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.
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.
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.
Polymer microgels with sizes of some tens to hundreds of micrometers can be formed with exquisite control by droplet‐based microfluidic templating. This study presents a systematic assessment of the effect of the premicrogel droplet size on the ability of production of such microgels. The focus is on two popular acrylamide‐derivatives at a fixed monomer concentration and external polymerization temperature. An exponential dependence of the success of droplet gelation on the droplet size is found, which can be rationalized in view of the balance between production and transfer of heat within and from the droplets on basis of a simple Arrhenius argument.
Amphiphilic block copolymers of 2,3,4,5,6‐pentafluorostyrene (PFS) and methacrylic acid (MAA) are synthesized via nitroxide‐mediated polymerization (NMP). It is established that to obtain a controlled copolymerization a minimum of 40 mol% of PFS is required, which is significantly greater than other copolymerization systems such as using 4.5–8 mol% styrene or 1 mol% of 9‐(4‐vinylbenzyl)‐9H‐carbazole to control the copolymerization of methacrylates. It is surmised that this lack of control is due to the reactivity ratios that favor the addition of MAA rather than PFS (rPFS = 0.14, rMAA = 6.97). However this reactivity ratio pair suggests that a one‐shot delayed injection approach can be utilized to synthesize almost pure block copolymers in one pot. Therefore, poly(PFS)‐b‐(PFS‐ran‐MAA) block copolymers are synthesized by a one‐shot delayed addition of MAA. While the concentration of irreversibly terminated chains is evident these results suggest a promising route to the synthesis of fluorinated amphiphilic block copolymers by NMP.
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.
Reversible‐deactivation radical polymerization (RDRP) techniques have received lots of interest for the past 20 years, not only owing to their simple, mild reaction conditions and broad applicability, but also their accessibility to produce polymeric materials with well‐defined structures. Modeling is widely applied to optimize the polymerization conditions and processes. In addition, there are numerous literatures on the kinetic and reactor models for RDRP processes, which show the accessibility on polymerization kinetics insight, process optimization, and controlling over chain microstructure with predetermined molecular weight and low dispersity, copolymer composition distribution, and sequence distribution. This review highlights the facility of the method of moments in the modeling field and presents a summary of the present state‐of‐the‐art and future perspectives focusing on the model‐based RDRP processes based on the method of moments. Summary on the current status and challenges is discussed briefly.
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.
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.
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.
The model and methodology for estimating diffusion‐controlled rate coefficients for the methyl methacrylate (MMA) polymerization system is extended to the vinyl acetate (VAc) case. Comparison of the kinetic behavior and termination rate coefficients (kt) of both monomers suggests that at low conversions the termination reaction is controlled by the chemical step, whereas at moderate and high conversions it is controlled by the diffusive step which in turn is determined by the segmental diffusion of the long radicals and not by the center of mass diffusion of short radicals. It is found that, for most of the conversion range, diffusion coefficient for VAc is lower than the one for MMA notwithstanding that ktVAc > ktMMA. An explanation of this apparent inconsistency on the base of the model results and in terms of segmental mobility is proposed.
The present work describes a kinetic approach which is able to predict how the internal surface area of polymer particles evolve during suspension copolymerization in the presence of porogen. For such a purpose, the concept of elementary gel structures has been introduced by modeling their surface area through the numerical fractionation technique. Thus, variables such as diluents composition, divinyl monomer concentration, and dilution degree could be assessed in the simulations. The present mathematical model is validated by using different experimental data from literature and a fair agreement is reached. Furthermore, the developed model is also capable of predicting the most significant copolymerization variables, e.g., conversion rate, concentration of species, and average molecular weights.
A kinetic model for the radical homopolymerization of acrylamide in aqueous solution is developed, incorporating propagation and termination rate coefficients as functions of monomer concentration and including the formation and reaction of midchain radicals based on the insights and measured rate coefficients from recent pulsed‐laser studies. The model successfully represents the batch conversion profiles measured using an in situ NMR technique between 40 and 70 °C with initial monomer concentrations of 5 to 40 wt%, as well as the associated polymer molar mass distributions. In particular, the model captures the decreased rate that occurs at lowered monomer concentrations as a result of the formation of less‐active midchain radicals by backbiting. Previous literature data are also well represented by the model.
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.
Phillips catalyst is one of the most significant industrial ethylene polymerization catalysts. Chemical modifications have been carried out to tune the Phillips catalyst performance and improve the polyethylene properties. After the modification of the catalyst by fluorine, the polyethylene product with higher molecular weight (MW) and narrower molecular weight distribution (MWD) is suitable for producing automobile fuel tanks. Vanadium containing Phillips catalyst enhances α‐olefin incorporation and MW regulation. In present work, fluorine modified and unmodified chromium–vanadium (Cr–V) bimetallic catalysts are prepared and explored. Compared with the fluorine‐free catalyst, the activities of F‐modified bimetallic catalysts slightly decrease with the increasing MW of the product and the hydrogen response increases slightly. Due to the synergistic effect of the chromium, vanadium and fluorine on the silica gel support, the short‐chain branch distribution (SCBD) of copolymers from F‐modified Cr–V bimetallic catalyst (Cr–V–F)600 is more beneficial than that of Cr–V bimetallic catalyst (Cr–V)600 and F‐modified Cr–V bimetallic catalyst (Cr–V–F)500. The fluorination of Cr–V bimetallic catalysts has not only preserved the high polyethylene activity of bimetallic active sites but also produced the advantage of the high MW ability from fluorine.
In this study a framework consisting of a computational fluid dynamics simulation coupled to a population balance model for the modeling of emulsion polymerizations is proposed. The combined approach is used to understand the impact of changing length and time scales, as well as mixing conditions on the particle size distribution (PSD) of a polymer latex under different conditions. It is shown that the effect of agitation rate can have a profound impact on the distribution of ionic species in the reactor, and thus on the evolution of the PSD.
A series of (SiO2/MgO/MgCl2)?TiClx Ziegler‐Natta polyethylene catalysts have been synthesized using water‐soluble Mg‐compounds such as magnesium acetate as Mg‐source at different TiCl4 treating temperatures. The catalyst shows the highest activity of homopolymerization when preparation temperature is 120 °C with only 1.25 of optimal Al/Ti molar ratio, which is much lower than industrial value, resulting in much lower catalyst preparation and polyethylene production cost. The operation condition is relatively moderate and the synthesized catalysts exhibit rather high activity, good hydrogen response, and copolymerization ability with high 1‐hexene incorporation. The polymers obtained from these catalysts have high molecular weight and medium molecular weight distribution. Compared with the conventional industrial Ti/Mg Ziegler‐Natta catalysts using the relatively expensive, anhydrous and moisture‐sensitive Mg‐sources, the most unique feature of our novel catalysts is the capability of utilization of any soluble Mg‐compounds under mild conditions and can achieve rather high activity with only a small amount of cocatalyst, hence show great potential for application in polyethylene industry.
A new way to fabricate monodisperse polymer particles in a microfluidic device without UV‐light and without the need for high temperatures is described in this article. By applying an activator regeneration by electron transfer ‐ atom transfer radical polymerization (ARGET‐ATRP) initiator system in a co‐capillary microfluidic setup and by separating the monomer mixture in an initiator and a catalyst phase, a fast polymerization of the droplets at low temperature without premature curing and thus clogging of the capillaries can be achieved. The influence of the flow rates on the particle sizes and their polydispersity as well as the controlled character of the polymerization are investigated. The particle size is well adjustable, but the reaction is not controlled due to the high radical concentration needed for rapid polymerization. In addition, particles with incorporated UV‐dyes are produced as a proof of concept at low temperature.
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.
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.
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