Cationic photoinitiators are also known as photoacid generators. Once the cationic photoinitiator absorbs UV radiation, the initiator molecule is converted into a strongly acidic substance, Lewis acid or brent acid, which initiates the polymerization reaction. Most cationic photoinitiators are sulfonates or iodides, so they consist of cationic and anionic pairs-each with a specific role in the reaction mechanism. The cationic portion of the photoinitiator molecule is responsible for absorbing UV radiation, while the anionic portion of the molecule becomes a strong acid after UV absorption. Example sulfonates and iodized salt are shown in Figure 1[1] below, with the cationic portion marked in blue and the anionic portion marked in red.
Figure 1. Two examples of typical cationic photoinitiator types: sulfonates and iodized salts.
Just as different free radical photoinitiators have different absorption spectra, cationic photoinitiators based on different molecular configurations around sulfur or iodine atoms also have different absorption spectra. Various anionic Metal halides will produce strong acids of varying strength, which is related to the rate of reaction (an increase in acid strength means an increase in the rate of reaction) . Hexafluoroantimonate ion is one of the strong Metal halides ions used in cationic photoinitiators to produce ~ -30 pKa! Don't be intimidated by the strong acid discussion -- these materials are in the range of 0.1 percent by weight in uv-curable resins. The reaction mechanism for the generation of strong acids from representative diaryliodonium salt molecules is shown in Figure 2[2] .
Figure 2. Cationic photoinitiators absorb ultraviolet radiation to produce excited electronic states. The excited electronic state makes the molecule undergo both homogeneous cleavage to produce several free radicals and heterocleavage to produce cationic matter. The hybrid product will react with the monomer to stabilize itself, producing a strong acid.
The Cationic polymerization mechanism is consistent with chain growth polymerization, including initiation, propagation, termination and chain transfer. Radical polymerization transfers free radicals from monomer to monomer during chain growth, while Cationic polymerization transfer charge from monomer to monomer during chain growth. Next week we'll talk about monomers for Cationic polymerization, but the most common monomers are Epoxide, specifically cyclic aliphatic Epoxide. For simplicity, in Figure 3 below, a ring-like ternary ring (oxirane) is used to demonstrate the Cationic polymerization mechanism of any epoxy-based material. Step 1 is to protonize the epoxy group from the photo-generated acid. Step 2 is the ring-opening of the proton epoxy group and charge transfer to an additional, new, epoxy group. Step 3 is a continuous chain growth until all the monomers are consumed and the charge is transferred to another chain or the reaction terminates.
Figure 3. Cationic polymerization mechanism.
Note: Source from POLYMER INNOVATION BLOG by Jeff Gotro, Wising Chem is intended to convey more information, if there is infringement please contact us to delete, thank you!
