Linear, high molecular weight polyethylene from a discrete, mononuclear phosphinoarenesulfonate complex of nickel(ii)

Origin of Suppressed Chain Transfer in Phosphinephenolato Ni(II)-Catalyzed Ethylene Polymerization

Recent breakthroughs in the generation of polar-functionalized and more sustainable degradable polyethylenes have been enabled by advanced phosphinephenolato Ni(II) catalysts. A key has been to overcome this type of catalysts' propensity for extensive chain transfer to enable formation of high-molecular-weight polyethylene chains. We elucidate the mechanistic origin of this paradigm shift by a combined experimental and theoretical study. Single-crystal X-ray structural analysis and cyclic voltammetry of a set of six different catalysts with variable electronics and sterics, combined with extensive pressure reactor polymerization studies, suggest that an attractive Ni-aryl interaction of a P-[2-(aryl)phenyl] is responsible for the suppression of chain transfer. This differs from the established picture of steric shielding found for other prominent late transition metal catalysts. Extensive density functional theory studies identify the relevant pathways of chain growth and chain transfer and show how this attractive interaction suppresses chain transfer.

Catalytic applications of zwitterionic transition metal compounds

This frontiers article highlights recent developments on the application of transition metal-based zwitterionic complexes in catalysis. Recent applications of selected zwitterionic catalysts in polymerization reactions, including the carbonylative polymerization of cyclic ethers, carbon-carbon coupling reactions, the asymmetric hydrogenation of unfunctionalized olefins, and the hydrofunctionalization of alkenes are reviewed. In addition, advances in the field of hydrogenation/dehydrogenation reactions related to energy applications, including the hydrogenation of CO₂ and the dehydrogenation of formic acid and N-heterocycles, the functionalization of CO₂ with amines and hydrosilanes, and the valorization of polyfunctional bio-based feedstocks, such as the dehygrogenation of glycerol to lactic acid or the reduction of levulinic acid into γ-valerolactone, are also described.

Neutral Nickel(II) Catalysts: From Hyperbranched Oligomers to Nanocrystal-Based Materials

Plastics materials are a vital component of modern technologies. They are applied, e.g., in construction, transportation, communication, water supply, or health care. Consequently, polyolefins-the most important plastics by scale-are produced in vast amounts by catalytic polymerization. Effective and selective as the catalysts used may be, their high sensitivity toward any polar compounds limits these methods to hydrocarbon reaction media and monomers like ethylene and propylene, respectively. This can be overcome by less oxophilic late transition metal catalysts, and here particularly neutral nickel(II) catalysts have seen major advances in the past few years. They stand out due to being capable of aqueous catalytic polymerizations. Aqueous polymerizations are benign processes that advantageously yield polymers in the form of particles. Moreover, these catalysts can incorporate polar monomers like acrylates, a realm previously restricted to noble metal catalysts. The introduction of polar moieties can induce properties like compatibility with metals or fibers in high performance composite materials or a desirable degradability.This Account provides a personal account of developments in the past decade. Prior findings are outlined briefly as a background. Aqueous polymerizations afford unique polyethylene morphologies as a result of the unusual underlying particle growth mechanism. Polymer single crystals are formed, which can be composed of a single ultrahigh molecular weight chain. This represents a completely disentangled state of such extremely long polymer chains, which has been long sought-after in order to overcome the difficult processing of high performance ultrahigh molecular weight materials. A key prerequisite for this approach and utilization of these catalysts, in general, is control of polymer branching and molecular weight. This is achieved via remote substituents on the Ni(II)-chelating ligand. Despite their distal position to the active site, weak secondary interactions control whether branching and chain transfer pathways compete very effectively with chain growth or are suppressed entirely. This provides access to hyperbranched oligomers, on the one hand, and enables living polymerizations to strictly linear high molecular weight polymer, on the other hand. Other advanced catalysts provide linear copolymers with in-chain polar monomer repeat units for the first time with non-noble metal active sites. Mechanistic studies further revealed that for copolymerizations with polar vinyl monomers the decisive limiting factor is irreversible termination reactions with neutral Ni(II) catalysts, rather than the well-recognized reversible blocking of coordination sites by the polar functional groups found for other types of catalysts. The mechanistic picture also implies the possibility of free-radical pathways, and their role in the formation of desirable polymer end groups and polymer blends is now being recognized. The area of neutral Ni(II) catalysts has progressed significantly in the entire range from fundamental mechanistic understanding, catalyst performance, and previously inaccessible polymer microstructures, and it is moving forward to materials through unique concepts. The unprecedented ability to incorporate functional groups into linear crystalline polyethylene also provides perspectives for much needed polyolefin materials that will not persist in the natural environment for several decades but that can be degraded by virtue of low levels of functional groups.

Ortho-Functionalized Dibenzhydryl Substituents in α-Diimine Pd Catalyzed Ethylene Polymerization and Copolymerization

Sterically bulky diarylmethyl-based ligands have received increasing attention in the field of late-transition-metal catalyzed olefin polymerization. Ortho-substituents may have a significant impact on the performance of diarylmethyl-based α-diimine Pd catalysts. In this contribution, a series of α-diimine Pd catalysts bearing ortho-methoxyl/hydroxyl functionalized dibenzhydryl units were prepared, characterized, and investigated in ethylene polymerization and copolymerization with methyl acrylate (MA). The catalytic performances were improved by introducing more ortho-substituents. The catalysts exhibited good thermal stabilities at high temperatures, producing branched polyethylenes. The catalysts bearing hydroxyl groups possessing intramolecular H-bonding, resulted in slightly higher incorporation ratios of MA unit when compared with the catalysts bearing methoxyl groups.

Ligand–metal secondary interactions in phosphine–sulfonate palladium and nickel catalyzed ethylene (co)polymerization

Ligand secondary interactions and Lewis acid modulation are simultaneously achieved in palladium and nickel catalyzed ethylene polymerization and copolymerization.

Camphor-based phosphine-carbonyl ligands for Ni catalyzed ethylene oligomerization

Ni and Pd complexes of camphor-based phosphine-carbonyl ligands containing biaryl moiety are designed and synthesized. The Ni complexes can catalyze ethylene oligomerization and generate waxy higher olefins as well as oily lower olefins.

Sterically very bulky aliphatic/aromatic phosphine-sulfonate palladium catalysts for ethylene polymerization and copolymerization with polar monomers

Copolymerization of ethylene and polar monomers with significantly high molecular weight and activity was achieved using bulky aliphatic/aromatic phosphine-sulfonate palladium catalysts.

Norbornene polymerization and copolymerization with 1-alkenes by neutral palladium complexes bearing aryloxide imidazolin-2-imine ligand

A series of novel aryloxide imidazolin-2-imine bidentate neutral Pd(ii) complexes with different substituents on the imidazolin-2-imine ligand were synthesized and characterized.

Phosphine phosphonic amide nickel catalyzed ethylene polymerization and copolymerization with polar monomers

The phosphine phosphonic amide ligand platform is highly versatile, with three positions that can be well-tuned.

ortho-Phenyl dialkylphosphonium sulfonate compounds: two rotamers in equilibrium

Two known and two new ortho-phenyl phosphonium–sulfonate compounds have been synthesized and analyzed in solution and in the solid state.

Influence of Ligand Backbone Structure and Connectivity on the Properties of Phosphine-Sulfonate Pd(II)/Ni(II) Catalysts

Phosphine-sulfonate based palladium and nickel catalysts have been extensively studied in ethylene polymerization and copolymerization reactions. Previously, the majority of the research works focused on the modifications of the substituents on the phosphorous atom. In this contribution, we systematically demonstrated that the change of the ligand backbone from benzene to naphthalene could greatly improve the properties of this class of catalysts. In the palladium system, this change could increase catalyst stability and polyethylene molecular weights. In the nickel system, this change could dramatically increase the polyethylene molecular weights. Most interestingly, the change in the connectivity of phosphine and sulfonate moieties to the naphthalene backbone could also significantly influence the catalyst properties.

Manipulation of polymer branching density in phosphine-sulfonate palladium and nickel catalyzed ethylene polymerization

A series of palladium and nickel catalysts bearing heteroaryl-aryl moieties were prepared and applied to ethylene polymerization and copolymerization reactions.

Phosphine-sulfonate-based nickel catalysts: ethylene polymerization and copolymerization with polar-functionalized norbornenes

A series of biaryl-based phosphine-sulfonate nickel catalysts are shown to mediate efficient ethylene polymerization and copolymerization with polar-functionalized norbornene comonomers.

Synthesis of high molecular weight polyethylene using iminopyridyl nickel catalysts

Polyethylene with a molecular weight of more than one million could be generated using iminopyridyl nickel catalysts.

Norbornene homopolymerization and copolymerization with ethylene by phosphine-sulfonate nickel catalysts

Phosphine-sulfonate nickel catalyzed norbornene homopolymerization and copolymerization with ethylene with an interesting response to B(C₆F₅)₃ additive.

Ortho-phosphinobenzenesulfonate: a superb ligand for palladium-catalyzed coordination-insertion copolymerization of polar vinyl monomers

Ligands, Lewis bases that coordinate to the metal center in a complex, can completely change the catalytic behavior of the metal center. In this Account, we summarize new reactions enabled by a single class of ligands, phosphine-sulfonates (ortho-phosphinobenzenesulfonates). Using their palladium complexes, we have developed four unusual reactions, and three of these have produced novel types of polymers. In one case, we have produced linear high-molecular weight polyethylene, a type of polymer that group 10 metal catalysts do not typically produce. Secondly, complexes using these ligands catalyzed the formation of linear poly(ethylene-co-polar vinyl monomers). Before the use of phosphine-sulfonate catalysts, researchers could only produce ethylene/polar monomer copolymers that have different branched structures rather than linear ones, depending on whether the polymers were produced by a radical polymerization or a group 10 metal catalyzed coordination polymerization. Thirdly, these phosphine-sulfonate catalysts produced nonalternating linear poly(ethylene-co-carbon monoxide). Radical polymerization gives ethylene-rich branched ethylene/CO copolymers copolymers. Prior to the use of phosphine-sulfonates, all of the metal catalyzed processes gave completely alternating ethylene/carbon monoxide copolymers. Finally, we produced poly(polar vinyl monomer-alt-carbon monoxide), a copolymerization of common polar monomers with carbon monoxide that had not been previously reported. Although researchers have often used symmetrical bidentate ligands such as diimines for the polymerization catalysis, phosphine-sulfonates are unsymmetrical, containing two nonequivalent donor units, a neutral phosphine, and an anionic sulfonate. We discuss the features that make this ligand unique. In order to understand all of the new reactions facilitated by this special ligand, we discuss both the steric effect of the bulky phosphines and electronic effects. We provide a unified interpretation of the unique reactivity by considering of the net charge and the enhanced back donation in the phosphine-sulfonate complexes.

2-[(Di-methyl-phenyl-phosphanyl-idene)aza-nium-yl]-5-methyl-benzene-sulfonate benzene monosolvate

The title compound, C15H18NO3PS·C6H6, is a rare example of a crystallographically characterized exocyclic phosphiniminium-arene-sulfonate zwitterion, which crystallises as its benzene solvate. The crystal structure shows that the N atom is protonated and that the iminium H atom forms both intra- and inter-molecular hydrogen bonds to the single-bonded sulfonate O atom in an R 2 (2)(4) graph-set motif. The dihedral angle between the aromatic rings in the main molecule is 89.49 (8)°.