Due to the longevity of the cationic active centers, cationic ring-opening photopolymerization can continue after illumination ceases. In addition, substantial reactivity enhancement for epoxides is realized through copolymerization with oxetanes. Here, the separate reactions of epoxide and oxetane moieties were resolved during illumination and subsequent dark cure via real-time Raman spectroscopy. Using oxetane additives, reactivity and conversion of 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (EEC) were improved during illumination and subsequent dark cure through modulation of the initial formulation viscosity and selection of the oxetane secondary functional groups. The largest enhancement in reactivity occurred with secondary groups comprising either aliphatic chains with their flexibility or hydroxyls with their chain-transfer capacity. In contrast, oxetanes containing UV-absorbing phenyl rings reduced the initiation efficiency, and difunctional oxetanes suppressed overall conversion through additional crosslinking. Although bulk conversion was directly related to initial formulation viscosity, the impact of the oxetane secondary functional groups was greater. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 1436-1445