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