Metallocenes in ethylene polymerization studied by cyclic and differential pulse voltammetry

Applied Catalysis A: General 344 (2008) 98–106

Contents lists available at ScienceDirect

Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Metallocenes in ethylene polymerization studied by cyclic and differential pulse voltammetry Fernando Silveira a, Lilian M.T. Simplı´cio b, Zeˆnis Novais da Rocha b, Joa˜o Henrique Zimnoch dos Santos a,* a b

Instituto de Quı´mica, UFRGS, Avenida Bento Gonc¸alves, 9500, Porto Alegre 91509-900, Brazil Instituto de Quı´mica, UFBA, Campus de Ondina, Salvador 40170-290, Brazil

A R T I C L E I N F O

A B S T R A C T

Article history: Received 15 December 2007 Received in revised form 29 March 2008 Accepted 4 April 2008 Available online 11 April 2008

The relationship between redox characteristics and catalytic activity in ethylene polymerization, with methylaluminoxane (MAO) as the cocatalyst, was evaluated for a series of homogeneous metallocenes (Cp2ZrCl2, (MeCp)2ZrCl2, (nBuCp)2ZrCl2, (iBuCp)2ZrCl2, (tBuCp)2ZrCl2, Cp2TiCl2, Cp2HfCl2, Et(Ind)2ZrCl2, Et(IndH4)2ZrCl2 and MeSi2(Ind)2ZrCl2). Catalytic activity is discussed in terms of the coordination sphere and metal center of the metallocene and compared with their electrochemical behavior in the presence of ethylene and MAO. The resulting polyethylenes were analyzed by GPC. The catalytic activity in the ethylene polymerization reaction is discussed in terms of half-life (t1/2) and electrochemical gap of the metallocene species. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Metallocene catalyst Cyclic voltammetry Differential pulse voltammetry Polymerization Polyethylene

1. Introduction Metallocene catalysts have been intensively investigated in the last 20 years both in industrial and academic laboratories. The single site catalyst nature allows the production of tailored polymers in terms of molecular weight, molecular weight distribution, stereoregularity and co-monomer incorporation [1]. The coordination sphere around the metal center, the presence or absence of bridge binding both rings, and the metal center itself affect and influence both the catalytic activity and the characteristics of the resulting polymers [2]. Tian and Huang [3] investigated the effect of the size and nature of the ligand bond to the aromatic ring in zirconocenes and in some hafnocenes, concluding that these ligands affect both catalytic activity and polymer molecular weight. Mo¨hring and Coville [4,5] discussed the steric effects of ligands and bridges in metallocene on the catalytic activity and polymer properties. Wang [6] studied the role of the bridge between the rings and concluded that it influences the available space for the coordination of the olefins, which in turn, affects catalytic activity. Most of these studies correlate molecular structure with catalytic activity. A few systematic studies have been devoted to

* Corresponding author. E-mail address: [email protected] (J.H.Z. dos Santos). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.04.006

evaluating changes in the electronic or steric properties of the catalyst along changes on the coordination sphere or on the metal center of the metallocene or even under experimental conditions. In a previous study, we correlated the binding energy, determined by X-ray photoelectron spectroscopy, of a series of metallocenes differing in steric hindrance with catalytic activity in ethylene polymerization [7]. In a continuation of that approach, the present paper comparatively discusses the electrochemical behavior of metallocenes, in the presence of the monomer and the cocatalyst, monitored by cyclic and differential pulse voltammetry. Cyclic voltammetry has been used as a tool for characterization of metallocenes and non-metallocene polymerization catalysts. In a pioneering study, Brintzinger’s group analyzed ansa-metallocenes by this technique [8]. Linear correlation between electrochemical (redox) gaps and absorption charge-transfer energies has been reported for Ti(IV), Zr(IV) and Hf(V) bis(cyclopentadienyl) dichlorides [9]. Recently, cyclic voltammetry has been employed to determine the electrochemical behavior of Ti and Zr-1,2 phenyledioxoborylcyclopentadiene complexes, [10] and to investigate the reactivity of Zr- and Ti alcooxide bidentate complexes in the presence of methylaluminoxane (MAO) and ethylene [11]. The catalytic species resulting from the interaction between [Me2Si(Cp)2ZrCl2] or [Cp2ZrCl2] with MAO or trimethyl aluminum (TMA) was also investigated by cyclic voltammetry [12]. In the present study, a series of metallocenes bearing different coordination spheres (Cp, MeCp, nBuCp, iBuCp, tBuCp, Ind, IndH4),

F. Silveira et al. / Applied Catalysis A: General 344 (2008) 98–106

bridges (Et, Me2Si) and metal centers (Zr, Ti, Hf) was comparatively analyzed by cyclic and differential pulse voltammetry. The catalysts were evaluated in ethylene polymerization and the resulting polymers were characterized by gel permeation chromatography (GPC). The correlation between electrochemical characteristics and catalysis activity or polymer properties is discussed. 2. Experimental

99

2.4. Polyethylene characterization Molecular weight distribution of polyethylene was measured using a Waters CV plus a 150 8C high-temperature GPC instrument equipped with viscosimetrical detector, and three Styragel HT type columns (HT3, HT4 and HT6) with an exclusion limit of 1  107 for polystyrene. 1,2,4-Trichlorobenzene was used as the solvent, at a flow rate of 1 cm3 min1. The analyses were performed at 140 8C. A universal calibration method with narrow polystyrene standards was used.

2.1. Materials 3. Results and discussion All the chemicals were manipulated under inert atmosphere using the Schlenk technique. MAO (Witco, 10.0 wt.% toluene solution, average molar mass 900 g mol1) and the metallocenes ([(MeCp)2ZrCl2], [(tBuCp)2ZrCl2], [(iBuCp)2ZrCl2], [(nBuCp)2ZrCl2], [Cp2ZrCl2], [Cp2TiCl2], [Cp2HfCl2], [Et(Ind)2ZrCl2], [Et(IndH4)2ZrCl2], [Me2Si(Ind)2ZrCl2] (Boulder or Aldrich)) were purified by recrystallization from stock solutions. Ethylene and argon (White Martins) were passed through a molecular sieve (13 A˚) prior to use. Toluene was purified by refluxing with sodium, followed by distillation under nitrogen just before use. Acetonitrile was refluxed for 6 h followed by distillation and kept under a molecular sieve. Tetrabutylammonium tetraflouroborate (Merck or Fluka) was purified through successive extractions with ethylacetate at 78 8C. Crystals were collected by filtration and vacuum dried. 2.2. Electrochemical and spectrophotometric measurements The cyclic and pulse differential voltammograms were recorded with a potentiostat/galvanostat (PARC, model 273). All experiments were carried out in dry inert atmosphere by bubbling high purity argon using a conventional three-electrode cell. Glassy carbon was used as the working electrode. An Ag/AgCl electrode was used as the reference electrode and a platinum wire as the auxiliary electrode. Ag/AgCl electrode was calibrated by ferrocene, which was used as an internal reference. These measurements versus the ferrocene (Fc+/Fc0) redox couple (+0.50 V in acetonitrile solutions) were reported versus an Ag/AgCl electrode. Electrochemical data were obtained using 0.1 mol L1 solutions of tetrabutylammonium tetrafluoroborate in acetonitrile as supporting electrolyte. In the cyclic or pulse voltammograms, neither anodic nor cathodic peaks were observed in the absence of metallocene in the studied potential range. All solutions were deaerated by bubbling high purity argon. The cocatalyst solutions were prepared with different Al/M ratios (M = Ti, Zr or Hf) and the cyclic voltammograms were recorded with a scan rate of 100 mV s1. The electronic absorption spectra were recorded in quartz cells in a Hewlett Packard Model 8453 spectrophotometer.

In the present study, the investigated metallocenes differ in terms of the coordination sphere (RCp, Ind and IndH4), the metal center (Zr, Ti or Hf) and the presence or absence of bridge (Et or Me2Si) between the cyclopentadienyl (Cp) rings. Thus, a series of 10 metallocenes were employed: 7 without bridge, namely, [(MeCp)2ZrCl2], [(nBuCp)2ZrCl2], [(tBuCp)2ZrCl2], [(iBuCp)2ZrCl2], [Cp2HfCl2], [Cp2ZrCl2] and [Cp2TiCl2]; and 3 bearing a bridge between the Cp rings, [Et(IndH4)2ZrCl2], [Et(Ind)2ZrCl2], and [Me2Si(Ind)2ZrCl2]. All these metallocenes were evaluated in ethylene polymerization with MAO as the cocatalyst. 3.1. Cyclic voltammograms of the metallocenes A typical profile of a metallocene cyclic voltammogram is presented in Fig. 1. All the complexes were shown to be electroactive in potential below 1.0 V and presented an anodic peak at positive potential. For [Cp2ZrIVCl2] (Fig. 1), the voltammogram illustrates cathodic signals at 1.7 V (Epc1), 1.9 V (Epc2) and 2.2 V (Epc3) which were attributed to the redox process centered on the metallic ion. At positive potential, an anodic peak at +1.71 V versus Ag/AgCl was observed. In order to demonstrate the stability of the complex, the profile of the pulse voltammogram was registered several times (5–6). The constancy in the voltammograms confirmed their stability under these experimental conditions (dry inert atmosphere). Redox properties of substituted zirconocene dichloride compounds have been already reported in the literature. A redox pair at ca. 2.1 and 2.4 versus Ag/AgCl range in tetrahydrofuran solutions has been measured for such systems [13]. For a series of non-bridged and bridged bent-sandwich zirconocene complexes, a reduction potential around 1.5 to 1.6 V and an

2.3. Polymerization reactions Polymerizations were performed in toluene (0.15 L) in a 0.30 L Pyrex glass reactor connected to a constant temperature circulator equipped with mechanical stirring and inlets for argon and ethylene. For each experiment, a mass of catalyst system corresponding to 105 mol L1 of metal (M = Ti, Zr or Hf) was suspended in 0.01 L of toluene and transferred into the reactor under argon. The polymerizations were performed at atmospheric pressure with ethylene at 60 8C for 30 min at [Al/M] = 1000, using MAO as the cocatalyst. Acidified (HCl) ethanol was used to quench the processes, and reaction products were separated by filtration, washed with distilled water, and finally dried under reduced pressure at 60 8C.

Fig. 1. Typical cyclic voltammogram of a metallocene [Cp2ZrCl2].

100

F. Silveira et al. / Applied Catalysis A: General 344 (2008) 98–106

Scheme 1. Global electrode process reactions for [L2ZrIVCl2].

Fig. 2. Differential pulse voltammograms of 1  103 mol L1 [(tBuCp)2ZrCl2]. v ¼ 100 mV s1 : (a) cathodic scan and (b) anodic scan.

oxidation potential higher than +1.0 V (1.2–1.8 V versus SCE) has been reported [14]. The reduction potential was assigned as a ZrIV/ III process and the anodic peak at positive potential as a ligand oxidation L2/L. Parallel studies have evidenced a linear correlation between electrochemical gap and absorption charge transfer [14]. In the present paper, for each cathodic peak, a corresponding anodic peak was observed which was assigned according to the literature [12]. El Murr et al. [15] studied the electrode mechanism of a [Cp2TiCl2] series and discussed it as a three-step process with one electron each. Scheme 1 presents an analogous proposition for [L2ZrIVCl2].

For the complexes bearing methyl, n-butyl, i-butyl or t-butyl groups in the cyclopentadienyl ring, the inductive effect favors a small potential shift in relation to the complex [Cp2ZrCl2], making the reduction process of Zr(IV/III) slightly less unfavorable (Eq. (1)). It is worth noting that a larger shift was observed for the t-butyl ligand, in which cathodic peaks were detected at the following: 1.5 V (Epc1), 1.9 V (Epc2), and 2.2 V (Epc3) (see Fig. 2). This Epc1 shift in the direction of making the Zr(IV) more prone to reduction is a consequence of the electron-donating role of the alkyl groups. Among them, t-butyl ligand more significantly enhances the electron density on the cyclopentadienyl ring. This renders the Zr(IV/III) reduction process a little less difficult. In this case, a wave at 1.8 V can probably be assigned to the reduction of Zr(III) that releases chloride ligand, in analogy to the titanocene complex [15]. In the present case, acetonitrile present in the medium can coordinate to the Zr(III). Nevertheless, future investigations are necessary to confirm this proposition. Voltammogram a of Fig. 3 shows the profile of the pulse voltammogram for the complex [Et(IndH4)2ZrIVCl2] with cathodic peaks at 1.65 V (Epc1), 1.9 V (Epc2) and 2.3 V. These values are very similar to those detected for [Cp2ZrIVCl2], which suggests similar stability for Zr(IV) and shows that each cathodic peak has a corresponding one as an anodic sign (voltammogram b), indicating that the electrode process of metallic center might be reversible according to the chemical reaction proposed in Scheme 1. For all the investigated metallocenes, differential pulse voltammograms run from 0 to +2.5 V potential show an anodic wave assigned to the oxidation of the ligand. These results are consistent with those previously reported for metallocene complexes [9,14,16]. The absence of a cathodic wave at this potential range in the present experimental conditions might be a consequence of the instability of the anionic zirconocene radical generated after the reduction of the ligand. Fig. 4 shows the profile of the pulse voltammogram of [Et(IndH4)2ZrIVCl2] at the 0–2.5 V range in cathodic and anodic scan. For the complex [Me2Si(Ind)2ZrIVCl2], a similar current versus potential curve to that reported for [Me2Si(Cp)2ZrIVCl2] [12] was observed: cathodic peaks were detected at 1.65, 1.85 and 2.2 V (shoulder). Table 1 presents the detected potentials for the different investigated metallocenes. For the complexes [Cp2ZrIVCl2], [(nBuCp)2ZrIVCl2], [(iBuCp)2ZrIVCl2], [(tBuCp)2ZrIVCl2] and IV [Et(IndH4)2Zr Cl2] (Table 1), the cathodic peak placed in the less negative potential (Epc1) is attributed to the electrode process in which Zr(IV) is reduced and engenders the formation of the

Fig. 3. Differential pulse voltammograms of 1  103 mol L1 [Et(IndH4)2ZrCl2]. v ¼ 72:73 mV s1 : (a) cathodic scan; (b) anodic scan.

F. Silveira et al. / Applied Catalysis A: General 344 (2008) 98–106

Fig. 4. Differential pulse voltammograms of 2.1  103 mol L1 [Et(IndH4)2ZrCl2]. v ¼ 100 mV s1 : (a) cathodic scan; (b) anodic scan. Scan range: 0–2.5 V vs. Ag/AgCl.

Table 1 Cathodic potentials on metal center, anodic potential on the ligand and ligand to metal charge transfer (LMCT) band for the investigated metallocenes Complex

Epc1 (V)

Epc2 (V)

Epc3 (V)

Epo (V)

LMCT (nm)

[Cp2ZrCl2] [(MeCp)2ZrCl2] [(nBuCp)2ZrCl2] [(iBuCp)2ZrCl2] [(tBuCp)2ZrCl2] [Et(IndH4)2ZrCl2] [Et(Ind)2ZrCl2] [Me2Si(Ind)2ZrCl2]

1.7 1.6 1.5 1.6 1.5 1.65 1.6 1.65

1.9 1.9 1.87 1.9 1.78 1.9 1.9 1.85

2.2 – – 2.2 2.2 2.2 2.2 2.2d

1.71, 1.86a,b 1.68 1.72 1.75 – 1.55 1.54, 1.36a,b 1.43, 1.38a,b

336, 338c 352 350 350 350 369 382, 435b,c 424, 454b,c

a b c d

Ref. [16]. In CH2Cl2. Ref. [14]. Shoulder.

complex ion [L2ZrIIICl2] (Eq. (1) in Scheme 1). In this complex, Zr(III) can be reduced with the involvement of one electron (Eq. (2)), justifying the cathodic peak designated as Epc2. In agreement with the literature, for the species [Cp2ZrIIICl2], there is the possibility of the cyclopentadienyl ring (Cp) releasing (Eq. (3)). Taking into account this possibility, the cathodic peak in the more negative potential (Epc3) for the complex mentioned above could be attributed to the reduction process Zr(III/II) in [CpZrIIICl2] (Eq. (4)). In analogy to the complex [Me2Si(Cp)2ZrIVCl2] [12], in the case of the metallocene bearing a coordinated indenyl [Me2Si(Ind)2ZrIVCl2], the cathodic peaks are detected at 1.65 and 1.85 V. The peak in less negative potential (1.65 V) is attributed to the reduction process of Zr(IV/III) of the initial complex (Eq. (1)), and the other, at 1.85 V, is assigned to reduction of Zr(III/II) in

101

[Me2Si(Ind)2ZrIIICl2] (Eq. (2)). It is worth noting that in these complexes, the possibility of detachment of L seems to be attenuated (shoulder at 2.2 V), probably due to the Et(Ind)2 and Me2Si moieties, which makes the ligand less labile. The electronic spectra in the UV–visible region show a ligand to metal charge transfer (LMCT) and internal ligand (IL) bands, as reported in the literature [17–19]. In Table 1, the LMCT bands can be assigned to Cp-Zr. The zirconocene transition bands in the UV– visible region were already interpreted on the basis of the MO theory [20,21]. For the metallocenes with alkyl groups in the cyclopentadienyl ligand, a bathochromic shift of the LMCT band is verified in relation to the energy of the referred band in [Cp2ZrCl2] that shows a smaller difference between the HOMO and LUMO orbitals, probably attributed to the decrease of energy of LUMO. This shift is coherent with that observed in the reduction potential of Zr(IV/III). Table 2 reports data for [Cp2MCl2] (M = Ti, Zr or Hf). For comparative reasons, data from the literature were also included. According to these results, Ti(IV) is much less stable than Hf(IV) and Zr(IV). The results for the LMCT band are in agreement with those reported in the literature [16]. The series of metallocene was further evaluated in ethylene polymerization with MAO as the cocatalyst. 3.2. Catalytic activity Fig. 5 shows the catalytic activity of these metallocenes, displayed by decreasing catalytic activity, for each of the following three groups: unbridged metallocenes, different metal center, and bridged metallocenes. According to Fig. 5, unbridged systems were shown to be more active than bridged metallocenes. Among the unbridged ones, the coordination sphere (ligand in the Cp ring) influences the catalytic activity. Ewen [22], Chien and Wang [23], Tian and Huang [3], Alt and Ko¨ppl [2], and Bialek et al. [24] studied the steric and electronic effects of alkyl ligands in Cp rings and their influence on the catalytic activity. Alkyl ligands increase electron donation to the Cp ring, stabilizing the cationic character of the metal center. Therefore, the presence of alkyl ligand engenders an increase in catalytic activity, which, on the other hand, is limited by the steric hindrance, caused by the ligands themselves, which influences the olefin coordination and insertion steps. This global effect will be reflected in the catalyst activity and the properties of the resulting polymers. The differences in catalytic activity are small, the electrondonating effect played by the ligand in the case of [(RCp)2ZrCl2], affords higher catalytic activity in comparison to that of [(Cp)2ZrCl2]. The differences among the ansa-metallocenes [Me2Si(Ind)2ZrCl2], [Et(IndH4)2ZrCl2] and [Et(Ind)2ZrCl2] are in terms of the saturation of the aromatic ring (IndH4 in comparison to Ind) and of the nature of the bridge (Me2Si or Et). ansaMetallocenes are characterized by lower activity in comparison to those metallocene systems without a bridge, due to the steric effect

Table 2 Cathodic potentials on metal center, anodic potential on the ligand and ligand to metal charge transfer (LMCT) band for Cp2MCl2 (M = Ti, Zr or Hf) Complex

Epc1 (V)

Epc2 (V)

Epc3 (V)

Epo (V)

LMCT (nm)

[Cp2ZrCl2] [Cp2TiCl2] [Cp2HfCl2]

1.7, 1.63a,b, 1.78a,c 0.6, 0.82a,b 1.55d, 1.93a,b

1.9 1.7 1.9e

2.2 2.1 2.1

1.71, 1.86a,b 1.4, 1.96a,b 1.78, 1.81a,b

336, 338 528, 515a 310, 307a

a b c d e

Ref. [16] in toluene. In CH2Cl2. In THF. Shoulder. Epc at 1.9 V—higher current intensity probably due to ZrIV/III.

F. Silveira et al. / Applied Catalysis A: General 344 (2008) 98–106

102

Fig. 5. Catalytic activity in ethylene polymerization of the homogeneous metallocenes.

of the bridge. The geometry around the metal center makes the structure more rigid, although the bite angle increases [3,4]. For [Et(IndH4)2ZrCl2] and [Et(Ind)2ZrCl2], the bite angle between the aromatic rings is very similar [3,4]. Nevertheless, the electronic effect caused by the ring saturation in the case of [Et(IndH4)2ZrCl2] engenders a higher electron-donating effect to the metal, favoring the olefin coordination and insertion steps, which in turn leads to an increase in catalytic activity as compared to that observed in [Et(Ind)2ZrCl2]. [Me2Si(Ind)2ZrCl2] presents a relatively higher bite angle in comparison to [Et(IndH4)2ZrCl2] and [Et(Ind)2ZrCl2] [3]. However, the presence of methyl groups might have some steric effect that might be responsible for the lower observed catalytic activity. Similar trends have been reported in the literature [3,5,7]. Considering the [Cp2MCl2] series, in which the metal center is Zr, Ti or Hf, the zirconocene exhibited the highest catalytic activity. ´ Agnillo et al. [25,26] compared [Cp2ZrCl2], [Cp2HfCl2] and D [Cp2TiCl2] and observed, under similar polymerization conditions, the same following trend found in this work: ½Cp2 ZrCl2  > ½Cp2 TiCl2  > ½Cp2 HfCl2  The differences in catalytic activity can be attributed to the stability of the M–C bond in the case of hafnocene [27], and to thermal lability of titanocene for temperatures higher than 40 8C [28]. It is worth noting that Ihm et al. [29] suggest that the low activity observed in the case of titanocene might be due to bimolecular deactivation reactions of these compounds with alkylaluminum cocatalyst during the polymerization reaction. The highest activity of [Cp2ZrCl2] can be attributed to the labile Zr– C bond that favors the polymeric chain transfer via b-hydrogen elimination, increasing the olefin coordination rate, which in turn leads to an enhancement of catalytic activity [2]. It is worth noting that the trend found in this work cannot be explained in terms of the cathodic potential (see Table 2). Nevertheless, according to the literature, zirconocene catalysts usually show higher activities for olefin polymerization than titanocene and hafnocene analogues due to the larger atom radius of zirconium than titanium and hafnium [6].

Comparing the cyclic voltammogram of Fig. 6 with that in Fig. 1, there is a small shift in the reduction signals of Zr(IV/III) species for the mixtures in which the molar ratio [Al/Zr] was 5, 10 and 15, attributed to the starting complex (paper in preparation) and another one close to 2.0 V. The wave around 2.0 V versus Ag/ AgCl involved in the electrochemical process can be assigned to the monomethylated species. The result is consistent with the substitution of a Cl by a methyl group that increases the electronic density on the Zr(IV), therefore the reduction process takes place in a more negative potential (Fig. 6). For this complex, in a higher molar ratio ([Al/Zr] = 30), there is a redox process with potential of 2.6 V, attributed to the reduction of Zr(IV/III) in the ionic pair [12]. The electrochemical behavior of the metallocenes was also investigated in the presence of MAO and ethylene. The potential scan was started in 0.0 V and moved to negative potential. The presence of a cathodic peak close to 900 mV, absent in the starting complex, was observed. In accordance with data obtained in the presence of ethylene, the cathodic wave was attributed to the Zr(IV) reduction with the coordinated ethylene. Since this solution contains both the metallocene and the cocatalyts, the species, which is capable of interacting with the olefin, should be that proposed in Scheme 2. This proposition is in accordance with the mechanism proposed in the literature in which the monomethylated species reacts with an olefin molecule [30]. It is worth mentioning that this proposition, for the sake of comprehension, is very simplistic since the species might be solvated either by MAO, or by TMA, or even by the solvent itself.

3.3. Voltammetric studies of the metallocenes under polymerization conditions Cyclic voltammetry of the metallocenes was also performed in the presence of the cocatalyst (MAO) and in an ethylene atmosphere. Fig. 6 shows the cyclic voltammogram of [Cp2ZrCl2] in the presence of increasing amounts of MAO (Al/Zr = 5, 10 and 15).

Fig. 6. Cyclic voltammogram of 1  103 mol L1 [Cp2ZrCl2] with [Al/Zr] = 5, 10 and 15. v ¼ 100 mV s1 .

F. Silveira et al. / Applied Catalysis A: General 344 (2008) 98–106

103

Scheme 2. Structure of monomethylated species.

Fig. 7. Differential pulse voltammograms of 2  103 mol L1 [Cp2ZrCl2] and [Cp2ZrCl2] in ethylene atmosphere. v ¼ 100 mV s1 . Cathodic scan.

Scheme 3. Proposed active species structure for polymerization of olefins.

The generation of the active species (Scheme 3) for zirconocene complexes demands the presence of [Al/Zr] molar ratios higher than 30. For the titanocene complex, a signal is observed after the addition of MAO at Al/Ti higher than 10, with Epc = 1.7 V. With the addition of MAO, the chloride atom can be released and a methyl group can be introduced on the Ti coordination sphere. As a result, the complex should present a potential reduction shift to become less negative. The signal at 1.7 V is very likely to be assigned to the formation of the active species, which is analogous to the proposition for a metallocene in which the cathodic wave at 1.7 V was attributed to Ti(IV) reduction in the ionic pair [Cp2TiCl2]+[MAOCl] [31]. In order to evaluate the behavior of the current–potential profile versus potential, each metallocene was put in the presence of MAO and ethylene. Ethylene is classified as a relative soft base, and Zr(IV) is a hard acid. The Zr(IV)-ethylene interaction is relatively weak. Nevertheless, the ethylene molecule is attracted, due to its electron density, to the Zr(IV), the charge/atomic radius ratio of which contributes to the intensity of the electron field, engendering an attractive interaction between this acid and the Lewis base. Fig. 7 shows the pulse voltammogram of [Cp2ZrCl2], before and after the addition of ethylene atmosphere. Comparing both voltammograms, the disappearance of the cathodic peaks regarding the reduction of Zr in the original complex (see Scheme 1), and the appearance of signals in more negative potentials, are in agreement with the modification of the electronic density around the Zr(IV) center, making the metal more prone to reduction. In other words, this fact is coherent with the relative stabilization of a hard acid (Zr(IV)) with a relatively soft base.

The successive voltammograms registered at the scan speed of 100 mV s1 show that the current of the peak centered around 900 mV decreases with time (Fig. 8). This fact is related to the possible instability of the species containing the methyl group and the coordination of the olefin with the metallic ion Zr(IV). The analysis of the current–potential alteration as a function of time was made in triplicate, with the use of the mathematical equation for a kinetic treatment of a process of first order. Fig. 9 exemplifies this relation to the [(tBuCp)2ZrCl2]. Table 3 presents data of the decomposition rate constant and half-life for the different complexes. The rate constant for the decomposition of the species that should be formed in the first step of the polymerization is 0.012 s1 for [Cp2ZrCl2], which is very close to that calculated for [(MeCp)2ZrCl2] (0.010 s1). These results suggest a small electronic effect of the methyl in the Cp group on the interaction with the Zr(IV)-olefin species. The presence of a butyl group in the Cp ring yields a smaller decomposition rate for the polymerization active species. Such behavior might be related to steric effect. In the case of nBu and iBu, their decomposition rates are very close. According to Table 3, for [Et(IndH4)2ZrCl2], the decomposition rate constant of the active species in the presence of ethylene and MAO suggests that the protonation of the indenyl ring renders this catalyst less stable than the analogous [(Et(Ind)2ZrCl2]. On the other hand, the relatively higher potential reduction values for Zr(IV/III) suggests higher stability for the former. For [Me2Si(Ind)2ZrCl2], the presence of the Si atom in the bridge affords a higher stabilization of Zr(IV), since the Zr(IV/III) reduction

Table 3 Decomposition rate constant (kobs) and half-life (t1/2) of the metallocenes in the presence of MAO and ethylene Metallocene

kobs

t1/2 (s)

[Cp2ZrCl2] [Cp2HfCl2] [Cp2TiCl2] [(MeCp)2ZrCl2] [(nBuCp)2ZrCl2] [(tBuCp)2ZrCl2] [(iBuCp)2ZrCl2] [Et(IndH4)2ZrCl2] [Et(Ind)2ZrCl2] [Me2Si(Ind)2ZrCl2]

0.0126  7.72  104 0.0113  5.72  104 0.026  1.70  103 0.0105  1.33  103 0.0121  1.11  103 0.0077  5.70  104 0.015  6.61  104 0.010  6.54  104 0.0037  3.96  104 0.005  6.67  104

55 61.3 26.6 66 57.3 90 44.7 69.3 186 137

104

F. Silveira et al. / Applied Catalysis A: General 344 (2008) 98–106

Fig. 8. Successive cyclic voltammograms of [(tBuCp)2ZrCl2] at Al/Zr = 10 with scan rate 100 mV s1. Ethylene atmosphere.

process is more difficult in this case than for [(Et(Ind)2ZrCl2]; its decomposition constant is ca. 5  103 s1, while in the case of [Et(Ind)2ZrCl2], it is 3.7  103 s1. The p acceptor character from the Si atom renders the metal center more acidic in the case of [Me2Si(Ind)2ZrCl2], therefore the interaction between the metal center and ethylene becomes more favorable. The stability of the catalyst species, expressed by the half-life (t1/2), can also be related to the catalytic activity. Considering the zirconocenes without bridge, a trend can be established, as shown in Fig. 10: catalytic activity enhances as the half-life (t1/2) increases. It is worth noting that although the reduction potentials (Epc1) for t-butyl and n-butyl are the same (Table 1), there is a significant difference in their half-life values (Table 3). Steric effects might play a relevant role in the stabilization of the former, which in turn could justify its higher catalytic activity in ethylene polymerization. 3.4. The relationship between catalytic activity and electrochemical behavior In the literature, the relationship between the LMCT band position of metallocene complexes and the differences in redox

Fig. 9. Relationship between ln(It  If) for the cathodic wave at 900 mV vs. time for [(tBuCp)2ZrCl2] at Al/Zr = 10 in ethylene atmosphere.

Fig. 10. Relationship between half-life (t1/2) and catalytic activity in ethylene polymerization.

potential metallic center and coordinated ligand in zirconocene complexes has already been discussed [14,16]. The same behavior was observed in the present study. Table 4 presents cathodic potentials centered on the metal (MIV/III) (Epc1), anodic potential (L/0) (Epo), electrochemical gap (Epo–Epc1), and absorption (LMCT) band values for zirconium complexes and [Cp2MCl2] (M = Ti, Zr or Hf), respectively. The LMCT is formally illustrated as a charge transfer ligand (HOMO)-metal (LUMO) and implies the reduction of the metal center and the ligand oxidation, assigned as ZrIV(h5L2)2 ! ZrIII(h5-L2). Thus, in the zirconocene complexes, excited states ZrIII-L are formed. Considering that the correlation between the charge-transfer absorption energies and the electrochemical gap [14,16] (difference between oxidation potential ligand and reduction potential Zr(IV)) is a consequence of the fact that the LMCT energy is the difference between the LUMO and HOMO orbitals, these orbitals must be the same involved in the redox process. In this paper, we introduced a relationship between the electrochemical gap and the catalytic activity. In the series [(RCp)2ZrCl2] and [L2ZrCl2] (L = Cp or R = Ind), a linear correlation was found (Fig. 11) that showed the effect of the electronic characteristics of the complexes on catalytic activity in ethylene polymerization. For the same family of catalysts ([(RCp)2ZrCl2], for instance), [(tBuCp)2ZrCl2] and [(nBuCp)2ZrCl2], although they bear roughly the same reduction potential, the catalytic activity is different, suggesting the importance of the steric effect on catalytic activity. Comparing [(iBuCp)2ZrCl2] and [(nBuCp)2ZrCl2], better catalytic activity was expected for the former. The small displacement observed in Epc1 and the higher electrochemical gap value (Table 4) for iBu indicates that it is a little more difficult to reduce Zr(IV) than in the case of [(nBuCp)2ZrCl2]. This effect should result in higher activity for this complex in comparison to [(n- or iBuCp)2ZrCl2]. In the case of [Cp2MCl2] complexes, although a linear correlation between electrochemical gap and LMCT band (Cp ! Zr) Zr) can be found, in accordance with similar previous results [9,16], the same correlation between G and catalysis activity was not found. This might be a consequence of bimolecular deactivation reactions in titanocenes.

F. Silveira et al. / Applied Catalysis A: General 344 (2008) 98–106

105

Table 4 Cathodic potentials centered on metal (MIV/III), anodic potentials (L/L0), electrochemical gap (G), electronic spectrum (l) and ligand to metal charge transfer (LMCT) in acetonitrile solution and catalyst activity Complex

Epc (MIV/III) (V)

Epo (V)

Ga (V)

l/E (LMCT) (cm1 eV1b)

Activity (kgPE mol M1 h1)

[(MeCp)2ZrCl2] [(nBuCp)2ZrCl2] [(iBuCp)2ZrCl2] [(tBuCp)2ZrCl2] [Cp2ZrCl2] [Cp2TiCl2] [Cp2HfCl2] [Et(IndH4)2ZrCl2] [Et(Ind)2ZrCl2] [Me2Si(Ind)2ZrCl2]

1.6 1.5 1.6 1.5 1.7 0.6 1.9 1.65 1.6 1.65

1.68 1.72 1.75 – 1.71 1.4 1.78 1.55 1.54 1.43

3.30 3.22 3.36 – 3.41 2.00 3.68 3.2 3.14 3.08

28,409 28,571 28,571 28,571 29,762 18,940 32,258 27,100 26,178 19,417

9432 7340 7917 8773 5750 1600 189 4200 3900 3400

a b c d e f g

(3.52) (3.54) (3.54) (3.54) (3.69), (2.35), (4.00), (3.36) (3.24), (2.41),

(3.668)d,g (2.407)f,g (3.974)g 22,989c,f (2.85)d,e 22,026c,f (2.73)d,e

G = DE(Epo  Epc). The absorption bands at lowest energy, ref. [32]. Ref. [14]. In CH2Cl2. Ref. [16]. In toluene. Ref. [9].

Fig. 11. (a) Plot of electrochemical gap G vs. activity of metallocenes [Cp2ZrCl2] and [(RCp)2ZrCl2]; (b) plot of electrochemical gap G vs. activity of metallocenes [Cp2ZrCl2] and [(RInd)2ZrCl2].

3.5. Influence of the metallocene coordination sphere, metal center and electrochemical behavior on the molecular weight of the resulting polymers Modification in the coordination sphere, as well as in the nature of the metal center, influence the properties of the resulting polymers [2,3]. In the present study, the metallocenes tested in ethylene polymerization produced polymers with the molecular weights presented in Table 5. Data of catalytic activity were also included. According to Table 5, among the zirconocenes without bridge, the largest molecular weight was observed for [Cp2ZrCl2]. For the systems [(tBuCp)2ZrCl2], [(iBuCp)2ZrCl2] and [(nBuCp)2ZrCl2], the combination of steric and electronic effects from the ligands in the Cp ring seems to influence the molecular weight of the resulting polymers. For instance, the largest electron-donation capacity due to the tertiary carbon in the tert-butyl makes the metal center less cationic. This can render the coordination steps and olefin insertion easier. This result is consistent with the t1/2: increasing the turnover of the catalytic cycle releases the growing polymeric chain sooner, and a polymer of lower molecular weight is formed (Table 5). For the systems with different metal centers and the same coordination sphere, the differences among the produced polymers depend on the nature of the central atom. The system

[Cp2HfCl2] presents the lowest activity of the series due to the slow coordination kinetics and insertion of the olefin [2]. However, this metallocene produces polymers with higher molecular weight than their zirconocene and titanocene analogues due to the stable Hf–C bond [27]. This fact is confirmed by the decomposition rate constant of the product formed in hafnocene-MAO-ethylene solution (see Table 3). On the other hand, the labile Zr–C bond with longer bond distance when compared to Hf–C, makes the chain-transfer reaction through b-hydrogen elimination easier, Table 5 Catalyst activity, molecular weight and polydispersity (PDI) of polymers obtained with metallocenes in ethylene polymerization System

Activity (kgPE mol M1 h1)

Mw (kg mol1)

PDI

[(MeCp)2ZrCl2] [(tBuCp)2ZrCl2] [(iBuCp)2ZrCl2] [(nBuCp)2ZrCl2] [Cp2ZrCl2] [Cp2TiCl2] [Cp2HfCl2] [Et(IndH4)2ZrCl2] [Et(Ind)2ZrCl2] [Me2Si(Ind)2ZrCl2]

9432 8773 7917 7340 5750 1600 189 4200 3900 3400

132 56 229 270 290 137 495 232 384 260

2.1 2.3 2.2 2.3 2.2 1.8 2.8 2.7 2.3 2.1

[M] = 105 M; [Al/M] = 1000; T = 60 8C; V = 0.15 L toluene; t = 30 min; P = 1 atm.

106

F. Silveira et al. / Applied Catalysis A: General 344 (2008) 98–106

effect caused by the coordination sphere around the metal center. Catalytic activity of the investigated metallocenes and the molecular weight of the obtained polyethylene with some catalysts seem to be dependent on the electronic features engendered by the ligands. Chemical groups bearing higher electron-donation capacity provide longer stability to the active site, influencing the half-life (t1/2). The electrochemical gap seems to be related to the catalysis activity as shown by the linear correlation found in the present study. The increase in half-life afforded higher catalytic activity, and the increase of the reduction potential (Epc) engenders the decrease in the molecular weight (Mw) of the resulting polymers. For the unbridged metallocenes, an increase in the electronic gap increased the Mw of polymers. Nevertheless, for bridged metallocenes, the steric effect must be taken into account.

Acknowledgements

Fig. 12. Relationship between electrochemical gap G vs. molecular weight of obtained polymers. Cp = [Cp2ZrCl2]; MeCp = [(MeCp)2ZrCl2]; iBu = [(iBuCp)2ZrCl2]; nBu = [(nBuCp)2ZrCl2]; Me2Si = [Me2Si(Ind)2ZrCl2]; EtIndH4 = [Et(IndH4)2ZrCl2]; and EtInd = [Et(Ind)2ZrCl2].

This work was partially financed by CNPq and PRONEX. We thank Ipiranga Petroquı´mica S.A. (Triunfo, Brazil) for GPC analyses.

References producing polymers with lower molecular weight than those produced by the analogue hafnocene [33]. Considering the systems with bridge, [Et(IndH4)2ZrCl2] and [Et(Ind)2ZrCl2] are very similar structurally, bearing small differences in the angle between the aromatic rings [6,4]. In this case, the electronic effect caused by the partial saturation of the aromatic rings of the system [Et(IndH4)2ZrCl2] seems to be responsible for the reduction of the molecular weight of the polymer formed by this system, in comparison with the polymer formed by the system [Et(Ind)2ZrCl2]. That is explained by the fact that the saturation of the indenyl ring increases the electron-donor effect to the metal, enhancing the kinetics of the olefin coordination and insertion steps, and therefore reducing the molecular weight of the formed polymer. The system [Me2Si(Ind)2ZrCl2], having a slightly larger bite angle between the indenyl rings as compared to the [Et(IndH4)2ZrCl2] and [Et(Ind)2ZrCl2] systems, produces polymers of intermediate molecular weight [6]. An attempt to investigate the relationship between the electrochemical gap and the molecular weight provided a linear correlation for unbridged complex (Fig. 12). Considering the [Cp2MCl2] series, the hafnocene exhibited the highest molecular weight and electrochemical gap. The trend found between these two parameters in these complexes shows that molecular weight decreases in the following order: [Cp2HfCl2] > [Cp2ZrCl2] > [Cp2TiCl2]. As observed previously for the catalytic activity, the electronic effects on the unbridged complexes can also explain the observed Mw. For the systems with bridge, other factors besides the electronic one might influence the resulting Mw, suggesting once again the importance of the steric effect in such systems. The reduction potential, the decomposition rate constant, the half-life and the electronic gap do not show any clear influence on the polydispersity of the polymers obtained under the present conditions. 4. Conclusions Cyclic and pulse voltammetry assisted to explain metallocene behavior in ethylene polymerization in terms of the electronic

[1] B.A. Krentsel, Y.V. Kissin, V.J. Kleiner, L.L. Stotskaya, Polymers and Copolymers of Higher a-Olefins, Hanser, Munich, 1997. [2] H.G. Alt, A. Ko¨ppl, Chem. Rev. 100 (2000) 1205–1221. [3] J. Tian, B. Huang, Macromol. Rapid Commun. 15 (1994) 923–928. [4] P.C. Mo¨hring, N.J. Coville, Coord. Chem. Rev. 250 (2006) 18–35. [5] P.C. Mo¨hring, N.J. Coville, J. Organomet. Chem. 479 (1994) 1–29. [6] B. Wang, Coord. Chem. Rev. 250 (2006) 242–258. [7] R. Guimara˜es, F.C. Stedile, J.H.Z. dos Santos, J. Mol. Catal. A: Chem. 206 (2003) 353– 362. [8] H. Schvemlein, W. Tritschler, H. Kiesele, H.H. Brintzinger, J. Organometal. Chem. 293 (1985) 353–364. [9] G.V. Loukova, V.V. Strelets, J. Organometal. Chem. 606 (2000) 203–206. [10] M.J. Davies, M.F. Lappert, Polyhedron 25 (2006) 397–405. [11] N.R.S. Basso, P.P. Greco, C.L.P. Carone, P.R. Livotto, L.M.T. Simplicio, Z.N. da Rocha, J.H.Z. dos Santos, J. Mol. Catal. A: Chem. 267 (2007) 129–136. [12] F.G. Costa, L.M.T. Simplı´cio, Z.N. da Rocha, S.T. Branda˜o, J. Mol. Catal. A: Chem. 211 (2004) 67–72. [13] J. Langmaier, Z. Samec, V. Varga, M. Hora´cek, R. Choukroun, K. Mach, J. Organomet. Chem. 584 (1999) 323–328. [14] G.V. Loukova, V.V. Strelets, Russ. Chem. Bull. 49 (2000) 1037–1039. [15] N. El-Murr, A. Chaloyard, J. Tirouflet, J. Chem. Soc., Chem. Commun. 10 (1980) 446–448. [16] G.V. Loukova, Chem. Phys. Lett. 353 (2002) 244–252. [17] V.V. Strelets, Coord. Chem. Rev. 114 (1992) 1–60. [18] D. Coevoet, H. Cramail, A. Deffieux, Macromol. Chem. Phys. 199 (1998) 1451– 1457. [19] M.R.M. Bruce, A. Kenter, D.R. Tyler, J. Am. Chem. Soc. 106 (1984) 639–644. [20] E. Giannetti, G.M. Nicoletti, R. Mazzocchi, J. Polym. Sci., Polym. Chem. Ed. 23 (1985) 2117–2134. [21] A. Vogler, H. Kunkely, Coord. Chem. Rev. 211 (2001) 223–233. [22] J. Ewen, Stud. Surf. Sci. Catal. 25 (1986) 271–292. [23] J.C.W. Chien, B.P. Wang, J. Polym. Sci. Part A: Polym. Chem. 28 (1990) 15–38. [24] M. Bialek, K. Czaja, A. Reszka, J. Polym. Sci. Part A: Polym. Chem. 43 (2005) 5562– 5570. [25] L. D´Agnillo, J.B.P. Soares, A. Pendilis, J. Polym. Sci. Part A: Polym. Chem. 38 (1998) 831–840. [26] L. D´Agnillo, J.B.P. Soares, A. Pendilis, Macromol. Chem. Phys. 199 (1998) 955– 962. [27] L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 100 (2000) 1253–1346. [28] W. Kaminsky, M. Miri, H. Sinn, R. Woldt, Makromol. Chem. Rapid Commun. 4 (1983) 417–421. [29] S.I. Ihm, K.J. Chu, J.H. Yin, in: K. Soga, M. Terano (Eds.), Studies in Surface Science and Catalysis Advisory, Proceedings of the International Symposium on Catalyst Design for Tailor-Made Polyolefins, Kanazawa, Japan, (1994), pp. 299–306. [30] C. Przybyla, B. Weimann, G. Fink, in: W. Kaminsky (Ed.), Metalorganic Catalysts for Synthesis and Polymerisation, Springer, Berlin, 1999, pp. 333–346. [31] J.-N. Pe´deutour, K. Radhakrishnan, H. Cramail, A. Deffieux, J. Mol. Catal. 185 (2002) 119–125. [32] M.R.M. Bruce, A. Sclathni, D.R. Tyler, Inorg. Chem. 25 (1986) 2546–2549. [33] A. Razavi, L. Peters, L. Nafpliotis, J. Mol. Catal. A: Chem. 115 (1997) 129–154.