and Stereoselective Polymerization of 1,3-Butadiene Initiated by Iron

Jun 29, 2016 - designed, synthesized, and applied in an iron catalyst as donors for 1,2-stereopolymerization of 1,3-butadiene in hexane. In combinatio...
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Highly 1,2 Regio- and Stereoselective Polymerization of 1,3Butadiene Initiated by Iron Catalysts with Pyridinyl Phosphate Weijing Pan,† Huafeng Chen,† Rui Sun,† Dirong Gong,*,† Xiayu Jia,§,∥ Yanming Hu,‡ and Xuequan Zhang*,‡ †

Department of Polymer Science and Engineering, Faculty of Materials Science and Chemical Engineering, Key Laboratory of Specialty Polymers, Ningbo University, Ningbo 315211, People’s Republic of China ‡ Key Laboratory of Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, People’s Republic of China § Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, No. 1799, Jimei Road, Xiamen, Fujian 361021, People’s Republic of China ∥ Ningbo Urban Environment Observation and Research StationNUEORS, Chinese Academy of Sciences, Ningbo, Zhejiang 315830, People’s Republic of China ABSTRACT: Three pyridinyl based compounds, diphenylpyridin-2-yl phosphate (DPPyP), phenyldipyridin-2-yl phosphate (PDPyP), and tripyridin-2-yl phosphate (TPyP), were designed, synthesized, and applied in an iron catalyst as donors for 1,2-stereopolymerization of 1,3-butadiene in hexane. In combination with AliBu3, the DPPyP assisted iron catalyst is capable of 1,2-syndiotactic polymerization (1,2-selectivity, syndiotacticity) of 1,3-butadiene with high activity reaching 258 480 kg(polymer)/mol(Fe)·h at a butadiene/Fe feeding ratio of 8000 at 50 °C. The 1,2-regioselectivity, activity, and the morphology of resultant polymers can be controlled by the donor structures. The thermal stability of the catalysts were significant because a remarkably enhanced activity was observed while the high 1,2-stereoselectivity was unaltered in the range 50−80 °C. This work enabled us to have access to a highly active, 1,2-syndiotactic and thermally robust catalyst with ease of catalyst preparation, which could match the availability in industrial application for 1,2-syndiotactic polybutadiene.

1. INTRODUCTION The fine control of regioselectivity and/or stereoselectivity is of prime importance in diene polymerization, because the resultant chemical structures are crucial to the polymer properties serving various applications. In particular, 1,3butadiene is an attractive monomer conveniently available from steam cracking, and its regioselective and/or stereoselective polymerization gives access to three different isomers, cis-1,4, trans-1,4, and 1,2 polymers, as well as their diversified combinations. 1,2-Polybutadiene can be incorporated in three configurations; of special note is the syndiotactic 1,2polybutadiene, which is a high-vinyl polybutadiene having a stereoregular structure where the side-chain vinyl groups are located alternately on opposite sides in relation to the polymeric main chain. As a result, syndiotactic 1,2-polybutadiene is a crystalline thermoplastic resin that uniquely exhibits the properties of both plastics and rubber,1 and therefore it has many uses. For example, films, fibers, and various molded articles can be made from syndiotactic 1,2-polybutadiene.2−5 It can also be blended into and co-cured with natural or synthetic rubbers in order to improve the properties thereof. Syndiotactic 1,2-polybutadiene can be made by solution, emulsion,6,7 or suspension8 polymerization, where typically © 2016 American Chemical Society

metal compounds are required for chain initiation and microstructure regulation. To date, various transition metal titanium, vanadium, and molybdenum based Ziegler−Natta catalysts have been exploited for the preparation of syndiotactic 1,2-polybutadiene.9 However, the majority of these catalyst systems have no practical utility; typically, they are of low catalytic activity or poor stereoselectivity and in some cases produce low molecular weight polymers or cross-linked polymers unsuitable for commercial use. Recently, 1,2-stereoselective control of polybutadiene has been reported by Ricci and co-workers 10−13 using new chromium and cobalt postmetallocene catalysts ligated with phosphine ligand; however, the tedious synthetic procedures with associated high cost of catalyst components could present challenges for industrial production. Yet two dominant industrial polymerization catalysts, Co(acac)3/AlEt3/CS214 and Co(acac)3/AlEt3/ H2O/PPh315 (halogenated solvent as polymerization medium), have presented serious problems. The obnoxious smell, high Received: Revised: Accepted: Published: 7580

April 3, 2016 June 17, 2016 June 29, 2016 June 29, 2016 DOI: 10.1021/acs.iecr.6b01274 Ind. Eng. Chem. Res. 2016, 55, 7580−7586

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Industrial & Engineering Chemistry Research volatility, low flash point as well as toxicity, carbon disulfide, and halogenated solvent are difficult and dangerous to use and require expensive safety measures to prevent even minimal amounts escaping into the atmosphere. Another issue is that the higher crystallinity and melting point of the resultant polymer from the former system render the polymer processing rather energy consuming and risk thermal cross-linking. The iron compounds as butadiene catalysts are beneficial due to the low cost and earth abundance, as well as the biocompatibility and negligible environmental impact. The development of iron catalysts has, in large measure, been driven by a growing appreciation of the role played by the donor in influencing the catalytic performance. In the early stage in the development of iron-based catalyst systems, the most commonly applied electron donors are anilines,16 1,10phenanthroline,17,18 and bipyridine,19 but they have very low catalytic activities and poor stereoselectivities, sometimes giving rise to oligomers, low molecular weight liquid polymers, or gels. The discovery of dialkyl(aromatic)phosphites (DAP, Scheme 1) has sparked tremendous interest in butadiene polymer-

promising catalysts, we designed pyridine based phosphates for iron catalysts in stereopolymerization (Scheme 2). It is anticipated that the greater electronic deficient donor frame might allow a more stable stereoenvironment to be maintained throughout the polymerization process, which could generate strictly stereorregulated polymers.

2. EXPERIMENTAL SECTION 2.1. Materials. Triphenylphosphate was purchased from TCI. 2-Hydroxypyridine and AliBu3 (1.1 mol/L in toluene) were purchased from Energy chemical. Diphenyl chlorophosphate, phenyl dichlorophosphate, and triethylamine were purchased from Alfa Aesar. Phosphoryl bromide was purchased from Acros Organics. Polymerization grade butadiene was purchased from Valley Gas Co., Ltd., and purified by nbutyllithium prior to use. All solvents used were purified by the standard procedures. 2.2. Synthesis and Characterization of Donor. The synthetic way for diphenylpyridin-2-yl phosphate (DPPyP) was described. Diphenyl chlorophosphate (2.96 g, 11.01 mmol) was then added dropwise to a solution of 2-hydroxypyridine (1.00 g, 10.56 mmol) and triethylamine (2.90 g, 28.66 mmol) in 40 mL of THF at 40 °C in a 100 mL round-bottom flask under vigorous stirring. The mixture was kept at 40 °C for 8 h, and a solution of Na2CO3 (5.0 g) in water (50 mL) was added. The filtrate was concentrated by a rotary evaporator, redissolved in dichloromethane, washed with water (3 × 15 mL), and dried by anhydrous Na2SO4 overnight. The final product was recrystallized from ethanol and collected as a white solid (DPPyP, 2.19 g, 63%). Compounds (PDPyP and TPyP) were obtained by a similar procedure with yields of 57 and 43%, respectively. Diphenylpyridin-2-yl phosphate (DPPyP): Yield: 63%. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.35 (dd, J = 4.9, 1.6 Hz, 1H, o-pyH), 7.76 (t, J = 7.7 Hz, 1H, p-pyH), 7.41−7.31 (m, 8H, o-/m-arylH), 7.26−7.17 (m, 3H, m-pyH/p-arylH), 7.05 (d, J = 8.2 Hz, 1H, m-pyH). 13C NMR (101 MHz, CDCl3, δ, ppm): 157.29 (d, J = 6.1 Hz), 150.59 (d, J = 7.4 Hz), 148.10 (s), 140.21 (s), 129.80 (s), 125.59 (s), 121.37 (s), 120.29 (dd, J = 10.0, 4.9 Hz), 113.33 (d, J = 7.7 Hz). 31P NMR (162 MHz, CDCl3, δ, ppm): −19.20 (s). Phenyldipyridin-2-yl phosphate (PDPyP): Yield: 57%. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.31 (d, J = 3.1 Hz, 2H, opyH), 7.77 (t, J = 7.7 Hz, 2H, p-pyH), 7.47−7.31 (m, 4H, o-/marylH), 7.18 (ddd, J = 17.2, 11.9, 7.8 Hz, 5H, m-pyH/p-arylH). 13 C NMR (101 MHz, CDCl3, δ, ppm): 157.32 (d, J = 6.0 Hz), 150.68 (d, J = 7.6 Hz), 148.00 (s), 140.17 (s), 130.28−128.85 (m), 125.55 (s), 121.32 (s), 120.60 (t, J = 11.7 Hz), 113.58 (d, J = 7.7 Hz). 31P NMR (162 MHz, CDCl3, δ, ppm): −20.37 (s). Tripyridin-2-yl phosphate (TPyP): Yield: 43%. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.29 (dd, J = 4.9, 1.7 Hz, 3H, opyH), 7.78 (t, J = 7.8 Hz, 3H, p-pyH), 7.25 (d, J = 8.2 Hz, 3H, m-pyH), 7.18 (dd, J = 7.2, 5.1 Hz, 3H, m-pyH). 13C NMR (101 MHz, CDCl3, δ, ppm): 157.40 (d, J = 5.9 Hz), 147.92 (s),

Scheme 1. Structure of DAP Investigated in Our Study

ization and led to very useful iron-based catalysts for synthesis of syndiotactic 1,2-polybutadiene possessing syndio arrangements (1,2-selectivity 87−95%; pentad rrrr denoted syndiotacticity 81−90%), atactic 1,2-polybutadiene (as elastomer, 1,2selectivity 55−87%) as well as their block copolymer and in situ formed blend by controlling the activators feeding at industrially favored polymerization conditions.20−23 Among the many advantages of these catalyst systems are their high activity and selectivity in nonaromatic and nonhalogenated solvents such as aliphatic and cycloaliphatic solvents, which are environmentally preferred, and the ease of preparation (readily available and/or high thermal and chemical stability of catalyst components) and easy handling of catalyst components. This resurrected interest in the chemistry of iron complexes as catalysts toward butadiene activation. With understanding of the factors responsible for both activity and selectivity, therefore, to enhance the potential for stereoselective insertion chemistry and with the ultimate view of tailoring new and more Scheme 2. Donors Investigated in This Study

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Industrial & Engineering Chemistry Research Table 1. Effect of Donors on Polymerization Behaviorsa microstructureb (%) run

donor

conv (%)

1,2

trans-1,4

Tmc (°C)

Xcd (%)

rrrre (%)

Lve

Mnf (104)

MWDg

1 2 3 4 5

nonef TPP DPPyP PDPyP TPyP

0 61.74 76.87 70.82 51.48

88.18 94.17 75.47 77.48

11.82 5.83 1.26 8.64

177.08 173.95 115.00 155.80

55.17 32.90 1.69 9.34

95.7 72.7 37.5 49.2

10.69 7.06 3.36 4.70

7.4 3.7 13.0 5.1

2.99 5.47 3.15 2.69

a Polymerization conditions: in hexane at 50 °C for 8 h, [Bd] = 2.0 mol/L, [Al]/[Fe] = 40, and [P]/[Fe] = 4 (mol/mol). bDetermined by NMR (1H and 13C) and IR. cMelting point, determined by DSC at heat rate of 10 °C/min. dEstimated by the formula of ΔH/ΔH0; ΔH was calculated by DSC and ΔH0 referred to standard enthalpy of 1,2-polybutadiene with 100% crystallinity, equal to 60.7 J/g. eNumber-average length of 1,2-units and syndiotactic configuration rrrr was calculated by 13C NMR. fGPC data in trichlorobenzene vs polystyrene standards. gCatalyst without donor.

140.07 (s), 121.23 (s), 113.82 (d, J = 7.7 Hz). 31P NMR (162 MHz, CDCl3, δ, ppm): −20.90 (s). 2.3. Procedure for Butadiene Polymerization. The polymerization was carried out with DPPyP as an example. The purified 1,3-butadiene was prepared in a hexane solution with a concentration of 2.0 mol/L (10 mL) in an oxygen- and moisture-free ampule with a rubber cap. The catalyst components Fe(acac)3 (0.1 mL, 0.1 mol/L) and DPPyP (0.5 mL, 0.08 mol/L) were injected and homogenized. AliBu3 (0.36 mL, 1.1 mol/L) was then injected into the ampule to trigger polymerization. The polymerization was maintained at 50 °C for 8 h. Methanol (2 mL) containing 2,6-di-tert-butyl-4methylphenol (3.0 wt %) was added to quench the polymerization. Then the mixture was poured into a large quantity of methanol to precipitate the white solid. The resultant polymer was repeatedly washed with methanol and dried under vacuum at 40 °C to constant weight. 2.4. Characterization. 1H NMR (400 MHz), 13C NMR (100 MHz), and 31P NMR (162 MHz) were recorded on a Varian Unity spectrometer in CDCl3 at room temperature for pyridinyl compounds or in o-C6D4Cl2 at 125 °C for polymers. Fourier transform infrared (FTIR) spectra and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of the samples were recorded on a Nicolet 6700. The proportion of 1,2, cis-1,4, and trans-1,4 units as well as the sequence lengths of 1,2-units (Lv) of polymer were determined as in reported literature.24−28 The differential scanning calorimetry (DSC) analyses of the samples were conducted under a nitrogen atmosphere by a PerkinElmer DSC8000. In order to erase previous thermal history, polybutadiene was first heated from 50 to 200 °C at a rate of 10 °C/min. Data were collected from the cycle cooled to 50 °C and reheated from 50 to 200 °C at 10 °C/min under nitrogen atmosphere. Number-average molecular mass (Mn) and molecular weight distribution (MWD) were determined at 150 °C by a PL-GPC 220 instrument (Polymer Laboratories, U.K.) equipped with two columns (10 μm o.d., 7.8 mm i.d., 300 mm length). HPLC grade 1,2,4-trichlorobenzene was used as the mobile phase at a flow rate of 1.0 mL/min. The calibration was made by polystyrene. X-ray spectra were carried out at Xenocs. The synchrotron radiation wavelength was 1.541 Å. Two-dimensional (2D) wide-angle X-ray diffraction (WXRD) patterns were collected with a resolution of 1024 × 1024 pixels (pixel size = 158.44 μm). The sample-to-detector distance was 262.5 mm, and the scattering angle was calibrated by silver behenate (AgBe). All Xray images were adjusted for background scattering, air scattering, and beam fluctuations.

3. RESULTS AND DISCUSSION 3.1. Donor Effect. Three compounds, DPPyP, PDPyP, and TPyP, were individually examined in stereospecific polymerization of 1,3-butadiene as donor; the polymerization parameters and results are collected in Table 1. The contrasting experiments in the presence of donor TPP or in the absence of any donor are also included. As shown in run 1 at Table 1, the absence of donor had no polymerization activity toward butadiene (Bd). With phosphate as donor, all catalysts actively transformed butadiene to 1,2-polybutadiene with controllable crystallinity. The DPPyP catalyst had a very high 1,2-selectivity of 94.17% and syndiotacticity of 72.0% calculated from 13C NMR (Figure 1), more 1,2-regio- and stereoselectivity than the

Figure 1. DPPyP.

13

C NMR of polybutadiene obtained from catalyst with

TPP system (88.18% of 1,2-selectivity, run 2, Table 1). Significantly, this high degree of stereocontrol did not come with a sacrifice in activity; an acceptable polymer yield of 76.87% was obtained (run 3, Table 1). To further probe the polymer characteristics, we investigated the thermal−physical properties of polymer using DSC and XRD. The obtained polymer has a melting point of 174 °C with a moderate crystallinity. The PDPyP and TPyP assisted catalysts, where phenyl groups were partially replaced with pyridine, were less productive and had polymer yields of 70.82 and 51.48%, respectively. The 1,2-selectivities were also reduced to 75.47 and 77.48% (runs 4 and 5, Table 1), respectively. The FTIR spectra (Figure 2) showed that the transmittance signal attributed to the cis-1,4 configuration at 735 cm−1 was not found in the DPPyP donated catalyst, but was visible in PDPyP and TPyP donated catalysts. Clearly, the presence of pyridine 7582

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Figure 2. FTIR spectra of polybutadiene obtained from different donors.

Figure 4. Two-dimensional WXRD of polybutadiene obtained from different donors.

moiety reduced the proportion of 1,2 insertion but increased cis-1,4. The monomodal gel permeation chromatographic (GPC) curves of polymers were all observed (Figure 3), and

Figure 5. X-ray spectra of polybutadiene obtained from different donors.

Figure 3. GPC of polybutadienes obtained from four catalysts.

no evidence for low-molecular-weight oligomeric materials could be obtained from any run. The molecular weights of polymers and their distributions were varied with donor in an irregular trend (runs 3−5, Table 1), indicating various competing reaction pathways could be operative, including chain propagation, transfer, and termination reactions influenced by the donor to different extents. The morphology of corresponding products became elastomeric with significantly weakened crystallinity, as seen from Figure 4. The crystallinity behaviors were studied by WXRD. Four characteristic diffraction peaks at 2θ = 13.4, 16.1, 21.1, and 23.6° ascribed to the 1,2-syndiotactic polybutadiene were observed.29 The indices for the four peaks from low to high angle correspond to (010), (200)/(110), (210), and (111)/ (201), respectively.30 As illustrated in Figure 4 and Figure 5, with TPP as donor, the product obtained had clearly four crystalline peaks, but the crystalline peak attenuated significantly in DPPyP donated catalyst. This was in good accordance with the DSC observation (Figure 6), where the polymers from DPPyP catalyst system had a lower melting point, which could be conducive to processing. In both cases of PDPyP and TPyP, the products had lower melting points, and were much more amorphous observed from WXRD, demonstrating that the catalytic species generated from PDPyP or TPyP were less 1,2-

Figure 6. DSC of polybutadienes obtained from different donors.

stereospecific and more 1,4-enchained, both leading to a decreased length of 1,2-stereo block observed in 13C NMR. In addition, the product with higher 1,2-unit content prepared with DPPyP catalyst had a lower stereoregularity and shorter block length of the 1,2-units than those from the TPP catalyst 7583

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Industrial & Engineering Chemistry Research system. This may also be relevant to the decreased block length of the 1,2-units and 1,2-selectivity from PDPyP and TPyP compared to TPP catalyst. 3.2. Cocatalyst Type and Loading. In addition to AliBu3, trimethylaluminum (TMA), triethylaluminum (TEA), diisobutylaluminum chloride (AliBu2Cl), and methyl aluminoxane (MAO) were also used individually as cocatalysts to see whether different alkylaluminum compounds have any influence on the polymerization. In this phosphate donated iron catalyst system, we found only AliBu3 serves as a suitable activator for butadiene polymerization. The TMA and TEA activated catalysts, however, had significantly low polymer yields, and the Lewis type chloride aluminum and MAO are both completely inactive to butadiene. It was reasoned that the bulky size and moderate alkylation capability could be wellsuited to generating 1,2-stereoselective active species. These results indicated the catalytic species responsible for chain propagation is characteristic of Ziegler−Natta type.33,34 It is reported that the cocatalyst loading is an important parameter influencing the polymerization behaviors for tertiary polymerization catalysts.21,31,32 The effects of Al/Fe molar ratios on the polymerization of butadiene with the Fe(acac)3/ AliBu3 catalysts are shown in Figure 7. The polymer yield was

Figure 8. Effect of [Bd]/[Fe] ratio on polymerization behaviors with DPPyP donated catalyst system. Polymerization conditions: in hexane at 50 °C for 8 h, [Bd] = 2.0 mol/L, [Al]/[Fe] = 40, and [P]/[Fe] = 4 (mol/mol).

6000, the yield was kept at a high value from 79.15 to 75.44% (Figure 8), and then declined to 68.87 and 66.26% at 7000 and 8000, respectively. The slightly decreased 1,2-selectivity also went along with increasing monomer feeding, reaching a constant value of about 90% at higher monomer addition. The crystallinity also had a slightly decreased trend and stabilized around 30%. Presumably, 1,4 isomers originated from η4butadiene coordination at iron center increased at higher monomer concentration.25,30,31 3.4. Thermal Stability. Deactivation of iron-based catalysts usually occurs at high reaction temperature, which has been a critical deficiency because of the highly exothermic reaction of polymerization as well as fast chain transfer at high temperature, and thus leading to undesirable and odorous oligomer or a rapid decay in activity.35−38 In the current catalyst system, strikingly, we found the catalytic reactions were significantly promoted when the reaction temperature was increased from 15 to 80 °C (Figure 9). The polymerization achieved a conversion of 78.45% at 80 °C just in 30 min, but only 5.24%

Figure 7. Effect of [Al]/[Fe] ratio on polymerization behaviors with DPPyP donated catalyst system. Polymerization conditions: in hexane at 50 °C for 8 h, [Bd] = 2.0 mol/L, [Bd]/[Fe] = 2000, and [P]/[Fe] = 4 (mol/mol).

significantly low, below 30, and then increased with increasing of cocatalyst loading and leveled off at [Al]/[Fe] of 40 (yield 76.87%). Surprisingly, the Al/Fe molar ratio did not have an obvious influence on 1,2-selectivity in a wide range of 20−60: a high 1,2-selectivity 87.8−94.2% remained unaltered. The higher Al/Fe molar ratio at 70 brought a slight deterioration of 1,2selectivity. These indicated the 1,2-stereospecificity is of significant stability against activator variation. However, the crystallinity was affected by lowering the Al/Fe molar ratio with 20.1% at [Al]/[Fe] of 20, reaching a maximum 32.9% at [Al]/ [Fe] of 40, and then declining with increasing [Al]/[Fe] molar ratio. 3.3. Effects of Catalyst/Monomer Ratio. A series of polymerization experiments by changing the amount of catalyst (monomer concentration was maintained at 2.0 mol/L) were carried out to further evaluate the performance of the DPPyP donated catalyst system. Some representative results are listed in Figure 8. Elevating the Bd/Fe molar ratios from 2000 to

Figure 9. Effect of temperature on polymerization behaviors with DPPyP donated catalyst system. Polymerization conditions: in hexane for 30 min, [Bd] = 2.0 mol/L, [Bd]/[Fe] = 2000, [Al]/[Fe] = 40, and [P]/[Fe] = 4 (mol/mol). 7584

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Industrial & Engineering Chemistry Research was obtained at 15 °C. More importantly, 1,2-structure content was increased as well, reaching values of 94.17% at 50 °C and 90.22% at 80 °C. In addition, higher crystallinity could be obtained at elevated temperatures. The fact that both high activity and excellent 1,2-selectivity for the polymerization of butadiene stayed intact at elevated temperatures in the DPPyP donated system indicated the donor was capable of effectively stabilizing the iron active center. In conclusion, we reported new phosphate (DPPyP, PDPyP, TPyP) donated iron catalysts that were very active for 1,2syndiotactic insertion polymerization of butadiene upon activation with AliBu3. The polymerization for the DPPyP donated catalyst was even more active and 1,2-selective than the TPP system that we reported previously, thus offering a feasible alternative to TPP. Further replacement of phenyl by pyridine did not obtain promoted activity and enriched selectivity. Significantly, high 1,2-selectivity of active species was intact irrespective of cocatalyst loading and monomer concentration. The thermal robustness at industrial favored polymerization temperature could have an advantage in utilizing this catalyst for commercial polymer synthesis.

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(10) Ricci, G.; Forni, A.; Boglia, A.; Motta, T.; Zannoni, G.; Canetti, M.; Bertini, F. Synthesis and X-ray structure of CoCl2(PiPrPh2)2 a new highly active and stereospecific catalyst for 1,2-polymerization of conjugated dienes when used in association with MAO. Macromolecules 2005, 38, 1064. (11) Ricci, G.; Battistella, M.; Porri, L. Chemoselectivity and stereospecificity of chromium(II) catalysts for 1,3-diene polymerization. Macromolecules 2001, 34, 5766. (12) Ricci, G.; Morganti, D.; Sommazzi, A.; Santi, R.; Masi, F. Polymerization of 1,3-dienes with iron complexes based catalysts: Influence of the ligand on catalyst activity and stereospecificity. J. Mol. Catal. A: Chem. 2003, 204−205, 287. (13) Ricci, G.; Forni, A.; Boglia, A.; Sonzogni, M. New chromium(II) bidentate phosphine complexes: synthesis, characterization, and behavior in the polymerization of 1,3-butadiene. Organometallics 2004, 23, 3727. (14) Hamada, H.; Sugiura, S.; Ueno, H. Process for the preparation of 1,2-polybutadiene. U.S. Patent 3,778,424, Dec 11, 1973. (15) Makino, K.; Komatsu, K.; Takeuchi, Y.; Endo, M. Process for the preparation of 1,2-polybutadiene. U.S. Patent 4,182,813, 1980. (16) Bell, A. J.; Ofstead, E. A. Syndiotactic 1,2-polybutadiene synthesis in an aqueous medium. U.S. Patent 5,011,896, Apr 30, 1991. (17) Wang, B.; Bi, J.; Zhang, C.; Dai, Q.; Bai, C.; Zhang, X.; Hu, Y.; Jiang, L. Highly active and trans-1,4 specific polymerization of 1,3butadiene catalyzed by 2-pyrazolyl substituted 1,10-phenanthroline ligated iron (II) complexes. Polymer 2013, 54, 5174. (18) Hsu, W.; Halasa, A. Preparation and characterization of crystalline 3, 4-polyisoprene. Rubber Chem. Technol. 1994, 67, 865. (19) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.; Stromberg, S.; White, A. J. P.; Williams, D. J. Iron and cobalt ethylene polymerization catalysts bearing 2,6-bis(imino)pyridyl ligands: Synthesis, structures, and polymerization studies. J. Am. Chem. Soc. 1999, 121, 8728. (20) Lu, J.; Hu, Y. M.; Zhang, X.; Bi, J. F.; Dong, W. M.; Jiang, L. S.; Huang, B. T. Fe(2-EHA)3/Al(i-Bu)3/hydrogen phosphite catalyst for preparing syndiotactic 1,2-polybutadiene. J. Appl. Polym. Sci. 2006, 100, 4265. (21) Zheng, W. J.; Wang, F.; Bi, J. F.; Zhang, H. X.; Zhang, C. Y.; Hu, Y. M.; Bai, C. X.; Zhang, X. Q. Synthesis and characterization of softhard stereoblock polybutadiene with Fe(2-EHA)3/Al(i-Bu)3/DEP catalyst system. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1182. (22) Gong, D.; Dong, W.; Hu, Y.; Bi, J.; Zhang, X.; Jiang, L. Syndiotactically enriched 1,2-selective polymerization of 1,3-butadiene initiated by iron catalysts based on a new class of donors. Polymer 2009, 50, 5980. (23) Gong, D.; Dong, W.; Hu, J.; Zhang, X.; Jiang, L. Living polymerization of 1,3-butadiene by a Ziegler-Natta type catalyst composed of iron(III) 2-ethylhexanoate, triisobutylaluminum and diethyl phosphite. Polymer 2009, 50, 2826. (24) Endo, K.; Uchida, Y. 1,2-polymerization of 1,3-butadiene with Cr(acac)3-alkylaluminum catalysts. J. Appl. Polym. Sci. 2000, 78, 1621. (25) Chatarsa, C.; Prasassarakich, P.; Rempel, G. L.; Hinchiranan, N. 1,3-Butadiene Polymerization Using Co/Nd-Based Ziegler/Natta Catalyst: Microstructures and Properties of Butadiene Rubber. Polym. Eng. Sci. 2015, 55, 14. (26) Furukawa, J.; Kobayashi, E.; Katsuki, N.; Kawagoe, T. 13C-NMR spectrum of equibinary (cis-1,4−1,2) polybutadiene. Makromol. Chem. 1974, 175, 237. (27) Zhou, Z.-n.; Xie, D.-m.; Zhang, J.-g.; Wu, Q.-y.; Feng, Z.-l. 13CNMR study of sequence structure of PBD. Acta Polym. Sin. 1983, 6, 438. (28) Cai, Z.; Shinzawa, M.; Nakayama, Y.; Shiono, T. Synthesis of Regioblock Polybutadiene with CoCl2-Based Catalyst via Reversible Coordination of Lewis Base. Macromolecules 2009, 42, 7642. (29) Napolitano, R.; Pirozzi, B.; Esposito, S. Structural studies on syndiotactic 1,2-poly(1,3-butadiene) by X-ray measurements and molecular mechanics calculations. Macromol. Chem. Phys. 2006, 207, 503.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Natural Science Foundation of China (Grants 21304050, 21507126), the Key Innovation Team of Zhejiang Province (2011R50001-07), the Natural Science Foundation of Ningbo (2016A610045), and the Open Research Fund of Key Laboratory of Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (KLSR 2015001). This work is also sponsored by the K. C. Wong Magna Fund at Ningbo University.

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DOI: 10.1021/acs.iecr.6b01274 Ind. Eng. Chem. Res. 2016, 55, 7580−7586