Polypropylene (PP, also: polypropene) is a thermoplastic polymer that can show different tacticality.

The most commercially interesting product is isotactic: it is a semi-crystalline polymer characterized by high tensile strength, low density, and good thermal and abrasion resistance.

The density of isotactic polypropylene is 900 kg/m³, and the melting point is often over 165 °C. Chemical properties, determined at the production stage, include stereoregularity, molecular mass, and polydispersion index. In contrast, the attactic product looks like a rubbery-looking material, and has little commercial interest (it has been used only as an additive).

Polypropylene has experienced great success in the plastics industry: many everyday items, from doormats to colanders to name a few, are made of polypropylene.