Britannica Sources used in this answer

How does polyethylene form from ethylene molecules?

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.

Polyethylene is formed through the polymerization of ethylene ( $C H_{2} = C H_{2}$ ), a gaseous hydrocarbon containing a carbon-carbon double bond.1

The process involves breaking this double bond to create an extra single bond, which allows each ethylene molecule to link to another, forming a long, repeating chain. This reaction is typically facilitated by catalysts or initiators.12

There are two primary methods for this transformation:

Chain-growth polymerization: Often initiated by free radicals, such as those from peroxides. An initiator radical adds to an ethylene molecule, creating a new reactive radical that subsequently adds to more ethylene molecules in succession until the chain is terminated. This method is used to produce low-density polyethylene (LDPE).21
Catalytic polymerization: Using Ziegler-Natta catalysts or metallocene catalysts, ethylene molecules coordinate to a metal center and insert into a growing hydrocarbon chain. This process is used to produce high-density polyethylene (HDPE), which features a linear structure.31