The research, published in Polymer Bulletin by Arina S. Garkul, Artem A. Barabanov, Vladimir A. Zakharov and Mikhail A. Matsko, examines propylene–ethylene copolymerization over supported titanium–magnesium catalysts, the workhorse Ziegler–Natta systems that dominate global polypropylene production. These catalysts consist of titanium chloride species dispersed on a magnesium chloride support, but they also carry a fourth, less obvious component: an internal electron donor, an organic molecule such as a phthalate, a succinate or a diether that is incorporated into the catalyst during synthesis. The donor is not a passive filler. It coordinates to magnesium chloride surfaces, shapes the coordination geometry of titanium active sites and suppresses those sites that would otherwise produce non-stereoregular, atactic polymer. In homopolymerization, the influence of donors on isotacticity has been studied extensively; what this new work adds is a systematic picture of how donor chemistry affects the way ethylene comonomer is distributed along the polymer chains.

The team prepared random copolymers containing between four and five mole percent ethylene using three titanium–magnesium catalysts that differed only in the composition of their internal stereoregulating donors: di-n-butyl phthalate, an aromatic phthalate ester long used in industrial formulations; 2-isopropyl-2-isopentyl-1,3-dimethoxypropane, a representative of the diether class that replaced phthalates in many modern catalyst generations; and diethyl 2,3-diisobutyl succinate, an aliphatic succinate ester. Holding the overall comonomer content constant across the three systems was a deliberate design choice. It meant that any differences uncovered in the distribution of ethylene within the polymer chains, rather than the average amount incorporated, could be attributed to the donor chemistry itself, a distinction that bulk characterization alone would completely miss.

To expose those hidden differences, the researchers turned to preparative temperature rising elution fractionation, a powerful technique that separates a copolymer into fractions according to crystallizability, which in turn reflects both molecular weight and the local content and placement of comonomer units. From each copolymer, five fractions were isolated that differed systematically in molecular weight and isotacticity. For every fraction, the team determined the ethylene content, the weight-average molecular weight, the melting temperature and the degree of crystallinity. This fraction-by-fraction dissection converts what appears to be a single, uniform material into a layered portrait of molecular populations, each carrying its own record of which catalytic sites created it and under what stereochemical constraints they operated.

The headline finding is a striking asymmetry in how ethylene distributes across the molecular weight spectrum. The three fractions characterized by high molecular weight, with weight-average molecular weights between roughly 210 and 660 kilograms per mole, and by high isotacticity, showed ethylene contents that were not only mutually consistent within each copolymer but also essentially independent of which internal donor had been used. These high-molecular-weight, highly isotactic chains are the product of the most stereospecific active sites on the catalyst, and the study demonstrates that whatever donor governs the catalyst, those sites incorporate ethylene at a similar and relatively restrained rate. In other words, the most selective sites behave with a kind of chemical uniformity that transcends the specific donor molecule bound nearby.

The picture changes abruptly in the two lower-molecular-weight fractions, where the weight-average molecular weights fell between about 90 and 180 kilograms per mole. Here, the ethylene content increased sharply, indicating that the less stereospecific active sites responsible for these shorter chains incorporate ethylene far more readily than the highly isospecific sites. This correlation between lower stereospecificity, lower molecular weight and elevated comonomer incorporation has long been suspected from studies of Ziegler–Natta catalysts, but quantifying it for copolymers with matched overall composition, and showing where the boundary lies between donor-independent and donor-dependent behavior, gives the observation a new level of rigor and practical relevance.

Carbon-13 nuclear magnetic resonance spectroscopy then took the analysis to the level of monomer sequences, the finest structural resolution available for these materials. For the low-molecular-weight copolymer fractions, the researchers quantified the content of uninterrupted ethylene triads, denoted EEE segments, mixed ethylene–ethylene–propylene segments, denoted PEE, and isolated ethylene units flanked by propylene on both sides, denoted PEP. The choice of internal donor had little effect on these sequences for the high-molecular-weight, highly isotactic fractions, but exerted a noticeable and statistically meaningful influence on the proportions of EEE, PEE and PEP segments in the low-molecular-weight fractions. Some donors produced low-molecular-weight material in which ethylene units cluster into short runs, while others favored a more isolated, statistically random placement of ethylene within the propylene matrix.

The practical significance of this sequence-level control should not be underestimated. In random propylene–ethylene copolymers, the thermal and mechanical properties are governed less by the average ethylene content than by how the ethylene is distributed. Long ethylene sequences can act as defects that depress melting temperature and crystallinity and can even form separate polyethylene-like crystalline domains, whereas isolated ethylene units subtly disrupt the polypropylene crystal lattice and produce the clarity and toughness that designers of transparent containers and hot-fill packaging demand. A catalyst manufacturer who can tune the donor to control whether low-molecular-weight chains carry clustered or isolated ethylene units thereby gains a direct lever over haze, stiffness, impact strength and sealing behavior, all without changing the reactor feed composition.

The study also connects to a broader industrial and scientific conversation. Phthalate donors, once ubiquitous, have faced scrutiny over health and environmental concerns, driving the shift toward succinates and diethers in commercial catalysts, and recent high-throughput, data-driven donor screening programs have accelerated the search for next-generation donor molecules. The Novosibirsk results add an important constraint to that search: any candidate donor must not only deliver high activity and isotacticity in homopolymerization but must also be judged by how it reshapes the comonomer sequence distribution in the less stereospecific fraction of the copolymer. The work was carried out within the governmental assignment of the Russian Ministry of Science and Higher Education for the Boreskov Institute of Catalysis, and the authors acknowledge contributions from colleagues responsible for catalyst synthesis, molecular weight distribution analysis and NMR spectroscopy.

What emerges from this research is a refined mental model of the Ziegler–Natta catalyst as an ensemble of active sites with distinct personalities. The most isospecific, high-molecular-weight sites are remarkably consistent incorperating ethylene regardless of the internal donor, while the less stereospecific, lower-molecular-weight sites are exquisitely sensitive to the donor’s chemical identity, changing both how much ethylene they incorporate and how those units are arranged along the chain. For a polymer industry that increasingly demands precisely engineered materials rather than commodity plastics, this donor-level control of molecular microstructure represents exactly the kind of fundamental insight that translates into better films, tougher moldings and clearer packaging, all flowing from a molecule that most consumers will never know exists.

Subject of Research: Effect of internal electron donor composition on the molecular structure of propylene–ethylene copolymers made with supported titanium–magnesium Ziegler–Natta catalysts

Article Title: Propylene–ethylene copolymerization over titanium–magnesium catalysts: the effect of the composition of an internal stereoregulating electron donor on the molecular structure of the copolymer

Article References: Garkul, A. S., Barabanov, A. A., Zakharov, V. A., & Matsko, M. A. (2026). Propylene–ethylene copolymerization over titanium–magnesium catalysts: the effect of the composition of an internal stereoregulating electron donor on the molecular structure of the copolymer. Polymer Bulletin, 83(11), Article 634. https://doi.org/10.1007/s00289-026-06688-9

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06688-9

Keywords: titanium–magnesium catalysts, Ziegler–Natta catalysts, propylene–ethylene copolymerization, internal electron donor, stereoregulating donor, random copolymer, temperature rising elution fractionation, carbon-13 NMR, monomer sequence distribution, isotacticity, molecular structure, polypropylene

Ziegler–Natta catalysts have dominated olefin polymerization since the 1950s, when Karl Ziegler and Giulio Natta discovered that transition metal compounds activated by organoaluminium cocatalysts could assemble ethylene and propylene into useful polymers under mild conditions. Modern industrial versions are far more sophisticated than the original mixtures. Today’s catalysts consist of titanium tetrachloride deposited on high-surface-area magnesium chloride, often modified with internal electron donors and activated in the reactor with triethylaluminium. The support is not a passive scaffold. Its porosity, particle morphology, and chemical composition determine how the catalyst particle fragments during polymerization, how monomer diffuses to active sites, and ultimately whether the growing polymer replicates the shape of its parent particle or collapses into fines and fouling.

The Iranian team obtained three industrial catalysts and subjected them to a battery of standard characterization techniques. Chemical composition was quantified using inductively coupled plasma and energy-dispersive X-ray spectroscopy, revealing differences in titanium loading and the presence of elements such as silicon, iron, and oxygen that trace back to each catalyst’s synthesis route. Porosity was mapped through Brunauer–Emmett–Teller analysis, yielding surface area, total pore volume, and average pore diameter for each sample. Field-emission scanning electron microscopy captured the external morphology of the catalyst grains, while laser diffraction provided particle size distributions, summarized through the d10, d50, and d90 percentiles and the SPAN index, a measure of distribution breadth calculated as (d90 minus d10) divided by d50.

With the catalysts fully profiled, the researchers turned to ethylene polymerization, and here the study’s experimental design deserves particular attention. Comparing catalysts is deceptively difficult because activity and polymer properties are entangled with the amount of catalyst used. The team therefore ran polymerizations in two distinct modes. In the first, equal catalyst loadings were used for all three catalysts, allowing an apples-to-apples comparison of intrinsic catalytic performance, including activity profiles over time and responses to hydrogen, the chain transfer agent that controls molecular weight in industrial reactors. In the second mode, catalyst amounts were deliberately adjusted so that each run produced polyethylene of similar yield. This equal-yield strategy removes the confounding effect of different polymer masses and enables a far more reliable comparison of the resulting polymer properties.

The polyethylene products were then interrogated with a suite of analytical techniques that mirror quality-control practice in a real polyethylene plant. Melt flow index measurements captured the processability of each resin, while the flow rate ratio, the ratio of melt flow indices measured at two different loads, served as a proxy for molecular weight distribution width. Differential scanning calorimetry provided melting temperatures and crystallinity, density measurements reflected short-chain branching content, and particle size distribution analysis of the polymer powder revealed how faithfully each catalyst particle had replicated its morphology during growth. Together, these measurements allowed the team to quantify hydrogen response, comonomer response, and polymer morphology for each catalyst.

The results establish clear structure–performance relationships. Catalysts with higher titanium content and more accessible active sites showed distinct activity signatures, while support porosity influenced how effectively the catalyst particle fragmented under the mechanical stress of growing polymer. This fragmentation process is critical: as polymer accumulates inside the catalyst grain, internal pressure cracks the support into billions of sub-particles, expanding the active surface area by orders of magnitude. Catalysts whose pores were too narrow or too sparse fragmented unevenly, producing polymer particles with broader size distributions and compromised morphology. Conversely, catalysts with well-balanced porosity yielded dense, regular polymer grains, the kind that flow smoothly through industrial loop reactors and extruders without generating fines that plug equipment.

Hydrogen response, a key operational lever in polyethylene manufacturing, also varied systematically with catalyst characteristics. Hydrogen acts as a chain transfer agent, capping growing polymer chains and lowering molecular weight, but its effectiveness depends on how readily it reaches the active sites and how those sites respond once activated. The equal-yield comparison revealed that differences in melt flow index and flow rate ratio among the three catalysts traced back to their structural parameters, meaning that a plant switching between catalyst grades should expect measurable shifts in resin grade behavior even under identical reactor conditions. Similarly, comonomer response, the catalyst’s willingness to incorporate alpha-olefins such as 1-butene or 1-hexene that introduce short-chain branches and tune density, differed among the three catalysts in ways that correlated with their physicochemical profiles.

Perhaps the most industrially resonant part of the study is its treatment of pre-polymerization. In commercial practice, catalysts are often pre-polymerized, meaning a small amount of monomer is polymerized onto the catalyst under gentle conditions before it enters the main reactor. This protective polymer skin moderates the initial activity burst, prevents catastrophic particle fragmentation, and improves morphology development. The team carried out propylene pre-polymerization on their catalysts and examined how this step altered subsequent ethylene polymerization behavior and polymer properties. Their findings highlight pre-polymerization as a genuine tuning strategy rather than a mere protective formality: the pre-polymer layer reshapes the kinetic profile and the final polymer characteristics, offering operators an additional degree of freedom in matching catalyst behavior to reactor demands.

The significance of this work lies in its provenance and its methodology. Much of the academic literature on Ziegler–Natta catalysis relies on laboratory-synthesized catalysts and small-scale reactors, which can diverge substantially from the behavior of commercial catalysts in full-scale gas-phase or slurry processes. By starting with three industrial catalysts, characterizing them rigorously, and comparing them under both equal-loading and equal-yield protocols, the researchers produced correlations that plant engineers can act on directly. The study also reinforces a growing consensus in the field, reflected in recent work on support morphology, catalyst fragmentation, and single-particle modeling, that the catalyst particle must be understood as a dynamic, evolving microreactor whose architecture at the start of polymerization echoes through every property of the final resin.

For an industry under pressure to produce more sustainable, higher-performance polyethylene with tighter process control, the message is clear: the path to better polymers runs through the catalyst particle itself. By establishing quantitative links between composition, porosity, particle size distribution, polymerization kinetics, and melt properties, this study provides a practical roadmap for catalyst selection, pre-polymerization strategy, and resin grade design. As polyethylene demand continues to grow and recyclability requirements tighten, understanding and engineering the earliest moments of polymer birth, at the titanium sites of a Ziegler–Natta catalyst, will remain one of the most consequential frontiers in industrial chemistry.

Subject of Research: The influence of TiCl4/MgCl2 Ziegler–Natta catalyst parameters on ethylene polymerization kinetics and polyethylene properties

Article Title: The Effect of Parameters of Ziegler-Natta Catalyst on the Polymerization Kinetics and Polyethylene Properties

Article References: The Effect of Parameters of Ziegler-Natta Catalyst on the Polymerization Kinetics and Polyethylene Properties. (n.d.). https://doi.org/10.1007/s10562-026-05517-y

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05517-y

Keywords: Ziegler–Natta catalyst, polyethylene, polymerization kinetics, ethylene polymerization, TiCl4/MgCl2 catalyst, pre-polymerization, melt flow index, catalyst porosity, particle morphology, hydrogen response, comonomer response, Catalysis Letters