Α-diimine Palladium Catalysts Research Articles

Double stimulus-responsive palladium catalysts for ethylene polymerization and copolymerization

The external stimulus response strategy has been evolved rapidly in the field of olefin polymerization. In this work, we modularly synthesized three types of double stimulus responsive α-diimine palladium catalysts, combining redox regulation and other regulation together, such as light, Lewis acid and alkali cations. The catalytic activities and the molecular weight of polyethylene products can be regulated for 4 times in ethylene polymerization. These palladium complexes were also used for the copolymerization reaction of ethylene and polar monomers, such as methyl 10-undecylenate and methyl acrylate, effectively regulating the catalytic activities, the molecular weight and polar monomer incorporation of the prepared copolymers. The research on these dual-regulated palladium complexes makes full use of prepared catalysts and provides new inspirations for regulating olefin polymerization.

Ligand pre-inserted α-diimine palladium catalysts and mechanism studies in olefin polymerization

In this work, ligand pre-inserted α-diimine palladium complexes were synthesized to catalyze the ethylene polymerization and copolymerization with polar monomers.

Concerted flexible and steric strategy in α-diimine nickel and palladium mediated insertion (Co-)Polymerization

To address the issues of poor thermostability of catalyst and low polymer molecular weight resulted from chain transfer at high temperature in olefin polymerization, versatile strategies have been utilized in the benchmark α-diimine nickel and palladium catalysts. In this contribution, a concerted flexible and steric strategy was highlighted in the α-diimine nickel and palladium catalysts. Compared to the seminal Brookhart catalyst, these newly generated nickel catalysts exhibited improved thermostability over a wide range of temperature from 30 °C to 130 °C, showed higher catalytic activities, and produced significantly higher (4.7–5.8 times) molecular weight polymers in ethylene polymerization, which are highly branched polyethylenes (76–110/1000C, SR = 72 %). Notably, high molecular weight polyethylene (up to 219 kg mol−1) was even afforded at 130 °C, which is extremely challenging when the flexible substituent is used in α-diimine system. The corresponding palladium analogues by using the same strategy further produced high molecular weight ethylene-methyl acrylate copolymers (up to 137 kg mol−1) at the difficult 70 °C in the copolymerization of ethylene and methyl acrylate. The incorporation of methyl acrylate could be tuned (0.6–2.9 mol%), where methyl acrylate units distribute both in the main chain and at the end of branches (15 %:85 %).

Unsymmetrical Strategy on α-Diimine Nickel and Palladium Mediated Ethylene (Co)Polymerizations.

Among various catalyst design strategies used in the α-diimine nickel(II) and palladium(II) catalyst systems, the unsymmetrical strategy is an effective and widely utilized method. In this contribution, unsymmetrical nickel and palladium α-diimine catalysts (Ipty/iPr-Ni and Ipty/iPr-Pd) derived from the dibenzobarrelene backbone were constructed via the combination of pentiptycenyl and diisopropylphenyl substituents, and investigated toward ethylene (co)polymerization. Both of these catalysts were capable of polymerizing ethylene in a broad temperature range of 0-120 °C, in which Ipty/iPr-Ni could maintain activity in the level of 106 g mol-1 h-1 even at 120 °C. The branching densities of polyethylenes generated by both nickel and palladium catalysts could be modulated by the reaction temperature. Compared with symmetrical Ipty-Ni and iPr-Ni, Ipty/iPr-Ni exhibited the highest activity, the highest polymer molecular weight, and the lowest branching density. In addition, Ipty/iPr-Pd could produce copolymers of ethylene and methyl acrylate, with the polar monomer incorporating both on the main chain and the terminal of branches. Remarkably, the ratio of the in-chain and end-chain polar monomer incorporations could be modulated by varying the temperature.

A comprehensive picture on chain walking olefin polymerization

α-Diimine nickel and palladium catalyzed α-olefin polymerization could produce polyolefins with complicated branching structures due to the chain walking feature. The thorough understanding of the polymer microstructure could provide a general picture of monomer enchainment including the insertion selectivity and chain walking pathways. By utilizing α-diimine nickel (DBB-Ipty-Ni) and palladium (DBB-Ipty-Pd) catalysts reported previously by us that show an excellent selectivity in ethylene polymerization, homopolymerizations of linear α-olefin such as 1-pentene, 1-hexene and 1-octene, and non-linear α-olefin 4-methyl-1-pentene (4M1P) were conducted. In-depth studies of the polyolefin microstructures revealed the enchainment of α-olefin monomers, in which the enchainment of 4M1P indicated a chain-end control on the insertion regio-selectivity. Moreover, internal olefins including cis-2-hexene and cis-3-hexene, and non-conjugated diolefin 1,7-octadiene were successfully polymerized by the α-diimine palladium catalyst. It is worth noting that the isomerization of internal olefins to 1-alkenes in polymerization was found. Chain walking cyclopolymerization of 1,7-octadiene occurred to produce an alternating cyclic copolymer bearing cis-five- and cis-six-membered rings on the polymer backbone. A rational enchainment mechanism was proposed to explain the formation of the special alternating cyclic chain structure. In addition, mechanical properties of poly(α-olefin)s were studied, among which poly(1-octene) exhibited the highest tensile stress-at-break (17.9 MPa) and strain-at-break (1480%), while poly(1-pentene) showed the best strain recovery (81%).

Dual roles of trifluoroborate in nickel-catalyzed ethylene polymerization: Electronic perturbation and anchoring for heterogenization

Brookhart-type α-diimine nickel and palladium catalysts have been extensively studied over the past several decades; however, the heterogenization of these metal complexes has received much less attention. In this contribution, we installed a trifluoroborate potassium substituent on an α-diimine framework. The ionic nature of trifluoroborate potassium endowed the α-diimine nickel complex with a strong affinity for the SiO2 support, while its electron-donating nature enhanced the catalyst stability and polyethylene molecular weight. In the presence of only 100 equiv. of Et2AlCl cocatalyst, the SiO2-supported catalyst demonstrated significantly better performance than its homogeneous analog during ethylene polymerization, with extremely high activity (1.42–6.53 × 107 g mol−1 h−1) and high thermal stability. The heterogeneous system led to the formation of high-molecular-weight polyethylenes (Mn 142,500–732,800 g/mol), narrow polydispersities (2.18–3.00), tunable branching densities (21–64 per 1000 carbon atoms), and great mechanical properties. Moreover, the efficient copolymerization of ethylene with comonomers such as methyl 10-undecenoate, 6-chloro-1-hexene or 5-hexenylacetate was achieved. These superior properties enabled by the trifluoroborate potassium moiety may inspire its applications in other polymerization catalyst systems.

Polymeric α-diimine palladium catalysts for olefin (co)polymerization

In this contribution, a series of α-diimine palladium complexes with two vinyl units were designed and synthesized. Tandem ring opening metathesis polymerization/cross metathesis of cycloolefins in the presence of these complexes as chain-transfer agents resulted in a series of well-defined polymeric α-diimine Pd catalysts with improved catalytic performances. These polymeric Pd complexes can catalyze polymerization of ethylene and result in increased molecular weight of polyethylene products compared with their molecular analogues. In the polymerization of 1-hexene, the polymeric catalysts led to less isomerization, resulting in increased conversions of 1-hexene and higher molecular weights. Moreover, polymeric palladium catalysts show improved tolerance to polar comonomers, leading to higher activities as well as higher molecular weights in ethylene–methyl acrylate copolymerization. Most importantly, tuning of catalyst performance can be achieved by varying the structure or amount of cyclic monomer used in the preparation of these well-defined polymeric catalysts starting from the simple homogeneous platform.

A Dual Steric Enhancement Strategy in α-Diimine Nickel and Palladium Catalysts for Ethylene Polymerization and Copolymerization

A Dual Steric Enhancement Strategy in α-Diimine Nickel and Palladium Catalysts for Ethylene Polymerization and Copolymerization

Investigations of ligand backbone effects on bulky diarylmethyl-based nickel(II) and palladium(II) catalyzed ethylene polymerization and copolymerization

Recently, the sterically hindered ortho-dibenzhydryl or diarylmethyl-derived (CHPh2 or CHAr2 on the N-aryl ring) α-diimine nickel(II) and palladium(II) catalysts have received much attention due to their unique catalytic properties for olefin (co)polymerization. In this work, the synthesis and characterization of bulky diarylmethyl-derived Ni(II) and Pd(II) ethylene (co)polymerization catalysts containing a series of backbone substituents (H, CH3 and acenaphthene) were described. Substituent effect of the backbone on catalyst activity and stability, polymer molecular weight, polymer branching density and polar monomer incorporation were investigated in detail. Overall, acenaphthene-based Ni3 showed the highest catalytic activity and produced the polymer with the highest molecular weight in Ni(II) system, while CH3-based Pd2 showed the best catalytic property in Pd(II) system. The H-substituted Ni(II) and Pd(II) catalyst was notable in its low activity or no activity compared to the other substituted Ni(II) and Pd(II) catalyst. Most interestingly, Pd2 demonstrated highest polymerization activities under gas-phase conditions through the self-supported catalytic mechanism.

Unprecedented Steric and Positioning Effects of Comonomer Substituents on α-Diimine Palladium-Catalyzed Vinyl Arene/CO Copolymerization

To better understand the substituent effects of vinyl arene, a series of substituted styrenes with different groups and locations (n-X-St, n = 2, 3, 4; X = H, Me, tBu, MeO, tBuO, F, Cl, and Br) were used as comonomers for palladium-catalyzed vinyl arene/CO copolymerization. Dibenzobarrelene-based α-diimine palladium catalyst Pd1 was capable of catalyzing alternating copolymerizations of substituted styrene comonomers and CO in a living fashion, which excluded the effect of the palladium catalyst. Electronic effects of comonomer substituents were quantitatively examined by Hammet constants (σ) of substituents and highest occupied molecular orbital (HOMO) energies of comonomers. Experimental results clearly showed that the turnover number (TON) of copolymerization, copolymer molecular weight, and stereoregularity were greatly affected by the inserted substituent. The steric effect of 4-substituents was presented and clearly proved in addition to a widely acceptable electronic effect. Strikingly, the unprecedented positioning effects of comonomer substituents were initially discovered. Whatever substituents were located on 3-position, vinyl arene/CO alternating copolymerization was greatly promoted because of the neglectable steric effect of 3-substituents.

The synergistic effect of rigid and flexible substituents on insertion polymerization with α-diimine nickel and palladium catalysts

The introduction of flexible cycloalkyl groups greatly enhanced the chain growth in the Ni(ii) catalytic system and facilitated the insertion of polar monomers in the Pd(ii) catalytic system.

Combining hydrogen bonding interactions with steric and electronic modifications for thermally robust α-diimine palladium catalysts toward ethylene (co)polymerization

Thermally robust α-diimine palladium catalysts are highly active for ethylene (co)polymerization at high temperatures by steric and electronic modifications in combination with hydrogen bonding interactions.

Reversion of the chain walking ability of α-diimine nickel and palladium catalysts with bulky diarylmethyl substituents

In general, α-diimine palladium species are more likely to undergo chain walking than the corresponding nickel species, resulting in more branched and topological polyethylene. Moreover, the ligand steric effects have a significant influence on the chain walking in α-diimine system. In this contribution, a series of acenaphthene-based α-diimine ligands bearing bulky diarylmethyl moieties with various electronic effects and the corresponding Ni(II) and Pd(II) complexes were synthesized and characterized. These Ni(II) complexes exhibit high activities in ethylene polymerization even at 80 °C, generating ultrahigh-molecular-weight polyethylenes with low branching density and high melting temperature. The corresponding palladium complexes display moderate activity, leading to semicrystalline polyethylene with low branching density and high melting temperature. Polar functionalized semicrystalline polyethylene with high melting temperature can also be obtained via copolymerization of ethylene with polar monomers using these palladium complexes. Moreover, the remote nonconjugated electronic substituents exert a great influence on the ethylene (co)polymerization. Most importantly, the chain walking ability of metal species can be controlled by changing the ligand steric environment, and the diarylmethyl substituents can even reverse the chain walking trend of palladium and nickel species.

Thermally robust α-diimine nickel and palladium catalysts with constrained space for ethylene (co)polymerizations

The axial and equatorial plane model has been widely accepted for α-diimine nickel and palladium catalysts of olefins polymerization. In this paper, dinaphthobarrelene backbone-based α-diimine nickel and palladium complexes with the constrained space were designed and synthesized from the viewpoint of three-dimensional (3D) space. The 3D-constrained microenvironment around the Ni/Pd metal center created by the bulky ligand substituents fully shielded the back and axial sites, which improved catalytic activity, thermal stability, and living fashion of catalysts. Additionally, enhanced tolerance towards polar groups in copolymerization of ethylene and polar monomers was realized by dinaphthobarrelene-derived α-diimine nickel and palladium catalysts.

A comprehensive picture on catalyst structure construction in palladium catalyzed ethylene (co)polymerizations

Late transition metal palladium catalysts chelated by benchmark α-diimine, phosphine-sulfonate, bisphosphine-monoxide ligands are of great interest for ethylene polymerization and challenging copolymerization with polar comonomers. Compared to numerous studies on the classic α-diimine and phosphine-sulfonate palladium catalysts, the new generation bisphosphine-monoxide palladium (BPMO-Pd) system is far less investigated. In this contribution, by skillfully designing a family of BPMO-Pd catalysts bearing differed steric and electronic P(III) phosphine groups, differed steric and electronic O = P(V) phosphine monoxide groups, and different connectivity of them, a comprehensive picture on catalyst structure construction is built for ethylene (co)polymerizations. This constructed catalyst structure-polymerization property relationship paves the way for the development of more efficient late transition metal catalysts for olefin (co)polymerization.

Front Cover Picture: Bis‐Alkoxycarbonylation of Acrylic Esters and Amides for the Synthesis of 2‐Alkoxycarbonyl or 2‐Carbamoyl Succinates (Adv. Synth. Catal. 3/2020)

The front cover picture, provided by Carla Carfagna et al., shows the first example of an oxidative bis-alkoxycarbonylation of acrylic esters and acrylic amides, leading to 2-EWG-substituted succinates. Despite the low reactivity of electron-deficient olefins in carbonylation reactions, the aryl α-diimine palladium(II) catalyst is able to promote the carbonylation of both the β- and the generally non-reactive α-positions of electron-poor alkenes, under mild reaction conditions (PCO=4 bar, T=20 °C). Further details of this work can be found in the full paper on pages 533–544 (D. Olivieri, R. Tarroni, N. Della Ca′, R. Mancuso, B. Gabriele, G. Spadoni, C. Carfagna, Adv. Synth. Catal. 2020, 362, 533–544; DOI: 10.1002/adsc.201900918).

Redox-switchable α-diimine palladium catalysts for control of polyethylene topology

A series of palladium complexes bearing ferrocene substituted α-diimine ligands was synthesized and investigated for ethene polymerization at different conditions to modulate the extent of “chain-walking” mechanism and regulate branching and topology of resulting polyethylenes. All ferrocene substituted complexes catalyzed living/controlled ethene polymerization. The ability of ferrocene substituted palladium complexes to provide polyethylenes with dendritic topology was proved by measuring their Mark-Houwink plots. In-situ chemical oxidation of ferrocene moieties via silver triflate was used to affect the catalyst electronic structure and support the “chain-walking” mechanism. Oxidation of palladium catalyst led to its destabilization while the catalytic activity of newly formed sites was substantially increased. It was demonstrated that catalyst oxidation is a powerful tool to regulate the topology of resulting polyethylenes.

Synthesis of functional and hyperbranched ethylene oligomers using unsymmetrical α-diimine palladium catalysts

A series of unsymmetrical α-diimine palladium complexes bearing low steric crowdedness on one side and bulky substituents on the other side were prepared and characterized. Activated with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate, these α-diimine Pd(II) complexes were used to catalyze ethylene oligomerization and cooligomerization with polar monomers including methyl acrylate (MA), acrylic acid (AA) and vinyltriethoxysilane (VTEoS). In ethylene oligomerization, the hyperbranched oil products were obtained with high activities (14.2–27.0 × 103 g mol−1 h−1). The remote substituent of bulky diarylmethyl moiety showed a significant effect on catalyst activity, while exhibit little variation on the molecular weight of ethylene oligomers. In ethylene-polar monomer cooligomerization, functional and hyperbranched oils were produced and polar monomer incorporations can be tuned by the perturbations of the remote substituent.

Living (co)polymerization of ethylene and bio-based furfuryl acrylate using dibenzobarrelene derived α-diimine palladium catalysts

Living (co)polymerizations of petroleum-based ethylene and bio-based furfuryl acrylate were realized using dibenzobarrelene-derived α-diimine palladium catalysts.

A continuing legend: the Brookhart-type α-diimine nickel and palladium catalysts

Here we will summarize some of the recent advances in α-diimine ligand developments, as well as some new and challenging monomers that this class of catalysts can address through these ligand improvements.