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

Nickel(II)-catalyzed living polymerization of diazoacetates toward polycarbene homopolymer and polythiophene-block-polycarbene copolymers

Diazoacetate polymerization has attracted considerable research attention because it is an effective approach for fabricating carbon–carbon (C–C) main chain polymers. However, diazoacetate polymerization based on inexpensive catalysts has been a long-standing challenge. Herein, we report a Ni(II) catalyst that can promote the living polymerization of various diazoacetates, yielding well-defined C–C main chain polymers, polycarbenes, with a predictable molecular weight (M_n) and low dispersity (M_w/M_n). Moreover, the Ni(II)-catalyzed sequential living polymerization of thiophene and diazoacetate monomers affords interesting π-conjugated poly(3-hexylthiophene)-block-polycarbene copolymers in high yields with a controlled M_n, variable compositions, and low M_w/M_n, although the structure and polymerization mechanism of the two monomers differ. Using this strategy, amphiphilic block copolymers comprising hydrophobic poly(3-hexylthiophene) and hydrophilic polycarbene blocks are facilely prepared, which were self-assembled into well-defined supramolecular architectures with tunable photoluminescence.

Diazoacetate polymerizations using inexpensive catalysts has been a long-standing challenge to achieve. Here the authors report a Ni(II) catalyst that can promote living polymerization of various diazoacetates, yielding well-defined carbon–carbon main chain polymers, polycarbenes.

Selective branch formation in ethylene polymerization to access precise ethylene-propylene copolymers

Polyolefins with branches produced by ethylene alone via chain walking are highly desired in industry. Selective branch formation from uncontrolled chain walking is a long-standing challenge to generate exclusively branched polyolefins, however. Here we report such desirable microstructures in ethylene polymerization by using sterically constrained α-diimine nickel(II)/palladium(II) catalysts at 30 °C–90 °C that fall into industrial conditions. Branched polyethylenes with exclusive branch pattern of methyl branches (99%) and notably selective branch distribution of 1,4-Me₂ unit (86%) can be generated. The ultrahigh degree of branching (>200 Me/1000 C) enables the well-defined product to mimic ethylene-propylene copolymers. More interestingly, branch distribution is predictable and computable by using a simple statistical model of p(1-p)ⁿ (p: the probability of branch formation). Mechanistic insights into the branch formation including branch pattern and branch distribution by an in-depth density functional theory (DFT) calculation are elucidated.

Selective branch formation from uncontrolled chain walking is a longstanding challenge to generate exclusively branched polyolefins. Here the authors report such desirable microstructures in ethylene polymerization enabled by a nickel catalyst at 30 °C–90 °C that fall into industrial conditions and mimic ethylene-propylene copolymers.

Chromium Complexes Supported by NNO-Tridentate Ligands: An Unprecedent Activity with the Low Requirement of MAO

The development of metal catalysts with high activity and thermal stability but low requirement of MAO as cocatalyst is highly desired for polyolefin industrial application. In this contribution, a series...

Nickel-catalyzed multicomponent reaction of dinitriles and hydrazine hydrochlorides with boronic acids: access to 1,3-diaryl-1H-pyrazol-5-amines and 4,5-dihydropyridazin-3(2H)-ones

Developing efficient methods to accommodate azaheterocycles has been the hotspot field in organic synthesis. Herein, we disclose a facile and expeditious avenue to synthesize 1,3-diaryl-1H-pyrazol-5-amines and 4,5-dihydropyridazin-3(2H)-ones via nickel-catalyzed addition,...

Commercially available palladium salts as practical and green single-component catalysts in the coordination polymerization of 1-chloro-2-phenylacetylenes in air

Commercially available Pd salts serve as single-component catalysts for coordination polymerization of 1-chloro-2-phenylacetylenes, which affords a new design concept of metal catalysts for coordination polymerization of disubstituted alkynes.

Polar additive triggered chain walking copolymerization of ethylene and fundamental polar monomers

The use of a polar additive efficiently triggers chain walking copolymerization of ethylene with a broad scope of fundamental polar monomers, which is long-sought in an α-diimine Pd(ii) system.

Polyoxometalate-based CuII/CoII complexes tuned using various metal–pyrazole loops: design, diverse architectures and catalytic activity toward the oxidation of thioether derivatives

Three new polyoxometalate-based complexes with unique metal–pyrazole loops were synthesized, showing various architectures and finally resulting in different catalysis performance.

High molecular weight PE elastomers through 4,4-difluorobenzhydryl substitution in symmetrical α-diimino-nickel ethylene polymerization catalysts

The following family of N,N-diaryl-2,3-dimethyl-1,4-diazabutadienes, ArN Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C(Me)C(Me)NAr (Ar = 2,6-Me₂-4-{CH(4-FC₆H₄)₂}C₆H₂L1, 2-Me-6-Et-4-{CH(4-FC₆H₄)₂}C₆H₂L2, 2,4-{CH(4-FC₆H₄)₂}₂-6-MeC₆H₂L3, 2,4-{CH(4-FC₆H₄)₂}₂-6-EtC₆H₂L4, 2,4-{CH(4-FC₆H₄)₂}₂-6-iPrC₆H₂L5), each incorporating para-substituted 4,4-difluorobenzhydryl groups but differing in the ortho-pairing, have been synthesized and used as precursors to their respective nickel(ii) bromide complexes, Ni1–Ni5. Compound characterization has been achieved through a combination of FT-IR, multinuclear NMR spectroscopy (¹H, ¹³C, ¹⁹F) and elemental analysis. In addition, L1, Ni1 and Ni5 have been structurally characterized with Ni1 and Ni5 revealing similarly distorted tetrahedral geometries about nickel but with distinct differences in the steric protection offered by the ortho-substituents. All nickel complexes, under suitable activation, showed high activity for ethylene polymerization with a predilection towards forming branched high molecular weight polyethylene with narrow dispersity. Notably the most sterically bulky Ni5, under activation with either EtAlCl₂, Et₂AlCl or EASC, was exceptionally active (0.9–1.0 × 10⁷ g of PE per (mol of Ni) per h) at an operating temperature of 40 °C. Furthermore, the polyethylene generated displayed molecular weights close to one million g mol⁻¹ (M_w range: 829–922 kg mol⁻¹) with high branching densities (86–102/1000 carbons) and a selectivity for short chain branches (% Me = 94.3% (EtAlCl₂), 87.2% (Et₂AlCl), 87.7% (EASC)). Further analysis of the mechanical properties of the polymers produced at 40 °C and 50 °C using Ni5 highlighted the key role played by crystallinity (X_c) and molecular weight (M_w) on tensile strength (σ_b) and elongation at break (ε_b). In addition, stress–strain recovery tests reveal these high molecular weight polymers to exhibit characteristics of thermoplastic elastomers (TPEs).

N,N-Diaryl-2,3-dimethyl-1,4-diazabutadiene-nickel complexes appended with 4,4-difluorobenzhydryl groups can efficiently mediate the formation of polyethylene elastomers with molecular weights approaching one million g mol⁻¹.

Isospecific (co)Polymerization of Unmasked Polar Styrenes by Neutral Rare-earth Metal Catalysts

Syndioselective polymerization of unprotected polar styrenes has achieved great success via specially designed catalysts and establishment of "self-assisted" theory. On contrary, isospecific polymerization of polar styrenes has remained less explored, which needs to break the dilemma of high selectivity and activity of the involved catalysts. Herein, we present new racemic ansa-bis(benz[e]indenyl) rare-earth metal complexes and their high activity and perfect isoselectivity ( mmmm > 99 %) for the polymerization of unmasked polar styrenes without any activators. Moreover, the copolymerizations of para / meta -methoxystyrenes with styrene give gradient and random copolymers, respectively. The insertion rate of polar monomers could be readily tuned in the range of 0-100 mol % by changing their loading ratios. The resultant isotactic polar polystyrenes are quantitatively transformed into hydroxyl or methylsulfonyl polystyrenes with high T g s. DFT calculations reveal the isospecific mechanim.

Direct copolymerization of ethylene with protic comonomers enabled by multinuclear Ni catalysts

Ethylene/polar monomer coordination copolymerization offers an attractive way of making functionalized polyolefins. However, ethylene copolymerization with industrially relevant short chain length alkenoic acid remain a big challenge. Here we report the efficient direct copolymerization of ethylene with vinyl acetic acid by tetranuclear nickel complexes. The protic monomer can be extended to acrylic acid, allylacetic acid, ω-alkenoic acid, allyl alcohol, and homoallyl alcohol. Based on X-ray analysis of precatalysts, control experiments, solvent-assisted electrospray ionization-mass spectrometry detection of key catalytic intermediates, and density functional theory studies, we propose a possible mechanistic scenario that involves a distinctive vinyl acetic acid enchainment enabled by Ni···Ni synergistic effects. Inspired by the mechanistic insights, binuclear nickel catalysts are designed and proved much more efficient for the copolymerization of ethylene with vinyl acetic acid or acrylic acid, achieving the highest turnover frequencies so far for both ethylene and polar monomers simultaneously.

Effect of para-Substituents in Ethylene Copolymerizations with 1-Decene, 1-Dodecene, and with 2-Methyl-1-Pentene Using Phenoxide Modified Half-Titanocenes-MAO Catalyst Systems

Effect of para-substituents in the ethylene (E) copolymerization with 1-decene (DC), 1-dodecene (DD), and with 2-methyl-1-pentene (2M1P) using a series of Cp*TiCl2 (O-2,6-i Pr2 -4-R-C6 H2 ) [R=H (1), t Bu (2), Ph (3), CHPh2 (4), CPh3 (5), SiMe3 (6), SiEt3 (7), and newly prepared 4-t BuC6 H4 (8) and 3,5-Me2 C6 H3 (9)]-MAO catalyst systems has been studied. The activities in these copolymerization reactions were affected by the para-substituent, and the SiMe3 (6), SiEt3 (7) and 3,5-Me2 C6 H3 (9) analogues showed the higher activities at 50 °C in the E copolymerization reactions with DC (1.06-1.44×106  kg-polymer/mol-Ti⋅h), DD (1.04-1.88×106  kg-polymer/mol-Ti⋅h) than the others, whereas no significant differences were observed in the comonomer incorporations. Complexes 6 and 7 also showed the higher activities at 50 °C in the E/2M1P copolymerization, and the 2M1P incorporation was affected by the para-substituent and the polymerization temperature; complex 9 showed better 2M1P incorporation at 25 °C.

Neutral Unsymmetrical Coordinated Cyclophane Polymerization Catalysts

Cyclophane structures can control steric pressure in the otherwise open spaces of square‐planar d⁸‐metal catalysts. This elegant concept was so far limited to symmetrical coordinated metals. We report how a cyclophane motif can be generated in ligands that chelate via two different donors. An ancillary second imine in the versatile κ²‐N,O‐salicylaldiminato catalyst type enables ring closure via olefin metathesis and selective double bond hydrogenation to yield a 30‐membered ring efficiently. Experimental and theoretical analyses show the ancillary imine is directed away from the active site and inert for catalysis. In ethylene polymerization the cyclophane catalyst is more active and temperature stable vs. an open structure reference, notably also in polar solvents. Increased molecular weights and decreased degrees of branching can be traced to an increased energy of sterically demanding transition states by the encircling cyclophane while chain propagation remains highly efficient.

Fluorinated α-Diimine Nickel Mediated Ethylene (Co)Polymerization

Fluorine substituents in transition metal catalysts are of great importance in olefin polymerization catalysis; however, the comprehensive effect of fluorine substituents is elusive in seminal late transition metal α-diimine catalytic system. In this contribution, fluorine substituents at various positions (ortho-, meta-, and para-F) and with different numbers (F_n ; n=0, 1, 2, 3, 5) were installed into the well-defined N-terphenyl amine and thus were studied for the first time in the nickel α-diimine promoted ethylene polymerization and copolymerization with polar monomers. The position of the fluorine substituent was particularly crucial in these polymerization reactions in terms of catalytic activity, polymer molecular weight, branching density, and incorporation of polar monomer, and thus a picture on the fluorine effect was given. As a notable result, the ortho-F substituted α-diimine nickel catalyst produced highly linear polyethylenes with an extremely high molecular weight (M_w =8703 kDa) and a significantly low degree of branching of 1.4/1000 C; however, the meta-F and/or para-F substituted α-diimine nickel catalysts generated highly branched (up to 80.2/1000 C) polyethylenes with significantly low molecular weights (M_w =20-50 kDa).

Synthesis of Nonalternating Polyketones Using Cationic Diphosphazane Monoxide-Palladium Complexes

Copolymerization of olefin with carbon monoxide has received considerable interest from both academia and industry, and the introduction of polar carbonyl group renders the resultant polyketones with excellent mechanical strength, crystallinity, photodegradability, hydrophilicity, surface, and barrier properties. However, most of the reported polyketones are difficult to be processed because of limited solubility in common solvents and high melting temperature (T_m ∼ 260 °C) resulting from the strictly alternative structure. Nonalternating copolymerization of ethylene with CO is a very promising method to circumvent the problem of processability of traditional perfectly alternating polyketone. In the contribution, the palladium coordinated diphosphazane monoxide substituted by strong electron-donating groups is discovered to be highly reactive for producing nonalternating polyketones, and up to 24.2% extra ethylene incorporation has lowered T_m values to 147 and 165 °C and further improved thermal stability (T_d ∼ 339 °C) of the resultant materials. Our data demonstrates that cationic palladium complexes can also exhibit excellent reactivity and an unprecedented nonalternating degree in this copolymerization.

Hydrogen-Bonding-Induced Heterogenization of Nickel and Palladium Catalysts for Copolymerization of Ethylene with Polar Monomers

The practical synthesis of polar-functionalized polyolefins using transition-metal-catalyzed copolymerization of olefins with polar monomers is a challenge; the use of heterogeneous catalysts is little explored. Herein, we report the synthesis of heterogeneous naphthoquinone-based nickel (Ni/SiO₂ ) and palladium (Pd/SiO₂ ) catalysts through hydrogen bonding interactions of the ligands with the silica surface. Ni/SiO₂ exhibits high activities (up to 2.65×10⁶  g mol^-1  h^-1 ) during the copolymerization of ethylene with 5-hexene-1-yl-acetate, affording high-molecular-weight (M_n up to 630 000) polar-functionalized semicrystalline polyethylene (comonomer incorporation up to 2.8 mol %), along with great morphology control. The resulting copolymers possess improved surface properties and great mechanical properties. Pd/SiO₂ can mediate ethylene copolymerization with polar monomers with moderate activity to produce high-molecular-weight copolymers with tunable comonomer incorporation.

Unprecedented Application of Covalent Organic Frameworks for Polymerization Catalysis: Rh/TPB-DMTP-COF in Polymerization of Phenylacetylene and Its Functional Derivatives

Covalent organic frameworks (COFs) are applied widely in organic catalysis; however, no precedent has been reported in polymerization catalysis. Herein, we report the new application of COFs for polymerization catalysis. Different amounts of homogeneous Rh catalyst are incorporated into the COF via post-treatment to give a series of TPB-DMTP-COF-X wt % Rh (b-e) containing varying amounts of Rh from 2.74 to 11.38 wt %. In contrast to the known Rh catalysts, TPB-DMTP-COF-X wt % Rh (b-e) display an uncommon synergistic effect and exceptional steric confinement effect of nanochannels. Therefore, they possess the advantages of both homogeneous catalysts in high activity and selectivity and heterogeneous catalysts in stability and recyclability with extremely high activity up to 1.3 × 10⁷ g·mol_Rh^-1·h^-1 and cis-selectivity up to 99% and can be readily recycled and reused five times in the polymerization of phenylacetylene and its derivatives, affording cis-transoidal polyphenylacetylene and its derivatives having helical structures, aggregation-induced emission properties, or fluorescence properties with narrow molecular weight distributions.

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.