Kinetic Study on Stereospecific Polymerization of 1,3-Butadiene

Article pubs.acs.org/IECR

Kinetic Study on Stereospecific Polymerization of 1,3-Butadiene Using a Nickel Based Catalyst System in Environmentally Friendly Solvent Archana Singh,† Ankur Chavda,‡ Subrahmanyam Nandula,† Raksh Vir Jasra, and Madhuchhanda Maiti* Reliance Technology Group, Vadodara Manufacturing Division, Reliance Industries Ltd., Vadodara-391346, Gujarat, India S Supporting Information *

ABSTRACT: The present work describes the stereospecific polymerization of 1,3-butadiene catalyzed by a nickel naphthenate/ triethylaluminum/boron trifluoride etherate catalytic system in a noncarcinogenic solvent mixture. The detailed study on the effect of various parameters on the polymerization and product characteristics is reported. The catalyst concentration and reaction temperature did not have any significant effect on microstructure. Moreover, the conversion was observed to be the maximum at a 1.07 boron trifluoride/alkylaluminum molar ratio. The effect of catalyst concentration showed that at lower concentrations, a lower number of active sites leads to higher molecular weight (Mw) while the polydispersity index (PDI) exhibited a reverse trend. The polymerization was first order with respect to monomer as well as catalyst concentration. The overall rate constant was found to be 0.032 s−1 mol−1 for this catalyst system. The activation energy of the polymerization was found to be 14.1 kcal mol−1.

1. INTRODUCTION Polybutadiene rubber (BR) is the second most important synthetic general-purpose rubber produced worldwide.1 The global consumption of BR is approximately around 2.8 million metric tons per year, and it is forecasted to have an average growth of around 4.0% per year from 2010 to 2015.2 Polybutadiene rubber is a homopolymer produced by polymerization of 1,3-butadiene (BD). The butadiene molecule may enter the polymer chain either by 1,4-addition or 1,2-addition. In 1,4-addition, the unsaturated bond may be either of cis or trans configuration. According to the content of cis-BR, it is commercially available in two main forms: low cis-BR and high cis-BR. The major use of high cis-BR is in tires, with over 70% of the polymer produced going into tire sidewalls and treads. Polymerization of BD to high cis-BR is commercially done by solution polymerization using Ziegler−Natta catalysts. Titanium (Ti),3 cobalt (Co),4 nickel (Ni),5 and neodymium (Nd)6 are the most commonly used metals in the Ziegler−Natta catalyst system for the commercial production of high cis-BR.7 Among the above-mentioned systems, the cobalt-based catalyst system got the earliest recognition at the commercial level, but the major studies have been done with aromatic solvents like benzene, which is not eco-friendly. The nickel based system overcomes this drawback and offers the advantage of an ecofriendly solvent. Besides, it gives higher conversion than the cobalt-based catalyst system.7 There are a few reports on the polymerization of 1,3butadiene using nickel-based catalyst systems.8−22 Jang et al. investigated the activation of a metal alkyl-free Ni-based catalyst with tris(pentafluorophenyl)borane in the polymerization of 1,3-butadiene.12 In an excellent review paper of Ricci et al., the authors discussed that the catalyst structure strongly affects the polymerization and stereoselectivity.13 Endo et al. studied the polymerization of BD with transition metals and methylaluminoxane (MAO) catalysts and observed the highest catalytic © 2012 American Chemical Society

activities with nickel(II) acetylacetonate in combination with MAO.14 Throckmorton studied the effect of cocatalyst on the preparation of a high cis-BR using a nickel based novel ternary catalyst system.16 Schroder et al. studied the molar mass distribution of polybutadiene, synthesized with nickel octanoate based Ziegler−Natta catalysts.17,18 Kwag et al. also studied the catalyst activation process of nickel naphthenate, 1,3-butadiene, boron trifluoride etherate, and triethyl aluminum by using X-ray absorption and crystal field spectroscopy and proposed an optimum model of the nickel active site with density functional theory.19 However, very little work has been reported about the kinetics of such nickel based Ziegler−Natta catalysts. Yoshimoto et al. studied the kinetics of the nickel based system but in the carcinogenic solvent benzene.20 Lee and Hsu investigated the kinetics of the nickel stearate-diethyl aluminum chloride-water system; but such systems yield less than 96% ciscontent.21,22 The commercially used nickel based catalyst system consists of nickel naphthenate, boron trifluoride etherate, and triethyl aluminum. However, the detailed kinetics of such a system in noncarcinogenic solvent is not available in the literature. Furthermore, the effect of some polymerization reaction parameters on the polymer properties needs to be reported. Hence, in the present work, we have tried to address these concerns using nickel naphthenate as a catalyst in a noncarcinogenic solvent n-heptane/toluene (50:50) mixture. Additionally, we have studied the effect of the catalyst concentration, boron trifluoride/alkylaluminum ratio, catalyst aging temperature, as well as reaction temperature on the polymerization and product characteristics. Received: Revised: Accepted: Published: 11066

January 3, 2012 May 16, 2012 August 9, 2012 August 9, 2012 dx.doi.org/10.1021/ie300010y | Ind. Eng. Chem. Res. 2012, 51, 11066−11071

Industrial & Engineering Chemistry Research

Article

2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION 3.1. Effect of Catalyst Aging on Polymerization of 1,3Butadiene. In the commercial process, catalyst aging plays a crucial role on butadiene polymerization. The rubber produced without-aging and after aging of the catalyst is compared in Table 1. The results show that the microstructure of the rubber

2.1. Materials and Methods. 1,3-Butadiene (BD), nickel naphthenate (NIC), triethyl aluminum (TEAL), boron trifluoride etherate (BRF) solutions, phosphate of polyoxyethylene alkyl phenyl ether (PPA), and 2,6-di-tert-butyl-4methylphenol (DTBPC) were obtained from Reliance Industries Limited (RIL), Vadodara. Toluene, n-heptane, and methanol were procured from S D Fine-Chem Ltd., India. Carbon disulfide (CS2) (spectroscopic grade) was obtained from Labort Fine Chem Pvt. Ltd., India. The mixture of reactants consists of a proportional amount of BD and solvent and will be herein after called “feed”. BD was purified by passing through two successive columns containing TEAL solution and molecular sieves 3A. This was then solubilized in dry n-heptane/toluene (50:50) mixed solvent to prepare feed. Polymerization was carried out in a jacketed 1 L laboratory glass reactor (Buchi AG USTER, Switzerland). All the glassware was oven-dried prior to use and used under strict exclusion of moisture. Only dried and purified solvents were used throughout the study.23 Utmost care was taken while handling pyrophoric alkylaluminums. The external solutions and reaction feed were prepared using a vacuum manifold. The microstructure of BR was determined using a Fourier transform infrared (FTIR) spectrophotometer (Perkin-Elmer 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.24 The molecular weight and molecular weight distribution were studied using gel-permeation chromatography (GPC, Perkin-Elmer series 200) with a refractive index (RI) detector at 30 °C. Samples were dissolved in HPLC grade tetrahydrofuran at a concentration of 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 a flow rate of 1.0 mL/min using polystyrene as the standard (molecular weight range was 582−6 00 000). 2.2. Polymerization of 1,3-Butadiene in n-Heptane/ Toluene (50:50) Mixed Solvents. In a typical polymerization process, 42 mmol per 100 g of monomer (mmphgm) of BRF was charged in a Buchi reactor and to it 39 mmphgm of TEAL was added. Then 3.8 mmphgm of NIC was added to it. This catalyst mixture was aged for 1 h at 20 °C. The temperature was maintained in the reactor with the help of a temperature bath. Then the earlier prepared 350 mL of feed (20% (w/w) of BD in dry solvent) was added to the reactor and the temperature was increased to 60 °C. The reaction was conducted for 3 h. Stirring was done at 200 rpm. Following this, the calculated amount of PPA was added to the reactor to kill the catalyst activity. The rubber was coagulated by adding a methanolic solution of 0.5% (w/v) DTBPC. The filtered rubber was washed with fresh methanol and kept for air drying overnight. The sample was cut into small pieces and oven-dried at 40 °C for 6 h under vacuum. The efficiency of the reaction was evaluated as the weight % of rubber produced with respect to the monomer fed. Reactions were carried out in a similar manner as mentioned above using desired catalyst/cocatalyst ratios.

Table 1. Effect of Catalyst Aging on Characteristics of Rubber characteristics conversion at 2 h, % microstructure cis, % trans, % vinyl, % density, gcm−3 Mooney viscosity, ML1+4 at 100 °C weight average molecular weight (Mw) polydispersity index

without aging

with aging

88

52

97.3 2.0 0.7 0.92 41 2.86 × 105 3.41

97.2 0.8 2.0 0.88 48 3.21× 105 2.86

is not affected by catalyst aging, as stereoselectivity of the active sites does not change with aging.25 However, the molecular weight and Mooney viscosity in the absence of catalyst aging appears at a lower side with a broad molecular weight distribution. The aging temperature plays an important role in butadiene polymerization. The plot of conversion vs aging temperature (Figure 1) illustrates that the conversion is maximum at the

Figure 1. Effect of aging temperature on conversion. Polymerization conditions: monomer = 2.64 mol/L, catalyst aging time = 60 min, reaction temperature = 60 °C, reaction time = 120 min, NIC = 3.8 mmphgm, TEAL = 39 mmphgm, BRF/TEAL = 1.07.

aging temperature of 20 °C. Above or below this particular temperature (20 °C), a significant drop is observed in butadiene conversion. This may be due to thermal degradation of catalytic species at a higher temperature.25 However, higher molecular weight is observed below or above 20 °C. 3.2. Effect of Catalyst Components on Polymerization of 1,3-Butadiene. Figure 2 shows the dependency of the butadiene monomer conversion upon the BRF/TEAL molar ratio. The conversion increases exponentially up to 56% at 39 mmphgm of TEAL (BRF/TEAL molar ratio = 1.07) and then decreases. Boron trifluoride reacts with triethyl aluminum in a stoichiometric ratio to generate active cocatalyst. Higher BRF is required because boron trifluoride as a Lewis acid will react with alkylnickel to give a stable complex which could be an active species as per the proposed mechanism by Furukawa.26 11067

dx.doi.org/10.1021/ie300010y | Ind. Eng. Chem. Res. 2012, 51, 11066−11071


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Figure 5 illustrates that the conversion shows an increasing trend with the TEAL/NIC ratio up to a certain specific molar

Figure 2. Effect of BRF/TEAL on conversion. Polymerization conditions: monomer = 3.22 mol/L, catalyst aging time = 60 min, catalyst aging temperature = 20 °C, reaction temperature = 60 °C, reaction time = 180 min, NIC = 3.8 mmphgm.

Figure 5. Effect of TEAL/NIC on conversion. Polymerization conditions: monomer = 3.22 mol/L, catalyst aging time = 60 min, catalyst aging temperature = 20 °C, reaction temperature = 60 °C, reaction time = 180 min, BRF/TEAL molar ratio = 1.07.

Figure 3 shows that the conversion increases with an increase in concentration of the catalyst as usual. Effect of catalyst

ratio (TEAL/NIC = 10). With a further increase in this ratio, the conversion starts falling down. 3.3. Effect of Reaction Temperature on Polymerization. Figure 6 illustrates that the monomer conversion and

Figure 3. Effect of catalyst (NIC) concentration on conversion.

concentration on the characteristics of the polymer is shown in Figure 4. At lower catalyst concentration, Mw is higher but

Figure 6. Effect of temperature on conversion, Mw and PDI. Polymerization conditions: catalyst aging time = 60 min, catalyst aging temperature = 20 °C, reaction time = 180 min, NIC = 3.8 mmphgm, BRF/TEAL molar ratio = 1.07, monomer = 2.64 mol/L.

molecular weight of butadiene rubber increases with increasing temperature. Whereas PDI shows a reverse trend, it decreases with increasing temperature. The reaction temperature was reported to have a relatively small effect on molecular weight in the solvent benzene.20 The microstructure of polymer synthesized using various catalyst concentrations at different temperatures is reported in Table 2. All the samples show a cis-content of >97%, with around 2% of the trans-content and balance being vinylcontent. Neither catalyst concentration nor temperature has any pronounced effect on the microstructure. Similarly, the microstructure of polymer was reported to be unaltered at different temperatures as well as catalyst concentrations in conventional solvent such as benzene.20 3.4. Kinetic Studies. The polymerization conditions for the kinetic studies were fixed as per the earlier study of single parameters. The conditions are unless otherwise mentioned: TEAL = 39 mmphgm, BRF/TEAL = 1.07, TEAL/NIC = 10,

Figure 4. Effect of catalyst (NIC) concentration on molecular weight and PDI.

decreases at a higher catalyst dose. At lower catalyst concentration, a lower number of active sites leads to higher molecular weight. A similar trend can be observed for this particular catalyst system in conventional solvent, benzene.20 The polydispersity index (PDI) shows a reverse trend. Higher PDI indicates that chain transfer is more favorable than propagation at higher catalyst concentrations. 11068



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Table 2. Microstructure of Different Polymers microstructure catalyst concentration (mmphgm)

temperature (°C)

cis (%)

trans (%)

vinyl (%)

2.8 3.8 4.8 3.8 3.8

60 60 60 55 65

97.6 97.2 97.4 97.7 97.4

1.7 2.0 1.8 1.6 1.9

0.7 0.8 0.7 0.7 0.8

catalyst aging temperature = 20 °C, catalyst aging time = 60 min, and reaction temperature = 60 °C. A typical plot of time vs conversion is shown in Figure 7.

Figure 8. 1st order plot of the polymerization (at monomer concentration 2.64 mol/L).

−ln(1 − x) = k[CC]b t

(4)

Figure 9 shows the dependence of the reaction rate on catalyst concentration at a constant initial monomer concen-

Figure 7. Monomer conversion as a function of the polymerization time.

Initially polymerization proceeds rapidly after that the rate gradually slows down. The curve does not pass through the origin due to slight formation of polymer during the heating period prior to the normal polymerization. The following equation is considered for the overall kinetics of the stereospecific polymerization of butadiene in solution.27 − dC M = k[CM]a [CC]b dt

Figure 9. 1st order plot of the polymerization of butadiene at various catalyst concentrations, at a monomer concentration = 2.64 mol/L.

tration (2.64 mol/L). The slope of the lines gives the value of the rate constant at each catalyst concentration. These rate constant values are plotted as a function of the catalyst concentration (keeping the molar ratio of catalyst-cocatalyst and promoter constant) (Figure 10). Since, the points

(1)

where −dCM/dt = rate of disappearance of monomer/rate of polymerization, k = rate constant, CM = monomer concentration, mol/L, a = reaction order with respect to monomer concentration, CC = catalyst concentration, b = reaction order with respect to catalyst concentration, and t = reaction time. The experimental time−conversion data for different monomer concentrations (Table S1 in the Supporting Information) were fitted, and a representative plot is shown in Figure 8. The linearity of this plot indicates that the reaction is truly first order with respect to the monomer concentration. With this justification, the reaction order was taken as first order (a = 1) with respect to monomer concentration, so eq 1 becomes

− dC M = k[CM][CC]b dt

Figure 10. Plot of k as a function of catalyst concentration. (2)

Differential eq 2 is integrated between CM0 and CM and rearranged.

( ) = k[C ] CM C M0

−ln

t

fall on a straight line, the conclusion can be made that b is equal to 1, i.e., the polymerization rate with respect to the catalyst concentration is of the first order. The first order reaction has been reported in the literature using a similar catalyst system but in a conventional solvent, benzene.20 The slope of the line

b

C

(3) 11069



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in Figure 10 is as follows: {(−ln(1 − x)/t)/[CC]} = k = 0.032 s−1 mol−1, which is the overall rate constant. 3.5. Determination of Activation Energy. The activation energy was determined according to the Arrhenius equation.27 k = f (T ) = A e−E / RT

properties (conversion, molecular weight, microstructure, and PDI) are investigated. The study shows that the catalyst aging temperature plays a crucial role in the final conversion. The molar ratio of promoter to cocatalyst and cocatalyst to catalyst are significant parameters in governing the polymerization. The microstructure of rubber is not affected by the catalyst concentration and reaction temperature. The detailed kinetics study in the n-heptane/toluene 50:50 solvent mixture illustrates that the polymerization is first order with respect to monomer concentration as well as catalyst concentration, with an overall rate constant of 0.032 s−1 mol−1 and activation energy of 14.1 kcal mol−1.

(5)

where T is the absolute temperature, E is the activation energy, and R is the universal gas constant = 1.98 cal K−1 mol−1. The polymerization reactions were carried out at 55, 60, and 65 °C at constant catalyst and monomer concentrations. The rate constant at different temperatures were calculated from the slope of the line plotted in Figure 11.

■

ASSOCIATED CONTENT

* Supporting Information S 13

C NMR spectra of a representative sample for the determination of the cis-content of rubber and the experimental time−conversion data for different monomer concentrations. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91-265-6693934. Present Addresses

Figure 11. Effect of temperature on reaction rate.

†

Institute of Technology, Nirma University, Sarkhej-Gandhinagar Highway, Ahmedabad-382481, Gujarat, India. ‡ Dharmsinh Desai University, Nadiad-387001, Gujarat, India.

The rate constant values are reported in Table 3. Figure 12 shows the plot of log k vs 1/T. The slope of the line = −E/2.3R = −3091.37. So, the activation energy E = 14 078 cal mol−1 = 14.1 kcal mol−1.

Notes

The authors declare no competing financial interest.

Table 3. Effect of Temperature on the Rate of Reaction temperature (K)

k (s−1)

1/T

log k

328 333 338

0.000 024 0.000 035 0.000 046

0.003 05 0.003 00 0.002 96

−4.6199 −4.4590 −4.3413

■

ACKNOWLEDGMENTS

■

REFERENCES

The authors thank Mr. Dinesh S. Pandya, Mr. Chirag S. Shah, Mr. Suraj K. Kusum, and Mrs. Swati G. Trivedi for their support. The authors are grateful to Reliance Industries Ltd. for their consent to publish this work.

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Figure 12. Plot of log k vs 1/T.

4. CONCLUSIONS In this work, we report the polymerization of 1,3-butadiene catalyzed by the nickel naphthenate/triethylaluminum/boron trifluoride etherate catalytic system. The effects of several parameters such as catalyst aging, concentration of catalyst components, and reaction temperature on the polymer 11070



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(8) Pawlow, J. H.; Hogan, T. E. Nickel Catalyst System for the Preparation of High cis Polybutadiene. U.S. Patent Application 20100093920, April 15, 2010. (9) Qin, Z.; Poulton, J. T.; Roggeman, D. M.; Matsuo, S. Nickel-Based Catalyst Composition. U. S. Patent Application 20080255327, October 16, 2008. (10) Rachita, M. J.; Johnson, S. E. Synthesis of 1,4-Polybutadiene. U.S. Patent 7,081,504, July 25, 2006. (11) Kitayama, T.; Shimizu, I.; Yoshimura, Y.; Kamata, M. Japanese Patent JP59117512, July 06, 1984. (12) Jang, Y.; Choi, D. S.; Han, S. Effects of tris(pentafluorophenyl) borane on the activation of a metal alkyl-free Ni-based catalyst in the polymerization of 1,3-butadiene. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 1164−1173. (13) Ricci, G.; Sommazzib, A.; Masic, F.; Ricci, M.; Bogliaa, A.; Leonea, G. Well-defined transition metal complexes with phosphorus and nitrogen ligands for 1,3-dienes polymerization. Coord. Chem. Rev. 2010, 254, 661−676. (14) Endo, K.; Uchida, Y.; Matsuda, Y. Polymerizations of butadiene with Ni(acac)2-methylaluminoxane catalysts. Macromol. Chem. Phys. 1996, 197, 3515−3521. (15) Porri, L.; Giarrusso, A. Comprehensive Polymer Science, Vol. 4; Pergamon Press Ltd.: Oxford, U.K., 1989. (16) Throckmorton, M. C. Butadiene polymerization with nickel compounds: Effect of cocatalysts. J. Appl. Polym. Sci. 1991, 42, 3019− 3024. (17) Schroder, K.; Schmitz, G.; Lechner, M. D.; Gehrke, K. Molar mass distribution of polybutadiene synthesized with nickel-based Ziegler-Natta catalysts. I. Catalytic Behaviors. Angew. Makromol. Chem. 1994, 218, 153−162. (18) Schroder, K.; Schmitz, G.; Lechner, M. D.; Gehrke, K. Molar mass distribution of polybutadiene synthesized with nickel-based Ziegler-Natta catalysts. II. Kinetic Study. Angew. Makromol. Chem. 1994, 218, 163−170. (19) 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. (20) Yoshimoto, T.; Komatsu, K.; Sakata, R.; Yamamoto, K.; Takeuchi, Y.; Onishi, A.; Ueda, K. Kinetic study of cis-1,4 polymerization of butadiene with nickel carboxylate/boron trifluoride etherate/triethylaluminum catalyst. Angew. Makromol. Chem. 1970, 139, 61−72. (21) Lee, D.; Hsu, C. C. Polymerization of butadiene in toluene with nickel(II) stearate-diethyl aluminum chloride catalyst. I. Catalytic behaviors. J. Appl. Polym. Sci. 1980, 25, 2373−2392. (22) Lee, D.; Hsu, C. C. Polymerization of butadiene in toluene with nickel(II) stearate-diethyl aluminum chloride catalyst. II. Kinetic study. J. Appl. Polym. Sci. 1981, 26, 653−666. (23) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals; Pergamon Press: London, 1988. (24) Ciampelli, F.; Morero, D.; Cambini, M. Some remarks on the infrared analysis of polyisoprenes. Makromol. Chem. 1963, 61, 250− 253. (25) Ivana, L. M.; Fernanda, M. B. C. Neodymium Ziegler−Natta catalysts: Evaluation of catalyst ageing effect on 1,3-butadiene polymerization. Eur. Polym. J. 2008, 44, 2893−2898. (26) Junji, F. Mechanism of diene polymerization. Pure Appl. Chem. 1975, 42, 495−508. (27) Levenspiel, O. Chemical Reaction Engineering, 3rd ed.; John Wiley & Sons: New York, 1999.

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Kinetic Study on Stereospecific Polymerization of 1,3-Butadiene

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