Zaccaria, Francesco (2018) Molecular olefin polymerization catalysts: applications of molecular design for properties modulation. [Tesi di dottorato]

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Olefin polymerization is an industrial sector of huge economic and social impact. In this context, molecular catalysts have drawn much attention since 1990s, as they can be used for the industrial production of advanced polymeric material which are hardly accessible by heterogeneous Ziegler-Natta technology. The growing commercial demand is pushing research toward the identification of novel active species for the synthesis of innovative (co)polymer architectures. Advances in experimental and especially computational technologies foster the ambition of catalyst development by design, but the passage from the 'classical' trial-and-error to a fully rational approach is hampered by the complex interactions between all species in the catalytic pool and the many complex subtleties that molecular design has to balance. This PhD thesis explores several issues related to rational catalyst development, trying to contribute to a more detailed understanding of the polymerization process by a combined experimental and computational approach. The first part of the work deals with the identification of structure/ properties correlations for the selective synthesis of specific products. In particular, Chapter 2 is dedicated to factors determining reactivity ratios in ethene/α-olefin copolymerization reactions. A suitable computational protocol to accurately reproduce comonomer affinities for a large variety of molecular catalysts is proposed, and utilized to draw tentative conclusions on entropic, electronic and steric effects determining comonomer affinity. Important kinetic considerations on the rate limiting step for chain propagation are provided, potentially explaining the occasionally non-trivial temperature dependence of copolymerization statistics. Due to the rapidly increasing demand, the production of comonomer (e.g. 1-hexene, 1-octene) feedstock has become an issue of growing relevance the polyolefin industry. Chapter 3 summarizes the work carried out during a three months internship at the Imperial College London (UK), working on chromium-catalyzed ethene oligomerization for the selective production of linear α-olefins (LAOs). The case of bis(benzimidazolyl)amine Cr-catalysts was investigated, providing supporting evidence for a mechanistic and kinetic model explaining alternating distribution of products. Along with the production of specific polymer architectures, activity and thermal stability are two key catalyst features to be considered in the design of novel active species, which are examined in the second part of this thesis. Chapter 4 describes a novel chain transfer to solvent process that was recently discovered by our group. This involves CH activation of the toluene solvent by Ti-phosphinimide catalysts, leading to benzyl terminated polypropylenes, and represents a reversible deactivation route being competitive at high temperatures and moderately high pressures. Extensive polymerization and DFT screenings were carried out, aiming at elucidating the mechanism and exploring the scope of this reaction. Finally, cocatalyst influence on polymerization performance is discussed in Chapter 5. Free-trimethylaluminum in commercial MAO solution was effectively trapped by addition of a hindered phenol, allowing to explore the properties of the oligomeric fraction of MAO by NMR spectroscopy. A phosphinimide half-titanocene was used as case study, taking advantage of the presence of a phosphorus atom in the ancillary ligand as spectroscopic probe for 31P NMR. Consequences of the absence of free trialkyl aluminum on the formation of dormant sites are evaluated and connections with the mechanism of chain transfer to solvent are highlighted. This work was carried out in collaboration with the group of Prof. Alceo Macchioni and Prof. Cristiano Zuccaccia at the University of Perugia.

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