How is titanium trichloride used to make polypropylene?

Titanium trichloride (${\text{TiCl}}_3$) is a key component in the production of polypropylene as part of a Ziegler-Natta catalyst system.12

To initiate the polymerization of propylene, titanium trichloride is combined with an organo-aluminum compound, such as triethylaluminum (${\text{Al(CH}}_2{\text{CH}}_3{)}_3$). This mixture functions as a heterogeneous catalyst, meaning it exists as a solid phase separate from the reaction mixture. During the process, the catalyst facilitates the stereoregular polymerization of propylene, allowing for the creation of polymers with highly ordered molecular structures, such as isotactic or syndiotactic polypropylene.1324

The catalytic activity occurs at specific sites on the surface of the titanium trichloride, where the monomer units are adsorbed and added to the growing polymer chain in a consistent spatial orientation. This ordered arrangement is essential for producing high-strength materials, as it enables the polymer chains to pack closely together, resulting in a higher degree of crystallinity compared to atactic (randomly oriented) polypropylene.21