Styrene-containing Phosphine-sulfonate Ligands for Nickel- and Palladium-catalyzed Ethylene Polymerization

A series of phosphine-sulfonate ligands bearing 2-, 3- and 4-vinylphenyl on the phosphorus atom were designed, synthesized, characterized and investigated in Ni- and Pd-catalyzed ethylene polymerization. The structure of the phosphine-sulfonate Pd complex bearing 2-vinylphenyl on the phosphorus atom showed 2,1-insertion for the 2-vinyl group. The phosphine-sulfonate Ni complex bearing 2-vinylphenyl resulted in significantly increased thermal stability and polyethylene molecular weights (Mn = 3.69×104 g·mol−1 at 80 °C) versus the counterparts bearing 3-/4-vinyl groups as well as previously reported phosphine-sulfonate Ni complexes bearing bulky biaryl substituents.

In this contribution, we wish to explore an alternative strategy by introducing a polymerizable moiety to the ligand structure. This way, a polymeric substituent could be generated in situ during polymerization and significantly alters ligand steric effects. A series of phosphine-sulfonate ligands bearing 2-, 3-and 4-vinylphenyl groups on the phosphorus atom (Chart 1, X) (Scheme 1) were designed, synthesized, characterized and investigated in Ni-and Pd-catalyzed ethylene polymerization. These phosphine-sulfonate nickel and palladium catalysts are well-defined single component catalysts. For the nickel catalysts, the dissociated ligand PPh 3 was used. For the palladium catalysts, the dissociated ligand DMSO (dimethylsulfoxide) was used. It is hypothesized that the copolymerization of the 2-vinyl group on ligand with ethylene can generate a bulky polymer substituent on ligand [45,[56][57][58][59][60][61][62][63] associated with enhanced steric effect. Consequently, the catalyst bearing 2-vinylphenyl group might result in increased molecular weight of polyethylene product compared with the catalysts bearing 3-or 4-vinylphenyl groups.

EXPERIMENTAL General
All experiments were carried out under dry nitrogen atmosphere using standard Schlenk techniques or in a glove-box. Deuterated solvents used for NMR were dried and distilled prior to use. 1 H-, 13 C-and 31 P-NMR spectra were recorded on a Bruker AscendTm 400 spectrometer at ambient temperature unless otherwise stated. The chemical shifts of the 1 H-and 13 C-NMR spectra were referenced to tetramethylsilane; the 31 P-NMR spectra were referenced to an external 85% H 3 PO 4 solution. Coupling constants are in Hz. Elemental analysis was performed by the Analytical Center of the University of Science and Technology of China. X-ray diffraction data were collected at 298 (2) K on a Bruker Smart CCD area detector with graphitemonochromated Mo Kα radiation (λ=0.071073 nm). The molecular weights and molecular weight distributions of the polymers were determined using gel permeation chromatography (GPC) with a PL-220 equipped with two Agilent PLgel Olexis columns at 150 °C using trichlorobenzene as a solvent. The calibration curve was constructed from polystyrene standards and was corrected for linear polyethylene by universal calibration using the Mark-Houwink parameters of Rudin: K=5.90×10 −2 cm 3 ·g −1 and R = 0.69 for polyethylene. Dichloromethane, THF, and hexanes were purified by solvent purification systems.

Synthesis of L1
At 0 °C, n BuLi (2.5 g·mol −1 , 16 mL, 40 mmol) was added slowly to a solution of benzenesulfonic acid (3.16 g, 20 mmol) in THF (50 mL  g·mol −1 in hexane, 8 mL, 20 mmol) was added dropwise. The resulting red solution was stirred for 1.0 h at −78 °C before the lithium [chloro(cyclohexyl)phosphino]benzenesulfonate was added dropwise. The mixture was stirred for another 24 h at room temperature. The volatiles were removed, and the residue was taken up in distilled water (150 mL). The mixture was acidified to pH ~2 with concentrated HCl/H 2 O solution, and extracted three times with CH 2 Cl 2 (total volume 250 mL). The extracts were combined, dried over MgSO 4 , and concentrated under vacuum. The crude product was recrystallized from dichloromethane/ether at room temperature. The resulting white crystals were filtered and dried to give the desired ligand L1 (4.56 g, 61%). 1

Synthesis of L2
Similar procedure to the above was employed except 3-bromostyrene was used. L2 was obtained as a light white solid (4.1 g, 55%). 1

Synthesis of L3
Similar procedure to the above was employed except 2-bromostyrene was used. L3 was obtained as a light white solid (5.8 g, 77%). 1

Synthesis of Pd1
Ligand L1 (200 mg, 0.53 mmol) was suspended in dioxane (3 mL). (TMEDA)PdMe 2 (134 mg, 0.53 mmol) was added at room temperature. After 5 min the evolution of gas stopped and the suspension turned clear. The solution was stirred for 2 h at room temperature. The resulting white precipitate was filtered, washed with diethyl ether and dried under reduced pressure. Then it was dispersed in 10 mL of DMSO at room temperature. The solvent was removed under reduced pressure at 70 °C. After removal of DMSO under reduced pressure, the resulting solid was dispersed in diethyl ether, and isolated by filtration to yield a gray solid Pd1 (203 mg, 67%). 1

Synthesis of Ni1
A suspension of L1 (200 mg, 0.53 mmol) and Na 2 CO 3 (169 mg, 1.53 mmol) in 15 mL of dichloromethane was stirred for 6 h at room temperature. trans-[(PPh 3 ) 2 Ni(Cl)Ph] (368 mg, 0.53 mmol) was added in small portions. Dichloromethane was added until the volume of the solution reached 20 mL, and the reaction mixture was stirred for 24 h at room temperature. The resulting yellow-orange mixture was filtered over Celite and the volatiles were removed under vacuum. Toluene (3 mL) was added to the orange residue to afford a slurry, then hexane (5 mL) was added and the mixture was stirred for 5 min. The precipitate was recovered by filtration, washed with hexane (3×10 mL) and dried for 20 h under dynamic vacuum to yield a yellow solid Ni1 (245 mg, 60%). 1

Procedure for Ethylene Polymerization
In a typical experiment, a 350 mL glass thick-walled pressure vessel was charged with 20 mL of toluene and a magnetic stir bar under an inert atmosphere. The vessel was pressurized with ethylene gas with stirring. Then catalyst (in 1 mL of CH 2 Cl 2 ) was injected to initiate polymerization and stirred continuously for the desired time. The polymerization was quenched by adding ethanol (30 mL) and the polymer was precipitated and dried overnight in vacuum at 50 °C.

Ethylene Polymerization
The results of the nickel and palladium catalyzed ethylene polymerizations are summarized in Table 1. The phosphine-sulfonate nickel and palladium catalysts are single component catalysts. They can mediate ethylene polymerizations in the absence of any co-catalysts or scavengers. [47−55] Catalysts Pd1 and Pd2 exhibited high catalytic activities in ethylene polymerization (activity >10 5 g·mol Pd −1 ·h −1 ), generating high molecular weight polyethylene (M n up to 3.35×10 4 g·mol −1 ) (Table1, entries 1 and 2). The above asymmetrically cyclohexyl/vinylphenyl phosphine-sulfonate based Pd catalysts resulted in higher molecular weights of polyethylene compared with symmetrical dicyclohexyl-or diphenyl-substituted phosphinesulfonate based Pd catalysts. [64] Catalyst Pd1 resulted in higher molecular weight of polyethylene compared with Pd2, which could be due to the electronic effects. 3-Vinyl group is more electron-withdrawing than 4-vinyl group (σ m of vinyl is 0.06 and σ p of vinyl is −0.04), [65] resulting in reduced electron density from the palladium center associated with decreased molecular weight of polyethylene. [50,66−68] Catalyst Pd3 showed no catalytic activity in ethylene polymerization (Table 1, entry 3), which could be due to the formation of stable five-membered ring in the structure of Pd3.
The nickel catalysts Ni1, Ni2 and Ni3 exhibited low catalytic activities in ethylene polymerization at room temperature (Table 1, entries 4−6), generating high molecular weight polyethylene (M n up to 2.51×10 5 g·mol −1 ). Similar to Pd1 and Pd2, catalyst Ni1 resulted in higher molecular weight of polyethylene compared with Ni2, which could be due to the electronic effects (for vinyl, σ m > σ p ). [50,65−68] Catalyst Ni2 resulted in low-melting branched polyethylenes (Table 1, entries 5 and 8). For the formation of the branched polyethylenes, the exact mechanism is currently unknown. Generally, the electronically unsymmetrical nature of phosphine-sulfonate ligands was believed to inhibit β-H (X) elimination, resulting in the formation of highly linear polyethylenes. [14] 3-Vinyl group is a more electron-withdrawing group compared with 4-vinyl group (σ m of vinyl is 0.06 and σ p of vinyl is −0.04). [65] Consequently, the 3-vinyl group may alleviate the electronic unsymmetry in the ligand, resulting in the formation of branched polyethylene. [52] The catalytic activities of the nickel catalysts increased significantly with increasing temperat-ure, while the polyethylene molecular weight decreased (Table 1, entries 7−9). Catalysts Ni1 and Ni2 showed moderate catalytic activities and generated low molecular weight polyethylene products (M n <1500 g·mol −1 ) at 80 °C (Table 1,  entries 7 and 8), indicating the β-H elimination was accelerated significantly with increasing temperature. Remarkably, catalyst Ni3 bearing 2-vinylphenyl exhibited high catalytic activity (activity=3.4×10 5 g·mol Ni −1 ·h −1 ) and resulted in much higher molecular weight of polyethylene (M n =3.69×10 4 g·mol −1 , M w =1.38×10 5 g·mol −1 ) ( Table 1, entry 9) compared with Ni1 and Ni2, indicating it is potentially suitable for industrially used gas-phase ethylene polymerization conditions (80−100 °C). [21,69] The polyethylenes′ molecular weights declined more than 2 orders of magnitude for Ni1 and Ni2 with increasing temperature. In contrast, the polyethylenes molecular weight was decreased less than 7 times for Ni3 (Table 1, 7 versus 4, 8 versus 5, 9 versus 6), demonstrating the great thermal stability of Ni3.
We do not currently fully understand the effect of the 2vinyl group in this system. Nevertheless, there are several possible explanations on the good catalytic performance of Ni3. First, the 2-vinyl group is more sterically hindered compared with 3-and 4-vinyl groups. However, Ni3 resulted in much higher molecular weight compared with the phosphine-sulfonate nickel catalysts bearing bulkier biaryl groups (M n up to 1.03×10 4 g·mol −1 , typically <10 4 g·mol −1 ) at 80 °C (Scheme 2). [48][49][50][51][52][53][54] Therefore, the superior catalytic performance of Ni3 cannot solely be explained by the steric hindrance of the 2-vinyl group. Second, the 2-vinyl group on ligand could be copolymerized with ethylene via nickel catalyzed polymerization, resulting in the formation of a bulky polymer substituent (Scheme 3a) associated with the enhanced steric effect. This was reported by literature evidences that some nickel complexes can efficiently catalyze copolymerization of ethylene with styrene. [57][58][59] As a result, Ni3 can generate high molecular weight polyethylene at 80 °C, since the sterically bulky polymer substituent at the axial position with respect to the nickel center suppressed β-H elimination during ethylene polymerization. The significantly increased PDI value also supported this hypothesis (Table 1,  entry 9). Third, the 2-vinyl substituent at the axial position might form a secondary interaction with the nickel center (Scheme 3b), preventing ethylene coordination and subsequently chain transfer to monomer. [2,3,10,50,70−72]

CONCLUSIONS
In summary, a series of phosphine-sulfonate ligands containing styrene moieties (L1, L2 and L3) and the corresponding nickel (Ni1, Ni2 and Ni3) and palladium (Pd1, Pd2 and Pd3) catalysts have been synthesized, characterized and investigated in ethylene polymerization. The syntheses of the ligands were easier than the traditional phosphine-sulfonate ligands bearing bulky biaryl or menthyl groups. Catalysts Ni1 and Pd1 bearing 4-vinyl group led to polyethylenes with higher molecular weights than those from Ni2 and Pd2 bearing 3-vinyl group, which could be due to the electronic effects (for vinyl, σ m >σ p ). The more electron-withdrawing 3-vinyl group resulted in reduced electron density from the metal center, resulting in decreased molecular weight of polyethylene. Pd3 bearing 2vinylphenyl showed 2,1-insertion for the 2-vinyl group, indicating that the 2-vinyl group was a polymerizable moiety during coordination-insertion polymerization. Catalyst Pd3 possesses no catalytic activity in ethylene polymerization, which could be due to the formation of the stable five-membered ring in the structure of Pd3. Catalyst Ni3 bearing 2-vinylphenyl resulted in higher activity and much higher polyethylene molecular weight (M n =3.69×10 4 g·mol −1 , M w =1.38×10 5 g·mol −1 ) compared with Ni1, Ni2 as well as previously reported nickel catalysts bearing bulky biaryl groups (M n up to 1.03×10 4 g·mol −1 , typically <10 4 g·mol −1 ) at 80 °C. [48][49][50][51][52][53][54] Some possible explanations on the superior catalytic performance of Ni3 have been discussed. It is envisaged that vinyl-containing ligands will be applicable to other catalytic systems for olefin polymerization and other types of organic transformations.