Pd Diimine Research Articles

Copolymerisation of 1-alkenes with bulky oxygen-containing olefins for dual-stage functionalisation of polyolefins

Pd–Diimine complexes catalyse the copolymerisation of 1-alkenes with olefins having a bulky oxygen-containing substituents, such as cyclic acetal or bicycloorthoester groups, affording the copolymers containing the comonomer unit up to 34%.

Pd-Promoted Copolymerization of Methallyl and Isoprenyl Ethers and Acetate with α-Olefins

1,1-Disubstituted ethylene derivatives with oxygen functionalities, such as methallyl and isoprenyl ethers and acetate, were employed as comonomers with α-olefins catalyzed by Pd–diimine complexes. A mixture of a chloro(methyl)palladium complex with α-diimine ligand having 2,4,6-trimethylphenyl groups at the coordinated N atoms (I) and Na+B{C6H3(CF3)2-3,5}4– catalyzes the copolymerization of 1-decene with methallyl tert-butyl ether (1) at 0 °C to produce copolymers with comonomer content of 9.7 mol %. Methallyl acetate (2) undergoes copolymerization with 1-decene to afford the corresponding copolymers with comonomer content of 1.0 to 3.3 mol %. Nuclear magnetic resonance analyses of the copolymers of 1-decene with 1 and 2 revealed that no significant β-alkoxy or β-acetate elimination takes place during the polymerization and that the insertion mode of the comonomer determines the location of the monomer on either the main chain or the terminal of the copolymer. Related compounds, isoprenyl tert-butyl ether (3), isoprenyl acetate (4), and isoprenyl trimethylsilyl ether (5), undergo copolymerization with 1-decene or 1-hexene to afford the corresponding copolymers with comonomer content of 1.0 to 6.2 mol %. The reaction with a higher concentration of a comonomer results in the formation of polymers with higher comonomer content.

"Living" Polymerization of Ethylene and 1-Hexene Using Novel Binuclear Pd⁻Diimine Catalysts.

We report the synthesis of two novel binuclear Pd–diimine catalysts and their unique behaviors in initiating “living” polymerization of ethylene and 1-hexene. These two binuclear catalysts, [(N^N)Pd(CH2)3C(O)O(CH2)mO(O)C(CH2)3Pd(N^N)](SbF6)2 (3a: m = 4, 3b: m = 6) (N^N≡ArN=C(Me)–(Me)C=NAr, Ar≡2,6–(iPr)2C6H3), were synthesized by simply reacting [(N^N)Pd(CH3)(N≡CMe)]SbF6 (1) with diacrylates, 1,4-butanediol diacrylate and 1,6-hexanediol diacrylate, respectively. Their unique binuclear structure with two identical Pd–diimine acrylate chelates covalently linked together through an ester linkage was confirmed by NMR and single crystal XRD measurements. Ethylene “living” polymerizations were carried out at 5 °C and under ethylene pressure of 400 and 100 psi, respectively, with the binuclear catalysts, along with a mononuclear chelate catalyst, [(N^N)Pd(CH2)3C(O)OMe]SbF6 (2), for comparison. All the polyethylenes produced with both binuclear catalysts show bimodal molecular weight distribution with the number-average molecular weight of the higher molecular weight portion being approximately twice that of the lower molecular weight portion. The results demonstrate the presence of monofunctional chain growing species resembling catalyst 2, in addition to the expected bifunctional species leading to bifunctional “living” polymerization, in the polymerization systems. Both types of chain growing species exhibit “living” characteristics under the studied conditions, leading to the simultaneous linear increase of molecular weight in both portions. However, when applied for the “living” polymerization of 1-hexene, the binuclear catalyst 3a leads to polymers with only monomodal molecular weight distribution, indicating the sole presence of monofunctional chain growing species. These two binuclear catalysts are the first Pd–diimine catalysts capable of initiating bifunctional ethylene “living” polymerization.

Synthesis and Electrochemistry of a Ferrocene Conjugate of a Pd(II) Diimine Catecholate Complex and Its Charge-Transfer Properties in the One-Electron-Oxidized Form

Abstract A new Pd complex Pd(tBu2bpy)(FcC) 2Fc (FcC = 4-ferrocenylcatecholato, tBu2bpy = 4,4′-di-tert-butyl-2,2′-dipyridyl) was synthesized and characterized using 1H and 13C NMR spectroscopy, and ESI-MS. Electrochemical analysis of 2Fc using cyclic voltammetry in CH2Cl2/nBu4NPF6 revealed two well-defined, quasi-reversible waves assigned to the sequential oxidation of the Pd(tBu2bpy)(C) and ferrocene (Fc) moieties. The potential splitting between the waves (519 mV) indicated that there was an electronic interaction between both moieties. The one-electron-oxidized species of 2Fc was investigated using DFT calculations and UV–vis–NIR spectroscopy, and its charge-transfer property was compared with that of Pt(tBu2bpy)(FcC) 1Fc, FcV (4-ferrocenylveratrole), and FcA (4-ferrocenylcatechol bis(acetate)). 2Fc+ was Fc+-centered, rather than semiquinone-ligand centered, and exhibited an ICT transition band from to the catecholate to the Fc+ moiety in the NIR region. The electronic coupling (HAB) value of 2Fc+ (790 cm−1) was placed between FcV+ (660 cm−1) and 1Fc+ (1050 cm−1). Electronic interactions between the organometallic center and the non-innocent moiety were modulated by substitution of the metal ion (M2+ = Pt2+, Pd2+) in this series.

Hyperbranched Polyethylene Ionomers Containing Cationic Tetralkylammonium Ions Synthesized by Pd–Diimine-Catalyzed Direct Ethylene Copolymerization with Ionic Liquid Comonomers

Synthesis of polyolefin-based ionomers through direct catalytic copolymerization of olefin with ionic comonomers has been challenging with no prior reports. We demonstrate in this article the first direct synthesis of a class of hyperbranched polyethylene ionomers containing positively charged tetralkylammonium ions and different counteranions (tetrafluoroborate, hexafluorophosphate, bis(trifluoromethane)sulfonimide, or hexafluoroantimonate) by copolymerization of ethylene with tetralkylammonium-containing acrylate-type ionic liquid comonomers (3–5, 7). The use of a Pd–diimine catalyst (1), which shows excellent stability toward the highly polar ionic group, is key to the direct synthesis. A detailed study on the catalytic copolymerization and the effects of the polymerization condition on the macromolecular chain parameters of the resulting ionomers has been performed. A range of ionomers differing in ion content and counteranion has been synthesized and characterized. Meanwhile, systematic studies on th...

Homo- and Co-polymerization of norbornene and methyl acrylate with Pd–diimine catalysts

Four Pd–diimine catalysts with varying ligand sterics {[RC6H3NC(R′)–C(R′) = NC6H3R] Pd(CH3) (N≡CMe)}SbF6 [1, R = 2,6-(iPr)2, R′ = CH3; 2, R = CH3, R′ = H; 3, R = 2,6-(iPr)2, R′ = H; 4, R = CH3, R′ = CH3] are investigated in this article for the coordinative vinyl polymerization of norbornene and its copolymerization with methyl acrylate. Vinyl-type polynorbornenes have been successfully obtained with all four catalysts. A systematic investigation of the effects of ligand sterics, polymerization temperature, and time on norbornene polymerization activity, catalyst stability, and polymer molecular weight has been undertaken. In addition to facilitating NB polymerization, catalyst 1 has also been discovered to efficiently initiate homopolymerization of methyl acrylate in the presence of a very small amount of norbornene and ambient light, rendering high-molecular-weight polymers. Radicals generated from the decomposition of Pd species are responsible for the acrylate homopolymerization. In addition, catalyst 1 has further been found to copolymerize norbornene and methyl acrylate in the absence of light via a coordinative mechanism to render low-molecular-weight copolymers, likely telechelic polynorbornenes end-capped with a methyl acrylate unit.

Cationic palladium(II)–acetylacetonate complexes bearing α-diimine ligands as catalysts in norbornene polymerization

A variety of palladium(II)–acetylacetonate complexes bearing α-diimine ligands were synthesized by the reaction of [Pd(acac)(MeCN)2]BF4 with N⁀N ligands. When activated with BF3·OEt2, these complexes provide access to a new class of alkylating agent free Pd–diimine catalyst systems. Catalyst screening for the vinyl addition polymerization of norbornene showed their high activities of 102–104kgpolmolcat−1h−1.

Noncovalent solubilization of multi-walled carbon nanotubes in common low-polarity organic solvents with branched Pd–diimine polyethylenes: Effects of polymer chain topology, molecular weight and terminal pyrene group

Noncovalent nonspecific solubilization of carbon nanotubes with common polymers without having any specific functionality is an important strategy for rendering debundled nanotube solutions for their processing and technological applications. Among the various polymers investigated thus far for noncovalent nonspecific nanotube solubilization, hyperbranched polyethylene (HBPE) featured with distinct highly compact dendritic chain architecture has been discovered to show outstanding performance in rendering stable nanotube solutions in common low-polarity organic solvents (including tetrahydrofuran (THF) and chloroform) at surprisingly high concentrations. To understand the mechanism of the nanotube solubilization with this unique class of polymers and to elucidate the effects of various macromolecular structural parameters, we have designed and synthesized in this work four sets of highly branched polyethylenes varying in chain topology, molecular weight, and end group. With these polymers, we have systematically investigated and compared their performance for the solubilization of multi-walled carbon nanotubes in common solvents including THF, chloroform, n-heptane, and toluene. We have found that these macromolecular structural parameters as well as the solvent play complex but sensitive roles in this noncovalent solubilization system. This work thus provides some valuable guidelines towards the design of optimum polymers for efficient noncovalent nonspecific solubilization of carbon nanotubes.

ChemInform Abstract: Designing Polyethylenes of Complex Chain Architectures via Pd—Diimine‐Catalyzed “Living” Ethylene Polymerization

AbstractReview: 97 refs.

Transition metal-catalyzed polymerization of polar allyl and diallyl monomers

Abstract

A conveniently synthesized polyethylene gel encapsulating palladium nanoparticles as a reusable high-performance catalyst for Heck and Suzuki coupling reactions

The synthesis of a polyethylene (PE) gel containing self-incarcerated palladium(0) nanoparticles is described. The PE gel matrix without any special functionality was synthesized by one-pot chain walking copolymerization of ethylene and 1,6-hexanediol diacrylate facilitated by a Pd–diimine complex (1). The Pd(II) species from 1 were immobilized in situ onto the gel matrix and further reduced to Pd(0) nanoparticles as a result of catalyst 1 deactivation during polymerization and methanol (a reducer) washing in the polymer purification process. The resulting Pd-containing PE gel (2) is shown to be a high-performance and facilely reusable heterogeneous catalyst for the Heck and Suzuki coupling reactions of iodobenzene or aryl bromides. An average TOF of 460 h−1 was achieved with an average 0.57 ppm Pd leaching in each cycle of the Heck reaction of iodobenzene when the PE gel 2 was recycled 10 times. A maximum TOF of 3.33 × 104 h−1 was reached and less than 0.64 ppm of Pd was leached in the Suzuki reactions of aryl bromides with water as solvent.

A Pd–diimine catalytic inimer for synthesis of polyethylenes of hyperbranched-on-hyperbranched and star architectures

A Pd-diimine catalytic inimer containing uniquely catalytic, initiating, and monomeric functionalities in one catalyst molecule is designed. It enables the first self-condensing vinyl copolymerization of ethylene, leading to polyethylenes of a hyperbranched-on-hyperbranched architecture. Meanwhile, it also facilitates the synthesis of star polyethylenes by tandem UV-coordinative polymerization.

Designing polyethylenes of complex chain architectures via Pd–diimine-catalyzed “living” ethylene polymerization

Polymer chain architecture is a critically important chain parameter governing intrinsically the properties and applications of polymers. The rapid developments in "living"/controlled polymerization techniques, particularly the controlled radical polymerization techniques, in the past two decades have enabled the precision synthesis of novel polymers having a great variety of complex yet well-defined chain architectures from various monomer stocks. For polyolefins synthesized via catalytic coordination polymerization, the design of complex chain architectures, however, has only started recently because of the relatively limited advancements in the catalytic "living" olefin polymerization technique. In this regard, the versatile Pd-diimine catalysts have provided some unprecedented opportunities, due to their outstanding features, in rendering successfully a novel class of polyethylenes of various new complex chain architectures through the "living" ethylene polymerization protocol. The complex chain architectures designed to date have included hyperbranched, hybrid hyperbranched-linear, block, gradient and block-gradient, star, telechelic, graft and comb, and surface-tethered polymer brushes. This Feature Article attempts to summarize the recent developments achieved in the area, with an emphasis on the synthetic strategies for the architectural design. These developments demonstrate the great potential for further advancements of this new exciting research area.

Tuning structural parameters of polyethylene brushes on silicananoparticles in surface-initiated ethylene “living” polymerization and effects on silica dispersion in a polyolefin matrix

Surface-initiated ethylene “living” polymerization with covalently tethered Pd–diimine catalysts represents a novel technique for covalent surface functionalization of silica nanoparticles with polyethylene (PE) brushes. In this paper, we report on the successful tuning of various structural parameters of PE brushes in this surface-initiated polymerization technique, including brush length, density, and topology. To control/reduce the brush density, the density of the surface-tethered acryloyl groups for catalyst immobilization is adjusted by using mixed silane agents comprised of effective 3-acryloxypropyltrichlorosilane and inert ethyltrichlorosilane at different compositions in the surface functionalization step, which in turn adjusts the density of immobilized catalysts for rendering PE brushes. This approach gives rise to low-polydispersity PE brushes of controllable densities at the polymerization condition of 27 atm/5 °C: 0.022–0.055 chains per nm2 on a precipitated silica (Silica-I) and 0.07–0.17 chains per nm2 on a fumed silica (Silica-II). The length of PE brushes is controlled by adjusting the polymerization time, with the highest brush length of about 45 kg mol−1 achieved at 6 h of polymerization at 27 atm/5 °C. Unlike the linear brushes with short branch structures obtained at 27 atm/5 °C, hyperbranched PE brushes with compact topology are obtained at 1 atm/25 °C, benefiting from the chain walking mechanism of the Pd–diimine catalyst. The PE-grafted silicas of varying brush density and length are subsequently used as nanofillers to construct polymer nanocomposites with an ethylene–α-olefin copolymer as the matrix polymer. The effects of brush length and density on the nanofiller dispersion and physical properties of the composites are examined. This represents the first study on polyolefin composites containing silica nanoparticles grafted with polyolefin brushes as nanofillers.

Alternating, gradient, block, and block–gradient copolymers of ethylene and norbornene by Pd–Diimine‐Catalyzed “living” copolymerization

AbstractWe demonstrate, in this article, the facile synthesis of a broad class of low‐polydispersity ethylene–norbornene (E–NB) copolymers having various controllable comonomer composition distributions, including gradient, alternating, diblock, triblock, and block–gradient, through “living”/quasiliving E–NB copolymerization facilitated with a single Pd–diimine catalyst (1). This synthesis benefits from two remarkable features of catalyst 1, its high capability in NB incorporation and high versatility in rendering E–NB “living” copolymerization at various NB feed concentrations ([NB]0) while under an ethylene pressure of 1 atm and at 15 °C. At higher [NB]0 values between 0.42 and 0.64 M, E–NB copolymerization with 1 renders nearly perfect alternating copolymers. At lower [NB]0 values (0.11–0.22 M), gradient copolymers yield due to gradual reduction in NB concentration, with the starting chain end containing primarily alternating segments and the finishing end being hyperbranched polyethylene segments. Through two‐stage or three‐stage “living” copolymerization with sequential NB feeding, diblock or triblock copolymers containing gradient block(s) have been designed. This work thus greatly expands the family of E–NB copolymers. All the copolymers have controllable molecular weight and relatively low polydispersity (with polydispersity index below 1.20). Most notably, some of the gradient and block–gradient copolymers have been found to exhibit the characteristic broad glass transitions as a result of their possession of broad composition distribution. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Stereo-controlled synthesis of polyolefins with cycloalkane groups by using late transition metals

Pd complexes with diimine ligands promoted the controlled cyclopolymerization of 1,6-dienes and 1,6,11-trienes to afford polymers containing 1,2-trans-cyclopentane groups with well-regulated stereochemistry. The polymerization proceeded with quantitative cyclization of the monomer, even under bulk conditions or in copolymerization reactions with ethylene and α-olefins. The polymerization of monomers with oligomethylene spacers yielded polymers with five-membered rings that are accurately distributed along the polymer chain. 4-Alkylcyclopentenes and alkenylcyclohexanes were also polymerized by Pd–diimine complexes to afford polymers with 1,3-trans-cyclopentane groups and 1,4-trans-cyclohexane groups, respectively. Pd complexes with a C2 symmetrical structure promoted the isospecific polymerization of 4-alkylcyclopentenes, and the resultant isotactic polymers showed liquid crystalline properties. The mechanism of the polymerization reaction has been revealed. Pd complexes with diimine ligands promote controlled cyclopolymerization of 1,6-dienes and 1,6,11-trienes to afford the polymers containing 1,2-trans-cyclopentane groups with well-regulated stereochemistry. Copolymerization of the dienes with ethylene and α-olefins also proceeds to give functionalized polyolefins. The polymerization of the monomers with oligomethylene spacer yields the polymers with five-membered rings in accurate density and distribution along the polymer chain. 4-Alkylcyclopentenes and alkenylcyclohexanes also get polymerized by the Pd complex to afford polymers with 1,3-trans-cyclopentane groups and 1,4-trans-cyclohexane groups, respectively. Isotactic polymer of 4-alkylcyclopentenes show liquid crystalline properties.

Synthesis and characterization of low- and medium-molecular-weight hyperbranched polyethylenes by chain walking ethylene polymerization with Pd–diimine catalysts

We report in this article the synthesis and characterization of a range of hyperbranched polyethylenes having various low and medium molecular weights by chain walking ethylene polymerization with Pd–diimine catalysts of reduced ligand steric crowdedness, which are intended for potential applications as novel synthetic base stocks. Four Pd–diimine catalysts featured with different ligand crowdedness, ([(RC6H3NC(R′)–C(R′)NC6H3R)Pd(CH3) (NCMe)]SbF6) (1, R = 2,6-(iPr)2, R′ = CH3; 2, R = CH3, R′ = H; 3, R = 2,6-(iPr)2, R′ = H; 4, R = CH3, R′ = CH3), were employed herein for ethylene polymerizations at different conditions. Generally, reducing ligand steric crowdedness (in the order 1 > 4 > 3 > 2) leads to decreased catalyst activity and dramatically reduced polymer molecular weight. As opposed to high-molecular-weight polymers (weight-average molecular weight (Mw): about 150 kg/mol) obtained with catalyst 1, low-molecular-weight polymers (Mw: below 1.0 kg/mol) were obtained with 2 and 3, and medium-molecular-weight polymers (Mw: about 25 kg/mol) were produced with 4. Despite their various reduced molecular weights, the polymers are all featured with highly branched chain structures with a total branching density of above 100 branches per 1000 carbons. The low- and medium-molecular-weight hyperbranched polymers synthesized with 2–4 display good potential for applications as synthetic base stocks. In comparison with three commercial poly(α-olefin) based synthetic base stocks, they exhibit similar thermal and viscosity properties. Meanwhile, it is also discovered that a subsequent one-pot hydrogenation step can be incorporated in the process after the Pd–diimine catalyzed polymerization step to render nearly fully saturated hyperbranched polymers without the use of additional hydrogenation catalysts.

“Arm-First” Synthesis of Core-Cross-Linked Multiarm Star Polyethylenes by Coupling Palladium-Catalyzed Ethylene “Living” Polymerization with Atom-Transfer Radical Polymerization

We report in this article the “arm-first” synthesis of core–shell-structured multiarm polyethylene (PE) star polymers having multiple linear-but-branched PE arms joined at a cross-linked polydivinylbenzene (polyDVB) core. This synthesis is achieved in two steps by coupling two mechanistically incompatible polymerization techniques, Pd-catalyzed coordinative ethylene “living” polymerization and atom-transfer radical polymerization (ATRP). Ethylene “living” polymerization was first carried out using a functionalized Pd–diimine catalyst, [(ArN═C(Me)–(Me)C═NAr)Pd(CH2)3C(O)O(CH2)2OC(O)C(CH3)2Br]+SbF6– (Ar = 2,6-(iPr)2C6H3) (1), to directly synthesize narrow-distributed PE macroinitiators (MIs) containing an end-capping 2-bromoisobutyryl group active for initiating ATRP. Featured with controllable molecular weights at low polydispersity, the PE MIs are employed subsequently in the second step to initiate cross-linking polymerization of divinylbenzene (DVB) via ATRP to obtain the core-cross-linked star polymers. As a demonstration, three PE MIs at different lengths (number-average molecular weight of 7.3, 10.3, and 13.7 kg/mol, respectively) were specifically synthesized and employed for star construction. In the ATRP step, the effects of the various polymerization parameters, including MI concentration, the molar ratio of DVB to MI, and MI length, on the polymerization kinetics, star yield, average arm number, and average molecular weights of the star polymers were investigated systematically. By controlling the polymerization parameters, a range of PE star polymers having narrow-distributed arm lengths (7.3–13.7 kg/mol) and controllable average arm numbers (ca. 5–43 per star) have thus been successfully synthesized. A study on dilution solution properties of these star polymers having various structural parameters reveals their spherical chain conformation and resemblance of rigid spheres and high-generation dendrimers, with their intrinsic viscosity depending primarily on arm length while not on arm number or polymer molecular weight.

Oxidation of Organometallic Platinum and Palladium Complexes Obtained from C−H Activation

η3-Cyclohexenyl and -indenyl Pt(II) and Pd(II) diimine complexes, which are generated via C−H activation of cyclohexene and indene by Pt and Pd hydroxy dimers, are selectively oxidized by Br2, Na2PtCl6, and CuCl2 to give halogenated organic products along with well-defined Pd(II) and Pt(II) species.

Binding of polar monomers in the complexes with organometallic ethylene polymerization catalysts — Natural orbitals for chemical valence and energy decomposition analysis

The binding mode of polar monomers in complexes with late-transition-metal catalysts for ethylene polymerization was studied by density functional theory (DFT) calculations. The Ziegler–Rauk energy decomposition scheme was used to characterize the geometry distortion and steric and orbital-interaction terms in the bonding energy, while Natural Orbitals for Chemical Valence (NOCV) were applied to describe the donation and back-bonding components of the bond between the monomer and the catalyst. The NOCV analysis allowed for comparison of the donor–acceptor properties of different monomers in the σ- and π-complexes. The complexes with the model, cationic Ni– or Pd–diimine catalysts, N^N–Ni(H)+ and N^N–Pd(H)+, and the neutral Ni–anilinotropone system, N^O–Ni(H), were investigated. The monomers studied included: simple olefins (Et and Pr); examples of oxygen- and nitrogen-containing polar monomers (methyl acrylate (MA), vinyl acetate (VAc), their fluorinated derivatives (FMA, FVAc), vinyl ether (VE), acrylonitrile (AN), and β-butenoic nitrile (BN); vinyl and allyl amines (VAm, PrAm); and a tertiary dimethyl vinyl amine (MVAm). The results demonstrate that the metal-based fragment has a significant influence on the relative stability of the σ- and π-complexes; the π-binding mode increases in the following order: N^N–Ni(H)+ < N^N–Pd(H)+ < N^O–Ni(H). The results of the Ziegler–Rauk bond-energy decomposition indicate that for some monomers (MA, FMA, VAc, AN, VAm, MVAm) the preference of the coordination mode has a steric (electrostatic and Pauli) origin. For other monomers (VE, FVAc, BN, PrAm) the changes in the orbital-interaction terms are important as well. The results of the NOCV analysis indicate that for both, σ- and π-coordination modes there exist components describing σ-donation and π-back-bonding. The sequences of σ-donor and π-acceptor properties of monomers in the π-complexes as well as σ-complexes are similar for the considered catalysts.