Phenoxysalicylaldimine-Bearing Chromium(III) Precatalysts for Ethylene Polymerization

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wileyonlinelibrary.com DOI: 10.1002/macp.201400223

Phenoxysalicylaldimine-Bearing Chromium(III) Precatalysts for Ethylene Polymerization

Margaret Yankey , Collins Obuah , James Darkwa*

A series of discrete chromium(III) precatalysts, {[Cr( κ 3 -L1 )] ( 1a.7H 2 O ), {[Cr( κ 3 -L2 )] ( 2a.5H 2 O ), {[Cr( κ 3 -L3 )] ( 3a.6H 2 O ), {[Cr( κ 3 -L4 )] ( 4a.10H 2 O ), {[Cr( κ 3 -L5 )] ( 5a.4H 2 O ) and in-situ-generated chromium(III) precatalysts {[Cr( κ 3 -L1 )] ( 1b ), {[Cr( κ 3 -L2 )] ( 2b ), {[Cr( κ 3 -L3 )] ( 3b ), {[Cr( κ 3 -L4 )] ( 4b ), {[Cr( κ 3 -L5 )] ( 5b ) bearing 2,4-di-tert-butyl-6-{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol ( L1 ), 2,4-di-tert-butyl-6-{[2-RR-pyrazol-1-yl-ethylimino]-methyl}-phenol, R = H ( L2 ), Me ( L3 ), Ph ( L4 ), and 2,4-di-tert-butyl-6-{[2-pyrid-2-yl-ethylimino]-methyl}-phenol ( L5 ) are developed. The discrete precatalysts are characterized by IR spectroscopy, elemental analysis, and mass spectrometry. The in-situ-generated precatalysts afford a one-step process for screening chromium(III) complexes as ethylene polymerization catalysts. Upon activation with EtAlCl 2, both types of precatalysts exhibit high catalytic activities up to 1.71 × 10 5 g mol(Cr) −1 h −1 for the formation of high-density polyeth-ylene with molecular weights as high as 3.48 × 10 3 g mol −1 . While the discrete precata-lysts produce monomodal active catalysts at all conditions, the in-situ-generated precata-lysts produce either monomodal or bimodal active catalysts, suggesting that, at certain conditions, two active catalysts are produced in the in situ reactions.

M. Yankey, C. Obuah, Prof. J. Darkwa Department of Chemistry, University of Johannesburg , Kingsway Campus, Auckland Park , Johannesburg 2006 , South Africa E-mail: [email protected]

with high selectivity and produces only a small amount of polyethylene. [ 3b,c, 5 ] However, other chromium(III) precatalysts bearing bi-dentate ligands such as NÔ-(salicylaldiminato, β-ketoiminato, hydroxyindanimine), [ 6 ] and N^N-(bis(imidazole), imino pyrrolide) [ 7 ] or tri-dentate ligands such as P^N^P (bridged-diphosphine), [ 8 ] N^N^N-(bis(imino)pyridines, 2-imino-1,10-phenanthrolines), [ 9 ] N^NÔ-(2-acetyl-6-iminopyridine, 2-(ethylcarboxylato)-6-iminopyridine), [ 9a , 10 ] or tetra-dentate N^N^N^N-(tris(2-pyridylmethyl)amine [ 11 ] and OÔÔÔ-(calix[4 and 6]arene) [ 12 ] produce mainly ethylene polymers.

The above ligands O- and N-donor salicylaldimine ligands have been studied extensively with groups 4 and 5 transition metals by Fujita [ 13 ] and others, [ 14 ] and with chromium(III) via high-throughput screening by Gibson and co-workers [ 6c , 15 ] to produce polyethylene. However, with chromium(III) as the precatalysts, the products of the ethylene reaction depend on the denticity of the

1. Introduction

Although a number of transition metal catalysts have been developed for the polymerization of α-olefi ns, [ 1 ] chromium(III) complexes remain one of the unique cata-lysts because of their versatility to produce ethylene oli-gomers and polymers. [ 2–4 ] The uniqueness of chromium(III) to produce both oligomers and polymers depends on the ligands on which the metal is supported. For example, Sasol technology has developed chromium(III) complexes with tridentate mixed donor ligands such as bis-phosphiny-lamines and bis-sulphanylamines that trimerize ethylene

Macromol. Chem. Phys. 2014, 215, 1767−1775

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salicylaldimine ligands. While the bidentate O^N-ligands produce polyethylene, [ 15a ] the tridentate O^N^N-ligands pro-duce ethylene oligomers. [ 15a , 16 ] We here report a series of tri-dentate O^N^N-salicylaldiminechromium(III) precatalysts, which upon activation with EtAlCl 2 , uniquely produced poly ethylenes. The nature of the salicylaldimine ligands allows for variations in the substituents on the ligands and also variations in the second N-donor atom by incorpo-rating either imidazolyl-, pyrazolyl-, or pyridyl-groups.

2. Experimental Section

2.1. Materials and Instrumentations

Syntheses of air- and moisture-sensitive compounds were carried out using standard Schlenk techniques and in a glove box. All sol-vents were dried and distilled prior to use. Tetrahydrofuran (THF) and hexane were dried over sodium, using benzophenone as an indicator; while dichloromethane was dried over phosphorous pentoxide. Ethylaluminum dichloride (EtAlCl 2 ) (1.8 M ) in toluene and other chemicals were obtained from Sigma–Aldrich and used as received. The salicylaldimineimidazolyl- ( L1 ) [ 17 ] and salicyl-aldiminepyrazolyl- ( L2-L4 ) [ 18 ] ligands were prepared according to literature procedures. NMR spectra were recorded on a Bruker 400 MHz instrument ( 1 H at 400 MHz and 13 C{ 1 H} at 100 MHz) at room temperature. Chemical shifts are reported in δ (ppm) and ref-erenced to the residual proton and carbon (7.24 and 77.0 ppm for CDCl 3 and 132.36 for o -dichlorobenzene-d 4 ) in the NMR solvents. Infrared (IR) spectra were recorded on a Bruker Tensor 27 equipped with a diamond ATR. Elemental analyses were performed on a Vario Elementar III microcube CHN analyzer, while mass spectra were recorded on a Waters API Quattro micro-spectrophotometer.

The crystal evaluation and data collection were performed on a Bruker SMART APEXII diffractometer with Cu K α ( λ = 1.54178 Å) radiation and the diffractometer to crystal distance of 4.03 cm. The initial cell constants were obtained from three series of ω scans at different starting angles. The refl ections were success-fully indexed by an automated indexing routine bui0lt in the APEXII program. The absorption correction was based on fi tting a function to the empirical transmission surface as sampled by multiple equivalent measurements. [ 19 ] The structures were solved by direct methods and refi ned by least-squares techniques using OLEX2: a complete structure solution, refi nement, and analysis program. [ 20 ] All non-hydrogen atoms were refi ned with anisotropic displacement coeffi cients. All hydrogen atoms were included in the structure factor calculation at idealized positions and were allowed to ride on the neighboring atoms with relative isotropic displacement coeffi cients. The refi ned structure was subjected to the check CIF criteria of the International Union of Crystallography (IUCr) and no major alerts came up.

2.2. Synthesis of 2,4-Di- tert -butyl-6-{[2-pyridin-2-yl-ethylimino]-methyl}-phenol (L5)

A mixture of 3,5-di- tert -butyl-2-hydroxybenzaldehyde (1.03 g, 4.39 mmol), and 2-(2-pyridyl)-ethylamine (0.54 g, 4.39 mmol) in

methanol with catalytic amount of formic acid was refl uxed for 24 h forming a yellow solution. The solvent was evaporated after the reaction time and the crude product was dissolved in dichlo-romethane. Solid NaHCO 3 was added to the dichloromethane solution and allowed to stir for 3–5 min to neutralize the formic acid. The mixture was then fi ltered and the solvent was evapo-rated to give analytically pure product as yellow oil. Yield: 1.43 g (97%). 1 H NMR, (CDCl 3 , J HH (Hz)): δ 1.27 (s, 9H, C 6 H 2 -4-C(C H 3 ) 3 ); 1.42 (s, 9H, C 6 H 2 -2-C(C H 3 ) 3 ); 3.16 (t, 2H, J HH = 7.2, C H 2 CH N py ); 4.00 (t, 2H, J HH = 7.2, CH N C H 2 ); 7.01 (d, 1H, J HH = 2.0, 3-C 6 H 2 OH); 7.16 (m, 1H, m -C 6 H 4 -N py ); 7.18 (s, 1H, 5-C 6 H 2 OH); 7.35 (d, J HH = 2.40, 1H, m -C 6 H 4 N py ); 7.61 (m, 1H, p -C 6 H 4 N py ); 8.29 (s, 1H,

C H N ); 8.56 (d, J HH = 4.40, 1H, o -C 6 H 4 N py ); 13 C{ 1 H} NMR (CDCl 3 ): δ 29.4 (C 6 H 2 3 C (CH 3 ) 3 ); 31.4 (C 6 H 2 -5-C( C H 3 ) 3 ); 34.1 (C 6 H 2 -3-C( C H 3 ) 3 ); 35.0 (C 6 H 2 -5- C (CH 3 ) 3 ); 39.5 ( C H 2 CH N py ); 59.0 ( CH N C H 2 ); 117.80 ( m - C 6 H 4 N py ); 121.5 ( m - C 6 H 4 N py ); 123.7 ( o-C 6 H 2 OH); 125.8 ( m - C 6 H 2 OH); 126.8 ( m - C 6 H 2 OH); 136.6 ( o - C 6 H 2 OH); 139.9 ( p - C 6 H 4 N py ); 149.2 ( p - C 6 H 2 OH); 158.0 ( o-C 6 H 4 N py ); 159.1 ( C 6 H 2 OH); 166.4 ( o-C 6 H 4 N py ); 166.4 ( C H N). IR (ATR) (cm −1 ): 1631 (s) ( ν C N ). Anal. Calc. for C 22 H 30 N 2 O: C, 78.06; H, 8.93; N, 8.28. Found: C, 78.39; H, 9.32; N, 8.24%.

2.3. Synthesis of Complexes 1a.7H 2 O-5a.4H 2 O

2.3.1. General Procedure

In a typical experiment, the chromium complexes were synthe-sized by adding [CrCl 3 (THF) 3 ] to a solution of the ligand in THF. The resulting solution was refl uxed at 60 °C for 24 h during which the color of the solution changed to yellowish brown. The solvent was evaporated and the crude product was recrystallized from dichloromethane and hexane. All complexes were obtained in moderate to good yields; details of each reaction are described below.

(2,4-Di-tert-butyl-6-{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol)-chromium(III) dichloride (1a.7H 2 O) : Complex 1a was prepared from L1 (0.20 g, 0.61 mmol) and [CrCl 3 (THF) 3 ] (0.23 g, 0.61 mmol) as a yellowish brown powder. Yield: 0.19 g (71%). IR (ATR) (cm −1 ): 3280 ( ν H2O ), 1614 (s) ( ν C N ). HRMS (ESI): m / z (%) = 413.1348 (100) [M-Cl] + . Anal. Calc. for C 20 H 28 Cl 2 CrN 3 O.7H 2 O: C 41.74, H 7.36, N 7.38%; Found: C 41.63, H 6.15, N 5.88%.

(2,4-Di-tert-butyl-6-[(2-pyrazol-1-yl-ethylimino)-methyl]-phenol)-chromium(III) dichloride ( 2a .5H 2 O) : Complex 2a was prepared from L2 (0.22 g, 0.66 mmol) and [CrCl 3 (THF) 3 ] (0.25 g, 0.66 mmol) as a yellowish brown powder. Yield: 0.21 g (71%). IR (ATR) data (cm −1 ): 3300( ν H2O ), 1615 (s) ( ν C N ). HRMS (ESI): m / z (%) = 413.1339 (100) [M-Cl] + . Anal. Calc. for C 20 H 28 Cl 2 CrN 3 O.5H 2 O: C 44.53, H 7.10, N 7.79%; Found: C 44.51, H 6.76, N 6.28%.

(2,4-Di-tert-butyl-6-{[2-(3,5-dimethyl-pyrazol-1-yl)-ethylimino]-methyl}-phenol)-chromium(III) dichloride ( 3a .6H 2 O) : Complex 3a was prepared from L3 (0.20 g, 0.57 mmol) and [CrCl 3 (THF) 3 ] (0.22 g, 0.57 mmol) as a yellowish brown powder. Yield: 0.24 g (88%). IR (ATR) (cm −1 ): 3231 ( ν H2O ), 1616 (s) ( ν C N ). HRMS (ESI): m / z (%) = 441.1643 (100) [M-Cl] + . Anal. Calc. for C 22 H 32 Cl 2 CrN 3 O.6H 2 O: C 45.13, H 7.57, N 7.18%; Found: C 45.18, H 6.66, N 7.69%.

(2,4-Di-tert-butyl-6-{[2-(3,5-diphenyl-pyrazol-1-yl)-ethylimino]-methyl}-phenol)-chromium(III) dichloride ( 4a .10H 2 O) : Complex 4a



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was prepared from L4 (0.15 g, 0.31 mmol) and [CrCl 3 (THF) 3 ] (0.12 g, 0.31 mmol) as a yellowish brown powder. Yield: 0.15 g, (80%). IR (ATR) (cm −1 ): 3120 ( ν H2O ), 1615 (s) ( ν C N ). MS (ESI): m / z (%) = 602 (10) [M] + , 566 (2) [M-Cl] + . Anal. Calc. for C 32 H 36 Cl 2 CrN 3 O.10H 2 O: C 49.17, H 7.22, N 5.38%; Found: C 48.07, H 6.32, N 5.14%.

(2,4-Di-tert-butyl-6-{[2-(3,5-diphenyl-pyrazol-1-yl)-ethylimino]-methyl}-phenol)-chromium(III) dichloride ( 5a .4H 2 O) : Complex 5a was prepared from L5 (0.55 g, 1.62 mmol) and [CrCl 3 (THF) 3 ] (0.61 g, 1.62 mmol) as a yellowish brown powder. Yield: 0.45 g, (62%). IR (ATR) (cm −1 ): 3313 ( ν H2O ), 1617 (s) ( ν C N ). HRMS (ESI): m / z (%) = 424.1378 (100) [M-Cl] + . Anal. Calc. for C 22 H 29 Cl 2 CrN 2 O.4H 2 O: C 49.63, H 7.00, N 5.26%; Found: C 50.36, H 6.76, N 4.74%.

2.4. General Procedure for Ethylene Polymerization

The polymerization was carried out in a 450 mL high-pressure reactor, which was preheated for 1 h at 200 °C under vacuum and cooled under nitrogen to the desired temperature before use. To an appropriate amount of precatalyst in a Schlenk tube was added degassed toluene (70 mL) and the solution was transferred into the reactor by a cannula. The reactor was then charged with ethylene (3 bar) for 5 min. In a second Schlenk tube containing degassed toluene (80 mL), an appropriate amount of ethy-laluminum dichloride (Al:Cr = 250:1–1000:1) was added with a syringe. The solution was then transferred via a cannula into the reactor. Immediately after the addition of the EtAlCl 2 solution, the reactor was charged with ethylene while the reactor tem-perature was maintained at the set temperature. The appropriate amount of ethylene was continuously supplied to maintain a constant pressure in the reactor during the polymerization reac-tion. After the set reaction time, the excess ethylene was vented and the reaction was quenched by adding 10 mL of acidifi ed methanol. The polymer formed was soaked in 3 M HCl for several hours after which it was fi ltered and washed with methanol, followed by diethyl ether, and dried under vacuum until a con-sistent weight was obtained.

3. Results and Discussion

3.1. Synthesis of the Ligands and Chromium Complexes (1a.7H 2 O-5a.4H 2 O)

The imidazolyl ligands, 2,4-di- tert -butyl-6-{[2-(1H-imi-dazol-4-yl)-ethylimino]-methyl}-phenol ( L1 ) [ 17 ] and the pyrazolyl ligands, 2,4-di- tert -butyl-6-{[2-pyrazol-1-yl-ethylimino]-methyl}-phenol ( L2 ), 2,4-di-tert-butyl-6-{[2-(3,5-dimethyl-pyrazol-1-yl)-ethylimino]-methyl}-phenol ( L3 ) and 2,4-di-tert-butyl-6-{[2-(3,5-diphenyl-pyrazol-1-yl)-ethylimino]-methyl}-phenol ( L4 ), were synthesized according to literature procedures. [ 18 ] The pyridyl ligand, 2,4-di-tert-butyl-6-{[2-pyridin-2-yl-ethylimino]-methyl}-phenol ( L5 ) was prepared following a similar procedure. [ 18 ] A typical condensation reaction between 3,5-di-tert-butyl-2-hydroxybenzaldehyde and 2-(2-pyridyl)-ethylamine afforded L5 as yellow oil in 97% yield. Compound L5 was characterized by a combination of IR, elemental analysis,

and 1 H/ 13 C{ 1 H} NMR spectroscopy. The compound showed the characteristic imine proton at 8.29 ppm while the eth-ylene linkers were observed as two triplets at 3.16 and 4.00 ppm. The phenoxy and pyridyl proton signals were observed between 7.16 and 8.56 ppm. FT-IR confi rmed the presence of the imine functional group at 1631 cm −1 while elemental analysis confi rmed the purity of the compound.

The chromium(III) precatalysts (1a. 7H 2 O -5a. 4H 2 O ) were synthesized in good yields as yellowish brown solids by reacting [CrCl 3 (THF) 3 ] with the corresponding salicylaldimine ligands L1 –L5 in a 1:1 ratio (Scheme 1 ). All the precatalysts were characterized by IR, elemental analyses, and in some cases by HRMS. IR data indicate that the imine-stretching frequencies in the complexes found between 1614 and 1617 cm −1 have shifted to lower frequencies compared with the corresponding stretching frequencies in the ligands, which were between 1630 and 1637 cm −1 . The chromium(III) precatalysts were, how-ever, very moisture sensitive and therefore satisfactory CHN analyses were diffi cult to obtain. Hence, one way of obtaining these chromium(III) precatalysts devoid of water was to generate them in situ.

3.2. Molecular Structure of L1

Although the synthesis of L1 has been previously reported, [ 17 ] its structure has never been reported, as such we have determined the structure of L1 and included it in this report. Colorless single crystals of L1 suitable for the X-ray structural determination was obtained by slow evap-oration of a solution of L1 in a mixture of dichloromethane and hexane at room temperature. The crystallographic data are in Table 1 , while Figure 1 shows the molecular structure of the compound with selected bond lengths and angles provided in the caption for Figure 1 . The molecular structure of L1 in Figure 1 shows two symmetrically gener-ated molecules. The molecule with suffi x “A” is symmetry generated with coordinates +X, 1+Y, +Z. The key aspects of the structure of L1 are the disposition of the phenolic OH group relative to the imine group. In the solid state, mole-cules of L1 are hydrogen bonded between the phenolic proton and the imine nitrogen. The bond length of the imino group in L1 is 1.265(3) Å, consistent with similar imine bond lengths found in the literature. [ 21 ]

3.3. Ethylene Polymerization

3.3.1. Ethylene Polymerization Using Discrete Chromium(III) Precatalysts (1a.7H 2 O–5a.4H 2 O)

Precatalysts (1a.7H 2 O–5a.4H 2 O) were screened for their activities towards the polymerization of ethylene after activation with EtAlCl 2 . To establish the best Al:Cr ratio for catalysts activity, we varied the precatalyst to co-catalyst



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ratio with 3a from 250:1–1000:1 (entries 1–4, Table 2 ) at 30 °C reaction temperature and 10 bar ethylene pressure for 1 h. The catalytic activities increased with increasing Al:Cr ratios (entries 1 and 2, Table 2 ), but decreased drastically when the Al:Cr ratio was further increased to 750:1 or

1000:1 (entries 3 and 4 in Table 2 ). Such effect of co-catalyst concentration on catalytic activity signifi es adduct forma-tion between catalyst and co-catalyst that reduces availability of active cata-lyst. This effect has been observed in pre-vious reports. [ 22 ] The highest activity of 1.22 × 10 5 g mmol Cr −1 h −1 was obtained at an Al:Cr ratio of 500:1 (entry 2, Table 2 ). We believe that the active species from all these precatalysts are similar to Cr-ethyl species reported by Braunstein and co-workers. [ 23 ]

The effect of varying the third donor atom in the salicylaldimine ligand from imi-dazolyl- to pyrazolyl- and to pyridyl- nitrogen was also investigated at Al:Cr ratio of 500:1, at 30 °C and 10 bar ethylene pressure (entries 2, 5–8, Table 2 ). Pre-catalyst 3a , which has the least σ-donor ability, exhib-ited the highest activity (1.22 × 10 5 g mol(Cr) −1 h −1 ; entry 2, Table 2 ) of all of the fi ve precatalysts. The activities of the remaining precatalysts were: 1a (1.07 × 10 5 g mol(Cr) −1 h −1 , entry 5, Table 2 ), 5a (0.97 × 10 5 g mol(Cr) −1 h −1 , entry 8, Table 2 ), 4a (0.62 × 10 5 g mol(Cr) −1 h −1 , entry 7, Table 2 ) and 2a (0.34 × 10 5 g mol(Cr) −1 h −1 , entry 6, Table 2 ). The salicylal-dimine ligands are Lewis bases, which donate electrons to form dative bonds with the chromium metal center. Hence the σ-donor ability of the binding atoms in the ligand could affect the electrophilicity of the chromium center and hence the reactivity of the resulting chro-mium catalyst. It is also possible that steric factors that lead to chain growth can affect both catalysts activity and polymer molecular weights. Precatalyst 3a , with methyl substituents, appears to have the best combina-tion of steric and electrophilicity compared with 4a and 2a ; hence, the best activity of the catalysts that have pyra-zolyl moieties. The better activity of 1a compared with 5a is simply related to the σ-donor abilities of imidazole and pyridine. Overall, the catalytic activities thus observed were 3a > 1a > 5a > 4a > 2a .

In order to investigate the effect of reaction conditions on the polymerization, we used precatalyst 3a to vary temperature, pressure, and time. The infl uence of the reaction temperature on 3a was investigated from 25 to 40 °C for 1 h, at an Al:Cr ratio of 500:1 and 10 bar (entries 2, 9–11, Table 2 ). The optimum temperature was found to be 30 °C (entry 2, Table 2 ). The catalytic activity, how-ever, decreased with increasing temperature (entries 10 and 11 in Table 2 ), most likely due to catalysts deactiva-tion at high temperatures. The molecular weights of the polyethylene produced were also observed to decrease with increase in temperature as a result of faster chain termination versus chain propagation. [ 1a ] However, the



Scheme 1. The synthesis of chromium(III) precatalysts 1a–5a .

Table 1. Crystal data collection and structural refi nement para-meters for L1 .

Parameter L1

Empirical formula C 40 H 59 N 6 O 2

Formula weight 655.93

Temperature [K] 100.04

Wavelength [Å] 0.71073

Crystal system Triclinic

Space group P 1

a [Å] 9.7473(8)

b [Å] 9.9603(6)

c [Å] 21.4674(16)

α [°] 89.824(5)

b [°] 89.931(6)

g [°] 66.331(4)

Volume [Å 3 ] 1908.9(2)

Z 2

Density (calculated)[Mg/m 3 ]

1.141

Absorption coeffi cient [mm −1 ]

0.553

F(000) 714

Goodness-of-fi t on F 2 0.963

Final R indices [I>2sigma(I)]

R 1 = 0.0537, wR 2 = 0.1225

R indices (all data) R 1 = 0.0889, wR 2 = 0.1341

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molecular weight distribution did not broaden and the Mw/ Mn remained in the range of 2–2.5. This suggests that catalyst 3a remains a single-site catalyst even at high polymerization temperatures.

It has been shown kinetically that the rate of poly-merization increases with increasing ethylene concentrations. [ 24 ] So ethylene pressure was increased from 5 bar through to 10, 15, and 20 bar at Al:Cr ratio of 500:1 for 1 h and at 30 °C (entries 2, 12–14, Table 2 ). Cat-alyst activity increased from 0.78 × 10 5 g mol(Cr) −1 h −1 at 5 bar to 1.22 × 10 5 g mol(Cr) −1 h −1 at 10 bar and 1.45 × 10 5 g mol(Cr) −1 h −1 at 15 bar (entries 2, 12, and 13, Table 2 ). The highest activity was observed at 15 bar. Fur-ther increase in ethylene pressure resulted in a decrease in activity (entry 14, Table 2 ), which could be attrib-uted to system saturation. The molecular weight of the polyethylene produced also increased with increase in ethylene pressure (entries 2, 12, and 13, Table 2 ) until 20 bar where it decreased (entry 14, Table 2 ).

The effect of time on activity using the same reaction conditions specifi ed for the pressure variation by var-ying time from 15 to 180 min (entries 2, 15–18, Table 2 ) was investigated. There was a drastic decrease in catalyst activity as time increased. At 30 min activity decreased to

1.65 × 10 5 g mol(Cr) −1 h −1 from 2.32 × 10 5 g mol(Cr) −1 h −1 at 15 min; this suggests that our catalyst has low stability. Similar observations have been made for other chromium(III) catalysts. [ 25 ]

In general, the polyethylenes produced have high molec-ular weights (e.g., 348.3 × 10 3 g mol −1 ) and melting points ( T m ) (133–140 °C) characteristic of high-density polyeth-ylene. Figure 2 is a typical high-temperature 13 C{ 1 H} NMR spectrum of the polyethylene produced confi rming that the polymers are indeed high-density polyethylene. [ 26 ] From the gel-permeation chromatography (GPC) data, 1a – 5a produce single-site catalysts that lead to the poly-ethylenes with moderate to high-molecular-weight poly-mers. We also observed that the molecular weight of the polyethylene produced decreased with increase in Al:Cr ratio, suggesting that chain transfer to aluminum occurs in this system during the ethylene polymerization reac-tion in addition to chain termination. [ 27 ]

3.3.2. Ethylene Polymerization Using In-Situ-Generated Salicylaldiminechromium(III) Precatalysts 1b–5b

The chromium(III) precatalysts were also generated in situ ( 1b – 5b ) in an effort to prevent the highly



Figure 1. Molecular structure of L1 shown with the 50% probability ellipsoids. Selected bond lengths (Å) and angles (°) are: N2 C18, 1.373(3); N2 C19, 1.317(3); N1 C16, 1.465(3); N1 C15, 1.265(3); O1 C1C, 1.358(3); C1C C7, 1.402(4); C1C C9, 1.411(3); C19 N2 C18, 105.7(2); C16 N1 C15, 119.2(2); O1 C1C C7, 119.8(2), O1 C1C C9, 119.5(2).

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Figure 2. 13 C{ 1 H} NMR spectrum of the polyethylene produced by chromium(III) complex 3a in toluene.

moisture-sensitive chromium precatalysts from absorbing water. It also afforded a one-pot synthesis of these chromium(III) catalysts. Active catalysts were generated by reacting the in-situ-generated chromium(III) complexes

( 1b – 5b ) with EtAlCl 2 (Table 3 ). The activities obtained with the in-situ-generated catalysts were higher than those observed for the equivalent discretely prepared chromium(III) precatalysts 1a.7H 2 O – 5a.4H 2 O . The

Table 2. Ethylene polymerization data using salicylaldiminechromium(III) precatalysts 1a .7H 2 O– 5a .4H 2 O and EtAlCl 2 . a)

Entry Catalyst Al:Cr Temp. Pressure Time Activity b) MMnn c) MMww c) MMww /MMnn c) T m d)

[°C] [bar] [min] [×10 3 g mol −1 ] [×10 3 g mol −1 ] [°C]

1 3a 250 30 10 60 0.91 46.6 155.8 3.3 138.0

2 3a 500 30 10 60 1.22 65.2 145.1 2.2 135.5

3 3a 750 30 10 60 1.09 38.9 110.1 2.8 132.8

4 3a 1000 30 10 60 0.38 56.1 155.5 2.8 135.0

5 1a 500 30 10 60 1.07 58.6 220.4 3.8 137.4

6 2a 500 30 10 60 0.34 51.9 106.2 2.0 136.2

7 4a 500 30 10 60 0.62 4.0 39.7 9.8 138.6

8 5a 500 30 10 60 0.97 21.4 82.8 3.9 133.1

9 3a 500 25 10 60 1.07 76.2 163.2 2.1 138.0

10 3a 500 35 10 60 1.01 81.1 348.3 4.3 135.8

11 3a 500 40 10 60 0.87 41.1 100.2 2.4 132.5

12 3a 500 30 5 60 0.79 48.4 128.8 2.7 135.1

13 3a 500 30 15 60 1.45 76.6 168.8 2.2 137.6

14 3a 500 30 20 60 1.23 63.4 140.4 2.2 136.3

15 3a 500 30 10 15 2.32 48.5 107.4 2.2 132.9

16 3a 500 30 10 30 1.65 55.8 114.2 2.1 134.1

17 3a 500 30 10 120 0.56 49.3 109.6 2.2 139.5

18 3a 500 30 10 180 0.11 24.7 100.1 4.1 135.1

a) Reaction conditions: toluene (150 mL), Cr(III) (11 μmol); b) Polymer activity = × 10 5 g mol(Cr) −1 h −1 ; c) Determined by GPC using poly-styrene standardy; d) Determined by DSC.

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high-density polyethylenes obtained with the in-situ pro-cess has T m values of 130–135 °C. The GPC data (Figures 3 and 4 ) of the polyethylenes from the in situ process have high molecular weight (33.9 × 10 3 g mol −1 ) and the poly-dispersity indices are broader (Figure 3 ) than the polymers from the discrete cat-alysts, indicating that there is more than one active site involved in the in situ polymerization reactions. Indeed some of the GPC data (Figure 4 ) are bimodal.

Here too polymerization reactions were performed by varying conditions to investigate the effect of these condi-tions on catalytic activity and products formed. Precatalysts and co-catalysts ratios were varied from 250:1 to 1000:1 (entries 1–4, Table 3 ) at 30 °C. The highest activity (1.71 × 10 5 g mmol(Cr) −1 h −1 ) was observed at Al:Cr ratio of 500:1 (entry 2, Table 3 ) and activity decreased at 750:1 and 1000:1. This trend is similar to the results obtained with the discrete chromium(III) precatalysts.

Reaction temperature was also sig-nifi cant on the catalytic activity and the properties of the polyethylenes obtained.

By varying the temperatures between 30 and 90 °C (entries 2, 15, and 16, Table 3 ), there was a major reduction in the catalyst activity from 1.71 × 10 5 to 0.72 × 10 5 and to 0.65 × 10 5 g mol(Cr) −1 h −1 at 30, 60, and 90 °C, respectively.



Table 3. Ethylene polymerization data using in-situ-generated salicylaldiminechromium(III) precatalysts 1b – 5b and EtAlCl 2 . a)

Entry Catalyst Al:Cr Temp. [°C]

Pressure [bar]

Time [min]

Activity b) MMnn c) MMww c) MMww/MMnn c) T m d)

[×10 3 g mol −1 ] [×10 3 g mol −1 ] [°C]

1 3b 250 30 10 60 1.49 33.9 75.6 2.2 134.4

2 3b 500 30 10 60 1.71 29.9 60.2 2.0 135.2

3 3b 750 30 10 60 1.22 17.6 96.1 5.5 132.5

4 3b 1000 30 10 60 0.90 19.5 192.5 n.d 132.2

5 1b 500 30 10 60 1.60 21.5 59.9 2.8 134.7

6 2b 500 30 10 60 0.94 11.3 233.4 20.6 132.1

7 4b 500 30 10 60 1.29 24.7 216.4 8.8 133.6

8 5b 500 30 10 60 1.35 13.1 118.1 9.0 133.6

9 3b 500 30 10 30 1.25 18.1 226.6 12.5 131.5

10 3b 500 30 10 120 1.07 32.3 415.9 12.8 132.6

11 3b 500 30 10 180 1.04 24.7 112.0 4.5 132.2

12 3b 500 30 5 60 1.58 19 92.8 4.9 132.1

13 3b 500 30 15 60 1.43 28.4 173.5 6.1 133.4

14 3b 500 30 20 60 1.33 29.4 644.5 21.8 133.8

15 3b 500 60 10 60 0.72 11.6 139.8 12.0 130.8

16 3b 500 90 10 60 0.65 5.8 34.7 5.9 129.6

a) Reaction conditions: toluene (150 mL), Cr(III) (11 μmol); b) Polymer activity = × 10 5 g mol(Cr) −1 h −1 ; c) Determined by GPC using polysty-rene standards; d) Determined by DSC.

Figure 3. GPC curves of polyethylenes produced using 3b and EtAlCl 2 at different Al:Cr ratios (entries 1–4, Table 3 ).

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Polymer molecular weights also decreased with increase in the reaction temperature. Interestingly at 60 °C, the molecular weight distribution was clearly bimodal, with more of the products in the low-molecular-weight region (Figure 4 ). At 90 °C, the molecular weight dis-tribution was much broader as shown in Figure 4 ; indicative of a faster rate of chain transfer versus chain propagation. [ 1a ] As reaction time increased, catalytic activity decreased (entries 2, and 9–11, Table 3 ) (Figure 5 )

as well as a decrease in the molecular weights of the polyethylenes. The Mw decreased with increasing reaction time from 226.5 × 10 3 g mol −1 at 30 min to 60.2 × 10 3 g mol −1 at 60 min (entries 2 and 9, Table 3 ), indicative of the active species deactivating over the prolonged reaction times, similar to observations made with the discrete chromium(III) precatalysts.

4. Conclusion

Two types of O^N^N salicylaldi-minechromium(III) precatalysts, one discrete and the other generated in situ, have been developed, which gen-erate very active catalysts for the selec-tive polymerization of ethylene instead of producing a mixture of oligomers

and polymers like most chromium(III) catalysts. The activities obtained with the in-situ-generated catalysts were higher than the catalysts generated from the dis-crete chromium(III) precatalysts. Both types of catalyst produced similar high-density polyethylenes with mod-erate to high molecular weights. Whereas the discrete chromium(III) precatalysts produced single-site catalysts in all cases, the in-situ-generated catalysts were not also always single-site.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author. Crystallographic data has been deposited with the Cambridge Crystallographic Data Centre with CCDC 998100 (L1). Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: +44–1223–336063; e-mail: [email protected] or http://www.ccdc.cam.ac.uk .

Acknowledgements: The authors acknowledge the DST-NRF Centre of Excellence in Catalysis (c*change) and the University of Johannesburg for fi nancial support for this project.

Received: April 29, 2014 ; Revised: May 13, 2014 ; Published online: June 30, 2014 ; DOI: 10.1002/macp.201400223

Keywords: chromium ; ethylene ; poly-merization ; salicylaldimine



Figure 4. GPC curves of polyethylenes produced using 3b and EtAlCl 2 at various reaction temperatures (entries 2, 15, and 16, Table 3 ).

Figure 5. GPC data of polyethylenes produced using 3b and EtAlCl 2 at different reaction times (entries 2 and 9–11, Table 3 ).

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Phenoxysalicylaldimine-Bearing Chromium(III) Precatalysts for Ethylene Polymerization

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