Adhesive and sealant chemists commonly formulate polymers with photoinitiators in order to design light-curing or light-sensitive polymer products. Although these are considered to be highly specialized and useful chemicals to a polymer chemist, there are actually many examples of photoinitiators in nature.
Photoinitiators, as the name suggests, are light-sensitive materials that will initiate a chemical reaction upon exposure to light. All UV curing resins utilize some form of a photoinitiator:
- In free radical curing systems, such as UV acrylates, the photoinitiator produces a free radical species that begins the free radical polymerization. These are radical photoinitiators.
- In cationic systems, such as UV cure epoxies, the photoinitiator produces a super acid (Brönsted or Lewis acid) that attacks nearby epoxy monomers, and initiates the subsequent cationic polymerization. These are cationic photoinitiators.
Hydrogen peroxide, nitrogen dioxide, and molecular oxygen are all examples of photoinitiators that are natural to our atmosphere and initiate reactions that help to control concentrations of greenhouse gases. Each of these compounds are examples of radical photoinitiators.
1. Hydrogen Peroxide
Hydrogen peroxide is spontaneously created in our atmosphere under normal conditions, and will produce hydroxyl radicals with UV light.
The process usually starts with molecular oxygen, which can easily abstract hydrogen atoms because of its relatively high electronegativity, readily forming HOO radicals. This radical can further obtain another hydrogen atom, this time forming hydrogen peroxide, H2O2. Peroxides are easily cleaved by UV light forming two hydroxyl radicals.
Hydroxyl radicals readily oxidize greenhouse gases and pollutants in the atmosphere and are the most widespread oxidizer in the troposphere.
2. Nitrogen Dioxide
Another example of an atmospheric photoinitiator is nitrogen dioxide. Photons that are 400 nm or less are able to photolytically cleave the compound into nitric oxide and a free oxygen atom.
As free oxygen atoms are highly reactive, they can also easily obtain hydrogen atoms from other compounds, such as water. In the case that it reacts with water, two hydroxyl radicals are formed.
3. Molecular Oxygen
Lastly, molecular oxygen is also a very good example of a natural photoinitiator, and is very important in maintaining levels of ozone in the atmosphere.
Upon exposure to radiation with wavelengths less than 240 nm, molecular oxygen splits into individual free radical atoms. These free radical oxygen atoms in turn combine with molecular oxygen to form ozone, which in turn, can be photolyzed back into free oxygen and molecular oxygen. Additionally, free oxygen atoms and ozone can react to form molecular oxygen.