Recent Advances in the Anionic Photocurable Epoxy/Thiol-Ene Systems

to several advantages including superior mechanical properties, chemical and heat resistance, low shrinkage, high adhesivity and low toxicity [1]. However, one of the main drawbacks is the inherent fragility due to the high level of crosslinking in these materials. Because of this aspect, several methods to improve their toughness have been reported [2,3]. Our research group focused on improving the toughness of photocurable epoxy resins by concurrently polymerizing them with a thiol-ene system [4,5]. Thiol-ene photopolymerization has recently been classified as “click” chemistry because virtually any kind of thiol can react very rapidly under mild conditions, with any unsaturated compound, even in the presence of oxygen, to produce polythioethers [6]. Recent investigations by our group have demostrated that simultaneous anionic polymerization of an epoxy resin and a thiol-ene system result in the formation of homogeneous polymeric materials with improved toughness [7]. This result is due to the flexible polythioethers that originate during thiol-ene photopolymerization. The epoxy/thiol-ene system was composed of an epoxy resin such as the diglycidyl ether of bisphenol A (DGEBA), and the thiol-ene system, The thiol-ene system included a curing agent of the tertiary amine type, such as N1, N1, N6, N6 tetrallyl hexane-1,6-diamine (ALA4) and a radical photoinitiator like dimethoxyphenyl acetophenone (DMPA) (Figure 1). The formulations were subjected to a simultaneous dual photothermal cure at 85°C and a 40mW/cm2, while varying the molar concentration of the thiol-ene system in the epoxy resin. This epoxy/thiol-ene system was demostrated to be highly reactive achieving full conversion in less than 10 minutes [8]. This reactivity can be described to the presence of several initiating species such as the tertiary amine, thiolate, and thioethers groups. They are all basic species that are able to react with the oxirane groups promoting their anionic ring opening polymerization (Figure 2). It can be seen that the tertiary amine attacks the epoxide groups which undergo Hoffman intramolecular re arrangement to produce vinyl alkoxide responsible for the initiation of the anionic ring opening polymerization of the oxirane groups. The tertiary amines are also able to undergo acid-base reaction with the thiol groups to produce thiolates. These species are basic enough to anionically attack the epoxide groups. At the same time the thiol groups can react with the double bonds of the curing agent to produce polythioethers. When the concentration of the polythioethers is significant, they can also attack the residual epoxy groups to achieve full conversion of the epoxide groups depending on curing conditions.