Ethylene Polymerization and Copolymerization with Polar Monomers Using Nickel Complexes Bearing Anilinobenzoic Acid Methyl Ester Ligand

Neutral nickel complexes containing an anilinobenzoic acid methyl ester ligand are prepared and applied for the ethylene polymerization and copolymerization with polar monomers. The complex C2 containing isopropyl substituent on the aniline ligand conducts ethylene polymerization with high activity and good thermal stability. Most importantly, the catalyst promotes the copolymerization of ethylene and polar monomers with high activity (up to 277 kg·mol−1·h−1), affording ester-functionalized semicrystalline polyethylene with reasonable polar monomer content (up to 3.20 mol %).


Introduction
Although polyolefins have been extensively used in a wide range of application, the nonpolarity of polyolefins limits their further application [1]. This limitation could be dramatically improved by the introduction of even a small amount of polar functional groups to the polyolefin backbone [2]. A pioneering discovery was reported by Brookhart and co-workers that the α-diimine-nickel/palladium catalysts can initiate direct copolymerization of ethylene with polar monomers in the 1990s [3]. Since then, late-transition-metal catalysts based on palladium complexes have been extensively studied for olefin copolymerization with polar monomers [4][5][6][7][8][9][10][11]. Recently, low cost nickel catalysts were also developed extensively for the synthesis of functionalized polyolefins, although their copolymerization activities and resulting copolymer molecular weight are far from those required for industrial application [12][13][14][15][16][17][18][19][20].

Materials
All experiments were performed under dry nitrogen atmosphere using standard Schlenk techniques or in a glovebox. All solvents were dried by the PS-MD-5 (Innovative Technology (China) Ltd., Hong Kong, China) solvent purification system. Ethylene was purified by passage through dehydration column of ZHD-20 and deoxidation column of ZHD-20A before using. Commercial polar monomers were distilled ov`er calcium hydride before using. The other reagents were purchased and used without purification.

Analytical Procedure
NMR spectra were measured on a Bruker Asend TM 600 spectrometer (Bruker, Karlsruhe, Germany). CDCl3 was employed as solvent and the central peak of the solvent was used as an internal reference (CDCl3, 7.26, {77.16} ppm). Differential scanning calorimeter (DSC) analyses were carried out on a TA Q2000 instrument (Waters, New Castle, DE, USA). The DSC curves of the samples were recorded under a nitrogen atmosphere at a heating rate of 10 °C·min −1 . The single crystals data was made on a Bruker APEX2 diffractometer (Bruker, Karlsruhe, Germany) using graphite monochromated with Mo Kα radiation (l = 0.71073 Å), and the measurement were mounted under nitrogen atmosphere at low temperature. Crystallographic data are summarized in Table S1. Molecular weight and molecular weight distribution of polymers obtained were tested by a polymer laboratory PL GPC-22 (Agilen, Santa Clara, CA, USA) at 150 °C using 1,2,4-trichlorobenzene as a solvent.

Materials
All experiments were performed under dry nitrogen atmosphere using standard Schlenk techniques or in a glovebox. All solvents were dried by the PS-MD-5 (Innovative Technology (China) Ltd., Hong Kong, China) solvent purification system. Ethylene was purified by passage through dehydration column of ZHD-20 and deoxidation column of ZHD-20A before using. Commercial polar monomers were distilled ov'er calcium hydride before using. The other reagents were purchased and used without purification.

Analytical Procedure
NMR spectra were measured on a Bruker Asend TM 600 spectrometer (Bruker, Karlsruhe, Germany). CDCl 3 was employed as solvent and the central peak of the solvent was used as an internal reference (CDCl 3 , 7.26, {77.16} ppm). Differential scanning calorimeter (DSC) analyses were carried out on a TA Q2000 instrument (Waters, New Castle, DE, USA). The DSC curves of the samples were recorded under a nitrogen atmosphere at a heating rate of 10 • C·min −1 . The single crystals data was made on a Bruker APEX2 diffractometer (Bruker, Karlsruhe, Germany) using graphite monochromated with Mo Kα radiation (l = 0.71073 Å), and the measurement were mounted under nitrogen atmosphere at low temperature. Crystallographic data are summarized in Table S1. Molecular weight and molecular weight distribution of polymers obtained were tested by a polymer laboratory PL GPC-22 (Agilen, Santa Clara, CA, USA) at 150 • C using 1,2,4-trichlorobenzene as a solvent.

General Polymerization Procedure
Atmospheric pressure polymerization was carried out in a 100 mL glass reactor equipped with a magnetic stirrer and the high pressure polymerization was performed in Parr Instrument Company autoclave. At first, the reactor was vacuumized at 110 • C and charged with nitrogen several times Polymers 2018, 10, 754 3 of 10 before polymerization procedure. Then the prescribed amounts of toluene, monomers and cocatalyst under nitrogen were charged into the reactor and polymerization was initiated by adding the catalyst solution at desired temperature and desired pressure. The polymerization was conducted for a certain time and terminated with acidic methanol. The polymer obtained was stirred overnight, collected by filtration, adequately washed with alcohol and acetone, and dried under vacuum at 80 • C overnight until a constant weight was reached.

Synthesis of Nickel Complex C1
A mixture of the ligand L1 (0.227 g, 1 mmol) and KH (0.044 g, 1.1 mmol) in 50 mL of THF was stirred for 3 h to afford potassium salts of L1. The THF solution of potassium salts was slowly dropped into a solution of trans-[Ni(PPh 3 ) 2 PhCl] (0.696 g, 1 mmol) in 5 mL of the same solvent. The mixture was stirred over night at 25 • C. The reaction mixture was filtered off under a nitrogen atmosphere (PTFE, 0.45 µm) and evaporated under vacuum. The solid thus obtained was purified by recrystallization with a mixture of toluene/hexane to afford crimson single crystals. Yield 0.305 g (49%). Elemental Analysis: C 38

Synthesis and Molecular Structure of Nickel Complexes
The synthesis procedure of the ligands L1-L2 and the corresponding nickel complexes C1-C2 is shown in Scheme 1. 2-Anilinobenzoic acid methyl ester ligands were easily obtained by the amination of methyl 2-bromobenzoate in the presence Pd(dba) 2 /dppf and Cs 2 CO 3 [48]. The nickel complexes were prepared from the reaction of corresponding potassium salt of ligand with 1 equiv of trans-[Ni(PPh 3 ) 2 PhCl] in good yields according to the literature procedure [49]. Complex C2 was synthesized in a similar way to that for Complex C1. C2 was purified by recrystallization with a mixture of THF/hexane to afford orange-red single crystals. Yield 0.291 g (41%). Elemental Analysis: C44H44NNiO2P, Calcd. C 74.59, H 6.26, N 1.98; Found:C 74.37, H 6.29, N 2.08.

Synthesis and Molecular Structure of Nickel Complexes.
The synthesis procedure of the ligands L1-L2 and the corresponding nickel complexes C1-C2 is shown in Scheme 1. 2-Anilinobenzoic acid methyl ester ligands were easily obtained by the amination of methyl 2-bromobenzoate in the presence Pd(dba)2/dppf and Cs2CO3 [48]. The nickel complexes were prepared from the reaction of corresponding potassium salt of ligand with 1 equiv of trans-[Ni(PPh3)2PhCl] in good yields according to the literature procedure [49]. The molecular structures of nickel complexes were determined by single-crystal X-ray diffraction analysis (Figure 1). Complexes C1 and C2 showed very similar structure in view of the bond distances and the bond angles and exhibited a six-membered (N, O) nickel chelate ring with a puckered conformation. Each nickel center adopted a square planar coordination geometry, and the phosphine occupied the trans position to the nitrogen. The molecular structures of nickel complexes were determined by single-crystal X-ray diffraction analysis (Figure 1). Complexes C1 and C2 showed very similar structure in view of the bond distances and the bond angles and exhibited a six-membered (N, O) nickel chelate ring with a puckered conformation. Each nickel center adopted a square planar coordination geometry, and the phosphine occupied the trans position to the nitrogen.

Synthesis and Molecular Structure of Nickel Complexes.
The synthesis procedure of the ligands L1-L2 and the corresponding nickel complexes C1-C2 is shown in Scheme 1. 2-Anilinobenzoic acid methyl ester ligands were easily obtained by the amination of methyl 2-bromobenzoate in the presence Pd(dba)2/dppf and Cs2CO3 [48]. The nickel complexes were prepared from the reaction of corresponding potassium salt of ligand with 1 equiv of trans-[Ni(PPh3)2PhCl] in good yields according to the literature procedure [49]. The molecular structures of nickel complexes were determined by single-crystal X-ray diffraction analysis (Figure 1). Complexes C1 and C2 showed very similar structure in view of the bond distances and the bond angles and exhibited a six-membered (N, O) nickel chelate ring with a puckered conformation. Each nickel center adopted a square planar coordination geometry, and the phosphine occupied the trans position to the nitrogen.

Ethylene Polymerization
The nickel complexes C1 and C2 were applied in ethylene polymerization. The results are summarized in Table 1. C1 containing nonsubstituted aniline ligand was inactive regardless of Polymers 2018, 10, 754 5 of 10 the cocatalyst used. In contrast, C2 containing isopropyl substituent on the aniline ligand in the presence of trialkylaluminum-free dried modified methylaluminoxane (dMMAO) or Ni(COD) 2 promoted ethylene polymerizaiton with almost the same activities. The results suggested that the substituent on the aniline ligand and the cocatalyst as a phosphine scavenger are necessary for ethylene homopolymerization. The similar effects of the ligand substituent were observed with both cationic and neutral nickel catalyzed ethylene polymerization [3][4][5][6][7][8][9][10]. The Ni(COD) 2 system gave high molecular-weight polyethylene (up to 76.3 kg·mol −1 ) with narrow molecular weight distribution, while the dMMAO system produced ethylene oligomers along with a small amount of polyethylene (entry 2, Table 1). The improved chain transfer rate in the dMMAO system may be attributed to the efficient dissociation of the phosphine, owing to the stronger Lewis acidity of dMMAO than that of Ni(COD) 2 . The same phenomenon was also reported by Kim et al. by using ketoenamine-based neutral nickel complexes, in which the use of MAO conducted ethylene oligomerization with high activity [50]. Polymerization activity is strongly depended on the polymerization temperature and the ethylene pressure. The decrease of the temperature to 20 • C resulted in the decrease of the activity in one order of magnitude (entry 5). Although activity decreased as the temperature was increased to 60 • C (entry 6), the catalyst still showed high activity of 336 kg·mol −1 ·h −1 , indicating good thermal stability of this catalytic system. The reason for the small decrease of the activity at 60 • C could be attribute to the reduced ethylene concentration at the high polymerization temperature [51]. The molecular weight increased with decreasing the polymerization temperature, indicating that the increase of chain transfer rate is more effective than chain growth rate with rising the polymerization temperature ( Figure 2). Further increase in activity and molecular weight was observed by the increase of ethylene pressure at 40 • C.

Copolymerization of Ethylene with Polar Monomers
The ethylene copolymerizations with polar monomers such as 5-hexene-1-yl-acetate (HAc), 5-norbornene-2-yl acetate (NBAc), vinyl acetate (VAc), and methyl acrylate (MA) were conducted ( Table 2). Although the Ni(COD)2 system showed slightly higher comonomer incorporation than the dMMAO system, the copolymerization activity of Ni(COD)2 system was much lower than that of the dMMAO system, affording lower molecular weight copolymers (entries 1 and 3). In general, the polymerization of the sterically bulky monomers such as higher α-olefins and norbornene catalyzed by the neutral nickel complexes require MAO or MMAO as the phosphine scavenger to provide reasonable monomer coordination site [10]. The incorporation of the copolymers obtained was investigated by 1 H NMR, and the typical 1 H NMR spectra of copolymers obtained by complex C2 are illustrated in Figure 3. The signals at 4.06 ppm and 2.05 ppm in copolymers were assigned for CH2 and CH3 protons of ester-group label as a and e, respectively. The microstructure of the E-HAc copolymer was also investigated by 13 C NMR to demonstrate the incorporation of polar monomers ( Figure S5) [16,52]. HAc incorporation was increased (up to 1.33 mol %, entry 4) by increasing HAc concentration in the feed, although copolymerization activity and copolymer molecular weight were decreased slightly. As compare to the results with complex F [47], anilinobenzoic acid methyl ester-ligated complex C2 showed slightly lower copolymerization activity and HAc incorporation.

Copolymerization of Ethylene with Polar Monomers
The ethylene copolymerizations with polar monomers such as 5-hexene-1-yl-acetate (HAc), 5-norbornene-2-yl acetate (NB Ac ), vinyl acetate (VAc), and methyl acrylate (MA) were conducted ( Table 2). Although the Ni(COD) 2 system showed slightly higher comonomer incorporation than the dMMAO system, the copolymerization activity of Ni(COD) 2 system was much lower than that of the dMMAO system, affording lower molecular weight copolymers (entries 1 and 3). In general, the polymerization of the sterically bulky monomers such as higher α-olefins and norbornene catalyzed by the neutral nickel complexes require MAO or MMAO as the phosphine scavenger to provide reasonable monomer coordination site [10]. The incorporation of the copolymers obtained was investigated by 1 H NMR, and the typical 1 H NMR spectra of copolymers obtained by complex C2 are illustrated in Figure 3. The signals at 4.06 ppm and 2.05 ppm in copolymers were assigned for CH 2 and CH 3 protons of ester-group label as a and e, respectively. The microstructure of the E-HAc copolymer was also investigated by 13 C NMR to demonstrate the incorporation of polar monomers ( Figure S5) [16,52]. HAc incorporation was increased (up to 1.33 mol %, entry 4) by increasing HAc concentration in the feed, although copolymerization activity and copolymer molecular weight were decreased slightly. As compare to the results with complex F [47], anilinobenzoic acid methyl ester-ligated complex C2 showed slightly lower copolymerization activity and HAc incorporation.
The nickel complex C2 was also capable of copolymerizing ethylene with 5-norbornene-2-yl acetate (NB Ac ) with good activity, generating E-NB AC copolymer with reasonable incorporation (3.20 mol %, in Figure S6)). Unfortunately, the commercial polar comonomers such as VAc, and MA shut down copolymerization activity (entries 6, 7). This was mainly caused by the coordination of the carbonyl oxygen atom to the nickel center to generate a stable six-membered nickel chelate compound [4].
Polymers 2018, 10, x FOR PEER REVIEW 7 of 10 mol%, in Figure S6)). Unfortunately, the commercial polar comonomers such as VAc, and MA shut down copolymerization activity (entries 6, 7). This was mainly caused by the coordination of the carbonyl oxygen atom to the nickel center to generate a stable six-membered nickel chelate compound [4].

Conclusions
In conclusion, neutral nickel complexes bearing anilinobenzoic acid methyl ester ligand have been synthesized and characterized. The steric effect on the aniline ligand played a significant role in the polymerization performances, in which isopropyl substituted complex C2 could enhance ethylene polymerization activity. The complex C2 was also able to promote copolymerization of ethylene with polar monomers with high activity (114-277 kg·mol −1 ·h −1 ) and reasonable polar monomer incorporation.

Conclusions
In conclusion, neutral nickel complexes bearing anilinobenzoic acid methyl ester ligand have been synthesized and characterized. The steric effect on the aniline ligand played a significant role in the polymerization performances, in which isopropyl substituted complex C2 could enhance ethylene polymerization activity. The complex C2 was also able to promote copolymerization of ethylene with polar monomers with high activity (114-277 kg·mol −1 ·h −1 ) and reasonable polar monomer incorporation.