Britannica Sources used in this answer

How does polypropylene differ from polyethylene?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram illustrating a homopolymer arrangement using polyvinyl chloride as an example, showing a chain of identical repeating vinyl chloride units with the chemical structure highlighted. — polymer Industrial polymers are synthesized from simple compounds joined together to form long chains. For example, polyvinyl chloride is an industrial homopolymer synthesized from repeating units of vinyl chloride.

Polypropylene and polyethylene are both members of the polyolefin family of resins, but they differ significantly in their chemical structure and physical properties.12

Chemical Structure

The primary difference lies in their molecular composition:
* Polyethylene is formed by the polymerization of ethylene ( $C H_{2} = C H_{2}$ ), resulting in a simple chain of carbon atoms with attached hydrogen atoms.2
* Polypropylene is formed by the polymerization of propylene ( $C H_{2} = C H C H_{3}$ ). Its structure features a carbon backbone with a pendant methyl group ( $C H_{3}$ ) attached to every other carbon atom.1

Physical Properties and Applications

Because of the methyl groups in its structure, polypropylene generally exhibits different performance characteristics compared to polyethylene:1
* Strength and Stiffness: Polypropylene is typically stronger, stiffer, and harder than polyethylene.1
* Thermal Resistance: Polypropylene has a higher melting point (approximately 170 °C or 340 °F) compared to polyethylene, which ranges from approximately 110 °C to 130 °C depending on the density.12
* Durability: Polypropylene possesses excellent fatigue resistance, allowing for the design of "self-hinged" containers. However, it is slightly more prone to oxidation than polyethylene unless stabilizers are added.1
* Recycling: They are categorized differently in recycling systems, with polypropylene marked as #5 and polyethylene marked as #2 or #4 depending on its density.12