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The cationic polymerization of diglycidyl ether of bisphenol-A (DGEBA) using a latent cationic thermal initiator, ammonium antimony hexafluoride, and the effect of addition of cycloaliphatic epoxy resin on the polymerization were studied with DSC, TGA, and hardness measurement. It was found that the cationically polymerized DGEBA at 0.5 phr initiator level gives the best compromise of chemical reactivity and optical properties, and therefore is suitable for light emitting diodes (LED) applications as an optically transparent encapsulant. This encapsulant, on one hand, is room temperature stable for at least six months, yet cures fast at low temperature, on the other hand. Moreover, the encapsulant has a high refractive index of 1.6 resulting from the high density of aromatic structure in the cross-linked network. Packaged with this encapsulant, the LED device is shown to exhibit an increased light output as demonstrated by both simulation and experimental measurement. The introduced system provides a new one-component encapsulant that is optically transparent, cures fast at low temperature with no sacrifice of room temperature stability, and has high refractive index at the same time.