Elsevier

Polymer

Volume 119, 16 June 2017, Pages 176-184
Polymer

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

https://doi.org/10.1016/j.polymer.2017.05.028Get rights and content

Highlights

  • Effect of alkylaluminum cocatalyst in Nd-based ternary catalyst was studied.

  • Isoprene polymerization results based on four catalyst systems shown different for the kind of alkylaluminum.

  • Local structures around Nd center of the four catalysts were obtained by XAFS technique.

  • The structural information is beneficial to elucidate the effect of alkylaluminum to the isoprene polymerization.

Abstract

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.

Introduction

The development of high-quality elastomers demands for high-performance synthetic rubbers. On one hand, tunable molecular weight (MW) and narrow molecular weight distribution (MWD) is an invariable goal in high polymer synthesis industry [1], [2], [3], 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 1,3-conjugated 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 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], [8], [9], [10] catalysts have contributed to the efficient formation of high-cis 1,4-polydienes.

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 [11], [12], [13], [14], [15], [16] 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 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.

Section snippets

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 n-Hexane 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

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

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

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).

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