Polymer capsules are formed following the impact of viscous poly-anion drops onto a deep pool of water-like poly-cation solution. During impact, an air bubble is often trapped on the drop/capsule surface, affecting the membrane quality locally. This entrapment has been experimentally examined for a typical anion-cation pair through detailed imaging of the impact crater. Experiments show that the bubble entrapment is caused by the collapse of the impact crater above the top surface of the submerged-and-reacting drop. Locally, the bubble pinch-off can be viewed as the continuous thinning of a gas filament driven by the rebounding bulk flow. In the low impact Weber number (We) regime, the data suggests that both surface tension and inertia govern the entrapment process, whereas, for higher Weber numbers the process is inertia-dominated. The transition condition is found to be coincident with the observation of the critical cone angle for the impact crater, seen in inviscid non-reacting drop impact.