Correlation of alkylaluminum cocatalyst in Nd-based ternary catalyst with the polymerization performance of isoprene

Polymer 119 (2017) 176e184

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Correlation of alkylaluminum cocatalyst in Nd-based ternary catalyst with the polymerization performance of isoprene Huilong Guo a, b, 1, Jifu Bi c, 1, Jiayi Wang b, Xuequan Zhang c, ***, Shichun Jiang a, **, Zhonghua Wu b, * a b c

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China Institute of High Energy Physics, Chinese Academy of Sciences, & Graduate University of Chinese Academy of Sciences, Beijing 100049, China Research Center of High Performance Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 November 2016 Received in revised form 26 April 2017 Accepted 12 May 2017 Available online 15 May 2017

Nd-based ternary catalyst systems are widely used for rubber industry. The effect of alkylaluminum cocatalysts has been investigated for decades. However, there is a lack of structural evidence, especially the quantitative result to illustrate the inner reasons. In the present paper, the effect of different alkylaluminum cocatalysts to the polymerization performance of isoprene has been studied. Catalysts adopted in the present paper all show high 1,4-cis selectivity (above 96%) of polyisoprene and kind of alkylaluminum is not the crucial factor to affect the microstructure of polyisoprene. By comparing the polymerization performance of isoprene and the local atomic structure around Nd centers, it can be concluded that the short carbon-chain in the alkylaluminum cocatalyst is helpful to increase the percent conversion of isoprene. While the carbon-chain length and number in each alkylaluminum cocatalyst can be used to modulate the molecular weight of the synthesized polyisoprene. All the results in the present paper provide a clue to improve the Nd-based ternary catalyst for rubber synthesis. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Correlation of alkylaluminum cocatalyst Polymerization performance XAFS

1. Introduction The development of high-quality elastomers demands for highperformance synthetic rubbers. On one hand, tunable molecular weight (MW) and narrow molecular weight distribution (MWD) is an invariable goal in high polymer synthesis industry [1e3], on which unremitting effort has been focused. On the other hand, natural rubber is a high-performance high-molecular compound. The limited supply [4] of natural rubber compels people to seek for better synthetic polyisoprene. With advances in the synthetic rubber industry, regioselective polymerization [5] of 1,3conjugated dienes has attracted much attention in recent decades. Certainly, suitable catalyst plays a vital role in the synthesis of high-performance rubber. In fact, Nd-based ternary catalyst systems are widely used for rubber industry. The three components

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Zhang), [email protected] (S. Jiang), [email protected] (Z. Wu). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.polymer.2017.05.028 0032-3861/© 2017 Elsevier Ltd. All rights reserved.

of the ternary catalyst are, respectively, the neodymium (Nd)-based [2,6,7] compounds used as the main catalytic precursors, the alkylaluminum used as cocatalyst, and the chlorine source used as another cocatalyst. Several types of Nd-based [3,4,8e10] catalysts have contributed to the efficient formation of high-cis 1,4polydienes. Although the Nd-based catalysts contribute mainly to the formation of high-cis 1,4-polydienes, the polymerization effect of dienes is tightly related to the variation of alkylaluminums. Up until now, the role of the alkylating cocatalyst has not been clarified fully from the structure change. A few researches [11e16] explored the impact of the alkylaluminum cocatalyst components by comparing the polymerization performances with different cocatalysts. Friebe et al. [17,18] compared the effect of triisobutylaluminum {Al(iBu)3} and diisobutylaluminum hydride {Al(iBu)2H} in an Nd-based ternary catalyst system, pointing out that the yield of polybutadiene decreased to 1/8 when Al(iBu)3 was replaced by Al(iBu)2H with the same molar loading. This difference could be explained by a significantly more facile substitution of a hydride moiety from Al(iBu)2H than an isobutyl group from either Al(iBu)2H or Al(iBu)3 by a living polybutadienyl chain. Some researches attempted to solve the structures of Nd complexes by using mass

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spectra [19,20], XRD [7,21,22], and NMR [23] techniques. Binnemans et al [24] studied a series of neodymium carboxylates from Nd(C3H9COO)3 to Nd(C14H29COO)3 by using single-crystal X-ray diffraction. It was concluded that the coordination sphere around neodymium (III) in the higher alkanoates is comparable to the surroundings of neodymium (III) in neodymium butyrate monohydrate where nine oxygen atoms are coordinated to the central Nd. It can be found that the existing researches cannot explain well the discrepant polymerization performances caused by different alkylaluminum cocatalysts. To understand the influence of alkylaluminum cocatalysts on the performance of Nd-based ternary catalysts, detailed structural information of Nd-based ternary catalyst containing different alkylaluminum cocatalysts is required. As far as we know, there are almost no reports about the structural difference of Nd-based ternary catalysts that contain different alkylaluminum cocatalysts. The quantitative structural analysis about Nd-based ternary catalysts is even scarce. However, the structure knowledge of Nd-based ternary catalysts is conducive to understanding the catalytic mechanism, improving the catalytic performance, and promoting their application in rubber synthesis industry. For the Nd-based ternary catalyst systems, their structure characterization is quite difficult because they are generally in liquid form without crystalline structures. The thornier problem is that the alkylaluminum component of Nd-based ternary catalyst is inflammable and explosive dangerous goods. Therefore, the structural characterization must be performed in waterless and airless environment. Perhaps, these severe measurement conditions leave the unclear liquid structures of the Nd-based ternary catalysts. To compare the effect of alkylaluminum components on the structure around Nd centers in Nd-based ternary catalyst, four kinds of alkylaluminum, i.e., triethylaluminum (AlEt3), Al(iBu)3, trioctylaluminum {Al(nOct)3}, and Al(iBu)2H, are used as the alkylaluminum component in the Nd-based ternary catalyst, respectively. Where, neodymium neodecanoate {Nd(vers)3} was used as the main catalyst. Chlorodiisobutyl aluminum {Al(iBu)2Cl} was used as the chlorine source. By comparing the structural changes of such four catalytic systems with the preparation steps, a possible link between the alkylaluminum components and the structure around the centered Nd is expected to be established. The local atomic structure around centered Nd atom in these catalysts is indispensable information to improve the catalytic performance and design new catalysts devoted to advanced rubber manufacture. X-ray absorption fine structure (XAFS) technique, including extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray absorption near-edge structure (XANES) spectroscopy, is especially suited for measurements of the local environment around metallic ions in solution. XAFS technique has been widely used to detect the local atomic structures in rare earth complexes [25,26]. The advantage of XAFS technique for solution structural study [27] is also borne out. However, XAFS study on the neodymium-based catalysts of rubber manufacturing is still scarce. As far as we know, only Kwag [28] et al. gave one research report in 2001. The main challenge for XAFS measurements is the inflammable and explosive risk of the neodymium-based ternary catalysts. Fortunately, we have successfully designed and applied a waterless and airless liquid cell [29] to the in-situ XAFS study of neodymium-based ternary catalysts. In this study, the isoprene (IP) polymerization experiments assisted by four ternary catalytic systems will be compared. In-situ XAFS technique is used to probe the local atomic structural changes around centered Nd atom. We expect that this study is helpful to understanding the catalytic mechanism.

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2. Experiments 2.1. Materials and sample preparation Nd(vers)3 was provided by Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Four kinds of alkylaluminum {AlEt3, Al(iBu)3, Al(nOct)3, and Al(iBu)2H}, Al(iBu)2Cl, IP, and n-Hexane were purchased from Akzo Nobel or Aldrich. Before nHexane was used as solvent and cleaning agent, it was dried by molecular sieves (4 Å) to remove the potential moisture. Nd(vers)3 was dissolved in hexane solvent to form the homogeneous and transparent solutions. Then alkylaluminum cocatalyst and chlorine source cocatalyst were, respectively added into the Nd(vers)3 hexane solution in two steps. In Step I, each of the four alkylaluminums was, respectively, added into the Nd(vers)3 hexane solution in a mole ratio of 10:1 to form mixture solutions I. In Step II, chlorine source cocatalyst Al(iBu)2Cl was added into mixture solutions I in a mole ratio of 3:10:1 to form mixture solutions II. The as-prepared samples are summarized in Table 1. For simplification, all the four alkylaluminum cocatalysts {AlEt3, Al(iBu)3, Al(nOct)3 and Al(iBu)2H} were marked as [Al], the chlorine source cocatalyst Al(iBu)2Cl was marked as [Cl], and Nd(vers)3 was marked as [Nd] in Table 1. 2.2. XAFS measurements X-ray absorption spectra of Nd LIII-edge were collected at beamline 4B9A of the Beijing Synchrotron Radiation Facility (BSRF) for the samples prepared in Step I, II, and III. A moistureproof and adjustable-thickness liquid sample-cell [29] was used for the insitu XAFS measurements. The Nd-LIII edge XAFS spectra of all the samples were collected in transmission mode at room temperature. The storage ring was run at 2.5 GeV with an electron current of about 100 mA. The incident X-ray beam was monochromatized by a Si(111) double crystal monochromator with energy resolution (DE/ E) of about 2
104. The X-ray intensities before and after sample absorption were monitored with ion chambers. The obtained XAFS data were analyzed with the IFEFFIT program [30]. FEFF code [31] was used to calculate the reference XAFS spectra. 2.3. IP polymerization reaction IP polymerization reactions were carried out in an ampoule under an atmospheric pressure of dry nitrogen. The ingredient mole ratio of Al(iBu)2Cl, alkylaluminum, and Nd(vers)3 in hexane solution were also set to 3: 10: 1, but the mole ratio of IP: Nd(vers)3 was set to 1000:1. In this sample preparation, hexane solvent and IP were first injected into the ampoule, then the ternary Nd-based catalyst (3: 10: 1) was added. The reaction solution was withdrawn by syringe, and the polymerization reaction was terminated and stabilized using methanol solution containing 2,6-di-t-butyl-4methylphenol (1 wt/v%) and a small amount of hydrochloric acid (2

Table 1 Sample ingredient of the Nd-based ternary catalyst hexane solutions. Step

Ingredienta

Mole ratio

I II

Nd(vers)3 [Al] þ [Nd] [Cl] þ [Al] þ [Nd]

10: 1 3: 10: 1

a Symbol [Al] stands for one of the four alkylaluminum cocatalysts {Al(iBu)3, AlEt3, Al(nOct)3 and Al(iBu)2H}, symbol [Cl] stands for the chlorine-source cocatalyst Al(iBu)2Cl, and [Nd] stands for Nd(vers)3.

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v/v%). The polymerization product was repeatedly washed with ethanol and then was cut into small pieces. Finally, the polymer was dried at 40  C in vacuum to constant weight. After correction for catalyst residues, the conversion percentage was gravimetrically determined. 2.4. Polymer characterization The microstructure of the as-prepared polymer was detected by nuclear magnetic resonance (NMR) and infrared spectroscopy (IR). 1 H and 13C NMR spectra of the polymer were collected by using magnetic resonance spectrometer (Unity-400 Nuclear, Varian, USA) at room temperature with TMS as internal standard and deuterated chloroform as solvent. IR spectra were measured with Vertex-70 FTIR spectrophotometer. Film samples were prepared on a KBr disc by casting carbon disulfide solution (ca. 2e8 mg/mL) of polymer. The number average molecular weight (Mn) and the weight average molecular weight (Mw) of the as-prepared polymer were measured at 30  C with the gel permeation chromatography (GPC) equipped with a Waters 515 HPLC pump, four columns (HMW 7 THF, HMW 6E THF
2, HMW 2 THF), and a Waters 2414 refractive index detector. Tetrahydrofuran was used as eluent at a flow rate of 1.0 mL/min. The calculated values of Mn and Mw/Mn were calibrated by using polystyrene. The sample (1.0 mg/mL) was injected after filtration through a 0.45 mm filtration kit. Narrow distribution polystyrene as internal standard was adopted to perform the curve correction. Intrinsic viscosity [h] was measured by Ubbelohde Viscometer at 30  C and methylbenzene was used as solvent. 3. Results and discussion 3.1. Polymerization of isoprene To compare the effect of alkylaluminums in a ternary catalyst system to the polymerization reaction of isoprene, a group of polymerization reactions of isoprene in hexane solution were performed. These polymerization reactions were catalyzed by the Nd(vers)3/alkylaluminum/Al(iBu)2Cl ternary catalyst and carried out under the same conditions (at 25  C for 3 h) except the replacement of alkylaluminum cocatalyst. The properties of the polymerized isoprene were compared between the four cocatalysts {AlEt3, Al(iBu)2H, Al(iBu)3, and Al(nOct)3} with respect to monomer conversion as listed in Table 2. In order to increase the amount of polymerization products, i.e., polyisoprene, the mole ratio of IP:[Cl]:[Al]:[Nd] was increased to 1000:3:10:1. The results demonstrate that the four Nd-based ternary catalysts containing different alkylaluminum cocatalyst all exhibit good catalytic activities for isoprene polymerization. Among the four alkylaluminum cocatalysts, Al(nOct)3 system has the lowest polymerization conversion that is about 90.8%. The other three cocatalysts {AlEt3, Al(iBu)2H, and Al(iBu)3} have higher and similar conversions which is around 95%. The catalytic activity of the Ndbased ternary catalyst increases in the order:

Al(nOct)3 < Al(iBu)2H < Al(iBu)3 < AlEt3 when the alkylaluminum ingredient was displaced. It seems that these alkylaluminums with short alkyl-chains are beneficial to the conversion. Wilson et al. [16] also studied the catalytic activity of Nd-based ternary catalysts, and found that the catalytic performance of Nd-based ternary catalysts including different alkylaluminum components follows: Al(nOct)3 > Al(iBu)2H ~ Al(iBu)3 > AlEt3. They noticed that these relative changes of catalytic performance could be different for different researchers. Generally speaking, the catalytic mechanism is quite complicated and even multifarious. A subtle variation of the cocatalytic component in the Nd-based ternary catalysts might result in the relative change of catalytic activity. Currently, the association of the alkylaluminum and the distribution caused by association [12] were used to explain the catalytic behaviors among different cocatalysts. The 1H NMR spectra of the polyisoprene obtained with different Nd-based ternary catalysts were shown in Fig. 1(a), which can be used to determine the contents of 1,4 and 3,4 units in polyisoprene. The peak at 5.12 ppm in the 1H NMR spectrum can be assigned to the olefinic protons of the 1,4 units, while these peaks at 4.76 and 4.68 ppm can be assigned to the olefinic protons of the 3,4 units [32]. The contents of 1,4 and 3,4 units could be determined from 1H NMR analysis respectively. Furthermore, the 1,4-cis and 1,4-trans units can be distinguished by the 13C NMR spectrum. The peaks at 135.4, 125.2, 32.4, 26.6, and 23.6 ppm in the 13C NMR spectrum can be assigned to the characteristics of the 1,4-cis units, while the peaks at 16.3 and 39.9 ppm can be assigned to the characteristics of the 1,4-trans units [33]. As an example, Fig. 1(b) shows one of the four 13C NMR spectra. Almost, there is no observable peaks corresponding to the 1,4-trans units in the obtained polyisoprene. Therefore, the contents of 1,4 units calculated from 1H NMR data are also the contents of 1,4-cis units. The results are shown in Table 2. The IR spectrum of the polyisoprene obtained with Nd(vers)3/ Al(iBu)3/Al(iBu)2Cl catalyst is shown in Fig. 2, which can be also used to estimate the content of 1,4-cis unit. The proportion of 1,4cis and 3,4 units can be determined from the absorption bands at 836 and 890 cm1 as indicated by the literature [34]. From the analysis of IR spectrum, the contents of 1,4-cis and 3,4 units were, respectively, determined to be 96.9% and 3.1%, supporting the NMR results. In this study, the as-prepared polyisoprene has high 1,4-cis selectivity (above 96%) regardless of the type of alkylaluminum. The obtained 1,4-cis repeating units contents of polyisoprene is around 97.5% (or 97.4%) when AlEt3 {or Al(nOct)3} was employed in the catalyst, while it is about 96.8% (or 96.7%) when Al(iBu)3 (or Al(iBu)2H) was used as the cocatalyst in the Nd-based ternary catalyst. These results are not completely the same with previous researches. Rocha et al. [12] used a Ziegler-Natta catalyst system to evaluate the effect of Al(iBu)3, tri(n-hexyl)aluminum {Al(nHex)3}, Al(nOct)3, and Al(iBu)2H cocatalysts on catalyst activity and polybutadiene characteristics. In such a research, the catalyst was constituted of neodymium versatate (main catalyst), an alkylaluminum compound (alkylating agent and cocatalyst), and tert-butyl

Table 2 Comparison of isoprene polymerization properties catalyzed by different alkylaluminum cocatalysts involved in Nd-based ternary catalysts. Isoprene was polymerized in hexane solution at 25  C for 3 h. The mole ratio of IP:[Cl]:[Al]:[Nd] is 1000:3:10:1. [Al]

Conversion (%)

[h] (dL/g)

Mn (
103)

Mw (
103)

Mw/Mn

1,4-cis (%)

AlEt3 (AlC6H15) Al(iBu)2H (AlC8H19) Al(iBu)3 (AlC12H27) Al(nOct)3 (AlC24H51)

95.7 94.1 94.7 90.8

1.77 1.43 3.06 1.65

63.7 11.1 186.4 87.2

407.0 355.4 948.8 396.8

6.4 32.0 5.1 4.6

97.5 96.7 96.8 97.4

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Fig. 1. (a) 1H NMR spectra (olefinic region) of the polyisoprene obtained with four different Nd(vers)3-based ternary catalysts; (b) 13C NMR spectrum of the polyisoprene obtained with the Nd(vers)3/Al(iBu)3/Al(iBu)2Cl catalyst.

Fig. 2. IR spectrum of the polyisoprene obtained with Nd(vers)3/Al(iBu)3/Al(iBu)2Cl catalyst.

chloride (chlorinating agent and cocatalyst). The obtained 1,4-cis selectivity of polybutadiene is around 97.5% when Al(nHex)3 or Al(nOct)3 was used as the alkylaluminum cocatalyst, but it is close to 98% as Al(iBu)3 or Al(iBu)2H was used. The authors considered that the difference of 1,4-cis selectivity of polybutadiene implies that the type of alkylaluminum component has slight influence on the catalyst stereospecificity. The slightly lower cis contents can be attributed to the longer alkyl chain of Al(nHex)3 or Al(nOct)3 component, which forms a larger obstacle to the orientation of butadiene monomers. In other words, the bulky alkyl groups in Al(nHex)3 and Al(nOct)3 seem to affect the coordination way of butadiene molecules. Furthermore, Jia et al. [13] considered that the bulky isobutyl group in Al(iBu)3 and Al(iBu)2H may make the surrounding space of the active site more crowded, thus the butadiene monomer coordinated to the central metal in a compacted h4- mode is more favorable to the formation of 1,4-cis units in the polymerization process of butadiene. They found that the difference of 1,4-cis selectivity among Nd-based ternary catalysts including different alkylaluminum component is more obvious, which changes from 65.1% to 83.2%. All these results demonstrate that the previous researches do not show the identical 1,4-cis selectivity and the same catalytic activity for Nd-based ternary catalysts containing different alkylaluminum cocatalyst. By comparing our research with the previous reports, the 1,4-cis selectivity difference of the polyisoprene polymerized by the Nd-

based ternary catalysts with different alkylaluminum cocatalysts is smaller than 1% in this study, which is in agreement with Rocha's result. However, the weight average molecular weight (Mw) and the number average molecular weight (Mn) of polyisoprene are strongly dependent on the alkylaluminum cocatalysts. In addition, the four catalysts produce polyisoprene with different MWD. As seen in Table 2, the highest Mw was obtained by Al(iBu)3 cocatalyst and the largest MWD was obtained by Al(iBu)2H cocatalyst. Polyisoprene with Al(iBu)2H system has broadest multimodal curves (Fig. 3), indicating that multiple kinds of active sites or strong chain transfer reaction are present in the Al(iBu)2H system. The Al(iBu)2H cocatalyst has the same alkyl-chain length with Al(iBu)3, but it is a diisobutylaluminum hydride. The fairly broad MWD illustrated that there is a mass of small molecules of polyisoprene in the polymer. This result demonstrates that the molecule of diisobutylaluminum cocatalyst is easier to transfer, which is not beneficial to the formation of high molecular weight polyisoprene. Although the obtained polyisoprene molar masses are not a consensus among this study and others, The lowest Mw and the largest Mw/Mn always occur in the case of Al(iBu)2H cocatalyst in the previous literature [13,16]. Such a coincidence was explained [12] as that the hydride from Al(iBu)2H is easier to transfer than the alkyl group from trialkylaluminum. Considering comprehensively the high percent conversion, the high 1,4-cis selectivity, and the high weight average molecular weight Mw, Al(iBu)3 could be one of the best cocatalysts in the Ndbased ternary catalysts for rubber synthesis. From the above discussion, it can be concluded that the alkylaluminum cocatalyst in Nd-based ternary catalyst indeed affects the polymerization properties of isoprene, but subtle structure change during polymerization process is desired for understanding the effect of cocatalyst. 3.2. XANES spectra To extract the structural information of the ternary catalysts, four groups of catalyst hexane solutions were prepared according to the composition proportion as listed in Table 1. In the four groups of catalyst solutions, AlEt3, Al(iBu)3, Al(nOct)3, and Al(iBu)2H were used as the alkylaluminum cocatalysts, respectively. In-situ XAFS measurements for the Nd LIII-edge have been performed. The normalized Nd LIII-edge XANES spectra are shown in Fig. 4. For comparison, the XANES spectra of pure Nd(vers)3 hexane solution as well as the binary hexane solutions of Nd(vers)3 and alkylaluminum cocatalysts are also shown in Fig. 4. It can be seen that all the samples have an intense white-line resonance, which can be

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Fig. 3. Molecular weight distributions of polyisoprenes obtained with different Nd(vers)3/alkylaluminums/Al(iBu)2Cl ternary catalysts.

bondings. Although the Nd-C bondings could behave as covalent character [28] instead of the complete ionic bond as in Nd-O bonding, the longer Nd-C bond-lengths than the Nd-O ones could lead a further transfer of electrons from central Nd to its C neighbors. However, when Al(iBu)2Cl were further added into the mixture hexane solutions of alkylaluminums and Nd(vers)3, the electronegative Cl came from Al(iBu)2Cl could also bonded to the central Nd. But the formation of Nd-Cl bonding will obviously reduce the Nd-O and/or Nd-C coordination number because of the larger Cl atom, as a consequence, the local coordination configuration around central Nd has to reconstruct. In this case, the outer Nd-Cl ionic bonding could change the electron distribution in the inner Nd-C covalent bonding. Consequently, the white-line position on the XANES spectra returns back to the original one as in pure Nd(vers)3 hexane solution. The white-line intensity reflects the electron state density. From Fig. 4, it can be found that all the four white-line intensities decrease with the adding of alkylaluminum to the Nd(vers)3 hexane solution. This decrease of Nd 5d electron state density could be attributed to the decrease of Nd-O ionic bonding component and the increase of Nd-C covalent bonding component. With a further adding of Al(iBu)2Cl to the hexane solutions of Nd(vers)3 and alkylaluminums, the white-line intensities show a further decrease. Perhaps, such a decrease could be attributed to the decrease of the total coordination number and the serious distortion of local configuration around Nd centers due to the formation of longer NdCl bond. Generally speaking, the white-line variation tendencies of XANES spectra are similar for the four groups of catalysts including different alkylaluminums AlEt3, Al(iBu)3, Al(nOct)3, and Al(iBu)2H. Certainly, more abundant details appear on the XANES spectra of the ternary catalysts including Nd(vers)3, alkylaluminum, and Al(iBu)2Cl. Such subtle changes perhaps imply the formation of NdCl-Al bonding. 3.3. Quantitative EXAFS analysis

Fig. 4. Normalized Nd LIII-edge XANES spectra. The sample information was listed in Table 1.

attributed to p3/2-5d electronic transition [35] of the central Nd atoms. When the four alkylaluminums were, respectively, added into the Nd(vers)3 hexane solutions, their white line position on the XANES spectra all show an up-shift of about 0.5 eV. However, when Al(iBu)2Cl were further added into the above four solutions, their white-line positions all shift down to almost the same position as in the pure Nd(vers)3 hexane solution. Generally, the shift of whiteline position reflects the change of electron transfer. Therefore, the up-shift of the white-line position demonstrates that further electron transfer from Nd to its neighbors occurred when alkylaluminums were added to the Nd(vers)3 hexane solutions. The subsequent down-shift of the white-line position demonstrates that the electron transfer from Nd to its neighbors was receded when Al(iBu)2Cl were further added into mixture hexane solutions of alkylaluminums and Nd(vers)3. These shifts of white-line position are tightly related to the coordination environment around central Nd atoms. When four alkylaluminums were, respectively, added to the Nd(vers)3 hexane solution, the C atoms came from the alkyls perhaps have bonded to the central Nd atoms. In other words, Nd-C bondings have partially replaced the original Nd-O

To get the quantitative structural information around the central Nd atoms, the EXAFS spectra of the four Nd-based ternary catalysts were analyzed. As an example, the normalized X-ray absorption curve of the ternary catalyst including Nd(vers)3, Al(nOct)3, and Al(iBu)2Cl is shown in Fig. 5 after removing the preedge background. For comparison, the X-ray absorption spectra of pure Nd(vers)3 hexane solution and the mixture hexane solution of Nd(vers)3 and Al(nOct)3 are also shown in Fig. 5. Well EXAFS oscillations can be observed from these X-ray absorption spectra which demonstrate that the in-situ XAFS measurements under waterless and oxygen-free conditions are successful. These approximate sinusoid oscillations imply that there is only the nearest near-neighbor contribution to the XAFS spectra. In other words, the intermediate-range and long-range ordered structures are not existent in these catalysts. From the inset of Fig. 5, it can be found that the EXAFS oscillation of the mixture hexane solution of Nd(vers)3 and Al(nOct)3 is more approximating to the one of the pure Nd(vers)3 hexane solution. It seems that the EXAFS oscillation of the ternary catalyst including Nd(vers)3, Al(nOct)3, and Al(iBu)2Cl has more details (as appeared at 6222 eV) than the two others. These subtle differences in EXAFS oscillations imply that the adding of cocatalysts Al(nOct)3 and Al(iBu)2Cl to the major catalyst Nd(vers)3 hexane solution must impact the near-neighbor structure around central Nd. Therefore, besides the Nd-O bondings from the major catalyst Nd(vers)3, the Nd-C and Nd-Cl bondings from the cocatalysts Al(nOct)3 and Al(iBu)2Cl have to be considered in the EXAFS analysis. The above discussion is also suitable for the other three groups of catalysts including AlEt3,

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181

Fig. 5. Normalized Nd LIII-edge X-ray absorption spectra of the catalysts including Nd(vers)3, Al(nOct)3, and Al(iBu)2Cl. The enlarged XANES spectra are shown in the inset.

Al(iBu)3, and Al(iBu)2H, respectively. The EXAFS oscillations were extracted by removing the postedge backgrounds with cubic spline function. Normalized k3weight XAFS signals in the k-region of 2e11 Å1 were Fouriertransferred to R-space. The obtained Fourier-transform (FT) spectra without phase-shift correction are shown in Fig. 6 (a1-a4) for the four ternary catalysts. As discussion above, only one coordination peak around 2.32 Å is visible in the four FT spectra, verifying the short-range ordered attribute of the four catalysts. This coordination peak was isolated in the R-region of 1.40e2.80 Å and Fourier-transformed to k-space to obtain the so-called single-shell EXAFS function as shown in Fig. 6 (b1-b4). Before EXAFS fitting, a double-electron transition [27] from 2p4d/5d2 superimposed at about 6.5 Å1 of the k-space was removed. All reference XAFS spectra of the possible Nd-O, Nd-C, and Nd-Cl bondings were calculated by using the FEFF code [31]. All the EXAFS spectra were fitted in both R-space and k-space. For the ternary systems, three sub-shells including Nd-C, Nd-Cl and Nd-Al have to be used to fit the EXAFS data after testing various fitting strategies. Finally, the Rspace and k-space fitting curves are also shown in Fig. 6(a1-a4) and Fig. 6(b1-b4), respectively. The fitting parameters are summarized in Table 3. For comparative purpose, our previous fitting results [29] for Al(iBu)3 system are also tirelessly given in Table 3. It can be found that the local atomic structures around Nd centers in the four ternary catalysts can be well described with Nd-C, Nd-Cl, and Nd-Al three kinds of atom-pairs. The carboxylate ligands get displaced upon addition of AlR3 and AlR2Cl [36] and the Nd-C and Nd-Cl form the nearest near-neighbors. That is to say, there are only C and Cl atoms directly coordinating to the central Nd in these Nd-based ternary catalysts. These C nearest near-neighbors can be attributed to the contribution of alkylaluminum cocatalyst, while those Cl nearest near-neighbors can be attributed to the contribution of Al(iBu)2Cl cocatalyst. However, the Al neighbors do not directly bonded to the Nd centers, but Nd center and Al neighbor form the Nd-Cl-Al coordination through an intermediate Cl as a “bridge” atom. The Nd-Al bond-length is about 3.13 or 3.15 Å, while the NdCl bond-lengths distribute in 2.87e2.89 Å. From a quasicrystal structure [37] of AlCl3, the Al-Cl bond-length with bridging Cl atom is about 2.29 Å. Therefore, the average bond angle of Nd-Cl-Al can be estimated to be about 74 , and the average included angle of ClNd-Al is about 44 . Although the Nd-Cl bond-lengths and Nd-Al

Fig. 6. Fourier transform spectra of Nd LIII-edge EXAFS data for ternary systems including AlEt3, Al(iBu)3, Al(nOct)3, and Al(iBu)2H (a1 to a4), respectively, as well as the corresponding first-shell EXAFS function k3c(k) (b1 to b4). Open circles represent the experimental values and solid lines represent the fitting curves.

distances are slightly different among the four catalysts, the NdCl-Al triangular configuration is almost unchanged. Averagely, there are 3 Al(iBu)2Cl molecules bonded to one central Nd atom through the Nd-Cl-Al triangular coordination configuration. That is to say, each Al(iBu)2Cl molecule contributes a Nd-Cl-Al bonding and the same chlorine source (Al(iBu)2Cl) used as cocatalyst has no significant structure difference in the four Nd-based ternary catalysts. Therefore, the performance difference of the four Nd-based ternary catalysts to the isoprene polymerization comes mainly from the contributions of different alkylaluminum cocatalysts.

3.4. Effect of alkylaluminums To cognize the effect of alkylaluminum cocatalysts to the catalytic performance of Nd-based ternary catalysts, four kinds of alkylaluminums were comparatively studied. Although the four alkylaluminums all contribute the Nd-C coordination to the central Nd atoms, the analysis for the differences of Nd-C coordination structures is still significant. From Table 3, it can be found that the Nd-C coordination number is greater than and approximating to 4

Table 3 Nd LIII-edge EXAFS fitting parameters of the Nd-based catalysts dissolved in hexane. N is the coordination number, R is the interatomic distance, s2 is the Debye-Waller factor, and DE0 is the shift of energy threshold. Ternary system

Bond

N

R (Å)

s2 (Å2)

DE0 (eV)

AlEt3

Nd-C Nd-Cl Nd-Al Nd-C Nd-Cl Nd-Al Nd-C Nd-Cl Nd-Al Nd-C Nd-Cl Nd-Al

4.5 3.0 2.2 2.8 3.0 2.5 2.9 2.9 2.2 4.2 2.9 1.5

2.58 2.88 3.15 2.59 2.87 3.13 2.58 2.87 3.15 2.64 2.89 3.13

0.0025 0.0048 0.0046 0.0019 0.0031 0.0032 0.0016 0.0025 0.0048 0.0021 0.0050 0.0026

1.7 9.6 0.1 0.9 10.6 7.0 1.2 7.1 7.5 4.5 10.6 7.5

Al(iBu)2H

Al(iBu)3 [29]

Al(nOct)3

182

H. Guo et al. / Polymer 119 (2017) 176e184

when AlEt3 or Al(nOct)3 was used the cocatalyst, but it is smaller than and approximating to 3 when Al(iBu)2H or Al(iBu)3 was used as cocatalyst. This difference of Nd-C coordination number reflects the different effects of four alkylaluminum molecules on the catalytic activity. Comparing the four alkylaluminum molecules, AlEt3 includes three ethyl groups, which has the shortest carbon-chains and the smallest molecular weight (114.16) among the four alkylaluminums. Al(nOct)3 includes three n-octyl groups, which has the longest carbon-chains and the largest molecular weight (366.64) among the four alkylaluminums. Al(iBu)3 includes three isobutyl groups, which has the moderate-length carbon-chains and the second largest molecular weight (198.32) among the four alkylaluminums. Al(iBu)2H includes two isobutyl groups with the same length of carbon-chains as in Al(iBu)3 and a single hydrogen atom bonded to Al. Its molecular weight (142.22) is the second smallest among the four alkylaluminums. Evidently, the Nd-C coordination number in the four Nd-based ternary catalysts is highly correlative to the carbon-chain configuration of the alkylaluminum cocatalysts. Because AlEt3 and Al(nOct)3 have only the line chains, while Al(iBu)3 and Al(iBu)2H have the branch chains, it is easy to understand that AlEt3 and Al(nOct)3 can form more Nd-C coordination numbers than Al(iBu)3 and Al(iBu)2H because the line carbon-chains have smaller steric hindrance than the branched carbon-chains. Because Al(iBu)3 and Al(iBu)2H have the same isobutyl groups, it is reasonable for both to form the same Nd-C coordination numbers as shown in Table 3. Therefore, the carbonchain configuration in alkylaluminum is important, but its length or the molecular weight is not an important factor to determine the Nd-C coordination number. Comparing with Table 2, it can be found that all the systems have fairly high 1,4-cis selectivity for the synthesized polyisoprene. As discussed above, the 1,4-cis selectivity difference of different systems is smaller than 1%. Therefore, such a 1,4-cis selectivity is possibly not just related to the Nd-C coordination numbers. The catalyst type, concentration and proportion of catalyst and monomer could also offset or cover up the influence of alkylaluminum. Seemingly, results from Rocha et al. [12] and Jia et al. [13] support that these alkylaluminums with branched carbon-chains are favorite to the 1,4-cis selectivity of polymers. However, even in Rocha et al.’s result, the 1,4-cis contents obtained from different systems were quite close. The monomers of butadiene and pentadiene are approximately linear molecules in the both previous reports, while the monomer of isoprene is a branched molecule in this study. That is to say, the 1,4-cis selectivity of polymer is not only related to the carbon-chain configuration of alkylaluminum cocatalyst, but also related to the monomer configuration of the polymer monomer. Summarizing the above discussion, it can be found that high cis-1,4 selectivity (above 96%) could obtained in the synthesis of polyisoprene catalyzed by Nd(vers)3-based ternary catalysts and kind of alkylaluminum is not a crucial role to affect the cis-1,4 selectivity in this paper. From Table 3, it can be found that when AlEt3, Al(iBu)3, and Al(iBu)2H were, respectively, used as the cocatalyst in the Nd-based ternary catalysts, the obtained Nd-C bond lengths (2.58, 2.58, and 2.59 Å) are very close, but the Nd-C bond length (2.64 Å) is obviously larger when Al(nOct)3 was used as the alkylaluminum cocatalyst. These Nd-C bond lengths satisfy the relation: AlEt3  Al(iBu)3 < Al(iBu)2H < Al(nOct)3. This result demonstrates that the longer of the carbon-chains in the alkylaluminum cocatalysts, approximately, the longer of the Nd-C bond lengths in the Nd-based ternary catalysts. Such a correlation can be attributed to that the interaction between Nd atom from Nd(vers)3 and the terminal C atom from the alkylaluminum cocatalysts with longer carbon-chains is relatively weaker than the one with shorter carbon-chains. In Rocha et al.’s research [12], a reaction temperature of 70  C was adopted and they also pointed out that the

decrease of the catalyst activity was along with the increasing alkyl group length. Comparing with Table 2, it can be found that AlEt3, Al(iBu)3, and Al(iBu)2H cocatalysts have higher percent conversions (95.7%, 94.7%, and 94.1%) in the synthesis of polyisoprene, while it is lower (90.8%) as Al(nOct)3 was used as the alkylaluminum cocatalyst. Seemingly, the Nd-C bond lengths have a negatively correlation with the percent conversions of isoprene polymerization. Therefore, it can be concluded that these alkylaluminum cocatalysts with shorter carbon-chains can produce higher percent conversion than the ones with longer carbon-chains in the synthesis of polyisoprene. As discussed above, Al(iBu)3 cocatalyst corresponds to the polymer with largest molecular weight. Al(iBu)2H has the same carbon-chain groups, but the obtained Mn and Mw are the smallest among the four alkylaluminum cocatalysts. The result is consistent with the other researches [12,13,16], which seems to be a consensus. This result demonstrates that the molecular weight of the polyisoprene is neither positively-related nor negativelyrelated to the carbon-chain length or the (line or branch) carbonchain configuration. Therefore, we guess that there is an optimized carbon-chain length to get the largest molecular weight of polyisoprene. Roughly, the optimized carbon-chain length could contain 3e4 carbon atoms. In other words, one of the tripropylaluminum, triisopropylaluminum, tributylaluminum, and triisobutylaluminum {Al(iBu)3} could be the best alkylaluminum cocatalyst to get the largest molecules of polyisoprene. Perhaps, this distinct molecular weight between Al(iBu)3 and Al(iBu)2H systems can be attributed to the different carbon-chain numbers in Al(iBu)3 and Al(iBu)2H. The alkylaluminum containing three isobutyl carbonchains is tendency to get the highest molecule-weight polyisoprene, while the alkylaluminum containing two isobutyl carbonchains (and one hydrogen) tends to get the lowest molecule-weight polyisoprene. Table 2 also illustrates that the largest MWD occurs at the case with Al(iBu)2H as the alkylaluminum cocatalyst. It seems that the polymerization process of isoprene is easier to be interrupted in the presence of Al(iBu)2H cocatalyst in the Nd-based ternary catalyst. This result implies that the effect of local atomic structure around Nd is not sensitive to the molecular weight of polyisoprene, but the kind of alkylaluminum cocatalyst and its number of carbon-chain group determine the final molecular weight. Friebe et al. [18] also compared the Al(iBu)3 and Al(iBu)2H cocatalysts and pointed out that the hydride group in Al(iBu)2H and the isobutyl group in Al(iBu)3 have substantially different activities in the substitution of the living polybutadienyl chains. In fact, a detailed mechanism to modulate the molecular weight of polyisoprene is still unclear. From this point of view, more and further studies are desired. Based on the polymerization measurements of isoprene and the local atomic structure information around Nd centers, the effect of alkylaluminum cocatalysts on the polymerization performance of isoprene has been discussed. Here, we have to emphasize that Nd is the active center [38] of the Nd-based ternary catalyst. The local atomic structures around Nd center determine the catalytic performance of the ternary catalyst to the polymerization of isoprene. As in our previous study [29], the original Nd-O bondings in Nd(vers)3 hexane solution was seriously reduced by adding alkylaluminum cocatalyst into the solution, and was almost completely destroyed by further adding chlorine source cocatalyst into the solution. It was the oligomeric-structure destruction of Nd(vers)3 in hexane solution that results in the main catalytic activity of the Ndbased ternary catalyst. Although the adding of all the alkylaluminums and chloroalkyl aluminum into the Nd(vers)3 hexane solution has been identified to form Nd-C and Nd-Cl bondings and destroy the oligomeric structures of Nd(vers)3 in hexane solution, the activated catalytic activity can still be modulated by different

H. Guo et al. / Polymer 119 (2017) 176e184

cocatalysts through the slight structure change around Nd center. Cocatalyst is often used for the molar-mass control of polymer and used as the scavengers [17] for impurities such as moisture, excess carboxylic acids etc. In this paper, the effect of alkylaluminum cocatalysts to the catalytic performance has been studied for the Ndbased ternary catalyst in rubber synthesis. This study demonstrates that the polymerization performance of isoprene can be appropriately modulated by changing the alkylaluminum component in the Nd-based ternary catalyst. 4. Conclusions The Nd-based ternary catalysts including different alkylaluminum compositions {AlEt3, Al(iBu)3, Al(nOct)3 and Al(iBu)2H} are compared from the local atomic structures around Nd center and the catalytic performances for isoprene polymerization. The conclusions can be summarized as follows: (1) The main catalytic activity of the Nd-based ternary catalyst is activated by the alkylaluminum and chlorine-source cocatalysts by destroying the oligomeric structure of Nd(vers)3 in hexane solution. Besides further decreasing the oligomeric structure, the participation of chlorine-source cocatalyst forms the Nd-Cl-Al coordination around Nd active center which is almost unchanged with the different kinds of alkylaluminum cocatalysts. Catalysts adopted in the present paper all show high 1,4-cis selectivity (above 96%) of polyisoprene. (2) The alkylaluminum components consist of n-alkyl groups can form four or more Nd-C bondings, while the alkylaluminum components consist of isoalkyl groups form three or less Nd-C bondings in the hexane solution of the Nd-based ternary catalyst. (3) The Nd-C bond lengths in the hexane solution of the Ndbased ternary catalyst are positively related to the carbonchain length of the alkylaluminum, but negatively related to the percent conversion of isoprene into polyisoprene. Therefore, the shorter carbon-chain length of the alkylaluminum component in the Nd-based ternary catalyst can get higher percent conversion of polyisoprene than the longer one. (4) The molecular weight of the synthesized polyisoprene have no obvious correlation with the Nd-C coordination structure, but are dependent on the alkyl carbon-chains in the alkylaluminum cocatalyst. It is estimated that trialkyl-aluminums with 3e4 carbons in each alkyl group have the largest molecular weight, but dialkyl-aluminum hydrides have the smallest molecular weight values and the largest molecular weight distribution. Acknowledgment This work was supported by the National Natural Science Foundation (Nos. U1232203, U1432104, 11405199, 21374077, 11305198, U1332107, 51203147, 51473156) of China, the CAS Hundred Talents Program (Y220011001), and the Jilin Provincial Research Fund for Basic Research, China (No. 20130102007JC). References [1] A. Fischbach, C. Meermann, G. Eickerling, W. Scherer, R. Anwander, Discrete lanthanide aryl (alk) oxide trimethylaluminum adducts as isoprene polymerization catalysts, Macromolecules 39 (2006) 6811. [2] C. Fan, C. Bai, H. Cai, Q. Dai, X. Zhang, F. Wang, Preparation of high cis-1, 4 polyisoprene with narrow molecular weight distribution via coordinative chain transfer polymerization, J. Polym. Sci. Part A Polym. Chem. 48 (2010)

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