Early Transition Metal Catalysts Research Articles

Accessing Functionalized Ultra-High Molecular Weight Poly(α-olefin)s via Hafnium-Mediated Highly Isospecific Copolymerization.

Inspired by the favorable impact of heteroatom-containing groups in phenoxy-imine titanium and late transition metal catalysts, a series of novel pyridylamido hafnium catalysts bearing ─OMe (Cat-OMe), ─CF3 (Cat-CF3), and ─C6F5 (Cat-C6F5) substituents are designed and synthesized. Together with the established hafnium catalysts Cat-H and Cat-iPr by Dow/Symyx, these catalysts are applied in the polymerization of α-olefins, including 1-hexene, 1-octene, and 4M1P, as well as in the copolymerization of these α-olefins with a specifically designed polar monomer. The enhancement of polymer molecular weight derived from catalyst modification and the incorporation of polar monomers is discussed in detail. Notably, the new catalysts are all highly active for α-olefins polymerization, with catalyst Cat-CF3 producing isotactic polymers with the highest molecular weight (Mw = 1649kg mol-1); in copolymerization with polar monomers, catalyst Cat-OMe yields isotactic copolymer with the highest molecular weight (Mw = 2990kg mol-1). Interestingly, catalyst Cat-C6F5 bearing a ─C6F5 group in the N-aryl moiety gives rise to poly(α-olefin) with reduced stereoselectivity. The findings of this study underscore the potential of heteroatom-containing groups in the development of early transition metal catalysts and the synthesis of polymer with novel structures.

Recent advancements in multinuclear early transition metal catalysts for olefin polymerization through cooperative effects

Abstract The pursuit of high-performance polyolefin materials has been an objective for chemists. Recently, the development of multinuclear catalysts has significantly enabled the preparation of high-performance polyolefin materials. In this review, we mainly focus on the cooperative effects of multinuclear early transition metal catalysts and provide a comprehensive summary of the research progress in binuclear and multinuclear early transition metal catalysts over the past decade. Compared with the catalytic performance and polymer structure of these catalysts with mononuclear catalysts, we have found that various factors, such as steric hindrance effect, electronic effect, heteroatom effect, hydrogen bonding interaction, the distance between metal centers in multinuclear metal catalysts, and the use of additives, have distinctive and significant impacts on catalyst performance. These effects give rise to notable cooperative effects. This review offers crucial insights and serves as a valuable reference for shaping the future development directions of binuclear and multinuclear early transition metal catalysts.

Polar Group-Promoted Copolymerization of Ethylene with Polar Olefins

Copolymerization of ethylene (E) and polar vinyl monomers is a great challenge but remains attractive, especially for early transition metal catalysts featuring strong oxophilicity. Herein, we present the copolymerization of E with 6-phenoxy-1-hexene (POH) by scandium precatalysts with different steric hindrances: (FluCH2CH2NHC)Sc(CH2SiMe3)2 (1), (FluCH2C5H4N)Sc(CH2SiMe3)2 (2), (FluCH2Ph)Sc(CH2SiMe3)2(THF) (3), and (C5Me4Ph)Sc(CH2C6H4NMe2-o)2 (4). In comparable conditions, precatalyst 1, activated by [Ph3C][B(C6F5)4] and AliBu3, shows high activity (1.4 × 105 g molSc–1 h–1) but gives a low POH incorporation ratio of 0.7 mol %. Under identical conditions, the activity of 2 drops to 0.1 × 105 g molSc–1 h–1. In contrast, the half-sandwich scandium precatalyst 3 displays a much higher activity (8.1 × 105 g molSc–1 h–1) and affords a copolymer with 7.0 mol % POH. The highest activity (18.0 × 105 g molSc–1 h–1) and incorporation ratio (8.6 mol %) are achieved by 4. After optimization of the reaction conditions, up to 16 mol % of POH could be incorporated into the copolymer produced with 4. The resultant copolymers are readily transformed into brominated products and, subsequently, ethylene-based ionomers featuring excellent mechanical properties. Density functional theory (DFT) simulations are employed to illustrate the mechanisms of polar group incorporation and the control of activity from kinetic, thermodynamic, and geometric viewpoints.

Microenvironment Modulation of Metal-Organic Frameworks (MOFs) for Coordination Olefin Oligomerization and (co)Polymerization.

The majority of commercial polyolefins are produced by coordination polymerization using early or late transition metal catalysts. Molecular catalysts containing these transition metals (Ti, Zr, Cr, Ni, and Fe, etc.) are loaded on supports for controlled polymerization behavior and polymer morphology in slurry or gas phase processes. Within the last few years, metal-organic frameworks (MOFs), a class of unique porous crystalline materials constructed from metal ions/clusters and organic ligands, have been designed and utilized as excellent supports for heterogeneous polymerization catalysis whose high density and uniform distribution of active sites would benefit the modulations of molecular weight distributions of high-performance olefin oligomers and (co)polymers. Impressive efforts have been made to modulate the microenvironment surrounding the active centers at the atomic level for improved activities of MOFs-based catalysts and controlled selectivity of olefin insertion. This review aims to draw a comprehensive picture of MOFs for coordination olefin oligomerization and (co)polymerization in the past decades with respect to different transition metal active centers, various incorporation sites, and finally microenvironment modulation. In consideration of more efforts are needed to overcome challenges for further industrial and commercial application, a brief outlook is provided.

Chain Transfer to Solvent and Monomer in Early Transition Metal Catalyzed Olefin Polymerization: Mechanisms and Implications for Catalysis

Even after several decades of intense research, mechanistic studies of olefin polymerization by early transition metal catalysts continue to reveal unexpected elementary reaction steps. In this mini-review, the recent discovery of two unprecedented chain termination processes is summarized: chain transfer to solvent (CTS) and chain transfer to monomer (CTM), leading to benzyl/tolyl and allyl type chain ends, respectively. Although similar transfer reactions are well-known in radical polymerization, only very recently they have been observed also in olefin insertion polymerization catalysis. In the latter context, these processes were first identified in Ti-catalyzed propene and ethene polymerization; more recently, CTS was also reported in Sc-catalyzed styrene polymerization. In the Ti case, these processes represent a unique combination of insertion polymerization, organic radical chemistry and reactivity of a M(IV)/M(III) redox couple. In the Sc case, CTS occurs via a σ-bond metathesis reactivity, and it is associated with a significant boost of catalytic activity and/or with tuning of polystyrene molecular weight and tacticity. The mechanistic studies that led to the understanding of these chain transfer reactions are summarized, highlighting their relevance in olefin polymerization catalysis and beyond.

Incorporation of Boronic Acid Functionality into Isotactic Polypropylene and Its Application as a Cross-Linking Point

Direct copolymerization of propylene and functional monomers using early transition metal catalysts is a prominent synthesis method to render a functionality to polypropylene maintaining high stereoregularity. In this study, we investigated the copolymerization of propylene and alkenylboronic acid moieties using the Zr complex and found that diaminonaphthalene (dan) is a good protective group for the boronic acid. Dan-protected boronic acid scarcely reduced the stereospecificity of the propylene polymerization so that the incorporation of 3.8 mol % comonomer still maintained a high melting point (107 °C). Moreover, deprotection of boronic acid amide proceeded along with the standard purification of polypropylene in acidic methanol. Boronic acid-functionalized isotactic polypropylene showed high Young’s modulus and strength probably because the copolymer was cross-linked upon molding via dehydrotrimerization of the boronic acids. Such cross-link was accelerated by the addition of a secondary amine.

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.

Mechanistic Insight into the Stereoselective Cationic Polymerization of Vinyl Ethers.

The control of the tacticity of synthetic polymers enables the realization of emergent physical properties from readily available starting materials. While stereodefined polymers derived from nonpolar vinyl monomers can be efficiently prepared using early transition metal catalysts, general methods for the stereoselective polymerization of polar vinyl monomers remain underdeveloped. We recently demonstrated asymmetric ion pairing catalysis as an effective approach to achieve stereoselective cationic polymerization of vinyl ethers. Herein, we provide a deeper understanding of stereoselective ion-pairing polymerization through comprehensive experimental and computational studies. These findings demonstrate the importance of ligand deceleration effects for the identification of reaction conditions that enhance stereoselectivity, which was supported by computational studies that identified the solution-state catalyst structure. An evaluation of monomer substrates with systematic variations in steric parameters and functional group identities established key structure-reactivity relationships for stereoselective homo- and copolymerization. Expansion of the monomer scope to include enantioenriched vinyl ethers enabled the preparation of an isotactic poly(vinyl ether) with the highest stereoselectivity (95.1% ± 0.1 meso diads) reported to date, which occurred when monomer and catalyst stereochemistry were fully matched under a triple diastereocontrol model. The more complete understanding of stereoselective cationic polymerization reported herein offers a foundation for the design of improved catalytic systems and for the translation of isotactic poly(vinyl ether)s to applied areas.

A concerted double-layer steric strategy enables an ultra-highly active nickel catalyst to access ultrahigh molecular weight polyethylenes

Both catalytic activity and polymer molecular weight are two crucial parameters in olefin polymerization catalysis. Differed from the superior feature of early transition metal catalysts, late transition metal nickel catalysts are usually more challenging to approach both of them at an ultrahigh level. In this contribution, by use of a concerted double-layer steric strategy a new conceptual α-diimine nickel catalyst was prepared to address the issues. The nickel catalyst featured highly thermally robust (0–150 °C), was ultra-highly active (a new level of 1.03 × 109 g mol−1 h−1) toward ethylene polymerization, and simultaneously produced ultrahigh molecular weight polyethylene product (UHMWPE, Mw = 4.2 × 106 g mol−1). Additionally, these obtained polyethylenes featured linear (2/1000C) to lightly branched (32/1000C) and could also be incorporated with a small amount of methyl 10-undecenoate.

Synthesis and characterization of post-metallocene titanium complexes of bidentate dicarboxylic acids and studies on the effect of ring size on their polymerization activity at room temperature in aqueous emulsion

The past decade has witnessed a remarkable growth in the polymerization reactions that are catalyzed by postmetallocene complexes. In this report, three early transition metal catalysts based on TiCl4 and aliphatic linear saturated dicarboxylic acids (Oxalic, Malonic and succinic acid) have been synthesized and characterized and their catalytic activity in emulsion polymerization have been studied. Upon treatment with NaBPh4, the complexes have been found to remain robust and highly active for the polymerization of Methylmethacrylate even at very low co-catalyst /catalyst ratios. Moreover, the polymers synthesized have been found to be isotactic rich.

The co-catalyst is the key

The synthesis of highly branched low molecular weight products — potential synthetic lubricants — directly from ethylene is challenging when highly efficient early transition metal catalysts are employed. Now, an alkane soluble co-catalyst permits the synthesis of such polyethylenes using a zirconium catalyst in a saturated hydrocarbon solvent, where competitive solvent coordination at the active site of the catalyst is relatively unimportant.

Highly branched polyethylene oligomers via group IV-catalysed polymerization in very nonpolar media

Early transition metal catalysts produce high-density and linear low-density polyethylenes with spectacular efficiency. Nevertheless, these catalysts are ineffective in producing low-density polyethylene homopolymers with large –(CH2)xCH3 branch densities (x ≥ 5) or low-molecular-mass (Mn < 1,200 g mol−1) highly branched polyethylenes (HBPEs). The latter are potential alternative synthetic lubricants that have eluded efficient catalytic synthesis. Here we report the synthesis of low-Mn HBPEs with 61–93 branches per 1,000 carbon atoms from abundant ethylene as the primary feedstock using a soluble, highly active ion-paired organozirconium catalyst in a saturated hydrocarbon solvent. The unprecedented activity and branch selectivity reflect previously unrecognized aspects of the cationic catalyst–counteranion pairing in nonpolar media and are characterized spectroscopically and quantum mechanically. The HBPE products are rheologically and tribologically attractive candidates for synthetic lubricants. Low-molecular-weight, highly branched polyethylenes are attractive candidates for synthetic lubricants, but their efficient production is constrained by a lack of effective catalytic methods. While conventional group IV transition metal catalysts produce diverse polyethylenes on a huge scale, they are unable to produce highly branched polyethylenes. Here a hydrocarbon-soluble organozirconium precatalyst and borate cocatalyst produce the desired polyethylenes with excellent activity and branch selectivity.

Syndioselective Polymerization of a BN Aromatic Vinyl Monomer

A new synthetic platform accessing stereoregular polar polyolefins is introduced. We showcase the stability of aromatic azaborine derivatives by demonstrating the syndioselective polymerization of BN 2-vinylnaphthalene (BN2VN) by a monocyclopentadienyl titanium complex. Homogeneous early transition metal catalysts are well-established systems for the synthesis of highly stereoregular syndiotactic polystyrene (sPS), but the oxophilic nature of these complexes results in catalyst decomposition with polar monomers. BN2VN’s compatibility with coordination polymerization, and its ability to intercept the mechanism of styrene polymerization, is attributed to its aromaticity. Stereoretentive postpolymerization oxidation of the organoborane side chain generates syndiotactic poly(vinyl alcohol) (sPVA), an example of PBN2VN’s potential for diverse postfunctionalization.

Exploring Ethylene/Polar Vinyl Monomer Copolymerizations Using Ni and Pd α-Diimine Catalysts.

The most ubiquitous polymer, polyethylene (PE), is produced either through a radical-initiated process or, more commonly, through a coordination/insertion process employing early transition metal catalysts, particularly titanium- and chromium-based systems. These oxophilic early metal catalysts are not functional-group-tolerant and thus cannot be used to synthesize copolymers of ethylene and polar vinyl monomers such as alkyl acrylates and vinyl acetate. Such PE copolymers have enhanced properties relative to PE and are made through radical polymerization processes, requiring exceptionally high pressures and temperatures. Copolymerizations of polar vinyl monomers with ethylene using more functional group-tolerant late metal catalysts potentially offer an attractive alternative for generating such value-added copolymers since ligand variations may provide more control of polymer microstructures and milder reaction conditions would apply. This Account describes our efforts, particularly through detailed mechanistic studies, to probe and develop this potential using Pd(II) and Ni(II) α-diimine catalysts. To inform discussions of the copolymerizations, we briefly review key aspects of ethylene homopolymerizations using these diimine catalysts. These include ligand designs that incorporate axial blocking groups that retard chain transfer and promote production of a high polymer rather than an oligomer. These ligand designs also lead to unique branched polyethylenes via migration of the metal along the chain ("chain-walking") prior to insertion. Mechanistic investigations of copolymerizations of ethylene with polar vinyl monomers using the diimine complexes have revealed several impediments to developing practical catalysts: (1) The polar group of the comonomer can coordinate strongly to the metal center, blocking coordination of ethylene. (2) Weak binding affinity of the polar monomer relative to ethylene can result in very low levels of comonomer incorporation. (3) A metal alkyl chain bearing a heteroatom, X, on the β-carbon atom can undergo β-X elimination leading to deactivation of the catalyst. (4) Stable chelate formation following insertion of a polar comonomer can greatly retard the rate of chain growth. (5) A metal alkyl chain bearing an electron-withdrawing heteroatom at the □-carbon atom can result in a high insertion barrier. A patent disclosure by the DuPont Versipol group and our extensive mechanistic studies reveal that, remarkably, vinyl trialkoxysilanes are ideal comonomers and circumvent all of the impediments noted above. The Pd-catalyzed copolymerization of vinyl trialkoxysilanes with ethylene produces highly branched, low molecular weight copolymers with activities comparable to those of analgous ethylene homopolymerizations. A 1,2- insertion of the vinyl silane results in the formation of a five-membered Pd-O(R)Si chelate which is readily opened by ethylene and thus does not reduce the rate of chain growth. β-Silyl elimination results in chain transfer and accounts for the lower molecular weight polymer. The nickel α-diimine-catalyzed copolymerizations produce high molecular weight copolymers with structures that vary from nearly linear to moderately branched. Both four- and five-membered chelates are catalyst resting states but are rapidly opened by ethylene, and thus turnover frequencies are only slightly reduced relative to ethylene homopolymerizations. Finally, a convenient and practical nickel-based system has been developed for the efficient synthesis of this copolymer which can be cross-linked to form PEX- b, a commercial PE plastic used for hot water plumbing pipes and power cable coatings.

Early transition metal-catalyzed C-H alkylation: hydroaminoalkylation for Csp3-Csp3 bond formation in the synthesis of selectively substituted amines.

Hydroaminoalkylation is a 100% atom economic method for forming Csp3-Csp3 bonds through C-H activation α to an amine and subsequent reaction with an alkene. When catalyzed by early transition metals, this reaction allows for alternative disconnections for the synthesis of structurally complex amines. This method avoids the installation of protecting groups or directing groups and does not require added oxidants, or photoredox catalysts. In this feature article, we discuss the various selectively substituted amines that can be accessed by hydroaminoalkylation, with a special focus on the development of early transition metal catalysts for their rapid, step and atom efficient assembly.

Base‐Metal Catalysis Marries Utilitarian Aspects with Academic Fascination

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Electrochemical Conversion of Cis,Trans-muconic Acid to Trans,Trans-muconic Acid and Trans-3-Hexenedioic Acid for Bio-Based Polymer Production

The production of biorenewable chemicals has significantly evolved over the past years and numerous processes either have reached commercial stage or are close to commercialization.1 Recently, the biological synthesis of diacids has been receiving greater attention for the manufacture of green polyesters and polyamides.2 Muconic acid, an unsaturated C6 diacid, is quickly evolving as a high interest intermediate for the production of a wide variety of renewable chemicals from biomass (Fig. 1). The increased interest is due to the increasing market size of adipic acid and the need for greener synthesis. Electrocatalytic hydrogenation (ECH) represents a promising approach to combine the fermentative synthesis of muconic acid with its chemical catalytic diversification to produce these value-added chemicals. ECH typically proceed by applying a -0.3 to -1.8 V potential vs. Ag/AgCl on a transition metal electrode. In contrast to high pressure hydrogenations using noble metals, the electrocatalytic hydrogenation of muconic acid is facilitated through the reduction of protons on the surface of base metal catalysts, yielding interesting selectivities and conversion rates. We demonstrate that the use of Earth-abundant catalysts such as Pb also mitigate the necessity to separate amino acids from the fermentation broth that have been shown to readily poison archetypical hydrogenation catalysts such as Ni, Pd, and Pt3. Herein we screen a wide variety of early, late, and post transition metal catalysts in an attempt to identify key parameters that govern ECH selectivity and faradaic efficiency. In particular, we differentiate the selected transition metals based on the free energy of hydrogen absorption ΔGH on ECH performance (Fig. 2). Our studies detail the synthesis of trans,trans-muconic acid and trans-3-hexenedoic acid, two bio-based intermediates for the production of polyamides and polyesters, using low hydrogen overpotential metal catalysts. In contrast, high hydrogen overpotential metals produced trans-3-hexenedioic acid with high yield and faradaic efficiency. Conversion and selectivities after 1 h reaction durations for two particular electrocatalysts are displayed in table 1. In all cases, the need to identify binding strengths of the organic reactant will necessitate further correlations that determine hydrogenation product selectivity.

Cycloolefin Copolymers by Early and Late Transition Metal Catalysts

AbstractThe complex [Pd(κ2‐P,O‐{2‐(2‐MeOC6H4)2P}C6H4SO3)Me(dmso)] (1) is investigated for the copolymerization of (E) with norbornene (N) and functionalized N derivatives affording P(E‐co‐N) in excellent yields. Copolymer molar masses are higher than those of PE and increase with N concentration. In addition, the complex [Ti(κ2‐N,O‐{2,6‐F2C6H3N = C(Me)C(H) = C(CF3)O})2Cl2] (2) is evaluated as catalyst for living E‐co‐N copolymerization upon activation with dried methylaluminoxane between 25 and 90 °C. Copolymerization at different [N]/[E] feed ratios affords stereoirregular alternating high molar mass P(E‐co‐N) with narrow molar mass distribution. P(E‐co‐N) living copolymerization is demonstrated by kinetics at 50 °C. Block copolymers are synthesized and fully characterized. magnified image

Supported single‐site catalysts for slurry and gas‐phase olefin polymerisation

AbstractEarly and late transition metal single‐site catalysts revolutionised the polyolefin manufacturing industry and research with their ability to make polymers with uniform microstructural properties. Several of these catalysts are currently used commercially to produce commodity and differentiated‐commodity resins. The key to their rapid success and industrial implementation resided in the fact that they could be used without major modifications in the polymerisation reactors that previously used heterogeneous Ziegler–Natta and Phillips catalysts. Since most of these industrial processes use slurry or gas‐phase reactors, the soluble single‐site catalysts must be supported on adequate carriers that ensure not only high activity, but also the formation of polymer particles with the proper morphology and bulk densities. In this article, we will review several methods used for supporting single‐site early and late transition metal catalysts on a variety of supports. © 2011 Canadian Society for Chemical Engineering

Supported hybrid early and late transition metal catalysts for the synthesis of polyethylene with tailored molecular weight and chemical composition distributions

Supported hybrid catalysts using metallocenes and a nickel diimine catalyst were synthesized and used for ethylene slurry polymerization and ethylene/1-hexene copolymerization. Two types of metallocenes, together with a nickel diimine catalyst were supported onto SiO2 through chemical bonding, and a borate compound was physisorbed for the activation of the catalysts. These supported hybrid catalysts had high catalyst activities and made free-flowing polymer particles. The chemical composition distribution, molecular weight averages and distributions of resultant polymers were controlled by catalyst structure and polymerization conditions such as reaction temperature and the use of α-olefin. According to GPC-IR, 13C NMR and CEF characterization results of some polymers, more 1-hexene was incorporated in the high molecular weight region, short chain branches were generated by the chain walking mechanism in low molecular weight region. The morphologies of the resulting particles were investigated by SEM.