SiO2‐supported Cr–V bimetallic catalyst can be used for producing bimodal polyethylene which can be applied for high‐performance pipe material. Alkyl aluminum are used to prereduce the bimetallic catalysts, and the effects of alkyl aluminum for the bimetallic catalyst are fully studied by catalyst characterization, polymerization kinetics, and the properties of polymer product by the comparison with the catalyst without prereduction. The result shows that the optimum polymerization activity is almost double after the catalyst is prereduced by triisobutylaluminum (TIBA), and the needed dosage of alkyl aluminum also is decreased significantly. The alkyl aluminum of the prereduced catalyst can also act as a chain transfer agent, significantly reducing the molecular weight of the polymer. The diethylaluminum chloride (DEAC) is mostly deactivating the Cr species during the ethylene polymerization. The synthesized catalysts, prereduced by TIBA, triethylaluminum (TEA), and DEAC, all exhibited good hydrogen response and comonomer interposition ability, which will be favorable for the further application of the bimetallic catalyst in the industrial field.
The present article reveals important roles of metal alkyl activators in tuning the performance of the Phillips catalyst in ethylene polymerization. The addition of aluminum alkyl aids the activation of the catalyst, while excess addition leads to the loss of the activity. The balance between the activation and deactivation depends on the type of employed aluminum alkyl, and tri‐n‐octylaluminum offers the most efficient catalyst usage by preferentially suppressing the deactivation. The passivation of aluminum trialkyl with a hindered phenol mildens not only the deactivation but also chain transfer reactions, leading to an increment of high molecular weight fractions.
Various MgCl2‐supported Ziegler–Natta (ZN) catalysts are synthesized with the intention to influence polymerization performance and 1‐butene incorporation in an ethylene copolymer. Modifications are introduced during different steps in the synthesis process, namely support preparation, titanation, and catalyst workup. While multiple different effects are observed upon modification, heat treatment during titanation shows the greatest impact. Increasing the heat‐treatment temperature increases polymerization activity. More importantly, the 1‐butene distribution can be shifted toward a more homogeneous profile. The amount of 1‐butene incorporated is similar to both for short‐ and for very long‐chain molecules. This behavior has so far been known only from metallocene‐based polyethylene and suggests that active sites are distributed more homogeneously in the ZN catalyst.
This article proposes a method to quantify the polymerization kinetics of ethylene and α‐olefins with commercial TiCl4/MgCl2 Ziegler–Natta catalysts. The method determines the leading apparent polymerization kinetic constants for each active site in a Ziegler–Natta catalyst by simultaneously fitting the instantaneous polymerization rate, cumulative polymer yield, and polymer molecular weight distribution measured at different times during a series of semi‐batch polymerization experiments. This approach quantifies the behavior of olefin polymerization with multisite catalysts using the least number of adjustable parameters needed to consistently model polymerization kinetics and polymer microstructural data.
This work aims at deriving analytical solutions for the molecular architecture of multi‐block polymer synthesized in a dual‐catalyst single CSTR. While the relevant equations are developed for homopolymerization, they can easily be extended to copolymerization. Special emphasis is placed on the quantities associated with each catalyst rather than the overall ones. However, if all rate parameters are available, the expressions can be used to calculate the properties of the material made by each catalyst as well as the overall ones under various process conditions. Given the reasonable assumption of large residence time, the solutions are simplified to elucidate the kinetics of chain‐shuttling involving two catalysts. It is shown that systems with low chain‐shuttling ability, if DPPn,0 ≠ DPQn,0, may exhibit significant deviation from Flory's most probable distribution. Furthermore, systems with high chain‐shuttling ability produce macromolecules with more uniform architecture and polydispersity index close to 2.
The effect of prepolymerization on ethylene homopolymerization and ethylene/1‐hexene copolymerization with a commercial TiCl4/MgCl2 catalyst was investigated and the apparent homo‐ and copolymerization rate constants were estimated by varying polymerization temperature, pressure, time, and 1‐hexene/ethylene molar ratio during the prepolymerization. The apparent rate constants for activation, propagation, and deactivation depend on the prepolymerization conditions, showing that the prepolymerization stage strongly regulates the behavior of the catalyst in the main polymerization. Interestingly, the surface morphology of the prepolymer particles correlates to and explains these changes in polymerization kinetics behavior.
In this article a systematic method is proposed to deconvolute the time‐dependent molecular weight distributions (MWD) and average comonomer fraction profiles of ethylene/1‐olefin copolymers made with heterogeneous Ziegler–Natta catalysts. These distributions with a high‐temperature gel permeation chromatography equipped with an infrared detector at four different polymerization times have been measured and used this information to infer how the fractions of polymer made on each site type varied with polymerization time. The model estimates here the minimum number of active site types needed to describe these copolymers, the MWD of polymer populations made on each site type, and their average comonomer fractions. This method is useful to quantify the microstructure of olefin copolymers made with multiple site type catalysts using the least number of adjustable parameters.
Cellulose nanocrystals (CNCs) are renewable, nontoxic and naturally available organic nanoparticles derived from cellulosic resources such as cotton and wood pulp. Poly(n‐butyl acrylate‐co‐methyl methacrylate)/CNC latexes are successfully synthesized via in situ emulsion polymerization. The effect of CNC loading on overall conversion, polymer particle size, glass transition temperature (Tg), gel content, latex viscosity, and storage and loss moduli of dried latex are studied. While the effect of CNC content on overall conversion, polymer particle size, and Tg of the resulting latexes is negligible, significant increase in gel content, latex viscosity, and storage and loss moduli are observed.
The preparation of forced gradient polymers has received considerable attention using batch reactors, while the preparation of usable quantities of forced gradient copolymers using continuous flow reactors has been hampered by the need to vary the composition of the monomer feedstock continuously during the reaction. A reactor that allows for addition of a monomer feedstock continuously at all points along the length of the reactor tubing allows for the preparation of forced gradient copolymers in continuous flow reactors, allowing for the scale‐up and bulk preparation of these polymers. This study reports here the initial investigation of preparing forced gradient copolymers using the reversible addition–fragmentation chain transfer methodology in tube‐in‐tube continuous flow reactors.
The reactivity ratios for the bulk free‐radical copolymerization of n‐butyl acrylate (BA)/n‐butyl methacrylate (BMA) are estimated at 80 °C. By performing a series of low conversion runs including replicate runs, the reactivity ratios are estimated as rBA = 0.460 and rBMA = 2.008. Runs to high conversions are then conducted at three different feed compositions (fBMA = 0.2, 0.5, and 0.8) to validate the reactivity ratios. The composition data from the high conversion experiments show good agreement with the estimated reactivity ratios in the integrated form of the Mayo–Lewis model. The molecular weight, gel content, and glass transition temperature of BA/BMA copolymers are also determined.
ArF candidate photoresist polymers have been synthesized by nitroxide mediated polymerization (NMP). Statistical copolymerizations of α‐gamma butyrolactone methacrylate, 3‐hydroxy‐1‐adamantyl methacrylate, and 2‐methyl 2‐adamantyl methacrylate with 5–10 mol% of controlling comonomers (i.e., styrene, p‐acetoxystyrene, 2‐vinyl naphthalene, acrylonitrile, and pentafluorostyrene), which are necessary for controlled polymerization of methacrylates by NMP with the unimolecular alkoxyamine initiator BlocBuilder, have been used. As little as 5 mol% controlling comonomer in the feed is demonstrated to be sufficient to produce linear evolution of number average molecular weight against conversion (X) up to X = 0.7 for relatively low target degrees of polymerization. All of the resulting copolymers have relatively low dispersities and show relatively low absorbance at 193 nm, comparable to other 193 nm candidate photoresists reported previously, with the exception of VN‐containing copolymer.
Significant progress has been made over the past 20–30 years in terms of the ability to develop and solve mechanistic models of emulsion polymerization processes, and in particular models for prediction of the particle size distribution. However, this does not imply that modeling of these economically important processes is by any means a “solved problem,” or that it is no longer necessary to perform fundamental research in this area. There are a number of areas where strong scientific work would increase the understanding of the process, including events in the aqueous phase, radical entry into growing particles, monomer partitioning, and especially the mechanisms and modeling of particle coagulation.
Multiscale mixing phenomena in stirred‐tank polymerization reactors are mainly caused by stir agitation, which performs a key function in macroscopic and microscopic flow fields. Both macroscopic and microscopic flow fields interact with each other and significantly affect the microstructure and product distribution of the resultant polymers. In this work, a computational fluid dynamics model combining the moment method used in the polymerization engineering field is implemented and validated using open data. Multiscale properties are characterized in terms of macroscopic mixing fields and the polymer microscopic structure of the atom transfer radical copolymerization system of methyl methacrylate and 2‐(trimethylsilyl) ethyl methacrylate. Agitation in a 3D stirred tank is also thoroughly studied by using the multiple reference frame approach, and the effects of several important parameters, such as impeller speed, impeller types, and feeding position, on the macroscopic and microscopic flow fields are investigated on the basis of the validated model. Interdependent relationships among agitation, multiscale flow fields, and polymerization are described clearly. The results highlight the function of stirring and provide useful guidelines for the scale‐up of stirred‐tank polymerization reactors.
Polyolefins (POs) are the largest polymer product in the world. The innovation in converting commodity olefin monomers to highly value added high‐performance POs has been and will continue to be a major theme in both academic research and industry practice. The excellent properties of POs can be achieved through precise engineering of their chain architectures, which largely involves control of the chain branching structures. Long‐chain branching is one of the most important parameters in the aspect of various chain branching structures. A huge amount of literatures have been reported to achieve better control of long‐chain branched structures over the last two decades. Recently, good effort has been made in reviewing all the major literatures and summarizing the catalytic systems and synthetic strategies for the controlled synthesis of long‐chain branched POs. This paper represents the first of the series, that is, controlled synthesis of long‐chain branched POs via single catalyst systems.
Two artificial neural network models (forward and inverse) are developed to describe ethylene/1‐olefin copolymerization with a catalyst having two site types using training and testing datasets obtained from a polymerization kinetic model. The forward model is applied to predict the molecular weight and chemical composition distributions of the polymer from a set of polymerization conditions, such as ethylene concentration, 1‐olefin concentration, cocatalyst concentration, hydrogen concentration, and polymerization temperature. The results of the forward model agree well with those from the kinetic model. The inverse model is applied to determine the polymerization conditions to produce polymers with desired microstructures. Although the inverse model generates multiple solutions for the general case, unique solutions are obtained when one of the three key process parameters (ethylene concentration, 1‐olefin concentration, and polymerization temperature) is kept constant. The proposed model can be used as an efficient tool to design materials from a set of polymerization conditions.
Strategies have been successfully developed for the monitoring of the homo and copolymerization of styrene and butyl acrylate in a miniemulsion system using near‐infrared (NIR) spectroscopy. Different concentrations of costabilizer, stearyl methacrylate, are tested to obtain the best stabilization condition. The spectral data are associated with the properties of the reaction medium, such as particle average diameter, conversion, number, and surface area of particles, through linear regression based on partial least squares. It is observed that the NIR spectrophotometer is sensitive to the dynamics of miniemulsion polymerization reactions, thus confirming the promising aspect of NIR technology for monitoring the latex properties.
Long‐chain branching plays an important role in the performance of polyolefins (POs). The existence of a very small amount of long‐chain branches (LCBs), i.e., <1 per 10 000 carbons, can significantly improve processability of the polyolefin materials, which is highly desired for those intractable polyolefins with narrow molecular weight distribution and high degree of crystallinity. Numerous literatures have been published on the controlled synthesis of long‐chain‐branched POs. In the previous paper, the major literatures of single catalyst systems have been summarized. This paper provides a comprehensive review for the binary and multiple catalyst systems and a brief summary of some other methods for the controlled synthesis of long‐chain‐branched POs. The controllability of long‐chain‐branched structures in the various preparation procedures with single or two reactor systems, and in one or two‐step processes, is analyzed and compared in‐depth.
Computational fluid dynamics (CFD) is used to study the gas–particle heat transfer in gas‐phase olefin polymerizations. Particularly, the effects of particle rotation on the gas–particle heat transfer coefficient and internal particle temperatures are evaluated, showing that particle rotation can exert a significant impact on observed temperature profiles, so that this effect should not be neglected during detailed CFD process simulations. As a consequence, particle rotation can lead to particle cooling and development of spherical gradient symmetry, validating the use of simpler modeling schemes that are based on reaction–diffusion in symmetrical spherical geometry.
In order to improve the branching ability of Phillips catalyst for ethylene polymerization, a new bimetallic Phillips catalyst was developed. In this work, a series of vanadium‐modified Phillips catalyst were prepared through co‐impregnation by varying the pH of the impregnation solution, the vanadium loading and the vanadium precursors. A method to prepare bimetallic Phillips catalyst having good catalytic performance was proposed. Catalyst characterization by diffuse reflectance UV‐Vis and X‐ray photoelectron spectroscopies clarified the key role of specific interaction between chromium and vanadium components for the improvements in branching ability and catalyst activity.
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