Flexible ligands in heterogeneous catalysts for olefin polymerization: Insights from spectroscopy

Heterogeneous catalysis for olefin polymerization is one of the most relevant industrial processes in terms of worldwide diffusion, material volume, and economic turnover. At the base of this process, two main families of multicomponent catalysts, the Ziegler-Natta and the Phillips ones, which have been developed independently since the 1950s and nowadays serve different sectors of the market. This review aims at unifying the picture of these catalysts, pointing out the often-neglected relevance of the ligands around the active sites, including not only the additives explicitly included in the general composition, but also some compounds that are generated in situ during the catalytic process by side reactions, and even the support material itself. All these components have a direct influence on the properties of the catalytic sites and, in turn, on the overall activity and on the properties of the produced polymers. A multi-technique spectroscopic investigation has the potential to shed light on the fleeting, but decisive interactions among all the components and their effect on the catalytic active sites, contributing to create a proper three-dimensional (nano-sized) environment and acting in a concerted way during the olefin polymerization. A few spectroscopic results obtained in our lab are reported, in order to show in parallel how the same principles of coordination chemistry can be effectively applied to both Ziegler-Natta and Phillips catalysts.