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In the early 1 it was reported that heating up or photolyzing a mixture of a difunctional thiol and a
difunctional ene resulted in the formation of linear polymers. This polymerization is unique in that it is a step growth type polymerization that proceeds by a free-radical chain mechanism. In the mid 1970s, in monumental work in the radiation curing field, Morgan and 3 showed that by using muttifunctional thiols and multifunctional enes, a crosslinked network characterized by high functional group conversion could be readily formed upon exposure to a high intensity medium pressure mercury lamp source. These systems were widely used in electronic coatings applications and as wear layers for clear coatings on residential floor tiles. The salient feature that made thiol-enes so popular was their rapid polymerization resulting in highly crosslinked networks that exhibit excellent physical and mechanical properties. Below are examples of a typical ultifunctional ene and a multifunctional thiol, the combination of which emerged as a commercially viable
resin system from this initial work. The triallylisocyanuarate shown below is simply the allophonate of
allylisocyanate. Interestingly, these resins systems were used commercially with benzophenone as the
photoinitiator. It has been generally accepted that upon excitation, benzophenone abstracts a hydrogen from
the thiol resulting in a thiyl radical capable of initiating the free-radical chain polymerization process. After the
initial flurry of industrial and academic interest in the 1970s that resulted from the work of Morgan and Ketley,
the use of thiol-enes as matrix resins for commercially viable photocurable systems waned, and thiol-ene
photocurable systems were rapidly replaced by acrylates. Today, thiol-enes are used on a small scale in
adhesives. Next we present a brief description of the general mechanism for the free-radical thiol-ene
polymerization process.
