Neutral Nickel Catalysts for Olefin Homo- and Copolymerization: Relationships between Catalyst Structures and Catalytic Properties

Efficient Suppression of Chain Transfer and Branching via Cs -Type Shielding in a Neutral Nickel(II) Catalyst

An effective shielding of both apical positions of a neutral NiII active site is achieved by dibenzosuberyl groups, both attached via the same donors' N-aryl group in a Cs -type arrangement. The key aniline building block is accessible in a single step from commercially available dibenzosuberol. This shielding approach suppresses chain transfer and branch formation to such an extent that ultrahigh molecular weight polyethylenes (5×106  g mol-1 ) are accessible, with a strictly linear microstructure (<0.1 branches/1000C). Key features of this highly active (4.3×105  turnovers h-1 ) catalyst are an exceptionally facile preparation, thermal robustness (up to 90 °C polymerization temperature), ability for living polymerization and compatibility with THF as a polar reaction medium.

Boosting the Thermal Stability of α-Diimine Palladium Complexes in Norbornene Polymerization from Construction of Intraligand Hydrogen Bonding and Simultaneous Increasing Axial/Equatorial Bulkiness

Increasing the thermostability of α-diimine late-transition-metal complexes and therefore rendering them more active at higher temperatures is of great importance, yet challenging for the olefin polymerization field. In the present research, a new family of α-diimine palladium complexes that can promote norbornene polymerization at high temperatures (up to 140 °C) is disclosed. Because of the conformational restriction caused by increasing the axial and equatorial bulkiness as well as the presence of intraligand H···F hydrogen bonds, N-aryl rotations can be efficiently restricted, therefore circumventing the deactivation of the active species at high temperatures. At 80-140 °C, these complexes can efficiently catalyze norbornene homopolymerizations, giving high catalytic activities up to 5.65 × 10⁷ g of PNB per mole Ni per hour and polymers with high molecular weights up to 37.2 × 10⁴ g/mol, which are highly superior to catalytic systems mediated by CF₃-free complexes. Moreover, these complexes could also afford medium catalytic activities in the presence of polar 5-norbornene-2-carboxylic acid methyl ester (NB-COOCH₃).

Toward the Copolymerization of Propylene with Polar Comonomers

Polyolefins are produced in vast amounts and are found in so many consumer products that the two most commonly produced forms, polyethylene (PE) and polypropylene (PP), fall into the rather sparse category of molecules that are likely to be known by people worldwide, regardless of their occupation. Although widespread, the further upgrading of their properties (mechanical, physical, aesthetic, etc.) through the formation of composites with other materials, such as polar polymers, fibers, or talc, is of huge interest to manufacturers. To improve the affinity of polyolefins toward these materials, the inclusion of polar functionalities into the polymer chain is essential. The incorporation of a functional group to trigger controlled polymer degradation is also an emerging area of interest. Currently practiced methods for the incorporation of polar functionalities, such as post-polymerization functionalization, are limited by the number of compatible polar monomers: for example, grafting maleic anhydride is currently the sole method for practical functionalization of PP. In contrast, the incorporation of fundamental polar comonomers into PE and PP chains via coordination insertion polymerization offers good control, making it a highly sought-after process. Early transition metal catalysts (which are commonly used for the production of PE and PP) display poor tolerance toward the functional groups within polar comonomers, limiting their use to less-practical derivatives. As late transition metal catalysts are less-oxophilic and thus more tolerant to polar functionalities, they are ideal candidates for these reactions. This Account focuses on the copolymerization of propylene with polar comonomers, which remains underdeveloped as compared to the corresponding reaction using ethylene. We begin with the challenges associated with the regio- and stereoselective insertion of propylene, which is a particular problem for late transition metal systems because of their propensity to undergo chain walking processes. To overcome this issue, we have investigated a range of metal/ligand combinations. We first discuss attempts with group 4 and 8 metal catalysts and their limitations as background, and then focus on the copolymerization of propylene with methyl acrylate (MA) using Pd/imidazolidine-quinolinolate (IzQO) and Pd/phosphine-sulfonate (PS) precatalysts. Each generated regioregular polymer, but while the system featuring an IzQO ligand did not display any stereocontrol, that using the chiral PS ligand did. A further difference was found in the insertion mode of MA: the Pd/IzQO system inserted in a 1,2 fashion, while in the Pd/PS system a 2,1 insertion was observed. We then move onto recent results from our lab using Pd/PS and Pd/bisphosphine monoxide (BPMO) precatalysts for the copolymerization of propylene with allyl comonomers. These P-stereogeneic precatalysts generated the highest isotacticity values reported to date using late transition metal catalysts. This section closes with our work using Earth-abundant nickel catalysts for the reaction, which would be especially desired for industrial applications: a Ni/phosphine phenolate (PO) precatalyst yielded regioregular polypropylene with the incorporation of some allyl monomers into the main polymer chain. The installation of a chiral menthyl substituent on the phosphine allowed for moderate stereoselectivity to be achieved, though the applicable polar monomers currently remain limited. The Account concludes with a discussion of the factors that affect the insertion mode of propylene and polar comonomers in copolymerization reactions, beginning with our recent computational study, and finishing with work from ourselves and others covering both comonomer and precatalyst steric and electronic profiles with reference to the observed regioselectivity.

2,4,6-Triphenylpyridinium: A Bulky, Highly Electron-Withdrawing Substituent That Enhances Properties of Nickel(II) Ethylene Polymerization Catalysts

The reactivity of Ni^II and Pd^II olefin polymerization catalysts can be enhanced by introduction of electron-withdrawing substituents on the supporting ligands rendering the metal centers more electrophilic. Reported here is a comparison of ethylene polymerization activity of a classical salicyliminato nickel catalyst substituted with the powerful electron-withdrawing 2,4,6-triphenylpyridinium (trippy) group to the -CF₃ analogue. The trippy substituent is substantially more electron-withdrawing (σ_meta =0.63) than the trifluoromethyl group (σ_meta =0.43) which results in a ca. 8-fold increase in catalytic turnover frequency. An additional advantage of trippy is the high steric bulk relative to the trifluoromethyl group. This feature results in a four-fold increase in polymer molecular weight owing to enhanced retardation of chain transfer. A significant increase in catalyst lifetime is observed as well.

Asymmetric hydrogenation catalyzed by first-row transition metal complexes

This review focuses on asymmetric direct and transfer hydrogenation with first-row transition metal complexes. The reaction mechanisms and the models of enantiomeric induction were summarized and emphasized.

Neutral and cationic methallyl nickel complexes in alkene activation: a combined DFT, ESI-MS and chemometric approach

Herein, we report a comparative study of ethylene activation and 1-hexene isomerization carried out with isomeric neutral and cationic methallyl nickel complexes L1Ni(η3-C3H5) and [L1Ni(η3-C3H5)][B(ArF)4] in the presence of borane co-catalysts.

Custom-made polar monomers utilized in nickel and palladium promoted olefin copolymerization

In this review, the functions of custom-made polar monomers are insightfully emphasized in the preparation of functional polyolefins.

Homo- and copolymerization of norbornene using tridentate IzQO palladium catalysts with dimethylaminoethyl as a side arm

Rigid IzQO–Pd catalysts were synthesized by Rh(iii)-catalyzed C–H/alkyne annulation and applied for the homo- and copolymerizations of norbornene with polar vinyl monomers.

Direct synthesis of various polar functionalized polypropylene materials with tunable molecular weights and high incorporation ratios

As compared with the classical α-diimine catalyst, iminopyridyl catalysts were observed to be highly efficient for the direct synthesis of polar functionalized polypropylene with tunable molecular weights and high incorporation ratios.

Asymmetric bis-salicylaldiminato binuclear titanium complexes for ethylene polymerization and copolymerization

Asymmetric binuclear Ti complexes were synthesized, which exhibited excellent activity for ethylene homopolymerization and copolymerization with 1-hexene or norbornene and high comonomer insertion efficiency under MMAO activation.

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.

The Effect of SiMe3 and SiEt3 Para Substituents for High Activity and Introduction of a Hydroxy Group in Ethylene Copolymerization Catalyzed by Phenoxide-Modified Half-Titanocenes

Remarkable effects of SiMe₃ and SiEt₃ para-substituents in the phenoxide-modified half-titanocenes, Cp*TiCl₂ (O-2,6-ⁱ Pr₂ -4-R-C₆ H₂ ) [R=SiMe₃ (6), SiEt₃ (7)], toward the catalytic activities in ethylene copolymerizations with 2-methyl-1-pentene, 1-decene, 1-dodecene and with 9-decen-1-ol (DC-OH) have been demonstrated. The activities by 6, 7 at 50 °C showed higher than those conducted at 25 °C in all cases in the presence of MAO cocatalyst. Efficient synthesis of high-molecular-weight (HMW) ethylene copolymers incorporating DC-OH (or 5-hexen-1-ol, HX-OH) has been attained in the copolymerization by 7, which showed better DC-OH (HX-OH) incorporation at 50 °C to afford the HMW copolymers, poly(ethylene-co-DC-OH)s, with high activities (activity 1.21-3.81×10⁵  kg-polymer mol^-1 -Ti h, M_n =6.55-10.0×10⁴ , DC-OH 2.3-3.6 mol %).

Ultrahigh Branching of Main-Chain-Functionalized Polyethylenes by Inverted Insertion Selectivity

Branched polyolefin microstructures resulting from so-called "chain walking" are a fascinating feature of late transition metal catalysts; however, to date it has not been demonstrated how desirable branched polyolefin microstructures can be generated thereby. We demonstrate how highly branched polyethylenes with methyl branches (220 Me/1000 C) exclusively and very high molecular weights (ca. 106  g mol-1 ), reaching the branch density and microstructure of commercial ethylene-propylene elastomers, can be generated from ethylene alone. At the same time, polar groups on the main chain can be generated by in-chain incorporation of methyl acrylate. Key to this strategy is a novel rigid environment in an α-diimine PdII catalyst with a steric constraint that allows for excessive chain walking and branching, but restricts branch formation to methyl branches, hinders chain transfer to afford a living polymerization, and inverts the regioselectivity of acrylate insertion to a 1,2-mode.

Nickel Catalyzed Olefin Oligomerization and Dimerization

Brought to life more than half a century ago and successfully applied for high-value petrochemical intermediates production, nickel-catalyzed olefin oligomerization is still a very dynamic topic, with many fundamental questions to address and industrial challenges to overcome. The unique and versatile reactivity of nickel enables the oligomerization of ethylene, propylene, and butenes into a wide range of oligomers that are highly sought-after in numerous fields to be controlled. Interestingly, both homogeneous and heterogeneous nickel catalysts have been scrutinized and employed to do this. This rare specificity encouraged us to interlink them in this review so as to open up opportunities for further catalyst development and innovation. An in-depth understanding of the reaction mechanisms in play is essential to being able to fine-tune the selectivity and achieve efficiency in the rational design of novel catalytic systems. This review thus provides a complete overview of the subject, compiling the main fundamental/industrial milestones and remaining challenges facing homogeneous/heterogeneous approaches as well as emerging catalytic concepts, with a focus on the last 10 years.

Copolymerization of Ethylene and Vinyl Fluoride by Self-Assembled Multinuclear Palladium Catalysts

The self-assembled multinuclear Pd^II complexes {(Li-OPO^OMe2)PdMe(4-5-nonyl-pyridine)}₄Li₂Cl₂ (C, Li-OPO^OMe2 = PPh(2-SO₃Li-4,5-(OMe)₂-Ph)(2-SO₃⁻-4,5-(OMe)₂-Me-Ph)), {(Zn-OP-P-SO)PdMe(L)}₄ (D, L = pyridine or 4-^tBu-pyridine, [OP-P-SO]³⁻ = P(4-^tBu-Ph)(2-PO₃²⁻-5-Me-Ph)(2-SO₃⁻-5-Me-Ph)), and {(Zn-OP-P-SO)PdMe(pyridine)}₃ (E) copolymerize ethylene and vinyl fluoride (VF) to linear copolymers. VF is incorporated at levels of 0.1–2.5 mol% primarily as in-chain -CH₂CHFCH₂- units. The molecular weight distributions of the copolymers produced by D and E are generally narrower than for catalyst C, which suggests that the Zn-phosphonate cores of D and E are more stable than the Li-sulfonate-chloride core of C under copolymerization conditions. The ethylene/VF copolymerization activities of C–E are over 100 times lower and the copolymer molecular weights (MWs) are reduced compared to the results for ethylene homopolymerization by these catalysts.

Synthetic investigations of low-coordinate (N-phosphino-amidinate) nickel chemistry: agostic alkyl complexes and benzene insertion into Ni-H

Treatment of (PN)NiX (X = NHdipp or OtBu; PN = N-phosphinoamidinate ligand) with Me2PhSiH in benzene solvent afforded the crystallographically characterized, antifacial-coordinated, dinuclear species 1, the formation of which corresponds to the hitherto unknown net Ni-H addition of two equivalents of the putative (PN)NiH intermediate across C[double bond, length as m-dash]C units within a single benzene molecule. Computational analysis supports the view of 1 as being comprised of two cationic (PN)NiII fragments ligated by a substituted butadiene dianion μ2-η3:η3-C6H82- bridging group. Also described is the formation and characterization of three-coordinate (PN)Ni(alkyl) complexes stabilized by β-agostic (alkyl = Et, 2; n-Bu, 3; n-hexyl, 4) or γ-agostic (alkyl = neopentyl, 5) interactions, and our efforts to employ 2 and 3 as synthons for the generation of (PN)NiHvia β-hydride elimination. Notably, compound 5 represents both the first crystallographically characterized three-coordinate Ni-alkyl complex featuring a heterobidentate ligation, and the first neutral γ-agostic NiII-alkyl complex.

Remote Perfluoroalkyl Substituents are Key to Living Aqueous Ethylene Polymerization

In various nickel(II) salicylaldiminato ethylene polymerization catalysts, which are a versatile mechanistic probe for substituent effects, longer perfluoroalkyl groups exert a strong effect on catalytic activities and polymer microstructures compared to the trifluoromethyl group. This effect is accounted for by a reduced electron density on the active sites, and is also supported by electrochemical studies. Thus, β‐hydride elimination, the key step of chain transfer and branching pathways, is disfavored while chain‐growth rates are enhanced. This enhancement occurs to an extent that enables living polymerizations in aqueous systems to afford ultra‐high‐molecular‐weight polyethylene for various chelating salicylaldimine motifs. These findings are mechanistically instructive as well as practically useful for illustrating the potential of perfluoroalkyl groups in catalyst design.

A simple and versatile nickel platform for the generation of branched high molecular weight polyolefins

The development of high-performance transition metal catalysts has long been a major driving force in academic and industrial polyolefin research. Late transition metal-based olefin polymerization catalysts possess many unique properties, such as the ability to generate variously branched polyolefins using only ethylene as the feedstock and the capability of incorporating polar functionalized comonomers without protecting agents. Here we report the synthesis and (co)polymerization studies of a simple but extremely versatile α-imino-ketone nickel system. This type of catalyst is easy to synthesize and modify, and it is thermally stable and highly active during ethylene polymerization without the addition of any cocatalysts. Despite the sterically open nature, these catalysts can generate branched Ultra-High-Molecular-Weight polyethylene and copolymerize ethylene with a series of polar comonomers. The versatility of this platform has been further demonstrated through the synthesis of a dinuclear nickel catalyst and the installation of an anchor for catalyst heterogenization.

Living coordination–insertion copolymerization of 1-hexene and ligated α-olefins using an α-diimine nickel catalyst and preparation of metal–ligand coordination crosslinked polymers

Metal–ligand coordination crosslinked polymers were prepared by coordination–insertion copolymerization of 1-hexene and ligated α-olefins.

Influence of initiating groups on phosphino-phenolate nickel catalyzed ethylene (co)polymerization

The influence of initiating groups on late transition metal catalyzed ethylene (co)polymerization was comprehensively studied for the first time.

Oligomerization and polymerization of 5-ethylidene-2-norbornene by cationic palladium and nickel catalysts

Nickel- and palladium-based catalyst systems were developed to convert 5-ethylidene-2-norbornene (ENB) to oligomers and polymers with highly controllable molecular weights.