Polymerization of 1,3-butadiene catalyzed by pincer cobalt(II) complexes derived from 2-(1-arylimino)-6-(pyrazol-1-yl)pyridine ligands

https://doi.org/10.1016/j.apcata.2013.04.026Get rights and content

Highlights

  • Well-defined cobalt pincer complexes were synthesized.

  • The complex/MAO selectively catalyzed cis-1,4 butadiene polymerization.

  • The polymer yields and selectivity can be controlled by the ligation environment.

  • 1,2 syndio-selectivity can be achieved by phosphine regulators.

Abstract

A new class of air stable and structurally well-defined cobalt complexes with unsymmetrical pincer type ligands ([2-(ArNdouble bondCMe)-6-(Py)C5H3N]CoCl2) (Ar = single bondC6H5, Py = pyrazol-1-yl, 5a; Ar = 2,4,6-Me3C6H2, Py = pyrazol-1-yl, 5b; Ar = 2,6-iPr2C6H3, Py = pyrazol-1-yl, 5c; Ar = single bondC6H5, Py = 3,5-Me2pyrazol-1-yl, 5d; Ar = 2,4,6-Me3C6H2, Py = 3,5-Me2pyrazol-1-yl, 5e; Ar = 2,6-iPr2C6H3, Py = 3,5-Me2pyrazol-1-yl, 5f; Ar = 2,6-iPr2C6H3, Py = 3,5-iPr2pyrazol-1-yl, 5g and [2-(Odouble bondCMe)-6-(3,5-diphenylpyrazol-1-yl)C5H3N]CoCl2 5h) were prepared and the molecular structures of 5a, 5c and 5f were determined by single crystal X-ray crystallography. Upon activation by methylaluminoxane (MAO) in toluene at room temperature, all complexes initiate polymerization of 1,3-butadiene (polymer yields: 65–99%), affording polybutadiene with excellent cis-1,4 regularity (97.5–98.7%). The polymer yields and properties in terms of molecular weight and distribution are well controlled by the substituents on iminoaryl rings and pyrazole rings. Selectivity switch from cis-1,4 to syndio-1,2 was also achievable by adding phosphine as microstructure regulator.

Introduction

Polymerization of 1,3-butadiene is an important process in the polymer industry as the resultant polymers possess versatile properties for a wide range of applications [1], [2], [3]. As these properties depend mainly on the microstructures generated during the polymerization, much effort has been devoted to designing highly regio- and/or stereoselective catalyst systems, to prepare polybutadienes with precisely tailor-made microstructures. The development of homogeneous Ziegler–Natta catalysts has realized regio- and stereospecific polymerization of 1,3-butadiene to produce cis-1,4- [4], [5], [6], [7], [8], [9], [10], [11], trans-1,4- [9], [12], [13], [14], [15], [16] and syndiotactic 1,2-polybutadienes [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. Among them, cis-1,4-polybutadiene is one of the most important products since it is employed as one of the major materials in the tire and automotive industries [32]. Cis-1,4-polybutadiene can also be copolymerized or blended with other compounds to prepare materials with improved mechanical properties, such as Styrene-Butadiene Rubber (SBR), High Impact PolyStyrene (HIPS), Acrylonitrile-Butadiene-Styrene (ABS) or Hydroxyl Terminated PolyButadiene (HTPB). As a result, transition metal complexes have been considerably explored as catalyst precursors to prepare the cis-1,4 polymer [33]. Ziegler–Natta catalysts of titanium [34], [35], [36], [37], [38], nickel [39], [40], [41], cobalt [17], [33], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51] and neodymium-based catalysts [2], [8], [11], [52], [53], [54], [55] activated with various organoaluminum reagents have been used in the production of cis-1,4-polybutadiene, with the neodymium-based ones being the main catalysts used for the production on the industrial scale. These catalysts offer high activity and selectivity (cis-1,4 > 97%) to provide polymers with desirable material properties such as excellent abrasion and cracking resistance and high tensile strength of the vulcanizates [6]. Their ill-defined and multi-site nature, nevertheless, causes relatively broad molecular weight distributions, and thus complicates the mechanistic study and hinder catalyst design and optimization. In order to gain better control over molecular weight and polymer compositions, academic and industrial research has focused on well-defined single-site catalysts in recent years, mainly based on rare earth metals and the first row transition metals. Various well-defined Lnsingle bondC bond containing complexes, such as lanthanide-based metallocene alkyl complexes, lanthanocene aluminates, alkyl bridged lanthanide carboxylates, and lanthanide alkyl complexes supported by versatile pincer ligands, are extensively investigated and developed [1], [2], [5], [56], [57], [58], [59]. With these active systems, excellent controls over the molecular weights as well as the microstructures of polymers are facilely achieved, though industrial applications have not yet been demonstrated. Meanwhile, late transition metal catalysts, such as iron, nickel and cobalt, have been received increased attention due to their good functional group tolerance, decreased air and moisture sensitivity and lower cost and environmental impact. In particular, cobalt-based catalysts have been demonstrated for high regio- and stereoselective polymerization of 1,3-butadiene [60].

The carboxylates (or acetylacetonate) [61], [62] and halides of cobalt [17], [31], [33], [42], [43], [44], [45], [47], [49] are the major classes of catalysts for producing cis-1,4-polybutadiene when activated by methylaluminoxane (MAO) or aluminum chlorides. Simple cobalt salts CoCl2, activated with organoaluminum compounds, are able to generate heterogeneous active species to polymerize 1,3-butadiene with cis-specific living polymerization. Interestingly, in the polymerization process, the selectivity can be switched and led to the formation of desired cis-1,4-1,2 block copolymer by addition of PPh3 [17], [47], [49]. Homogeneous systems such as Co(acac)3 [61] and Co(Salen)2 [42], [63] in combination with MAO or ethyl aluminum sesquichloride (EAS) were also reported to catalyze cis-regiospecific polymerization of 1,3-butadiene. More recently, research efforts in this field have been gradually shifted on the design of well-defined, homogeneous and high active CoCl2 complexes supported with various tridentate ligands, such as bis(imino)pyridine, bis(imidazolyl)amine and bis(imidazolyl)pyridine, and the high cis-1,4 selectivity and activity have been maintained [9], [16], [33], [44], [64], [65], [66], [67], [68]. Despite these achievements, further exploration of well-defined homogeneous catalyst systems with high cis-1,4 selectivity and thermo-, air and moisture stability has remained a fascinating and challenging subject. We are interested in pincer ligated complexes in a general formula of [(NNL)M] because their coordination geometries and electronic properties (and thus the reactivity of the metal center) could be modified by tuning the structures of the arms and substituents [60]. When the pyridyl-supported pyrazolyl-imine system is adopted, the large π-system is believed to increase the Lewis-acidity of the metal center to facilitate the interactions between the metal and the monomer which may enhance the activity [69]. Herein, we describe the synthesis and characterization of a new class of cobalt complexes with pyridyl-supported pyrazolyl-imine ligands and their applications in the 1,3-butadiene polymerization.

Section snippets

Synthesis and characterization of complexes

The cobalt complexes 5a–5g were obtained by reacting CoCl2 with one equivalent of corresponding ligands at room temperature (Scheme 1). These complexes were characterized by IR, and elemental analysis, as well as single crystal X-ray analysis of the selected compounds. The Cdouble bondN (arylimino) stretching bands of free ligands at 1649–1631 cm−1 shift toward lower frequencies by 7–23 cm−1 when coordinated to cobalt. The structures of 5a, 5c and 5f were further confirmed by X-ray crystallography.

The

Conclusion

In summary, a serial of new cobalt complexes carrying unsymmetrical 2-(1-arylimino)-6-(pyrazol-1-yl) pyridine were prepared and fully characterized. The complex 5a with the smallest-sized ligand was isolated as an ion-pair in the form of [L2Co]2+[CoCl4]2−, while complex 5f showed two slightly different geometries in the crystal unit cell. All the complexes converted butadiene selectively to cis-1,4-polybutadiene with moderate to high monomer conversion upon activation with MAO. Studies on the

General procedure and materials

CoCl2 and anilines were purchased from Alfa Aesar. 6-Bromo-2-pyridinecarboxylic acid, MAO, pyrazole, 3,5-dimethylpyrazole and butadiene (25 wt% in toluene) were available from Aldrich. Phosphines were purchased either from Strem or Aldrich. Dioxane was dried over CaH2. Toluene and tetrahydrofuran (THF) were dried with Na/benzophenone and distilled prior to use. 1H and 13C spectral were recorded on a Bruker AV400 spectrometer at 25 °C with CDCl3 as solvent (400 MHz for 1H NMR, 100 MHz for 13C NMR).

Acknowledgement

We are grateful for the generous financial support from King Abdullah University of Science and Technology.

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