In free-radical polymerization, which species propagates the chain growth?

• A. Carbocations.
• B. Carbanions.
• C. Neutral molecules.
• D. Free radicals.

Answer: (D)

Propagation in free-radical polymerization is driven by a radical at the growing chain end. That reactive end adds to the carbon–carbon double bond of a monomer, forming a new bond and producing a new radical at the chain terminus. This radical chain end then repeats the same step, so the chain grows one monomer unit at a time. Carbocations and carbanions are features of ionic polymerization mechanisms, not the radical pathway, and neutral molecules lack the unpaired electron needed to keep the chain moving. So the moving, chain-extending species is the free radical at the end of the growing chain.