Coordination−Insertion Copolymerization of Fundamental Polar Monomers

Aqueous Keto-Polyethylene Dispersions from Catalytic Copolymerization of Ethylene and Carbon Monoxide in Water

Water-soluble [P,O]Ni(II) catalysts enable the direct catalytic nonalternating copolymerization of fundamental comonomers ethylene and carbon monoxide (CO) in water as an environmentally friendly reaction medium. This yields stable aqueous dispersions of high molecular weight polyethylene containing ∼1 mol % of largely isolated in-chain keto groups in the form of particles with sizes between 100 nm and 1 μm. The intermediate species of chain growth resulting from incorporation of polar comonomers are amenable to specific chain termination pathways in conjunction with water.

Aqueous Coordination-Insertion Copolymerization for Producing High Molecular Weight Polar Polyolefins

Hydrocarbons, when used as the medium for transition metal catalyzed organic reactions and olefin (co-)polymerization, are ubiquitous. Environmentally friendly water is highly attractive and long-sought, but is greatly challenging as coordination-insertion copolymerization reaction medium of olefin and polar monomers. Unfavorable interactions from both water and polar monomer usually lead to either catalyst deactivation or the formation of low-molecular-weight polymers. Herein, we develop well-behaved neutral phosphinophenolato nickel catalysts, which enable aqueous copolymerization of ethylene and diverse polar monomers to produce significantly high-molecular-weight linear polar polyolefins (219-549 kDa, 0.13-1.29 mol %) in a single-component fashion under mild conditions for the first time. These copolymerization reactions occur better in water than in hydrocarbons such as toluene. The dual characteristics of high molecular weight and the incorporation of a small amount of functional group result in improved surface properties while retain the desirable intrinsic properties of high-density polyethylene (HDPE).

Ethylene/Polar Monomer Copolymerization by [N, P] Ti Complexes: Polar Copolymers with Ultrahigh-Molecular Weight

A series of novel titanium complexes (2a-2e) bearing [N, P] aniline-chlorodiphenylphosphine ligands (1a-1e) featuring CH3 and F substituents have been synthesized and characterized. Surprisingly, in the presence of polar additive, the complexes (2a-2e) all displayed high catalytic activities (up to 1.04 × 106 gPolymer (mol·Ti)-1·h-1 and produced copolymer with the ultrahigh molecular weight up to 1.37 × 106 g/mol. The catalytic activities are significantly enhanced by introducing electron-withdrawing group (F) into the aniline aromatic ring. Especially, the increase in activity based on different complexes followed the order of 2e > 2d > 2c > 2b > 2a. Simultaneously, density functional theory (DFT) calculations have been performed to probe the polymerization mechanism as well as the electronic and steric effects of various substituents on the catalyst backbone. DFT computation revealed that the polymerization behaviors could be adjusted by the electronic effect of ligand substituents; however, it has little to do with the steric hindrance of the substituents. Furthermore, theoretical calculation results keep well in accordance with experimental measurement results. The article provided an appealing design method that the employment of fluorine atom as electron-withdrawing to be studied is the promotive effect of transition-metal coordination polymerization.

Synthesis and Deconstruction of Polyethylene-type Materials

Polyethylene deconstruction to reusable smaller molecules is hindered by the chemical inertness of its hydrocarbon chains. Pyrolysis and related approaches commonly require high temperatures, are energy-intensive, and yield mixtures of multiple classes of compounds. Selective cleavage reactions under mild conditions ( Collapse

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Grants

• H2020 European Research Council

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Affiliation(s)

Simon T. Schwab Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitätsstraße 10, 78464 Konstanz, Germany
Maximilian Baur Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitätsstraße 10, 78464 Konstanz, Germany
Taylor F. Nelson Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitätsstraße 10, 78464 Konstanz, Germany
Stefan Mecking Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitätsstraße 10, 78464 Konstanz, Germany

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Ambient Catalytic Spinning of Polyethylene Nanofibers

A novel single-atom Ni(II) catalyst (Ni-OH) is covalently immobilized onto the nano-channels of mesoporous Santa Barbara Amorphous (SBA)-15 particles and isotropic Anodized Aluminum Oxide (AAO) membrane for confined-space ethylene extrusion polymerization. The presence of surface-tethered Ni complexes (Ni@SBA-15 and Ni@AAO) is confirmed by the inductively coupled plasma-optical emission spectrometry (ICP-OES) and X-ray photoelectron spectroscopy (XPS). In the catalytic spinning process, the produced PE materials exhibit very homogeneous fibrous morphology at nanoscale (diameter: ~50 nm). The synthesized PE nanofibers extrude in a highly oriented manner from the nano-reactors at ambient temperature. Remarkably high Mw (1.62×106  g mol-1 ), melting point (124 °C), and crystallinity (41.8 %) are observed among PE samples thanks to the confined-space polymerization. The chain-walking behavior of surface tethered Ni catalysts is greatly limited by the confinement inside the nano-channels, leading to the formation of very low-branched PE materials (13.6/1000 C). Due to fixed supported catalytic topology and room temperature, the filaments are expected to be free of entanglement. This work signifies an important step towards the realization of a continuous mild catalytic-spinning (CATSPIN) process, where the polymer is directly synthesized into fiber shape at negligible chain branching and elegantly avoiding common limitations like thermal degradation or molecular entanglement.

Keto-Polyethylenes with Controlled Crystallinity and Materials Properties from Catalytic Ethylene-CO-Norbornene Terpolymerization

Recent advances in Ni(II) catalyzed, nonalternating catalytic copolymerization of ethylene with carbon monoxide (CO) enable the synthesis of in-chain keto-functionalized polyethylenes (keto-PEs) with high-density polyethylene-like materials properties. Addition of norbornene as a bulky, noncrystallizable comonomer during catalytic polymerization allows tuning of the crystallinity in these keto-PE materials by randomly incorporated norbornene units in the polymer chain, while molecular weights are not adversely affected. Such crystallinity-reduced keto-PEs are characterized as softer materials with better ductility and may therefore be more suited for, e.g., potential film applications.

Norbornene Copolymerization with Polar Monomers Catalyzed by Palladium Catalysts Containing Imidazolidin-2-imine/Guanidine Ligands

The development of a palladium catalyst that has enhanced catalytic performance, such as low aluminum cocatalyst loading, good copolymerization ability, high molecular weight, and excellent solubility of the (co)polymers, is still a challenge in norbornene copolymerizations. Here, a series of PdCl2 and PdMeCl complexes containing differently substituted anilines and imidazolidin-2-imine/guanidine ligands was successfully synthesized and characterized. X-ray diffraction analysis results revealed that these Pd complexes adopted an almost square-planar geometry, and the six-membered chelate ring showed structural distinctions as compared to traditional N^N-based α-diimine and β-diimine Pd complexes. These Pd complexes were activated by EtAlCl2 and then exhibited moderate activity (104-105 g mol-1 h-1) and good thermal stability (up to 90 °C) for norbornene polymerization to produce high-molecular-weight PNBs (Mn up to 96.4 kg mol-1) with narrow polydispersities (PDI as low as 1.39). These Pd complexes also exhibited good polar group tolerance in the copolymerization of norbornene with methyl 5-norbornene-2-carboxylate and methyl 10-undecenoate, in which the activity was achieved up to 7.04 × 104 g mol-1 h-1. It furnished polar functionalized norbornene-based copolymers with high molecular weight (Mn up to 63.1 kg mol-1), narrow PDI, reasonable polar monomer incorporation, and good solubility. These Pd catalysts exhibited an enhanced copolymerization ability to produce PNB or NB-based copolymers, representing significant progress in this field.

Recent Advances in Synthesis of Non-Alternating Polyketone Generated by Copolymerization of Carbon Monoxide and Ethylene

The copolymers of carbon monoxide (CO) and ethylene, namely aliphatic polyketones (PKs), have attracted considerable attention due to their unique property and degradation. Based on the arrangement of the ethylene and carbonyl groups in the polymer chain, PKs can be divided into perfect alternating and non-perfect alternating copolymers. Perfect alternating PKs have been previously reviewed, we herein focus on recent advances in the synthesis of PKs without a perfect alternating structure including non-perfect alternating PKs and PE with in-chain ketones. The chain structure of PKs, catalytic copolymerization mechanism, and non-alternating polymerization catalysts including phosphine-sulfonate Pd, diphosphazane monoxide (PNPO) Pd/Ni, and phosphinophenolate Ni catalysts are comprehensively summarized. This review aims to enlighten the design of ethylene/CO non-alternating polymerization catalysts for the development of new polyketone materials.

Copolymerization of Ethylene with Functionalized 1,1-Disubstituted Olefins Using a Fluorenylamido-Ligated Titanium Catalyst

Considering the sustainability of material development, coordination polymerization catalysts effective for 1,1-disubstituted olefins are receiving a great deal of attention because they can introduce a variety of plant-derived comonomers, such as β-pinene and limonene, into polyolefins. However, due to their sterically encumbered property, incorporating these monomers is difficult. Herein, we succeeded in the copolymerization of ethylene with various hydroxy- or siloxy-substituted vinylidenes using a fluorenylamido-ligated titanium catalyst-MMAO system. This is the first example of ethylene/polar 1,1-disubstituted olefins' copolymerization using an early transition metal catalyst system. The polymerization proceeded at room temperature without pressurizing ethylene, and high-molecular-weight, functionalized polyethylene was obtained. The obtained copolymer showed a reduced water contact angle compared with that of the ethylene/isobutene copolymer, demonstrating the increment in hydrophilicity by hydroxy groups.

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.

Acrylate-Induced β-H Elimination in Coordination Insertion Copolymerizaton Catalyzed by Nickel

Polar monomer-induced β-H elimination is a key elementary step in polar polyolefin synthesis by coordination polymerization but remains underexplored. Herein, we show that a bulky neutral Ni catalyst, 1Ph, is not only a high-performance catalyst in ethylene/acrylate copolymerization (activity up to ∼37,000 kg/(mol·h) at 130 °C in a batch reactor, mol % tBA ∼ 0.3) but also a suitable platform for investigation of acrylate-induced β-H elimination. 4Ph-tBu, a novel Ni alkyl complex generated after acrylate-induced β-H elimination and subsequent acrylate insertion, was identified and characterized by crystallography. A combination of catalysis and mechanistic studies reveals effects of the acrylate monomer, bidentate ligand, and the labile ligand (e.g., pyridine) on the kinetics of β-H elimination, the role of β-H elimination in copolymerization catalysis as a chain-termination pathway, and its potential in controlling the polymer microstructure in polar polyolefin synthesis.

Recent Advancements in Mechanistic Studies of Palladium- and Nickel-Catalyzed Ethylene Copolymerization with Polar Monomers

The introduction of polar functional groups into polyolefin chain structures creates opportunities to enhance specific properties, such as adhesion, dyeability, printability, compatibility, thermal stability, and electrical conductivity, which widen the range of potential applications for these modified materials. Transition metal catalysts, especially late transition metals, have proven to be highly effective in copolymerization processes due to their reduced Lewis acidity and electrophilicity. However, when compared to the significant progress and summary of synthetic methods, there is a distinct lack of a comprehensive summary of mechanistic studies pertaining to the catalytic systems involved in ethylene copolymerization catalyzed by palladium and nickel catalysts. In this review, we have provided a comprehensive summary of the latest developments in mechanistic studies of ethylene copolymerization with polar monomers catalyzed by late-transition-metal complexes. Experimental and computational methods were employed to conduct a detailed investigation of these organic and organometallic systems. It is mainly focused on ligand substitution, changes in binding modes, ethylene/polar monomer insertion, chelate opening, and β-H elimination. Factors that control the catalytic activity, molecular weight, comonomer incorporation ratios, and branch content are analyzed, these include steric repulsions between ligands and monomers, electronic effects arising from both ligands and monomers, and so on.

Using a single complex to predict the reaction energy profile: a case study of Pd/Ni-catalyzed ethylene polymerization

Mechanism-driven catalyst screening could be greatly accelerated by quantitative prediction models of the reaction energy profile. Here, we propose a novel method for molecular representation, taking palladium- and nickel-catalyzed ethylene polymerization as model reactions. The geometric parameters (GPfra) and electron occupancies (EOfra) from the non-ligand fragment of the η3-complex were extracted as the molecular descriptors, followed by constructing the reaction energy profile prediction models on the basis of various regression algorithms. The models showed great accuracy with respect to both theoretical and experimental data. More importantly, the models are convenient for training and utilization. On one hand, all the features were easily captured from the single η3-complex. On the other hand, further investigation also demonstrated that the models could be constructed with a small training sample size. We believe that our featurization method could possibly be generalized to more organometallic reactions and paves the way to efficient catalyst design.

Controlled Radical Polymerization of Acrylates and Isocyanides Installs Degradable Functionality into Novel Copolymers

Installing ketones into a polymer backbone is a known method for introducing photodegradability into polymers; however, most current methods are limited to ethylene-carbon monoxide copolymerization. Here we use isocyanides in place of carbon monoxide in a copolymerization strategy to access degradable nonalternating poly(ketones) that either maintain or enhance the thermal properties. A cobalt-mediated radical polymerization of acrylates and isocyanides synthesizes nonalternating poly(acrylate-co-isocyanide) copolymers with tunable incorporation using monomer feed ratios. The kinetic product of the polymerization is a dynamic β-imine ester that tautomerizes to the β-enamine ester. Hydrolysis of this copolymer affords a third copolymer microstructure─the elusive nonalternating poly(ketone)─from a single copolymerization strategy. Analysis of the copolymer properties demonstrates tunable thermal properties with the degree of incorporation. Finally, we show that poly(acrylate-co-isocyanide) and poly(acrylate-co-ketone) are photodegradable with 390 nm light, enabling chain cleavage.

Photodegradable polar-functionalized polyethylenes

The degradation of plastics has attracted much attention from the global community. Polyethylenes (PEs), as the most abundant synthetic plastics, are most frequently studied. PE is non-degradable and non-polar because of the sole presence of the pure hydrocarbon components. Concurrent incorporation of both in-chain cleavable and functional groups into the PE chain is an effective pathway to overcome the non-degradable and non-polar issue; however, the method for achieving this pathway remains elusive. Here, we report a strictly non-alternating (>99%) terpolymerization of ethylene with CO and fundamental polar monomers via a coordination-insertion mechanism using late transition metal catalysts, which effectively prevents the formation of undesired chelates originating from both co-monomers under a low CO concentration. High-molecular-weight linear PEs with both in-chain isolated keto (>99%) and main-chain functional groups are prepared. The incorporation of key low-content isolated keto groups makes PEs photodegradable while retaining their desirable bulk material properties, and the introduction of polar functional groups considerably improves their surface properties.

Stabilization of propene molybdenum and tungsten half-sandwich complexes by intramolecular coordination of a thioether function

This study reports the stabilizing effect of an intramolecularly coordinated thioether function in propene complexes of the general formula [{η⁵:κS-C₅H₄(CH₂)₂SR}M(CO)₂(η²-C₂H₃Me)][BF₄] (M = Mo, W; R = Et, Ph). They are formed by protonation of allyl analogues [{η⁵-C₅H₄(CH₂)₂SR}M(CO)₂(η³-C₃H₅)] by tetrafluoroboric acid in non-coordinating solvents. In contrast to analogues with unsubstituted Cp ligands, these propene complexes are isolable in a pure form and characterized by NMR spectroscopy. The molybdenum compounds are stable at low temperature and the propene ligand can easily be exchanged by thioethers or acetonitrile. Several representatives of the reaction products were characterized by X-ray structure analysis. The stabilization effect in tungsten complexes [{η⁵:κS-C₅H₄(CH₂)₂SR}W(CO)₂(η²-C₂H₃Me)][BF₄] (R = Et, Ph) was unusually high. The compounds are long-term stable at room temperature and do not undergo ligand exchange reactions even with strong chelators such as 1,10-phenanthroline. The molecular structure of the tungsten propene complex was confirmed by X-ray diffraction analysis on a single crystal.

Recent Advances in Transition Metal-Based Catalysts for Ethylene Copolymerization with Polar Comonomer

The synthesis of polar functionalized polyolefin (PFP) offers improvement in mixing properties, polymer surface, and rheological properties with the potential of upgraded polyolefins for modern and ingenious applications. The synthesis of PFP from metal-based catalyzed olefin (non-polar in nature) copolymerization with polar comonomers embodies energy-efficient, atom-efficient, and apparently an upfront methodology. Despite their outstanding success during conventional polymerization of olefin, 3^rd and 4^th group (early transition metal)-based catalysts, owing to their electrophilic nature, face challenges mainly due to Lewis basic sites of the polar monomers. On the contrary, late transition metal-based catalysts have also made progress, in recent years, for PFP synthesis. The recent past has also witnessed several advancements in the development of dominating palladium-based catalysts while their lower resistance towards ligand functional groups has limited the practical application of abundant and cheaper nickel-based catalysts. However, the relentless efforts of the scientific community, during the past half-decade, have indicated rigorous progress in the development of nickel-based catalysts for PFP synthesis. In this review, we have abridged the recent research trends in both early as well as late transition metal-based catalyst development. Furthermore, we have highlighted the role of transition metal-based catalysts in influencing the polymer properties.

Mechanistic study to reveal steric and electronic aspects involved in the formation of microstructures during Pd-catalyzed olefin/divinyl formal copolymerization: reactivity to catalyst choice

The advancement of metal-catalyzed copolymers is a formidable challenge for achieving distinct catalytic properties to compete with existing plastic polymers in industrial commodities. Herein, we reveal the roles of electronic and steric environments in the thermodynamic preference of microstructures in ethylene/divinyl formal (DVF) co-polymerization using a Pd catalyst under mild conditions to accommodate the respective industrial applicabilities. The insertion products of DVF result in the alteration of the steric crowding, ultimately favoring the efficient formation of cyclic units having potential applications in the manufacture of high-strength fibers. More specifically, to achieve an improved yield of the end copolymer, we tuned the catalytic activity and regioselectivity through a variety of catalysts during ethylene-DVF co-polymerization. The naphthalene-bridged (P^O)PdMe catalyst was found to be promising in terms of the least hindered (buried volume of 47.8%) environment with the thermodynamic preference of 2,1-insertion with an energy of 5.1 kcal mol^-1 among all the Pd-metal based catalysts. The highest activity with moderate energy barriers of the proposed catalyst will open new avenues for achieving a variety of potential applications, which is typically not possible using existing polymerization techniques.

Direct Synthesis of Partially Chain-Straightened Propylene Oligomers and P-MA Co-Oligomers Using Axially Flexible Shielded Iminopyridyl Palladium Complexes

In this study, a series of partially chain-straightened propylene oligomers and functional propylene−methyl acrylate (P-MA) co-oligomers were synthesized with 8-alkyl-iminopyridyl Pd(II) catalysts. The molecular weight and polar monomer incorporation ratio could be tuned by using Pd(II) catalysts with various 8-alkyl-naphthyl substituents (8-alkyl: H, Me, and n-Bu). In propylene oligomerization, all the 8-alkyl-iminopyridyl Pd(II) catalysts convert propylene to partially chain-straightened (119−136/1000 C) oligomers with low molecular weights (0.3−1.5 kg/mol). Among the catalysts, Pd1 with non-substituent (H) on the ligand showed the highest activity of 5.4 × 104 g/((mol of Pd) h), generating oligomers with the lowest molecular weight (Mn: 0.3 kg/mol). Moreover, polar-functionalized propylene-MA co-oligomers with very high incorporation ratios (22.8−36.5 mol %) could be obtained in the copolymerization using these 8-alkyl-iminopyridyl Pd(II) catalysts. Additionally, Pd1 exhibited the best performance in propylene-MA copolymerization as it displayed the highest MA incorporation ratio of up to 36.5 mol%. All the three catalysts are capable of generating partially chain-straightened P-MA co-oligomers and the activities decrease gradually while the molecular weight increases with the increasing steric hindrance of the alkyl substituent (H < Me < n-Bu). Compared to Pd4 with the rigid 8-aryl substituent, the flexible 8-alkyl-iminopyridyl Pd(II) catalysts (Pd1-3) not only showed much higher activities in the propylene oligomerization, but also yielded P-MA co-oligomers with significantly higher incorporation ratios in the propylene co-oligomerization.

Density Functional Theory Analysis of the Copolymerization of Cyclopropenone with Ethylene Using a Palladium Catalyst

Density functional theory has been used to elucidate the mechanism of Pd copolymerization of cyclopropenone with ethylene. The results reveal that introducing ethylene and cyclopropenone to Pd catalyst is thermodynamically feasible and generates the α,β-unsaturated ketone unit (UnitA). Cis-mode insertion and Path A_1a are the most favorable reaction routes for ethylene and cyclopropenone, respectively. Moreover, cyclopropenone decomposition can generate CO in situ without a catalyst or with a Pd catalyst. The Pd-catalyzed decomposition of cyclopropenone exhibits a lower reaction barrier (22.7 kcal/mol) than its direct decomposition. Our study demonstrates that incorporating CO into the Pd catalyst can generate the isolated ketone unit (UnitB). CO is formed first; thereafter, UnitB is generated. Therefore, the total energy barrier of UnitB generation, accounting for the CO barrier, is 22.7 kcal/mol, which is slightly lower than that of UnitA generation (24.0 kcal/mol). Additionally, the possibility of copolymerizing ethylene, cyclopropenone, and allyl acetate (AAc) has been investigated. The free energy and global reactivity index analyses indicate that the cyclopropenone introduction reaction is more favorable than the AAc insertion, which is consistent with the experimental results. Investigating the copolymerization mechanism will help to develop of a functionalization strategy for polyethylene polymers.

Electronic Tuning of Sterically Encumbered 2-(Arylimino)Pyridine-Nickel Ethylene Polymerization Catalysts by Para-Group Modification

A collection of five related 2-(arylimino)pyridines, 2-{(2,6-(CH(C6H4-p-F)2)2-4- RC6H2)N=CMe}C5H4N, each ortho-substituted with 4,4′-difluorobenzhydryl groups but distinct in the electronic properties of the para-R substituent (R = Me L1, Et L2, i-Pr L3, F L4, OCF3 L5), were prepared and combined with (DME)NiBr2 to form their corresponding LNiBr2 complexes, Ni1–Ni5, in high yields. All the complexes were characterized by FT-IR, 19F NMR spectroscopy and elemental analysis, while Ni5 was additionally the subject of an X-ray determination, revealing a bromide-bridged dimer. The molecular structure of bis-ligated (L4)2NiBr2 (Ni4’) was also determined, the result of ligand reorganization having occurred during attempted crystallization of Ni4. On activation with either EtAlCl2 or MMAO, Ni1–Ni5 exhibited high catalytic activities (up to 4.28 × 106 g of PE (mol of Ni)−1 h−1 using EtAlCl2) and produced highly branched polyethylene exhibiting low molecular weight (Mw range: 2.50–6.18 kg·mol−1) and narrow dispersity (Mw/Mn range: 2.21–2.90). Notably, it was found that the type of para-R group impacted on catalytic performance with Ni5 > Ni4 > Ni3 > Ni1 > Ni2 for both co-catalysts, underlining the positive influence of electron withdrawing substituents. Analysis of the structural composition of the polyethylene by 1H and 13C NMR spectroscopy revealed the existence of vinyl-end groups (–CH=CH2) and high levels of internal unsaturation (–CH=CH–) (ratio of vinylene to vinyl, range: 3.1:1–10.3:1) along with various types of branch (Me, Et, Pr, Bu, 1,4-paired Me, 1,6-paired Me and LCBs). Furthermore, reaction temperature was shown to greatly affect the end group type, branching density, molecular weight and in turn the melting points of the resulting polyethylenes.

Selective dehydrogenation of small and large molecules by a chloroiridium catalyst

The dehydrogenation of abundant alkane feedstocks to olefins is one of the mostly intensively investigated reactions in organic catalysis. A long-standing, pervasive challenge in this transformation is the direct dehydrogenation of unactivated 1,1-disubstituted ethane, an aliphatic motif commonly found in organic molecules. Here, we report the design of a diphosphine chloroiridium catalyst for undirected dehydrogenation of this aliphatic class to form valuable 1,1-disubstituted ethylene. Featuring high site selectivity and excellent functional group compatibility, this catalytic system is applicable to late-stage dehydrogenation of complex bioactive molecules. Moreover, the system enables unprecedented dehydrogenation of polypropene with controllable degree of desaturation, dehydrogenating more than 10 in 100 propene units. Further derivatizations of the resulting double bonds afford functionalized polypropenes.

Recent Advances in the Copolymerization of Ethylene with Polar Comonomers by Nickel Catalysts

The less-expensive and earth-abundant nickel catalyst is highly promising in the copolymerization of ethylene with polar monomers and has thus attracted increasing attention in both industry and academia. Herein, we have summarized the recent advancements made in the state-of-the-art nickel catalysts with different types of ligands for ethylene copolymerization and how these modifications influence the catalyst performance, as well as new polymerization modulation strategies. With regard to α-diimine, salicylaldimine/ketoiminato, phosphino-phenolate, phosphine-sulfonate, bisphospnine monoxide, N-heterocyclic carbene and other unclassified chelates, the properties of each catalyst and fine modulation of key copolymerization parameters (activity, molecular weight, comonomer incorporation rate, etc.) are revealed in detail. Despite significant achievements, many opportunities and possibilities are yet to be fully addressed, and a brief outlook on the future development and long-standing challenges is provided.

Highly Active and Thermally Robust Nickel Enolate Catalysts for the Synthesis of Ethylene-Acrylate Copolymers

The insertion copolymerization of polar olefins and ethylene remains a significant challenge in part due to catalysts' low activity and poor thermal stability. Herein we demonstrate a strategy toward addressing these obstacles through ligand design. Neutral nickel phosphine enolate catalysts with large phosphine substituents reaching the axial positions of Ni achieve activity of up to 7.7×10³  kg mol^-1  h^-1 (efficiency >35×10³  g copolymer/g Ni) at 110 °C, notable for ethylene/acrylate copolymerization. NMR analysis of resulting copolymers reveals highly linear microstructures with main-chain ester functionality. Structure-performance studies indicate a strong correlation between axial steric hindrance and catalyst performance.

Controlled Cobalt-Mediated Free-Radical Co- and Terpolymerization of Carbon Monoxide

While controlled free-radical polymerizations are established for a vast range of vinyl monomers, they have not been reported for carbon monoxide, although it is a unique monomer that forms in-chain keto groups which can promote, for example, desirable photo-degradability in polyethylenes. We report organometallic-mediated radical copolymerization of carbon monoxide with ethylene initiated by an organocobalt^III compound to keto-modified polyethylenes with up to 15 mol % ketone repeat units. Terpolymerization with 2-methylene-1,3-dioxepane affords polyethylenes with in-chain ester and keto groups. Compared to ethylene homopolymerization, the controlled character of the copolymerization is strongly enhanced by the Lewis base function of carbon monoxide, which suppresses multiple unfavorable termination pathways.

Linear Block Copolymer Synthesis

Block copolymers form the basis of the most ubiquitous materials such as thermoplastic elastomers, bridge interphases in polymer blends, and are fundamental for the development of high-performance materials. The driving force to further advance these materials is the accessibility of block copolymers, which have a wide variety in composition, functional group content, and precision of their structure. To advance and broaden the application of block copolymers will depend on the nature of combined segmented blocks, guided through the combination of polymerization techniques to reach a high versatility in block copolymer architecture and function. This review provides the most comprehensive overview of techniques to prepare linear block copolymers and is intended to serve as a guideline on how polymerization techniques can work together to result in desired block combinations. As the review will give an account of the relevant procedures and access areas, the sections will include orthogonal approaches or sequentially combined polymerization techniques, which increases the synthetic options for these materials.

Molecularly Defined Polyolefin Vitrimers from Catalytic Insertion Polymerization

Vitrimers can combine the advantageous properties of cross-linked materials with thermoplastic processability. For the prominent case of polyethylene, established post-polymerization introduction of cross-linkable moieties results in extremely heterogeneous compositions of the chains. Here, we report the generation of functionalized polyethylenes directly by catalytic insertion polymerization, with incorporated cross-linkable aryl boronic esters or alternatively acetal-protected groups suited for cross-linking with difunctional boronic esters. In addition to the desired homogeneous in-chain distribution, the reactive cross-linkable groups are enriched at the chain ends. This enables the incorporation of all chains in the network, as also supported by simulations of all chains' compositions. The uniform molecular composition of the chains reflects in resulting vitrimers' material properties, particularly lack of leaching with solvents. At the same time, cross-linking is indeed fully reversible and the vitrimers can be recycled.