Highly Branched Unsaturated Polyethylenes Achievable Using Strained Imino-cyclopenta [ b ] pyridyl-nickel Precatalysts

Technology, Hangzhou 310014, China. Email: xinquan@zjut.edu.cn b Key laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Email: whsun@iccas.ac.cn c School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China. E-mail: cyguo@ucas.ac.cn dDepartment of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, UK. E-mail: gas8@leicester.ac.uk e State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China. † Electronic Supplementary Information (ESI) available: Ethylene polymerization with Ni1–Ni4/MMAO; and CCDC 1519413 and1519414for complexes Ni1and Ni2. See DOI: 10.1039/x0xx00000x Received 00th January 20xx, Accepted 00th January 20xx


Introduction
With the aim of introducing subtle control over the microstructure of a polyethylene, late transition metal αdiimino-nickel/palladium 1 and bis(imino)pyridyl-iron/cobalt precatalysts 2 have shown great promise for realizing this goal.For instance, the polyolefinic materials generated by nickel precatalysts can show branching properties that are a characteristic of linear low density polyethylene, 1 while linear high density polyethylene is a hallmark of the materials obtained by the bis(imino)pyridyl-iron/cobalt precatalysts.2b,2d With a view to gaining a deeper understanding of the mechanism of polymerization, coupled with the growing commercial applications of these polyethylene grades, research groups in both industry and in academia have been exploring the effects of modifications to the ligand framework, the nature of the late transition metal center and/or the polymerization conditions.1f,3 In this regard, α-diimino-nickel systems have been at the forefront of many of the key developments.
With an eye to developing new or improved polyethylene properties through microstructural modifications, we have disclosed a range of cycloalkyl-fused pyridine ligand sets that have served as compatible chelating supports for a variety of nickel(II) precatalysts including 8-arylimino-5,6,7trihydroquinolynickel (A, Chart 1) 4,5 and 9-arylimino-5,6,7,8tetrahydrocyclohepta[b]pyridyl-nickel (B, Chart 1). 6Indeed A and B not only display high activities but also confer some distinct effects on the structural properties of the so-formed polyethylenes.These effects being ascribed to the cyclictension imparted by the presence of the respective cyclohexyl and cycloheptyl fused rings present within the ligand frame.For example, the use of B leads to a higher branching content when compared with A, while both display similar polyethylene molecular weights.4a,5 Moreover, variations in the electronic and steric influences notably affect the degree of branching, particularly with respect to the long chain branches.4b,4c By contrast nickel precatalysts containing the smaller cyclopentyl counterparts have proved more synthetically challenging.Indeed, difficulties associated with the preparation of 7-aryliminocyclopenta[b]pyridines from their cyclopenta [b]pyridine-7-one precursor have so far precluded their synthesis; an observation that is likely due to imine-enamine tautomerization in the product and ensuing decomposition pathways. 7To block this tautomerization, we have developed related precatalysts incorporating multiple substitutions at the α-carbon to the imine.For example, the gem-dimethyl containing cobalt and chromium complexes C (Chart 1) can readily be synthesized, 7,8 and moreover these precatalysts mediate the formation of polyethylenes with narrow polydispersity and high linearity. 4,7,8Similar gemdimethyl substitution is also possible in the cyclohexylcontaining nickel complex D (Chart 1) 9 and again high activity and single-site behavior are a feature.Furthermore, when compared with A, 4a D affords polyethylene waxes exhibiting lower molecular weight and higher levels of branching, including a higher ratio of long chain branches.In this report, we develop the dialkyl-substitution strategy by applying it to the synthesis of a range of 7-arylimino-6,6dimethylcyclopenta[b]pyridyl-nickel complexes (Chart 1).On activation with either MAO or MMAO, these precatalysts exhibit high activities towards ethylene polymerization and produce polymers with lower molecular weight, narrower polydispersities and a higher ratio of long chain branches when compared with cyclohexyl-containing D (Chart 1). 9 A detailed microstructural study of the polyethylenes is reported using high temperature 1 H NMR, 13 C NMR and DEPT135 13 C NMR spectroscopy.Interestingly, the unsaturated content within the resultant polyethylenes is clearly observable with both vinyl and vinylene groups apparent.To the best of our knowledge, this is the first disclosure of such polyolefinic materials being generated in an ethylene homopolymerization using a single metal catalyst, with copolymerization of ethylene and 1-alkene representing the alternative previous approach. 10Besides, such polyethylene macromers are in high demand for long chain branched copolymers, functionally modifiable polymers as well as coating materials. 11We also present a mechanism that accounts for the observed microstructural properties, while the approximate number of regenerated active species are evaluated on the basis of the molecular weight of the polyethylene obtained.

Synthesis and characterization
The (3) with the corresponding aniline in o-dichlorobenzene in good yields (Scheme 1).4a,7,12 Ketone 3 is not commercially available and was synthesized in a two-step procedure by firstly gemdimethylating 2-chloro-cyclopenta[b]pyridin-7-one (1) 7 at the 6-position to give 2-chloro-6,6-dimethylcyclopenta[b]pyridin-7one (2) and then 2 could be de-chlorinated to form 3 by its hydrogenation in the presence of palladium on carbon.All the organic compounds were characterized by 1 H NMR, 13 C NMR and FT-IR spectroscopies and elemental analyses.
Single crystals of Ni1 and Ni2 suitable for the X-ray diffraction determinations were obtained by slow diffusion of diethyl ether into methanol solutions containing the corresponding complex.The structures of the two complexes are different and will be discussed separately.Views of Ni1 and Ni2 are shown in Figs. 1 and 2 respectively; selected bond lengths and angles are tabulated for both in Table 1.chloride complexes previously reported by our group. 7,8In addition, the Nimino-aryl ring is inclined close to perpendicular with respect to the N1, N2, N1 i , N2 i plane (dihedral angles of 89.50°).No intramolecular contacts of note were apparent.Unlike Ni1, Ni2 adopts a centrosymmetric chloride-bridged dinuclear structure with only one NN-chelating ligand present per nickel center.The geometry around each nickel atom can be described as distorted octahedral, with the nitrogen atoms of the bidentate ligand disposed trans to the bridging chlorides and cis to one terminal chloride and one methanol ligand.The O1−Ni1−Cl2 angle (173.11(5)o ) shows some deviation from the expected 180°.The presence of the O-bound methanol ligand was unexpected and derives from the solvent of crystallization.The Ni−Npyridine bond distance (2.0650(2) Å) in Ni2 is comparable with that observed in Ni1 (2.0676(13) Å), while the Ni−Nimine distance (2.1925(19) Å) is slightly shorter than that observed in Ni1 (2.2952( 14) Å); a similar observation has been highlighted in 8-arylamino-5,6,7-trihydroquinolyl-nickel dichlorides.4c As with Ni1, the Nimino-aryl ring planes are inclined towards perpendicular with respect to the plane formed by N1, N2, Cl1, and Cl1 i (dihedral angle = 83.61°).Similar structural features have been observed elsewhere for nickel complexes bearing either cyclic or acyclic iminopyridine derivatives.5a,6a,13 Fig. 2 ORTEP drawing of complex Ni2.Thermal ellipsoids are shown at the 30% probability level and H atoms are omitted for clarity.
Despite using a 1:1 molar ratio of NiCl2•6H2O to ligand it was unexpected to observe that both Ni1 and Ni3 adopt bisbidentate structures instead of the predicted mono-bidentate as seen in Ni2 and Ni4.It is uncertain as to the origin of this difference in coordination seen in the solid state, but it may be due to a combination of the relative bulkiness of the orthosubstituents (R 1 = Me for Ni1 vs. R 1 = Et for Ni2) and ligand/solvent competition for coordination to the metal.In related structures a preference for mono-bidentate coordination has been noted. 15

Ethylene polymerization: co-catalyst screen
In order to determine the most efficient co-catalyst for use in the ethylene polymerization studies, Ni3 was initially examined in conjunction with four different alkylaluminium reagents namely methylaluminoxane (MAO), modified methylaluminoxane (MMAO), ethylaluminumsesquichloride (EASC, Et3Al2Cl2) and diethylaluminium chloride (Et2AlCl); the conditions and results are summarized in Table 2. On the basis of the catalytic activities obtained, MAO and MMAO were identified as the most compatible co-catalysts delivering the best overall performances.Hence, all subsequent studies focused on MAO and MMAO (see ESI) as the co-catalysts.

Using MAO as co-catalyst
To establish the optimal conditions for the polymerization using a catalyst composed of Ni3 and MAO, the molar ratio of Al/Ni, reaction temperature and run time were systematically investigated; the results are summarized in Table 3.Firstly, with the run time set at 30 minutes and the temperature at 30 o C, the molar ratios of Al/Ni were varied between 1750 to 2750 (entries 1-5, Table 3).As the molar ratio was increased, the catalytic activity reached a maximum of 4.68 × 10 6 g(PE)•mol -1 (Ni)•h -1 with the ratio at 2250 (entry 3, Table 3).Subsequently, the activity gradually decreased as the Al/Ni ratio was further increased.The trend in molecular weight of the resultant polymers obtained at different Al/Ni ratios is rather irregular, nevertheless the molecular weight distribution (Mw/Mn = 1.45-1.51) is particularly narrow and unimodal as shown by the GPC curves (Fig. 3).Notably, when compared with the polyethylenes obtained using cyclohexylcontaining D (Chart 1), 9 those afforded using Ni3/MAO display narrower polydispersities.
Secondly, with the Al/Ni ratio fixed at 2250 and the reaction time at 30 minutes, the temperature was varied between 20 and 60 o C (entries 3, 6-9, Table 3).The highest activity for Ni3/MAO of 5.02 × 10 6 g(PE)•mol -1 (Ni)•h -1 was achieved at 20 o C (entry 6, Table 3).Raising the temperature further led to a gradual lowering in activity (entries 7 −9, Table 3) and at 60 o C a significant drop was observed.This lowering in activity could be attributed to two possible reasons: 1) a reduction in the number of catalytically active species which is most pronounced at 60 o C (310 h -1 ), indicative of significant degradation; 4a,14 2) the solubility of ethylene generally decreases with an increase in temperature. 15With regard to the polyethylene obtained, lower temperature leads to higher molecular weight material (1.56 Kg•mol -1 at 20 o C) with a relatively high melt temperature (Tm = 71.2o C).When the temperature was increased, the resultant polyethylene displayed lower molecular weight and a lower melt temperature (Tm = 59.1 − 66.4 o C); this molecular weight/temperature correlation is reflected in the GPC curves shown in Fig. 4 and can be attributed to a higher rate of chain termination occurring at elevated temperature as compared to chain propagation (entries 3, 6 -9, Table 3). 4,6hirdly, with the Al/Ni ratio fixed at 2250 and the temperature at 20 o C, the run time was varied between 5 minutes and two hours.On inspection of the data it is apparent that Ni3/MAO required an induction period of 30 minutes following addition of co-catalyst at which point it reaches an optimum catalytic activity of 5.02 × 10 6 g(PE)•mol - 1 (Ni)•h -1 (entry 6, Table 3). 4,5,6On extending the reaction time beyond this period (entries 13 − 15, Table 3), the catalytic activities slightly decreased as the active species started to deactivate. 16For example, after 1 hour the activity was 4.31 × 10 6 g(PE)•mol -1 (Ni)•h -1 (entry 14, Table 3), whilst after 2 hours it had reduced to only 2.51 × 10 6 g(PE)•mol -1 (Ni)•h -1 .Indeed the difference in the number of functional active species between 30 minutes and 2 hours is approximately 892 h -1 , highlighting the slow deactivation process occurring (entries 6 and 15, Table 3).
In general, the least sterically hindered precatalysts with the most electronically donating groups at the para position (e.g.CH3 instead of H) favored high catalytic efficiency. 17For instance, Ni3 displayed the highest catalytic activity (entry 6, Table 3) due to a combination of the least sterically bulky ortho groups and the positive inductive effect imparted by the methyl group; similar findings have been reported elsewhere.2b,17,18 3).

Microstructural properties of the polyethylenes
The microstructural characteristics of the polyethylenes produced using Ni3/MAO (entry 6, Table 3) have been the subject of a high temperature 1 H NMR, 13 C NMR and DEPT135 13 C NMR spectroscopic study (recorded in deuterated 1,1,2,2tetrachloroethane at 100 °C).In the 1 H NMR spectrum (Fig. 5), the peaks centered around 5.90 and 5.00 ppm, with a relative peak area ratio of 1:2, were unambiguously assigned to the vinyl group (-CH=CH2) located at the end of the polymer chain.19b,19c This assignment is confirmed in the 13 C NMR spectrum (Fig. 6) which shows peaks around 114.7 and 139.9 ppm which are characteristic of the carbon atoms belonging to a vinyl end group.In addition to this vinyl group, a multiplet was observed at 5.49 ppm in 1 H NMR spectrum attributable to the presence of a vinylene (-CH=CH-) group present within the polymer chain.10b He et al. investigated a similar type of polyethylene microstructure that in their case was produced by the copolymerization of ethylene with 1-octene.10a At this stage, we were not able to specify the exact position of the vinylene group in the polymer chain.However, the integral ratio of all respective double bond protons showed that one of the alkyl chains positioned on either side of the vinylene contained at least two or more carbons.For example, the integral ratio of the Ha+Hb+Hc+Hc´/Hd protons equates to 5/8 instead of 5/9 illustrating that there is no -CH3 group attached to the -CH=CH-group.Furthermore, the 13 C NMR spectrum (Fig. 6) reveals peaks around 131 and 124 ppm that could be assigned to two vinylenic carbons with their difference in chemical shift distinguishing the two different chains attached to either side of vinylenic group.In the DEPT135 13 C NMR spectrum (Fig. 7), three positive peaks at 139, 131 and 123 ppm were observed for the -CbH, -CcH and -Cc´H carbons, respectively, and one negative peak at 114 ppm for the -CaH2 carbon atom.To the best of our knowledge, Ni1 -Ni4 are the first catalyst systems in ethylene polymerization that produce polyethylenes with double bonds either at the terminal position or within the chain.The mechanistic implications are discussed in detail later (vide infra).
To further confirm the generality of this catalyst family to form related microstructures, samples of polyethylenes obtained using Ni3/MAO at 60 °C (entry 9, Table 3) and Ni2/MAO at 20 °C (entry 17, Table 3) were characterized by high temperature 1 H NMR spectroscopy (Fig. 5).While little difference in chemical shifts was observed in their 1 H NMR spectra, the ratio of vinylene groups (-CH=CH-) increased at higher temperature: (-CH=CH-)/(H2C=CH-) = 2.2/1.0 with Ni3/MAO at 60 °C and 1.3/1.0 with Ni2/MAO at 20 °C (determined by the integration of corresponding peaks).This is likely due to the higher rate of termination at higher temperature and hence enhanced possibilities to generate a vinylene units within the polymer chain.On the other hand, for the sample obtained at 20 °C, the ratio of vinylene to vinyl groups is slightly decreased (Fig. 5).It is worthy of note that such polyethylenes have industrial demand due to their broad applications including in the production of long chain copolymers, functional polymers as well as coating materials. 11Fig. 5 1 H NMR spectra of the polyethylenes obtained using Ni3/MAO and Ni2/MAO at 20 or 60°C (entries 6, 9 and 17, Table 3) Fig. 6 13 C NMR spectrum of the polyethylene obtained using Ni3/MAO at 20 °C (entry 6, Table 3) On the basis of the encouraging results reported above using Ni3/MAO, the microstructural properties of the polyethylenes obtained using Ni3 with other co-catalysts (viz.MMAO, Et2AlCl) were also investigated.A detailed polymerization study using MMAO as co-catalyst has also been conducted (see ESI, Table S1).Ni1 -Ni4/MMAO showed very similar polymerization performances to that observed with MAO and indeed Ni3/MMAO again exhibited the highest activity (up to 4.66 × 10 6 g(PE)•mol -1 (Ni)•h -1 ), albeit under slightly different optimized conditions (30 °C, 2000 Al/Ni molar ratio, 30 minutes).For purposes of comparison, samples of polymer obtained using Ni3/MMAO and Ni3/Et2AlCl at 30 °C were also subject to a high temperature 1 H NMR spectroscopic study (Fig. 8).The chemical shifts of all the peaks are very similar to the spectrum of the polyethylene obtained using Ni3/MAO, however there are variations in the relative ratio of vinylene groups present: (-CH=CH-)/(H2C=CH-) = 1.15/1.0for Ni3/MMAO and 0.89/1.0for Ni3/Et2AlCl (determined by the integration of corresponding peaks).The microstructure of the polyethylene was further elucidated by a branch number determination based on data acquired from 13 C NMR spectra (Fig. 6 and Fig. 9) and interpreted using methods described elsewhere.10b, 20 The polyethylene produced by Ni3/MAO possessed 37 branches per 1000 carbons which included 22.8% methyl branches, 5.0% ethyl branches and 72.2% of long chain branches.In comparison with 8-arylimino-7,7-dimethyl-5,6-dihydroquinolynickel precatalysts (D, Chart 1), 9 a notably higher proportion of long chain branches was obtained, this observation being attributable to the more constrained geometry of the current precatalysts.Moreover, less branches were observed when put alongside literature reports even though they display similar Tm values.4b,4c,4d,6 A segment of the structure of the polyethylene obtained is represented in Fig. 6.By contrast, in the polymer obtained using Ni3/MMAO (Fig. 9), a modest reduction in the numbers of branches is observed when compared with the material obtained using Ni3/MAO: 31 branches per 1000 carbons including 27.2% methyl branches, 5.4% ethyl branches and 67.4% longer chain branches.Fig. 9 13 C NMR spectrum of the polyethylene obtained using Ni3/MMAO at 30 °C (entry 3, Table S1)

Mechanistic implications
Despite extensive evidence being available from theoretical and some experimental measurements concerning the mechanism of ethylene polymerization, 21 many questions still remain to be fully answered.For example, i) what relationship between the chelating ligand and the metal determines the polymer properties produced, 22 ii) what factors, either electronic, steric or both, are responsible for generating branched polymers, 1,4,5,6 iii) why do Fe and Co precatalysts tend to give polymers with uniquely vinyl end groups.2d,10,23 Bearing in mind these questions, 1,13,24 we postulate a mechanistic pathway that accounts for both the terminal and non-terminal double bonds identified in the polymers found in this study (Scheme 2).
The proposed mechanism comprises three conventional steps namely chain propagation, chain isomerization and chain termination.Following initial coordination of ethylene at the vacant site and insertion of the coordinated ethylene into the Ni-R bond, multiple coordination-insertion steps proceed leading to chain growth (propagation step).Chain isomerization is the key step that defines the microstructure with regard to the type of branch and how many branches there are.According to Brookhart's mechanistic studies, 21e,21f,21g,21h,24d chain isomerization proceeds by elimination of a β-agostic hydrogen from the newly formed chain, subsequent rotation of the chain by 180° around the Mη 2 -olefin bond followed by reattachment of the hydrogen to the terminal carbon atom of the polymer.From our viewpoint we consider that, in addition to β-agostic hydrogen interactions, γ-agostic, δ-agostic or greater are likely to take place along the growing chain.Then, following rotation and migration of the hydrogen to the terminal carbon of the polymer chain, methyl, ethyl, propyl and longer chain branches would be formed depending upon which type of agostic interaction initially occurs.19a,25 In particular, we propose that during chain isomerization, a cyclic intermediate (I, Scheme 2) forms between the agostic hydrogen, metal center and the terminal carbon of the growing chain which is then followed by a rotation of the polymer chain by 180° to generate intermediate II (Scheme 2), which could be reversible with I.In this scenario, the metal can chain walk on the growing polymer via agostic interactions, rotation and finally transfer of hydrogen to the detached end of the chain; this cycle can then repeat on either side of the growing chain.24d This phenomenon would be recognized by the presence of an internal vinylene group.At the end, intermediate III (Scheme 2) can have three possible ways to deliver the final microstructure: 1) either terminate by β-hydrogen assistedelimination following pathway A to generate the internal vinylene, 2) repeat the cycle by following pathway C to afford more branches in the polymer and 3) terminate the growing polymer chain with a terminal vinyl group using pathway B by β-hydrogen assisted-elimination and again initiate a new chain.
Moreover, the low molecular weight of the polymer and the high catalytic activity emphasizes the high rate of βelimination and high number of active species as mentioned in Table 3.
precatalysts developed in this study show larger bite angles, N1-Ni1-N2 (79.49(5) o in Ni1 and 80.75 (7) o in Ni2), than in the cyclohexyl[b]pyridyl-nickel precatalysts (A, Chart 1)  ). 9 This constraining of the NN-ligand is likely to create a more accessible metal center leading to rapid ethylene coordination and higher rates of chain isomerization and termination.

Conclusions
Nickel(II) chloride complexes, Ni1−Ni4, bearing strained NNchelating imino-cyclopenta[b]pyridines are disclosed, which in the presence of MAO (or MMAO) form unusual unsaturated polyethylenes containing vinyl or vinylene unsaturated functional groups and high levels of long chain branches.All precatalysts showed high activities with Ni3 the standout performer producing polyethylenes of low molecular weight and narrow polydispersities.An alternative mechanism has been proposed to account for the vinylene groups (and alkyl branches) identified within the polyethylenes which could form the basis for future theoretical as well as experimental studies.These types of unsaturated materials could serve as macromers for generating polymers with long chain branching or could be used for further functionalization.This new approach is a cost efficient and potentially industrially applicable route for making materials related to those obtained by the copolymerization of ethylene and an α-olefin.

General procedures
All manipulations involving air-and/or moisture sensitive compounds were performed in an atmosphere of nitrogen using standard Schlenk techniques.Toluene was refluxed over sodium-benzophenone and distilled under nitrogen before use.Methylaluminoxane (MAO, 1.46 M in toluene) and modified methylaluminoxane (MMAO, 1.93 M in n-heptane) were purchased from Akzo Nobel Corp. Diethylaluminium chloride (Et2AlCl, 1.17 M in toluene) and ethylaluminium sesquichloride (EASC, 0.87 M in n-hexane) were purchased from Acros Chemicals.High purity ethylene was purchased from Beijing Yanshan Petrochemical Co. and used as received.Other reagents were purchased from Aldrich, Acros or local suppliers.NMR spectra were recorded on a Bruker DMX 400 MHz instrument at ambient temperature using TMS as an internal standard.IR spectra were recorded on a Perkin-Elmer System 2000 FT-IR spectrometer.Elemental analysis was carried out using a Flash EA 1112 microanalyzer.Molecular weights and molecular weight distributions (MWD) of the polyethylenes were determined using a PL-GPC220 GPC/SEC high temperature instrument; three 300 × 7.5 mm PL gel 10 μ m MIXED-BLS columns connected in series were employed.The GPC measurements were undertaken at 150 °C with a flow rate of 1.0 ml/min using 1,2,4-trichlorobenzene (TCB) as the eluent.Data collection and handling were carried out using Cirrus GPC Software and Multi Detector Software; data were collected at 1 point per second.The calibrant used for constructing conventional calibration was polystyrene (Calibration KitS-M-10 from the Polymer Labs Company).The true average molecular weights of the polyethylene are generated by inputting the Mark-Houwink constants of the polyethylene; K of 0.727 and α of 40.Samples were dissolved at a concentration of 0.5 to 2.5 mg ml −1 depending on the molecular weights.The melting points of the polyethylenes were measured from the second scanning run on a PerkinElmer TA-Q2000 differential scanning calorimetry (DSC) analyzer under a nitrogen atmosphere.In the procedure, a sample of about 5.0 mg was heated to 140 °C at a rate of 20 °C/min and kept for 2 min at 140 °C to remove the thermal history and then cooled at a rate of 20 °C/min to -40 °C. 1 H NMR, 13 C NMR and DEPT135 13 C NMR spectra of the polyethylenes were recorded on a Bruker DMX 300 MHz instrument at 100 °C in deuterated 1,1,2,2-tetrachloroethane with TMS as an internal standard.

X-ray crystallographic studies
Single crystals of Ni1 and Ni2 suitable for the X-ray diffraction determination were obtained by slow diffusion of diethyl ether into a methanol solution of the complex at room temperature.The X-ray studies were carried out on a Rigaku Saturn724 + CCD with graphite-monochromatic Mo-Kα radiation (λ = 0.71073 Å) at 173 (2) K; cell parameters were obtained by global refinement of the positions of all collected reflections.Intensities were corrected for Lorentz and polarization effects and empirical absorption.The structures were solved by direct methods and refined by full-matrix least squares on F 2 .All hydrogen atoms were placed in calculated positions.Structure solution and refinement were performed by using the SHELXL-97 package. 26Details of the X-ray structure determinations and refinements are provided in Table 4.

Fig. 1
Fig. 1 ORTEP drawing of complex Ni1.Thermal ellipsoids are shown at the 30% probability level and H atoms are omitted for clarity.

a
Reaction conditions: 5 μmol of Ni, 10 atm of ethylene, 100 mL total volume of toluene; b 10 6 g(PE)•mol -1 (Ni)•h -1 ; c Determined by DSC; d Determined by GPC; e N (active species) = the number of polymer molecules / the number of Ni complex molecules; f Conditions: 5 atm of ethylene.

Table 3
Polymerization of ethylene in the presence of MAO a

Table 4
Crystal data and structure refinement for Ni1 and Ni2•2CH3OH