Non-traditional Ziegler-Natta catalysis in α-olefin transformations: reaction mechanisms and product design

Introduction

α-Olefins are very important large-scale raw materials used in the chemical industry. Nowadays the chemical industry applies α-olefins as co-monomers for the production of medium and high molecular weight, low density polyethylenes and polypropylenes [1], [2], [3], functional polymers [4], oils and lubricants [5], [6], [7], depressants [8], [9], dispersants [10], and other products. The discovery of selective methods of ethylene tri- and tetramerization [11], [12], [13], [14], [15], [16], [17], [18] makes synthesis and processing of 1-hexene and 1-octene more attractive and cost-effective.

Traditionally, polyolefins are produced by means of heterogeneous Zigler-Natta catalysts. For production of advanced polyolefins the single-site zirconocene catalysts of various structures are often used [19]. Most zirconocenes are able to catalyze the polymerization of olefins and produce medium and high molecular weight polymers only after activation with considerable excess (up to 10³–10⁴ eq.) of methylalumoxane (MAO) [20], [21], [22], [23].

Low molecular weight products of α-olefin transformations (dimers, oligomers) are also of interest for chemical industry [24]. These products can be prepared by means of zirconocene catalysts activated with minimal (1–100 eq.) amounts of MAO [25], [26], [27], [28]. However, only several zirconocene complexes were studied under these conditions and the product composition is strictly dependent on the type of zirconocene used [25], [26], [27], [28].

In the present work we generalize the results obtained by our group in the study of the zirconocene catalyzed olefin oligomerization. The paper focuses on questions regarding the reaction mechanism, rational design of zirconocene pre-catalysts, as well as prospective uses of α-olefin products.

Reaction mechanism and Zirconocene dichloride activation

Assuming that selective dimerization of α-olefins occurs only in the presence of zirconocene dichlorides, activated by several eq. of MAO, Bergman [25] suggested that the catalytic particle in the dimerization is not the classical zirconocene cation, but a hydride complex containing a Cl atom in the Zr atom’s coordination sphere. Considering recent works, which studied the formation of bridged Zr–Al complexes [29], [30], [31], [32], their role in alkene polymerization [33], [34], [35], as well as the nature of ion pairs [34], [35], [36], [37], and mechanisms of chain propagation and termination [38], [39] we proposed a dimerization scheme [28] (Fig. 1b), which compliments the traditional mechanism of zirconocene catalysis.

Fig. 1:

Traditional (a) and non-traditional (b) ways of zirconocene dichloride – MAO catalysis.

Taking this concept into account, one can propose that for zirconocenes with relatively weak electron-donating η⁵-ligands (cyclopentadienyl, mono-substituted cyclopentadienyl), in alkyl complex B (Fig. 1b), which is formed within the first α-olefin insertion, the Cl coordination to zirconium is retained. After the insertion of a second molecule of α-olefin, β-hydride elimination is carried out easily through transition state С thanks to the assistance of Al. This leads to a formation of the α-olefin dimer and cation complex A, which is our proposed dimerization catalytic particle. The hypothesis regarding direct presence of coordinated Cl is an important addition to the traditional cationic polymerization mechanism. We should note that there are a few examples of single-site olefin oligomerization processes where the direct participation of Cl has been established, e.g. in Cr-catalyzed ethylene trimerization [40], [41], [42]. A number of experimental results support this hypothesis. We have determined that (η⁵-C₅H₅)₂ZrF₂ – as opposed to (η⁵-C₅H₅)₂ZrCl₂ – in the presence of 10 eq. of MAO catalyzes the oligomerization of 1-hexene. Furthermore, the dimerization selectivity in the presence of (η⁵-C₅H₅)₂ZrCl₂ is influenced only by R₂AlCl: the increase of the Al_Cl/Zr ratio almost fully suppresses the formation of higher oligomers, and the addition of soluble ionic chlorides does not affect the product distribution. Another supporting factor for our hypothesis that the reaction mechanism is dependent upon the zirconocene structure is that is that R₂AlCl does not visibly observably influence the properties of bis-indenyl complexes – both, in the presence and absence, of additional quantities of R₂AlCl, polymer products of same molecular mass are formed.

Therefore, we propose that a weak, reversible coordination of R₂AlCl to zirconocene cation results in oligomer formation. In the case of complexes with highly electron-donating η⁵-ligands, there is no Zr–Cl–Al coordination (the acidity of Zr center is poor), and the interaction with an α-olefin goes by the traditional cation pathway with polymer product formation. Therefore we can control the direction of α-olefin transformation catalyzed by zirconocene dichlorides in the minimal presence of MAO. The instruments for such reaction management are the design of the zirconocene structure, variation of the MAO/Zr ratio, as well as the introduction of additional quantities of chain termination regulator R₂AlCl into the reaction mixture.

According to the Fig. 1b, the dimerization/oligomerization/polymerization processes starts after the zirconium hydride A forms. Treatment of a zirconocene dichloride with any methyl-aluminum derivative, for example, with MAO is not the best way to generate Zr–H particles. It was reported that zirconocene dichloride (η⁵-C₅H₅)₂ZrCl₂, activated with 10 eq. of MAO, catalyzed the formation of vinylidene dimers of α-olefins with very low rates. The reaction was characterized by a long induction period and took tens of hours to complete [25], [26]. We repeated the dimerization experiment for 1-hexene, described in [25], using analogs of (η⁵-C₅H₅)₂ZrCl₂ – bis-cyclopentadienyl and bis-indenyl ansa-complexes Me₂C(η⁵-C₅H₄)₂ZrCl₂ and Me₂C(η⁵-C₉H₆)₂ZrCl₂ – as pre-catalysts and 10 eq. of MAO as an activator. In both cases, even after 2 h at 60 °С, there was no observed signifciant monomer conversion.

In order to solve the problem of pre-catalyst activation, we developed a two-step method [28]. In the beginning, LZrCl₂ reacted with 10–20 eq. of triisobutylaluminium (TIBA) to form soluble isobutyl derivative D (Fig. 1b) and Zr–Al hydride complex E therefrom [43], [44], [45], [46]. We observed that the intermediate E was not able to oligomerize an α-olefin, however, the reaction starts immediately after introduction of minimal amounts of MAO, which transforms complex E into the catalytic particle A.

The newly developed activation method for zirconocene dichlorides of various structures allowed us to carry out a comparative study of the catalytic activities of a wide range of complexes. Based on results of this study, we elaborated effective synthetic approaches to different types of α-olefin products: vinylidene dimers, oligomers and polymers.

α-olefin dimerization

Zirconocene-catalyzed dimerization of α-olefins (Scheme 1) has been known since the late 1980s [47]. The reaction is carried out in the presence of (η⁵-C₅H₅)₂ZrCl₂ (1), activated by a minimal quantity of methylalumoxane (MAO). By now, zirconocene-catalyzed dimerization is the most effective method to prepare α-olefin dimers with the vinylidene fragment >C=CH₂ [25], [26], [27], [28], [48], [49], [50], [51], [52].

Scheme 1:

Dimerization of 1-hexene (a), structural formulas of zirconocene dichlorides studied in this reaction (b) and selected geometric parameters of zirconocene geometry (c) [28].

Only a few zirconocene dichlorides have been studied as catalysts of α-olefin dimerization. It has been determined that n-alkyl-substituted complexes [for example, (n-BuC₅H₄)₂ZrCl₂] somewhat outperform 1 by productivity, while losing in selectivity [49]. The increase in size of alkyl substitutes in cyclopentadienyl rings serves to worsen both parameters. We have recently carried out a comparative study [28] of a series of different geometry zirconocenes 1–8 in 1-hexene dimerization (Scheme 1a, b). It was found that dimerization selectivity drops after replacement of the Cp fragment with a more electron-donating indenyl ligand (2), or upon introduction of bulky substituents into the Cp ring (3). Comparing the catalytic performances of 1 and bis-cyclopentadienyl ansa-complexes 4–8 allowed us conclude that an increase in the bridge length leaded to an increase of productivity and selectivity of hexene dimerization. This effect can be explained in the terms of geometric parameters of zirconocenes, the dihedral angle β between the cyclopentadienyl rings that is related with Coordination Gap Aperture (CGA, Scheme 1c) [53]. This dihedral angle is correlated to the catalytic activity of complexes 4–8; as β decreases, the productivity of the metallocene catalyst increases. Among zirconocenes with a minimal dihedral angle β (1, 7, 8) complex 8 having three-membered Me₂SiOSiMe₂ bridge has demonstrated the highest selectivity and productivity (Table 1). The results presented in Table 1 are clearly illustrated in Fig. 2.

Fig. 2:

Product distribution in catalytic experiment with 1-hexene in the presence of zirconocenes 1–8. Reaction conditions: neat monomer, 60°C, 1 h, 0.05 mol% of zirconocene, 20 eq. of TIBA and 10 eq. of MAO.

In order to demonstrate the general character of α-olefins dimerization catalyzed by 8, we studied the reactivity of various substrates (Scheme 2). The reactions were carried out in the presence of 1–2 eq. of Et₂AlCl, 20 eq. of TIBA and 10 eq. of MAO. Isolated yields of linear α-olefin dimers were over 90%. It is remarkable that 8 effectively catalyzes the dimerization of sterically hindered α-olefins. For example, the dimer of 3-methyl-1-butene (14) was produced with yield of 78% (when catalyzed with 1 the yield of 14 was only 3%) [25]), the dimer of vinylcyclopentane (16) was synthesized with a yield of 84% (25% in the literature [51]). Ethers, thioethers, amines and pyrrole derivatives did not enter in the reaction, but dimers of allylfuran 20 and allylthiopenes 21 and 22 were isolated with low (20) or moderate (21, 22) yields.

Scheme 2:

Preparation of R–CH=CH₂ dimers using precatalyst 8. Reaction conditions: 0.05 mmol of zirconocene, 100 mmol of substrate, 60°C; 1 mmol of TIBA (20 min, first activation step), 0.5 mmol of MAO (4 h, second activation and oligomerization step, exept 20–22, see Experimental).

Dimers of α-olefins are structural analogs of isobutylene and can be successfully used in the synthesis of analogs of isobutylene-based products. Due to a highly branched structure with long alkyl fragments, α-olefin dimers are prospective starting compounds for the synthesis of functional derivatives with regulated lipophilicity, such as analogs of 2-ethylhexanol, 2-ethylhexanoic and 2-ethylhexylphosphoric acids. However, the scientific and patent literature contains only few examples of transformations of vinylidene dimers [61], [62], [63], [64], [65], [66], [67]. For example, 1-decene dimer was used in Friedel-Crafts alkylation of benzene [61] and N,N-diphenylamine [62] to give the mixture of branched alkylarenes. Epoxydation of 1-decene dimer by H₂O₂ [63] allowed to get an epoxide, which was transformed into a corresponding aldehyde via acid-catalyzed dehydration [64]. The catalytic processes are illustrated by hydroformylation [65], hydroaminomethylation [66] and hydrocarbomethoxylation [67] with relatively low yields and selectivities.

In order to estimate the synthetic potential of α-olefin dimers, we carried out some catalytic, electrophylic and radical transformations of 2-butyl-1-octene (9) (Fig. 3). We prepared the lipophylic 2-butyl-1-octanol and its methacrylic ester 23, which is the prospective monomer for the preparation of hydrocarbon-soluble polyacrylates. Chlorosilane 24 was obtained by hydrosilylation of 9 in the presence of the Karstedt catalyst. Hydrocarbomethoxylation product 25 was synthesized in high yield and very high regioselectivity using bulky diphosphine/Pd catalyst [68], [69], [70]. The possibility of very effective radical addition to vinilydene fragment was illustrated by reactions of 9 with hydrosulfite (26), H₃PO₂ (prospective phosphonic 27 and phosphinic 28 acids), thiol (29) and thioacetic acid (30). The last product was converted into branched thiol 31. Ene reaction with maleic anhydride leads after hydrolysis to dicarboxylic acid 33. Characteristics of the products 23–33 are given in Experimental.

Fig. 3:

2-Butyl-1-octene derivatives and their prospective applications.

α-olefin oligomerization

Practical interest toward oligomers of higher α-olefins is due to the fact that the products of their hydrogenation (commercially known as PAO, from “poly-α-olefins”) are the main components of synthetic motor oils and low temperature lubricants, the properties of which outperform those of naturally occurring mineral oils. The most important characteristics of motor oils are the pour point (PP) and the ability to resist large reductions in viscosity with increasing temperature. It is desirable to have the lubricant flow easily upon “cold starting” an engine, but at high operating temperatures the oil should have a high enough viscosity to maintain a fluid film between moving parts. The temperature dependence of the viscosity of a lubricant is typically characterized by the viscosity index (VI). The VI is determined using a standard method (ASTM D-2270) in which the kinematic viscosity of an oil at 40°C (KV⁴⁰) and 100°C (KV¹⁰⁰) is compared against the viscosities of two reference oils.

The tribological characteristics and physical properties of PAOs depend on their molecular structure [71]. For high molecular weight hydrocarbons, these structures can be categorized as linear, short-chain branched, alicyclic, long-chain branched, and “star-like” (Scheme 3). The type of molecular structure is determined by the hydrocarbon source. Mineral oils, isolated directly from crude oil, contain significant quantities of linear and cyclic hydrocarbons, characterized by very high pour points and low viscosity indexes. Currently, most PAOs are produced with electrophilic oligomerization of 1-decene. Even when using a relatively “soft” catalyst, such as BF₃/ROH, this process is accompanied by the isomerization of the carbon skeleton, with a partial formation of short-chain branched hydracarbons characterized by low VI [72]. Long-chain branched and star-like products have the best characteristics; their synthesis is based on coordination oligomerization of α-olefins.

Scheme 3:

Main structural types of hydrocarbons – PAO components.

Janiak and Blank [73] gave a review of α-olefin metallocene oligomerization, but they sum up the data mostly about the synthesis of high molecular weight oligomers. Besides, it has been experimentally determined that 1 and its substituted analogs such as (n-BuC₅H₄)₂ZrCl₂, are not useful for preparation of α-olefin oligomers [74]. An effective oligomerization catalyst should primarily provide the formation of oligomers, characterized by a given P_n in the range of 3–10. It also should not form the polymers, as well as α-olefin isomerization byproducts, such as 2-alkenes.

In [75], it has been shown that of ansa-zirconocenes 34 (Scheme 4) with alkyl groups in the α-position relative to the bridge catalyzed the formation of higher oligomers of α-olefins. The difficulty in synthesizing complex 34 pushed us toward searching for alternative catalytic systems, which also contain alkyl substituents in the α-position to the bridge fragment. We chose ansa-zirconocenes 35 and 36 as promising catalysts with one-carbon bridges (Scheme 4) and studied their catalytic activity in oligomerzation of 1-hexene. Complex 35, synthesized first by Erker and coll [76], demonstrated high activity. The products obtained in the presence of 35 were a mixtures of dimer and high oligomers. However, during the course of the reaction, about 20% of initial 1-hexene was transformed into 2-hexene which is an unwanted products of the process. Complex 36 is newly obtained, its synthesis and X-Ray structure determination are described in our recent work [77]. This complex, despite its structural similarity to 35, demonstrated a higher productivity, catalyzing dimer and higher oligomer formation in approximately equal amounts; the partial loss of the monomer through 2-hexene formation was less than 2%. 1-Octene and 1-decene in the presence of 36 also formed a mixture of dimers and oligomers with a high fraction of desired product and minimal 2-alkene content. Table 2 shows the results of 1-hexene oligomerzation experiments for 35, 36 and reference compounds 1 and 4, including GC data for oligomer ratios. As far as the reaction mixtures after the oligomerization in the presence of 36 contain the compounds of definite structures only (there is no isomerization products as it was within BF₃ or AlR₃ assisted oligomerization) we managed to isolate the individual trimers, tetramers and pentamers of 1-hexene, 1-octene, and 1-decene (Table 3), to hydrogenate them in order to estimate the viscosity characteristics of these individual compounds.

Scheme 4:

Zirconocenes studied in 1-hexene oligomerization.

An alternative way to prepare the hydrocarbons of definite structure, which can be considered as promising PAOs, consists of vinylidene dimers RR′C=CH₂ dimerization in the presence of acids (Scheme 5) to give long-chain branched “dimers of dimers”. It was shown that the dimer of 1-decene in the presence of EtAlCl₂/silica forms C₄₀-hydrocarbons with the yield of ~80% at room temperature [49]. We have also studied the acid-catalyzed dimerization of α-olefin dimers and found out that EtAlCl₂ in the presence of tertiary alkyl chlorides, for example, tert-BuCl, is a much more effective catalyst of this process, providing ~90% conversion after 1 h even at −30°C. Carrying out the reaction at lower temperatures allowed us to prevent the isomerization of the skeleton of the long chains within vinylidene dimers RR′C=CH₂ dimerization.

Scheme 5:

Dimerization of α-olefin dimers.

We compared the viscosities and physical properties of the oligomers obtained by us with known characteristics of commercially available PAOs, produced by means of acid catalysis. The results are shown in Table 4. It can be seen that the decene-based PAOs have the best characteristics. However, the octene-based PAOs, obtained by coordination oligomerization, in contrast to the products of 1-hexene oligomerization, match the technical requirements of PAO engine oils (Fig. 4). It is important that the octene-based PAOs obtained by coordination oligomerization outperform significantly 1-decene oligomers, obtained via AlCl₃ catalysis, and their characteristics are similar to that of commercial 1-decene derived products, synthesized by means of soft electrophilic catalyst BF₃/ROH.

Fig. 4:

Viscosity characteristics of 1-decene (green) and 1-octene (red) oligomers.

α-olefin polymerization

According to the concept represented in Fig. 1b, large MAO/Zr ratios (higher than 100/1) are not compulsory to ensure a normal course of the zirconocene catalyzed α-olefin polymerization process. The only requirement, which should be met in this case, namely, the ligand at Zr should be able to decrease the ability of the metal center to coordinate ClAlR₂ or any other organoaluminium particle. This can be implemented by applying the ligands at Zr with improved electron-donating ability, for example, by indenyls with alcoxy-groups or by cyclopentadienyls/indenyls with condensed pyrrole of thiophene rings. In our pioneering work on heterocenes (zirconocenes prepared from heterocyclic ligand) we found that the zirconocenes derived from heterocyclic indene such as indeno[1,2-b]indole demonstrated industrially satisfactory productivity in the polymerization of ethylene when activated by 15 eq. of MAO only [80]. Later we shown that in propylene and 1-butene polymerization zirconocene derived from methoxyindene and dithiophenocyclopentadienes demonstrated high productivity in the presence of 100–200 eq. of MAO.

During the present study we compared the catalytic properties of a series of conventional indenyl-derived zirconocenes and electron-reach zirconocenes 37 [81], [82] and 38 [83], [84], [85], [86], [87] (Scheme 6) in hexene-1 polymerization. The Al_MAO/Zr=20 was taken and the catalyst was prepared via two-stage activation (TIBA + MAO). We have found that the nature of the ligand influenced mainly the molecular mass of the polyhexene so obtained and both 37 and 38 allowed to prepare the polymers with a relatively high M_n.

Scheme 6:

1-Hexene conversions and characteristics of polymers obtained using different zirconocene catalysts. Reaction conditions: 0.05 mmol of zirconocene, 200 mmol of 1-hexene, 60°C; 1 mmol of TIBA (30 min, first activation step), 1 mmol of MAO (4 h, second activation and oligomerization step).

Another application of zircononcene catalysis at low Al_MAO/Zr ratios is the preparation of polymer composite materials. We have shown that zirconocene 38 was effective in forming composites based on α-olefin-diene co-polymers and microparticles of silica-coated Fe₃O₄ [88]. We have also demonstrated that this composite material represented a thermodegradable ferromagnetic absorbent for oil spill recovery.

Conclusion

Thus, α-olefins R–CH=CH₂ can be transformed into dimers, oligomers and polymers via metallocene catalysis in the presence of small (10–20 eq.) excess of MAO. The product composition depends on the nature of the zirconocene being used. For example, complexes of the bis-cyclopentadyenyl type primarily catalyze dimerization with a formation of vinylidene dimers, R–C(H=CH₂)CH₂CH₂R. The introduction of substituents into the cyclopentadienyl rings, or replacement of one cyclopentadienyl fragment with an indenyl ligand shifts the direction of the process toward the formation of oligomer mixtures. For bis-indenyl complexes, polymerization is the preferred mode of chemical transformation. We proposed effective catalysts for each type of processes. Bis-cyclopentadyenyl ansa-zirconocene 8 with Me₂SiOSiMe₂ bridge is effective for selective α-olefins dimerization. The ansa-complex containing shortest C1 bridge and α-alkyl substituent, 36, is applicable for low P_n oligomer formation. The “heterocene” 38 containing dithiophenocyclopentadienyl fragment represents very performant catalyst of hexane-1 polymerization even at Al_MAO/Zr=20.

We assume that the direction of α-olefin transformation mostly depends on the zirconocene reaction center’s ability to coordinate the R₂AlCl or any other organoaluminium particle that occurs after pre-catalyst activation. The realization of such coordination accelerates β-hydride elimination which leads to chain termination. For this reason, the reversible and relatively weak coordination of a organoaluminium particle to Zr-center is a factor, which influences the balance of chain propagation/chain termination and, as such, manages the degree of oligomerization. In the absence of coordination or if such coordination is relatively weak the process goes according to the classical cation mechanism and the polymer forms.

Products of α-olefin transformations have far-reaching prospective applications. Dimers of α-olefins can be used as substrate for the synthesis of various products with controlled lipophilicity, such as selective rare earth element extractans, detergents, plasticizers, adhesives, and surface modifiers, monomers for the production of motor oils and lubricants, as well as polar polymeric depressors. Oligomers of α-olefins are important for production of PAOs. α-Olefin polymers and co-polymers show promise as thickeners and absorbents.

Experimental