What is chain transfer in free-radical polymerization?

In free-radical polymerization, chain transfer is a process where the active radical site on a growing polymer chain is transferred to another molecule, effectively terminating the growth of the original chain while initiating a new one.1

This reaction can occur in two primary ways:

• Intramolecular transfer (Backbiting): Because growing chains are flexible, the radical at the chain end may curl back and abstract a hydrogen atom from a position within the same chain. This creates a new radical site in the middle of the chain, leading to the formation of branches, such as those found in low-density polyethylene.1
• Intermolecular transfer: The radical site may also be transferred to a separate molecule, such as a solvent or another polymer chain.12

When choosing solvents for industrial polymerization, it is critical to select those that do not readily undergo chain-transfer reactions, as these processes can alter the desired molecular weight and structure of the final polymer.2