4.3 THIOL-ENE SYSTEMS

The thiol-ENE curing reaction is also a radical induced process, however, proceeds via a step growth type polyaddition in contrast to the radical induced chain growth polymerization. This process has the distinct advantage that it is not oxygen inhibited , unlike the acrylate based polymerizations. The thiol-ENE UV curable coatings are based on a mixture of a polyene and multifunctional thiols that undergo a polyaddition reaction upon UV irradiation. The UV irradiation produces a photoinitiator radical (PI ), which in a preliminary step abstracts hydrogen from a thiol under formation of a thionyl radical (R S ). The step growth process, which is propagated by chain transfer reaction involving as the key molecule the thionyl radical (R S ) is depicted in Figure 4.27. The basic principles and almost all aspects of this type of coating system have been reviewed recently.66 ^,67

The general mechanism shown in Figure 4.27 also explains why the oxygen does not have a detrimental effect on the curing process. After the formation of a thionyl radical (R S ) by hydrogen abstraction of the photoinitiator radical, the thionyl radical adds to an ENE double bond and forms a new sigma bond and a carbon centered radical. This addition reaction is the first and already crucial step for the crosslinking process. The second step is the subsequent hydrogen abstraction from a further thiol molecule by the carbon centered radical produced in the first step and formation of another thionyl radical. In the presence of air, the...