Ziegler-Natta Catalysts

Ziegler-Natta catalysts, discovered in the years 1953–1954, account today for a production volume of ~65 million tons of polyolefins, including mainly polyethylene and polypropylene products. Since their first discovery, the development of Ziegler-Natta catalysts has been relentless, and their evolution is the result of the exploitation, starting from the mid-1960s, of four major breakthroughs: the active form of MgCl₂; the stereoregulating effect of electron donors (isotactic polypropylene); the chemical route to the active form of MgCl₂; and, finally, the control of the morphology of the catalyst/polymer particles. Thus, the most advanced Ziegler-Natta catalysts are today prepared starting from controlled morphology MgCl₂, or its precursors, TiCl₄, or other titanium derivatives and, especially for isotactic polypropylene, electron donors. With respect to the first-generation of Ziegler-Natta catalysts based on TiCl₃, MgCl₂-based catalysts are not only much more efficient in terms of productivity ( both increased number of active centers and value of their propagation constant), but they also offer an unprecedented level of customization to meet any process (slurry, bulk, or gas-phase) or product requirements (polyethylene, polypropylene, and their copolymers). Actually, MgCl₂-supported catalysts are complex but also versatile systems whose architecture (particle shape, size, size distribution, and porosity) and performances (activity, hydrogen response, stereocontrol, and capability to tune polymer MW and MWD) can be directed acting on both the morphology of the MgCl₂ precursors and the nature of the electron donors.

Many mechanistic aspects of the Ziegler-Natta catalysis have been elucidated, eg, the mechanism of polymer particle growth, the role of prepolymerization, the activating effect of hydrogen in propylene polymerization, and the stereoblock nature of isotactic polypropylene. On the other hand, many other aspects are still open to debate or, at least, still need clarification; eg, the exact nature and number of active centers and their mechanism of deactivation, the intimate mechanism of action of electron donors, the difference between the species that are active in ethylene or propylene polymerization, and the comonomer effect in ethylene copolymerization.

Thus, there is still a lot of room for further investigation and improvements in Ziegler-Natta catalysts. A big step forward in this field would be the generation of catalysts combining the economics and morphological features of MgCl₂-based systems with the peculiarities of single-site systems.

Keywords: ziegler-natta; heterogeneous catalysts; polyethylene; polypropylene; propylene–ethylene copolymers; electron donors; polymer growth; stereospecific polymerization