How is polypropylene made from propylene?

Polypropylene is produced through the polymerization of propylene, a gaseous hydrocarbon obtained from the thermal cracking of petroleum fractions such as naphtha, ethane, propane, and butane.1

The process involves breaking the carbon-carbon double bond in the propylene molecule ($C{H}_2=CHC{H}_3$), allowing thousands of these molecules to link together into a long, chainlike polymer. To produce the commercially significant isotactic form—where all pendant methyl groups are arranged on the same side of the carbon backbone—manufacturers use Ziegler-Natta catalysts at low temperatures and pressures.12

These catalysts, which typically combine a transition metal compound with an organo-aluminum compound, enable the creation of stereoregular structures that provide the material with high strength and crystallinity. More recently, highly reactive metallocene catalysts have also been developed for this purpose.2