Anionic UV curing in physically obstructed shadow zones remains fundamentally challenging. Here, we demonstrate room-temperature shadow curing through anionic chain-growth polymerization of episulfide resins initiated by a coumarin-based photobase generator (Cou-DCMC) that releases a guanidine base upon UV irradiation. Shadow curing was achieved using an optimized episulfide formulation comprising a thiol initiator, Cou-DCMC, and high-dielectric-constant solvents, with controllable cure-propagation lengths depending on UV intensity and temperature. Comparison with conventional epoxy–thiol systems confirmed that chain-growth polymerization is essential for shadow-zone curing; episulfide systems achieved shadow curing, whereas epoxy–thiol systems undergoing step-growth polymerization failed to cure in shadow zones despite higher epoxy conversions. Shadow-cured materials exhibited comparable morphology and thermal properties in both directly UV-exposed regions and shadow zones. This work establishes a strategy for UV shadow curing by combining highly reactive episulfide resins with Cou-DCMC, enabling nearly uniform network formation in regions with limited light accessibility.
Inoue et al. (Tue,) studied this question.