Increasing the thermal stability of α-diimine nickel (II) catalyst by camphorquinone as a great backbone and investigating the steric effects of this backbone in ethylene polymerization

In the α-diimine catalyst system, catalyst design with high thermal stability through ligand modifications is very challenging. This paper reports the preparation of three camphyl-based ligands with diverse steric effect and their Ni (II) complexes. To evaluate the backbone and ligand steric effects these nickel complexes were used as catalysts in the polymerization of ethylene. The camphyl-based catalysts show high thermal stability with good catalyst activity up to 75 °C. In ethylene polymerization using bulky isopropyl substitution in the ortho position of the aniline moieties, it is achievable to tune the catalytic activities (6 × 105 g/mol Ni.h), polymer molecular weights (16 × 104 g/mol), and polymer melting temperatures (to 117.5 °C) over an extensive range.


Introduction
Recently, late transition metal catalysts are the most important strategy for the synthesis of polyolefins in industry and academia [1][2][3].Brookhart-type catalysts have exhibited a high ability to obtain hyperbranched and branched polyethylene [4][5][6][7].Systematic studies of backbone electronic [8,9] and steric [10,11] effects in late transition metalcatalyzed ethylene copolymerization and polymerization have been investigated in various catalytic systems.The polymer topologies (dendritic, hyperbranched, and linear) and microstructures can be managed through controlled polymerization conditions such as solvent, reaction temperature, ethylene pressure, and dose of catalyst [12][13][14].
In the 1990s, Brookhart and co-workers indicated that Pd(II) and Ni(II) catalyst bearing sterically bulky α-diimine backbone can catalyze olefin homopolymerization and copolymerization with a polar monomer such as vinyl ketone or acrylate monomers [15,16].Since Brookhart's initial report (Scheme 1, I) [16] extensive studies have been conducted to investigate the α-diimine systems and to control the ethylene insertion, especially via the phenyl ortho-positions modification (Scheme 1, I−III) [17,18] containing backbone adjustments, electronic perturbations and steric tuning (Scheme 1, IV) [19].Modification of aryl-ortho groups has a significant influence on the thermal stability of the catalyst as well as polymerization activities [20][21][22].Also, the substitution of N − phenyl groups plays an important role to control both the place of the incoming monomer after chain-walking and the regiochemistry of ethylene insertion [11,23].
Recently, the effect of catalyst backbone on the thermal stability and the reactivity of ethylene polymerization has been much considered [24,25].One of the most critical challenges of the α-diimine complex is its low thermal stability.The Ni-and Pd-based complexes quickly decompose at temperatures over 60 °C.This instability is related to increasing C-H activation and associative chain transfer that is caused by the increasing aniline moieties rotations from the perpendicular [26,27].Modifying ligand backbone and N-aryl moiety substitutions can increase the thermal stability of the α-diimine complex.The presence of bulky substituents in the ortho position of the N-aryl moiety improves the thermal stability [28][29][30].ArN = C(H) (H)C = NAr, ArN = C(Me) (Me)C = NAr, acenaphthene, and camphorquinone have been used as mostly α-diimine backbones to study the effect of backbone on the catalytic 430 Page 2 of 15 behavior of α-diimine complex.Among these, camphorquinone has higher thermal stability than other backbones due to its rigid and bulky bicyclic-substituted and can be active in ethylene polymerization up to 80 °C [31][32][33].
Lately, we have reported the synthesis of α-diimine nickel (II) camphyl-based with isopropyl (iPr), methyl (Me), and hydrogen (H) substituents in the ortho position of N-aryl moiety (Scheme 2).The steric effects of thermostable Scheme 1 Some of Brookhart's types of ligands reported in the literature Scheme 2 α-diimine Ni(II) complexes based camphyl backbone camphyl-based Ni (II) complexes have been successfully examined in ethylene polymerization.Box-Behnken statistical design (BBD) was used to choose the optimal polymerization conditions.BBD confirmed the effectiveness of pressure (P Ethy.), temperature (T), and Al/Ni ratio as essential factors in ethylene polymerization.

Materials
All manipulations were performed under a purified and inert atmosphere by nitrogen line or glovebox techniques.All chemicals, trimethylaluminum (TMA, 2 M in hexane), modified methylaluminoxane (MMAO by 7% wt Al in toluene) were purchased from Sigma-Aldrich Co. Dichloromethane (DCM) were dried over calcium hydride.Toluene (> 99.5%) was dried by distillation over sodium and benzophenone.Aniline, 2 6-dimethylaniline, and 2 6-diisopropylaniline were distilled under reduced pressure.Nitrogen and ethylene were dried through columns containing active silica gel, anhydrous potassium hydride, and a molecular sieve.

Characterization
FT-IR analysis was carried out using a BRUKER-IFS48 spectrophotometer (Germany). 1HNMR, 13 CNMR, H-H COSY data of ligands were determined on Bruker 400 MHz Ultra-Shield (USA) instruments at 30 °C, using CDCl 3 and tetramethylsilane (TMS).DSC was conducted with a Mettler-Toledo DSC1 (USA).Thermograms were recorded at -100 ºC to 200 ºC under an N 2 atmosphere at heating rates of 10 ºC/min.Thermal analysis (TGA) was measured using STA 1500 Rheometric Scientific (England).The contents of camphyl-based Ni(II) complexes were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using Thermo electron corporation make, (IRIS Intrepid 11 XDL instrument).Elemental analysis (C, H, N) was performed on a Vario EL series elemental analyzer.EDS was performed in SEM to investigate the chemical composition of the final catalyst.

Synthesis of camphyl-based ligands and corresponding Ni(II) complexes
All ligands synthesized were characterized by FT-IR, 1 HNMR, 13 CNMR, H-H COSY and elemental analysis.

Synthesis of H 3
Following the above process, H 3 was purified by silica gel (column chromatography) using petroleum ether/ethyl acetate (15:1) as eluent.The final product with 35.3% yield was obtained as yellow crystals (in ethanol). 1

Synthesis of C 1
Ligand H 1 (1.0 mmol, 0.44 g) and (DME)-NiBr 2 (1.0 mmol, 0.308 g) were added to DCM (25 mL) at 25 °C and stirred for 12 h under a nitrogen atmosphere.The reaction mixture was filtered and washed with hexane (10 mL × 3) and dried in a vacuum.The final solid product with 74.2% yield was obtained.See FT-IR of C 1 in the Supporting Information (Fig. S9).

Synthesis of C 2
Following the above process, C 2 was obtained in 80.2% yield.See FT-IR of C 2 in the Supporting Information (Fig. S10).

Synthesis of C 3
Following the above process, C 3 was obtained in 87.5% yield.

Ethylene polymerization
All polyethylene was synthesized using a lab-scale polymerization setup.The setup had a 100 mL stainless steel reactor, a catalyst injector, and a magnetic agitator.An oil circulator between the reactor's inner and outer walls controlled the polymerization reaction's temperature.Before all polymerization reactions, the reactor was purged with pure nitrogen at 110 °C for 2 h and vacuumed to guarantee the absence of moisture and oxygen.Then, the reactor was charged with 19 mL of dry toluene and the required amount of the MMAO, and the solution was degassed.The reactor was warmed to a favorable temperature.To initiate ethylene polymerization, 5 μmol of the appropriate α-diimine complex was suspended in 1 mL toluene and injected into the reactor.In order to monitor the rate of ethylene uptake, a mass flow controller rate was conducted, and consumed ethylene was recovered to keep the reactor at constant pressure.After 1h, gas was vented and the reaction quenched by ethanol-HCl (95:5).The obtained polymer was washed several times with ethanol and dried in a vacuum oven.

Design of experiment
Recently, surface methodology (RSM) was used by many researchers in order to optimize and develop reaction engineering.The choice of an allowable experimental design is a necessary consideration in experimental optimization.Design Expert (v12.0;Stat-Ease, Inc.) and Box-Behnken Design (BBD) method was applied to obtain the optimum P i,removal .Here, a three-factor three-level BBD (Table 1) for RSM was used to define the optimum conditions and study the effect of T poly., Al/Ni ratio, and P ethylene , on the yield of polymerization and catalyst activity.

Characterization of camphyl-based ligands and their complexes
The synthesis process of ligands H 1 , H 2 , and H 3 and their complexes C 1 , C 2 , and C 3 are depicted in Scheme 3. Brookhart-type α-diimine was obtained through a condensation reaction between diketone and primary anilines using the acid as a catalyst.Since camphyl-based ligands have a bulky backbone, so they can't be synthesized under similar conditions.In order to solve this problem, first primary aniline derivatives were treated with TMA to form an aminoalane dimer.Aminoalane can effectively convert bulky ketones into imines because Al-O bond is very stronger than Al-N bond.Therefore, camphyl-based ligands were achieved with good yields.H 1 , H 2 , and H 3 were confirmed by FT-IR, 1 HNMR, 13 CNMR, H-H COSY, and CHN.The camphyl-based Ni(II) complexes were simply obtained from the reaction of the H 1 , H 2 , and H 3 with (DME)NiBr 2 in high yields.The final Ni(II) complexes were characterized by elemental analysis, ICP, and Far-FTIR.Since nickel is a paramagnetism element, high-resolution NMR analysis can't be obtained from Ni(II) complexes.

FT-IR spectroscopy
In order to characterize the functional groups of ligands and their complexes, FT-IR analysis was carried out.The most important functional group in a camphorquinone is C = O, whose specific peaks can be seen at 1750 and 1770 cm −1 (Fig. 1A).The removal of the C = O peaks at 1750 and 1770 cm −1 and appeared of the C = N peaks in 1430, 1433, 1653, 1687 cm −1 , and C-N peaks at 1018, 1193 cm −1 confirms  the binding of camphorquinone to aniline and the synthesis of camphyl-based α-diimine ligand (Fig. 1B).Far-FTIR can be used to assure the ligand binding to the metal center.The peaks at 216, 281 cm −1 and 440, 581 cm −1 , were assigned to Ni-Br and Ni-N bond in α-diimine Ni(II)complex, respectively (Fig. 1C).See FT-IR of H 1 and H 2 and C 1 and C 2 in the Supporting Information (Figs.S7-S8 and S9-S10, respectively).

EDS and ICP analysis
EDS analysis was employed to investigate the chemical composition of final complexes (C 1 -C 3 ) (Fig. 3).The presence of C and N elements in H 1 , H 2 , and H 3 shows the reaction between aniline and camphorquinone has progressed well.Further, the presence of Ni and Br in the EDS spectra confirms the immobilization of Ni on the α-diimine ligands and the formation of the final complexes.The ICP analysis result proved the desired wt% Ni (II) in C 1 -C 3 (Table 2).

BBD analysis
In order to obtain the highest yield of polyethylene, polymerization conditions were investigated by BBD.In BBD, the Al/Ni ratio, P ethylene , and T poly were elected as independent variables (Table 3).The ANOVA analysis in Table 4 shows, these "P value " < 0.05 for the "Model" and the model terms of F 1 , F 2 , F 3 , F 1 F 2 , F 1 F 3 , F 2 F 3 demonstrate that the model were significant.In addition to the high value of "Pvalue", the insignificance of "Lack of fit" indicated that the used model was fit.Equation (1) was obtained with the system response variation with operating factors: (1) Equation ( 1) where F 1 is the temperature, F 2 is the pressure, and F 3 is Al/Ni ratio.
As shown in Table 4, the R 2 of the model was 92.69% which indicates the actual amounts of the yield were in good agreement with the predicted model (Fig. 4). Figure 5 shows the effects of the T poly., Al/Ni ratio, and P ethylene on the polymer yield and catalyst activity by the 3D response surface plots.The results of the 3D surface plot show P ethylene plays a more significant role in the catalyst activity and polymer yield than the Al/Ni ratio and T poly .By increasing the P ethylene , its solubility in toluene and its concentration around the metal center should increase.The increase in temperature leads to a decrease in the solubility of ethylene in solvent and consequently decreases catalyst activity.
Figure 6 exhibits the effect of Al/Ni ratio, P ethylene , and T poly.on the yield of the polyethylene, separately.P ethylene was the most effective factor in increasing polyethylene yield.The contour plots (Fig. 7) confirm the results of investigating the effects of different operating factors.

The thermal stability of camphyl-based Ni(II) complexes in ethylene polymerization
As shown in Table 3, camphyl-based catalysts still have high activity even at 75 °C.Camphorquinone as bulky framework prevents the rotation of the C Ar -N bond in two ways: Therefore, catalyst decomposition was retard and camphylbased catalysts showed high thermal stability.
In order to further investigate the thermal stability of the camphyl-based Ni (II) complexes, thermal gravimetric analyses (TGA) were conducted and portrayed in Fig. 8.The weight loss at 400-430 °C is attributed to the decomposition of the catalyst, which indicates the high stability of the camphylbased catalysts (Fig. 8).

The steric effect of backbones on the catalytic activity of camphyl-based Ni(II) complexes in ethylene polymerization
To investigate the steric structure camphyl-based ligands on ethylene polymerization, the complexes C 1 -C 3 were used, together with MMAO, under similar conditions prepared by BBD.The ethylene polymerization data at optimum conditions are listed in Table 5.
C 1 -C 3 were highly active in the polymerization of ethylene, activity being in the range of 2-6 (× 10 5 g mol −1 h-1 ).As shown in Table 5, the activity of C 1 complex is less than C 2 and C 3 complexes, establishing that the existence of steric substituent in ortho-aryl positions is efficient in protecting Ni(II) as active centers and increasing activity.Complex C 3 with 2,6-diisopropyl substituent demonstrated higher activity and yielded higher M w of polyethylene than complex C 2 with 2,6-dimethyl substituent and complex C 1 without substituent.The steric hindrance in the ortho position of the aniline moieties in complex C 3 leads to more protection of the ligand and metal center axial position.Consequently, the chain propagation rate increases relative to its chain transfer.Therefore, the rate of polymerization, activity, and molecular weight of the final polymer also increases.Despite the fact that complex C 1 has no substitution in the ortho position, it is active in the polymerization of ethylene which can be attributed to a significant steric effect of the camphorquinone.Therefore, the metal center is active in ethylene polymerization even without bulky substitution in the ortho position.Thermal properties in DSC curves show α-diimine Ni(II) complexes (C 1 -C 3 ) produced PEs with a broad melting peak which was related to the branched structure of PEs (Fig. 9).The result show, C 1 as a catalyst with a camphyl backbone and without substitutions in the ortho position, is active in ethylene polymerization.This activity is related to the camphorquinone structure that shows the C(22) (in 1 HNMR) hiding behind one of the aryl rings and C(23) and C(24) (in 1 HNMR) are oriented toward the axial position, which can prevent the rotation of C Ar -N bond or potential fluctuation.Therefore, the steric backbend effects lead to the stability of the metal center and its activity in ethylene polymerization.Also, C 3 catalyst with isopropyl substitutions in the ortho position produces higher activity and M w than the C 2 with methyl substitutions in the ortho position.

Fig. 4
Fig. 4 Actual yields of the polyethylene versus corresponding predicted ones

Fig. 5
Fig. 5 3D surface plot shows the polyethylene yields as a function of the Al/Ni ratio, P ethylene (atm.), and T poly.(°C)

Fig. 6
Fig. 6 Main effects of the Al/ Ni ratio, T poly., and P ethylene on the yield

Table 1
Experimental ranges and levels of the Al/Ni ratio (F 1 ) and pressure (F 2 ) and temperature (F 3 )

Table 2
The ICP analysis

Table 3
The experiments of BBD and the ethylene polymerization results

Table 5
The polymerization result of C 1 -C 3