How is high-density polyethylene made with Ziegler-Natta catalysts?

High-density polyethylene (HDPE) is produced through the polymerization of ethylene at low temperatures and pressures using Ziegler-Natta catalysts.12

These catalysts, typically composed of a mixture of a transition metal compound (such as titanium trichloride) and an organometallic compound (such as an alkyl aluminum derivative), facilitate the growth of the polymer chain. In this process, the polymer chain grows directly from the surface of the catalyst through the successive insertion of ethylene molecules. Unlike free-radical initiators, these catalysts are not consumed during the reaction.32

Because the Ziegler-Natta process produces a linear molecular structure without branches, the polymer chains can pack closely together. This results in a highly crystalline material characterized by high strength, moderate stiffness, and a higher melting point than branched versions of polyethylene. Once the polymerization is complete, the resulting polymer chains detach from the catalyst surface.124