Dual-Cure Coatings: Spiroorthoesters as Volume-Controlling Additives in Thiol–Ene Reactions

Most thiol–ene systems exhibit shrinkage during cross-linking, potentially resulting in micro-cracks and delamination. Oligocyclic monomers like spiroorthoesters (SOEs), on the contrary, show expansion during the ring-opening polymerization. In this communication, a photocurable thiol–ene system composed of a trifunctional thiol, a bisfunctional allyl-bisphenol A compound, and an SOE compound bearing one olefin function shows expansion in the range from −3.07 to +1.70 vol% if the SOE content is increased from 0–30 wt%. Network formation can be accomplished under visible light if a radical as well as a cationic photoinitiator (dual-cure mechanism) and a sensitizer are used. The elasticity of the cured resin increases upon the addition of the SOE; correspondingly, the glass-transition temperature shows a (minor) decrease from 16 to 3 °C. A tailor-made combination of the allyl-bisphenol A compound (90 wt%) and the SOE (10 wt%) yields networks that are volume-neutral during curing.