Elucidating the Key Role of Phosphine−Sulfonate Ligands in Palladium-Catalyzed Ethylene Polymerization: Effect of Ligand Structure on the Molecular Weight and Linearity of Polyethylene

Abstract

The mechanism of linear polyethylene formation catalyzed by palladium/phosphine−sulfonate and the effect of the ligand structure on the catalytic performance, such as linearity and molecular weight of the polyethylene, were reinvestigated theoretically and experimentally. We used dispersion-corrected density functional theory (DFT-D3) to study the entire mechanism of polyethylene formation from (R 2 PC 6 H 4 SO 3 )PdMe(2,6-lutidine) (R = Me, t-Bu) and elucidated the key steps that determine the molecular weight and linearity of the polyethylene. The alkylpalladium ethylene complex is the key intermediate for both linear propagation and β-hydride elimination from the growing polymer chain. On the basis of the key species, the effects of substituents on the phosphorus atom (R = t-Bu, i-Pr, Cy, Men, Ph, 2-MeOC 6 H 4 , biAr) were further investigated theoretically to explain the experimental results in a comprehensive manner. Thus, the experimental trend of molecular weights of polyethylene could be correlated to the ΔΔG ⧧ value between (i) the transition state of linear propagation and (ii) the transition state of the path for ethylene dissociation leading to β-hydride elimination. Moreover, the experimental behavior of the catalysts under varied ethylene pressure was well explained by our computation on the small set of key species elucidated from the entire mechanism. In our additional experimental investigations, [o-Ani 2 PC 6 H 4 SO 3 ]PdH[P(t-Bu) 3 ] catalyzed a hydrogen/ deuterium exchange reaction between ethylene and MeOD. The deuterium incorporation from MeOD into the main chain of polyethylene, therefore, can be explained by the incorporation of deuterated ethylene formed by a small amount of Pd−H species. These insights into the palladium/phosphine−sulfonate system provide a comprehensive understanding of how the phosphine−sulfonate ligands function to produce linear polyethylene.

Olefins and Vinyl Polar Monomers: Bridging the Gap for Next Generation Materials

“…[26] Olefin polymerization involving these catalysts produce much more linear polymers than their Brookhart counterparts,a st he combination of as trong and sterically bulky s donor and aw eak s donor promote linear polyethylene formation. [27] Copolymerization of ethylene and polar vinyl monomers with these catalysts has been investigated, [28] and polymers with up to 7% polar monomer incorporation within the main polymer chain, not exclusively at the end, are obtained. [29] Recent advances have investigated nickel catalysts containing phosphine-sulfonate ligands with varied sterics, [30] electronics, [31] and ligand substituents [32] which have afforded catalysts with higher olefin polymerization rates compared to their palladium counterparts with similar levels of polar monomer incorporation.…”

Recent Progress in Ethylene Polymerization Catalyzed by Ni and Pd Catalysts

“…Ligands do have critical influences on the ethylene polymerization process, thus greatly influencing the microstructures of the resulting polymers . Ligands such as α‐diimines,[4f], , [15b], , , phosphine‐sulfonates,, [15a], salicylaldimines,, , [15d], [19a], and iminopyridines, forming five‐ or six‐membered chelate rings with the metal center are still common for catalyst design because of their higher stability, and modification of ligands in substituents and backbone is a common – but not easy – way to design new, high‐performance catalysts. In fact, there are still some examples of failed attempts to design catalysts with special performances, as can be seen from recent reports …”

“…Electronic effects mainly originate from the electron‐donating or electron‐withdrawing natures of substituents whereas steric effects are related to interatomic distances and angles, the sizes of substituents, the ionic radii of the transition metals, and so on. [15c] Usually, bulky substituents near the metal can block the axial site of the metal center, thus suppressing chain transfer[19b], , [27l] and affording polymers with higher molecular weights . Electronic perturbation of ligands can modify the net charge at the metal center, to yield polymers with a variety of microstructures (including changing the insertion locations of polar comonomers for copolymers) and can markedly influence the activities of ethylene polymerization.…”

“…With regard to characterization methods, electron paramagnetic resonance (EPR) spectroscopy is popular as a useful method for studying the mechanisms of Ni‐catalyzed ethylene polymerization and the nature of active species . Density functional theory (DFT) methods, as theoretical computational methods, have been widely used in combination with experimental results to develop polymerization mechanisms at the molecular level . For data analysis, the response surface method (RSM) has been successfully applied to the modelling of the relationships between multiple operational parameters and results…”

Ethylene (co)Oligomerization by Phosphine‐Pyridine Based Palladium and Nickel Catalysts

“…The phosphine sulfonate ligands combine the strong σ‐donating phosphine and weak σ‐donating sulfonate groups. This electron‐unsymmetric feature was generally believed to be able to inhibit β‐H (X) elimination, enabling the formation of linear polymers and ethylene‐polar monomer copolymers . However, Jordan group and Nozaki group showed that the change of the phosphine group to a stronger σ‐donating carbene group led to inactive palladium catalysts (Scheme , II and III ) ,.…”