Ethylene oligomerization, homopolymerization and copolymerization by iron and cobalt catalysts with 2,6-(bis-organylimino)pyridyl ligands

doi:10.1016/j.ccr.2005.12.018

Coordination Chemistry Reviews

Volume 250, Issues 11–12, June 2006, Pages 1391-1418

https://doi.org/10.1016/j.ccr.2005.12.018 Get rights and content

Abstract

In this review are highlighted the key advances that have occurred in the discovery and development of 2,6-bis(imino)pyridyl iron(II) and cobalt(II) catalysts for the transformation of ethylene into linear and branched homopolymers or into α-olefins with either Schulz–Flory or Poisson distribution. Particular attention has been paid to studies of the electronic and geometrical structure of both supporting ligands and metal complexes as well as to the mechanisms of precatalyst activation, chain-propagation and chain-transfer.

Introduction

The polymerization of ethylene was discovered in 1933, but the first generation of effective transition metal polymerization catalysts was developed only 20 years later by Ziegler and Natta [1], [2]. The Ziegler–Natta (ZN) catalysts are based on early transition metals such as titanium, zirconium and vanadium, and polymerize ethylene at relatively low pressures and temperatures [3]. Soon after the initial discoveries of ZN catalysts, efforts were made to develop homogeneous models of the heterogeneous catalysts that would prove more amenable to mechanistic studies. In 1957, Natta and Breslow independently reported that TiCl₂(Cp)₂ could be activated for olefin polymerization by Et₃Al or Et₂AlCl. These soluble catalysts were able to polymerize ethylene although with much lower activities as compared to heterogeneous systems, but they were inactive for propene [4], [5].

The polyolefin scenario changed dramatically in the early 1980s when Sinn and Kaminsky reported that partially-hydrolyzed AlMe₃ was able to activate biscyclopentadienyl derivatives (metallocenes) of group 4 metals for the polymerization of both ethylene and α-olefins [6]. The partially-hydrolyzed AlMe₃ product is known with the name of methylaluminoxane (MAO) and its discovery was a real breakthrough as it allowed for a much better control of the properties of polyethylene (PE) and polypropylene (PP) while maintaining or even improving the catalytic productivity.

Following the development of MAOs, group 4 metallocenes and half-sandwich amide complexes (constrained geometry catalysts) (Scheme 1) have provided the most impressive results and the use of these single-site catalysts for the production of PE and PP is an industrial reality [7], [8].

Until a few years ago there were relatively few reports on late transition metal complexes capable of efficiently catalyzing the polymerization of ethylene and α-olefins. A major and common drawback of these catalytic systems was a higher rate of chain-transfer as compared to early metal catalysts. The discovery of new ligand systems and activators has contributed to overcome this gap and make late transition metal catalysts as efficient as metallocenes and even more versatile.

In 1995, Brookhart and co-workers synthesized a new class of Ni^II and Pd^II polymerization catalysts stabilized by bulky α-diimine ligands [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Ni^II catalysts of this type are unique in polymerizing ethylene to give a variety of materials, ranging from highly viscous liquids to rubbery elastomeric materials, to rigid linear polyethylenes (Scheme 2).

The methyl precursors can be straightforwardly employed in ethylene homopolymerization, while the bis-halide derivatives need the cooperation of an activator such as MAO to promote polymerization. Turnover frequencies (TOFs) up to 4 × 10⁶ mol ethylene (mol catalyst × h)⁻¹ (1.1 × 10⁵ kg PE (mol catalyst × h)⁻¹) are common for cationic Ni^II catalysts, thus approaching the activity of metallocenes [23].

A new class of neutral Ni^II catalysts stabilized by salicylaldiminato ligands was independently reported by Johnson [24] and Grubbs [25], [26] in 1998 (Scheme 3). These innovative precursors give from moderately branched to linear polyethylene materials with properties that can be finely tuned by varying the nature and size of the L, R, R¹, R² groups. Typical TOFs in ethylene polymerization are 10⁵ at 17 bar. The catalysts may contain either σ-organyl or η³-allyl ligands and are generally activated by Lewis-acid co-catalysts such as B(C₆F₅)₃ or B(PPh)₃.

In 1998, Brookhart, Bennett and Gibson independently discovered that five-coordinate 2,6-bis(arylimino)pyridyl Fe^II and Co^II dihalides, activated by MAO, are effective catalysts for the conversion of ethylene either to high-density polyethylene or to α-olefins with Schulz–Flory distribution (Scheme 4) [27], [28], [29], [30], [31], [32]. Remarkably, the productivities were as high as those of most efficient metallocenes.

The advantages of these Fe and Co catalysts over other types of single-site Ziegler–Natta catalysts for ethylene homopolymerization (e.g., metallocenes, constrained geometry early transition metal complexes) are manifold, spanning from the ease of preparation and handling to the use of low-cost metals with negligible environmental impact. Another intriguing feature of bis(organylimino)pyridyl Fe^II and Co^II precursors (organyl = aryl, alkyl) is provided by the facile tuning of their polymerization activity by simple modifications of the ligand architecture. It has been shown, in fact, that the size, nature and regiochemistry of the substituents in the iminoaryl groups are of crucial importance in controlling the polymerization [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52] and oligomerization [39], [44], [46], [48], [49], [52], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64] of ethylene. Moreover, due to the good compatibility with various early and late metal copolymerization catalysts, 2,6-bis(arylimino)pyridyl Fe^II and Co^II dihalides can be used as oligomerization catalysts in tandem catalytic systems for the production of branched PE as well as in reactor blending to give PE with controlled molecular weight distribution [56], [57], [58].

The catalysts for α-olefins production have experienced a development similar to that of ethylene polymerization catalysts. Originally linear α-olefins were produced by the Ziegler (Alfen) process which consists of a controlled oligomerization of ethylene in the presence of AlEt₃ at 90–120 °C at a monomer pressure of 100 bar [65], [66], [67]. Common industrial catalytic systems for ethylene oligomerization still comprise alkylaluminum compounds or their combinations with early transition metal compounds [e.g., TiCl₄]. However, but well-defined late transition metals such as Ni^II and Pd^II in conjunction with chelating ligands [65], [66], [67], Fe^II dihalides modified with 2,6-bis(organylimino)pyridines [68], [69] and Co^II dihalides modified with iminopyridines constitute a valid, in some cases, better alternative [70], [71], [72], [73], [74].

In this article, we have reviewed the activity of 2,6-bis(organylimino)pyridyl Fe^II and Co^II dihalides as catalyst precursors for the homopolymerization, oligomerization and copolymerization of ethylene. In an attempt of correlating structure and activity, we have focused much of our attention on the many structural variations that feature these ligands. To the best of our knowledge, review articles covering this specific subject have not appeared elsewhere.

Section snippets

Synthesis of 2,6-bis(organylimino)pyridine ligands

Most 2,6-bis(arylimino)pyridyl ligands are commonly prepared by condensing 2,6-bis(acetyl)pyridine with 2 equiv. of the required aniline in the presence of an acid co-catalyst (route a, Scheme 5). The use of 2,6-bis(formyl)pyridine leads to aldimine derivatives [27], [28], [29], [30], [31], [32].

The method of reacting 2,6-bis(acetyl)pyridine, first, with 1 equiv. of a substituted aniline and then with 1 equiv. of either a primary amine or a different aniline (route b, Scheme 5) has been developed

Synthesis of 2,6-bis(organylimino)pyridine iron(II) and cobalt(II) catalyst precursors

The synthesis of the Fe^II and Co^II catalyst precursors is straightforward and involves the plain addition of the solid ligands to n-BuOH or THF solutions of either anhydrous or hydrated dihalides (Scheme 9) [27], [28], [29], [30], [31], [32]. Irrespective of the metal, the dihalides are sparingly soluble in aromatic hydrocarbons, while they dissolve fairly well in polar solvents. The solids are rather air-stable, whereas they decompose in solution unless protected by an inert gas atmosphere.

The

Principal activators of 2,6-bis(organylimino)pyridine Fe^II and Co^II catalyst precursors

At present, MAO and modified methylaluminoxanes (MMAO), commonly with 20–25% Al(i-Bu)₃, are the most widely used activators for 2,6-bis(organylimino)pyridine Fe^II and Co^II dihalides. For the sake of simplicity, MAO is commonly referred to as linear chain or cyclic rings [ single bond Al(Me)O]_n containing three-coordinate aluminum centers, yet the true structure of MAO is still a matter of debate [104]. It may be a dynamic mixture of linear-, ring- and cage-complexes, all formed from methyl aluminoxane

Ethylene polymerization by 2,6-bis(arylimino)pyridyl iron and cobalt catalysts

The acronyms HDPE, LDPE (low-density polyethylene) and LLDPE (linear low-density polyethylene) define the three major classes of commercial PE. HDPE is a linear semicrystalline homopolymer produced commercially by ZN or chromium-based coordination polymerization technology. LLDPE is a random copolymer of ethylene and α-olefins (1-butene, 1-hexene or 1-octene) prepared using ZN, chromium or metallocene catalysts. LDPE is a branched ethylene homopolymer generally obtained by high-temperature and

Ethylene oligomerization by 2,6-bis(arylimino)pyridyl Fe^II and Co^II catalysts

α-Olefins are currently produced at a rate of more than 2 × 10⁶ t/year predominantly through the oligomerization of ethylene. These linear oligomers are extensively used for the preparation of detergents, plasticizers and, most importantly, as comonomers in the polymerization of ethylene to give LLDPE. A successful example of late transition metal technology to α-olefin production is the SHOP process that uses Ni^II stabilized by chelating monoanionic P,O-ligands [65], [66], [67], [68], [69].

It is

Proposed mechanisms for activation, initiation, chain-propagation and chain-transfer in ethylene polymerization/oligomerization catalyzed by 2,6-bis(organylimino)pyridyl Fe^II and Co^II precursors

Unlike α-diimine Ni^II and Pd^II precursors, no 2,6-bis(imino)pyridyl Fe^II or Co^II catalyst with either alkyl or hydride co-ligands polymerizes ethylene in the absence of aluminum activators. This has certainly contributed to overshadow the catalytically active species as the excess of activator makes in situ spectroscopic studies unable to provide useful information. Moreover, the catalyst precursors are not amenable to be studied by NMR spectroscopy due to their paramagnetic nature. Therefore,

Simultaneous oligomerization/polymerization of ethylene by C₁-symmetric 2,6-bis(organylimino)pyridyl Fe^II precursors

As unequivocally demonstrated in previous sections, the number, nature and position of the substituents on the aryl rings play a crucial role in driving the catalytic activity of 2,6-bis(organylimino)pyridyl Fe^II precursors selectively towards either polymerization or oligomerization. However, it is also possible to achieve the simultaneous production, in the same reactor, of both HDPE and α-olefins using a single C₁-symmetric [2,6-bis(arylimino)pyridyl]iron catalyst [49] Scheme 27 shows

2,6-Bis(arylimino)pyridyl Fe^II catalysts for the production of α-olefins with a Poisson distribution

The ability of 2,6-bis(arylimino)pyridyl Fe^II alkyls to terminate propagation by chain-transfer to aluminum has been exploited by Gibson to produce α-olefins with a Poisson distribution via iron-catalyzed polyethylene chain growth on zinc and related metals [61], [134]. To this purpose was initially used the polymerization catalyst precursor FeCl₂L (L = 2,6-(2,6-(i-Pr)₂C₆H₃N double bond CMe)₂C₅H₃N) in combination with MAO and ZnEt₂ (>500 equiv.) (Scheme 31).

Later, other 2,6-bis(arylimino)pyridyl Fe^II and Co^II

Heterogenized 2,6-bis(organylimino)pyridyl Fe^II and Co^II catalysts

Despite BP-Amoco have recently announced a joint agreement aimed at commercializing HDPE prepared with Fe^II catalysts [135], the industrial application of the 2,6-bis(organylimino)pyridyl Fe^II and Co^II precursors in continuous flow processes (gas phase or slurries) is still problematic due to extensive reactor fouling and the high exothermicity of the polymerization process. In order to overcome reactor fouling as well as increase the catalyst lifetime, the heterogenization of the homogeneous

2,6-bis(organylimino)pyridyl Fe^II and Co^II catalysts in reactor blending and tandem copolymerization reactions

A method for controlling the molecular weight (MW) and the molecular weight distribution (MWD) of polyolefins involves combining two or more types of catalysts in a single reactor to produce polymers with different MW and MWD. This method, known as multi-component polymerization or reactor blending, has achieved considerable industrial attention as it is capable of producing easily polymers with good properties by using just a single polymerization process [146], [147], [148], [149], [150],

Conclusions

The discovery that iron(II) and cobalt(II) dihalides modified with 2,6-bis(imino)pyridyl ligands are very active catalysts for the polymerization and oligomerization of ethylene, on activation by MAO, has represented one of the major breakthroughs in catalysis over the last years. Like metallocenes, a substantial contribution to the success of these catalysts has been provided by the availability of MAO and related activators. Indeed, 2,6-bis(imino)pyridyl Fe^II and Co^II dihalides constituted a

Acknowledgments

The EC contract NMP3-CT-2005-516972 (NANOHYBRID) and the COST Action D17 action are thanked for financial support.

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¹: Present address: Chemistry Department, University of Warwick, UK.

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ReviewEthylene oligomerization, homopolymerization and copolymerization by iron and cobalt catalysts with 2,6-(bis-organylimino)pyridyl ligands

Abstract

Introduction

Section snippets

Synthesis of 2,6-bis(organylimino)pyridine ligands

Synthesis of 2,6-bis(organylimino)pyridine iron(II) and cobalt(II) catalyst precursors

Principal activators of 2,6-bis(organylimino)pyridine FeII and CoII catalyst precursors

Ethylene polymerization by 2,6-bis(arylimino)pyridyl iron and cobalt catalysts

Ethylene oligomerization by 2,6-bis(arylimino)pyridyl FeII and CoII catalysts

Proposed mechanisms for activation, initiation, chain-propagation and chain-transfer in ethylene polymerization/oligomerization catalyzed by 2,6-bis(organylimino)pyridyl FeII and CoII precursors

Simultaneous oligomerization/polymerization of ethylene by C1-symmetric 2,6-bis(organylimino)pyridyl FeII precursors

2,6-Bis(arylimino)pyridyl FeII catalysts for the production of α-olefins with a Poisson distribution

Heterogenized 2,6-bis(organylimino)pyridyl FeII and CoII catalysts

2,6-bis(organylimino)pyridyl FeII and CoII catalysts in reactor blending and tandem copolymerization reactions

Conclusions

Acknowledgments

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Review
Ethylene oligomerization, homopolymerization and copolymerization by iron and cobalt catalysts with 2,6-(bis-organylimino)pyridyl ligands

Principal activators of 2,6-bis(organylimino)pyridine Fe^II and Co^II catalyst precursors

Ethylene oligomerization by 2,6-bis(arylimino)pyridyl Fe^II and Co^II catalysts

Proposed mechanisms for activation, initiation, chain-propagation and chain-transfer in ethylene polymerization/oligomerization catalyzed by 2,6-bis(organylimino)pyridyl Fe^II and Co^II precursors

Simultaneous oligomerization/polymerization of ethylene by C₁-symmetric 2,6-bis(organylimino)pyridyl Fe^II precursors

2,6-Bis(arylimino)pyridyl Fe^II catalysts for the production of α-olefins with a Poisson distribution

Heterogenized 2,6-bis(organylimino)pyridyl Fe^II and Co^II catalysts

2,6-bis(organylimino)pyridyl Fe^II and Co^II catalysts in reactor blending and tandem copolymerization reactions