Polyolefin Catalysis Research Articles

Perspectives on Polyolefin Catalysis in Microfluidics for High-Throughput Screening: A Minireview

Polyolefins are the largest produced plastics in the world, wherein continuous stirred tank reactors and fluidized bed reactors are traditionally employed for commercial production. The operating condition, reaction kinetics, and molecular interactions inside the reactor strongly affect the polyolefin properties, which require a stringent process control in conventional procedures. Understanding the catalytic pathway, behavior of polymer particles, and effect of reactor conditions is essential for designing specific polymer properties, namely, the molecular weight, chain length, polydispersity, etc. Microfluidics can play a significant role in designing polymers tailored to the user needs. Smaller channel dimensions help obtain uniform reaction conditions over the length of the microfluidic reactor in a controlled environment. With real-time monitoring techniques in microfluidics, even single-particle growth of polymer can be studied to understand the parameters affecting the polymer properties. High-throughput microfluidics can help catalyst screening in a short duration with less consumption of reagents, generating less waste. When supplemented with efficient machine-learning algorithms, automated high-throughput microfluidics has the potential to rapidly optimize the process and facilitate the development of new knowledge even with a limited data set. When trained on data sets generated using microfluidic experiments that are designed efficiently with working knowledge of the process, machine-learning algorithms can provide the relationship between the multivariable parameters space and polymer properties, which is not possible with the traditional statistical methods and interpolation techniques. The rise in the utilization of microfluidics, with the advancement of machine-learning algorithms, for polyolefin catalysis, highlights the importance of microfluidics for catalyst discovery, parameter optimization, and understanding the reaction pathway for producing polymers with specific properties for specialized applications.

Solution-Phase Conformational/Vibrational Anharmonicity in Comonomer Incorporation Polyolefin Catalysis.

The prediction of comonomer incorporation statistics in polyolefin catalysis necessitates an accurate calculation of free energies corresponding to monomer binding and insertion, often requiring sub-kcal/mol resolution to resolve experimental free energies. Batch reactor experiments are used to probe incorporation statistics of ethene and larger α-olefins for three constrained geometry complexes which are employed as model systems. Herein, over 6 ns of quantum mechanics/molecular mechanics (QM/MM) molecular dynamics is performed in combination with the zero-temperature string method to characterize the solution-phase insertion barrier and to analyze the contributions from conformational and vibrational anharmonicity arising both in vacuum and in solution. Conformational sampling in the solution-phase results in 0-2 kcal/mol corrections to the insertion barrier which are on the same scale necessary to resolve experimental free energies. Anharmonic contributions from conformational sampling in the solution phase are crucial energy contributions missing from static density functional theory calculations and implicit solvation models, and the accurate calculation of these contributions is a key step toward the quantitative prediction of comonomer incorporation statistics.

Polyolefin catalysis of propene, 1-butene and isobutene monitored using hyperpolarized NMR.

Polymerization reactions of the dissolved gases propene, 1-butene, and isobutene catalyzed by [Zr(Cp)2Me][B(C6F5)4] were characterized using in situ NMR. Hyperpolarization of 13C spins by the dissolution dynamic nuclear polarization (DNP) technique provided a signal enhancement of up to 5000-fold for these monomers. For DNP hyperpolarization, liquid aliquots containing monomers were prepared at a temperature between the freezing point of the solvent toluene and the boiling point of the monomer, mixed with the polarizing agent α,γ-bis-diphenylene-β-phenylallyl free radical, and subsequently frozen. The hyperpolarized signals after dissolution enabled the observation of reaction kinetics, as well as polymer products and side products within a time of 30 s from the start of the reaction. The observed kinetic rate constants for polymerization followed a decreasing trend for propene, 1-butene, and isobutene, with the lowest rate constant for the latter explained by steric bulk. For all reactions, partial deactivation was further observed during the measurement time. The line shape and the chemical shift of the monomer signals with respect to a toluene signal were both dependent on catalyst concentration and reaction time, with the strongest dependence observed for isobutene. These changes are consistent with the characteristics of a rapid binding and unbinding process of the monomer to the catalyst occurring during the reaction.

Activation of homogenous polyolefin catalysis with a machine-assisted reactor laboratory-in-a-box (μAIR-LAB)

Catalysis discovery is typically limited to specialized labs – this work demonstrates an Artificially Intelligent Microreactor Lab in a Box applied to investigate the chemistry of different co-catalysts for zirconocene-catalyzed olefin polymerization.

A Career in Catalysis: Max McDaniel

Still going strong in his seventies, some of the work of Dr. Max McDaniel is briefly described in this account. Regarded as a titan of polyolefin catalysis by his industrial and academic colleagues, he arguably stands among the topmost important scientists to ever make contributions to the polyolefin industry. Several of the highlights and insights he has contributed to the field over his long and prolific career are summarized.

Tuning the Relative Energies of Propagation and Chain Termination Barriers in Polyolefin Catalysis through Electronic and Steric Effects

A computational exploration of the predicted molecular weights for Ti‐ and Zr‐catalyzed olefin polymerizations shows that there are considerable opportunities for electronic tuning. Ligand variation mainly affects the propagation rate, whereas chain transfer to the monomer is hardly affected by electronic factors. The results are analyzed in terms of the effects of ligand variation on the relative energies of “connected couples” of reactant local minima and the corresponding transition states on the basis of the Hammond postulate and the Curtin–Hammett principle. For the constrained‐geometry catalysts (CGCs) and bis(amido) systems studied, better donating ligands increase the preference for a “planar” metal environment and produce higher molecular weights.

Catalysis design: from Ti-bis(enolato-imino) to Ti-bis(enolato-azo) in living olefin polymerization at DFT level

Reaction mechanisms of both propagation and termination steps in homogeneous polyolefin catalysis are investigated at DFT level. Starting from existent catalysts, we propose the design of a new Ti-bis(enolato-azo) catalytic family. We considered a number of hydrogenated or fluorinated models in aromatic group and chelating ring. Fluorination in the aromatic rings is able to control the propagation steps by activating the metal site, while hydrogenation of the ligand can inhibit the termination transfers. So, we have optimized the structure of the catalyst to give it the highest reactivity towards polymerization and the lowest one towards termination, by means of the meta-F substitutions in the aromatic group, and –CH2CF3 on the chelating ring. Thus, the developed bis(enolato-azo) models seem to be interesting for experimental investigations as new living catalysts.

Group 4 metal complexes for homogeneous olefin polymerisation: a short tutorial review

The discovery of transition metal-catalysed polymerisation of olefins in the 1950s by Ziegler and Natta was of huge importance. Over the last 30 years, the interest in homogenous polymerisation has not only grown but has changed focus from primarily studying the metallocene complexes of Group 4 to widespread exploration of post-metallocene systems. This is a consequence of extensive patenting of the metallocene catalyst systems, as well as for general interest in the improvement of polyolefin catalysis. The plastics industry produces more than 107 tonnes of polyethylene, polypropylene and polystyrene each year worldwide, and the study of well-defined homogenous catalysts is invaluable in uncovering the mechanism of polymerisation and the fundamental properties of the active species. This short Tutorial Review gives an overview of homogenous transition metal Ziegler–Natta catalysis in general as well as an overview of a selection of Group 4 post-metallocene catalysts. An overview of the synthesis and reactions of alkyl cations relevant to olefin polymerisation is also given. Finally, this review summarises recent advances in bimetallic catalysts for olefin polymerisation.

Solid-State NMR Investigations of a MgCl2·4(CH3)2CHCH2OH Molecular Adduct: A Peculiar Case of Reversible Equilibrium between Two Phases

MgCl2·xROH molecular adducts are extensively employed as a support material for Ziegler-Natta polyolefin catalysis. However, their structural properties are not well understood. Recently, we reported on the preparation of an isobutanol adduct, MgCl2·4(CH3)2CHCH2OH (MgiBuOH) ( Dalton Trans. 2012 , 41 , 11311 ), which is very sensitive to the preparation conditions, such as the temperature and refluxing time. For the present study, the structural properties of MgiBuOH adducts prepared under different conditions have been investigated thoroughly by solid-state NMR and nonambient XRD. Formation of two phases has been confirmed, and in situ variable temperature solid-state NMR measurements confirm the coexistence of two phases as well as the oscillation from one to another phase. It is expected that such molecular adducts could have a significant role in organic transformation reactions due to an oscillating structural component. An understanding of phase oscillation with the Mg(2+) ion as the central metal ion might shed some light toward understanding various biological and structural functions.

Silica-supported (n BuCp)2 ZrCl2 : effect of catalyst active center distribution on ethylene-1-hexene copolymerization

Metallocenes are a modern innovation in polyolefin catalysis research. Therefore, two supported metallocene catalysts—silica/MAO/(nBuCp)2ZrCl2 (Catalyst 1) and silica/nBuSnCl3/MAO/(nBuCp)2ZrCl2 (Catalyst 2), where MAO is methylaluminoxane—were synthesized, and subsequently used to prepare, without separate feeding of MAO, ethylene–1-hexene Copolymer 1 and Copolymer 2, respectively. Fouling-free copolymerization, catalyst kinetic stability and production of free-flowing polymer particles (replicating the catalyst particle size distribution) confirmed the occurrence of heterogeneous catalysis. The catalyst active center distribution was modeled by deconvoluting the measured molecular weight distribution and copolymer composition distribution. Five different active center types were predicted for each catalyst, which was corroborated by successive self-nucleation and annealing experiments, as well as by an extended X-ray absorption fine structure spectroscopy report published in the literature. Hence, metallocenes impregnated particularly on an MAO-pretreated support may be rightly envisioned to comprise an ensemble of isolated single sites that have varying coordination environments. This study shows how the active center distribution and the design of supported MAO anions affect copolymerization activity, polymerization mechanism and the resulting polymer microstructures. Catalyst 2 showed less copolymerization activity than Catalyst 1. Strong chain transfer and positive co-monomer effect—both by 1-hexene—were common. Each copolymer demonstrated vinyl, vinylidene and trans-vinylene end groups, and compositional heterogeneity. All these findings were explained, as appropriate, considering the modeled active center distribution, MAO cage structure repeat units, proposed catalyst surface chemistry, segregation effects and the literature that concerns and supports this study. While doing so, new insights were obtained. Additionally, future research, along the direction of the present work, is recommended. © 2013 Society of Chemical Industry

An improved phase‐inversion process for the preparation of silica/poly[styrene‐co‐(acrylic acid)] core–shell microspheres: synthesis and application in the field of polyolefin catalysis

AbstractSpherical and well‐dispersed silica/poly[styrene‐co‐(acrylic acid)] (SiO2/PSA) core–shell particles have been synthesized using an improved phase‐inversion process. The resulting particles were successfully used as supports for polyolefin catalysts in the production of polyethylene with broad molecular weight distribution. Through the vapor phase, instead of the liquid phase in the traditional process, a non‐solvent was introduced into a mixture of micrometer‐sized SiO2 and PSA solution. The core–shell structure of the resulting SiO2/PSA microspheres was confirmed using optical microscopy, scanning electron microscopy, Fourier transfer infrared spectrometry, thermogravimetric analysis and measurement of nitrogen adsorption/desorption isotherms. In order to avoid agglomeration of particles and to obtain a good dispersion of the SiO2/PSA core–shell microspheres, the non‐solvent was added slowly. As the concentration of PSA solution increased, the surface morphology of the core–shell particles became looser and more irregular. However, the surface area and the pore volume remained the same under varying PSA concentrations. The SiO2/PSA core‐shell microspheres obtained were used as a catalyst carrier system in which the core supported (n‐BuCp)2ZrCl2 and the shell supported TiCl4. Ethylene/1‐hexene copolymerization results indicated that the zirconocene and titanium‐based Ziegler–Natta catalysts were compatible in the hybrid catalyst, showing high activities. The resulting polyethylene had high molecular weight and broad molecular weight distribution. Copyright © 2010 Society of Chemical Industry

Weak Attractive Ligand–Polymer and Related Interactions in Catalysis and Reactivity: Impact, Applications, and Modeling

The notion of weak attractive ligand-polymer interactions is introduced, and its potential application, importance, and conceptual links with "cooperative" ligand-substrate interactions are discussed. Synthetic models of weak attractive ligand-polymer interactions are described, in which intramolecular weak C-H...F-C interactions (the existence of which remains contentious) have been detected by NMR spectroscopy and neutron and X-ray diffraction experiments. These C-H...F-C interactions carry important implications for the design of catalysts for olefin polymerization, because they provide support for the practical feasibility of ortho-F...H(beta) ligand-polymer contacts proposed for living Group 4 fluorinated phenoxyimine catalysts. The notion of weak attractive noncovalent interactions between an "active" ligand and the growing polymer chain is a novel concept in polyolefin catalysis.

Neutron and X‐ray Diffraction and Spectroscopic Investigations of Intramolecular [CH⋅⋅⋅FC] Contacts in Post‐Metallocene Polyolefin Catalysts: Modeling Weak Attractive Polymer–Ligand Interactions

A family of Group 4 post-metallocene catalysts, supported by fluorine-functionalized tridentate ligands with the fluorine substituent in the locality of the metal center, is described. For the first time, the contentious C-H...F-C interaction has been characterized by a neutron diffraction study, which has allowed the position of the hydrogen atoms to be accurately determined. The nature of the weak intramolecular C-H...F-C contacts in these complexes in solution and the solid state was probed by using multinuclear NMR spectroscopy in tandem with neutron and X-ray crystallography. Evidence is presented to demonstrate that the spectroscopic C-H...F-C coupling occurs "through-space" rather than "through-bond" or by MF coordination. The titanium catalysts exhibit excellent activities and high co-monomer incorporation in olefin polymerization. The observed intramolecular C-H...F-C interactions are important with regards to potential applications in polyolefin catalysis because they substantiate the proposed ortho-F...H(beta) ligand-(polymer chain) contacts derived from DFT calculations for the remarkable fluorinated phenoxyimine Group 4 catalysts. Compared with agostic and co-catalyst...metal contacts, weak attractive noncovalent interactions between a polymer chain and a judiciously designed "active" ligand is a new concept in polyolefin catalysis.

An Atomistic Investigation of Solubility and Diffusion of Ethylene in Polyethylene Confined in a Pore

The solubility and diffusivity of ethylene in a polymer‐filled nanopore is investigated. The goal is to ascertain whether the thermodynamic and transport properties of the ethylene‐polyethylene system are influenced by the confinement in the nanopore. The nanopore is representative of the smallest pores present in current supported polyethylene catalysts. Advanced modeling techniques are used to generate a single atomistic polyethylene chain confined in the pore. Subsequently, Monte Carlo and Molecular techniques are employed to determine thermodynamic (phase equilibrium) and transport properties of the ethylene‐polyethylene system for two different pore sizes. For the smallest pore, both types of properties differ from their values in macroscopic bulk systems. Such differences are shown to be a consequence of confinement in the nanopore and can be of relevance in catalyst design and in mesoscopic and macroscopic modeling of heterogenous polyolefin catalysis. #An earlier version of this paper was presente...

Alternating copolymerization of alkenes and carbon monoxide catalyzed by cationic palladium complexes

AbstractIn the late 1940s, Reppe discovered a nickel catalyst for the co‐oligomerization of ethene and carbon monoxide. Since then, various groups have made attempts to develop more efficient catalysts for this reaction. The recent discovery of a new class of very active palladium catalysts now allows, for the first time, ready access to a large variety of high‐molecular‐weight alkene/carbon‐monoxide co‐ and terpolymers. The catalysts comprise a cis‐coordinated palladium(II) species associated with weakly or non‐coordinating anions. Their discovery has made it possible to develop a viable process for the commercial production of the first members of the new family of CARILON‐olefin/carbon monoxide copolymers.In this brief review, we highlight some key phenomena in polyketone catalysis. The elementary reaction steps in the palladium‐catalyzed polyketone formation during chain propagation, initiation and termination are outlined. The role of bidentate ligands and the reasons for perfect alternation in polymer chain growth are dicussed. The attractive feature of the catalysts, that they also catalyze the co‐ and terpolymerization of olefins higher than ethene with carbon monoxide, is briefly noted. A parallel between polyketone catalysis by cationic palladium complexes and modern polyolefin catalysis by cationic metallocene complexes is suggested.