Phase separation polymerization of dicyclopentadiene has been characterized from initiation to bulk material formation for the first time via in operando fluorescence microscopy imaging. The morphology of the precipitated polymers at early reaction stages persists in the bulk polymer after completion of the reaction. Two-fluorophore experiments revealed the mechanistic origin of the "dumbbell" morphology as physical strand aggregation/precipitation rather than chemical attachment and revealed that strand aggregation was slow and irreversible relative to precipitation. These data highlight the complementary information available through the single-particle sensitivity and in operando microscopy nature of this technique.