ISSN 15600904, Polymer Science, Ser. B, 2013, Vol. 55, Nos. 7–8, pp. 460–466. © Pleiades Publishing, Ltd., 2013. Original Russian Text © D.F. Grishin, E.S. Kotlova, I.D. Grishin, 2013, published in Russian in Vysokomolekulyarnye Soedineniya, Ser. B, 2013, Vol. 55, No. 8, pp. 1115–1121.

POLYMERIZATION

Features of Acrylonitrile Radical Polymerization in the Presence of Iron Carbonyl Complexes D. F. Grishin, E. S. Kotlova, and I. D. Grishin Nizhni Novgorod State University, pr. Gagarina 23, Nizhni Novgorod, 603950 Russia email: [email protected] Received November 2, 2012; Revised Manuscript Received February 4, 2013

Abstract—It was shown that mono and binuclear carbonyl complexes combined with carbon tetrachloride were effective initiators of the radical polymerization of acrylonitrile. The mechanism of initiation of acry lonitrile polymerization in the presence of the investigated iron complexes was studied. DOI: 10.1134/S1560090413070038

One of the urgent directions of the chemistry of highmolecularmass compounds is the synthesis of functional polymers based on nitrogencontaining monomers, including Nvinylpyrrolidone, acrylamide and its analogs, and acrylonitrile [1]. Composite materials based on these compounds meet the demands of modern hightech industry and have widespread practical application. In particular, PAN is intensively used as a precursor in the production of stateoftheart carbon fibers. In this case, only poly mers with specified molecularweight characteristics and particular chain microstructures may be used in the production of quality products [2]. There are several approaches to the synthesis PAN with the required characteristics. Note the leading developments made by Russian scientists via the intro duction of reversiblechaintransfer agents (dithioesters, trithiocarbonates) [3] that allow effi cient control over the molecularweight parameters of the precursor PAN. The studies of foreign investiga tors are mainly connected with the use of metalcom plex systems based on compounds of transition met als—in particular, copper and iron halogenides [4, 5]—in combination with various donor ligands and also the use of cobalt complexes [6]. An alternative, but poorly known, approach to the radical polymerization of vinyl monomers with the aim to produce polymers with a particular combina tion of properties and characteristics is the application of initiating systems based on halogencontaining compounds and carbonyl derivatives of transition metals [7, 8]. In this context, the purpose of the present study is to investigate the reactivities of the

mono and binuclear iron complexes of η5cyclopen tadienyldicarbonyl iron chloride (CpFe(CO)2Cl)

Fe CO

Cl CO

and the dimer η5cyclopentadienyldicarbonyl iron (Cp2Fe2(СО)4) CO

CO Fe

Fe CO

CO

during the polymerization of acrylonitrile as a polar vinyl monomer. EXPERIMENTAL Acrylonitrile (AN) was dried over calcium hydride and distilled under atmospheric pressure, and a frac tion with a boiling temperature of 77–78°С was col lected. Freshly distilled monomer was used in the experiments. The organic solvents DMFA and DMSO were purified via common techniques. Carbon tetra chloride was used as an initiator. The Cp2Fe2(CO)4 iron complex was a commercial product; it was recrys tallized from hexane. The chlorinecontaining deriva tive CpFe(CO)2Cl was obtained through a wellknown technique [9]. The physicochemical constants of the solvents and metal complexes corresponded to the lit erature data [10, 11]. The samples were prepared for polymerization as follows: Exact amounts of the metal complex, mono mer, initiator, and solvent were placed into glass bulbs and freed from oxygen via triple degassing of the reac

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tion mixture frozen in liquid nitrogen to a residual pressure less than 1.3 Pa, and polymers were synthe sized at a particular temperature. After the completion of polymerization, the bulbs were frozen in liquid nitrogen; opened in air; and, if required, diluted with DMFA and deposited in water. The polymer was fil tered on a glass filter, washed with water, and dried in vacuum to a constant weight. The conversion of the monomer was determined gravimetrically. The molecularmass characteristics of PAN were determined via gel permeation chromatography on a Knauer setup featuring a linear column with a 2 × 10 6 Da separation limit (Phenomenex, Nucleogel GPCM10, United States) and a K2301 RI Detector differential refractometer at 40°С in DMFA contain ing 10 mmol LiBr. The molecular mass of PAN was calculated with the use of PMMA standards via the universal calibrating dependence and with literature data on the coefficients of the Mark–Kuhn–Houwink equation [12] through the formula 1 + α PMMA log M PAN =   log M PMMA 1 + α PAN K PMMA 1 +  log   K PAN 1 + α PAN Chromatographic data were interpreted with the program ChomGate. Voltammetric studies were performed in an inert atmosphere in a threeelectrode cell with the use of an IPC Pro potentiostat. A disk platinum electrode served as an indicating electrode (d = 2 mm), a plati num wire whose surface area substantially exceeded the working area of the indicating electrode served as an additional indicating electrode, and a saturated sil ver chloride electrode (Ag|AgCl, KCl) isolated from the organic phase with the use of agar bridge with KClO4 served as a comparison electrode. Tetrabutylam moniumhexafluorophosphate (Acros Organics) was used as a background electrolyte. The values of the oxidation and reduction potentials were determined relative to the potential of ferrocene oxidation of the used internal standard and introduced into the test solution at the final stage of analysis. Infrared spectra were registered on an Infralum FT801 Fourier transform infrared spectrometer that used KBr tablets. The error in determining the numer ical values of the absorption bands did not exceed ±0.05 cm–1. The polymers were analyzed via the method of highperformance liquid chromatography (HPLC) on a Knauer Smartline liquid chromatograph with an S3600 diodematrix detector and a Kromasil 605CN POLYMER SCIENCE

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250 × 4.6 analytical column. The eluent was a mixture of nhexane and dichloromethane (3 : 1), and the flow rate was 1 mL/min. RESULTS AND DISCUSSION The production of PAN with the given characteris tics is restricted, on one hand, by the high reactivity of the monomer and, on the other hand, by the insolubil ity of the forming polymer both in a monomer medium and in several organic solvents [13]. In this case, the solvents that allow AN polymerization under homogeneous conditions (for example, DMFA and DMSO) and the monomer itself are polar molecules that favor the formation of complexes with the com pounds of some transition metals, a circumstance that significantly restricts the selection of the metalcom plex components initiating the systems and the cata lytic compositions. Note that a number of iron compounds, in partic ular, halogenated iron derivatives, can be used as cata lysts of the polymerization processes in polar media [5, 14]. In this context, the investigation of the activities of iron carbonyl complexes of mono and binuclear structures (CpFe(CO)2Cl, Cp2Fe2(CO)4) with carbon tetrachloride during AN polymerization is very urgent, especially for the development of stateofthe art techniques of functionalpolymer synthesis under the conditions of radical initiation. Study of the Mechanism of Acrylonitrile Polymerization Initiation in the Presence of Iron Complexes At the first stage, the behavior of Cp2Fe2(СО)4 and CpFe(СО)2Cl was studied during heating (70°С) in an organicsolvent medium. The reaction products were analyzed via HPLC, the times of holding of the com pounds were compared, and the compounds were identified on the basis of UVspectrum data and infra red spectroscopy. According to the literature data [15], Fe–Fe bond breaking in the binuclear iron complex is possible only under photolysis conditions. The holding of the Cp2Fe2(СО)4 binuclear derivative in hexane at 70°С leads to its destruction, an outcome that points to the thermal stability of the complex. It is remarkable that this compound is stable also in the medium of polar acrylonitrile even at heating to 70°С, a result that is confirmed by HPLC data and infrared spectroscopy. IR ν(С=О), cm–1: 1976 (w), 1955 (s), 1939 (s), 1907 (w), 1770 (s), 1760 (s) [16]. At the same time, it was established that the CpFe(СО)2Cl complex exhibits lower thermal stabil ity. As a result of its thermolysis in hexane at 70°С, the binuclear complex Cp2Fe2(СО)4, which is detected via HPLC in studies of the reaction products, is formed. In this case, the chromatograms have two peaks that correspond to Cp2Fe2(СО)4 and CpFe(СО)2Cl. UV (CpFe(CO)2Cl) λmax, nm: 286, 340, 395. The heating 2013

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of the metal complex apparently leads to abstraction of the halogen atom from CpFe(СО)2Cl, and the subse

quent dimerization with the formation of the binuclear derivative proceeds according to the following reaction.

CO

CO t

Fe

Fe

Fe

Cl

CO

CO

CO

However, in contrast to the binuclear derivative, the CpFe(СО)2Cl complex in polar media, including acrylonitrile, favors the formation of a number of iron nitrile derivatives, which were identified with the use of infrared spectroscopy. IR (KBr) ν, cm–1: 2075 (s), 2021 (s), 1995 (w) (C=O); 2125 (w) (C=N) [17]. Thus, it is evident that the iron chloride–containing derivative is partially deactivated in a polar medium, whereas the binuclear metal complex Cp2Fe2(СО)4 is quite stable. As noted above, the presence of carbon tetrachlo ride in the system is required to initiate polymeriza tion. With allowance for this fact, the features of the

Fe

Fe

.

R CH2 CH

CN

CN

Here, R· are carbon centered radicals (·CCl3) initiat ing polymerization. However, the polymerization initiation by the 18 electron halogencontaining complex CpFe(СО)2Cl in the presence of CCl4 is of greater interest. On the basis of the performed investigations, we assume the decomposition of the complex, halogen atom abstraction (ligand dissociation), and the follow ing interaction of the CpFe(CO)2 particle that origi nated with CCl4 and the formation of carbon centered radicals initiating polymerization.

CO

CO

.

Fe CO

CO

+ CCl3

+M

.

Pn

Cl

Here, M is a monomer molecule and Pn is a polymer macroradical. It was established that the binuclear iron metal complex Cp2Fe2(СО)4 is more effective at the initia tion stage than the iron mononuclear halogencon

+ R.

Fe CO

R + CH2 CH

+ CCl4

behavior of the iron complexes used during the addi tion of CCl4 were investigated. The analysis of the model reaction products via HPLC established that the oxidative interaction of Cp2Fe2(СО)4 with carbon tetrachloride in hexane leads to the formation of the chlorinecontaining CpFe(СО)2Cl complex, in addition to the initial com plex, in the reaction products. The isolated product was identified also with the use of infrared spectros copy. IR ν(С=О), cm–1: 2052 (s), 1996 (s) [18]. Thus, on the basis of the experimental data and lit erature data [19], the oxidation of the binuclear deriv ative by alkylhalogenide can be presented as follows.

+ R Hal

CO

.

Fe

CO

CO

CO CO

CO

CO

(1)

Fe

CH2=CH–CN

CO

Hal

(2)

Polymerization

taining derivatives. This result correlates well with the literature data [20, 21] and the belowpresented inves tigation results of polymerization of AN in the pres ence of the studied iron compounds. Note that, in polar media, in particular, those used during PAN synthesis, there is a high probability of the coordination of the nitrile groups to the iron atoms of oxidized metal complexes along with the formation of derivatives that cannot undergo a reversible redox interaction with the halogencontaining compound [22]. As a result, iron complexes can no longer be active in the regulation of polymerchain growth, an outcome that correlates well with the analysis results of the molecularmass characteristics of the synthesized PAN samples. The instability of the oxidized forms of the metal complex is confirmed by the irreversibility of the elec trochemical oxidation of the compounds studied in polar media. Cyclic voltammetry (CVA) makes it pos sible to model singleelectron transfer, which is a key stage of the polymerization in the presence of organo metallic compounds.

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(b)

1

2 µA

2 µA 1

2 2

3

3 −300

−100

100

300

500 E, mV

500

700

900

1100

1300 E, mV

Fig. 1. Voltammetric patterns of 5 × 10–3 M solutions of (a) Cp2Fe2(СО)4 and (b) CpFe(СО)2Cl complexes registered on a disk platinum electrode in (1) acrylonitrile, (2) 1,2dichloroethane, and (3) methyl methacrylate. The potential sweep rate was 100 mV/s. The background electrolyte was Bu4NPF6 (0.5 mol/L).

Figure 1 shows CVA curves registered for the iron complexes Cp2Fe2(СО)4 and CpFe(СО)2Cl in various media. Voltammetric patterns obtained for metal complexes in AN show irreversible anode peaks of oxi dation that reflect the character of the behavior of compounds during polymerization initiation. The process of electrochemical oxidation is equivalent to the interaction of compounds studied with carbon tet rachloride during heating. In this case, the absence of a reversible peak of reduction unambiguously verified the interaction between the oxidized form of a com plex and the investigated monomer with the formation of a nitrile derivative [23]. In the medium of such a monomer as styrene or methylmethacrylate, whose polymerization is quite effectively controlled by the systems based on carbonyl derivatives of iron [21, 24], the reversible redox transi tions observed in the medium of 1,2dichloroethane were registered for the studied compounds (Fig. 1). In nonpolar media, the oxidation process of both the binuclear complex Cp2Fe2(СО)4 and the mononu clear chlorinecontaining complex CpFe(СО)2Cl is singleelectron, a result that is verified by the differ ence between the potentials of oxidation and reduc tion of the compound: At a rate of change in the potential in 1,2dichloroethane of 100 mV/s, ΔE Cp2Fe 2 = 74 mV and ΔE CpFeCl = 80 mV. Thus, the compositions based on the mono and binuclear iron carbonyl complexes Cp2Fe2(СО)4 and CpFe(СО)2Cl in the presence of carbon tetrachloride undergo redox transformations leading to the genera tion in the system of radicals that can initiate polymer ization. The features of the radical polymerization of AN in the presence of the aforementioned systems are analyzed below, where consideration is given to the influence of the initiatingsystem components and the POLYMER SCIENCE

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solvent on the polymerization kinetics and molecular weight characteristics of PAN. Influence of the Composition of the Initiating System and the Nature of the Solvent on Acrylonitrile Polymerization As a result of the above experiments, we established that the systems based on the iron derivatives CpFe(CO)2Cl and Cp2Fe2(CO)4 along with carbon tetrachloride reveal high activities in the initiation of AN polymerization. It turned out that the solution polymerization of AN during the action of these initiating systems pro ceeds without gel formation. With the use of the binu clear complex Cp2Fe2(CO)4, the limiting conversion of monomer is achieved for 20 h (Fig. 2, 1 curve). At the same time, during polymerization under the action of CpFe(СО)2Cl and CCl4, it significantly decreases (3 curve). The observed lower rate of AN polymeriza tion with the use of CpFe(СО)2Cl correlates well with the above results of the investigation of the initiating capabilities of the studied iron complexes. With a twofold increase in the CpFe(СО)2Cl con tent of the initiating system, both the process rate and the PAN yield increase up to 60% (Fig. 2, curve 2). We attribute this outcome to the increase in the concen tration of radicals formed during initiation. In addition, a change in the ССl4 concentration leads to a change in the polymerization rate and directly affects the molecularmass characteristics of polymers (see below). It was established that, in the case of systems based on the binuclear derivative Cp2Fe2(CO)4, during an increase in the amount of CCl4 amount in the system at a constant molar ratio of [Cp2Fe2(CO)4] : [AN], the polymer yield increases over a preset time (Fig. 3). This result is likely due to an increase in the amount of active centers that form 2013

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GRISHIN et al. Conversion, % 80

Conversion, % 1

80

1

2 3

2

60

60 3

40

40

20

20

0

40

20

60 Time, h

0

Fig. 2. Acrylonitrile polymerization in a DMSO solution at Т = 70°С and [AN] = 4 mol/L in the presence of 0.25 mol % CCl4 plus (1) 0.0625 mol % Cp2Fe2(СО)4, (2) 0.125 mol % Cp2Fe2(CO)4, or (3) 0.0625 mol % CpFe(CO)2Cl.

Table 1. Molecularweight characteristics of PAN synthesized in a DMSO solution ([AN] = 4 mol/L, in the presence of met al complexes and CCl4 (0.25 mol %), Т = 70°C) Mn ×10–3

Mw /Mn

Cp2Fe2(СО)4 0.0625 mol % 31

47.8

3.3

50

44.9

3.1

58

46.8

3.2

73

44.2

3.1

CpFe(СО)2Cl 0.0625 mol % 23

41.3

2.2

36

36.7

2.3

46

38.3

2.5

CpFe(СО)2Cl 0.125 mol % 38

38.4

2.2

49

35.8

2.1

60

32.7

2.0

60 Time, h

Fig. 3. Acrylonitrile polymerization in a DMSO solution at Т = 70°С and [AN] = 4 mol/L in the presence of 0.0625 mol % Cp2Fe2(СО)4 plus (1) 0.5, (2) 0.25, or (3) 0.0625 mol % CCl4.

during the interaction of the organometallic derivative with haloalkyl. A direct influence of the type of solvent on the polymerization rates and molecularmass characteris tics of the synthesized polymers was found (Tables 1, 2). It is known that DMFA, in comparison to other organic solvents used in solvent polymerization of AN, differs by its greater capacity to dissolve the polymer [25].

Conversion, %

40

20

However, changing the solvent led to significant decreases in the process rate and monomer conversion relative to those for the aboveconsidered AN poly merization in the DMSO solution under the same conditions and to decreases of the molecular weight of the synthesized PAN (Tables 1, 2). These results cor relate well with the data of classical radical polymer ization of AN in DMSO and DMFA [25]. It is known that the addition of an excess of the reducing agent to the initiating systems based on metal complexes makes it possible to transform a complex into its initial state and, as a result, to increase the rate of the controllable process. An example is ascorbic acid, in whose presence a significant acceleration of the controllable radical polymerization is observed [26]. It was established that the introduction into the Cp2Fe2(СО)4–ССl4 system of a twofold molar excess of ascorbic acid with respect to the metal complex leads to slight acceleration of polymerization. (The limiting monomer conversion of ~80% is achieved over 60 h.) In this case, the process proceeds in a sta tionary mode at a constant concentration of reactive centers, as is indicated in Fig. 4 by the linear depen dence of ln[M]0/[M] on time. Analysis of the MolecularWeight Characteristics of the Synthesized Polymers Analysis of the polymers synthesized via GPC showed that the PAN synthesis conditions, in particu lar, the compositions of the initiating systems and the types of solvents, substantially influence polymer molecular weight (Tables 1, 2). The presented data show that PAN samples synthesized in DMFA have lower molecular weight and a narrower molecular weight distribution (Table 2) than those of the poly

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mers obtained in DMSO (Table 1). The observed dependences are easily explained by the active partic ipation of DMFA in the reactions of chain transfer. A 1.5fold decrease in the molecular weight of PAN is promoted by the introduction of ascorbic acid into the system. In this case, the polydispersity coefficients of the samples obtained in DMSO solutions decrease (Fig. 5, Mw/Mn = 1.6–1.7 and Mw/Mn ≥ 3.0 in the presence and the absence of ascorbic acid, respec tively). Such an influence of ascorbic acid on the molecularweight characteristics of the synthesized samples is likely due to the possible weak inhibition of the radical polymerization and the high activity of ascorbic acid in the processes of chain transfer [27]. Analysis of the molecularweight characteristics of PAN samples obtained in DMSO at different ratios of the initiatingsystem components makes it possible to conclude that, with an increase in the monomer con version, there are no increases in the molecularmass characteristics for the processes of controllable radical polymerization [28, 29] (Table 1). Thus, the proposed compositions based on iron derivatives and carbon tet rachloride can be considered only as systems initiating AN polymerization, not as agents of controllable syn thesis of macromolecules. Note that PAN samples synthesized in the presence of iron complexes have a unimodal, but relatively wide (Mw/Mn ≥ 2.0), molecularweight distribution (Table 1). At the same time, in contrast to AN polymers syn thesized with the use of classical radical initiators [30] and characterized by high percentages of crosslinked macromolecules, PAN samples obtained in the pres ence of the proposed metal complexes have good sol ubility in DMSO and DMFA. PAN samples obtained at high ССl4 concentrations have lower molecularweight values (Fig. 5, curves 2–

465

Table 2. Results of acrylonitrile polymerization in a DMFA solution ([AN] = 4 mol/L, in the presence of metal complexes (0.0625 mol %) and CCl4 (0.25 mol %), T = 70°C) Time, h

Conversion, %

Mn × 10–3

Mw /Mn

Cp2Fe2(СО)4 0.75

6

22.7

1.7

3.0

29

25.4

1.7

18.5

66

20.8

1.8

CpFe(СО)2Cl 11.0

20

24.1

1.5

34.0

30

23.2

1.5

4). This circumstance may be due to both an increase in the amount of active polymerization centers and an increase in the probability of chain transfer to CCl4 (Сs = 1.2 × 10–4) [31]. As a consequence, the molecu larmass distribution of the PAN synthesized in DMSO remains quite wide (Mw/Mn ≥ 2.0) relative to that of the polymers synthesized under conditions of controllable radical polymerization [32, 33]. Thus, it was established that the compositions based on the mono and binuclear iron carbonyl com plexes CpFe(СО)2Cl and Cp2Fe2(СО)4 in the pres ence of carbon tetrachloride are effective initiators of the radical polymerization of acrylonitrile. The use of the investigated compositions makes it possible to modify the molecularmass characteristics of poly acrylonitrile synthesized under conditions of radical initiation via variation in the qualitative and quantita

ln[M]0/[M] 2.0

1 2 3

1.5

4

1.0 0.5

0

20

40

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6

7 logM

Fig. 5. GPC curves of PAN (80% conversion) synthesized in DMSO solutions at Т = 70°С and [AN] = 4 mol/L in the presence of 0.0625 mol % Cp2Fe2(СО)4 plus (1) 0.25 mol % CCl4 and 1.25 mol % ascorbic acid, (2) 0.5 mol % CCl4, (3) 0.25 mol % CCl4, or (4) 0.0625 mol % CCl4.

Fig. 4. Dependence of ln[M]0/[M] on time during acry lonitrile polymerization in a DMSO solution at [AN] = 4 mol/L in the presence of 0.0625 mol % Cp2Fe2(СО)4, 0.25 mol % CCl4, and 1.25 mol % ascorbic acid. POLYMER SCIENCE

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tive composition of the initiating compositions and in the solvent selection.

14. C. Hou, R. Qu, C. Sun, C. Ji, C. Wang, L. Ying, N. Jiang, F. Xiu, and L. Chen, Polymer 49, 3424 (2008).

ACKNOWLEDGMENTS

15. T. V. Nikitina, Methods of Organoelement Chemistry: Iron Organometallic Compounds, Moscow: Nauka, 1985 [in Russian].

We thank E.V. Geras’kina for performing some of the analyses. This study was supported by the federal target pro gram Scientific and Educational Specialists of Innova tion Russia.

16. A. J. Dixon, M. W. George, C. Hughes, M. Poliakoff, and J. J. Turner, J. Am. Chem. Soc. 114, 1719 (1992). 17. C.H. Lai, W.Z. Lee, M. L. Miller, J. H. Reibenspies, D. J. Darensbourg, and M. Y. Darensbourg, J. Am. Chem. Soc. 120, 10103 (1998). 18. G. A. Donna and K. W. Barnett, Inorg. Chem. 17, 2827 (1978).

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