photoinitiated

Photoinitiated refers to reactions or processes that are started by exposure to light, typically through a photoinitiator that absorbs photons to generate reactive species such as free radicals or cations. In polymer chemistry, photoinitiators are used to initiate polymerization upon irradiation with UV or visible light, enabling rapid curing at ambient temperatures and spatial control.

Two main classes exist: Type I photoinitiators undergo cleavage upon light absorption to yield reactive radicals directly; Type II photoinitiators require a co-initiator (often a secondary amine) that reacts with the excited photosensitizer to produce initiating radicals. Photosensitizers can extend the usable wavelength range by absorbing light and transferring energy or electrons to the initiator. Norrish-type I and II photoinitiators are well-known in literature.

Applications include UV- and visible-light curing of coatings, inks, dental resins, adhesives, as well as photopolymerization in hydrogels, 3D printing resins, and microfabrication. Common examples include camphorquinone in dental materials and benzoin ethers in UV-curable coatings.

Advantages include rapid curing, room-temperature operation, reduced solvent use, and on-demand curing with light. Limitations involve limited penetration depth for UV light, potential toxicity or odor of certain initiators, color formation, and sensitivity to ambient light and oxygen for some systems.

Development of initiators and sensitizers continues to expand visible-light systems, improve biocompatibility, and tailor spectral and kinetic properties.