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Ziegler–Natta catalysis is a very important industrial process for the production of polyolefins. However, the catalysts are not well-understood at the molecular level. Yet, atomic-scale structural information is of pivotal importance for rational catalyst development. We applied a solid-state NMR/density functional theory tandem approach to gain detailed insight into the interactions between the catalysts’ support, MgCl2, and organic electron donors. Because of the heterogeneity of the samples, large line widths are observed in the carbon spectra. Despite this, good agreement between experimental and computational values was reached, and this shows that 1,3-diether based donors coordinate at (110) surface sites, while phthalates are less selective and coordinate at both (104) and (110) surface sites.

Since 1963, when Karl Ziegler and Giulio Natta were jointly awarded the Nobel Prize for their discoveries of the catalytic polymerization of olefins with Ti-chlorides and Al-alkyls, heterogeneous Ziegler-Natta (ZN) catalysts have become the main catalysts for the industrial production of polyolefins. Despite of the relevance of ZN catalysts for the large-scale production of polyolefins, a clear mechanistic understanding of these catalysts is still incomplete due to the elusive nature of the active site structures. Over the last two decades, researchers have used density functional theory (DFT) methods to clarify the polymerization mechanisms and to identify the nature of the active sites, unraveling the influence of supports, cocatalysts, and the effect of internal and external donors on the polymerization processes. Major efforts were dedicated to understanding the origin of stereoselectivity in α-olefin polymerization as well as the termination reactions mechanisms, and the role that impurities can play in heterogeneous ZN catalysis. Here, we review the DFT studies on heterogeneous ZN catalysts and suggest promising areas for future research.

Effect of hydrogen and alkoxysilanes as external donor were investigated using two commercial Ziegler-Natta catalysts (TiCl4/MgCl2•nEtOH and TiCl4/phthalate type/MgCl2) in ethylene polymerization. These catalysts were analyzed by EDX, FTIR and XRD measurements. Furthermore, catalytic activity was tested. According to the results, when alkoxysilane was introduced into the system, catalytic activity reduced. Compared these two alkoxysilanes, cyclohexylmethyldimethoxysilane (CHMDMS) can decrease activity more than dimethoxydimethylsilane (DMDMS) did. This was because CHMDMS has more bulky hydrocarbon groups than DMDMS. This led to restrict the direction of monomer insertion to active sites. In addition, when hydrogen was added into the system, CHMDMS is more sensitive to hydrogen than DMDMS. However, CHMDMS showed lower hydrogen response than DMDMS with increased hydrogen concentration. This was owing to its hydrogen sensitivity. Among the two catalysts, catalyst having internal donor can decrease sensitivity of catalyst activity in the system with hydrogen. The obtained polymers were further characterized by DSC and GPC.

Five new alkoxysilanes with different sizes of hydrocarbon substituents were first synthesized and employed as external donors for propylene polymerization with a Ziegler–Natta catalyst. The effects of these and industrial alkoxysilanes with different sizes of hydrocarbon substituents on the microstructure of polypropylene were studied by SSA and 13C NMR. The results showed that isotactic sequence length of polypropylene increased with the size of R2 substituent on alkoxysilanes in the five donor systems, similar to the regularity of molecular weight, isotacticity, and thermal properties of polypropylene. Although the polypropylene produced by double cycloalkane substituents on alkoxysilanes had lower lamellae thickness L1, its contents were the highest. Moreover, excess large volume of hydrocarbon substituents on alkoxysilanes might be detrimental to improving isotactic sequence length of polypropylene. Polypropylene prepared by MIPDMS had a more uniform distribution of the stereodefects, indicating that...

Varies phthalate-free internal electron donors used in Ziegler-Natta catalysts for propylene polymerization were reviewed. Different catalysts with 1,3-diether, diol esters and butanedioic diester as internal electron donors were compared and discussed. Other kinds of electron donors were also involved. In view of the increasing requirement for clean polypropylene, development and prospect of electron donors used in Ziegler-Natta polypropylene catalyst system were outlined.

Summary Diolefins can be used as comonomers in propylene copolymerizations in order to modify some of the final properties of the obtained poly(propylene) (PP) resins. Although reduction of catalyst activity can be expected when such copolymerizations are performed with standard heterogeneous Ziegler- Natta catalysts, copolymers containing 1,5-hexadiene (HD) and 1,7-octadiene (OD) can be produced at high rates when the comonomer content is sufficiently low (below 2 mol%); however, the presence of very small amounts of 5-ethylidene-2-norbornene (ENB) leads to complete catalyst inhibition. It is shown that incorporation of dienes lead to decrease of the melting point of the final product, especially in presence of ENB. When compared to HD, addition of OD causes more pronounced modification of the melt flow index and weight-average molecular weight, although the xylene solubles is less sensitive to the comonomer feed when the comonomer composition is low. Obtained results indicate that small amounts of dienes can be used at plant site to modify the final properties of PP resins produced with standard heterogeneous Ziegler-Natta polymerizations.

Optimization of operational conditions for the preparation of Ultrahigh-molecular-weight polyethylene (UHMWPE)/Graphene nanocomposites with Ziegler–Natta catalyst was carried out via response surface methodology (RSM). This study deals with the optimization of process variables to optimize the productivity and molecular weight. A three-factor, three-level Box–Behnken design with temperature (X1), monomer pressure (X2), and [Al]/[Ti] molar ratio (X3) as the independent variables were selected for the study. The dependent variables were productivity and molecular weights of the final nanocomposites. It was developed by using the three parameters at three levels including 50, 60, and 70°C for temperature; 4, 6, and 8 bar for pressure; and 176, 318, and 460 for [Al]/[Ti] molar ratios. The optimum reaction conditions derived via RSM were: temperature 60°C, pressure 8 bar, and [Al]/[Ti] molar ratio 242. Productivity and molecular weight were 2107 g PE/mmol Ti.h and 3.7 × 106 g/mol, respectively, under optimum c...

Polyolefin/clay nanocomposites are mainly prepared by melt mixing, solution, and in situ polymerization approaches. Among them, the last one is more attractive because the dispersion obtained through in situ polymerization is the most efficient, particularly, in the fully exfoliated polyolefin/clay nanocomposite formation. The clay-supported coordination catalysts, i.e. Ziegler–Natta, metallocene, and late transition metal catalysts, were applied to olefins polymerization in order to produce the polyolefin/clay nanocomposites. This review presents the studies reported on the use of the clay-supported Ziegler–Natta catalysts for polymerizing the olefins, which provided that detailed examples reported in the open literature. The emphasis is placed on the production of the polyolefin/clay nanocomposites via in situ polymerization.

Recently we successfully established the first structure–performance relationships between the catalyst surface area and propylene polymerization activity using novel MgO/MgCl2/TiCl4 core–shell catalysts with non-porous and non-fragmentable features. In the present paper, we have addressed the physical and chemical natures of these novel model catalysts in comparison with typical Ziegler–Natta catalysts, by means of comprehensive characterization and analyses. It was clarified that the MgO/MgCl2/TiCl4 core–shell catalysts offer an ideal and powerful tool to address relationships between the support architectures and polymerization performance, which had been long un-clarified. Unique structure of MgO/MgCl2/TiCl4 core–shell catalysts but yet identical surface chemistry with typical Ziegler–Natta catalyst offers an ideal and powerful tool to address relationships between the support architectures and polymerization performance.

High-performance Ziegler–Natta catalysts with ill-defined structural hierarchy were studied over multi-length scales. In this study, multilateral characterization was performed to address quantitative structure–performance relationships in ethylene/1-hexene copolymerization with Mg(OEt)2-based Ziegler–Natta catalysts. Macroscopic characteristics of the catalysts (e.g., particle size and meso- and macropore volumes) were greatly affected by structures of Mg(OEt)2 precursor particles, while microscopic characteristics (e.g., micropore volume and chemical composition) were hardly influenced. Ethylene/1-hexene copolymerization results suggested the significance of monomer diffusion: The activity was enhanced for smaller catalyst particles, while the 1-hexene incorporation was improved with larger meso- and macropore volumes.

Effect of AlEt3 on 1-hexene polymerization with MgCl2-supported Ziegler–Natta catalyst was studied. Complete activation of active centers (C*) producing medium to low molecular weight polymer requires relatively higher Al/Ti than C* producing high molecular weight polymer. Raising Al/Ti from 30 to 300 caused continuous shifting of the active center distribution from centers of smaller number, fast chain propagation and slow chain transfers to centers of larger number, slow chain propagation and fast chain transfers. Chain transfer with AlEt3 is more evident in C* producing lower molecular weight polymer.

The discovery of Ziegler − Natta catalysts has been one of the greatest developments in technology for the synthesis of stereoregular polymers in both academy and industry since the 1950s. In particular, the development of the fourth generation Ziegler − Natta catalyst with MgCl2 as support brings a revolutionary improvement to the properties of manufactured polyolefins and stimulates people to explore the stereospecific polymerization of α-olefin and diene monomers, which supplies the power to synthesize a new type of polyolefin materials. Although research on single-site catalysts has attracted a lot of attention in recent years, the ‘old’ and conventional Ziegler − Natta catalysts are still being developed vigorously due to their unique industrial advantages such as low cost, high catalytic efficiency and high stereospecificity. In this mini-review, we mainly summarize the development of the conventional supported Ziegler − Natta catalyst system and the stereoregular polyolefins synthesized with supported Ziegler − Natta catalysts. © 2013 Society of Chemical Industry

Summary Comprehensive polymer architectural studies were carried out on a group of Ziegler-Natta (Z-N) polyethylene homopolymers via size-exclusion chromatography coupled with multi-angle light scattering (SEC-MALS) and rheology. Although Z-N resins are traditionally viewed as linear polymers, both SEC-MALS and rheology results strongly indicate that there are topological variations, presumably long-chain branching (LCB) or possibly hyper-branches, in this group of supposedly linear Z-N homopolymers. These Z-N homopolymers exhibit very complex elution behavior that resulted in anomalous elution profiles in SEC. It appeared that some high molecular weight (MW) and long-chain branched components followed a non-size exclusion separation mechanism on SEC columns, co-eluting with low MW and linear polyethylene components, causing a delayed elution. The possible origin of LCB in these Z-N polymers is also discussed.

The formation of complexes in catalysts of the Ziegler-Natta type are studied virtually in situ using Mossbauer spectroscopy in combination with a quick-freezing technique. Information on the dynamics of the processes involved is acquired without perturbing the state of the systems under study. The mechanism of formation and the nature of the catalytically active complexes are investigated by a method that allows the condition of the catalyst to be observed without removing it from the reaction medium.

Copolymerization of propylene and ethylene and terpolymerization of propylene, ethylene, and 1-butene were carried out to compare the characteristics of diether- and phthalate-based Ziegler-Natta catalysts in a reaction system of pilot scale. The ethylene incorporation with the diether-based catalyst was higher but the 1-butene incorporation was lower compared with those of the phthalate-based catalyst. In the case of copolymers from the diether-based catalyst, melting behavior, determined through differential scanning calorimetry (DSC), showed a distinct shoulder peak and lots of nuclei were formed during crystallization. The diether-based catalyst led to polymers having blockier ethylene sequences compared with those of the phthalate-based catalyst; the highly crystallizable fraction (HIS) containing blockier ethylene sequences was produced with the diether-based catalyst. These results seem to be the result of regio-irregular characteristics of the diether-based catalyst. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 851-859, 2013

A rigorous method is developed to extract kinetic parameters describing catalyst activity from experimental polymerization rate profiles for Ziegler–Natta catalysts. The kinetic scheme used correlates the oxidation state with catalyst activity, eliminating the need for multiple site types to describe rate profiles. The method is applied to data from kinetic experiments performed in a lab-scale semi-batch reactor. Four parameters are required to describe the activity of the catalyst: the polymerization rate constant and three site transformation rate constants. The model is able to reproduce changes in the rate profiles in response to reactant concentrations. The fits of the model to the data are comparable to those in similar studies.

Fifty years after the Nobel Prize was awarded to Ziegler and Natta, the transition-metal catalyzed polymerization of olefins remains one of the most important reactions conducted on the industrial scale and the subject of industrial as well as academic research. This introductory article will provide a short historical review of the discovery of catalyzed olefin polymerization by Ziegler, Natta, and others and its development in the years following the Nobel Prize, as well as giving insight into Ziegler-Natta polymerization for the nonspecialist.

The success of polyolefins is governed to a large extent by the development of robust and versatile catalysts offering excellent morphology control. This review highlights the major evolution steps made in the polyolefin catalyst systems in terms of productivity and possibilities to control the molecular architecture of both polypropylene and polyethylene. Starting from the initial TiCl3-types, the continuous improvement of the Ziegler-Natta catalysts in terms of performance and cost is the major factor behind their wide market penetration. On the other hand, metallocene and the other “single-site” catalysts enable an unprecedented fine-tuning of chain microstructure by ligand design. In this article, special emphasis is placed on the influence of catalyst type on polymer structure characteristics such as molecular weight distribution, stereoregularity, and comonomer distribution and, ultimately, on the end-use properties of polyolefins. It is the excellent balance among price, performance, and processability that will further strengthen the position of polyolefins as a dominant class of materials in the polymer industry.

In heterogeneous Ziegler-Natta catalysts for olefin polymerization, isolation of a single type of active sites is a kind of ambition, which would solve long-standing questions on the relationship between active site and polymer structures. In this paper, polypropylene produced by TiCl3/MgCl2 model catalysts with minimum Ti heterogeneity was analyzed by combined solvent fractionation and the two-site statistical model. We found that the active sites of the model catalysts were classified into only three types, whose proportions were dependent on the Ti dispersion state. The addition of external donors not only newly formed highly isospecific sites, but also altered the stereochemical nature of the other active sites.

The different ways of initiating the polymerization of an unsaturated monomer such as styrene or ethylene are described. Brief accounts of free radical, cationic, anionic and organometallic initiators are included. The free radical mechanism for the polymerization of styrene is treated in a little more detail, as are the so-called Ziegler/Natta catalysts for alkene polymerization.

Industrial Ziegler–Natta catalysts for polypropylene production are complex formulations with a reputation for being “black boxes”. In this paper, we report the results of an extensive investigation of the three latest commercial generations, performed with advanced high-throughput experimentation tools and methods. The thus obtained database of structure–property relationships, of extraordinary width and depth, provided a high-definition picture of the screened systems, allowing us to highlight important details of their inner workings. In particular, the delicate relationship between surface coverage and lateral steric pressure on the stereoselectivity of the catalytic species, as well as the role of chemisorbed donor and Al-alkyl species on said parameters for different formulations, was revealed.

Using 2-thiophenecarbonyl chloride (TPCC) as a quenching agent, the number of active centers in propylene polymerization with MgCl2-supported Ziegler–Natta catalysts were determined by measuring the sulfur content of the quenched polymer. Under suitable conditions, the thiophenecarbonyl-labeled polymer chains were stable in the reaction system when TPCC/Al > 1. The number of active centers was found to increase in the first 5 minutes of propylene polymerization, accompanied by rapid decrease of the propagation rate constant kp. Diffusion barrier from the polymer covering the catalyst fragments is thought as the main reason of the rapid decay of kp in the initial stage.

On March 11, 1954 Giulio Natta synthetized isotactic polypropylene. It was the first synthesis, in laboratory, of a polymer having a sterically ordered structure, similar to those found in some natural polymers like, for instance, natural rubber and cellulose. This meaningful aspect was emphasized by Prof. Fredga of the Swedish Academy in his speech at the customary ceremony of Nobel Prize on December 10, 1963, when he stated “Natta has broken the monopoly of nature”. With the polypropylene synthesis Natta discovered the “stereospecific polymerization” defined by Toblosky (Princeton University) and by Flory, one of the most eminent scientist in the macromolecular chemistry and future Nobel laureate in Chemistry (1974), a “revolution” in the macromolecular field. Natta was recognized as the “father” of stereospecific polymerization. The Journal of Polymer Science, vol. 51 (1961), dedicated to Natta, reports “Seldom a scientific contribution aroused such a deep and fundamental interest and was followed by such a rapid technical development as the series of publications by Professor Natta and his coworkers on the stereospecific polymerization. Yet Professor Natta has succeeded in maintaining undisputed leadership in this field of polymer chemistry and continues to surprise his colleagues with new and unexpected results”. The revolution brought by Natta in the field of macromolecular chemistry involved the entire scientific and industrial world specialized in that area. Soon after 1954-55 a lot of universities and industrial laboratories around the world started to work on activities concerning the novel stereospecific polymerization. The results of these studies are reported in about a thousand scientific publications and some hundreds of patents issued from the Milan Polytechnic Institute of Industrial Chemistry between 1955 and 1977 (concentrated in 1955-57). The importance of polypropylene from the commercial point of view is evident from the fact that the world production of various types of polypropylene now stands at about 60 million tons/year. It is estimated that the related global economic value approaches the second place over all synthetic products, after polyethylene, together with ammonia, and before other products such as urea, polymers of styrene and vinyl chloride, nylons, etc. For their part 1,4-cis polybutadiene and copolymers based on ethylene and propylene occupy respectively the second and the third place in terms of worldwide production and market value among synthetic elastomers, after styrene-butadiene rubbers (SBR). More information concerning the personality and the scientific and didactic activities of Giulio Natta and his relations with the scientific and industrial world are reported on the internet site www.giulionatta.it containing the “Giulio Natta Archive” (more than 40.000 pages).

The authors thank the PSMN, CINES, and IDRIS for the computational resources.; International audience; The Ziegler-Natta polymerization is one major example of application of catalysis in industry. Since the first discovery of Ziegler and Natta, several modifications of the catalyst have been developed, in order to improve its performance. Nowadays, a typical Ziegler-Natta catalyst for polyethylene synthesis consists of a precatalyst, composed of TiCl4 supported on MgCl2 in the presence of a Lewis base, activated by organoaluminum. The atomic-scale characterization of the Ziegler-Natta catalyst is crucial for further improvement of the catalyst. Here, the precatalyst TiCl4-MgCl2 with EtOH as internal. Lewis base is characterized combining solid-state NMR spectroscopy and periodic density functional theory calculations. From total energy and NMR spectra, eight surface species were proposed showing EtO- ligands on the Ti and EtOH/EtO- on the surface Mg species, These species lead to a complete interpretation of the NMR two-dimensional spectra. Hence a detailed molecular scale description of the precatalyst was obtained.

High temperature two-dimensional liquid chromatography (HT 2D-LC) was recently introduced as a new technique to analyze the heterogeneities with regard to composition and molar mass present in model blends of polyolefins and various olefin copolymers. The method uses graphite as stationary phase and solvent gradients of 1-decanol 1,2,4-trichlorobenzene as mobile phase for the compositional separation. With the aim to maximize the chromatographic resolution, the influence of the separation's temperature in the first dimension was evaluated: approaching the -temperature of polyethylene (PE) in 1-decanol selectively enhances the retention of higher molar mass PE standards while that of the lower molar mass ones is hardly affected. A bimodal ethylene/1-butene copolymer and its temperature rising elution fraction (TREF) fractions were separated by HT 2D-LC. For the first time, both axes of the contour plot were calibrated with regard to chemical composition and molar mass, respectively. Prefractionation of the bulk sample by TREF enhances the detectability of separated components of the 2D separation. The influence of the separation temperature, that is, working around the theta-temperature of PE in 1-decanol, can be used to enhance the chromatographic resolution of the 2D chromatography.

In this work, we present a systematic DFT analysis of the effect of surface coverage on the coordination properties of alkoxysilanes to the (104) and (110) surfaces of MgCl2. Furthermore, we invest...

The impact of support architectures on the olefin polymerization performance of heterogeneous Ziegler–Natta catalysts has been scarcely understood due to the complexity of pore system and the fragmentation-induced structural changes during polymerization. In this communication, a series of novel core–shell MgO/MgCl2/TiCl4 catalysts were synthesized by utilizing poreless single-crystal MgO nanoparticles as a non-fragmentable core material. Employing these catalysts, we have successfully established a structure–performance relationship (SPR) that the propylene polymerization activity is perfectly proportional to the catalyst surface area for the first time. On the other hand, polymer properties were found to be decided by active site natures independently of the surface area.

Systematic periodic density functional calculations were conducted to clarify the mechanism for donors to exert steric and electronics influences on propylene polymerization using heterogeneous Ziegler–Natta catalysts. It was concluded that TiCl 4 preferentially adsorbs as mononuclear species on the MgCl 2 (1 1 0) surface, and the coadsorption of donors with it is energetically viable. The coadsorption of donors on the (1 1 0) surface reinforces the electron density of the Ti mononuclear species and sterically transfers the underlying C 2 symmetry to convert the originally aspecific mononuclear species into isospecific one. The nearest coadsorption of ethylbenzoate (EB) not only sterically induces the isospecificity of the Ti mononuclear species but also electrostatically improves the regiospecificity in propylene insertion. In addition, EB prevents sterically demanding chain transfer to propylene, increasing the molecular weight of the produced polypropylene. Thus theoretically derived “coadsorption model” is highly consistent with a variety of experimentally known facts and believed to be useful for the ab initio prediction of new donor structures.

The melt fracture of high-density polyethylenes (HDPEs) is studied primarily as a function of molecular weight and its distribution for broad molecular weight distribution metallocene and Ziegler–Natta catalyst resins. It is found that sharkskin and other melt fracture phenomena are very different for these two classes of polymers, although their rheological behaviors are nearly the same for many of these. Moreover, the metallocene HDPE shows significant slip at the die wall without exhibiting stick-slip transition. Important correlations are derived between the critical conditions for the onset of melt fracture and molecular characteristics.

The article presents experimental data on the formation rates of active centers in polymerization reactions of ethylene with solid Ti-based Ziegler–Natta catalysts (γ and δ forms of TiCl 3 , TiCl 2 ) and five types of supported catalysts, TiCl 4 /MgCl 2 /SiO 2 , TiCl 3 /MgCl 2 /SiO 2 and TiCl 4 /MgCl 2 /diisobutyl phthalate. Effects of the following factors on the center formation are discussed: reaction temperature, monomer concentration, the amount of cocatalyst, the initial valence state of Ti atoms in the catalyst, pre-reactions of catalysts with cocatalysts, the presence of molecular hydrogen, effects of 1-alkenes in ethylene polymerization reactions. A combination of the results produces the basis for the formation mechanism of active centers in Ziegler–Natta catalysts, which involves reduction of numerous Ti species on the catalyst surface to II Ti and oxidation addition of alkenes to II Ti with the generation of IV Ti-based polymerization centers.

Specific surface area based on the BET method in N 2 adsorption has historically posed poor correlation with olefin polymerization activity of heterogeneous Ziegler-Natta (ZN) catalysts. We have investigated the validity of the BET surface area for ZN catalysts based on the α S -plot method, where non-porous core–shell MgO/MgCl 2 /TiCl 4 catalysts were used as a reference material. The α S -plots for typical industrial ZN catalysts clarified the presence of two classes of micropores and resultant difficulty to isolate a multilayer adsorption region for the BET method. The results cast doubt on the application of the BET method to evaluate the surface area of heterogeneous ZN catalysts.

Ziegler Natta (ZN) catalysts are used to produce a wide range of polyolefins on an industrial scale. It has long been known that polymer produced on these catalysts have a relatively wide distribution in chain lengths; values for the Poly-Dispersity Index (PDI) are generally well above 2, the value that would be expected if a single catalyst site type were active. These catalysts are conceptualised as containing a number of different active sites, which produce polymer with different average molecular weights. No physical explanation for the existence of different site types has been offered. In fact it has been shown that there is a close link between activity and oxidation state, and that only a single type of polymerising site is present. In this work, we propose that sites differ not in propagation rate but in the rate at which chain growth is terminated by a range of terminating agents, including hydrogen, monomer and co-monomer. Each terminating agent is considered to act at a particular “pseudo-site”. The ability of this interpretation to simulate a wide range of polymer properties is demonstrated. The pseudo-sites model is then used to investigate experimentally-determined chain length distribution data for ethylene-1-butene copolymer produced in lab reactors. It is shown that this new view of the heterogeneity of ZN active sites can successfully reproduce key polymer properties, including PDI and mean chain lengths. Based on these results, it is proposed that the pseudo-sites model can be used to consolidate experimental data into a single set of parameters to describe the behaviour of a Ziegler-Natta catalyst for polyolefin production.

Two isotactic polypropylene (iPP) samples (PP-A and PP-B) were obtained by utilizing two different heterogeneous Ziegler-Natta catalysts in a given polymer- ization system. The molecular structure and conformational behavior of the samples were studied. The results of determi- nation of xylene soluble material (XS) and 13 C NMR showed that the average isotacticity of the samples were nearly same. However, the results of high-resolution high-temperature 13 C NMR (HRHT 13 C NMR) and successive self-nucleation and annealing (SSA) fractionation revealed that the amount of high isotacticity of PP-B was lower than that of PP-A, and the amount of relative medium and low isotacticity of PP-B was higher than PP-A, indicating that the stereodefect distri- bution of PP-B was more uniform than PP-A. The calculation of average meso sequence length from SSA was found to be in good agreement with that calculated from the results of HRHT 13 C NMR. Moreover, Fourier transformation infrared was utilized to study the influence of stereodefect distribu- tion on the conformational behavior of iPP. The result sug- gested that the molecular conformation of the PP-B was more disordered than PP-A, the regularity of molecular structure for PP-B was lower than that of PP-A. The related action mechanism and the influences of which on crystalliza- tion behavior were discussed. V C 2012 Wiley Periodicals, Inc. J Appl Polym Sci 125: 3076-3083, 2012

The influence of the structure of external alkoxysilane donors on stereoslectivity and molecular weight distributions in MgCl2-supported Ziegler–Natta catalysis has been examined. We shall demonstr...

Silica-supported titanium(IV) chloride is readily reduced by Mashima and co-workers' reagent (1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene) to afford materials active in ethylene polymerisation without need of aluminum alkyl cocatalyst.

Several organohalide (HC) compounds were tested as promoters for a Ti-based Ziegler-Natta (Z- N) catalyst at different polymerization conditions. Results show that the intensity of the promoting effect depends on the nature and amount of the promoters. A proper amount, especially optimum amount, of aliphatic type organohalides leads to a strong productivity, and aromatic ones leads to a weak productivity improvement; however, 3-chloro-1-propene poison the catalyst even at lower HC/ Ti molar ratios. Among studied compounds, chlorocyclo- hexane has the best activity promotion effect at HC/Ti molar ratio of 128, and with this as a promoter, the activ- ity increases over 85%. To understand more details about this phenomenon, the rate of polymerization during time and the effect of polymerization conditions (the tempera- ture and hydrogen partial pressure) on the performance of chlorocyclohexane as the most effective promoter were studied. Finally, to explore the mechanism of reactivation of catalyst species by organohalides, molecular modeling was employed and a new oxidation-addition mechanism was proposed, which basically consisting of homolytic breaking of CACl bond in organohalides. It was found that reoxidation of the catalyst, restoring active center, by Cl rich organohalides is energetically more favored. V C 2011

E-mail:lijw@mail.buct.edu.cn(Received May 10, 2011; Revised June 13, 2011; Accepted June 22, 2011)요약. 지글러-나타 촉매를 사용하여 에틸렌을 중합함으로써 다양한 공단량체의 종류와 조성을 갖는 이중 분자량 분포의 고밀도 폴리에틸렌이 합성되었다. 이들의 구조와 물성을 GPC, NMR, DSC, 인장 측정기를 이용하여 연구하였다. 에틸렌/1-헥센 공중합체가 에틸렌/1-부텐 공중합체보다 비슷한 조성을 가질 경우 높은 인장강도와 파단연신율을 가짐을 확인하였다. 분자량은 고분자의 공단량체 비율이 증가할수록 감소하였다. 짧은 곁사슬은 결정화도에 영향을 주어 결과적으로이중 분자량 분포를 갖는 고밀도 폴리에틸렌의 모폴로지와 기계적 물성에 영향을 미쳤다. SSA로 처리 후 다수의 발열 곡선이 관찰되었으며 이는 주로 에틸렌 배열길이와 라멜라 두께의 불균일성에 기인한다. 분포지수의 차이로부터 공단량체의 조성이 높은 폴리에틸렌의 SCB 분포가 균일도를 향상시킴을 알 수 있었다.주제어: 이중 분자량 분포 고밀도 폴리에틸렌, 짧은 곁사슬, 구조, 기계적 물성ABSTRACT. Bimodal high-density polyethylenes with different comonomer type and content were synthesized by poly-merization of ethylene using Ziegler-Natta catalyst. Their structure and properties were studied using GPC, NMR, DSC andtensile test. It was found that ethylene/1-hexene copolymer exhibits higher tensile strength and elongation at break than thatof ethylene/ 1-butylene copolymer with similar comonomer content. The molecular weight decreases as the comonomer con-tent of the polymer increases. Short chain branching affects the crystallinity and thus the morphology and consequently themechanical properties of the corresponding bimodal high-density polyethylenes. After SSA treated, the multiple endothermicpeaks were observed. Multiple endothermic peaks are mainly attributed to the heterogeneity of ethylene sequence length andlamellar thickness. The difference of broadness index indicates that SCB distribution of polyethylene containing highercomonomer content has improved uniformity.Keywords: Bimodal high-density polyethylene, Short chain branching, Structure, Mechanical Properties

In this work, we report on the structure and formation energy of uncovered MgCl2 crystallites of different shapes (hexagonal and square), sizes (up to crystallites composed of 157 MgCl2 units), and edges (crystallites presenting the (104) and (110) edges). Both uncovered crystallites and crystallites covered by dimethyl ether were considered. Our results indicate that the formation energy of uncovered crystallites, irrespective of shape, size, and edges, linearly depends on the density of vacancies (measured as the ratio between the number of Mg vacancies and the number of MgCl2 units in the crystallite) and that larger crystallites that present (104) edges are favored. In the case of crystallites completely covered by dimethyl ether, our results indicate that the formation energy of crystallites, again irrespective of shape, size, and edges, inversely depends on the dimethyl ether/Mg ratio. As opposed to uncovered crystallites, in the presence of dimethyl ether, smaller crystallites presenting (110) edge...

The mechanical and rheological properties of ethylene α-olefin copolymers are governed by a molecular and supramolecular structure that depends on its molecular weight characteristics, comonomer concentration, and the uniformity of branching distributions for polymer chains with different molecular weights (compositional heterogeneity). Using state-of-the-art methods, we study the parameters of molecular weight distribution, molecular structure, and compositional heterogeneity of ethylene hexene-1 copolymers with different compositions, produced over highly active supported Ti-Mg and V-Mg Ziegler-Natta catalysts, and show the possibility of controlling these parameters to improve the quality of the resulting polymers.

This communication reports a strategy for scale-up of an in situ polymerization technique for polyolefin-based nanocomposites preparation, taking layered silicate (clay) and multi-walled carbon nanotubes (MWCNTs) as examples of nanofillers. The strategy is realized by transforming the nanofillers into granular "nanosupports" for Ziegler-Natta catalysts. With a catalyst to polymer replication effect on particle morphology, the in situ prepared nanocomposites are of controlled granular particle morphology. With the polymer particle morphology controlled, the in situ polymerization technique becomes suitable for industrial olefin polymerization processes for mass production of polyolefin nanocomposites.