A family of catalysts developed in the 1950s for the mass production of polyethylene and polypropylene.

Karl Ziegler and Giulio Natta recieved the Nobel Prize in 1963 for the discovery of these chemicals and their applications. Initially, Ziegler was only able to make wax like low molecular weight polyethylene. Natta experimented with many further formulations of the catalysts and became aware that they could polymerize propylene. Along with Earl Tupper, Wallace Carothers, Paul Flory, and Leo Bakeland, they are the most famous polymer chemists of all time.

All of the catalysts are made from a mixture of titanium tetrachloride and an alkylated metal like triethyl aluminum.

Metallocene catalysts have emerged as a fierce rival because they operate with a single active site and thus can produce polymers with a lower polydispersity and higher molecular weight which result in better properties. They can also be used to make fancy copolymers.

Another competitor are Phillips catalysts. They are composed of chromium oxide on a high surface area silica substrate. Less branching occurs in the polyethylene formed by this catalyst. The solid silica substrate makes recovery of the catalyst easier, and thus using it makes for a cheaper process.

Ziegler would charge exhorbitant fees to demonstrate the use of his catalysts without explaining how they worked. He would charge even more to give an explanation or to help set up a manufacturing plant.

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