Photopolymerization

Photopolymerization is a light-initiated polymerization process in which monomers or oligomers containing reactive unsaturated groups are cured or solidified by exposure to light, usually in the ultraviolet or visible range. The process relies on photoinitiators that absorb light and generate reactive species (radicals or cations) to start chain growth. Common monomers include acrylates and methacrylates, as well as epoxies and other multifunctional materials. Light sources include LEDs and lamps chosen to match the initiator’s absorption spectrum, with cure rate controlled by light dose, initiator concentration, and system geometry.

The mechanism involves photoexcitation of the initiator, which cleaves or rearranges to form initiating species. In free-radical photopolymerization, radicals add to carbon–carbon double bonds, propagating chains until termination by combination, disproportionation, or chain transfer. Oxygen can quench radicals, reducing cure efficiency. In cationic photopolymerization, photoinitiators generate strong acids or alkyl cations that open cyclic monomers (such as epoxides) and propagate through cationic routes. Many systems employ sensitizers or co-initiators to extend reactivity into visible light.

Two main categories dominate: free-radical photopolymerization (widely used for acrylates and methacrylates) and cationic photopolymerization (common with epoxy and cycloaliphatic monomers). Visible-light and dual-cure systems broaden applicability and penetration depth, improving practicality for thicker samples.

Applications include dentistry (light-cured composites), additive manufacturing (stereolithography, DLP, and other photopolymer-based 3D printing methods), coatings, inks, adhesives, and microfabrication. Advantages are rapid, room-temperature curing and energy efficiency; drawbacks can include shrinkage stress, residual monomer, and oxygen inhibition in radical systems.