Ethylene polymerization using N-Heterocyclic carbene complexes of silver and aluminum

ABSTRACT Various transition metal catalysts have been utilized for ethylene polymerization. Silver catalysts have attracted less attention as the catalysts, but are potential for production of high molecular weight polyethylene. Herein, we report that silver complexes with various N-heterocyclic carbene (NHC) ligands in combination with modified methylaluminoxane (MMAO) afford polyethylene with high molecular weight (melting point over 140°C). SEM observation showed that the produced polyethylene has ultra-high molecular weight. NMR investigation of the reaction between the silver complexes with organoaluminums indicate that the NHC ligands transfer from the silver complex to aluminum to produce NHC aluminum complexes. Ph3C[B(C6F5)4] abstract methyl group from the NHC aluminum complex to afford cationic aluminum complex. The NHC aluminum complex promoted ethylene polymerization in the presence of Ph3C[B(C6F5)4] and organoaluminums. NHC ligand also promoted ethylene polymerization in combination with MMAO to produce polyethylene with high melting point (140.7°C). Thus, the aluminum complexes are considered to be the actual active species in silver-catalyzed ethylene polymerization.

Silver is another minor transition metal that is known to promote ethylene polymerization. Although active species of most of the above-mentioned olefin polymerization catalysts are cationic alkyl species, such species is not common in silver complex, as monovalent species is common in silver. Silver complexes are often light sensitive and need care in handling. These would be the possible reason why silver catalysts have attracted less attention as olefin polymerization catalysts. However, Jin reported that trinuclear silver complex with pyridyl-substituted N-heterocyclic carbene (NHC) ligand shows high activity for ethylene polymerization in the presence of methylaluminoxane (MAO) (Scheme 1) [33]. The produced polyethylene is rarely soluble in common organic solvents, and no further characterization of the produced polyethylene has been demonstrated. The detailed mechanism of the polymerization, such as active species of the polymerization, is not clear, either. In the present paper, we examined ethylene polymerization by silver as well as copper and gold complexes with various NHC ligand in the presence of MAO. cocatalyst (Table 1). Figure 1 shows the NHC silver complexes used in this study. First, we attempted to synthesize trinuclear silver complex with 1-methyl-3-(pyridylmethyl)imidazolylidene ligand reported by Jin [33], but it was not successful. Thus, we synthesized some mononuclear silver complexes with various NHC ligands, including 1-methyl-3-(pyridylmethyl)imidazolylidene ligand, and examined them for ethylene polymerization. Silver complex with 1-methyl-3-(pyridylmethyl) imidazolylidene ligand (1-Ag) brought about polymerization in the presence of modified methylaluminoxane (MMAO) [34]. The activity was 0.103 g mmol Ag −1 h −1 (Table 1, run 1), which is much lower than that by trinuclear silver complex reported by Jin (470 g mmol Ag −1 h −1 ) [33]. Silver complexes with 1-tert-butyl-3-(pyridylmethyl)imidazolylidene ligand (2-Ag) and 1,3-bis-(pyridylmethyl)imidazolylidene ligand (3-Ag) were also effective for the polymerization (runs 2 and 3), although their activity was lower than 1-Ag with N-methyl group (run 1). Silver complex with 1-methyl-3-benzyl-imidazolylidene ligand (4-Ag), without pyridyl group, also produced the polymer in lower catalytic activity (run 4).
Differential scanning calorimetry (DSC) analysis of the polyethylene obtained by the silver complexes show their melting point at 138-143°C (1st heating), which are characteristic of ultra-high molecular weight polyethylene with highly linear structure [35]. Thus, the low solubility of the produced polyethylene would be because they are ultra-high molecular weight. MMAO-12, which has octyl-Al bond, was also effective as cocatalyst, and afforded polyethylene with T m = 141.4°C (run 8). Cob-web morphology was observed in scanning electron microscope (SEM) image of the obtained polyethylene ( Figure 2), which is characteristic of polyethylene powders as polymerized [36]. Decrease in concentration of the silver complex (1-Ag) resulted in almost comparable catalytic activity (runs 9 and 1) and formation of polyethylene with T m = 141.4°C. However, decrease in concentration of MMAO lead to significant decrease in catalytic activity (runs 10 and 1).
MAO in combination with 1-Ag produced polyethylene with higher catalytic activity to 1-Ag/MAO (runs 11 and 9). In this case, again decrease in catalytic activity was observed when the concentration of MAO was decreased (runs 12, 13, and 11). Increase in ethylene pressure has negative effect on catalytic activity (run 14). 2-Ag/diethylaluminum chloride (DEAC) showed much lower activity than 2-Ag/MMAO, and afforded polyethylene with lower melting point (runs 15). The actual catalytic species in the present silver/organoaluminum catalyst systems is considered to be a cationic aluminum complex, formed by the ligand transfer from silver to aluminum (vide infra). The lower Lewis acidic character of DEAC compared to MAO and/or the presence of chloride on the cationic aluminum species would be the possible reason for the low activity. In order to promote efficient ligand transfer from silver to aluminum before the polymerization, aging the catalyst was conducted at 30°C for 18 h before starting polymerization. However, it showed small positive effect on the catalyst activity (run 16 and 2).
In addition to NHC silver complexes, NHC copper and NHC gold complexes (6-Cu and 6-Au) were also examined for the ethylene polymerization (runs 13 and 14). Actually, both copper and gold complexes were effective for the ethylene polymerization in the presence of MMAO. The copper complex showed slightly higher catalytic activity than the corresponding silver complex, whereas the activity of the corresponding gold complex was much lower. The melting points of the produced polyethylenes obtained by Cu, Ag, and Au complexes, however, were above 139°C, and formation of ultra-high molecular weight polyethylene was indicated in all these cases.
Several copper complexes have been reported to promote ethylene polymerization in the presence of organoaluminums [29][30][31]. In these cases, however, the actual active species is proposed to be organoaluminum complex formed via ligand transfer from the copper complex to the organoaluminum [32]. There has been no report on gold catalyst capable of promoting ethylene polymerization. Thus, it is anticipated that the active species in the present polymerization is NHC organoaluminum complex, formed via transfer of NHC ligand from the silver, copper, or gold complex to aluminum complex, rather than the organosilver, organocopper, or organogold complex.

NHC ligand transfer from silver to aluminum
In order to confirm that the NHC aluminum complex actually forms in the present polymerization, the NMR study of the reaction between NHC silver complex and organoaluminum was conducted. Thus, (IPr)AgCl (6-Ag) (0.12 mmol) and i Bu 3 Al (0.12 mmol) were mixed in C 6 D 6 (0.5 mL) and reacted at room temperature. 13    It is anticipated that the IPr ligand, dissociated from the silver center, is transferred to aluminum. 1 H NMR spectrum of the mixture after 24 h showed a new doublet signal at δ −0.33, which is assigned to AlCH 2 CH(CH 3 ) 2 attached to IPr ( Figure S1). The spectrum also showed a singlet signal at δ 6.65 as well as triplet signals at δ 7.39, 7.21, and 7.03 with intensity ratio of 1.00 : 0.88 : 3.96. This result indicates that (IPr)AliBu 3 complex, once formed via the ligand transfer from Ag to Al, is transformed to zwitter ionic species. It has been reported that the reaction of IPr with R 3 Al affords (IPr)AlR 3 , which is easily transformed to zwitterionic species due to the steric repulsion between bulky NHC ligand with alkyl group on aluminum center [37,38]. The similar reaction also took place in the present reaction. The slow ligand transfer reaction from Ag to Al would be also due to the steric bulkiness of the i Bu 3 Al and IPr ligand.
Thus, the reaction of less sterically hindered (IMes) AgCl (5-Ag) with Me 3 Al was conducted. (IMes)AgCl (5-Ag) (0.18 mmol) and Me 3 Al (0.18 mmol) were reacted in toluene (1.8 mL) at r.t. for 12 h. After filtration, the filtrate was concentrated and cooled to −20°C to afford white powder. Figure 4 shows the 1 H NMR spectra of the produced powder (Figure 4 (i)) and (IMes)AlMe 3 complex prepared by the reaction of IMes with Me 3 Al according to the procedure reported by Dagorne (Figure 4 (ii)) [39,40].
Chemical shift of the signals due to IMes ligand of the product are close to that of (IMes)AlMe 3 . The signal at δ −0.78 is assigned to AlMe group. Intensity ratio of the signal (e) and that of IMes ligand (b) is 6.0:4.0, which indicates that two methyl group is bonded to Al. Thus, it is speculated that the complex formed by the reaction of (IMes)AgCl and Me 3 Al is (IMes)AlMe 2 Cl (eq. 1). Thus, IMes ligand transfers from silver to aluminum. It has been reported recently that the reaction of (IMes)AgCl and AlCl 3 affords (IMes)AlCl 3 via ligand transfer from silver to aluminum [41].
Progress of the ligand transfer reaction was monitored by 1 H NMR ( Figure S2). (IMes)AgCl (5-Ag) (0.047 mmol) and Me 3 Al (0.12 mmol, 2 M toluene solution) were mixed in C 6 D 6 (0.45 mL) and reacted at room temperature (naphthalene (0.025 mmol) was added as internal standard). After the reaction time of 1 h, new signals were observed at δ −0.77 and −0.78 ppm, which are due to (IMes)AlMe 2 Cl formed by the ligand transfer from 5-Ag to AlMe 3 , and (IMes) AlMe 3 formed by the reaction between (IMes)AlMe 2 Cl and AlMe 3 , respectively. The relative intensity of the signal at δ 5.94 due to imidazolium group and those due to AlMe is 2.0:3.8. The intensity of the signals due to AlMe becomes larger after 2 h (2.0:6.3) and  after 3 h (2.0:11.0). Thus, the ligand transfer reaction from (IMes)AgCl to AlMe 3 seems to complete in 3 hours, and it is speculated that the reaction is much faster in the presence of large excess of organoaluminum with respect to the silver complex.
Synthesis of the similar NHC aluminum complex by the reaction of silver complex with 1-methyl-3-(pyridylmethyl)imidazolylidene ligand (1'-Ag) (0.30 mmol) with Me 3 Al (0.9 mmol, 2 M toluene solution) was conducted similarly to the synthesis of (IMes)AlMe 2 Cl. 1 H NMR spectrum of the reaction mixture (reaction time = 21 h) showed the signals at δ 8.39, 5.06, and 3.14, which are assigned to vinylene, CH 2 -py, and N-Me groups the 1-methyl-3-(pyridylmethyl)imidazolylidene ligand, as well as the signal at δ −0.61, which is assigned to the AlMe group. The intensity ratio of the signal due to CH 2 -py and the signal due to AlMe is 2.0:5.9, which indicates formation of (NHC) AlMe 2 Cl complex ( Figure S3). However, minor signals are observed at 7.5 to 2.5 ppm as well as 0.3 to −0.3 ppm, which is due to the presence of other organoaluminum species, such as those with AlMe 3 coordinated to the pyridyl group. The higher activity of the silver complexes with pyridylmethyl group would be due to the cooperation between the aluminum centers locating on imidazolylidene group and pyridyl group. Attempted isolation of the product by recrystallization was not successful.
Activity of (IMes)AlMe 2 Cl (5-AlCl)/NaBARF/ i Bu 3 Al was lower than (IMes)AlMe 3 /Ph 3 C[B(C 6 F 5 ) 4 ]/Me 3 Al (run 8), which would be due to slower formation of cationic aluminum complex in the reaction between (IMes) AlMe 2 Cl and NaBARF (BARF = [B{C 6 H 3 (CF 3 ) 2 -3,5} 4 ]) in toluene. The IMes ligand (5) was also employed in combination with MMAO for the ethylene polymerization, where polyethylene was obtained in higher yield (run 9). The melting point of the produced polyethylene was observed at 140.7°C, which is higher than that obtained by (IMes)AlMe 3 /Ph 3 C[B(C 6 F 5 ) 4 ]/Me 3 Al (133.3°C). Thus, MAO might play an important role for the production of higher molecular weight polyethylene. Compared to the silver complex/MAO systems, the transition-free Al-based catalyst systems showed lower catalytic activity. Silver is considered to enhance activity of the Al catalyst, but its exact role is not clear at present.

Conclusion
NHC silver complexes promote ethylene polymerization in the presence of MMAO. The produced polyethylene is rarely soluble in common organic solvent, but DSC analysis and SEM observation indicate ultra-high molecular weight of the produced polyethylene. Investigation of the reaction between the NHC silver complex with organoaluminum indicates the NHC ligand transfer from silver to aluminum takes place. Thus, the NHC aluminum complex is considered to be the actual active species of the polymerization. The present catalyst must be also valuable from a viewpoint of transition-metal-free olefin polymerization catalyst. However, the use of NHC silver complexes has advantage to easily introduce less stable NHC ligand to aluminum center.

General method
All manipulations of air-and water-sensitive compounds were carried out under nitrogen. NMR spectra were recorded on JEOL JNM-ECZ500R spectrometer at 20°C (in C 6 D 6 ) or 130°C (in C 2 D 2 Cl 4 ). 1 H (500 MHz) and 13 C{ 1 H}(125 MHz) NMR chemical shifts were referenced to C 6 H 6 (δ 7.16) in the C 6 D 6 solvent or C 2 H 2 Cl 4 (δ 5.91) in the C 2 D 2 Cl 4 solvent for 1 H and C 6 D 6 (δ 128) or C 2 D 2 Cl 4 (δ 74.2) for 13 C. High temperature GPC A morphology of the polyethylene was observed as a secondary electron image using a Hitachi S-4800 field-emission scanning electron microscope (SEM) operated at an accelerating voltage of 1.0 kV and a working distance of 8.4 mm. The sample was coated with Pt -Pd before the observation. DSC was recorded on Seiko DSC6200R instruments (1 st heating and cooling, heating rate = 10°C/min, cooling rate = −20°C/min).

Reaction of NHC or NHC silver complex with organoaluminum
NHC or NHC silver complex was placed in 50 mL Schlenk flask. The reaction vessel was dried in vacuo, and N 2 gas was purged. Toluene was added to the flask, and the mixture was stirred overnight at room temperature. The reaction mixture was filtered through celite and filtrate was concentrated under reduced pressure. The solution was recrystallized at −20℃ to afford the crystal of NHC aluminum complexes.

Polymerization reactions
NHC silver complex was placed in 100-mL autoclave and was dried in vacuo. The reaction vessel was purged with 1 MPa ethylene and degassed. This process was repeat three times. And then, toluene was added to the reaction vessel and 1 MPa ethylene was purged. After stirred at 750 rpm for 20 h, the reaction mixture was poured to HCl/MeOH, and formed white precipitate was collected by filtration.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This work was supported by a JSPS KAKENHI Grant Numbers or JP18H02017 and JP20H00386.

Data availability statement
The data that support the findings of this study ( 1 H NMR spectra and DSC charts) are available in the supporting information https://doi.org/10.6084/m9.figshare.23585295.

References
[1] Hakim S, Nekoomanesh M, Shahrokhinia A. The effect of mixed and individual silane external donors on the stereo-defect distribution, active sites and properties of polypropylene synthesized with fourth generation