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Although Ziegler-Natta catalysts were discovered more than four decades ago, many aspects of their characteristics are not fully understood. The effect of the support particle size on the final pro...

Many researchers have studied the effects of donors, promoters and alkyl aluminum compounds on the polymerization catalysts, the polymerization process and the final properties of polymers. Neverth...

The scientific and technical advances in the field of polymer science has been abundant in recent years. Amongst the various polymeric materials available in market, synthesis of polyolefins has been in the forefront since decades. A major challenge in this domain remains in attaining stereoregular polyolefins especially polypropylene (PP) and significant efforts were carried out by synthesizing various internal donors (ID) aiding the catalysts involved in producing them. This short review gives an overview of i) various generations of Ziegler–Natta (ZN) catalyst systems ii) general classes of ID that has been demonstrated by the researchers over the past decades iii) their influence on PP isotacticity and polymer properties. The coordination modes of different donor classes on supported ZN system and comparative study especially between phthalate and diether ID classes were also addressed here. This review also presents the studies carried out on phthalate catalyst structure analysis, detailed comparison study on phthalate and diether IDs in terms of PP isotacticity, regioselectivity, hydrogen response, and also their cross combination study and competitive behavior. Further a brief description on other structurally varied IDs like malonates, maleates, silyl diol esters, bifunctional donors, multi ether donors demonstrated for isotactic PP were also presented. Studies conducted on compatibility of incorporation of two different classes of IDs on a single supported ZN system for the fundamental understanding of the catalyst behavior; and also on how mixed donor approach enables in tuning the catalyst for polymer properties were also presented. This review also provides an opportunity to the young minds and the basic researchers from academic point of view by and large to create new polymeric materials with useful properties or modify the existing materials for new applications by incorporating new IDs for further improvisation of the stereo regularity in obtaining the polymers. Graphic Abstract

Chain entanglement was very important for adjusting the processability and mechanical property of nascent ultrahigh molecular weight polyethylene (UHMWPE). So far, it is still a mystery to unravel ...

Bromoalkyl-functionalized poly(olefin)s were synthesized by copolymerization of 4-(4-methylphenyl)-1-butene with 11-bromo-1-undecene using Ziegler–Natta polymerization. The resulting bromoalkyl-fun...

The innovation of Ziegler-Natta (Z-N) catalysts opened a novel aspect of chemistry in terms of polyolefin formation. Significant progress of more than fifty years in catalytic olefin polymerization simplified polyolefin production by optimizing the reaction conditions and internal donors. In this review, we particularly emphasized on recent development on different internal donor based Z-N catalyst systems, the role of external electron donors and co-catalyst chemistry. Furthermore, the Ziegler-Natta catalyst system, polymerization reaction, and polymer process technologies gradually upgrading to produce different polyolefin products. In particular, polyolefin fibers, composites, foam, blends and polyolefin-based 3D-printing products targeted in this review for substantial benefits of both society and highly diversified industries.

In this contribution, novel bio-derived rosin acid ester compounds have been developed and used as internal electron donors to prepare MgCl2 supported titanium catalysts for propylene polymerization. The results of the polymerization showed that tri-n-amyl maleate, triiso-amyl maleate and tri-n-heptyl maleate as internal electron donors in Ziegler-Natta catalysts all have high polymerization activity and stereoregularity. In particular, the Ziegler-Natta catalyst with tri-n-amyl maleate as the internal electron donor has the highest activity (up to 33.4 kgPP·gCat−1·h−1) for propylene polymerization and the isotacticity of polypropylene is as high as 96.8% determined by 13C NMR. Simultaneously, the obtained polypropylene with a relatively broad molecular weight distribution of 9.0 and melt flow rate of 160.3 g/10 min, which is beneficial to processing and has practical industrial application value when used as the basic material of melt blown cloth for medical mask.

The majority of EPDM materials are produced by traditional Ziegler–Natta/Vanadium catalyst and process technology. In recent decades, EPDM metallocene catalyst technologies have increased the effic...

It was well known that the defects in the polymer chains could significantly influence the crystallization process of semi-crystalline polymers. In this work, the role of defect distribution in the melt memory effects was studied by comparatively investigating a pair of Ziegler-Natta and metallocene-made isotactic polypropylene (ZN-iPP and M-iPP) fraction specimens with the same total defect content which were obtained from two raw parent samples through the solvent extraction. The characterization results of melting behaviors, polymorphic structures, and the successive self-nucleation and annealing analysis demonstrated that these two fractions exhibited the different distribution of defects, i.e., the defect distribution in the M-iPP specimen was random while that in the ZN-iPP specimen was nonstatistical. The results of self-nucleation (SN) experiments indicated that the M-iPP exhibited much wider temperature range of Domain II than the ZN-iPP with similar total defect content, which indicated that the M-iPP showed a much stronger melt memory effect. Considering the effect of the microstructure on the crystallization behavior and the sequences containing defects could be excluded from the crystalline regions during the formation of crystal, we speculated that the random distributed defects in M-iPP could form a continuous interface between the crystalline regions and amorphous phase which could cause a heterogeneous melt and strengthen the melt memory effect.

Medium- and high-resolution SEM analysis of several Ti-based MgCl2-supported Ziegler–Natta catalysts and isotactic polypropylene produced with them is carried out. Each catalyst particle, 35–55 μ in size, produces one polymer particle with an average size of 1.5–2 mm, which replicates the shape of the catalyst particle. Polymer particles contain two distinct morphological features. The larger of them are globules with Dav ∼400 nm; from 1 to 2 × 1011 globules per particle. Each globule represents the combined polymer output of a single active center. The globules consist of ∼2500 microglobules with an average size of ∼20 nm. The microglobules contain several folded polymer molecules; they are the smallest thermodynamically stable macromolecular ensembles in propylene polymerization reactions. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3832–3841

Isotactic polypropylene (i-PP) is mainly produced with heterogeneous Ziegler-Natta (ZN) catalyst systems. Direct structural information on the active species is very difficult to achieve; as a matter of fact, the 13C NMR microstructure of the polymer has long been used as an indirect "fingerprint". For a long time, however, the intrinsically poor sensitivity of natural abundance 13C NMR spectroscopy hampered a complete evaluation of the microstructure, and many rare albeit important details escaped detection with standard tools and methods. The most prominent case is that of the regiodefects, which are specially important because they dictate ZN catalyst response to molecular hydrogen used as a chain transfer agent. In recent papers we have reported on the quantification of regioirregular 2,1 units in ZN i-PP by means of high-temperature cryoprobe 13C NMR. Here we shift the focus onto the stereochemical environment of such units, which was highlighted by comparing the spectra of suitable ZN PP fractions with those of model PP samples made with molecular catalysts of different structures and symmetries. The results lend support to our three-site model of ZN catalytic species, originally based on the stereochemistry of regioregular PP chains/blocks.

This study focuses on gas-phase polymerization of ethylene using the titanium-based Ziegler–Natta catalysts prepared from different magnesium sources including MgCl2 (Cat A), magnesium powder (Cat B), and Mg(OEt)2 (Cat C). During polymerization, different cocatalysts were also used. It was found that Cat C with triethylaluminum as a cocatalyst exhibited the highest activity. This was likely attributed to optimal distribution of active sites on the catalyst surface. It can be observed by increased temperature in the reactor due to highly exothermic reaction during polymerization. By the way, the morphologies of the polymer obtained from this catalyst were spherical, which is more preferable. Besides the catalytic activity, crystallinity and morphology were also affected by the different magnesium sources used to prepare the catalysts.

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.

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...

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.

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

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.

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.

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

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

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.

Polypropylene homopolymer samples were prepared with a Ziegler–Natta catalyst using two different external donors, namely diphenyldimethoxysilane (DPDMS), and methylphenyldimethoxysilane (MPDMS). Each donor was used in varying molar ratios to the catalyst in order to prepare samples for physical testing. The polymers were fully characterised and also fractionated by preparative TREF with characterisation of the fractions. In terms of the polymerisation reactions the DPDMS external donor exerts greater influence at the active sites than the MPDMS and produces polymer of higher molar mass and lower polydispersity. The physical properties of the polymers were investigated using microhardness measurements. It is revealed that the microhardness is strongly dependent on the stereoregularity of the samples.

Deconvolution of the MWD of a polymer produced by multi-site catalysts into independent Flory modes is the first step in modeling the polymerization process. A new deconvolution procedure for GPC data is developed that does not require an a priori assumption concerning the nature of the discrete distribution and can be used with a continuous distribution. The MWD measured via GPC is a linear function of the individual catalytic sites, but it is numerically ill-conditioned, preventing direct inversion of the GPC data. Tikhonov regularization has been developed to uniquely invert the MWD. Applying the regularizing method to a polyethylene produced via a Ziegler-Natta catalyst, seven discrete sites were found, and the kinetic constant ratios were determined for each of these sites.

Two types of industrial heterogeneous Ziegler-Natta propylene polymerization catalysts, TiCl3 type and MgCl2 supported TiCl4 type, were examined by paired interacting orbitals (PIO) analysis proposed by Fujimoto et al.. The ligand located trans to propylene was found to obstruct the insertion but was essential for the stereospecific insertion of propylene. The ligand located trans to the alkyl group of the active site was found to be necessary for activating the alkyl-Ti bond. The regioselectivity and stereoselectivity of the iso-specific polymerization site on the heterogeneous Ziegler-Natta catalyst system were confirmed. The polymerization activity of MgCl2 supported TiCl4 catalyst was larger than that of the TiCl3 catalyst.

The initial stages of gas-phase polymerizations of propylene and ethylene are analyzed using a fixed bed stopped flow reactor. The very early development of particle morphology and polymer properties is analyzed for three different commercial catalyst systems: MgCl2- and SiO2-supported Ziegler–Natta and SiO2-supported metallocene. It is shown that, depending on the operating conditions, distinct nonuniform catalyst fragmentation patterns can develop, confirming different scenarios described by published fragmentation models. In addition, it is shown that the molecular weight distributions and polymer yields obtained during the very early stages of the polymerization suggest the existence of significant temperature gradients inside the growing polymer particles. Finally, it is shown that the ratio of catalyst to glass beads in the bed can have a pronounced effect on the evolution of the polymerization reaction. This can be interpreted in terms of the significant temperature difference between the polymer particles and the gaseous monomer stream. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers

The deactivation behaviors of TiCl3/MgCl2 model catalysts with molecularly dispersed TiCl3 were investigated to clarify the role of the dispersion state of the Ti species in the deactivation of MgCl2-supported Ziegler-Natta (ZN) catalysts for propylene and ethylene polymerization. The propylene and ethylene polymerization activities of clustered Ti species supported on MgCl2 were approximately one tenth of those of isolated Ti species, which is indicative of the significance of the aggregation of Ti species in the deactivation of ZN catalysts for olefin polymerization. Moreover, the time-course depression of monomer consumption for the clustered Ti species was slower than that for the isolated Ti species in propylene polymerization, whereas the dispersion state barely affected the depression rate in ethylene polymerization. The reduction rate of the Ti species was concluded to be important for the time-course depression of propylene consumption, whereas ethylene polymerization was insensitive to the oxidation state of the Ti species.

A novel method based on a reaction between the growing polymer chain and cinnamoyl chloride (CC) was developed to determine the number of active centers (C p ) in olefin polymerization with supported Ziegler-Natta catalysts. The reaction of titanium-polymeryl bonds in the active centers of MgCl2/ID/TiCl4-AlR3 type catalysts with CC was very fast at 20∼80 °C. Cinnamoyl labeled polymer chains were formed, which were stable in the reaction system for at least 10 min when the CC/Al molar ratio was > 1. The number of active centers was determined by measuring the number of carbonyl groups in the polymer with the UV-vis method. The C p values of the 1-hexene polymerization systems with the addition of hydrogen were also measured.

Heterogeneous Ziegler-Natta precatalysts (with phthalate as internal donor) were modified by treatments with various Lewis acids (MCln with M = Ga, Sn, Si, and Sb and n = 3, 4, or 5) before their use in the polymerization of propylene. If performed on previously “detitanated” precatalysts, treatments with SnCl4 and SiCl4 lead to a slight activation but especially to an increase of the tacticity whereas GaCl3 and SbCl5 treatments deactivate the catalyst. The modification method applied to conventional unmodified precatalysts gave similar trends. A significant increase of tacticity (and/or of Tm) and a narrowing of the molecular weight distribution were observed in the case of SnCl4 and SiCl4 treatments. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2631–2635, 2010

National Natural Science Foundation of China [20406016]; Fujian Petrochemical Company of SINOPEC

Experimental data on the molecular weight distribution (MWD) of polyethylene (PE) produced over a broad number of Ziegler-Natta catalysts differing in composition and preparation procedure are presented. These catalysts include nonsupported TiCl3 catalyst, four types of supported titanium-magnesium catalysts (TMC) differing in the content of titanium and the presence of various modifiers in the composition of the support, and a supported catalyst containing VCl4 as an active component instead of TiCl4. The studied catalysts produce PE with different molecular weights within a broad range of polydispersity (Mw/Mn = 2.8–16) under the same polymerization conditions. The heterogeneity of active sites of these catalysts was studied by deconvolution of experimental MWD curves into Flory components assuming a correlation between the number of Flory components and the number of active site types. Five Flory components were found for PE produced over nonsupported TiCl3 catalysts (Mw/Mn = 6.8), and three–four Flory components were found for PE produced over TMC of different composition. A minimal number of Flory components (three) was found for PE samples (Mw/Mn values from 2.8 to 3.3) produced over TMC with a very low titanium content (0.07 wt %) and TMC modified with dibutylphtalate. It was shown that five Flory components are sufficient to fit the experimental MWD curve for bimodal PE (Mw/Mn = 16) produced over VMC. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

The dispersion state of surface Ti species is one of the most important factors for polymerization properties of MgCl_2-supported heterogeneous Ziegler-Natta catalysts. In this paper, ethylene-propylene copolymerization was conducted using a novel Ziegler-Natta model catalyst having “isolated” and “clustered” TiCl_3 molecules on MgCl_2. At a lower ethylene/propylene feed ratio, the dispersion state of the Ti species was found to stronglyaffect both the composition and sequence distribution of copolymers. The “isolated” Tispecies gave a relatively random copolymer and clustering of Ti species obviously enhanced the blockiness. The addition of an external donor basically deteriorated the random copolymerization character of the “isolated Ti species, but the degree of the deterioration depended on the kind of donors. On the contrary, the balance between themonomer feed and consumption dominated the copolymerization performance at a higher ethylene/propylene feed ratio to obscure the effects of the dispersion state.

The unprecedented ability of neodymium-based metal organic frameworks (MOFs) as polymerisation pre-catalysts towards isoprene is reported. Combined with methylaluminoxane (MAO) or modified MAO (MMAO), they afford mainly cis-selective polyisoprene, up to 90.7%. Both the activity and the selectivity are tentatively ascribed to the intrinsic microstructure of the starting materials. Compared to conventional carboxylates, MOFs associated to an Al co-catalyst are less active but the selectivity is found to be higher, and it may be modified by controlling the access to the pores, which would be favored at higher temperatures. Some residual crystalline MOF remains disseminated within the polymer matrix, as shown by X-ray diffraction and X-ray absorption spectroscopy studies.

In this work, an alternative formulation of the Population Balance Model (PBM) is proposed to simplify the mathematical structure of the reactor model. The method is based on the segregation approach applied to the recently developed unsteady state Residence Time Distribution (RTD). It is shown that the model can predict the performance of a reactor system under unsteady flow and composition conditions. Case studies involving time-varying catalyst flowrates, reactor temperature and reactor pressure were simulated and found to predict reactor performance with reasonable accuracy. The model was used to propose a grade transition strategy that could reduce transition time by as much as two hours.

Effects of external donor induced direct contact without alkylaluminum were studied for Ziegler-Natta catalysts containing monoester and diester as internal donors on the catalytic performances such as activity and stereoselectivity. Traditional slurry polymerizations were carried out for rather short period after direct contact for a variety of time. GC analysis was also performed to observe replacement phenomena of donors during direct contact. It was postulated that the different performances of the catalysts were greatly affected by the pair of internal and external donors.

The performance of MgCl 2 -supported Ziegler-Natta catalysts for propylene polymerization is highly dependent on the molecular structures of donors. Density functional calculations were conducted to examine the molecular behaviors of ethylbenzoate (EB), which coadsorbed with an active Ti species during the propylene insertion. It was found that the degree of the molecular flexibility of EB dominated the catalytic nature of the Ti species. The Ti species with geometrically fixed EB was highly isospecific but poorly active, while that with free EB was active but non-specific. An appropriate confinement of the flexibility was proposed to be essential for active and isospecific propylene insertion, which might be achieved by steric interference between EB molecules adsorbed close to each other on the MgCl 2 surface.

A systematic consideration of different Ti(IV) and Ti(III) species on the (104) and (110) MgCl2 surfaces has been implemented within DFT using cyclic boundary conditions. Some new mononuclear and dinuclear surface complexes of Ti(IV) and Ti(III) were obtained due to implication of zip coordination mode. A possible spin state of dinuclear Ti(III) species was thoroughly studied: antiferromagnetic (ESR silent) state proved to be the most preferable in a number of cases. The zip antiferromagnetic Ti2Cl6 complexes residing on the dominant (104) MgCl2 surface make it possible to rationalize the fact that the most part of Ti(III) incorporated in activated MgCl2 is ESR silent. Besides, these species produce aspecific active sites, thus explaining that aspecific centers significantly prevail over stereospecific one according to kinetic data on the simplest TiCl4/MgCl2 + AlR3 system.