Brookhart Catalyst Research Articles

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 %).

New advances in olefin homo and copolymerization using neutral, single component palladium/nickel complexes ligated by a phosphine-sulfonate

Use of catalysts for olefin homo- and copolymerization has received growing attention in recent decades. Most polyolefins especially polyethylene and polypropylene are currently produced in the presence of transition-metal catalysts. So far, late transition-metal catalysts particularly nickel and palladium-based complexes have attracted great attention and various ligands have been employed in the structure of these catalysts. In this contribution, nickel and palladium-based complexes with symmetrical cationic α-diimine (as Brookhart catalysts) and non-symmetrical neutral phosphine-sulfonate ligands (as Drent catalysts) have been extensively utilized as olefin (co)polymerization catalysts. Unlike classic α-diimine complexes, the generation of highly linear ethylene/polar monomer copolymers, prepared in the absence of co-catalyst by phosphine-sulfonate palladium/nickel catalysts poses an interesting case. The outstanding features of phosphine-sulfonate-based catalysts motivated many researchers to focus on these catalysts. In this context, several review articles have covered various aspects of this class of catalysts. Considering the importance of phosphine-sulfonate-based catalysts, in this mini-review, we wish to address recent advances of the neutral, single component palladium/nickel complexes bearing phosphine–sulfonate ligands for olefin (co)polymerization with polar monomers.

Facile Access to Polar-Functionalized Ultrahigh Molecular Weight Polyethylene at Ambient Conditions

Facile Access to Polar-Functionalized Ultrahigh Molecular Weight Polyethylene at Ambient Conditions

Highly resilient polyethylene elastomers prepared using α‐diimine nickel catalyst with highly conjugated backbone

An α‐diimine nickel catalyst (Cat‐1) with highly conjugated acenaphthylene backbone was synthesized and used in ethylene polymerization. Compared with typical Brookhart catalyst B‐Cat, Cat‐1 could produce polyethylene with both higher molecular weight (up to 5.1 × 105 g mol−1) and branching density (up to 135 branches per 1000 carbons) in good polymerization activity. The polyethylene prepared using Cat‐1 with higher molecular weight and branching density possessed exceptional mechanical properties (ultimate tensile strength and elongation at break could reach 12.08 MPa and 1148%) and outstanding elastomeric recovery as compared with the polymer prepared using B‐Cat. Cat‐1 was synthesized using a simple reaction route and the polymerization conditions were suitable for the industrial polymerization process. The novel nickel catalyst Cat‐1 is promising for producing highly resilient polyethylene elastomers industrially.

Introducing electron-donating substituents on ligand backbone of α-diimine nickel complex and the effects on catalyst thermal stability in ethylene polymerization

A novel α-diimine nickel pre-catalyst Cat.1 with two electron-donating methyl substituents on the ligand backbone was synthesized and investigated for ethylene polymerization using methylaluminoxane as cocatalyst. Cat.1 exhibited higher polymerization activity and produced highly branched polyethylene with slightly higher molecular weight than that of the typical Brookhart catalyst (B-Cat) at elevated temperature. By tracing the changes of active center number in the polymerization process, Cat.1 was found to have higher ratio of activated nickel ([C∗]/[Ni]), longer catalyst lifetime and larger chain propagation rate constant (kp) as compared with B-Cat. The computational studies demonstrated the electron-donating methyl substituents can make a more electron-rich ligand to influence the catalytic properties of Cat.1. Stronger interactions between the electron-rich ligand and the nickel center should be the main reasons for longer catalyst lifetime and enhanced thermal stability of Cat.1. The production of polyethylene with higher molecular weight by Cat.1 can be explained by its larger kp value and more stable transition state for ethylene insertion than for chain transfer reaction due to the electron-donating ligand. Overall, the results of this work clearly support the importance of ligand electronic effects on the thermal stability and intrinsic reactivity of active centers in α-diimine Ni(II) catalysts.

Efficient reductive depolymerization of hardwood and softwood lignins with Brookhart's iridium(iii) catalyst and hydrosilanes

Efficient catalytic reduction of lignin model molecules and reductive depolymerization of softwood and hardwood lignins is presented with the iridium based Brookhart's catalyst and hydrosilanes R3SiH as reductant.

Efficient Modulation of Polyethylene Microstructure by Proper Activation of (α-Diimine)Ni(II) Catalysts: Synthesis of Well-Performing Polyethylene Elastomers

The activation of a prototypical nickel(II) Brookhart catalyst by either methylalumoxane (MAO) or diethylaluminum chloride (AlEt2Cl) under a variety of conditions showed that a proper choice of the mode of activation is a powerful tool to modulate the polymer microstructure. In particular, use of AlEt2Cl instead of MAO resulted in the production of more branched polyethylenes with a higher content of long chain branches and even some “branches on branches”. Characterization of these materials by NMR, thermal, X-ray diffraction, and mechanical analyses provided insight into the relationships between the microstructure and the crystallization behavior and the elasticity of the polymers. For these branched polyethylenes, a transition from plastomeric toward elastomeric behavior occurs for branch concentrations much lower than for ethylene–propylene copolymers and like those observed for ethylene copolymers with bulkier comonomers. For elastomeric materials, reduction of branch concentration implies two relev...

α‐Diimine–Palladium Complexes Incorporated in Vinylic‐Addition Polynorbornenes: Synthesis and Catalytic Activity

α‐Diimine polymeric ligands have been synthesized using the bicyclic norbornane structure, present in vinylic‐addition polynorbornene (VA‐PNB). The VA‐PNB–diimine ligands have been prepared by functionalization of the copolymer obtained by Ni‐catalyzed polymerization of norbornene and norbornenylcarbonate. Immobilized palladium complexes of the type VA‐PNB–diimine–PdX2 have been prepared, and their catalytic activity has been tested. The trifluoroacetato complex (X = CF3COO) can be used as a recyclable precatalyst in the Suzuki reaction. It is the source of minute amounts of homogeneous palladium active species, which carry out the catalysis with high turnover numbers. The recovered polymeric complex can be reused several times with no significant loss of activity. The polymeric analogue to Brookhart's catalyst, VA‐PNB–diimine–PdMeCl, can also polymerize ethylene, although it is less active than its monomeric counterparts.

Insight into the Mechanism of Thermal Stability of α-Diimine Nickel Complex in Catalyzing Ethylene Polymerization

The union of experimental and computational methods can accelerate the development of polymerization catalysts for industrial applications. Herein, we report complementary experimental and computational studies of the thermal stability of α-diimine nickel complexes by using thermally stable Cat. 1 and a typical Brookhart catalyst (B-Cat) as models. Experimentally, we found that many more nickel atoms could be activated for Cat. 1 at elevated temperature during the ethylene polymerization process compared to those for B-Cat. Computationally, first-principle calculations showed that the decomposition energies of Cat. 1 were found to be higher than those of B-Cat, contributing to the activation observed for Cat. 1. We found that the presence of ethydene evidently affected the conformation of C1–N1–Ni–N2–C2 five-membered ring (where the nickel center is located) of Cat. 1, turning the envelope conformation (B-Cat) into a half-chair conformation (Cat. 1). According to calculations, the decomposition energy of ...

Preparation of novel cationic acetylacetonate complexes of palladium with diimine ligands

Catalytic systems based on palladium and nickel complexes with α-diimine ligands are known as the Brookhart catalysts [1, 2]; they are well known in modern practice of laboratory and industrial homogeneous catalysis of oligoand polymerization of lower alkenes. The strong point of such catalysts is their stability towards polar functional groups of the monomer, allowing preparation of hyperbranched polyolefines. Significant part of palladium complexes with α-diimine ligands described in the literature are neutral (halogenide or alkyl) complexes of palladium that generally require activation with organoaluminum compounds to initiate the active cationic sites [2]. A common feature of preparations of the known cationic palladium complexes is that they are multi-stage processes. On top of that, expensive starting materials are normally used (salts of silver and thallium) as well as magnesiumortincontaining chemicals [1]. There have been no examples of preparation of cationic acetylacetonato complexes of palladium with αdiimine ligands.

Polymerization of ethylene by (α-Diimine) nickel catalyst and statistical analysis of the effects of reaction conditions

This article regards the ethylene polymerization catalyzed by a nickel catalyst and activated by ethylaluminum sesquichloride (EASC). The effects of the reaction conditions [polymerization temperature, cocatalyst (EASC) concentration, and ethylene concentration] on the average molecular weights of the final polymers and reaction yields were evaluated with the help of empirical statistical models. It is shown that reaction temperature and cocatalyst (EASC) concentration exert the most important effects on average molecular weights and catalyst activity. The polydispersities of the obtained polyethylenes are larger than the polydispersities of polyethylenes obtained with typical Brookhart catalysts. The analysis of polymer branching frequencies shows new types of short branching and significant amounts of long branches, which may explain the relatively large polydispersities of the obtained polymer samples. POLYM. ENG. SCI., 50:1797–1808, 2010. © 2010 Society of Plastics Engineers

On‐line monitoring of Brookhart polymerization by electrospray ionization mass spectrometry

For the first time, an electrospray ionization (ESI) mass spectrometer coupled on-line with a microreactor permitted the fast screening of polymerization growth of alkenes in the liquid phase mediated by palladium Brookhart catalyst and allowed the direct mass spectrometric identification of alkyl chains still bonded to Pd. The method also allowed us to intercept the catalytically active cationic intermediates of the Brookhart polymerization of alkenes, and to prove on-line their catalytic activity through ion/molecule reactions. The technique is fast and sensitive, requires no sample preparation and manipulation, and is therefore suitable for the fast screening of catalyst/polymer growth by MS identification, with potential applications in areas such as new catalyst development, polymer chemistry, and product quality monitoring in manufacturing.

Computational Insight into Catalytic Control of Poly(ethylene−methyl acrylate) Topology

Drent et al. have recently reported on a neutral Pd(II)-based complex with a bidentate sulfonate ligand that is able to copolymerize ethylene (E) and methacrylate (MA) into linear EMA polymers. We report here on a theoretical study that examines the Drent system and compares it to the cationic Brookhart catalyst, which affords branched EMA polymers with MA units at the end of the branches.

Possible Side Reactions Due to Water in Emulsion Polymerization by Late Transition Metal Complexes II: Deactivation of the Catalyst by a Wacker-Type Reaction

The transition metal catalyzed ethylene polymerization in water suspensions has been increasingly successful in the last couple of years. Water, however, adversely affects the polymerization process by deactivating the catalyst in a Wacker-type reaction. The cationic Pd(II)-diimine Brookhart catalyst 1 is studied here by a combination of quantum mechanics and molecular mechanics to determine the nature of the decomposition reaction. The study considers the decomposition process to take place in two stages. In the first stage, a coordinated olefin is attacked by a hydroxide group to ultimately produce a β-hydroxy-ethyl complex 2. The second step represents a decomposition to acetaldehyde and Pd(0), the latter in the form of palladium black. For the attack of OH on the coordinated ethylene in 1, both an internal path involving transfer from a palladium-coordinated OH group produced from hydrolysis of a Pd−CH3 bond and an external path based on attack of H2O on coordinated ethylene were considered. Both path...

(E)−(Z) Selectivity in the Polymerization of 2-Butene Promoted by Ni(II) Brookhart-Type Catalysts

We offer a rationalization of the different reactivity observed in the polymerization of (E)- and (Z)-2-butene isomers with (α-diimine)Ni(II) complexes, known as “Brookhart catalysts”. Quantum mechanics calculations suggest that the two isomers of 2-butene coordinate similarly to the metal and that the slower reactivity of the (Z) isomer is related to stronger steric interactions at the transition state for insertion into the Ni−chain bond. The similar coordination energies suggested that the presence of the (Z) isomer should not inhibit reactivity of the (E) isomer. This point was confirmed with polymerization experiments performed with a 40% (Z)-2-butene:60% (E)-2-butene mixture.

A computational study of iron-based Gibson–Brookhart catalysts for the copolymerisation of ethylene and 1-hexene

A combined QM/MM study of the ethylene/1-hexene copolymerisation with bisiminepyridine iron(II) is presented. It has been found experimentally that these catalysts do not copolymerise ethylene with 1-hexene. Based on the mechanism of propagation and termination processes proposed by Deng et al., we have performed calculations for the ethylene and 1-hexene monomers in order to give a suitable explanation to the experimental findings. The propagation reaction is divided into two fundamental steps: the backside monomer capture and the backside monomer insertion. The energy barriers for these steps are, respectively, 2.11 and 0.59kcal/mol for the ethylene monomer and 6.62 and 5.43kcal/mol for 1-hexene. Additionally, the backside π-complex formation for 1-hexene is an endothermic process by 0.72kcal/mol. Therefore, the ethylene propagation reaction is very favourable as compared to the 1-hexene propagation. In the same way, the termination reaction is also divided into two elementary steps: the frontside monomer capture and the β-hydrogen transfer steps. The associated energy barriers for these two processes are, respectively, 5.83 and 6.55kcal/mol for the ethylene monomer and 6.03 and 8.38kcal/mol for 1-hexene. So, the differences between the rate-limiting step of the propagation and termination energy barriers are 4.44kcal/mol for the ethylene and 1.76kcal/mol for the 1-hexene monomer. These facts are in good agreement with concurrent experimental results. Furthermore, the role of the bulky ancillary ligands has been analysed.

Computational studies of the Brookhart's type catalysts for ethylene polymerization. 1. Effect of the active site conformations on the catalyst activities

DFT calculations have been carried out on the cationic species for the two different Brookhart's catalyst systems: [{ArNCH–HCNAr}NiR″] + ( 3a) and [{ArNCMe–MeCNAr}NiR″] + ( 3b) (where Ar={2,6-C 6H 3(Me) 2} and R″=Me). These calculations reveal that the conformation of aryl groups attached to nitrogen atoms could provide a suitable explanation for the large experimental differences found in the ethylene polymerization activity.