Let's trace Free Radical Polymerization of Ethene to make Polyethene. Step 1 (Initiation): An initiator (like Peroxide) breaks apart to form a radical (R•). R• hits an Ethene molecule. Step 2 (Propagation): The Ethene molecule becomes a radical. It bumps into another Ethene. They link up. The radical now sits at the end of the new two-unit chain. This keeps happening: R-CH2-CH2• + CH2=CH2 -> R-CH2-CH2-CH2-CH2•. Step 3 (Termination): The process ends when two long chains bump into each other and their lonely electrons pair up, sealing the chain forever.