Palladium/IzQO-Catalyzed Coordination–Insertion Copolymerization of Ethylene and 1,1-Disubstituted Ethylenes Bearing a Polar Functional Group

Polyethylene Materials with Tunable Degradability by Incorporating In-Chain Mechanophores

Polyolefins are recognized as fundamental plastic materials that are manufactured in the largest quantities among all synthetic polymers. The chemical inertness of the saturated hydrocarbon chains is crucial for storing and using polyolefin plastics, but poses significant environmental challenges related to plastic pollution. Here, we report a versatile approach to creating polyethylene materials with tunable degradability by incorporating in-chain mechanophores. Through palladium-catalyzed coordination/insertion copolymerization of ethylene with cyclobutene-fused comonomers, several cyclobutane-fused mechanophores were successfully incorporated with varied insertion ratios (0.35-26 mol %). The resulting polyethylene materials with in-chain mechanophores exhibit both high thermal stability and remarkable acid resistance. Upon mechanochemical activation by ultrasonication or ball-milling, degradable functional units (imide and ester groups) are introduced into the main polymer chain. The synergy of mechanochemical activation and acid hydrolysis facilitates the efficient degradation of high molecular weight polyethylene materials into telechelic oligomers.

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.

Metal-Free Amino(hetero)arylation and Aminosulfonylation of Alkenes Enabled by Photoinduced Energy Transfer

β-(Hetero)arylethylamines are privileged structural motifs found in many high-value organic molecules, including pharmaceuticals and natural products. To construct these important molecular skeletons, previous methods are mainly achieved by amino(hetero)arylation reaction with the aid of transition metals and preactivated substrates. Herein, we report a metal-free and photoinduced intermolecular amino(hetero)arylation reaction for the single-step installation of both (hetero)aryl and iminyl groups across alkenes in an efficient and regioselective manner. This method shows broad scope (up to 124 examples) and excellent tolerance of various olefins─from the simplest ethylene to complex multisubstituted alkenes can all participate in the reaction. Furthermore, aminosulfonylation of alkenes can be also conducted in the presence of sodium bisulfite as the SO2 source.

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.

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.

Multivariate Linear Regression Models to Predict Monomer Poisoning Effect in Ethylene/Polar Monomer Copolymerization Catalyzed by Late Transition Metals

This study combined density functional theory (DFT) calculations and multivariate linear regression (MLR) to analyze the monomer poisoning effect in ethylene/polar monomer copolymerization catalyzed by the Brookhart-type catalysts. The calculation results showed that the poisoning effect of polar monomers with relatively electron-deficient functional groups is weaker, such as ethers, and halogens. On the contrary, polar monomers with electron-rich functional groups (carbonyl, carboxyl, and acyl groups) exert a stronger poisoning effect. In addition, three descriptors that significantly affect the poisoning effect have been proposed on the basis of the multiple linear regression model, viz., the chemical shift of the vinyl carbon atom and heteroatom of polar monomer as well as the metal-X distance in the σ-coordination structure. It is expected that these models could guide the development of efficient catalytic copolymerization system in this field.

Copolymerization of ethylene with non-vinyl polar monomers

Introduction of functional groups on polyethylene endows it with a higher surface property and thus various catalysts have been developed for the copolymerization of ethylene with polar vinyl monomers. Aside from vinyl monomers, however, other classes of polar monomers have not found application in the copolymerization with ethylene. Here, in this short review article, our latest studies on catalyst development aiming at the use of non-vinyl polar monomers and the properties of the resulting copolymers are summarized.

Terpolymerization of Ethylene and Two Different Methoxyaryl-Substituted Propylenes by Scandium Catalyst Makes Tough and Fast Self-Healing Elastomers

The terpolymerization of a non-polar olefin (such as ethylene) and two different polar functional olefins in a controlled fashion is of great interest and importance but has hardly been explored to date. We report for the first time the terpolymerization of ethylene (E) and two different methoxyaryl-substituted propylenes (A^R1 P=hexylanisyl propylene; A^R2 P=methoxynaphthyl propylene or methoxypyrenyl propylene) by a half-sandwich scandium catalyst. The terpolymerization took place in a sequence-controlled fashion, affording unique multi-block copolymers composed of two different ethylene-alt-methoxyarylpropylene sequences E-alt-A^R1 P (soft segments) and E-alt-A^R2 P (hard segments) and relatively short ethylene-ethylene (EE) blocks (crystalline segments). The terpolymers exhibited excellent elasticity and unprecedented self-healing as a result of microphase separation of nanodomains of the crystalline EE segments and the hard amorphous E-alt-A^R2 P segments from a very flexible E-alt-A^R1 P matrix, demonstrating unique synergy of the three different components.

Influences of Ligand Backbone Substituents on Phosphinecarbonylpalladium and -nickel Catalysts for Ethylene Polymerization and Copolymerization with Polar Monomers

A series of phosphinecarbonylpalladium and -nickel catalysts bearing various substituents on the ligand backbone were prepared, characterized, and used in ethylene polymerization and copolymerization with polar monomers. The Pd and Ni catalysts can achieve high activities as well as high polymer molecular weights in both ethylene polymerization and copolymerization with polar monomers. The electron-donating group from the carbonyl side can effectively increase the polymer molecular weights. Utilization of a cyclic backbone structure can increase the catalytic activities at the expense of the polymer molecular weights. Moreover, installation of a pyridyl moiety in the ligand backbone can enable Lewis acid responsiveness and can enhance the polymerization activities. These results suggest the importance of the ligand backbone for the properties of catalysts in ethylene polymerization and copolymerization reactions.

A disubstituted-norbornene-based comonomer strategy to address polar monomer problem

The transition-metal-catalyzed copolymerization of olefins with polar comonomers is a direct strategy to access polar-functionalized polyolefins in an economical manner. Due to the intrinsic poisoning effect of polar groups towards Lewis acidic metal centers and the drastic reactivity differences of polar comonomers versus non-polar olefins, it is challenging to develop catalysts that provide the desired polymer molecular weight, comonomer incorporation, and activity. In this contribution, we tackle this issue from a comonomer perspective using 5,6-disubstituted norbornenes, which are highly versatile, easily accessible, inexpensive, and capable of introducing two functional groups in a single insertion. More importantly, they are only mildly poisoning due to the presence of long spacers between double bonds and polar groups, and are not prone to β-hydride elimination due to their cyclic structures. As strong π-donors, they can competitively bind to metal centers versus olefins. Indeed, phosphine-sulfonate palladium catalysts can catalyze the copolymerization of ethylene with 5,6-disubstituted norbornenes and simultaneously achieve a high polymerization activity, copolymer molecular weight, and comonomer incorporation. The practicality of this system was demonstrated by studying the properties of the resulting polymers, copolymerization in hydrocarbon solvents or in bulk, recovery/utilization of unreacted comonomer, molecular weight modulation, and large-scale synthesis.

Functional Polyethylenes by Organometallic-Mediated Radical Polymerization of Biobased Carbonates

Partly or fully renewable (co)polymers are gaining interest in both academia and industry. Polyethylene is a widely used polymer, classically derived from fossil fuels, with a high versatility stemming from the introduction of comonomers altering the mechanical properties. The introduction of renewable functionalities into this polymer is highly attractive to obtain functional, tunable, and at least partially renewable polyethylenes. We herein report the introduction of biosourced cyclic carbonates into polyethylene using organometallic-mediated radical polymerization under mild conditions. Molecular weights of up to 14 600 g mol^-1 with dispersities as low as 1.19 were obtained, and the cyclic carbonate content could be easily tuned by the ethylene pressure during the polymerization. As a proof of concept, the hydrolysis of the cyclic carbonates of a representative copolymer was explored, and it provided polyethylene-bearing vicinal diols, with a hydrolysis degree of 71%. Given the multitude of chemoselective modifications possible on cyclic carbonates as well as the fact that many allylic- and alkylidene-type cyclic carbonates are accessible from renewable resources, this work opens up an avenue for the design of functional and more sustainable polyethylenes.

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

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

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

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

Ortho-Functionalized Dibenzhydryl Substituents in α-Diimine Pd Catalyzed Ethylene Polymerization and Copolymerization

Sterically bulky diarylmethyl-based ligands have received increasing attention in the field of late-transition-metal catalyzed olefin polymerization. Ortho-substituents may have a significant impact on the performance of diarylmethyl-based α-diimine Pd catalysts. In this contribution, a series of α-diimine Pd catalysts bearing ortho-methoxyl/hydroxyl functionalized dibenzhydryl units were prepared, characterized, and investigated in ethylene polymerization and copolymerization with methyl acrylate (MA). The catalytic performances were improved by introducing more ortho-substituents. The catalysts exhibited good thermal stabilities at high temperatures, producing branched polyethylenes. The catalysts bearing hydroxyl groups possessing intramolecular H-bonding, resulted in slightly higher incorporation ratios of MA unit when compared with the catalysts bearing methoxyl groups.

Pyridine-Chelated Imidazo[1,5-a]Pyridine N-Heterocyclic Carbene Nickel(II) Complexes for Acrylate Synthesis from Ethylene and CO2

Nickel(II) dichloride complexes with a pyridine-chelated imidazo[1,5-a]pyridin-3-ylidene py-ImPy ligand were developed as novel catalyst precursors for acrylate synthesis reaction from ethylene and carbon dioxide (CO2), a highly promising sustainable process in terms of carbon capture and utilization (CCU). Two types of ImPy salts were prepared as new C,N-bidentate ligand precursors; py-ImPy salts (3, 4a–4e) having a pyridine group at C(5) on ImPy and a N-picolyl-ImPy salt (10) having a picolyl group at N atom on ImPy. Nickel(II) complexes such as py-ImPyNi(II)Cl2 (7, 8a–8e) and N-picolyl-ImPyNi(II)Cl2 (12) were synthesized via transmetalation protocol from silver(I) complexes, py-ImPyAgCl (5, 6a–6e) and N-picolyl-ImPyAgCl (11). X-ray diffraction analysis of nickel(II) complexes (7, 8b, 12) showed a monomeric distorted tetrahedral geometry and a six-membered chelate ring structure. py-ImPy ligands formed a more planar six-membered chelate with the nickel center than did N-picolyl-ImPy ligand. py-ImPyNi(II)Cl2 complexes (8a–8e) with tert-butyl substituents exhibited noticeable catalytic activity in acrylate synthesis from ethylene and CO2 (up to 108% acrylate). Interestingly, the use of additional additives including monodentate phosphines increased catalytic activity up to 845% acrylate (TON 8).

Palladium-Catalyzed Insertion of Ethylene and 1,1-Disubstituted Difunctional Olefins: An Experimental and Computational Study

Insertion or coordination copolymerization of ethylene with di-substituted olefins is challenging and the choice of di-substituted mono-functional olefin versus di-substituted di-functional olefin (DDO) appears to be decisive. Here we show that DDO-inserted species are amenable to ethylene insertion and polymerization. DDOs such as 2-acetamidoacrylic acid (AAA), methyl 2-acetamidoacrylate (MAAA), and ethyl 2-cyanoacrylate (ECA) were treated with palladium complex [{P∧O}PdMe(L)] (P∧O=κ² -P,O-Ar₂ PC₆ H₄ SO₂ O with Ar=2-MeOC₆ H₄ ; L=C₂ H₆ OS) and the existence of respective insertion intermediates in moderate yield (up to 37 %) was established. These intermediates were exposed to ethylene and corresponding ethylene-inserted products were isolated and characterized. A careful comparison with three model compounds confirmed ethylene insertion and polymerization. Thus, the combined experimental and computational investigations show that DDO-inserted species can undergo ethylene insertion and polymerization.

Indole-bridged bisphosphine-monoxide palladium catalysts for ethylene polymerization and copolymerization with polar monomers

Novel indole-based C–N-bridged bisphosphine-monoxide palladium catalysts enable ethylene polymerization with completely different activities and molecular weights and copolymerization with polar comonomers.

Carbonylative cycloaddition between two different alkenes enabled by reactive directing groups: expedited construction of bridged polycyclic skeletons

A novel palladium-catalyzed highly selective hydrocarbonylative cycloaddition reaction with two different alkenes in the presence of CO enabled by a reactive directing-group is developed, which offers efficient access to bicyclo[2,2,2]lactones.

Neutral Imino-Methyl Benzenesulfonate-Ligated Pd(II) Complexes and Implications in Ethylene Polymerization

A reaction between sodium 2-formylbenzenesulfonate and aniline revealed the near-quantitative (91%) formation of sodium-2-((phenylimino)methyl)benzenesulfonate L1. The identity of L1 was unambiguously ascertained using spectroscopic and analytical methods. The scope of this methodology was widened and various electron-donating amines were treated with sodium 2-formylbenzenesulfonate, and a small library of 6 imine ligands L2-L6 was generated. When L2 was treated with [(COD)PdMeCl], instead of the anticipated [L2PdMe(DMSO)] complex, the formation of [(DMSO)2Pd2Cl2Me2] Pd-Dim was observed. Nevertheless, the desired imino-methyl benzenesulfonate-ligated palladium complex [L2PdMe(Lu)] C1 was obtained by in situ abstraction of chloride and addition of bulky 2,6-lutidine as the donor group. The observation of characteristic Pd-Me protons at 0.06 ppm and the corresponding carbon at -8.1 ppm indicated the formation of C1. These 1D NMR observations were corroborated by 2D C-H correlation spectra and mass analysis, and the existence of C1 was unambiguously ascertained. Along the same lines, L4 and L5 were treated with a palladium precursor to produce [L4/5PdMe(Lu)]-type complexes C2-C3 in 55-84% yield, and their identity was established by using a combination of spectroscopic tools, analytical methods, and single-crystal X-ray diffraction. The synthetic utility of C1-C3 has been demonstrated by utilizing these complexes in the insertion polymerization of ethylene to polyethylene.

Emerging Palladium and Nickel Catalysts for Copolymerization of Olefins with Polar Monomers

Transition-metal-catalyzed copolymerization of olefins with polar monomers represents a challenge because of the large variety of substrate-induced side reactions. However, this approach also holds the potential for the direct synthesis of polar functionalized polyolefins with unique properties. After decades of research, only a few catalyst systems have been found to be suitable for this reaction. Some major advances in catalyst development have been made in the past five years. This Minireview summarizes some of the recent progress in the extensively studied Brookhart and Drent catalyst systems, as well as emerging alternative palladium and nickel catalysts.