Ethylene Polymerization by Sterically and Electronically Modulated …mslab.polymer.pusan.ac.kr/english/international/2008/159.pdf · 2008. 1. 11. · Ethylene Polymerization by Sterically

Ethylene Polymerization by Sterically and ElectronicallyModulated Ni(II) a-Diimine Complexes

BIJAL KOTTUKKAL BAHULEYAN, GI WAN SON, DAE-WON PARK, CHANG-SIK HA, IL KIM

Division of Chemical Engineering, Pusan National University, Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea

Received 1 March 2007; accepted 7 October 2007DOI: 10.1002/pola.22450Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A series of highly active ethylene polymerization catalysts based onbidendate a-diimine ligands coordinated to nickel are reported. The ligands are pre-pared via the condensation of bulky ortho-substituted anilines bearing remote push–pull substituents with acenaphthenequinone, and the precatalysts are prepared viacoordination of these ligands to (DME)NiBr2 (DME ¼ 1,2-dimethoxyethane) to formcomplexes having general formula [ZN ¼ C(An)-C(An) ¼ NZ]NiBr2 [Z ¼ (4-NH2-3,5-C6H2R2)2CH(4-C6H4Y); An, acenaphthene quinone; R, Me, Et, iPr; Y ¼ H, NO2,OCH3]. When activated with methylaluminoxane (MAO) or common alkyl alumi-niums such as ethyl aluminium sesquichloride (EAS) all catalysts polymerizeethylene with activities exceeding 107 g-PE/ mol-Ni h atm at 30 8C and atmosphericpressure. Among the cocatalysts used EAS records the best activity. Effects of remotesubstituents on ethylene polymerization activity are also investigated. The change inpotential of metal center induced by remote substituents, as evidenced by cyclicvoltammetric measurements, influences the polymerization activity. UV–visible spec-troscopic data have specified the important role of cocatalyst in the stabilization ofnickel-based active species. A tentative interpretation based on the formation ofactive and dormant species has been discussed. The resulting polyethylene was char-acterized by high molecular weight and relatively broad molecular weight distribu-tion, and their microstructure varied with the structure of catalyst and cocatalyst.VVC 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1066–1082, 2008

Keywords: catalysis; cocatalysts; ethylene; ligands; metal-organic catalysts/organo-metallic catalysts; polymerization; polyolefins; transition metal chemistry; Ziegler–Natta polymerization

INTRODUCTION

Rapid advances after Ziegler and Natta’s discov-ery in catalytic olefin polymerization, includingmetallocene revolution, gave an upward thrust tothe polyolefin industry. Even though highly activehomogeneous single site metallocene/methylalu-

minoxane (MAO) system gave luxury of tailoringthe microstructure of polymers, their high oxophi-licity and inability to incorporate polar monomerspaved the way for systems based on late transi-tion metals, which is now flourishing to thebest.1–3 The most commercially advanced cata-lysts of this type are diimine complexes of Pd(II)/Ni(II) and Fe(II)/Co(II) complexes with bis(imi-no)pyridyl ligands.4,5 Among them, nickel cata-lyst attained special interest because of theirtunable polymerization activity and polymermicrostructure by simple modification of theligand architecture. Catalysts devoid of bulky

This article contains supplementary material availablevia the Internet at http://www.interscience.wiley.com/jpages/0887-624X/suppmat.

Correspondence to: I. Kim (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 1066–1082 (2008)VVC 2007 Wiley Periodicals, Inc.

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substituents in ortho aryl positions have beenfound to oligomerize ethylene selectively to a–ole-fins, and hence these substituents were recog-nized to be the key factor for late transition-metalcatalysts to obtain high molecular weight poly-mers.6–8 Degree of polymerization and activitydepend greatly on the substitution pattern of arylgroups, that the bulky substituent effects be-came more obviously apparent. The bulky Ni(II)catalysts have been shown to catalyze a-olefinpolymerization in a living fashion at lowtemperature.9

Mechanistic details of polymerization includingthe role of bulky substituents on diimine ligandshave been thoroughly investigated both experi-mentally and theoretically.10,11 Lowering of elec-tron density on metal center through the additionof ligands with electron-withdrawing groups wasfound to produce more active catalysts.7(a) Effec-tive tuning of electronic environment of metalcenter was also achieved by varying the para andortho substituents.12 Acenaphthene diimine tran-sition metal complexes with olefinically substi-tuted aryl groups, which function as remote sub-stituents, were also found active towards olefinpolymerization/oligomerization.13

Even though various bulky aryl substitutedcatalysts have been studied,1–13 none of the sys-tems gave much information about how the com-bined effect of steric bulkiness along with elec-tronic variation affects ethylene polymerization.Our special interest lies on effective and system-atic analysis of the electronic perturbation causedby various substitutions on the bulky diimineligands and their effect on polymerization behav-iors. We synthesized a series of very bulkydiimine nickel catalysts with remote push–pullsubstituents (Fig. 1) and investigated their per-formance as ethylene polymerization catalystscombined with MAO or common alkyl alumi-niums such as ethyl aluminium sesquichloride(EAS) and triethyl aluminium (TEA). These pre-catalysts are different from the original Broo-khart’s diimine compounds (used as control,circled part in Fig. 1) in that they are even bulkierand they bear electronically modified ligands at aremote position.

EXPERIMENTAL

Materials

All reactions were performed under a purifiednitrogen atmosphere using standard glove box

and Schlenk technique. Polymerization grade ofethylene (SK, Korea) was purified by passing itthrough columns of Fisher RIDOXTM catalystand molecular sieve 5 A/13X. Organic solventswere purified by known procedures and storedover molecular sieves (4 A). All reagents used inthis study were purchased from Aldrich Chemi-cal and used without further purification.

Characterization

1H-NMR, 13C-NMR spectra of ligands wererecorded on a Varian Gemimi-2000 (300 MHz, 75MHz) spectrometer. All chemical shifts werereported in parts per million (ppm) relative to re-sidual CHCl3 (d 7.24) for 1H and CDCl3 (d 77.00)for 13C. 1H and 13C spectra of polyethylene (PE)were taken in C6H4Cl2 at 135 8C on Inova-500(500 MHz, 125 MHz) spectrometer. Analyticalthin layer chromatography was conducted usingMerck 0.25 mm silica gel 60F precoated alumi-num plates with fluorescent indicator UV254.Purification of ligands was carried out with aCombi-Flash (Companion) auto-column machine.Elemental analysis was carried out using VarioEL analyzer and UV–visible spectra wererecorded on a Shimadzu UV-1650PC UV–visiblespectrophotometer. Mass spectra of catalystswere recorded using positive fast atom bombard-ment (FAB) methods on JEOL JMS-AX505WA,HP 5890 Series II spectrometer. Electrochemical

Figure 1. Catalyst precursors under investigationfor ethylene polymerization having general formula[ZN ¼ C(An)-C(An)¼NZ]NiBr2, [Z ¼ (4-NH2-3,5-C6H2R2)2CH(4-C6H4Y); An ¼ acenaphthenequinone;R ¼ iPr; Y ¼ H, NO2, OCH3]. Circled part is Brook-hart’s catalyst used as control. Polymerization condi-tions: toluene solvent ¼ 80 in a 250 mL glass reactor,temperature ¼ 30 8C, catalyst ¼ 2.5 lmol, and atmos-pheric pressure of monomer.

ETHYLENE POLYMERIZATION BY Ni(II) �-DIIMINE COMPLEXES 1067

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

studies were performed using Kosentech ModelCV-104 (S. Korea) and EG & G PAR model273A (USA) Potentiostat/Galvanostat. A three-electrode system used in the entire study con-sisted of a glassy carbon working electrode, anAg/AgCl reference electrode, and a platinumcounter electrode.

Molecular weight and molecular weight dis-tribution of PE were determined by GPC (PL-GPC220/FTIR, 135 8C) in 1,2,4-trichlorobenzeneusing polystyrene columns as standard. Theintrinsic viscosity was measured in decalin at135 8C using an Ubbelohde viscometer and theaverage molecular weight was calculated.14

Thermal analysis of PE was carried out by dif-ferential scanning calorimeter (Perkin-ElmerDSC, model: Pyris 1) at 10 8C/min heating rateunder nitrogen atmosphere. The results of thesecond scan were reported to eliminate the dif-ference in sample history. The branching num-bers for PE were determined by 1H NMR spec-troscopy using the ratio of number of methylgroups to overall number of carbons and werereported as branches per thousand carbons.15

Polymerization

Ethylene polymerizations were performed in a250-mL round-bottom flask equipped with amagnetic stirrer and a thermometer outside.High dilution techniques were adopted toreduce the monomer mass transport effect. Af-ter adding the catalyst, reactor was chargedwith toluene (80 mL) by using syringe andimmersed in a constant temperature bath pre-viously set to desired temperature. The mostreproducible reaction temperature for runs con-ducted at 30 8C was maintained. When reactorwas equilibrated with bath temperature, ethyl-ene was introduced into the reactor afterremoving nitrogen gas under vacuum. When nomore absorption of ethylene into toluene wasobserved, required amount of cocatalyst wasinjected into reactor and then polymerizationwas started. Polymerization rate was deter-mined at every 0.01 s from the rate of consump-tion, measured by a hotwire flow meter (model5850 D from Brooks Instrument Div.) connectedto a personal computer through an A/D con-verter. Polymerization was quenched by theaddition of methanol containing HCl (5 v/v %)and then unreacted monomer was vented. Thepolymer was washed with an excess amount ofmethanol and dried in vacuum at 50 8C. To

make a worthy comparison of the effect of cata-lyst structure on catalytic activity and polymerstructure and properties, all data were collectedunder similar conditions.

Synthesis of Primary Ligands (1a-i)

General procedure: As summarized in Figure 2,all ligand precursors were prepared by similarprotocol based on the modification of reportedprocedure.16 As an example, 4,40-((4-nitro-phenyl)methylene)bis(2,6-diisopropylaniline) wasprepared by reacting 2,6-diisopropylaniline withp-nirobenzaldehyde. 2,6-Diisopropylaniline washeated to 130 8C under inert atmosphere towhich p-nitrobenzaldehyde dissolved in 3 mLconc. HCl and 40 mL THF (for up to 25 mmolbatch; otherwise mentioned) were added over aperiod of 1 h. The reaction mixture was refluxedfor 24 h. After cooling to room temperature, itwas basified with aq. NaOH and extracted withCHCl3. The combined organic layers werewashed with water and brine, respectively. Thesolution was dried over MgSO4 and added pen-tane to precipitate the product. It was purifiedby recrystallization twice in CHCl3 using pen-tane to afford the product as pure crystals.

4,40-(phenylmethylene)bis(2,6-dimethylaniline) (1a)

Light blue 1a crystals were obtained by reacting2,6–dimethylaniline (2.72 mL, 22 mmol) withbenzaldehyde (1.02 g, 10 mmol) in 61% (2.03 g)yield.

1H-NMR (300 MHz, CDCl3) d 2.11 (s, 12H,CH3

methyl), 3.47 (s, 4H, NH2), 5.26 (s, 1H, CH),6.69 (s, 4H, Harom), 7.12–7.24 (m, 5H, Harom);

13C-NMR (75 MHz, CDCl3) d 17.92, 55.66, 121.62,125.77, 128.20, 129.33, 129.38, 134.08, 140.81,145.06. Anal. Calcd for C23H26N2: C, 83.59; H,7.93; N, 8.48. Found: C, 83.55; H, 7.90; N, 8.50.

4,40-(phenylmethylene)bis(2,6-diethylaniline) (1b)

Blue 1b solid was obtained by reacting 2,6–diethylaniline (3.42 mL, 22 mmol) with benzal-dehyde (1.02 g, 10 mmol) in 60% (2.34 g) yield.

1H-NMR (300 MHz, CDCl3) d 1.15 (t, 12H,CH3

ethyl), 2.45 (quart, 8H, CH2ethyl), 3.50 (s, 4H,

NH2), 5.31 (s, 1H, CH), 6.75 (s, 4H, Harom),7.14–7.24 (m, 5H, Harom);

13C-NMR (75 MHz,CDCl3) d 13.45, 24.68, 55.23, 122.58, 125.83,127.96, 128.08, 129.40, 134.58, 140.65, 146.38.

1068 BAHULEYAN ET AL.


Anal. Calcd for C27H34N2: C, 83.89; H, 8.87; N,7.25. Found: C, 83.81; H, 8.92; N, 7.28.

4,40-(phenylmethylene)bis(2,6-diisopropylaniline) (1c)

Dark blue 1c crystals were obtained by reacting2,6–diisopropylaniline (4.15 mL, 22 mmol) withbenzaldehyde (1.02 g, 10 mmol) in 60% (2.67 g)yield.


ipr), 2.88 (sep, 4H, CHipr), 3.61 (s, 4H, NH2),5.32 (s, 1H, CH), 6.78 (s, 4H, Harom), 7.10–7.22(m, 5H, Harom);

13C-NMR (75 MHz, CDCl3) d22.24, 28.15, 55.63, 124.04, 125.58, 127.95,129.73, 132.28, 135.75, 138.42, 146.00. Anal.

Calcd for C31H42N2: C, 84.11; H, 9.56; N, 6.33.Found: C, 84.10; H, 9.58; N, 6.31.

4,40-((4-nitrophenyl)methylene)bis(2,6-dimethylaniline) (1d)

Yellowish 1d powder was obtained by reacting25 mmol (3.09 g) of 2,6-dimethylaniline with12.5 mmol (1.89 g) of p-nitrobenzaldehyde in57% (2.67 g) yield.


methyl), 3.47 (s, 4H, NH2), 5.35 (s, 1H, CH), 6.69(s, 4H, Harom), 7.12–7.24 (m, 4H, Harom);

13C-NMR(75 MHz, CDCl3) d 17.92, 55.90, 121.62, 125.77,128.20, 129.38, 134.08, 140.81, 145.06, 155.94.Anal. Calcd for C23H25N3O2: C, 73.57; H, 6.71; N,11.19. Found: C, 73.55; H, 6.73; N, 11.18.

Figure 2. Synthesis of ligands and catalysts: (i) HCl, THF; (ii) Acenaphthenequi-none, MeOH;/Hþ (iii) (DME)NiBr2, CH2Cl2.



4,40-((4-nitrophenyl)methylene)bis(2,6-diethylaniline) (1e)

Brownish yellow 1e needles were obtained byreacting 25 mmol (3.89 g) of 2,6-diethylanilinewith 12.5 mmol (1.89 g) of p-nitrobenzaldehydein 58% (3.15 g) yield.



NH2), 5.37 (s, 1H, CH), 6.75 (s, 4H, Harom),7.14–7.24 (m, 4H, Harom);

13C-NMR (75 MHz,CDCl3) d 13.45, 24.68, 56.27, 125.58, 125.83,129.40, 132.36, 134.58, 141.12, 146.38, 156.12.Anal. Calcd for C27H33N3O2 C, 75.14; H, 7.71;N, 9.74. Found: C, 75.15; H, 7.68; N, 9.75.

4,40-((4-nitrophenyl)methylene)bis(2,6-diisopropylaniline) (1f)

Brownish orange 1f crystals were obtained byreacting 50 mmol (9.44 g) of 2,6-diisopropylani-line with 25 mmol (3.78 g) of p-nitrobenzalde-hyde (6 mL conc. HCl and 60 mL THF weretaken) in 54% (6.61 g) yield.


ipr), 2.88 (sept, 4H, CHipr), 3.61 (s, 4H,NH2), 5.38 (s, 1H, CH), 6.78 (s, 4Harom), 7.10–7.22 (m, 4H, Harom);

13C-NMR (75 MHz, CDCl3)d 22.24, 28.15, 56.69, 124.04, 125.58, 129.73,132.28, 132.54, 140.12, 146.00, 155.34. Anal.Calcd for C30H39N3O2 C, 76.07; H, 8.30; N, 8.87.Found: C, 76.05; H, 8.33; N, 8.85.

4,40-((4-methoxyphenyl)methylene)bis(2,6-dimethylaniline) (1g)

Creamy white 1g powder was obtained by react-ing 25 mmol (3.09 g) of 2,6-dimethylaniline with12.5 mmol (1.7 g) of p-methoxybenzaldehyde in67% (3.03 g) yield.


methyl), 3.47 (s, 4H, NH2), 3.52 (s, 3H,OCH3), 5.21 (s, 1H, CH), 6.69 (s, 4Harom), 7.12–7.24 (m, 4H, Harom);

13C-NMR (75 MHz, CDCl3)d 17.20, 54.59, 61.74, 114.97, 122.50, 129.16,130.45, 131.12, 140.00, 141.40, 158.90. Anal.Calcd for C24H28N2O C, 79.96; H, 7.83; N, 7.77.Found: C, 79.99; H, 7.80; N, 7.75.

4,40-((4-methoxyphenyl)methylene)bis(2,6-diethylaniline) (1h)

Creamy white 1h powder was obtained by react-ing 25 mmol (3.89 g) of 2,6-diethylaniline with12.5 mmol (1.7 g) of p-methoxybenzaldehyde in65% (3.40 g) yield.



NH2), 3.61 (s, 3H, OCH3), 5.25 (s, 1H, CH), 6.75(s, 4H, Harom), 7.14–7.24 (m, 4H, Harom);

13C-NMR (75 MHz, CDCl3) d 13.15, 24.22, 54.59,61.74, 114.96, 118.95, 122.50, 137.10, 140.00,141.40, 146.10, 158.90. Anal. Calcd for C28H36

N2O C, 80.73; H, 8.71; N, 6.72. Found: C, 80.69;H, 8.75; N, 6.70.

4,40-((4-methoxyphenyl)methylene)bis(2,6-diisopropylaniline) (1i)

Creamy white 1i powder was obtained by react-ing 50 mmol (9.44 g) of 2,6-diisopropylanilinewith 25 mmol (3.40 g) of p-methoxybenzalde-hyde (6 mL conc. HCl and 60 mL THF weretaken) in 61% (7.20 g) yield.


ipr), 2.85 (sept, 4H, CH2ipr), 3.61 (s, 4H,

NH2), 3.8 (s, 3H, OCH3), 5.25 (s, 1H, CH), 6.78(s, 4H, Harom), 7.10–7.22 (m, 4H, Harom);

13C-NMR (300 MHz, CDCl3) d 23.84, 27.57, 54.60,61.68, 114.96, 127.3, 129.10, 131.45, 139.14,140.10, 151.65, 158.95. Anal. Calcd forC32H44N2O C, 81.31; H, 9.38; N, 5.93. Found: C,81.33; H, 9.34; N, 5.95.

Synthesis of Ligands (2a-i)

General procedure (Fig. 2): To a methanol solutionof the corresponding ligand precursors (1a-i), wasadded acenaphthenequinone (ANQ) in 2:1 ratioand a catalytic amount of formic acid. The reactionmixture was stirred overnight at room tempera-ture. For nitro-substituted ligand synthesis themixture was stirred overnight at 50 8C. After thereaction, methanol was removed under vacuumand the resulting product was purified via columnchromatography [ethyl acetate/n-hexane-silicagel]. The solvent was evaporated to get the productand it is then dried at 50 8C under vacuum.

N,N0-(acenaphthylene-1,2-diylidene)bis(4-((4-amino-3,5-dimethylphenyl)(phenyl)methyl)-2,6-dimethylaniline) (2a)

Orange 2a powder was obtained by reactingANQ (0.54 g, 3 mmol) with 1a (1.98 g, 6 mmol)in 63% (1.53 g) yield.

1H-NMR (300 MHz, CDCl3) d 2.11–2.39 (ds,24H, CH3

methyl), 3.57 (s, 4H, NH2), 5.53 (s, 2H,CH), 6.81–7.02 (8H, Harom), 7.22–7.39 (10H,Harom),7.61–7.91 (6H, Harom);

13C-NMR (75



MHz, CDCl3) d 18.20, 18.98, 55.97, 114.34,121.93, 122.89, 124.73, 126.00, 129.56, 130.11,130.34, 131.26, 132.11, 132.46, 133.71, 138.23,139.48, 141.50, 141.63, 143.11, 150.77, 161.12.Anal. Calcd for C58H54N4: C, 86.31; H, 6.75; N,6.94. Found: C, 86.32; H, 6.72; N, 6.94.

N,N0-(acenaphthylene-1,2-diylidene)bis(4-((4-amino-3,5-diethylphenyl)(phenyl)methyl)-2,6-diethylaniline) (2b)

Orange 2b solid was obtained by reacting 6mmol (2.32 g) of 1b and 3 mmol (0.54 g) of ANQin 62% (1.7 g) yield.

1H-NMR (300 MHz, CDCl3) d 1.10–1.49 (dt,24H, CH3

ethyl), 2.52 (m, 16H, CH2ethyl), 3.61 (s,

4H, NH2), 5.57 (s, 2H, CH), 6.81–7.02 (8H,Harom), 7.22–7.39 (10H, Harom), 7.81–7.93 (6H,Harom);

13C-NMR (75 MHz, CDCl3) d 13.28,14.21, 24.56, 25.05, 56.48, 114.27, 122.31,123.47, 127.27, 128.93, 130.12, 130.34, 131.33,132.36, 132.56, 133.78, 134.67, 138.77, 139.73,140.26, 140.67, 145.65, 149.97, 161.15. Anal.Calcd for C66H70N4: C, 86.23; H, 7.68; N, 6.09.Found: C, 86.26; H, 7.70; N, 6.03.

N,N0-(acenaphthylene-1,2-diylidene)bis(4-((4-amino-3,5-diisopropylphenyl)(phenyl)methyl)-2,6-diisopropylaniline) (2c)

Orange 2c solid was obtained by reacting 6mmol (2.66 g) of 1c with 3 mmol (0.54 g) ofANQ in 60% (1.85 g) yield.

1H-NMR (300 MHz, CDCl3) d 0.95-1.30 (dd,48H, CH3

ipr), 2.98 (m, 8H, CHipr), 3.69 (s, 4H,NH2), 5.60 (s, 2H, CH), 6.81–7.02 (8H, Harom),7.22–7.39 (10H, Harom), 7.51–7.91 (6H, Harom);13C-NMR (75 MHz, CDCl3) d 22.75, 23.91, 28.77,29.51 56.92, 114.76, 123.57, 124.78, 125.94,127.54, 128.12, 128.54, 129.89, 130.34, 131.69,132.15, 134.49, 135.97, 138.54, 140.16, 141.75,145.18, 149.73, 160.92. Anal. Calcd for C74H86

N4: C, 86.16; H, 8.41; N, 5.43. Found: C, 86.15;H, 8.43; N, 5.41.

N,N0-(acenaphthylene-1,2-diylidene)bis(4-((4-amino-3,5-dimethylphenyl)(4-nitrophenyl)-methyl)-2,6-dimethylaniline) (2d)

Orange 2d powder was obtained by reacting6 mmol (2.07 g) of 1d with 3 mmol (0.54 g) ofANQ 57% (1.52 g) yield.

1H-NMR (300 MHz, CDCl3) d 2.95-3.25 (ds,24H, CH3

methyl), 4.35 (s, 4H, NH2), 6.05 (s, 2H,

CH), 7.35–7.42 (8H, Harom), 7.62–7.78 (10H,Harom), 8.62–8.82 (6H, Harom);

13C-NMR (75MHz, CDCl3) d 21.31, 21.38, 58.10, 115.62,123.25, 124.70, 124.78, 126.65, 127.98, 128.38,129.57, 130.65, 131.10, 131.56, 132.62, 137.38,138.30, 139.65, 141.15, 146.50, 159.25, 163.60.Anal. Calcd for C58H52N6O4 C, 77.65; H, 5.84;N, 9.37. Found: C, 77.64; H, 5.86; N, 9.36.

N,N0-(acenaphthylene-1,2-diylidene)bis(4-((4-amino-3,5-diethylphenyl)(4-nitrophenyl)-methyl)-2,6-diethylaniline) (2e)

Orange 2e solid was obtained by reacting 6mmol (2.58 g) of 1e with 3 mmol (0.54 g) ofANQ in 55% (1.65 g) yield.

1H-NMR (300 MHz, CDCl3) d 1.22–1.53 (dt,24H, CH3


4H, NH2), 5.75 (s, 2H, CH), 6.61–7.02 (8H,Harom), 7.32–7.69 (10H, Harom), 8.11–8.25 (6H,Harom);

13C-NMR (75 MHz, CDCl3) d 17.81, 17.86,27.35, 27.21, 60.52, 115.43, 124.03, 124.94,125.35, 127.89, 128.45, 128.91, 129.63, 130.49,131.20, 131.74, 132.71, 135.06, 137.44, 138.50,140.15, 145.95, 158.85, 163.95. Anal. Calcd forC66H68N6O4 C, 78.54; H, 6.79; N, 8.33. Found: C,78.52; H, 6.80; N, 8.31

N,N0-(acenaphthylene-1,2-diylidene)bis(4-((4-amino-3,5-diisopropylphenyl)(4-nitrophenyl)-methyl)-2,6-diisopropylaniline) (2f)

Orange 2f solid was obtained by reacting 6mmol (2.93 g) of 1f and 3 mmol (0.54 g) of ANQin 51% (1.7 g) yield.


ipr), 2.98 (m, 8H, CHipr), 3.69 (s, 4H,NH2), 5.80 (s, 2H, CH), 6.81–7.02 (8H, Harom),7.22–7.39 (10H, Harom), 7.71–7.91 (6H, Harom);13C-NMR (75 MHz, CDCl3) d 26.65, 26.68, 32.46,32.52, 60.11, 114.11, 122.42, 124.21, 124.82,125.95, 127.36, 128.75, 129.65, 130.75, 131.12,131.92, 132.85, 135.76, 137.35, 138.95, 141.45,146.13, 159.65, 163.15. Anal. Calcd for C74H84

N6O4: C, 79.25; H, 7.55; N, 7.49. Found: C,79.27; H, 7.53; N, 7.48.

N,N0-(acenaphthylene-1,2-diylidene)bis(4-((4-amino-3,5-dimethylphenyl)(4-methoxy-phenyl)-methyl)-2,6-dimethylaniline) (2g)

Orange 2g powder was obtained by reacting6 mmol (2.16 g) of 1g with 3 mmol (0.54 g) ofANQ in 70% (1.80 g) yield.



1H-NMR (300 MHz, CDCl3) d 2.00–2.33 (ds,24H, CH3

methyl), 3.55 (s, 4H, NH2), 3.95 (s, 6H,OCH3), 5.32 (s, 2H, CH), 6.41–7.12 (8H, Harom),7.22–7.42 (10H, Harom), 7.85–7.95 (6H, Harom);13C-NMR (300 MHz, CDCl3) d 19.10, 19.04,55.14, 55.10, 113.42, 121.15, 123.30, 124.25,124.85, 126.67, 128.10, 129.54, 130.67, 131.82,131.62, 132.21, 134.00, 137.25, 138.56, 140.53,147.50, 158.20, 161.70. Anal. Calcd for C60H58

N4O2 C, 83.11; H, 6.74; N, 6.46. Found: C, 83.09;H, 6.75; N, 6.46.

N,N0-(acenaphthylene-1,2-diylidene)bis(4-((4-amino-3,5-diethylphenyl)(4-methoxyphenyl)-methyl)-2,6-diethylaniline) (2h)

Orange 2h solid was obtained by reacting6 mmol (2.5 g) of 1h with 3 mmol (0.54 g) ofANQ in 67% (1.95 g) yield.

1H-NMR (300 MHz, CDCl3) d 1.08-1.36 (dt,24H, CH3


4H, NH2), 4.92 (s, 6H, OCH3), 5.35 (s, 2H, CH),6.61–7.21 (8H, Harom), 7.22–7.49 (10H, Harom),7.72–7.92 (6H, Harom);

13C-NMR (300 MHz,CDCl3) d 14.10, 14.34, 25.10, 25.21, 55.05, 55.21,113.71, 123.62, 124.76, 127.10, 129.18, 129.66,130.05, 130.27, 131.37, 131.66, 132.21, 132.56,135.21, 137.51, 139.75, 141.12, 147.20, 158.11,161.51. Anal. Calcd for C68H74N4O2 C, 83.40; H,7.62; N, 5.72. Found: C, 83.38; H, 7.61; N, 5.74.

N,N0-(acenaphthylene-1,2-diylidene)bis(4-((4-amino-3,5-diisopropylphenyl)(4-methoxy-phenyl)-methyl)-2,6-diisopropylaniline) (2i)

Orange 2i powder was obtained by reacting 8mmol (3.42 g) of 1i and 4 mmol (0.73 g) of ANQin 66% (2.90 g) yield.


ipr), 3.00 (m, 8H, CHipr), 3.72 (s, 4H,NH2), 3.95 (s, 6H, OCH3), 5.40 (s, 2H, CH),6.71–7.32 (8H, Harom), 7.32–7.53 (10H, Harom),7.75–7.95 (4H, Harom);

13C-NMR (300 MHz,CDCl3) d 23.21, 23.25 29.10, 29.16, 55.28, 55.56,113.72, 123.71, 124.22, 125.11, 128.00, 129.12,129.86, 130.01, 130.21, 131.32, 131.76, 132.22,132.76, 135.31, 138.41, 140.90, 145.83, 158.11,162.15. Anal. Calcd for C76H90N4O2: C, 83.63; H,8.31; N, 5.13. Found: C, 83.64; H, 8.29; N, 5.12.

Synthesis of Catalysts (3a-i)

General procedure (Fig. 2): To a CH2Cl2 (20 mL)solution of ligand was added 1:1 equivalent of

(DME)NiBr2 (DME ¼ 1,2-dimethoxyethane) andstirred overnight at 30 8C. As the reaction pro-ceeded, (DME)NiBr2 was completely dissolved.The resulting product was precipitated and thentitrated in ether. The final product was driedunder vacuum at 60 8C after filtering and wash-ing. In this way we could get brown colored pre-catalysts in high yield above 90%. Since thesecomplexes are paramagnetic, a high resolutionNMR spectroscopic analysis was not feasible.

3a. R ¼ Methyl, Y ¼ H: Anal. Calcd forC58H54N4 Br2Ni: C, 67.92; H, 5.31; N, 5.46.Found: C, 67.81; H, 5.40; N, 5.41. MS (FABþ):m/z ¼ 945 (Mþ�Br).

3b. R ¼ Ethyl. Y ¼ H: Anal. Calcd forC66H70N4 Br2Ni: C, 69.67; H, 6.20; N, 4.92.Found: C, 69.74; H, 6.32; N, 4.85. MS (FABþ):m/z ¼ 1057 (Mþ�Br).

3c. R ¼ isopropyl. Y ¼ H: Anal. Calcd forC74H86N4 Br2Ni: C, 71.10; H, 6.93; N, 4.48.Found: C, 71.15; H, 6.87; N, 4.54. MS (FABþ):m/z ¼ 1170 (Mþ�Br).

3d. R ¼ Methyl, Y ¼ NO2: Anal. Calcd forC58H52Br2N6NiO4: C, 62.90; H, 5.10; N, 7.34.Found: C, 62.82; H, 5.15; N, 7.36. MS (FABþ):m/z ¼ 1035 (Mþ�Br).

3e. R ¼ Ethyl, Y ¼ NO2: Anal. Calcd forC66H68Br2N6NiO4: C, 64.93; H, 5.93; N, 6.68.Found: C, 64.89; H, 5.87; N, 6.71. MS (FABþ):m/z ¼ 1147 (Mþ�Br).

3f. R ¼ isopropyl, Y ¼ NO2: Anal. Calcd forC74H84Br2N6NiO4: C, 66.63; H, 6.62; N, 6.13.Found: C, 66.71; H, 6.56; N, 6.11. MS (FABþ):m/z ¼ 1260 (Mþ�Br).

3g. R ¼ methyl, Y ¼ OCH3: Anal. Calcd forC60H58Br2N4NiO2: C, 66.74; H, 5.78; N, 5.02.Found: C, 66.81; H, 5.71; N, 4.99. MS (FABþ):m/z ¼ 1005 (Mþ�Br).

3h. R ¼ Ethyl, Y ¼ OCH3: Anal. Calcd forC68H74Br2N4NiO2: C, 68.47; H, 6.57; N, 4.56.Found: C, 68.40; H, 6.62; N, 4.54. MS (FABþ):m/z ¼ 1117 (Mþ�Br).

3i. R ¼ isopropyl, Y ¼ OCH3: Anal. Calcd forC76H90Br2N4NiO2: C, 69.91; H, 7.22; N, 4.18.Found: C, 69.98; H, 7.15; N, 4.15. MS (FABþ):m/z ¼ 1230 (Mþ�Br).

RESULTS AND DISCUSSION

Catalyst Preparation and Characterization

The easily varied steric and electronic propertiesof a-diimine ligands are an important feature of



the nickel a-diimine catalyst systems.1 Their syn-thesis involves the condensation of a diketonewith 2 equiv of an alkyl- or arylamine. A series ofa-diimine ligands with sterically and electroni-cally modified substituents could be successfullyprepared by reacting suitably designed bulky an-iline with ANQ in high yield (Fig. 2). The stericbulkiness of ligand was controlled by modifyingalkyl groups in aniline and the electronic prop-erty was controlled by changing hydride group tonitro or alkoxy group in a remote position. Suita-ble crystals of primary ligands (1a-i) for X-raystudies were able to grown. Figure 3 shows astructure of nitro substituted 1f crystal as a rep-resentative example. When 2(g-i) was preparedat normal conditions, high temperature (50 8C)was required for 2(d-f) ligands.

Effect of these push–pull substituents wasprima-facie suspicious since they exist inextreme position from the metal center. But pre-liminary observations gave satisfactory indica-tion about the remote effect. 13C NMR signalsfor a tertiary carbon atom at 55.56, 56.92, and60.11 ppm for 2i, 2c, and 2f ligands, respec-tively, indicate the relevance of these remotepush–pull substituents on ligand system. Theelectron withdrawing NO2 group shifts thispeak downfield whereas the electron releasing -OCH3 group shifts it upfield. In addition Ni(II)

a-diimine complexes could be obtained withalmost stoichiometric yield by reacting theresulting ligands with (DME)NiBr2. Even if wefailed to obtain suitable crystals for X-ray stud-ies, elemental analysis and FAB mass spectrom-etry analysis showed all Ni(II) a-diimine com-plexes were successfully synthesized.

These remote substituents (Y ¼ H, NO2, OCH3

in Fig. 1) may change the orientation of catalyststructure and hence the potential of metal centerwhich severely affects polymerization activity,that has been proved by cyclic voltammetricmeasurements. Electrochemical characterizationof nickel complexes having different substituentgroups was carried out in acetone containingBu4NClO4 (0.1 M) as a supporting electrolyte.17

Kandaswamy and Krishnapriya17(a) have shownthat cyclic voltammograms of mononuclear com-plexes at negative potential involved a singleelectron transfer. In the presence of cocatalysts,immediate precipitation and high adsorption atthe electrode surface occurred on applying thepotential, which made it difficult to interpret thecyclic voltammogram. Oxidation potentials fornickel complexes having ��H, ��OCH3, and��NO2 substituent groups were determined to be0.835, 0.840, and 0.768 V, respectively versus Ag/AgCl reference (Fig. 4). The shift in cyclic volt-ammograms (curve a,b,c in Fig. 4) is due to aperturbation on the nickel centre caused byremote substituents. This result strongly sup-ports the influence of these remote substituentson nickel center and hence the polymerization ac-tivity. The separation between redox peaks for allcomplexes was found to be larger than the theo-retical values for a reversible electron transfer,indicating that electron transfer of nickel com-plexes is quasi-reversible.17(e)

Molecular models of all complexes were builtby using hyperchem software. Geometry optimi-zation and molecular mechanics (MM) calcula-tions were performed to mimic the structure ofthe complexes. It has been reported that macro-cyclic ligands will enhance the coordination sta-bility for metal complexes and the rigid framework of ligands prohibits free rotation of arylnitrogen bonds which allow the catalyst to makehigh molecular weight polymers at elevatedtemperature.18 Considering these facts and toeliminate the interactions of free NH2 groupduring polymerization, we tried to cyclyze ourligands by reacting it with dihaloalkanes con-taining various alkyl chain lengths likeX(CH2)nX where n ¼ 2–5 and X ¼ Br, Cl.

Figure 3. X-ray crystal structure of the primaryligand, 4,40-((4-nitrophenyl)methylene) bis(2,6-diiso-propylaniline) (1f). Hydrogen atoms omitted forclarity. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com.]



Unfortunately all cyclization methods wereunsuccessful. Since these ligands are having twochiral centers, in Figure 1, different orientations(RR, SR, SS configurations) have considered formolecular mechanics calculation and the modelsobtained were shown in Figure 5. From geome-try optimization we found that SR configura-tions were having the least energy values for allsystems when compared with RR and SS whichstates that two amino groups, indicated by thetwo nitrogen atoms N1 and N2 in Figure 5,were very far from each other which made themacrocycle synthesis impossible. The detailedcalculations are given in the Table 1. These chi-ral centers and rotational flexibility of theentire system gave an unfavorable situation toform suitable single crystals for X-ray analysis.Different parameters like surface area (A), Vol-ume (V), polarizability (P), Energy (E),N��Ni��N angle of our catalysts were comparedwith Brookhart’s system (control). For catalysts3c (Y ¼ H), 3f (Y ¼ NO2) and 3i (Y ¼ OCH3)the N��Ni��N angle was found to be 89.68,89.58 and 89.63 degree respectively when com-pared with control (89.87 degree). It is clearthat the N��Ni��N angle was greatly affectedby ligand bulkiness and remote substituents.

The decrease in N��Ni��N bong angle indicatesthat sterically crowded aryl groups are pointingtowards the metal centre rather than outwardswhich seriously affected the polymerizationbehavior.

Effect of the Modification of Ligand Substituents onEthylene Polymerization

The Ni(II) a-diimine catalysts was rejuvenatedwhen Brookhart and coworkers reported thatthey were highly active catalysts for the poly-merization of ethylene and a-olefins.1(a) Themain features of original a-diimine polymeriza-tion catalysts (circled part in Fig. 1) are, highlyelectrophilic metal centers and sterically bulkya-diimine ligands.1 The electrophilicity of themetal center results in rapid rates of olefininsertion, which can be modified by attachingpush–pull substituents on ligands. The use ofbulky ligands favors insertion over chain trans-fer. Accordingly, reduction in steric bulk greatlyaffects the molecular weight of PE. When thesteric effects are reduced significantly, theincreased chain-transfer rates lead to the pro-duction of linear a-olefins instead of high molec-ular weight polymer.1(a)

Table 2 summarizes the results of ethylenepolymerizations carried out at atmosphericpressure of ethylene. Catalytic activity waschanged by modifying the ortho substituents ofN-aryl rings from methyl to isopropyl groups. Aconsiderable drop in the activity of complexesbearing isopropyl groups was observed whencompared to complexes bearing methyl groups.For example, catalyst 3d (Entry 7) bearingortho methyl substituents on the N-aryl ringshowed an activity (as average rate of polymer-ization (Rp,avg) for 30 min) of 3.30 3 108 g-PE/mol Ni h atm at 30 8C combined with 300equivalents of MAO, catalyst 3e bearing ethylsubstituents showed an activity of 2.54 3 108 g-PE/ mol Ni h atm, and isopropyl groups substi-tuted 3f showed an activity of 2.43 3 108 g-PE/mol Ni h atm at the same conditions. Similarresults were observed with other systems, 3(a-i), also. At normal temperature deactivation ofthe catalyst was comparatively small, which isclearly understood from polymerization activitycurves in Figures 6 and 7. The structure ofNi(II) a-diimine complexes of this study is simi-lar to that of original Brookhart’s complexes inthat the steric bulk around the metal center,

Figure 4. Cyclic voltammograms of various catalystprecursors having general formula: [ZN ¼ C(An)-C(An) ¼ NZ]NiBr2, Z ¼ (4-NH2-3,5-C6H2R2)2CH(4-C6H4Y); An, acenaphthenequinone; R, iPr; and (a) Y¼ NO2 (3f), (b) Y ¼ OCH3 (3i) and (c) Y ¼ H (3c) inacetone at 25 8C. Scan rate is 50 mV/s.



which is key to retarding chain transfer toobtain high molecular weight polymer,1 andthey are different from Brookhart’s complexes

in that they bear two more aryl rings. One ofthe aryl rings was utilized for the electronicmodification of metal center and the other aryl

Table 1. Optimized Parameters [Surface area (A), Volume (V), Polarizability (P), Energy (E), N��Ni��N BondAngle] for RR, SR, and SS Configuration of Catalysts 3c (Y ¼ H), 3f (Y ¼ NO2), and 3i (Y ¼ OMe), respectively

Catalyst Config. Mass (amu) A (A2) V (A)3 P (A)3 E (kJ/mol) N��Ni��N (degree)

Control 719.22 611.8 1478.7 68.9 46.72 89.873c RR 1250.04 1222.2 2930.0 135.9 76.04 89.84

SR 1203.6 2902.4 75.34 89.68SS 1210.1 2913.5 75.48 89.69

3f RR 1340.04 1347.7 3057.1 139.6 81.83 89.71SR 1353.1 3058.6 74.92 89.58SS 1358.2 3055.5 75.36 89.31

3i RR 1310.09 1345.5 3079.4 140.8 85.50 89.87SR 1354.5 3085.7 77.61 89.63SS 1342.1 3075.4 85.72 89.87

Figure 5. Molecular models for different configurations (R and S) of catalysts pre-cursors having general formula: [ZN ¼ C(An)-C(An) ¼ NZ]NiBr2 [Z ¼ (4-NH2-3,5-C6H2R2)2CH(4-C6H4Y); An, acenaphthenequinone; R, iPr]. (a) RR of 3c (Y ¼ H) (b)SS of 3c (Y ¼ H), (c) SR of 3c (Y ¼ H), (d) SR of 3f (Y ¼ NO2), (e) SR of 3i (Y ¼OCH3). Hydrogen atoms omitted for clarity. N1 and N2 are two nitrogen atoms inamino groups.



Table 2. Ethylene Polymerization Results Carried Out at Atmospheric Pressure of Ethylene in a 250 mL GlassReactor Containing 80 mL of Toluene Solvent

EntryCatalyst (lmol)/cocatalyst (Al/Ni) Tp (8C) Activitya Mn

b (10�3) PDIb Mvc (10�3) Tm

d (8C) Branches/1000 Ce

1 3a (7.5)/MAO (500) 10 93.39 52.7 3.75 n.d.f 126 202 3a (7.5)/MAO (500) 30 100.06 26.1 3.58 n.d. 124 443 3b (7.5)/MAO (500) 30 61.97 n.d. n.d. 179.7 123 454 3c (7.5)/MAO (500) 30 43.28 48.9 2.99 n.d. 118 965 3c (2.5)/MAO (300) 30 22.06 59.0 2.62 n.d. 120 536 3c (2.5)/EAS (300) 30 91.16 56.9 2.73 n.d. 117g 1037 3d (2.5)/MAO (300) 30 33.01 n.d. n.d. 145.0 126 38 3e (2.5)/MAO (300) 30 25.39 n.d. n.d. 170.0 122 129 3f (2.5)/MAO (300) 10 27.36 140.5 3.68 n.d. 119 34

10 3f (2.5)/MAO (300) 30 24.25 54.6 4.63 n.d. 115 8111 3f (2.5)/MAO (300) 50 2.60 n.d. n.d. 98.0 107 8912 3f (2.5)/EAS (300) 30 98.36 52.2 3.57 n.d. 119g 13013 3i (2.5)/EAS (300) 30 78.68 49.4 4.08 n.d. 118g 9914 3i (2.5)/MAO (300) 30 50.07 48.4 5.70 n.d. 118 6315 3c (2.5)/TEA (300) 30 2.01 n.d. n.d. n.d. n.d. n.d.16 Control/MAO (300) 30 33.16 158h 2h 11h 88g

a Average rate of polymerization as 107 g-PE/mol Ni h atm.b Determined by GPC.c Determined by viscometry (Ubbelohde viscometer).d Determined by DSC (8C).e Branches per 1000 carbon atoms determined by 1H NMR.f Not determined.g Broad melting curves were obtained.h Average values from refs. 4(a) and 29.

Figure 6. Rate of polymerization (Rp) versus timeplot of ethylene polymerization catalyzed by variouscatalyst precursors having general formula: [ZN ¼C(An)-C(An) ¼ NZ]NiBr2, Z ¼ (4-NH2-3,5-C6H2R2)2CH(4-C6H4Y); An, acenaphthenequinone; R, iPr and(a) Y ¼ NO2 (3f), (b) Y ¼ H (3c), (c) Y ¼ OCH3 (3i),and (d) Control with EAS (Al/Ni ¼ 300). Polymeriza-tion conditions: toluene solvent ¼ 80 mL in a 250 mLglass reactor, temperature ¼ 30 8C, catalyst ¼ 2.5lmol, and atmospheric pressure of monomer.

Figure 7. Rp versus time plot of ethylene polymer-ization catalyzed by various catalyst precursors hav-ing general formula: [ZN ¼ C(An)-C(An) ¼ NZ]NiBr2,Z ¼ (4-NH2-3,5-C6H2R2)2CH(4-C6H4Y); An, acenaph-thenequinone; R, iPr and (a) Y ¼ OCH3 (3i), (b) Con-trol (c) Y ¼ H (3c) and (d) Y ¼ NO2 (3f) with MAO(Al/Ni ¼ 300). Polymerization conditions: toluene sol-vent ¼ 80 mL in a 250 mL glass reactor, temperature¼ 30 8C, catalyst ¼ 2.5 lmol, and atmospheric pres-sure of monomer.



ring gave extra hindrance around the metalcenter.

When effect of ortho substituents followed ageneral trend except in activity as discussed ear-lier, effect of remote substituents dramaticallyaltered the catalytic activity. Generally, for latetransitional metal catalysts, lowering of electrondensity on the metal center through the additionof ligands with electron-withdrawing groupsproduces more active catalysts.7(a),12(a) This isactually reversed for metallocenes in whichstrong electron donating groups will enhance thecatalytic activity.19 We have reported that toostrong electron donating substituents like ben-zyloxy and allyloxy groups lead to a reduction inthe electrophilic metal center, resulting indecreased coordinating power with respect toethylene monomer and thus cause a lower propa-gation rate for substituted pyridine-bis-iminoiron(II) and cobalt(II) complexes.20 The elec-tronic effects of a-diimine ligands with variouspush–pull substituents on oligomerization ofethylene have been reported by Brookhart andSvejda7(a) with a special exception to methoxysubstituted catalyst which performed better ac-tivity. Reason for this was the formation of Lewisacid–base complexes of methoxy group withexcess aluminium cocatalyst that reduces theelectron density on ligand and therefore leads toa more electrophilic metal center for high poly-merization activity. Same effect was nourishedwith our system 3i/MAO having methoxy groupin the extreme remote position to metal center(Fig. 7). But similar effect was not observedwhen EAS was used as cocatalyst (Fig. 6), indi-cating that such an acid–base complex formationwas not possible with EAS. It is generallyassumed that some of the aluminium centers inMAO have an exceptionally high propensitytowards the electron rich species and have atendency to form ion pairs or adducts.1(h) Eventhough remote substituent effects were undoubt-edly established, all those were competed withthe complicated cocatalyst effects which made itdifficult to give ecumenical explanations. Thefree amino groups on the catalytic system mayalso interact with various cocatalysts used.21

Even though there will be certain interactionswith free amino groups and cocatalysts, weexpect a similar environment for all catalystsduring polymerization which can be consideredas a ‘‘constant effect’’. The detailed mechanisticpathway leading to present results is underinvestigation and will be reported.

Effect of the Type and Amount of Cocatalyst onEthylene Polymerization

The choice of aluminium cocatalyst has a signifi-cant effect on ethylene polymerization reactions.Even though the effect of cocatalysts like MAOremains a ‘‘black box,’’ it was proved that theamount of alkyl aluminiums or aluminoxaneaffects the molecular weight and molecularweight distribution of polymers.1(h) We accom-plished ethylene polymerization with differentcocatalysts such as MAO, EAS, and TEA. Notmuch study has been reported on EAS as a coca-talyst for homogeneous catalyst system in ethyl-ene polymerization. Recently, we have exploredthe use of EAS as an effective cocatalyst to phe-noxy-imine nickel(II) complexes for ethylene oli-gomerization.22 We clearly explained how eitherof the two modes of complexation, EAS could beadsorbed to the active metal centers. Interest-ingly we found that all our nickel diimine cata-lysts under discussion showed highest activityfor ethylene polymerization with EAS (Fig. 6).

Each catalyst differing only at remote substit-uent gave extremely different behavior on vary-ing the cocatalyst. By earlier reports about met-allocene catalysts, changes in cocatalyst greatlyaffected the catalyst’s performance and proper-ties of polymer obtained.23 On comparing theactivity of remote substituted catalysts, nitrosubstituted catalyst 3f gave highest activity inEAS (Fig. 6) of the order of 9.84 3 108 g-PE/ molNi h atm (Entry 12) while methoxy substitutedcatalyst 3i gave lowest activity (7.86 3 108 g-PE/mol Ni h atm, Entry 13). This can be explainedwith the known facts for a late transition metalcatalyst bearing strong electron withdrawing/donating groups as general trend discussed ear-lier. From these results it is clear that EAS can-not form an effective Lewis acid–base complexwith methoxy group as with MAO. Note that thesame catalyst, 3i, was a best candidate withMAO (5.01 3 108 g-PE/ mol Ni h atm, Entry 14)due to the special effect of methoxy group on cat-alyst system as reported by Brookhart and Svej-da,7(a) even though its activity was lower thanwith EAS. The high activity of all our catalystsin EAS medium over MAO can be attributedto any type of catalyst–cocatalyst interactionswhich are still not clear. Even though MAO is asuperior alkylating agent and has a greatercapacity to produce and stabilize cation likecomplexes, its bulky structure along with thebulkiness of the catalyst may be one of the main



reasons for this effect. Eminently, Caruthersproved that the cocatalysts effects were primarilystructural rather than electronic.24 All our sys-tems responded very badly with TEA (Entry 15).Generally a threshold amount of cocatalyst isneeded for the effective activation of catalyst,presumably to scavenge impurities that maypoison the active catalyst. From our repeatedexperiments it was observed that the active spe-cies for ethylene polymerization were generatedafter 22 equivalents (eq.) of EAS and 25eq. ofMAO so that a small amount of cocatalyst mighthave utilized for scavenging property asexpected. The role of cocatalysts on active siteformation has been thoroughly investigated byUV–visible spectroscopic measurements.

Spectroscopic Study of Active Sites

UV–visible spectroscopic investigation of severalmetallocene/MAO and late metal systems hasbeen proved that it is an efficient method tostudy the catalytic mechanism for single-sitecatalyst with strong conjugated system.25–27 Toconfirm the specific role of cocatalysts on poly-merization rate through active species formed, aseries of UV–visible spectroscopic analysis of

catalytic system were implemented in toluene at20 8C (Fig. 8 through Fig. 11).

Absorption spectrum of a-diimine ligand andcorresponding nickel complex did not show anycharacteristic peak between 400 and 850 nmwhereas addition of cocatalyst to toluene solutionof nickel complex resulted in the formation ofnew absorption bands, one centered at 700 nmwhereas the other peaks of much lower intensitywere observed at 480–520 nm. The absorptionmaximum around 520 nm is of activated speciesand that around 700 nm is due to the deactivatedspecies, formed by partial reduction of Ni(II) spe-cies. Detailed investigations on a-diimine Ni(II)complexes have been reported by Cramail andcoworkers.27 Luo et al.26(a) has reported that theabsorptions at 545 and 660 nm were of activeand inactive species, respectively. UV spectrademonstrate the effect of MAO and EAS towardsremote substituted catalysts. Evidently EAS gen-erates more active species than MAO, which isakin to our polymerization data. It is clear fromFigure 8 that the deactivated species were morewhen MAO was used as the cocatalyst, as indi-cated by the absorption at 700 nm.27 Instead in Fig-ure 9, the dormant species are too less when com-pared to the active species (500 nm). Comparing

Figure 8. UV/VIS absorption spectra of reactionmixtures of various catalyst precursors having gen-eral formula: [ZN ¼ C(An)-C(An) ¼ NZ]NiBr2, Z ¼ (4-NH2-3,5-C6H2R2)2CH(4-C6H4Y); An, acenaphthenequi-none; R, iPr and (a) Y ¼ H (3c), (b) Y ¼ NO2 (3f) and(c) Y ¼ OCH3 (3i) with MAO (Al/Ni ¼ 100) in tolueneat 20 8C.

Figure 9. UV/VIS absorption spectra of reactionmixtures of various catalyst precursors having gen-eral formula: [ZN ¼ C(An)-C(An) ¼ NZ]NiBr2, Z ¼ (4-NH2-3,5-C6H2R2)2CH(4-C6H4Y); An, acenaphthenequi-none; R, iPr and (a) Y ¼ H (3c), (b) Y ¼ NO2 (3f) and(c) Y ¼ OCH3 (3i) with EAS (Al/Ni ¼ 100) in tolueneat 20 8C.



entries 10 and 12 in Table 2, the activity of cata-lyst 3f with EAS was almost four times higherthan with MAO. Similar effects were observedwith catalysts 3c and 3i. Figure 10 illustratesthe activity difference between MAO and EASmore clearly in which the absorption maximumat 500 nm for 3i/EAS system is high (curve c)and the deactivation is less (peak at 700 nm)when compared with 3i/MAO system (curve b).Figure 11 illustrates the effect of 3f catalysttowards the cocatalysts in which for MAO theformed active species were deactivated resultingin lower activity.

Polymer Characterization

Molecular weight of PE produced by each cata-lyst decreased with increase in temperature. Aswe could not detect any special effect of temper-ature for remote substituents except pursuing ageneral trend,28 the experiments were limited to10, 30 and 50 8C. Branching of carbon atomsincreased with temperature while polymer mo-lecular weight and melting point decreasedalong with the diminished activity (Entry 9, 10).This is due to the deactivation of active speciesat high temperature as expected. At higher tem-peratures, barrier for b-H activation transition

state may be lower resulting in more methylbranched polymers. This b–hydride eliminationreaction has reported to be unimolecular whileat lower temperature bimolecular reactionoccurs leading to a linear PE.10(b) We observedan abrupt decrease in branching of carbonatoms on polymers obtained, with decreased mo-lecular weight, on reducing the steric bulkinessof diimine ligands (Table 2). For example, cata-lyst 3f (Entry 10) produced a polymer of 81branches per 1000 carbon atoms, were reducedto 3 for methyl substituted catalyst 3d (Entry 7)under similar conditions. Recent reports haveproved that successive migratory insertion andethylene trapping leads to a linear polymerwhile insertion following chain running leads tothe introduction of branches in polymer chainand the extend of chain running prior to inser-tion is proportional to the number of branches.28

The molecular weight of polymers increasedwith ortho substituent bulkiness as per previousreports28 (Table 2). For example, methyl substi-tuted catalyst (3a, Entry 2) produced polymerswith Mn ¼ 26,100 where as isopropyl substitutedcatalysts (3c, Entry 4) gave Mn ¼ 48,900. Bulkygroups in the axial position of metal center willlead to a decrease in rate of chain transfer rela-tive to the rate of propagation, resulting in high

Figure 10. UV/VIS absorption spectra of (a) catalystprecursor having general formula: [ZN ¼ C(An)-C(An)¼ NZ]NiBr2, Z ¼ (4-NH2-3,5-C6H2R2)2CH(4-C6H4Y);An, acenaphthenequinone; R, iPr and Y ¼ OCH3 (3i),(b) 3i/MAO (Ai/Ni ¼100) and (c) 3i/EAS (Ai/Ni ¼100)in toluene at 20 8C.

Figure 11. UV/VIS absorption spectra of (a) catalystprecursor having general formula: [ZN ¼ C(An)-C(An)¼ NZ]NiBr2, Z ¼ (4-NH2-3,5-C6H2R2)2CH(4-C6H4Y);An, acenaphthenequinone; R, iPr and Y ¼ NO2 (3f),(b) 3f/EAS (Ai/Ni ¼100) and (c) 3f/MAO (Al/Ni ¼100)in toluene at 20 8C.



molecular weight polymers. For nickel catalystsit was suggested that a direct b-H transfer tomonomer, rather than associative displacementof unsaturated polymer chain from the metalcenter by ethylene, was responsible for retarda-tion of chain transfer.1(c),28 Another interestingpoint we observed was the broad molecularweight distributions ranging from 3.5 to 5.7. Thismay be attributable to the presence of any for-eign active species by the interaction of cocatalystwith various substituents in catalysts. Anotherpossibility is the different orientation of catalystsdue to two chiral carbon atoms from bulky triarylligands. However, we were not able to detect anybimodal feature by GPC. It was noted that PEobtained by methoxy substituted catalyst 3i/MAOshowed highest molecular weight distribution of5.7 (Entry 14). But with 3i/EAS it was 4.08. Notethat the interaction of MAO with methoxy sub-stituent was more effective than with EAS.7(a)

The effect of remote substituents on molecularweight was investigated under identical reactionconditions by keeping isopropyl as ortho substitu-ent. Branching of carbon atoms per 1000 carbonsdid not show any regular trend according to thepush–pull substituent order even though it gavemoderately branched polymers. But in almost allcases extent of branching depends on the cocata-lyst used. For example, when 3c/EAS (Entry 6)produced 103 branches, 3c/MAO (Entry 5) gaveonly 53 branches. A similar trend was noticedamong the catalysts 3f, and 3i by varying thecocatalyst in which EAS gave more branched PEthan MAO. 13C spectrum of PE of 3i/MAO systemindicates the presence of long chain branching bya peak around 14.8 ppm.29 Polymers obtainedfrom methyl ortho substituted catalyst showedmelt transitions between 122 and 130 8C, indicat-ing the linearity of polymers, where as with bulkysubstituted catalyst Tm varies from 115 to 122 8Cwhich is the characteristics of highly branchedpolymers. The differences in thermal propertiesof PE produced by these catalysts having varioussubstituents are mostly attributed to the varia-tions in degree of short chain branching. Themelting behavior of PE is mainly related to shortchain branching density. An increase in shortchain branching density decreases the lamellarthickness of crystal structure thereby lowers themelting temperature of polymer. PE produced byall catalysts with EAS exhibited very broad melt-ing curves with low melting point especially forhighly branched samples. PE obtained with MAOshowed very sharp melting peaks with high melt-

ing temperature, which is typical for high-densityPE. The physical appearance of PE varied withthe nature of cocatalyst; MAO system producedpowdered PE where as EAS system gave whiterubbery material.

CONCLUSIONS

Newly synthesized catalysts with bulky substi-tuted a-diimine ligands polymerize ethylenewith high activity than reported system (control)and the activity along with polymer propertieswere tuned according to different cocatalysts toyield high molecular weight PE with high PDI.Polymerization activity varied with substituentson the catalyst, even though they are at remoteposition from the metal center. Influence of suchremote substituents on nickel center was sup-ported by cyclic voltammetry measurements.Catalytic active species resulting from the acti-vation of nickel complex by cocatalysts wereidentified by UV–visible spectroscopy and foundthat EAS generates more active species thanMAO, which was reluctant towards deactivation.The special effect of methoxy-substituted cata-lyst with MAO was not observed with EAS. Ourcatalyst on combination with EAS gave highlybranched polymers when compared to MAO. Wewere able to design highly active catalystswhose steric and electronic environment can beeasily tuned according to the requirement. Alsowe explored EAS as an effective and cheapercocatalyst than MAO which will be an asset tothe search for commercially advanced catalysts.

This work was supported by Ministry of Commerce,Industry and Energy. The authors are also grateful tothe BK 21 Project and the Center for Ultramicro-chemical Process Systems (ERC), the National CoreResearch Center Program from MOST and KOSEF(R15-2006-022-01001-0), and the NRL Program.

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