What is a photoinitiator?

• A. A substance that initiates chain initiation upon irradiation by visible or ultraviolet light.
• B. A substance that absorbs light but does not participate in the reaction.
• C. A catalyst that speeds reactions without light.
• D. A solvent that enhances diffusion.

Answer: (A)

A photoinitiator is a chemical that, when exposed to light, converts that light into reactive species that start a polymerization reaction. The essential idea is that the initiator doesn’t just sit and absorb light; it activates to produce radicals (or other reactive species) that kick off chain initiation, which then leads to propagation and polymer growth. Some photoinitiators split to form radicals directly when they absorb a photon (Type I), while others work with a co-initiator to generate radicals (Type II). This light-triggered generation of initiating species is what distinguishes a photoinitiator from merely absorbing light, from a catalyst that speeds reactions without light, or from a solvent that merely affects diffusion.