Retarding Effect of Aromatic Solvents on Cobalt(II) - American

Sep 16, 2010 - Reliance Technology Group, Vadodara Manufacturing DiVision, Reliance Industries Ltd., Vadodara-391346,. Gujarat, India, and Institute o...
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Ind. Eng. Chem. Res. 2010, 49, 9648–9654

Retarding Effect of Aromatic Solvents on Cobalt(II)-Based Catalyst System during Synthesis of High cis-1,4-Polybutadiene Archana Singh,† Siddharth Modi,† N. Subrahmanyam,† Pradip Munshi,‡ Vinod K. Upadhyay,‡ Raksh Vir Jasra,‡ and Madhuchhanda Maiti*,‡ Reliance Technology Group, Vadodara Manufacturing DiVision, Reliance Industries Ltd., Vadodara-391346, Gujarat, India, and Institute of Technology, Nirma UniVersity, Sarkhej-Gandhinagar Highway, Ahmedabad-382481, Gujarat, India

The present study discloses the influence of aliphatic (cyclohexane) and aromatic solvents with different π-electron densities viz. toluene, o-xylene, ethylbenzene (EB), and p-diethylbenzene (PDEB) on the cobaltcatalyzed high cis-polymerization of 1,3-butadiene. Comparative study in cyclohexane and toluene has been conducted in view to understand the effect of solvent on catalyst reactivity, molecular weight of the polymer, and polymerization kinetics. At given reaction conditions conversion is highest (68%) in cyclohexane, followed by toluene (27%) and EB (8%). Respective polymerization rate constants are found to be 0.0170, 0.0020, and 0.0008 min-1. Higher viscosity average molecular weight (M) of polymer observed in toluene (M ) 4.68 × 105) compared to that in cyclohexane (M ) 2.37 × 105) indicates that the presence of lower number of active sites leads to higher molecular weight. Cyclohexane shows activity at a lower water:alkylaluminum ratio than does toluene; as the π-electron of toluene reduces the Lewis acidity of hydrolyzed diethylaluminum chloride (DEAC) more than cyclohexane, where influence of π-electron is absent. The effect of different solvents has also been demonstrated by respective shifting of wavelength (λ) of solvent π f π*, cobalt 4A2 f 4T1(P), as well as 4T1 g (F) f 4T1 g (P) and activated butadienyl π(HOMO) f π*(LUMO) transitions. 1. Introduction Butadiene rubber (BR) is the second largest volume synthetic rubber produced worldwide, next to styrene-butadiene rubber (SBR). The major use of high cis-BR is in tires, and more than 70% of the polymer produced goes into tire-treads and sidewalls. This is due to its excellent abrasion resistance, or less tread wear, and low rolling resistance, which leads to good fuel economy.1 The global demand for rubber can be estimated at about 30 million tonnes by 2020 with synthetic rubber share going up to nearly 67%.2 In other words, even a small improvement in the process can be of great benefit because of the huge demand of high cis-BR. This necessitates continuous research in this area. Polymerization of 1,3-butadiene (BD) to high cis-BR is commonly done by solution polymerization technique using Ziegler-Natta catalysts. The catalysts are based on titanium, cobalt, nickel, or neodymium in presence of alkylaluminum as a cocatalyst.3-8 Among them, a cobalt-based system has received the earliest recognition at the commercial level and thus a sizable literature is also available.7-12 With cobalt-based systems, benzene is the most commonly used solvent.7-12 However, primarily because of environmental and health concerns of benzene, attempts were made in finding alternative solvents.13-21 In that aspect, different aliphatic, aromatic, and olefinic solvents are being tried as alternatives. But polymerization rate in aliphatic solvents, e.g., cycloalkanes and reactivity of catalysts in aromatic solvents except benzene are the critical issues.14-19 In addition, there are only a few open references available detailing the effect of polymerization parameters in cycloalkane solvents.13,20,21 As study of solvent is an important aspect of the process,22 we are motivated to understand the effect of different solvents * Corresponding author. E-mail: [email protected]. Fax: +91-265-6693934. † Nirma University. ‡ Reliance Industries Ltd.

in butadiene polymerization.13,23 There are reports available citing consequences of different solvents, but the actual reason has not been disclosed anywhere. Gippin revealed that polarity of the solvent is inversely related to the rate of polymerization.12 Without elaborating, author also mentioned that solvent polarity should not have any influence on the microstructure of the polymer. Porri et al. remarked in their work on transition-metalbased catalyst system, while reasoning the stereoregularity, that donating solvent may take part in coordinating vacant site of the metal.24 In a separate report, he also discussed the comparative effect of solvents with increasing basicity.25 Sluggish rate of reaction in solvents with higher electron density was interpreted by the coordinating effect of the aromatic solvent. Distinctly, Mello and co-workers evaluated effect of the solvents with neodymium catalyst system.26 However, the study remained focused on only two solvents, namely, hexane and cyclohexane. This study also could not add information beyond Porri’s clarification. Therefore, the cause of influence of solvents in butadiene polymerization is still elusive. Ricci et al. discovered a new cobalt phosphine complex CoCl2(PiPrPh2)2 as an efficient catalyst for the preparation of 1,2 syndiotactic polybutadiene, one of the diene polymers of industrial interest.27 The metal complex associated with methylaluminoxane was found to be the active catalyst for the reaction. The authors also observed the effect of solvents that resulted in high productivity in heptane compared to toluene, for the same reason as mentioned earlier by Porri et al. However, to the best of our knowledge, solvent coordination to the metal center has not been directly shown so far. In the present work, we have tried to address this concern by screening various solvents and studying spectroscopically to understand the interaction of solvent with respective reaction components. The absorbance maxima (λmax) in UV-visible spectroscopy of different solvents in reaction mixture helped us to discuss the possible interactions with solvent molecules affecting rate of the reaction. UV-visible spectroscopic study

10.1021/ie101324d  2010 American Chemical Society Published on Web 09/16/2010

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of solvents in butadiene polymerization has not been looked into before and thus leaves great opportunity for us to explore. Additionally, we have studied the effect of catalyst concentration, water:alkylaluminum ratio (W/Al), and chain transfer agent on polymerization of BD in cyclohexane and toluene. The selected studies in toluene reported by Honig et al. are shown to be in agreement with our values.28,29 Noticeably, important variables like water:alkylaluminum ratio, molecular weight, polydispersity index (PDI), and rate constants are studied as valuable information for butadiene polymerization. 2. Experimental Section 2.1. Materials and Methods. 1,3-Butadiene (BD), ethylbenzene (EB), p-diethylbenzene (PDEB) were obtained from Reliance Industries Limited, India. Cobalt(II) octanoate,1, was procured from Maldeep Catalysts Pvt. Ltd., India. Diethylaluminum chloride (DEAC) solution and triethylaluminum (Et3Al) solution were obtained from Akzo Nobel, India. Ditertiary-butylp-cresol (DTBPC) was procured from Quality Industries, India. The mixture of reactants consists of proportional amount of BD and solvent will be herein after called “feed”. BD was purified by passing through two successive columns containing DEAC solution and molecular sieves 3A. This was then solubilized in respective solvents to prepare feed. Different solvents viz., cyclohexane, toluene, tetrahydrofuran (HPLC grade) were procured from Labort Fine Chem. Pvt. Ltd., India and o-xylene was obtained from Merck Specialties Pvt. Ltd., India. Methanol was procured from Fisher Scientific, India. Polymerization was carried out in a jacketed 1 L laboratory glass reactor (Buchi SFS, Switzerland). All the glasswares were oven-dried prior to use and used under strict exclusion of moisture. Only dried and purified solvents were used throughout the study.30 Utmost care was taken while handling pyrophoric alkylaluminums. External solutions and reaction feed were prepared using vacuum manifold. Microstructure of BR was determined using a Fourier Transformation Infrared (FTIR) spectrophotometer (PerkinElmer 1600 Series) in the range of 600-1100 cm-1 by dissolving the rubber sample in carbon disulfide (75 mg in 10 mL of carbon disulfide), according to the literature reported Morero method.31 Perkin-Elmer UV/vis/NIR (Lambda 19) spectrometer was used for obtaining the UV spectra. PDI was determined using gel permeation chromatography (GPC, Perkin Elmer series 200, USA). 2.2. Polymerization of 1,3-Butadiene in Toluene. In a typical polymerization process, 350 mL of feed (21% (w/w) of BD in solvent) was taken in a Buchi reactor. Water as a promoter (W/Al ) 0.3) was added into the feed under an inert atmosphere. The cocatalyst, DEAC, 15 mmol per 100 g of monomer (mmphgm), was then added gently into the above mixture under stirring. The addition of reactant was completed by introducing the catalyst, 1, (0.04 mmphgm), whereupon the reaction was considered to be started. The temperature was maintained at 26 ( 1 °C. Stirring was done at 200 rpm. After 60 min, the reaction was terminated by adding methanolic solution of 0.5% (w/v) DTBPC, while the rubber was found to be precipitated. The filtered rubber was washed with fresh methanol and kept for air drying overnight. Then the sample was cut into small pieces and oven-dried at 40 °C for 6 h under a vacuum. The efficiency of the reaction was evaluated as the weight % of rubber produced with respect to the monomer fed (eq 1). Reactions in different solvents were carried out in similar

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manner as mentioned above using desired catalyst:cocatalyst ratios. % conversion )

produced × 100 ( rubber monomer fed )

(1)

2.3. Viscosity Average Molecular Weight. The viscosity average molecular weight (M) of the sample was determined from the intrinsic viscosity ([η]) using the Mark-Houwink equation, eq 2. [η] ) kMR

(2)

Where k and R are Mark-Houwink constants; k ) 0.000305, and R ) 0.725 for the solvent toluene and polybutadiene rubber at 30 °C.32 2.4. UV-Visible Spectroscopy. The solutions for spectroscopic study were prepared in a 100 mL three necked round bottomed flask attached with vacuum manifold maintaining the ratio of 1:BD ten times higher than what was used in polymerization reaction, to observe the measurable absorbance. Desired solvent was chosen as per the requirement of the study. A stock solution of 1 used in polymerization was prepared in the concentration of 0.54 g in 100 mL of solvent. BD was dissolved in chilled solvent in the concentration of 17 g in 100 mL. DEAC was diluted to a concentration of 12.2 g in 100 mL of solvent. The desired proportion of the above solutions was mixed in a round-bottomed flask and transferred to a UV cell in a nitrogen atmosphere for recording the spectrum. 2.5. Gel Permeation Chromatography (GPC). Polydispersity index of the polymers was determined using GPC with refractive index (RI) detector at 30 °C. Samples were dissolved in HPLC grade tetrahydrofuran at concentration 60 mg/5 mL. For proper dissolution of the sample, reasonable sonication was performed. Response of the desired molecular weight was achieved in Mixed-Bed PLgel, 5 µm, 300 mm ×7.5 mm at flow rate of 1.0 mL/min using polystyrene as standard (molecular weight range was 582 to 6 × 105). The GPC procedure established was able enough to distinctly show the desired peak to be well-separated from solvent peak. 3. Results and Discussion 3.1. Comparative Study of BD Polymerization in Cyclohexane and Toluene. While searching for an alternative, successful solvent other than benzene for polymerization of BD, toluene and cyclohexane come as the second best candidates as far as industrial applications are concerned. In that aspect, at first, we have studied polymerization in cyclohexane and toluene. Polymerization in toluene has already been reported but when compared with cyclohexane, we found substantial difference between the solvents in terms of reactivity, molecular weight, and other important parameters. The results show that the polymer microstructure is not influenced markedly by changing solvents, as disclosed by Gippin.12 In both the cases, high cis (g95%) polymer was obtained. The trend of activity in both the solvents was almost similar. However, a significant decrease (3-fold) in the catalyst activity has occurred by changing the solvent from cyclohexane (aliphatic) to toluene (aromatic) as seen in Figure 1a. The minimum catalyst concentration of 0.03 mmphgm was used in toluene; below this concentration, the yield was observed to be very low. Additionally, in cyclohexane, the reaction was observed to be spontaneous above 0.05 mmphgm of 1. Hence, the run was taken up to this catalyst concentration. Both the solvents show a similar downward trend in a molecular weight

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Figure 1. (a) Comparative catalyst reactivity in cyclohexane and toluene solvents and (b) effect of cyclohexane and toluene solvents on molecular weight of polymer. Feed, 21% (w/w); water/DEAC, 0.3; DEAC/1, 375; 26 ( 1 °C; 1 h.

Figure 2. Effect of W/Al ratio on (a) conversion and (b) molecular weight of polymer. Feed, 21% (w/w); DEAC, 15 mmphgm; 1, 0.04 mmphgm; 26 ( 1 °C; 1 h.

vs catalyst concentration plot. To achieve polymer conversion similar to cyclohexane, we require a higher catalyst dose in the case of toluene. On the other hand, molecular weight shows a higher value in toluene at a particular catalyst concentration (Figure 1b). This result corroborates the hypothesis that at definite monomer concentration, having fewer available active sites provokes chains to grow to have a higher molecular weight.33 Apart from catalyst concentration, another important factor in the polymerization system of 1 and DEAC is the water concentration. Water, acting as a promoter, is known to be required to produce different Lewis acidity12 or aloxane34 type of structure generated after reacting with DEAC. Moreover, addition of water initiates selective hydrolysis of DEAC by sequential removal of alkyl group and thereby increasing the Lewis acidity, as observed by Gippin.12 Figure 2a shows the conversion as a function of water concentration in toluene as well as in cyclohexane. In both the solvents, in terms of the commonly used water:alkylaluminum (W/Al) ratio, conversion and molecular weight pass through maxima (Figure 2a,b), indicating a certain degree of modification by water on DEAC is required to achieve the best efficiency. The W/A1 ratio was varied by changing the water concentration at fixed alkylaluminum concentration. In the case of cyclohexane, the maximum is achieved at lower W/Al ratio with higher conversion. Distinctly, in the case of toluene, the maximum is achieved at higher W/Al ratio but with lower conversion. This clearly indicates that the Lewis acidity of hydrolyzed DEAC (at certain W/Al) has been affected greatly by solvent. In other ways, toluene, having π-electrons, as solvent reduces the Lewis acidity

of hydrolyzed DEAC more than cyclohexane, where the influence of π-electrons is absent. That is perhaps the reason why cyclohexane shows activity at lower W/Al ratio than does toluene. Difference in solubility of water in cyclohexane (0.01 g/100 mL, 20 °C) and toluene (0.033 g/100 mL, 25 °C) should not be the reason in this case because the water used in the reaction condition is within the solubility limit. Thus, the maximum conversion can be observed in case of cyclohexane. The decrease in molecular weight with increasing water can be observed in both the cases, which is in line with the work by Saltman and Kuzma.35 Water concentration beyond the critical value allows complete hydrolysis of aluminoxane framework, reducing the Lewis acidity greatly. In the commercial process of synthesis of cis-1,4-polybutadiene using this catalyst system, an additional chain transfer agent is usually employed to regulate the molecular weight in the desirable range of 0.2-0.4 millions for applications in automobile tires. A number of organic compounds, e.g., hydrogen, ethylene, propylene, alpha-olefins and allene, nonconjugated dienes, etc., are known to be used for this purpose.14 The effect of 1,5-cyclooctadiene (COD), as polymer chain regulator, is studied in this paper (Figure 3a,b). Up to a certain concentration of COD, the conversion is not affected significantly but molecular weight is observed to drop continuously. After this critical concentration of COD, both conversion and molecular weight decrease. Effect of chain regulator is more pronounced in toluene than in cyclohexane. With the addition of 0.2% COD with respect to monomer, the decrease in molecular weight is more significant in the case of toluene (48%) than in cyclohexane (3%). This behavior can be attributed to

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Figure 3. Effect of chain transfer agent on (a) conversion and (b) molecular weight of polymer. Feed, 21% (w/w); DEAC, 15 mmphgm; 1, 0.04 mmphgm; W/Al, 0.3; 26 ( 1 °C; 1 h.

Figure 4. Conversion of BD in different solvents at a given condition. Feed, 21% (w/w); DEAC, 15 mmphgm; 1, 0.04 mmphgm; W/Al, 0.3; 26 ( 1 °C; 1 h.

Figure 5. Conversion of BD as a function of time in different solvents. Feed, 21% (w/w); DEAC, 15 mmphgm; 1, 0.04 mmphgm; W/Al, 0.3; 26 ( 1 °C; 1 h.

the fact of the chain transfer reaction being more favored than propagation in presence of aromatic solvent.36 This is supported by broadening of the molecular weight distribution (PDI value changes from 2.1 in cyclohexane to 3.8 in toluene at same COD dosing) in changing the polymerization solvent. The above difference between cyclohexane and toluene can be explained by intervention of π-electrons in toluene. From the above studies, the concentrations of catalyst components and chain transfer agent were optimized. A noticeable point is that, as explained earlier, the number of active sites is the same but the accessibility of the active centers is lower in case of toluene. At a given monomer concentration, aromatic solvent comes in competition for coordination to the active center. Because of this competition of the solvent molecules with monomer at similar concentration, the reaction rate decreases and favors chain transfer. 3.2. Kinetic Behavior in Different Solvents. Progress of reaction in solvents having different electron-donating groups (+I effect), e.g., toluene, o-xylene, EB, and PDEB, under similar reaction conditions was also studied. The polymerization was conducted for 1 h. The final conversion of BD to polybutadiene is shown in Figure 4. Highest conversion is achieved with aliphatic solvent, cyclohexane, having no π-electrons. Conversion is observed to decrease with the use of aromatic solvent, toluene, and further with EB, matching with their increasing trend in π-electron density.37 Reaction is extremely slow for the solvents having disubstituted benzene, e.g., o-xylene and PDEB, and at studied conditions, no polymer formation occurs. A representative plot of conversion vs time in different solvents

Table 1. Effect of Solvents on Reaction Ratea solvent

rate constant, k (min-1)

cyclohexane toluene ethyl benzene

0.0170 0.0020 0.0008

a Feed, 21% (w/w); DEAC, 15 mmphgm; 1, 0.04 mmphgm; W/Al, 0.3; 26 ( 1 °C; 1 h.

is shown in Figure 5. The overall rate constant of the reaction in different solvents are listed in Table 1. Rate constants are determined considering the reaction to be first-order according to the information revealed earlier.11 It can be seen that the polymerization rate follows the order cyclohexane > toluene > EB, in reverse order with their π-electron density. Reaction is considered to be pseudo-first-order in toluene according to Honig et al.28,29 This also signifies that the parameter related to π-donation ability of the solvent is the determinant factor for controlling the reaction progress. Molecular weight of polymer as a function of time and conversion are described in Figure 6a,b respectively. Molecular weight and conversion increase rapidly at the initial stage of polymerization and then become plateau. At any time, conversion of BD is higher in cyclohexane than toluene. 3.3. UV Spectroscopy. To understand the effect of solvent on BD polymerization, we have attempted to study the interaction of catalyst, cocatalyst and different solvents using UVspectroscopy, Table 2. Interaction of π-electrons reflects in their absorption maxima, the shift of which is being monitored through UV spectroscopy. Electronic spectra of blue solution

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Figure 6. (a) Molecular weight of polymer as a function of time and (b) molecular weight vs conversion plot. Feed, 21% (w/w); DEAC, 15 mmphgm; 1, 0.04 mmphgm; W/Al, 0.3; 26 ( 1 °C. Table 2. UV-Visible Absorption of Different Solvent Combinationsa entry

solvent

1 2 3 4 5 6 7 8 9

toluene cyclohexane EB PDEB o-xylene toluene cyclohexane EB PDEB

a

catalyst co-catalyst monomer absorption (λ, nm) 1 1 1 1 1 1 1 1 1

DEAC DEAC DEAC DEAC

BD BD BD BD

Scheme 1. Proposed Reaction Path for High cis-1,4Polybutadiene Using 1, DEAC System

619, 575, 542, 285 612, 575, 550, 269 625 630 621 512, 288 525 500, 287 492, 304

Temperature 25 °C, λ scanned 250-800 nm.

Figure 7. UV-visible spectra of 1 in toluene, cyclohexane, PDEB, and 1-DEAC-BD system in toluene, cyclohexane, PDEB.

of 1 in toluene shows high energy peak at 619 nm, entry 1, Table 2 (herein after it will be designated according to E1-T2). A few representative spectra are given in Figure 7. This transition corresponds to tetrahedral configuration of respective transitions 4A2 f 4T1(P).38,39 In cyclohexane, the same band appears at 612 nm (E2-T2) (Figure 7). The red shift of the respective band from 612 to 619 nm occurs while the solvent was changed from cyclohexane to toluene because of stabilization of 4T1(P) in comparison to 4A2, resulting in lower energy of transition.40,41 The red shift of the aforesaid band is seen for other solvents in accordance to their electron density, e.g., EB, PDEB, and o-xylene (E3-T2 to E5-T2). This can be attributed to π-electron donation of the solvent molecule to 4T1(P), which becomes prominent at higher electron density.24 Upon addition of DEAC and BD in reaction, the color changes from blue to pale red. The lowest energy band for the toluene solution appears at 512 nm (E6-T2). DEAC (Et2AlCl) is reported to react with butadiene, generating butadienyl ion, which forms a complex with cobalt, replacing one octanoate.12 The species formed binds with another monomer of butadiene,

Scheme 1, attaining the coordination number 6.12,34,42,43 This indicates that cobalt most likely gains octahedral structure, as cubic structure (with 6 coordination number) is not feasible. The band at 512 nm also supports the literature value for 4T1g(F) f 4T1g(P) transition in the octahedral environment of Co(II).38 In this case, the effect of solvent is manifested even more prominently (Figure 7). Contrary to earlier observation, blue shift is seen in presence of Et2AlCl and butadiene while solvents were changed to higher polarity (E8-T2, E9-T2). This may be ascribed to the fact that because of the nonbonding nature of the interaction with the polar solvent, 4T1g(F) is more stabilized than 4T1g(P), leading to high energy transition. Thus a shift in λ for cyclohexane, toluene, EB, and PDEB with increasing electron density, is observed at 525, 512, 500, and 492 nm, respectively. In addition, a red shift of octanoate ligand, 269 to 285 nm, while the solvent is changed from cyclohexane to toluene has been observed. This is likely to be the π f π* transition and in accordance with the literature, that coordinating solvent has high influence in shifting the absorption maxima.44,45 In case n f π* transition had occurred, blue shift would have been observed on changing the solvent polarity from cyclohexane to toluene. The n f π* is not observed in our case, most probably because of its low intensity, which could not be captured by spectrophotometer. The red shift could be reasoned for attractive polarization between solvent and absorber that lowers the energy levels of π* resulting into low energy transition (E1, E2-T2).46 Analogous observations can be made in the case of other solvents as well (E3, E4, E5-T2). Besides, when possible interactions between the solvents and cocatalyst were investigated, we found remarkable differences

Ind. Eng. Chem. Res., Vol. 49, No. 20, 2010 Scheme 2. Possible Solvent Interaction with Catalyst That Hinders Monomer Binding to the Active Center

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Acknowledgment The authors thank Dr. K.V.V.S.R. Murthy, Mr. Dinesh S. Pandya, Mr. Chirag S. Shah, Mr. Pankaj C. Chawla, Mr. Suraj K. Kusum, and Mrs. Swati G. Trivedi for their support. The authors are grateful to Reliance Industries Ltd. for providing financial support to this work. Literature Cited

in the UV absorption of the solutions containing Et2AlCl and triethylaluminum (Et3Al). A band was observed at 265 nm when BD and Et2AlCl are added in toluene; but in the case of Et3Al, the band appears at 262 nm, probably corresponding to π f π* transition of the butadienyl unit. Furthermore, a light yellow coloration occurred when Et3Al was added to BD, unlike Et2AlCl, where no color change was observed. Probable reason may be because of π donation from solvent molecule to LUMO of butadienyl anion unit (π*), which effectively lowers the π (HOMO) f π* (LUMO) transition energy.44 Because of weaker Lewis acidity nature of Et3Al than Et2AlCl, creation of butadienyl anion as well as attractive polarization of solvent molecule are rather energy intensive.12 This becomes prominent in case of relatively electron-dense molecule like EB, PDEB, and o-xylene, where the values are 287, 300, and 304 nm, respectively, in the presence of Et2AlCl and BD. This emphasizes our hypothesis of solvent π-donation. Though we have used Et3Al as purchased, the occurrence of color may not be ruled out from the impurity present even at trace level. Nevertheless, several reactions with different stock of Et3Al showed similar observation. Thus it can be seen in Figure 7 that the extent of band shifting is a function of the electron density of the solvents. Therefore, it is evident from above observations that π electrons of aromatic solvents are in the mode of donation to Co, the metal center.34 In effect, such interaction lessens the partial positive charge on metal, which discourages butadiene monomer to propagate the chain length, Scheme 2. Thus the rate of reaction in aromatic solvent is severely deterred in comparison to the solvents like cyclohexane. This becomes so prominent that in the presence of PDEB and even o-xylene, no polymeric product was observed. The problem for highly electron-dense solvents is that the allowance of monomer onto active site of catalyst gets hindered. In view of the above, it is summarized that the solvents having higher electron density have genuine influence on the active site of the catalyst, retarding the efficiency greatly. 4. Conclusions In conclusion, solvents having π-electrons have marked effect on the activity of catalyst compared to aliphatic solvents (cyclohexane) as the medium of polymerization of BD by DEAC-cobalt catalyst system. The π-donation indicated by W/Al ratio studies has been confirmed by UV-vis spectroscopy and rate constant measurements. The rate of polymerization is inversely related to the π-electron density of the solvent. Higher molecular weight with broad molecular weight distribution is, however, possible to attain by using aromatic solvent. Therefore, considering the above consequences, judicious selection of solvents need to be exercised for purpose of solvent modification to the existing manufacturing process.

(1) Blow, C. M.; Hepburn, C. Rubber Technology and Manufacture; Butterworth Scientific: London, 1982. (2) Ramamurthy, K. World synthetic rubber scenario with special reference to asia pacific region. In India International Rubber Conference; Udaipur, India, Nov 1-3, 2007; Mohan Lal Sukhadia University: Udaipur, India, 2007; paper no. 10A. (3) Friebe, L.; Nuyken, O.; Obrecht, W. Neodymium-based ZieglerNatta catalysts and their application in diene polymerization. AdV. Polym. Sci. 2006, 204, 1–154. (4) Rao, G. S. S.; Upadhyay, V. K.; Jain, R. C. Polymerization of 1,3butadiene with neodymium chloride/2-ethylhexanoate/triethylaluminium catalyst system. Angew. Makromol. Chem. 1997, 251, 193–205. (5) Kwag, G.; Lee, J. G.; Lee, H.; Kim, S. Study of the active site and activation process of Ni-based catalyst for 1,3-butadiene polymerization using X-ray absorption and crystal-field spectroscopies. J. Mol. Catal. A: Chem. 2003, 193, 13–20. (6) Pires, N.; Ferreira, A.; Lira, C.; Coutinho, P.; Nicolini, L.; Soares, B.; Coutinho, F. Performance evaluation of high cis-1,4-polybutadienes. J. Appl. Polym. Sci. 2006, 99, 88–99. (7) Boor, J. Ziegler Natta Catalysis and Polymerizations; Academic Press: New York, 1979. (8) Saltman, W. M. The Stereo Rubbers; John Wiley and Sons: New York, 1977. (9) Carlson, G. J.; Berkeley, W. D.; Higgins, T. L.; Cerrito, E.; Charles, H. U.S. Patent 3 066 127, 1962. (10) Upadhyay, V. K.; Sivaram, S. Effect of reaction parameters on the solution polymerization of 1,3-butadiene using cobalt (II)-2-ethylhexoatediethylaluminium chloride catalyst. Indian J. Technol. 1991, 29, 579–583. (11) Gippin, M. Polymerization of butadiene with alkylaluminium and cobalt chloride. Ind. Eng. Chem. Prod. Res. DeV. 1962, 1, 32–39. (12) Gippin, M. Stereoregular polymerization of butadiene with alkylaluminum chlorides and cobalt octoate. Ind. Eng. Chem. Prod. Res. DeV. 1965, 4, 160–167. (13) Cass, P.; Pratt, K.; Mann, T.; Laslett, B.; Rizzardo, E. Replacement of benzene with regulators for the catalyzed polymerization of 1, 3-butadine to high cis-1, 4-polybutadiene. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2244–2255. (14) Odar, J. U.S. Patent 4 314 045, 1982. (15) Odar, J. U.S. Patent 4 224 426, 1980. (16) Lasis, E.; U.S. Patent 4 242 478, 1980. (17) Van der Arend, J. C. M.; De Boer-Wildschut, M.; van der Huizen, A. A.; Kersten, M. J. E. U.S. Patent 5 691 429, 1997. (18) Tsujimoto, N.; Yano, T.; Akikawa, K.; Kotani, C.; Tsukahara K.; U.S. Patent 5 905 125, 1999. (19) Odar, J.; U.S. Patent 4 303 769, 1981. (20) Cass, P.; Pratt, K.; Fairhall, K.; Laslett, B.; Rizzardo, E. Activecenter equilibrium in Ziegler-Natta butadiene polymerization. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2256–2261. (21) Cass, P.; Pratt, K.; Mann, T.; Laslett, B.; Rizzardo, E.; Burford, R. Investigation of methylaluminoxane as a cocatalyst for the polymerization of 1,3-butadiene to high cis-1,4-polybutadiene. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 3277–3284. (22) Capello, C.; Fischer, U.; Hungerbu¨hler, K. What is a green solvent? A comprehensive framework for the environmental assessment of solvents. Green Chem. 2007, 9, 927–934. (23) Upadhyay, V. K.; Sivaram, S. Solvent effects in the polymerization of 1, 3- butadiene with Cobalt (II) 2-ethylhexoate- diethyl aluminum chloride catalyst. Indian J. Technol. 1987, 25, 669–673. (24) Porri, L.; Giarrusso, A.; Ricci, G. Recent views on the mechanism of diolefin polymerization with transition metal initiator systems. Prog. Polym. Sci. 1991, 16, 405–441. (25) Ricci, G.; Boffa, G.; Porri, L. Polymerization of 1,3-dialkenes with neodymium catalysts. Some remarks on the influence of the solvent. Makromol. Chem., Rapid Commun. 1986, 7, 355–359.

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ReceiVed for reView June 21, 2010 ReVised manuscript receiVed August 6, 2010 Accepted August 28, 2010 IE101324D