Modulating Polyolefin Copolymer Composition via Redox-Active Olefin Polymerization Catalysts

Customizing Polymers by Controlling Cation Switching Dynamics in Non-Living Polymerization

Controlling the chain growth process in non-living polymerization reactions is difficult because chain termination typically occurs faster than the time it takes to apply an external trigger. To overcome this limitation, we have developed a strategy to regulate non-living polymerizations by exploiting the chemical equilibria between a metal catalyst and secondary metal cations. We have prepared two nickel phenoxyphosphine-polyethylene glycol variants, one with 2-methoxyphenyl (Ni1) and another with 2,6-dimethoxyphenyl (Ni2) phosphine substituents. Ethylene polymerization studies using these complexes in the presence of alkali salts revealed that chain growth is strongly dependent on electronic effects, whereas chain termination is dependent on both steric and electronic effects. By adjusting the solvent polarity, we can favor polymerizations via non-switching or dynamic switching modes. For example, in a 100:0.2 mixture of toluene/diethyl ether, reactions of Ni1 and both Li⁺ and Na⁺ cations in the presence of ethylene yielded bimodal polymers with different relative fractions depending on the Li⁺/Na⁺ ratio used. In a 98:2 mixture of toluene/diethyl ether, reactions of Ni2 and Cs⁺ in the presence of ethylene generated monomodal polyethylene with dispersity <2.0 and increasing molecular weight as the amount of Cs⁺ added increased. Solution studies by NMR spectroscopy showed that cation exchange between the nickel complexes and alkali cations in 98:2 toluene/diethyl ether is fast on the NMR time scale, which supports our proposed dynamic switching mechanism.

Distorted Sandwich a-Diimine Pd(II) Catalyst: Linear Polyethylene and Synthesis of Ethylene/Acrylate Elastomers

The introduction of m-xylyl substituents to α-diimine Pd(II) catalyst promotes living ethylene polymerization at room temperature and low pressure to yield high molecular weight polyethylene (PE) with low branching (<17/1000C). m-Xylyl groups provide a highly effective blockage to the axial sites of the catalytic center and form a distorted sandwich geometry. The shielding prevents chain-transfer and easy accessibility of polar monomers, leading to a living polymerization. Conducting a light irradiation as part of the one-step metal-organic insertion light initiated radical (MILRad) process leads to diblock copolymers of ethylene and acrylates. Incorporation of different acrylate block sequences can significantly modify the mechanical and chemical properties of block copolymers which can be modulated to be a hard plastic, elastomer, or semi-amorphous polymer.

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).

Nickel complexes based on BIAN ligands: transformation and catalysis on ethylene polymerization

Treatment of bis(arylimino)acenaphthene (ArBIAN) with Ni(COD)2 in toluene afforded dmpBIANNi(COD) (2a, dmp = 2,6-Me2C6H3) and dippBIANNi(COD) (2b, dipp = 2,6-iPrC6H3), respectively, in moderate yields. Complexes 2a and 2b can be oxidized by a small amount of oxygen at low temperature leading to oxygen-bridged dinuclear Ni(ii) complexes (dmpBIANNi)2(μ-O)2 (4a) and (dippBIANNi)2(μ-O)2 (4b), respectively, as a purple powder. The reaction of ArBIAN with 0.5 equiv of Ni(COD)2 or Ni(Ph3P)4 gave bisligated complexes (dmpBIAN)2Ni (3a) and (dippBIAN)2Ni (3b), which can be considered as Ni(0) complexes supported by two neutral BIAN ligands. Oxidation of the bisligated nickel complexes 3a and 3b with [Cp2Fe][B(C6F5)4] afforded cationic Ni(i) complexes [(dmpBIAN)2Ni][B(C6F5)4] (5a) and [(dippBIAN)2Ni][B(C6F5)4] (5b), respectively, in which the Ni(i) centre is chelated by two neutral Ar-BIAN ligands. These complexes were characterized by NMR and IR spectroscopy and DFT calculation, and the molecular structures of 3b, 4b, and 5b were well established by X-ray diffraction analysis. These complexes were evaluated as catalysts for ethylene polymerization in which 2b showed high activity in the presence of AlMe3. 13C NMR analysis of polymers showed that the 2b/AlMe3 catalytic system gave less-branched polymers when compared to that obtained with dippBIANNiBr2 under the same conditions.

A switchable dimeric yttrium complex and its three catalytic states in ring opening polymerization

A dimeric yttrium phenoxide complex can be oxidized in a stepwise fashion to access three oxidation states. The three states show different activity in the ring opening polymerization of cyclic esters and epoxides.

"Bulky-Yet-Flexible" α-Diimine Palladium-Catalyzed Reductive Heck Cross-Coupling: Highly Anti-Markovnikov-Selective Hydroarylation of Alkene in Air

To pursue a highly regioselective and efficient reductive Heck reaction, a series of moisture- and air-stable α-diimine palladium precatalysts were rationally designed, readily synthesized, and fully characterized. The relationship between the structures of the palladium complexes and the catalytic properties was investigated. It was revealed that the"bulky-yet-flexible"palladium complexes allowed highly anti-Markovnikov-selective hydroarylation of alkenes with (hetero)aryl bromides under aerobic conditions. Further synthetic application of the present protocol could provide rapid and straightforward access to functional and biologically active molecules.

A mechanistic study of microstructure modulation in olefin polymerizations using a redox-active Ni(ii) α-diimine catalyst

Density functional theory and experimental evidence provide insight into the mechanism of polyolefin microstructure modulation using redox-active Ni(ii) α-diimine catalysts.

Light-mediated olefin coordination polymerization and photoswitches

This review outlines photoswitchable, transition metal-based olefin coordination polymerization catalysts ranging from homogeneous to heterogeneous, and monometallic to bimetallic regimes.

Ethylene polymerization of nickel catalysts with α-diimine ligands: factors controlling the structure of active species and polymer properties

α-Diimine and related complexes of late transition metals such as palladium and nickel have been attracting continuing interest as single-site catalysts of ethylene homopolymerization to branched polyolefins, having challenging mechanical properties. The state-of-the art catalysts demonstrate promising catalytic activities, and enhanced thermal stabilities, affording polyethylenes with a variable degree of branching and, in addition, are able to incorporate polar co-monomers into polyethylene structures. At the same time, fundamental understanding of the structure-reactivity relationships of such catalysts mostly remains at the phenomenological level, due to the lack of experimental data on the solution structures of intermediates that drive the polymerization process. In this perspective, we discuss recent advances of α-diimine nickel based catalysts of ethylene polymerization, focusing on the relationships between the catalyst structures on the one hand, and their thermal stabilities and properties of the resulting polyethylene, on the other hand. In addition, some intriguing novel mechanistic findings of these catalyst systems are presented.

Conversion of aldimines to secondary amines using iron-catalysed hydrosilylation

Iron-catalyzed hydrosilylation of imines to amines using a well-defined iron complex is reported. This method employs relatively mild conditions, by reaction of imine, (EtO)3SiH in a 1 : 2 ratio in the presence of 1 mol% precatalyst ([BIAN]Fe(η6-toluene), 3, BIAN = bis(2,6-diisopropylaniline)acenaphthene) at 70 °C. A broad scope of imines was readily converted into the corresponding secondary amines without the need for precatalyst activators.

Photochemical regulation of a redox-active olefin polymerization catalyst: controlling polyethylene microstructure with visible light

The utility of photoredox chemistry is expanded to include microstructural control of polyolefins.

Accessing multiple polyethylene grades via a single redox-active olefin polymerization catalyst

Access to more than one polyethylene grade is realized via utilization of a Ni-based olefin polymerization catalyst bearing a redox-active α-diimine ligand.

Elastomeric polyethylenes accessible via ethylene homo-polymerization using an unsymmetrical α-diimino-nickel catalyst

Elastomeric polyethylenes, with good elastic recovery and high elongation at break, have been prepared using a new family of unsymmetrical α-diimino-nickel catalysts that display high activities and high thermal stability.

Redox control in palladium catalyzed norbornene and alkyne polymerization

Switchable polymerization of norbornene, 5-norbornene-2-yl acetate and 1-chloro-1-octyne could be realized by using two palladium complexes (NHC)Pd(allyl)Cl (NHC = 1,3-Ar₂-naphthoquinimidazolylidene, Ar = 2,6-Me₂-C₆H₃, 2,6-ⁱPr₂-C₆H₃) bearing a redox-active naphthoquinone moiety.