g of EBriB mol) and 20 ml of DMF were placed in a three-necked round bottom flask, and then g of FeCl 3 6H 2 O ( mol), g of succinic acid ( mol), and g of VC ( mol) were added in the presence of air. Then the flask was placed in a thermostated oil bath, and the temperature was controlled at 90 C. After the desired polymerization time, the flask was cooled by immersing it into iced water. The reactant was pored into a large amount of methanol for precipitation. The obtained SAN copolymer was dried at 60 C in vacuo for 24 h. Monomer conversion was determined by gravimetry. The M n(th) of SAN can be calculated by the following equation: M n(th) =([St/AN] 0 /[I]) W St/AN x. Where, [St/AN] 0 is the initial concentration of St/AN, [I] is the concentration of EBriB and W St/AN is the molecular weight of St and AN, x is the monomer conversion. Characterization The molecular weight and molecular weight distribution of the polymer were determined with a Waters 1515 gel permeation chromatography (GPC) equipped with refractive index detector, using HR1, HR3, and HR4 column with molecular weight range ,000 calibrated with polystyrene standard sample. Polystyrene standards were used to calibrate the columns. THF was used as a mobile phase at a flow rate of 1.0 ml/min and with column temperature of 30 C. NMR spectrum were recorded on a Bruker 400MHz Spectrometer instrument using d 6 -DMSO as the solvent References [1] Liu, D.; Padias, A. B.; Hall, H. K. Jr. Macromolecules, 1995, 28, 622. [2] Shi, Y.; Wu, Y.; Hao,J.; Li, G. J Polym Sci Part A: Polym Chem. 2005, 43, 203. [3] Janovi, Z.; Tomaek, Lj.; Malavai, T. J. Macromol. Sci., Pure Appl. Chem. 1996, 33,735. [4] Kaim, A.; Oracz, P. Polymer. 1998, 39,3901. [5] Tsarevsky, N. V.; Sarbu, T.; Göbelt, B.; Matyjaszewski, K. Macromolecules. 2002, 35, [6] Al-Harthi, M.; Sardashti, A. Soares, J B.P.Polymer. 2007, 48,1954. [7] Megiel, E.; Kaim, A. J. Polym. Sci. Part A: Polym Chem. 2008, 46,1165. [8] Plichta, A.; Li, W.; Matyjaszewski, K. Macromolecules. 2009, 42, [9] Listak, J.; Jakubowski, W.; Mueller, L.; Plichta, A.; Matyjaszewski, K.; Bockstaller, M. R. Macromolecules. 2008, 41, [10] Min, K.; Gao, H.; Matyjaszewski, K.. J. Am. Chem. Soc. 2005, 127,3825. [11] Jakubowski,W.; Matyjaszewski, K. Angew. Chem. Intt. Edit. 2006,45, [12] Dong, H.; Matyjaszewski, K. Macromolecules, 2008, 41, [13] Kwak, Y.; Magenau, A. J. D. Matyjaszewski, K. Macromolecules. 2011, 44, 811. [14] Min, K.; Gao, H.; Matyjaszewski, K. Macromolecules. 2007, 40,1789. [15] Magnus, J.; Daniel, N.; Ove, N.; Eva, M. Eur. Polym. J. 2009, 45, [16] Pietrasik, J.; Dong,H.; Matyjaszewski, K. Macromolecular. 2006,39,6384. [17] Ando, T.; Kamigaito, M.; Sawamoto, M. Macromolecular. 1997,30, [18] Matyjaszewski, K.; Wei,M.; Xia,J.; McDermott, N. E. Macromolecular. 1997,30, [19] Teodorescu, M.;Matyjaszewski, K.;Gaynor, SG. Macromolecules. 2000,33,2335. [20] Xue, Z.; Lee, B. W.; Noh, S.K. Polymer. 2007, 48,

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