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How do free radicals begin the polymerization of ethylene?
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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.
Free-radical polymerization of ethylene begins with the use of an initiator, such as a peroxide, which decomposes to form highly reactive species known as free radicals. These radicals contain an unpaired electron and are typically denoted as R·.12
The initiation process occurs in two primary steps:
- Radical Formation: The initiator breaks down (often through the cleavage of an oxygen-oxygen bond in a peroxide) to produce free radicals.12
- Chain Initiation: The initiator radical (R·) adds to the ethylene monomer (CH₂=CH₂). This reaction breaks the double bond of the ethylene molecule, creating a new, larger radical with the unpaired electron positioned on a carbon atom.1
Once this initial radical is formed, it reacts with subsequent ethylene molecules in a series of chain-propagation steps, where ethylene units are added successively to the growing chain. This process continues until the chain is eventually terminated through reactions such as the combination of two chains or disproportionation.12