Bis(benzothiazolyl) titanium, zirconium and vanadium complexes as catalysts for ethylene polymerization

European Polymer Journal 68 (2015) 385–397

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Bis(benzothiazolyl) titanium, zirconium and vanadium complexes as catalysts for ethylene polymerization Hamdi Ali Elagab, Helmut G. Alt ⇑ Laboratorium für Anorganische Chemie, Universität Bayreuth, Postfach 10 12 51, D-95440 Bayreuth, Germany

a r t i c l e

i n f o

Article history: Received 6 March 2015 Received in revised form 2 May 2015 Accepted 17 May 2015 Available online 18 May 2015 Keywords: Bis(benzothiazolyl) complexes of Ti, Zr, V Effect of the bridge Ethylene polymerization catalysts Bimodal resins Single-reactor technology

a b s t r a c t 13 titanium, zirconium and vanadium complexes with 1,1-bis(benzothiazolyl)methane, 1,2-bis(benzothiazolyl)-ethane, 1,2-bis(benzothiazolyl)benzene and 2,6-bis(benzothiazolyl) pyridine ligands were synthesized and characterized. After activation with methylalumoxane (MAO), these complexes proved as good catalysts for ethylene polymerization in spite of the hetero atoms in the catalyst molecules and the oxophilicity of the metals. Structure–property-relationship studies indicated that the bridging unit connecting the two heterocyclic moieties has a strong influence on the performance of the catalysts. Different interactions of the catalyst cation and the bulky MAO anion, due to the various hetero atoms, could be responsible for this behavior. The produced polyethylenes show bimodal or multimodal molecular weight distributions indicating more than one active site in the polymerization process. Therefore such mononuclear catalysts are good candidates for single-reactor bimodal or multimodal polyethylene technology. The polymerization activity of the catalyst system [1,2-bis(benzothiazolyl)benzene zirconium tetrachloride] 13/MAO was investigated under different polymerization conditions. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Olefin polymerization is still a fascinating area in research because of the innumerable possibilities the various catalysts offer [1–3 and references therein]. Structure–property-relationship studies reveal that tiny changes in a catalyst or cocatalyst molecule can have a very strong influence on the polymerization process [4,5]. So far hetero atoms in a ligand of early transition metal complexes were disadvantageous for catalytic applications because their Lewis basic character and the oxophilicity of these metals can reduce the activities of such catalysts drastically [5,6]. Benzothiazole contains a heterocyclic sulfur atom and a pseudo-imidazole functional group. A review of the literature revealed that 1,2-bis(2-benzothiazolyl)benzene and 1,2-bis(2-benzo-thiazolyl)ethane are frequently used as ligands and a considerable number of complexes with late transition metals are reported [7–12]. Also, the nickel(II), cobalt(II) and copper(II) coordination chemistry of some tetradentate ligands involving benzothiazole functional groups has been published [13]. In all cases involving benzothiazoles as functional groups, the ligands behave as nitrogen donors, except in a few cases involving bridging benzothiazole [14] in which it is assumed to behave as a bidentate ligand involving both N and S donation. The ligand 2,6-bis(2-benzothiazolyl)pyridine [15,16] has been shown to behave as an N-3 donor in its complexes with manganese(II), iron(II) and nickel(II). For ethylene polymerization catalysis, only 1,2-bis(benzimidazolyl)-benzene copper(II) complexes were reported [17]. The vanadium complexes of bis(benzimidazole)amine tridentate ligands [N, N, N], are ethylene polymerization catalysts after activation with alkylaluminum compounds [18] and 2,6-bis(2-benzimidazolyl)pyridine zirconium dichloride/ MAO polymerizes ⇑ Corresponding author. E-mail address: [email protected] (H.G. Alt). http://dx.doi.org/10.1016/j.eurpolymj.2015.05.017 0014-3057/Ó 2015 Elsevier Ltd. All rights reserved.

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H.A. Elagab, H.G. Alt / European Polymer Journal 68 (2015) 385–397

methylacrylate [19]. We have shown recently [20–22] that bis (benzimidazolyl, benzothiazolyl, and benzoxazolyl) benzene complexes of titanium, zirconium and vanadium can be activated with methylalumoxane and then be applied as attractive catalysts for ethylene polymerization. Herein we report the first zirconium, titanium and vanadium complexes of 1,2-bis(ben zothiazolyl)benzene, 1,2-bis(benzothiazolyl)ethane, 1,1-bis(benzothiazolyl)methane, and 2,6-bis(benzothiazolyl) pyridine. Structure–property-relationship studies describe the influence of the bridging unit in the ligands on the catalyst performance. 2. Results and discussions 2.1. General synthesis of the ligand precursors The condensation reaction of a dicarboxylic acid or an acid anhydride in preheated polyphosphoric acid is a well established procedure for the preparation of the imidazole based ligand precursors [23,24] in moderate to high yields (Scheme 1). 2.2. Synthesis of coordination compounds 2.2.1. Synthesis of titanium and zirconium complexes The complexes were synthesized according to Scheme 2 and characterized by 1H NMR, 13C NMR and mass spectrometry (Table 1). The titanium and zirconium complexes were prepared by ligand displacement reactions. The reaction of the B

N

SH B

S

S

O PPA / 175°C, 3-5h

O NH2

O

1 (B = CH2) 2 (B = CH2CH2) 3 (B = 1,2-C6H4)

4 (B = 4-Me,1,2-C6H3) 5 (B = 2,6-py) Scheme 1. Synthesis of compounds 1–5.

B

B S

S N

N

MCl4(THF)2, CH2Cl2 (M =Ti, Zr) (M = V) or MCl3, Et2O

S

S N

N

r. t., 24h MCln

Complex No. 6 7 8 9 10 11 12 13 14 15 16 17 18

Bridging unit B CH2 CH2 CH2 CH2CH2 CH2CH2 CH2CH2 C6H4 C6H4 C6H4 4-Me- 1,2-Ph 4-Me- 1,2-Ph 4-Me- 1,2-Ph 2,6-Py

M Ti Zr V Ti Zr V Ti Zr V Ti Zr V V

Scheme 2. Synthesis of coordination compounds 6–18.

N

387

H.A. Elagab, H.G. Alt / European Polymer Journal 68 (2015) 385–397 Table 1 Ethylene polymerization activities of complexes 6–18/MAO. Catalyst No.

6

7

7

8

8

9

10

11

12

12

13

13

13

14

14

15

16

17

18

Al:M T °C Al:Ti 1000:1 50 °C Al:Zr 1000:1 50 °C Al:Zr 1000:1 40 °C Al:V 2500:1 50 °C Al:V 1000:1 50 °C Al:Ti 2500:1 50 °C Al:Zr 2500:1 50 °C Al:V 2500:1 50 °C Al:Ti 2500:1 20 °C Al:Ti 2500:1 50 °C Al:Zr 1000:1 40 °C Al:Zr 2500:1 40 °C Al:Zr 2500:1 50 °C Al:V 1000:1 20 °C Al:V 2500:1 50 °C Al:Ti 2500:1 50 °C Al:Zr 2500:1 50 °C Al:V 2500:1 50 °C Al:V 2500:1 50 °C

Activity (kg/mol cat. h)

Mn (g/mol)

Mw (g/mol)

PD

279

80,000

527,900

6.6

41

83,500

362,900

4.3

22

n.d.

n.d.

n.d.

380

250,100

2,883,600

11.5

493

14,200

42,000

3.0

155

70,360

373,250

5.3

102

398,900

1,762,700

4.42

208

140,020

460,100

3.3

862

3470

1,512,260

436

220

237,300

1,349,420

5.6

1311

149,350

1,371,580

9.18

450

66,250

1,273,130

19.2

132

70,560

462,380

6.6

54

458,200

1,389,040

3.03

442

456,400

2,134,690

4.5

260

327,200

3,344,210

10.2

128

143,800

1,580,130

11.0

241

98,400

1,638,340

16.6

284

275,300

1,652,780

6.0

a = All polymerization reactions were carried out in 250 ml pentane with MAO as cocatalyst (Al:M = 2500:1) at 50 °C, 10 bar ethylene pressure, 1 h reaction time.

tetrahydrofuran adducts of zirconium and titanium tetrachloride with the corresponding ligand precursor in methylene chloride results in an immediate color change and the complexes could be isolated in very high yields (80–95%). The complexes were characterized by NMR, mass spectrometry and elemental analysis. 2.2.2. Synthesis of the vanadium complexes The vanadium complexes were synthesized by dissolving vanadium trichloride in diethyl ether followed by the addition of the ligand precursor with constant stirring overnight. The product yields range within 60–78%.

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2.3. Characterization Since the complexes did not crystallize properly, they were characterized spectroscopically. 2.3.1. 1H and 13C NMR spectroscopy The ligand precursors 1–5 and their titanium and zirconium complexes were characterized by 1H and 13C NMR spectroscopy. The vanadium complexes, due to their paramagnetism, were characterized by mass spectrometry. The 1H NMR spectrum of compound 3 (see Fig. 1) shows six resonance signals for the 12 protons due to the symmetry of the molecule. The doublet with unresolved long range couplings at d = 8.01 ppm [3JH,H = 7.6 Hz] can be assigned to H6 of the benzothiazole phenyl ring. The multiplets at d = 7.93–7.90 and 7.61–7.58 ppm belong to an AA0 BB0 spin system and represent the protons in the positions 2 and 1. The doublet with unresolved long range couplings at d = 7.76 ppm [3JH,H = 7.6 Hz] is assigned to the protons H3. H5 and H4 appear as doublets of doublets at d = 7.43 and 7.33 ppm [3JH,H = 7.6 Hz]. The 13C NMR spectrum for compound 3 (Fig. 2) shows ten resonance signals at d = 166.4, 153.4, 136.6, 133.5, 131.6, 131.5, 127.2, 126.3, 123.8 and 121.9 ppm for the carbon atoms C4, C10, C5, C3, C1, C2, C8, C7, C9 and C6. The 1H NMR spectrum of complex 12 (Fig. 3) shows five sets of resonance signals. The doublet at d = 8.01 ppm [2H, 3 JH,H = 7.8 Hz] can be assigned to the protons H6 and the doublet for H3 at d = 7.90 ppm is superimposed by the AA0 BB0 pattern of H2 (m, d = 7.91 ppm) and H1 (m, d = 7.71 ppm). H4 and H5 are observed as doublets of doublets at d = 7.45 [2H, 3 JH,H = 7.8 Hz] and 7.38 ppm [2H, 3JH,H = 7.8 Hz]. The 13C NMR spectrum of 12 shows separate signals for each magnetically different carbon atom as indicated in Fig. 4 due to the dissymmetry of the ligand. 2.3.2. Mass spectrometry The mass spectrum of compound 3 is shown in Fig. 5. The molecular ion peak is detected at m/z = 344. The loss of one sulfur atom results in an ion at m/z = 311. The ion at m/z = 278 corresponds to the loss of two sulfur atoms and the peak with m/z = 222 is assigned to M°+-C6H4NS. The ion with m/z = 108 can be assigned to the fragment C6H4S.

Fig. 1. 1H NMR spectrum of compound 3 in chloroform-d1.

Fig. 2.

13

C NMR spectrum of compound 3 in chloroform-d1.

H.A. Elagab, H.G. Alt / European Polymer Journal 68 (2015) 385–397

389

Fig. 3. 1H NMR spectrum of complex 12 in DMSO-d6.

Fig. 4.

13

C NMR spectrum of complex 12 in DMSO-d6.

Fig. 5. Mass spectrum of compound 3.

The mass spectrum of complex 12 (see Fig. 6) shows the molecular ion peak at m/z = 534 with relatively low intensity. The peak at m/z = 464 represents the loss of two chloride ions. The base peak at m/z = 344 is representing the mass of the free ligand. Generally, it can be observed that the weak dative bonds involved in the formation of the complexes is responsible for relatively low intensities of the fragmentation pattern due to premature thermal decomposition in the spectrometer.

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H.A. Elagab, H.G. Alt / European Polymer Journal 68 (2015) 385–397

Fig. 6. Mass spectrum of complex 12.

2.4. Polymerization results The complexes of titanium, zirconium, and vanadium with ligands derived from 1,1-bis(benzothiazolyl)methane, 1,2-bis(benzothiazolyl)ethane, 1,2-bis(benzothiazolyl)benzene and 2,6-bis(benzothiazolyl)pyridine compounds were activated with methylalumoxane (MAO) in toluene solution assuming a mechanism as proposed for the activation of metallocene [25,26] and 2,6-bis(imino)pyridine iron complexes [27]. The homogeneous catalyst solutions were used for ethylene polymerization reactions. They showed variable activities for ethylene polymerization under different polymerization conditions (co-catalyst concentration, polymerization temperature). The activities are greatly influenced by the ligand environment and the nature of the metal atom. The activities of the titanium complexes (Fig. 7) were found to follow the order 6 > 12 > 9. This order of activities may be accounted for by the size of the chelate ring formed, since it is known that five and six membered chelate rings are more stable than seven membered rings. The higher activity of complex 6 compared to 12 and 9 was therefore understandable. On the other hand, for the titanium complexes 9 and 12, although both forming seven membered chelate rings, the activity of 12 is higher than for 9 due to the rigid structure formed by the phenylene bridge compared to a methylene bridge. For zirconium complexes derived from the above mentioned ligands (Fig. 7) the order of activity is controlled by the size of the zirconium atom compared to the size of chelating rings. The six membered chelate complex 1,1-bis(benzothiazolyl)methane zirconium tetrachloride 7 shows a lower activity compared to the 1,2-bis(benzothiazolyl)ethane zirconium complex 10 and the 1,2-bis(benzothiazolyl)benzene zirconium complex 13 due to steric factors. The overall lower activities of the zirconium complexes compared to the titanium complexes could be caused by thermodynamically stronger metal carbon bonds in the various polymerization steps slowing down kinetics. The vanadium complexes (Fig. 7) showed the highest activities among the catalyst series. The reason could be that vanadium has the lowest oxophilicity of these three metals and therefore disadvantageous side reactions deactivating active centers are comparatively low. Catalyst activities follow in general the same order of activity observed for the titanium complexes. Catalysts 8/MAO and 14/MAO show higher activities than 11/MAO derived from the 1,2-bis(benzothiazolyl)ethane ligand. The higher activity of 14/MAO (442 kg PE/mol cat.h) compared to 8/MAO (379.8 kg PE/mol cat.h) may arise from the ligand structure. The vanadium complex 18 with a 2,6-bis(benzothiazolyl)pyridine ligand shows an activity of 284.4 [kg PE/mol cat.h] which is lower than the activities of complexes 8 and 14. This difference may be accounted for by structural differences around the active centers. Introduction of a methyl group in the 1,2-phenylene bridge affects the activity of the vanadium 500 442

Activity [kg PE/mol cat.h]

450 380

400 350 300

284

279

250

220

200

208

155

150

132 102

100 41

50 0

6

9

12

7

10

13

8

11

14

18

Fig. 7. Polymerization activities of bis-benzothiazolyl titanium complexes 6, 9, 12, the zirconium complexes 7, 10 and 13 and the vanadium complexes 8, 11, 14 and 18 (Al:M = 2500:1, T = 50 °C).

391

Activity [kg PE/mol cat.h]

H.A. Elagab, H.G. Alt / European Polymer Journal 68 (2015) 385–397 500 450 400 350 300 250 200 150 100 50 0

442 380

8

284

241

208

11

14

17

18

Activity [kg PE/mol cat.h]

Fig. 8. Structure activity relationships for vanadium complexes 8, 11, 14, 17, 18/MAO (V:Al = 1:2500, T = 50 °C).

1000 900

861

800 700 600 500

442

400 300

219

200

54

100 0 12/20°C

12/50°c

14/20°C

14/50°C

Activity [Kg PE/mole cat.h]

Fig. 9. Effect of polymerization temperature on activities of catalysts 12, 14/MAO.

45 40 35 30 25 20 15 10 5 0

41

22

7/50°C

7/40°C

Fig. 10. Effect of polymerization temperature on activity of catalyst 7/MAO.

complex 17 compared to the unsubstituted complex 14. The methyl group due to its electronic effect cuts the activity of the catalyst system to half of the value obtained with the unsubstituted complex (Fig. 8). Applying complexes 12 and 14, the temperature dependence of polymerization activities was investigated. Under the same polymerization conditions (cocatalyst concentration and reaction temperature), the titanium complex showed a higher activity at low temperature, while the vanadium complex showed higher activity at an elevated temperature (Fig. 9). The decrease of activity with increasing temperature could be due to thermal decomposition of the catalyst. Applying complex 7 under the same cocatalyst concentration the activity decrease as the polymerization temperature increases (Fig. 10). Using the catalyst system 13/MAO, the effects of cocatalyst concentration and polymerization temperature on the catalyst activity were studied. At 40 °C and an Al:Zr ratio of 1000:1, the catalyst system shows an activity of 1311 [kg PE/mol cat.h]. Increasing the ratio to 2500:1, the activity dropped to 450 [kg PE/mol cat.h]. Too high cocatalyst concentrations obviously block the access of the monomer to the active center. Increasing the temperature to 50 °C under the same cocatalyst concentration, a further drop in activity of the catalyst system was observed. This behavior of the catalyst may be caused by thermal decomposition of the active sites at higher temperatures. The higher cocatalyst concentration may also block some of the active sites leading to an overall decrease in catalyst activity [24] (Fig. 11). In a similar manner, an increase of the cocatalyst in the system 8/MAO, results in an activity decrease at the same polymerization temperature (Fig. 12). GPC analysis of the polyethylenes produced with benzothiazole based complexes revealed that the symmetric catalysts systems were capable to produce resins with very high molecular weights [28,29] associated with broad or even bimodal molecular weight distributions (Fig. 13). The bimodality may arise from the fact that the MAO counterion induces adducts with different symmetries of the active sites in the activation process [30] interacting differently with the hetero atoms of

H.A. Elagab, H.G. Alt / European Polymer Journal 68 (2015) 385–397

Activity [Kg PE/mol cat.h]

392

1400

1311

1200 1000 800 600

450

400

132

200 0 13/40°C/1:1000

13/40°C/1:2500

13/50°C/1:2500

Activity [Kg PE / Mol Cat.h]

Fig. 11. Effect of temperature and cocatalyst concentration on the activity of the catalyst 13/MAO.

600 500

493 380

400 300 200 100 0

8/1000:1(Al:V)

8/2500:1(Al:V)

Fig. 12. Effect of cocatalyst concentration on the activity of catalyst 8/MAO.

Fig. 13. HT-GPC profile of polyethylene produced with 12/MAO.

the heterocyclic ligands. The catalyst system 10/MAO produces a polyethylene resin with very high molecular weight (Mw = 1.8  106 g/mol, PD = 4.42, Fig. 14). Figs. 15 and 16 show the GPC profiles of polyethylenes produced with the catalyst systems 13/MAO and 14/MAO. The molecular weight Mw of the polymer produced with 13/MAO was determined as 1.3  106 g/mol, (PD = 19.22). The catalyst system 17/MAO produced polyethylene with Mw = 1.6  106 g/mol (PD = 16.64). DSC measurements (see Table 2) showed that the catalyst systems produced HDPE with a high degree of crystallinity. For example the melting and crystallization temperatures of the polyethylenes produced with the catalyst system 13/MAO was 139.7 and 120.7 °C and the degree of crystallinity a = 12.1% (see Fig. 17). 3. Experimental All experimental work was routinely carried out using Schlenk technique unless otherwise stated. Dried and purified argon was used as inert gas. n-Pentane, diethyl ether, toluene and tetrahydrofuran were purified by distillation over Na/K alloy. Diethyl ether was additionally distilled over lithium aluminum hydride. Methylene chloride was dried over

H.A. Elagab, H.G. Alt / European Polymer Journal 68 (2015) 385–397

393

Fig. 14. HT-GPC profile of polyethylene produced with 10/MAO.

Fig. 15. HT-GPC profile of polyethylene produced with 13/MAO (40 °C, Al:Zr = 2500:1).

phosphorus pentoxide and in a second step over calcium hydride. Methanol and ethanol were dried over magnesium. 0 Deuterated solvents (CDCl3, DMSO-d6) for NMR spectroscopy were stored over molecular sieves (3 Å A). Methylalumoxane (30% in toluene) was purchased from Crompton (Bergkamen) and Albemarle (Baton Rouge, USA/ Louvain – La Neuve, Belgium). Ethylene (3.0) and argon (4.8/5.0) were supplied by Rießner Company (Lichtenfels). All other starting materials were commercially available and were used without further purification. The titanium tetrachloride and zirconium tetrachloride adducts were synthesized via published procedures [31].

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Fig. 16. HT-GPC profile of polyethylene produced with 17/MAO.

Table 2 Thermal analysis data for representative polymer samples produced with complexes 6, 8, 13, 14, 16/MAO (all polymerization reactions were carried out in 250 ml pentane, 10 bar ethylene pressure, 1 h reaction time). Complex No.

6

8

13

14

Al:M T °C Al:Ti 1000:1 50 °C Al:V 1000:1 50 °C Al:Zr 2500:1 50 °C Al:V 2500:1 50 °C

Tm °C

Tx °C

a%

135.6

118.3

30.1

139.2

119.7

26.7

139.7

120.7

12.1

136.9

119.3

24.6

Tm = melting temperature, Tx = crystallization temperature, a = degree of crystallinity.

3.1. NMR spectroscopy The spectrometers Varian Inova 300/400 M Hz and Bruker ARX 250 were available for recording the NMR spectra. The samples were prepared under inert atmosphere (argon) and routinely recorded at 25 °C. The chemical shifts in the 1H NMR spectra are referred to the residual proton signal of the solvent (d = 7.24 ppm for CDCl3, d = 2.5 ppm for DMSO) and in 13C NMR spectra to the solvent signal (d = 77.0 ppm for CDCl3, d = 39.5 ppm for DMSO-d6). 3.2. Mass spectrometry Mass spectra were routinely recorded at the Zentrale Analytik of the University of Bayreuth with a VARIAN MAT CH-7 instrument (direct inlet, EI, E = 70 eV) and a VARIAN MAT 8500 spectrometer.

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Fig. 17. DSC curve of polyethylene produced with the catalyst system 13/MAO.

3.3. Gel permeation chromatography (GPC) GPC measurements were routinely performed by SABIC Company (Riyadh, Saudi Arabia). 3.4. Elemental analysis Elemental analyses were performed with a VarioEl III CHN instrument. The calibration compound was acetamide. 3.5. General procedures for the syntheses of the complexes 3.5.1. Syntheses of organic compounds 1–5 2-aminothiophenol (0.05 mol) was mixed with a dicarboxylic acid or an acid anhydride (0.025 mol) and the mixture was poured in 50 ml of preheated (100 °C) polyphosphoric acid. The mixture was stirred and heated at 175 °C for 3–5 h. The reaction mixture was then poured into ice cold water and allowed to stand overnight. The precipitate was removed by filtration and washed several times with diluted sodium hydrogen carbonate solution and finally with water. The reaction product was then air dried and weighed. The products were characterized by NMR and mass spectroscopy and representative examples were characterized by elemental analysis. 3.5.1.1. Characterization of compound 1. 1H NMR: 8.04(d,2H), 7.95(d,2H), 7.47(t,2H), 7.43(t,2H), 5.05(s,2H,CH2), 13C NMR: 167.0, 153.2, 135.9, 127.0, 125.9, 123.2, 122.9, 38.5. Elemental analysis (calc: C 63.8, H 3.5, N 9.9; found: C 63.6, H 3.4, N 10.2). Mass spectrum: 282 M°+ (100%). 3.5.1.2. Characterization of compound 2. 1H NMR: 7.98(d,2H), 7.82(d,2H), 7.44(t,2H), 7.37(t,2H), 3.74(s,4H). 153.5, 135.5, 126.4, 125.3, 123.0, 121.9, 33.6. Mass spectrum: 296 M°+ (100%).

13

C NMR: 169.6,

3.5.1.3. Characterization of compound 3. 1H NMR: d = 8.01 (d,2H), 7.92–7.90(dd,2H), 7.76(d,2H), 7.61–7.58(dd,2H), 7.44(t,2H), 7.33(t,2H). 13C NMR: 166.4, 153.436, 136.6, 133.5, 131.6, 131.5, 127.2, 126.3, 123.8, 121.9 ppm. Elemental analysis (calc: C 69.8, H 3.5, N 8.1; found: C 69.7, H 3.5, N 8.2). Mass spectrum: 344 M°+ (100%). 3.5.1.4. Characterization of compound 4. 1H NMR: 8.01–7.95(m,2H), 7.84–7.79(m,3H), 7.74(s,1H), 7.48–7.44(m,3H), 7.38–7.33(m,2H), 2.50(s,3H, CH3). 13C NMR: 166.6, 166.5, 153.6, 153.5, 141.1, 136.8, 136.8, 133.5, 131.8, 131.2, 131.1, 131.0, 126.3, 126.2, 125.5, 125.4, 123.6, 123.5, 121.7, 121.6, 21.3. Mass spectrum: 358 M°+ (100%). 3.5.1.5. Characterization of compound 5. 1H NMR: 8.41(d,2H), 8.09(d,2H), 8.00–7.95(m,3H), 7.51(t,2H), 7.42(t,2H). 13C NMR: 168.8, 154.5, 151.5, 138.4, 136.6, 126.6, 126.15, 124.0, 122.2, 122.1. Elemental analysis (calc: C 66.1, H 3.2, N 12.2; found: C 66.3, H 3.2, N 11.9). Mass spectrum: m/z = 345 M°+ (100%).

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3.5.2. Titanium complexes To an amount of 0.87 g (2.6 mmol) TiCl4(THF)2 in dichloromethane was added an amount of 2.6 mmol of the solid ligand precursor. The reaction mixture was stirred over night at room temperature, filtered and washed several times with the dichloromethane, then with pentane and dried under vacuum and weighed (70–80%) yield. The products were characterized by NMR and mass spectrometry and representative examples were characterized by elemental analysis. 3.5.2.1. Bis(benzothiazolyl) methane titanium chloride complex(6). 1H NMR: 8.02(d,2H), 7.92(d,2H), 7.45(t,2H), 7.38(t,2H), 5.02(s,2H,CH2). 13C NMR: 167.0, 153.2, 135.8, 127.0, 125.9, 123.2, 122.9, 38.4. Mass spectrum: 472 M°+ (2), 436 M°+-Cl (5), 401 M°+-2Cl (10), 365 M°+-3Cl (5), 282 M°+-TiCl4 (100). 3.5.2.2. Bis(benzothiazolyl) ethane titanium chloride complex 9. 1H NMR: 8.02(d,2H), 7.99(d,2H), 7.44–7.35(m,4H), 3.68(s,4H,2CH2). 13C NMR: 170.6, 153.3, 135.3, 126.9, 125.8, 123.1, 122.9, 33.1. Mass spectrum: 487 M°+ (1), 453 M°+-Cl (5), 418 M°+-2Cl (10), 344 M°+-4Cl (80), 296 M°+-TiCl4 (75). 3.5.2.3. Bis-(benzothiazolyl) benzene titanium chloride complex 12. 1H NMR: 8.05(d,2H), 7.97–7.94(m,4H), 7.77–7.74(dd,2H), 7.50(t,2H), 7.44(t,2H), 13C NMR: 166.4, 153.4, 136.6, 133.5, 131.7, 131.6, 127.3, 126.4, 123.8, 123.0. Mass spectrum: 534 M°+ (10), 498 M°+-Cl, 463 M°+-2Cl (70), 427 M°+-3Cl, 344 M°+-TiCl4 (100). 3.5.2.4. Bis-(benzothiazolyl)-4-methylbenzene titanium chloride complex 15. 1H NMR: 8.02(t,2H), 7.92(t,2H), 7.82(d,1H), 7.73(s,1H), 7.53(d,1H), 7.49–7.43(dd,2H), 7.41–7.35(dd,2H), 2.47(s,3H,CH3), 13C NMR: 166.5, 166.4, 153.4, 153.4, 141.6, 136.6, 136.5, 133.5, 132.1, 132.0, 131.6, 130.8, 127.1, 127.2, 126.3, 126.2, 123.8, 123.7, 122.9, 122.8, 21.5. Mass spectrum: 548 M°+ (1), 512 M°+-Cl (2), 496 M°+-Cl–CH3 (5), 441 M°+-3Cl (3), 405 M°+-4Cl (2), 358 M°+-TiCl4 (100). 3.5.3. Zirconium complexes To an amount of 0.45 g (1.2 mmol) ZrCl4(THF)2 in dichloromethane was added an amount of 1.2 mmol of the solid ligand precursor. The reaction mixture was stirred over night at room temperature and then filtered. The residue was washed several times with dichloromethane, then with pentane, dried under vacuum and weighed. The products were characterized by NMR and mass spectroscopy and representative examples were characterized by elemental analysis. 3.5.3.1. Bis(benzothiazolyl) methane zirconium chloride complex 7. 1H NMR: 7, 8.10(d,2H), 8.01(d,2H), 7.53(t,2H), 7.45(t,2H), 5.10(s,2H,CH2). 13C NMR: 167.1, 153.4, 135.9, 127.0, 126.0, 123.3, 123.0, 38.5. Mass spectrum: 517 M°+ (1), 478 M°+-Cl (10), 444 M°+-2Cl (20), 406 M°+-3Cl (5), 282 M°+-ZrCl4 (100). Elemental analysis (calc: C 35.0, H 1.9, N 5.4; found: C 35.0, H 2.0, N 5.4). 3.5.3.2. Bis(benzothiazolyl) ethane zirconium chloride complex 10. 1H NMR: 8.03(d,2H), 7.92(d,2H), 7.49–7.35(m,4H), 3.71(s,4H,2CH2). 13C NMR: 170.7, 152.8, 135.3, 126.7, 125.7, 122.9, 122.7, 33.0. Mass spectrum: 530 M°+ (1), 493 M°+-Cl (5), 456 M°+-2Cl (5), 389 M°+-4Cl (20), 296 M°+-ZrCl4 (100). Elemental analysis (calc: C 36.3, H 2.3, N 5.3; found: C 37.3, H 2.5, N 5.4). 3.5.3.3. Bis-(benzothiazolyl) benzene zirconium chloride complex 13. 1H NMR: 8.01(d,2H), 7.92–7.90(m,4H), 7.72–7.70(dd,2H), 7.45(t,2H), 7.38(t,2H). 13C NMR: 166.4, 153.4, 136.6, 133.5, 131.6, 131.5, 127.2, 126.3, 123.8, 122.9. Mass spectrum: 577 M°+ (1), 506 M°+-2Cl (3), 470 M°+-3Cl (2), 433 M°+-4Cl (5), 344 M°+-ZrCl4 (100). Elemental analysis (calc: C 41.6, H 2.1, N 4.6; found: C 42.0, H 2.0, N 4.7). 3.5.3.4. Bis-(benzothiazolyl)-4-methylbenzene zirconium chloride complex 16. 1H NMR: 8.00(t,2H), 7.92(t,2H), 7.81(d,1H), 7.72(s,1H), 7.53(d,1H), 7.46(d,2H), 7.38(d,2H), 2.44(s,3H,CH3). 13C NMR: 166.5, 166.4, 153.4, 153.4, 141.6, 136.6, 136.5, 133.4, 132.1, 132.0, 131.6, 130.8, 127.2, 127.1, 126.3, 126.2, 123.8, 123.7, 122.9, 122.8, 21.5. Mass spectrum: 593 M°+ (3), 558 M°+-Cl (2), 522 M°+-2Cl (5), 485 M°+-3Cl (2), 471 M°+-3Cl–CH3 (2), 450 M°+-4Cl (5), 358 M°+-ZrCl4 (100). 3.5.4. Vanadium complexes To an amount of 0.41 g (2.6 mmol) VCl3 in ether was added an amount of 2.6 mmol of the ligand precursor. The reaction mixture was stirred over night at room temperature. The product was filtered and washed several times with ether and pentane, dried under vacuum and weighed. The products were characterized by mass spectroscopy and representative examples were characterized by elemental analysis. 3.5.4.1. Bis(benzothiazolyl) methane vanadium chloride complex 8. Mass spectrum: 439 M°+ (2), 402 M°+-Cl (5), 366 M°+-2Cl (5), 331 M°+-3Cl (10), 282 M°+-VCl3 (100). 3.5.4.2. Bis(benzothiazolyl) ethane vanadium chloride complex 11. Mass spectrum: 454 M°+ (5), 416 M°+-Cl (2), 383 M°+-2Cl (3), 296 M°+-VCl3 (70).

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3.5.4.3. Bis-(benzothiazolyl) benzene vanadium chloride complex 14. Mass spectrum: 501 M°+ (1), 465 M°+-Cl (3), 430 M°+-2Cl (2), 394 M°+-3Cl (1), 344 M°+-VCl3 (100). 3.5.4.4. Bis-(benzothiazolyl)-4-methylbenzene vanadium chloride complex 17. Mass spectrum: 515 M°+ (2), 478 M°+-Cl (1), 464 M°+-Cl–CH3 (3), 443 M°+-2Cl (3), 407 M°+-3Cl (3), 392 M°+-3Cl–CH3 (2), M°+-358 M°+-VCl3 (100). Elemental analysis (calc: C 48.9, H 2.7, N 5.6; found: C 48.6, H 2.8, N 5.2). 3.5.4.5. Bis(benzothiazolyl)pyridine vanadium chloride complex18. Mass spectrum: 501 M°+ (1), 465 M°+-Cl (2), 430 M°+-2Cl(2), 393 M°+-3Cl (2), 345 M°+-VCl3 (60). Elemental analysis (calc: C 45.4, H 2.2, N 8.4; found: C 45.6, H 2.3, N 8.4). 3.6. Polymerization of ethylene in a 1 l Büchi autoclave An amount of 2–5 mg of the desired complex was suspended in 5 ml of toluene. Methylalumoxane (30% in toluene) was added resulting in an immediate color change. The mixture was added to a 1 l Schlenk flask filled with 250 ml n-pentane. This mixture was transferred to a 1 l Büchi laboratory autoclave under inert atmosphere and thermostated. An ethylene pressure of 10 bar was applied for one hour. The polymer was filtered over a frit, washed with diluted hydrochloric acid, water, and acetone, and finally dried in vacuo. 4. Conclusions It is surprising that the bridging unit in the investigated chelating complexes has such a strong influence on the performance of the generated catalysts because this moiety is far away from the active center. Since the methylene unit and the phenylene unit gave the best results, it is likely that the rigidity of the generated catalyst cation and its interaction with the MAO anion are the responsible parameters. The appearance of bimodal or multimodal resins is indicative for more than one active site generated in the activation process. The bulky MAO counter anion could undergo different interactions with the hetero atoms of the catalyst ligands and produce adducts with different symmetries. This is a convincing example that the cocatalyst can play an essential role in the polymerization process. Such mononuclear catalysts are good candidates for single-reactor bimodal or multimodal polyethylene technology. Acknowledgements We thank SABIC company (Riyadh, Saudi Arabia) for GPC measurements and University of Juba (Khartoum, Sudan) for the financial support of H.A.E. for the first year. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]

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