A polymerization unit converts propylene and butylene into a gasoline blend stock by joining two olefin molecules to form one larger olefin called polymer gasoline.

Polymerization has largely been displanted by alkylation units which have the advantages of using isobutane as well as an olefin (expanding the resulting volume of gasoline) and resulting in a higher quality product.

Dimerization is a form of polymerization that uses only propylene as its olefin feed.