Switching from ethylene polymerization to nonselective oligomerization over a homogeneous model catalyst: A triphenylsiloxy complex of chromium(VI)

Journal of Organometallic Chemistry 699 (2012) 48e55

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Switching from ethylene polymerization to nonselective oligomerization over a homogeneous model catalyst: A triphenylsiloxy complex of chromium(VI) Pengyuan Qiu a, Ruihua Cheng a, Zhen Liu a, Boping Liu a, *, Boris Tumanskii b, Moris S. Eisen b, ** a b

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, People’s Republic of China Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Technion City, Haifa 32000, Israel

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 September 2011 Received in revised form 15 October 2011 Accepted 19 October 2011

SiO2-supported silyl chromate catalyst (UCC S-2 catalyst) is an important heterogeneous catalyst for the commercial production of polyethylene (PE). However, it has not been fully investigated during the past several decades. A large obstacle has been the lack of homogeneous model systems for this chromiumbased heterogeneous catalyst. In this work, bis(triphenylsilyl)chromate (BC), as a homogeneous model for UCC S-2 catalyst, was studied for its polymerization behavior with aluminum alkyl cocatalysts, namely, triisobutylaluminum (TIBA) and methylaluminoxane (MAO). Moreover, activation of BC was investigated by NMR and ESR spectroscopy, radical trapping on fullerene, and MALDI-TOF mass spectroscopy. In the case of TIBA, only an activity of ethylene polymerization were observed; In the case of MAO, BC exhibits an interesting transformation from ethylene polymerization (Al/Cr
200) to nonselective oligomerization (Al/Cr  500) with an increase of Al/Cr molar ratio. The obtained PE display a switching from a unimodal molecular weight distribution (MWD) (Al/Cr
100) to a bimodal MWD (Al/ Cr  200), and the obtained oligomers are main even-numbered linear a-olefins (C6eC16). The butyl radical and [Cr(h6-arene)2]þ cation was identified during activation of BC with TIBA, and the [Cr(h6arene)2]þ cation was proved to be inactive for ethylene polymerization by temperature-dependent ESR experiments. Ó 2011 Elsevier B.V. All rights reserved.

Keywords: Bis(triphenylsilyl)chromate Homogeneous model UCC S-2 catalyst Ethylene polymerization Nonselective oligomerization

1. Introduction Chromium-based heterogeneous polyethylene catalysts, including Phillips Cr/SiO2 catalyst [1e5] and SiO2-supported silyl chromate catalyst (UCC S-2 catalyst) [6e8], have long been among the most important industrial catalysts for the polymerization of ethylene. Until now, more than 50% of the world’s high density polyethylene (HDPE) are still produced by these two chromiumbased catalysts [5]. UCC S-2 catalyst was commercialized by Union Carbide Corporation (UCC) with a gas phase UNIPOL process for HDPE production. Until now, UCC S-2 catalyst is still producing several million tons of HDPE, which has desirable polymer chain characteristics meeting different market demanding [9,10]. During the past several decades, more attention has been paid by the academic community to the Phillips Cr/SiO2 catalyst [2e5]; however, UCC S-2 catalyst has not been fully investigated due to its complexity [6e8,10,11]. * Corresponding author. Tel./fax: þ86 21 64253627. ** Corresponding author. Tel.: þ972 4 8292680; fax: þ972 4 8295703. E-mail addresses: [email protected] (P. Qiu), [email protected] (R. Cheng), [email protected] (Z. Liu), [email protected] (B. Liu), botoshan@tx. technion.ac.il (B. Tumanskii), [email protected] (M.S. Eisen). 0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2011.10.033

Traditionally, UCC S-2 catalyst is directly prepared from a triphenylsiloxy complex of chromium(VI) namely bis(triphenylsilyl) chromate (BC) using thermally treated silica gel as support by wet impregnation method as shown in Scheme 1 [8]. As the starting material for UCC S-2 catalyst, BC is a well-defined homogeneous model for UCC S-2 catalyst due to its molecular structural characteristics. In 1970, Baker and Carrick reported [12] that BC can catalyze the polymerization of ethylene under atmospheric pressure and at room temperature with aluminum alkyl cocatalysts. Unfortunately, no further investigations about the polymerization behavior, as well as activation reaction of BC with different aluminum alkyl cocatalysts, were reported. In our recent work, a novel Cr(II) complex [(Ph3SiO) Cr$(THF)]2(m-OSiPh3)2 (1) was successfully synthesized as a potential homogeneous model for the prereduced UCC S-2 catalyst and structurally characterized. Moreover, 1/MAO exhibits an unexpected transformation from ethylene polymerization (Al/Cr
100) to nonselective ethylene oligomerization (Al/Cr  200) with an increase of the Al/Cr molar ratio (Al/Cr: 50e1000) [13]. In view of bearing the same Ph3SiO-ligands (Scheme 2), it would be very interesting to confirm if BC as a hexavalent chromium-based homogeneous model for the UCC S-2 catalyst, exhibits a similar

P. Qiu et al. / Journal of Organometallic Chemistry 699 (2012) 48e55

49

Scheme 1. A preparation method of UCC S-2 polyethylene catalyst.

transformation behavior or not. Moreover, it would be also very important to study the activation reaction of BC with aluminum alkyl cocatalysts. Therefore, NMR, ESR spectroscopy, radical trapping on fullerene, and MALDI-TOF mass spectroscopy [14,15] where used for elucidating on the activation reaction. Herein, a triphenylsiloxy complex of chromium(VI) namely BC, as a homogeneous model for the UCC S-2 catalyst, was studied on its ethylene polymerization behavior, and the activation reaction of BC with aluminum alkyl cocatalysts was also investigated by NMR, ESR spectroscopy, radical trapping on fullerene, and MALDI-TOF mass spectroscopy. As expected, BC does exhibit a similar ethylene polymerization/oligomerization behavior to complex 1 in the presence of aluminum alkyl cocatalysts. Much deeper understanding of ethylene polymerization/oligomerization behavior and activation reaction of this hexavalent chromium-based homogeneous model system for the UCC S-2 catalyst will be demonstrated. 2. Experimental section 2.1. General procedures All manipulations with air-sensitive materials were performed with the exclusion of oxygen and moisture in Schlenk-type glassware and a high-vacuum (106 Torr) line. For storage of materials, a nitrogen-filled Vacuum Atmospheres glovebox with a mediumcapacity recirculator (1e2 ppm O2) was used. The gases (argon and nitrogen) were purified by passage through a MnO oxygenremoval column and a Davison 4 Å molecular sieve column, and ethylene gas was purified by passage through a Davison 4 Å, 13 molecular sieves and Q-5 reactant catalyst (13 wt% of copper(II) oxide on alumina, Aldrich). Analytically pure solvents were distilled under N2 from Na/K alloy (toluene). All solvents for vacuum-line manipulations were stored under vacuum over Na/K alloy.

Bis(triphenylsilyl)chromate (Aldrich, 96%), TIBA (Acros, 1.1 M in toluene) and C60 (Aldrich, 98%) were stored and used in the glovebox as received. MAO (Witco) was prepared from a 30% suspension in toluene by evaporation of the solvent at 25  C/ 105 Torr in vacuum. Toluene-d8 was dried with Na/K alloy and transferred under vacuum; CDCl3 (CIL 99.8%) was used as received. The NMR measurements for BC (and BC/TIBA) were conducted in Teflon J. Young valve-sealed NMR tubes after vacuum transfer of the solvent in a high-vacuum line and recorded on Bruker Avance 300 MHz spectrometer. ESR spectra were recorded on a Bruker EMX-10/12 X-band (n ¼ 9.2 GHz) digital ESR spectrometer equipped with a Bruker N2-temperature controller. All spectra were recorded at a microwave power of 1e10 mW, 100 kHz magnetic field modulation of 1.0 G amplitude. The digital field resolution was 2048 points per spectrum, allowing all hyperfine splittings to be measured directly with an accuracy better than 0.2 G. Spectral processing and simulation were performed with Bruker WIN-EPR and SimFonia software. The g factor values were determined using 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) as reference (g ¼ 2.0058). MALDI-TOF LDþ and LD experiments were performed on a Waters MALDI mass spectrometer. 2.2. Ethylene polymerization/oligomerization The polymerization/oligomerization of ethylene was studied using BC as the catalyst precursor, which was activated by TIBA or MAO. The polymerization/oligomerization was performed in a 100 mL stainless steel reactor equipped with a magnetic stirrer. The reactor was charged with a certain amount of BC, cocatalyst, and toluene (10 mL) inside a glovebox. The reactor was filled with ethylene at a pressure of 20 atm. The pressure was kept constant for 30 min (or 90 min), at which point the temperature was rapidly reduced to 0  C and the reaction was quenched by exhausting the unreacted ethylene in a well-ventilated hood, followed by the introduction of H2O/MeOH (10%) to decompose the aluminum alkyl cocatalyst. Then the organic and aqueous phases were separated from the polymer. The polymer was washed with HCl/MeOH (10%), MeOH, H2O and dried at 60  C for 24 h under reduced pressure before the final mass was weighed. Ratios (or selectivities) of oligomers were obtained by GC by using standard references. The catalytic activity was determined by integrating the intensity of the olefinic NMR resonances versus the methyl group of the toluene solvent [16]. 2.3. Ethylene polymerization under ESR monitoring

Scheme 2. The comparison of BC with complex 1.

A Teflon J. Young valve-sealed NMR tube was charged with BC, cocatalyst, and a solvent inside a glovebox. The NMR tube was connected to a high-vacuum line, frozen, pumped, thawed, and

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P. Qiu et al. / Journal of Organometallic Chemistry 699 (2012) 48e55

Table 1 Results of ethylene polymerization run using BC activated by aluminum alkyl cocatalysts.a Entry

Cocat. ([Al]/[Cr])

Amt. of PE (g)

Activity 1 ) (g mmol1 cr h

Mwb (g mol1)

PDI (Mw/Mn)

Tmc ( C)

1 2 3 4 5 6 7 8 9 10 11

TIBA TIBA TIBA TIBA TIBA TIBA MAO MAO MAO MAO MAO

e 1.10 2.06 3.87 0.72 0.29 e 2.37 2.62 5.03 5.00

e 23.2 43.6 81.9 15.2 6.1 e 50.2 55.4 106.4 105.8

e 87 75 71 149 176 e 317 296 419 331

e 1.70 2.00 2.30 1.80 1.70 e 2.32 2.66 3.19 2.20

e 135.0 140.3 132.0 132.0 133.2 e 140.9 139.3 138.9 138.5

(0.5) (1) (2) (4) (10) (20) (2) (4) (10) (20) (30)

000 000 000 000 000 000 000 000 000

Standard conditions: T ¼ 22  C, V ¼ 10 mL, P ¼ 20 atm, catalyst: 20 mg, time: 90 min. b Determined by GPC. c Determined by DSC. a

filled with ethylene to 1 atm. The ESR measurements at different temperatures were performed immediately after the sample preparation. 2.4. Characterization of the oligomers and the polymers NMR measurements of the oligomers were conducted in CDCl3 and recorded on a Bruker Avance 300 MHz spectrometer. Chemical shifts for 1H NMR were referenced to internal solvent resonances and reported relative to methyl group of toluene. The percentage of vinyl termination was determined by 1H NMR according to the following formula: vinyl content ¼ 1  [(If  1.5Ia)/2If], where If and Ia are the relative intensities for terminal methyl and terminal vinyl groups of linear oligomers, respectively [17]. GC analyses were carried out on a Varian 3900 GC. The column had a length of 30 m and internal diameter of 0.25 mm. A gradient oven temperature program, going from 40  C (for 2 min) to 280  C at a rate of 10  C min1 and holding at the final temperature for 3 min, was employed. Linear a-olefins (C6H12, C8H16, C10H20, C12H24, C14H28, C16H32) and toluene were calibrated by blank GC experiments. Molecular weights of polymer were determined by the GPC method on a Waters-Alliance 2000 instrument using 1,2,4-

trichlorobenzene as a mobile phase at 160  C and referenced to polystyrene standards. Melting crystallization behavior of the polymers was examined using a Thermal Analysis Q200 differential scanning calorimeter. Three runs (heating-cooling-heating) at a rate of 10  C min1 in the range 30e180  C were performed for each sample of polymer. The second heating exothermal peak temperature was taken as a melting point.

3. Results and discussion 3.1. Ethylene polymerization/oligomerization studies As reported [12], BC can catalyze the polymerization of ethylene at somewhat harsh conditions (T > 130  C, P ¼ 350e1500 atm) without any aluminum alkyl cocatalysts. However, under our reaction conditions (20 atm and room temperature), BC alone was inactive for the ethylene polymerization in the absence of aluminum alkyl cocatalysts. Therefore, aluminum alkyl cocatalysts were used for testing the polymerization activity of BC in this work. Preliminary ethylene polymerizations over BC were carried out under relatively low Al/Cr molar ratios. As shown in Table 1, BC exhibits a low activity in the polymerization of ethylene when activated with aluminum alkyl cocatalysts. In the case of TIBA (Al/ Cr: 0.5e20), the polymerization activity first increases, and then decreases with an increase of the Al/Cr molar ratio; the highest 1 polymerization activity (81.9 gPE mmol1 Cr h ) was obtained at an Al/Cr molar ratio of 4. A similar dependence of the polymerization activity on the Al/Cr molar ratio was reported over a POSSchromate/TMA catalyst system (Al/Cr: 1e10) [18]. In the case of MAO, increasing the Al/Cr molar ratio (from 2 to 30) also led to an increase in the polymerization activity, following a slight decrease, 1 and the highest polymerization activity (106.4 gPE mmol1 Cr h ) was observed at an Al/Cr molar ratio of 20. Generally, TIBA is considered to be a much stronger reducing agent than MAO, so the highest polymerization activity of BC with TIBA was obtained under a much lower Al/Cr molar ratio (Al/Cr ¼ 4) than MAO (Al/Cr ¼ 20). Unexpectedly, BC/MAO exhibits a higher activity of ethylene polymerization than BC/TIBA. Actually, MAO has never been reported as an aluminum alkyl cocatalyst for UCC S-2 and Phillips Cr/SiO2 industrial polyethylene catalysts. It is noteworthy to mention that a longer polymerization time (90 min) and a larger amount of

Table 2 Results of ethylene polymerization/oligomerization run using BC.a Entry

Cocat. ([Al]/[Cr])

PE (g)

Mw (g mol1)

PDI (Mw/Mn)

Tm ( C)

1 Activityb (g mmol1 ) cr h

Oligomerc (g)

C6

C8

C10

C12

C14

C16

1 2 3 4 5f

TIBA (4) TIBA (10) TIBA (500) MAO (100) MAO (200)

1.58 0.71 Traceg 1.20 0.81

201 90 e 152 103

0 0 e 0 0

e e e e e

e e e e e

e e e e e

e e e e e

e e e e e

e e e e e

e e e e e

MAO (500)

0.64

129.8

386

2.4

12.5

25.7

26.8

18.3

9.5

4.3

90.3

7f

MAO (1000)

0.43

129.2

690

5.0

9.5

19.2

18.4

25.3

14.6

7.3

81.6

8f

MAO (1500)

0.55

3.0 2.4 e 2.6 2.1 1.1 2.1 1.6 2.8 1.2 2.8 1.4

132.8 132.9 e 134.7 131.7

6f

69 000 158 000 e 191 000 240 000 5400 254 000 3000 295 000 1900 313 000 1500

127.4

413

2.7

10.5

23.5

11.4

26.2

15.1

8.0

84.6

a b c d e f g

Standard conditions: T ¼ 22  C, V ¼ 10 mL, P ¼ 20 atm, catalyst ¼ 10 mg, time ¼ 30 min. Activity by adding polymerization activity to oligomerization activity. By integration of the NMR olefinic resonances with respect to the Me of the toluene solvent. By GC, values of C4 are not given due to volatility, remainder is C4 and C18þ. By integration of the NMR olefinic resonances. Bimodal distribution of molecular weights from GPC analysis. Less than 0.05 g.

Oligomers distributiond (%)

Vinyle (mol%)

P. Qiu et al. / Journal of Organometallic Chemistry 699 (2012) 48e55

catalyst (20 mg) were used in these preliminary ethylene polymerization experiments. Thereafter, the polymerization/oligomerization behavior of BC activated by aluminum alkyl cocatalysts were tested in the subsequent experiments under the following reaction conditions (polymerization time: 30 min, amount of catalyst: 10 mg) as to complex 1 in the previous report [13]. A summary of the ethylene polymerization/oligomerization results is given in Table 2. In the presence of TIBA, BC exhibits a similar ethylene polymerization behavior (Table 2, entries 1e2) to that of the preliminary polymerization experiments (Table 1, entries 4e5) at low Al/Cr molar ratios (Al/Cr ¼ 4 and 10), and the obviously higher polymerization activities (Table 2, entries 1e2) should be attributed to a shorter polymerization time (30 min). A higher Al/Cr molar ratio solely led to deactivation of the polymerization activity. In the case of MAO, solid PE was obtained at relatively low Al/Cr molar ratios (Al/Cr
200). With a further increase of Al/Cr molar ratio (Al/Cr  500), liquid oligomers were surprisingly obtained together with a small amount of solid PE. Therefore, BC/MAO exhibits a transformation from ethylene polymerization (Al/Cr
200) to nonselective oligomerization (Al/Cr  500) with an increase of Al/Cr molar ratios. The polymerization activity over BC/ MAO catalyst system decreased with an increase of Al/Cr molar ratio (Al/Cr: 100e1000), indicates that larger amount of MAO possibly induces a switching from an active site for ethylene polymerization to an active site for nonselective oligomerization. The obtained liquid oligomers from BC/MAO were characterized by 1H NMR and GC. 1H NMR results revealed liquid oligomers with high contents of terminal vinyl groups (>81%) (Fig. 1); this describes essentially linear a-olefins, and therefore the predominant chain transfer mechanism is the b-hydrogen elimination in the BC/MAO catalyst system, although chain transfer to aluminum

51

of MAO has been reported in some metallocene/MAO catalyst systems [19e23]. GC results (Fig. 2) suggested that the obtained oligomers were main even-numbered linear a-olefins (C6eC16). The obtained PE were characterized by GPC and DSC. The GPC analysis of the obtained PE from BC/MAO (Al/Cr ¼ 100) shows a unimodal MWD (Table 2, entry 4) with an average molecular weight of 191 000 g mol1. With an increase of Al/Cr molar ratio (Al/ Cr  200), the GPC analysis shows a bimodal MWD (Table 2, entries 5e8) with an average molecular weight of 2950 and 275 500 g mol1, respectively. Moreover, increasing the Al/Cr molar ratio increased the Mw values of the high molecular weight fractions with their corresponding PDI between 2.1 and 2.8 (Table 2, entries 5e8). Conversely, the Mw values of the low molecular weight fraction decreased with the Al/Cr molar ratio increasing, and their corresponding PDI were found to be in the range from 1.1 to 1.6 (Table 2, entries 5e8). Similar bimodal MWD of PE was also reported recently for a tris(pyrazolyl)methane-chromium(III) complex with MAO as the cocatalyst [24]. The Tm values (Table 2, entries 1e2, 4e5) of the PE are similar to that of HDPE around 132  C. As for BC/MAO system, increasing the Al/Cr molar ratio led to a slightly decrease in the Tm values, which can be explained by the introduction of some branching to PE chains arising from the in-situ copolymerization of the obtained oligomers with ethylene. In summary, we successfully corroborated that the BC/MAO catalyst system exhibited a similar ethylene polymerization/oligomerization transformation behavior as compared to 1/MAO [13]. 3.2. Activation of bis(triphenylsilyl)chromate As mentioned above, the ethylene polymerization/oligomerization behavior of BC was tested with aluminum alkyl cocatalysts,

Fig. 1. Typical 1H NMR spectrum of the obtained oligomers with terminal vinyl groups produced with BC/MAO (Table 2, entry 8); the “*” markers in the 1H NMR spectrum stand for resonances of CDCl3 and toluene.

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Fig. 2. Typical GC chromatogram of the liquid oligomers produced with BC/MAO (Table 2, entry 8).

and it was found that BC exhibited a very similar ethylene polymerization/oligomerization behavior to complex 1. Namely, in the case of MAO, BC presents a switching from ethylene polymerization to nonselective oligomerization with an increase of Al/Cr molar ratio; In the case of TIBA, only an activity of ethylene polymerization was observed at low Al/Cr molar ratios, and further increasing the Al/Cr molar ratio solely led to deactivation of polymerization activity. In our recent work, the activation of 1 with MAO was studied by NMR and ESR spectroscopy [13]. Herein, the activation of BC was also investigated by NMR and ESR spectroscopy; moreover, radical trapping on C60 fullerene, and MALDI-TOF mass spectroscopy experiments were also used. The 1H NMR spectrum of BC and its mixture with TIBA is presented. In Fig. 3a, 1H NMR spectrum of BC shows the Ph3SiO-ligands in the usual pattern of signals, and the assignments are well in agreement with Osborn’s report [25]. However, these signals disappeared when TIBA was added (Fig. 3b), and a similar disappearance result was also observed in the 13C NMR spectrum (Fig. 4). The

signal disappearance is plausible attributed to the spin delocalization effects of the paramagnetic low valent chromium species, which were produced during the activation of BC with TIBA. Therefore, the main chromium species (low valence and paramagnetism) in BC/TIBA system may contain two Ph3SiO-ligands, or the diamagnetic species containing Ph3SiO-ligands should give rise to similar signals to BC in both 1H and 13C NMR spectra. It is noteworthy that no precipitation was observed during the activation reaction. The similar NMR results were obtained in BC/MAO catalyst system. The activation of BC with TIBA was also characterized by ESR spectroscopy. It can be seen from Fig. 5a that a very characteristic isotropic, hyperfine structure multiplet was observed. This hyperfine structure originates from the coupling of the electron spin of magnetic isotope 53Cr(I)þ nucleus and with 10 equivalent protons from two toluene molecules (or arenes of the dissociated Ph3SiOligands) [13,26]. This coupling should bring about a multiplet with 11 lines, all of these peaks were observed with the exception of the

Fig. 3. 1H NMR spectra of BC and its mixture with TIBA (Al/Cr ¼ 4) in toluene-d8; (a) 1H NMR spectrum of BC; (b) 1H NMR spectrum of BC activated by TIBA (Al/Cr ¼ 4). The resonances assigned to toluene-d8 are marked with asterisks.

P. Qiu et al. / Journal of Organometallic Chemistry 699 (2012) 48e55

Fig. 4. 13C NMR spectra of BC and its mixture with TIBA (Al/Cr ¼ 4) in toluene-d8; (a) resonances assigned to toluene-d8 are marked with asterisks.

13

C NMR spectrum of BC; (b)

53

13

C NMR spectrum of BC activated by TIBA (Al/Cr ¼ 4). The

outer two peaks, which might be too weak to be observed. The isotropic values g ¼ 1.995 and aH (101H) ¼ 3.62 G as well as aCr(53Cr) ¼ 18.3 G are well in line with values for the cationic [Cr(h6toluene)2]þ species [13,26,27]. In order to further confirm the

Fig. 5. (a). ESR spectrum of BC activated by TIBA (Al/Cr ¼ 4) in toluene at 290 K. For determination of the g factor spectrum recorded with 2,2,6,6-tetramethylpiperidine-Noxyl (TEMPO, g ¼ 2.0058), the three lines of TEMPO are marked with asterisks. (b). Simulation of ESR spectrum by SimFonia using parameters obtained by experiment (aH (1H) ¼ 3.62 G, aCr (53Cr) ¼ 18.3 G, g ¼ 1.995).

Fig. 6. ESR and MALDI-TOF monitoring of the mixture of BC with fullerene C60 activated by TIBA (Al/Cr ¼ 4): (a) ESR spectrum at 290 K in toluene; (b) MALDI-TOF spectrum.

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P. Qiu et al. / Journal of Organometallic Chemistry 699 (2012) 48e55

Fig. 7. Monitoring ethylene polymerization over BC activated by TIBA (Al/Cr ¼ 4) in toluene by temperature-dependent ESR spectroscopy (from 220 K to 350 K). For determination of the g factor spectrum recorded with TEMPO (g ¼ 2.0058), the three lines of TEMPO marked with asterisks. A) 220 K; B) 250 K; C) 270 K; D) 290 K; E) 320 K; F) 350 K.

assignment, ESR simulation was carried out by using SimFonia software [14] on the basis of the obtained parameters by ESR spectrum (Fig. 5a). The simulated ESR spectrum (Fig. 5b) is in excellent agreement with ESR experimental spectrum (Fig. 5a). Again, aforementioned hyperfine structure multiplet was proved to be assigned to the cationic [Cr(h6-arene)2]þ species. Herein, these results indicated that small amounts of BC were reduced to Cr(I)þ species, which were coordinated by two molecules of toluene (or arenes of the dissociated Ph3SiO-ligands) to produce the cationic [Cr(h6-arene)2]þ species. The similar ESR results for the same cationic [Cr(h6-arene)2]þ species were also obtained in the BC/MAO and 1/MAO [13] catalyst systems at different Al/Cr molar ratios. Alkyl radical is known as an important intermediate during the activation reaction between transition metal-based polyolefin catalysts and metal alkyl cocatalysts. However, it is difficult to be characterized by spectroscopic methods due to its high reactivity and short life time. Recently, fullerene C60 [28,29], as an effective radical trapping agent, was used to study on the activation reaction and olefin polymerization in homogeneous catalyst systems [14,15]. In this work, an investigation to confirm the possibly generated alkyl radicals during the activation of BC with TIBA by fullerene radical trapping combined with ESR as well as MALDI-TOF mass spectroscopy experiments was performed. It can be seen from Fig. 6 that a new ESR signal (Fig. 6a) of the multiple addition paramagnetic adduct of butyl radicals to fullerene was successfully observed compared with the one without fullerene C60, and the addition of butyl radicals to fullerene was confirmed by MALDI-TOF analysis (Fig. 6b). Therefore, we considered that the butyl radical intermediate were generated during the reduction and alkylation of BC with TIBA. Similar ethyl and methyl radical formation have been previously reported during activation reaction in ZieglereNatta catalyst system (TiCl3/TiCl4 þ AlEt3) [30] and in nonmetallocene/MAO catalyst system [15], respectively. As above-mentioned, the cationic [Cr(h6-arene)2]þ species in BC/ TIBA catalyst system was identified by ESR spectroscopy. Generally,

this 17-electron sandwich complex was supposed to be inactive for polymerization of ethylene [31]. However, in the presence of TIBA, [Cr(h6-toluene)2]þI was reported to be active for polymerization of ethylene under 40 atm and 80  C, even higher polymerization temperature [32]. In order to understand the identity (active species or not) of the cationic [Cr(h6-arene)2]þ species in BC/TIBA catalyst system, a temperature-dependent ESR experiment (220e350 K) was performed for monitoring the ethylene polymerization (performed in the NMR tube). From Fig. 7, the multiplet (g ¼ 1.995) of the cationic [Cr(h6-arene)2]þ species remains unchanged [33], indicating that this kind of species is not active at this reaction conditions for the polymerization of ethylene. No other ESR signals were observed during the raising temperature process (220e350 K), although the solid PE was observed at 290 K. This result indicates that the active species for ethylene polymerization cannot be observed by ESR spectroscopy. Therefore, the valence state of active species in BC/ TIBA catalyst system might be Cr(II), which is always ESR silent [34,35]. However, Cr(III) cannot be also excluded to be the valence state of active species on the basis of the limit experimental results, because Cr(III) species were generally observed by ESR spectroscopy at very low temperature up to 77 K [26,36]. In summary, BC, as a homogeneous model for UCC S-2 catalyst, exhibits an interesting switching behavior from ethylene polymerization to nonselective oligomerization using MAO as a cocatalyst. In view of the similarity of molecular structural characteristics as well as ethylene polymerization behavior (with or without aluminum alkyl cocatalysts) [8,12], we infer that a similar switching behavior might be also observed over UCC S-2 catalyst activated with MAO. These systematic confirmation experiments over the UCC S-2 catalyst/MAO system are now in progress in our laboratory. 4. Conclusion In this work, a triphenylsiloxy complex of chromium(VI), namely bis(triphenylsilyl)chromate (BC), was utilized as a homogeneous

P. Qiu et al. / Journal of Organometallic Chemistry 699 (2012) 48e55

model for UCC S-2 catalyst, and its ethylene polymerization/oligomerization behavior and activation reaction in the presence of aluminum alkyl cocatalysts were studied. As expected, BC/MAO catalyst system exhibits a transformation from ethylene polymerization (Al/Cr
200) to nonselective ethylene oligomerization (Al/ Cr  500) with an increase of Al/Cr molar ratios. The obtained liquid oligomers contain the high contents of even-numbered linear aolefins (C6eC16), and the obtained PE display a switching from unimodal (Al/Cr
100) to bimodal MWD (Al/Cr  200). However, BC/TIBA only exhibits an activity of ethylene polymerization at the low Al/Cr molar ratios; higher Al/Cr molar ratio solely leads to deactivation of the polymerization activity without a transformation into ethylene oligomerization. Therefore, BC exhibits a very similar ethylene polymerization/oligomerization behavior to that of the complex 1. In addition, a butyl radical and a cationic [Cr(h6-arene)2]þ species were produced during the activation of BC with TIBA, and the [Cr(h6-arene)2]þ cation was proved to be inactive for ethylene polymerization by temperature-dependent ESR experiments. The similar characterization results were obtained during the activation of BC with MAO as the cocatalyst. Further clarification for the activation mechanism and the nature of active sites in these hexavalent and divalent Cr-based homogeneous models for UCC S-2 catalyst through combined experimental and theoretical investigations are still in progress, which will be available in a forthcoming publication. Acknowledgement National Natural Science Fund of China (No. 20774025) and Key Projects in the National Science & Technology Pillar Program during the 11th Five-Year Plan Period (2007BAE50B04) are thanked for financial support. This work is also financially supported by the Research Program of the State Key Laboratory of Chemical Engineering, the Program of Introducing Talents of Discipline to Universities (B08021) and the Fundamental Research Funds for the Central Universities. Dr I. Barzilai (Technion-Israel Institute of Technology) is thanked for GC measurements. Dr. B. Tumanskii is grateful for support from the Center for Absorption in Science, State of Israel Ministry of Immigrant Absorption. References [1] J.P. Hogan, R.L. Banks, Polymers and production thereof, US2825721 (1958). [2] A. Clark, Catal. Rev. 3 (1) (1970) 145e173. [3] M.P. McDaniel, Adv. Catal. 33 (1985) 47e98.

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