Cationic
Cationic Systems
Among different types of monomers which can be polymerized by cationic way, only those
functions with epoxy or vinyl ethers have been widely used in cured resins.
- Vinyl ethers
Among the monomers polymerise by cationic, vinyl ethers and propenyl are considered the
most reactive. In the presence of a sulfonium salt, of the photopolymerization of
triethyleneglycol divinyléther (Figure 7) runs as fast as a diacrylate, reaching a conversion rate
close to 100% within a few tenths of a second. The high reactivity of these monomers from the
highly electronegative character of the double bond and stabilization of the carbocation by
resonance:
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Epoxides
In the case of tétrafonctionnels epoxy monomers or oligomers epoxides téléchéliques, curing
develops in three dimensions and leads to a strongly bridged polymer. The rate of
polymerization is much lower than that of acrylate monomers, mainly due to a lower value of the
rate constant of propagation kp. Monomer aliph dicycloépoxyde is one of the most used
because of its high reactivity. The addition of difunctional monomers époxysilicones helped
increase the speed of polymerization and the final conversion rate, because of the greater
mobility of network chains. The crosslinked polymer has excellent mechanical properties,
combining flexibility and hardness and a very good resistance to solvents and heat.
Cationi
c photoamorceur
Salts diaryliodonium or triarylsulfonium give Brönsted acids in presence of a hydrogen donor
molecule and the photolysis. These acidic protons are capable of initiating cationic
polymerization of monomers which do not polymerize by free radicals such as vinyl ethers or
heterocyclic monomers (epoxides, lactones, cyclic ethers, époxysilicones).
These two types of photoamorceur act on different chemical functions. In addition, the
photoamorceur cationic are insensitive to oxygen, which inhibits radical reactions on the surface
and lead to systems "live" that continue to respond as long as monomers even in the dark.
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