Search Results (88)

Maintaining a stable melt flow index (MFI) is a critical objective in industrial polypropylene production because MFI directly reflects polymer molecular weight and strongly influences downstream processing performance. Although the effects of catalyst formulation and hydrogen concentration on polypropylene properties are well established, the operational origins of residual fluctuations in MFI under highly stable industrial conditions remain poorly understood. In this work, the relationships between feedstock quality, process operation, and residual MFI variability were investigated during the production of a commercial polypropylene grade in an industrial gas-phase reactor. A dataset comprising 61 industrial observations was assembled by integrating laboratory quality measurements with operational variables related to hydrogen concentration, catalyst management, reactor hydrodynamics, thermal behavior, productivity, and fouling. In parallel, the concentrations of key catalyst inhibitors, including carbon oxides, sulfur compounds, water, oxygen, acetylene, methylacetylene, propadiene, arsine, and phosphine, were quantified before and after the use of a modified zeolite-based purification system. The purification process reduced catalyst poisons to ppb levels, producing polymer-grade propylene with monomer purity exceeding 99.95 wt.%. Under these highly controlled conditions, the production campaign exhibited remarkable quality stability, with an average MFI of 3.03 g/10 min and a coefficient of variation of only 6.63%. Principal component analysis revealed that two dominant operational domains could describe 86.49% of the total process variability. The first domain was associated with reactor hydrodynamics, fouling behavior, and thermal conditions, whereas the second domain was governed by catalyst-system variables and hydrogen-mediated chain-transfer mechanisms. Variable importance in projection analysis identified Plate Fouling Factor (VIP = 2.17), Production Rate (VIP = 1.33), and H₂/C₃ Ratio (VIP = 1.17) as the variables most strongly associated with residual MFI fluctuations. The results demonstrate that once feedstock-related disturbances are effectively minimized, residual MFI variability arises from interactions among the hydrodynamic, thermal, and catalytic operational domains rather than from a single controlling parameter. These findings provide new insights into process–quality relationships in industrial polypropylene manufacturing and establish a practical framework for identifying the operational origins of subtle fluctuations in polymer quality in highly stabilized production systems. Full article

This study examines the influence of the chemical nature and molecular structure of external electron-donor compounds on the slurry synthesis of isotactic polypropylene in a hydrocarbon diluent using a titanium–magnesium Ziegler–Natta catalyst. Propylene polymerization was carried out at constant temperature, pressure, hydrogen concentration, and fixed molar ratios of the catalyst system components. Alkoxysilanes with different structures were used as external donors: dicyclopentyldimethoxysilane, cyclohexylmethyldimethoxysilane, diisobutyldimethoxysilane, and diethylaminotriethoxysilane; di-n-butyl phthalate was used as a reference compound. It was shown that external donors decrease catalytic activity relative to the donor-free system but increase stereospecificity, as indicated by a lower xylene-soluble fraction and a higher isotactic index of polypropylene. Dicyclopentyldimethoxysilane demonstrated the strongest stereoregulating effect, providing the lowest content of xylene-soluble polymer. The donor structure significantly affected the molecular weight, rheological, and thermal characteristics of polypropylene, including melt flow rate, viscosity-average molecular weight, melting temperature, melting enthalpy, and crystallinity. Comparison with literature data for catalyst systems differing in internal and external donors, as well as in synthesis conditions, showed that catalyst activity and the stereospecificity of the catalyst system may be determined not only by process parameters but also by the electron-donor environment of the active sites. The results support established concepts of external-donor action in Ziegler–Natta catalysis and provide a comparative assessment of alkoxysilane donors and di-n-butyl phthalate under slurry polymerization conditions. Full article

Ziegler-Natta (Z-N) catalysts for propylene polymerization were prepared in situ using dibutyl phthalate (DNBP) or 9,9-bis(methoxymethyl)fluorene (BMMF) as internal electron donors (IDs) by treating the support precursors (Mg(OEt)₂ or MgCl₂·2.5EtOH) or MgCl₂ complex solutions with TiCl₄ respectively. In this study, eight Z-N catalysts containing two types of IDs were prepared via different preparation routes and systematically characterized with modern analytical techniques. The results indicated that, even with the same IDs, the catalysts prepared by different methods exhibited significant differences in chemical composition, particle size distribution, catalytic activity and stereoselectivity. The properties of polypropylene (PP) were largely influenced by the preparation route of the catalysts. Particularly, the catalysts obtained by the reprecipitation method showed the highest catalytic activity and the smallest MgCl₂ particle size. The distribution of stereoselective active centers in the catalysts was simultaneously affected by the preparation method and the type of IDs. In addition, the melting point (T_m) of PP could be used as an effective indicator to evaluate the relative content of the highly isotactic active centers in the catalysts. This study provides valuable insights into the rational design of Z-N catalysts for propylene polymerization, highlighting the critical role of the preparation methodology in tailoring the catalyst properties and active center distribution. Full article

A ligand-free Ni(acac)₂/EASC Ziegler–Natta catalytic system was developed for the efficient synthesis of low molecular weight liquid polybutadiene (LPB) featuring high 1,4 content. The influences of key polymerization parameters, including Al/Ni ratio, polymerization temperature, monomer-to-catalyst ratio ([Bd]/[Ni]), and external donors, were systematically investigated to elucidate structure–reactivity relationships. Increasing the Al/Ni ratio significantly enhances catalytic activity while promoting chain transfer reactions, leading to reduced molecular weights and broader molecular weight distributions, with minimal impact on overall 1,4 selectivity. Polymerization temperature strongly affects both activity and stereoselectivity; elevated temperatures accelerate chain transfer processes and broaden dispersity, while inducing a shift from kinetically favored cis-1,4 insertion toward increased trans-1,4 incorporation. Variation of the [Bd]/[Ni] ratio provides an effective handle for molecular weight regulation, where higher ratios favor chain propagation over chain transfer, affording higher molecular weights but lower monomer conversion. Notably, the system maintains consistently high 1,4 content (>98%) across a wide range of conditions. In contrast, the introduction of external donors markedly affects catalytic behavior depending on their coordination ability. Strongly coordinating O- and S-containing donors partially deactivate the catalyst and significantly shift regioselectivity toward 1,2-vinyl incorporation (up to ~20%), while N- and P-containing donors are well tolerated and can increase molecular weight by suppressing chain transfer pathways, which also results in products with higher 1,2 content. Full article

In this contribution, two silsesquioxane–cyclopentadienyl titanium complexes featuring one or two chloride ancillary ligands, [Ti(η⁵-C₅H₄SiMeO₂Ph₇Si₇O₁₀-κO)Cl₂] (1) and [Ti(η⁵-C₅H₄SiMe₂OPh₇Si₇O₁₁-κ²O₂)Cl] (2), were synthesized and evaluated in the Ziegler–Natta polymerization of styrene and the ring-opening polymerization (ROP) of L-lactide, respectively. Complex 1, activated with methylaluminoxane (MAO), catalyzed the syndiotactic polymerization of styrene with turnover frequencies up to 28 h⁻¹, affording polymers with narrow dispersity, low number-average molecular weights (M_n = 5.2–8.2 kDa), and high stereoregularity, as confirmed by ¹³C NMR. Complex 2, in combination with benzyl alcohol, promoted the ring-opening polymerization of L-lactide in solution at 100 °C, achieving conversions up to 95% with good molecular weight control (M_n close to theoretical, Đ = 1.19–1.32). Under melt conditions at 175 °C, it converted up to 3000 equiv. of monomer within 1 h. Kinetic analysis revealed first-order dependence on monomer concentration. The results highlight the ability of these complexes to produce syndiotactic polystyrene with narrow molecular weight distributions and to catalyze controlled ROP of L-lactide under both solution and melt conditions. Computational studies provided insight into key structural and energetic features influencing reactivity, offering a framework for further catalyst optimization. This work broadens the application scope of silsesquioxane–cyclopentadienyl titanium complexes and supports their potential as sustainable and versatile catalysts for both commodity and biodegradable polymer synthesis. Full article

An analysis was conducted to investigate how reaction system turbulence affects the butadiene-isoprene copolymerization in the presence of the TiCl₄ + Al(i-Bu)₃ catalytic system. A model was developed, which integrates CFD simulations of TiCl₄ + Al(i-Bu)₃ particle breakage based on population balance equations with the kinetic modeling of the butadiene-isoprene copolymerization. It was established that an increase in turbulent kinetic energy leads to a reduction in catalyst particle size, an increase in active site concentration, an acceleration of the copolymerization process, and a decrease in the average molecular weights of the copolymer. Furthermore, catalytic activity correlates with both the average and maximum values of turbulent kinetic energy in the reaction system, whereas the effect of the average residence time of catalytic particles under turbulent conditions is insignificant. Based on these results, recommendations were provided for optimizing the impact of reaction system turbulence on TiCl₄ + Al(i-Bu)₃ particles to enhance the butadiene-isoprene copolymerization rate and achieve precise control over the molecular weight characteristics of the copolymer. The findings of this study can be applied to optimize the synthesis technology of the cis-1,4 butadiene-isoprene copolymer, which is used in the production of frost-resistant rubber. Full article

The coordination chemistry of benzimidazole ligands combines σ donation and π backbonding. Owing to this electronic flexibility, benzimidazole ligands stabilize both electron deficient and electron-rich transition states in the catalytic cycle of Ziegler-Natta polymerizations. In this study, Fe(III) and Ni(II) complexes of 2-substituted-benzimidazoles were tested as catalysts for ethylene and 1-butene oligomerization. The tests realized in toluene yielded mainly butenes and minor amounts of hexenes. When dichloromethane was used as solvent, a tandem reaction took place and 1-butene produced by ethylene dimerization was further oligomerized, yielding octenes and dodecenes as main products. All tested catalysts exhibited moderate selectivity for 1-octene, indicating 1-ω enchainment in 1-butene dimerization. Beyond catalytic tests, a theoretical study of the ligand 2,2′-(furan-2,5-diyl)bis(1H-benzimidazole) confirmed the planar structure of this compound as evidenced by NMR spectroscopy. Full article

This study reports the synthesis of high cis-1,4 polybutadiene with a narrow molecular weight distribution (Đ < 2.0) by coordinative chain transfer polymerization (CCTP) using a homogeneous ternary NdV₃/diisobutyl aluminum hydride (DIBAH)/dimethyldichlorosilane (Me₂SiCl₂) Ziegler–Natta catalyst system. The polymerization parameters, notably the monomer-to-initiator ratio ([M]/[Nd]) and the halogen-to-initiator ratio ([Cl]/[Nd]), were systematically varied to define the CCTP operational window. It was found that CCTP conditions are achieved only at low [M]/[Nd] ratios (<2500) and intermediate [Cl]/[Nd] ratios between 1.0 and 2.0, facilitating the production of polymers with molecular weights below 32 kDa and narrow dispersity. Increasing these ratios beyond these thresholds potentially induces the formation of insoluble, hyper-halogenated catalytic species and increases medium viscosity, which significantly broadens the molecular weight distribution (Đ > 4.0) and impairs CCTP control. These findings challenge previous assumptions that higher halogen concentrations are necessary for CCTP, thereby providing important mechanistic insights for tuning active species and achieving improved polymer architecture. The work demonstrates a viable pathway to control polymer microstructure and molecular weight in neodymium-based CCTP, which is critical for design of high-performance elastomeric materials. Full article

In this work, a theoretical study of the isoprene polymerization process was carried out using a cis-stereospecific neodymium Ziegler–Natta catalyst. The results obtained allowed us to conclude that the main reasons for the formation of highly stereoregular cis-1,4-polyisoprene are the lower activation energy of the monomer addition process in the cis-conformation (~50 kJ/mol) and the low probability of anti-syn-isomerization of the terminal unit of the growing macrochain. It was shown that for asymmetric isoprene, the most probable is the formation of macromolecules in which the monomer units are added according to the “tail-to-head” type. Full article

Linear Low-Density Polyethylene (LLDPE) is a versatile polyolefin made by copolymerizing ethene with minor amounts of a 1-alkene. The short side chain branches in the comonomer units partly hinder the ability of the polyethylene main chain to crystallize, thus providing a way to fine-tune material properties between the extremes of a thermoplastic and a moderate elastomer. In this function, higher 1-alkenes such as 1-hexene or 1-octene are more effective than shorter homologs like propene or 1-butene, because their alkyl substituents are fully incompatible with the polyethylene lattice. On the other hand, the former comonomers are also more expensive and, above all, poorly reactive with heterogeneous Ziegler–Natta (ZN) catalysts, the workhorses of the polyolefin industry; as a matter of fact, they can only be used with technologically more demanding molecular catalysts. The molecular kinetic factors governing this important and complicated catalytic reactivity are still poorly understood, and perusal of the literature led us to conclude that data reliability is often questionable due to experimental limitations in reaction equipment and protocols, particularly in academic laboratories. In this study, we made use of a state-of-the-art High-Throughput Experimentation workflow to measure the reactivity ratios with ethene of two representative higher 1-alkenes, namely 1-hexene and 1-decene, in the presence of a variety of well-defined molecular catalysts of metallocene and post-metallocene nature comparatively with a typical MgCl₂/TiCl₄ ZN catalyst for polyethylene application. We found that the two comonomers react almost identically with molecular catalysts, whereas a major decrease in reactivity for 1-decene compared with 1-hexene was observed idiosyncratically for the ZN catalyst. In our opinion, the overall results suggest that in the latter case, surface effects can be dominant over direct comonomer interactions with the coordination sphere of the active metal in dictating the observed molecular kinetic behavior. Full article

Divinylisoprene rubber, a copolymer of butadiene and isoprene, is used as raw material for rubber technical products, combining isoprene rubber’s elasticity and butadiene rubber’s wear resistance. These properties depend quantitatively on the copolymer composition, which depends on the kinetics of its synthesis. This work aims to theoretically describe how the monomer mixture composition in the butadiene–isoprene copolymerization affects the activity of the TiCl₄-Al(i-C₄H₉)₃ catalytic system (expressed by active sites concentration) via kinetic modeling. This enables development of a reliable kinetic model for divinylisoprene rubber synthesis, predicting reaction rate, molecular weight, and composition, applicable to reactor design and process intensification. Active sites concentrations were calculated from experimental copolymerization rates and known chain propagation constants for various monomer compositions. Kinetic equations for active sites formation were based on mass-action law and Langmuir monomolecular adsorption theory. An analytical equation relating active sites concentration to monomer composition was derived, analyzed, and optimized with experimental data. The results show that monomer composition’s influence on active sites concentration is well described by a two-step kinetic model (physical adsorption followed by Ti–C bond formation), accounting for competitive adsorption: isoprene adsorbs more readily, while butadiene forms more stable active sites. Full article

Ultra-high-molecular-weight polyethylene (UHMWPE) is a pivotal material in engineering and biomedical applications due to its exceptional mechanical strength, wear resistance, and impact performance. However, its extreme melt viscosity, caused by extensive chain entanglements, severely limits processability via conventional melt-processing techniques. Recent advances in catalytic synthesis have enabled the production of disentangled UHMWPE (dis-UHMWPE), which exhibits enhanced processability while retaining superior mechanical properties. Notably, heterogeneous catalytic systems, utilizing supported fluorinated bis (phenoxy-imine) titanium (FI) catalysts, polyhedral oligomeric silsesquioxanes (POSS)-modified Z-N catalysts, and other novel catalysts, have emerged as promising solutions, combining structural control with industrial feasibility. Moreover, optimizing polymerization conditions further enhances chain disentanglement while maintaining ultra-high molecular weights. These systems utilize nanoscale supports and ligand engineering to spatially isolate active sites, tailor the chain propagation/crystallization kinetics, and suppress interchain entanglement during polymerization. Furthermore, characterization techniques such as melt rheology and differential scanning calorimetry (DSC) provide critical insights into chain entanglement, revealing distinct reorganization kinetics and bimodal melting behavior in dis-UHMWPE. This development of hybrid catalytic systems opens up new avenues for solid-state processing and industrial-scale production. This review highlights recent advances concerning interaction between catalyst design, polymerization control, and material performance, ultimately unlocking the full potential of UHMWPE for next-generation applications. Full article

In this study, density functional theory (DFT) was used to analyze the processes that govern the interactions among triethylaluminum (TEAL), the Ziegler–Natta (ZN) catalyst, and the inhibitory compounds dimethylamine (DMA) and diethylamine (DEA) during olefin polymerization. The structural and charge characteristics of these inhibitors were examined through steric maps and DFT calculations. Combined DFT calculations (D3-B3LYP/6-311++G(d,p)) and IR spectroscopic analysis show that the most efficient way to deactivate the ZN catalyst is via the initial formation of the TEAL·DMA complex. This step has a kinetic barrier of only 27 kcal mol⁻¹ and a negative ΔG, in stark contrast to the >120 kcal mol⁻¹ required to form TEAL·DEA. Once generated, TEAL·DMA adsorbs onto the TiCl₄/MgCl₂ cluster with adsorption energies of −22.9 kcal mol⁻¹ in the gas phase and −25.4 kcal mol⁻¹ in n-hexane (SMD model), values 5–10 kcal mol⁻¹ more favorable than those for TEAL·DEA. This explains why, although dimethylamine is present at only 140 ppm, its impact on productivity (−19.6%) is practically identical to that produced by 170 ppm of diethylamine (−20%). The persistence of the ν(Al–N) band at ~615 cm⁻¹, along with a >30% decrease in the Al–C/Ti–C bands between 500 and 900 cm⁻¹, the downward shift of the N–H stretch from ~3300 to 3200 cm⁻¹, and the +15 cm⁻¹ increase in ν(C–N) confirm Al←N coordination and blockage of alkyl transfer, establishing the TEAL·DMA → ZN pathway as the dominant catalytic poisoning mechanism. Full article

Polybutadiene (PB) and polyisoprene (PI) rubbers are indispensable synthetic elastomeric materials widely used in tires, footwear, hose, belts, sealants, electricity, construction, and other applications. Nowadays, PB and PI elastomers are produced from butadiene (BD) and isoprene (IP) monomers via transition-metal-mediated coordination polymerization. Transition metal catalytic systems consist of a precise characteristic structural unit at the molecular level: well known as “single-site catalysts” (SSCs). These have experienced a revolutionary advance in the recently developed conjugated dienes synthetic rubber method. Among the SSCs, a class of rare-earth, metal-centered half-sandwich molecule has been identified as a high-performance catalytic system for conjugated dienes polymerization. These novel half-sandwich rare-earth (HSRE) catalytic systems exhibit several irreplaceable advantages compared with the conventional Ziegler–Natta-type catalytic systems. These HSRE catalytic systems can create novel conjugated diene rubbers (CDRs) with high catalytic reactivity, high stereoselectivity, an adjustable polymer chain microstructure, and high molecular weights and are considered to be the next generation of ecofriendly and economic catalytic systems for industrial applications. This paper delivers a concise review of some important synthetic methods for representative HSRE complexes with characteristic structures and of the utilization of some HSRE catalytic systems for the preparation of high-performance CDRs, especially highly stereoregular PI and PB materials. Full article

Organic electron donors are essential components of Ziegler-Natta (ZN) catalysts to produce isotactic polypropylene. In particular, aromatic or aliphatic diesters are widely used as ‘Internal Donors’ (ID) in MgCl₂/ID/TiCl₄ precatalyst formulations. Diesters are reactive with AlEt₃ (by far the most common ZN precatalyst activator) and are partly removed from the solid phase in the early stages of the polymerization process; this is detrimental for catalyst functioning, and a surrogate donor (‘External Donor’ (ED), usually an alkoxysilane) is added to the system to restore performance. Recent studies, however, demonstrated that even in cases where most of the diester is extracted by AlEt₃, the active sites retain a ‘memory’ of it in several aspects of the catalytic behavior (such as, e.g., the average productivity and the polydispersity index of the polymer produced). Considering that the residual diester is always in molar excess with respect to the active Ti, one may speculate that long-lasting interactions between the latter and diester molecules can occur. In turn, this should imply that the reactivity of AlEt₃ is different with binary MgCl₂/ID or ternary MgCl₂/ID/TiCl₄ mixtures. In this work, the latter hypothesis was explored for a library of diester IDs with large structural diversity. In line with the anticipation, the fractional amount of ID extracted by AlEt₃ was generally lower for ternary mixtures, although to an extent exquisitely dependent on diester structure. Full article