What chemical initiates the polymerization reaction?

• A. Catalyst
• B. Photoinitiator
• C. Initiator
• D. Activator

Answer: (B)

In light-curing systems, polymerization starts when a light-activated species generates the reactive centers that kick off chain growth. The photoinitiator is the molecule designed to do just that: it absorbs the lamp’s energy (UV or visible light) and splits into free radicals. Those radicals rapidly attack the monomer double bonds, creating active sites that propagate the polymer chains and cure the resin into a solid.

A catalyst simply speeds up a reaction that’s already underway; it doesn’t by itself start the polymerization when light is applied. An initiator is a general term for a substance that begins polymerization, but in typical light-cured systems the initiating species is specifically the photoinitiator, which responds to light. An activator isn’t the species that directly starts the chain reaction in this context.