It is a nice question. To my experience, the ammonium hydroxide (as a polymerization catalyst) is added to the solution for promoting polymerization process. This is to be done by reducing the time needed for polymerization to be activated. It is also known that polymer is more stable when ammonium is added.
Please note that the amount of ammonium (free or hydroxide) to be added in the polymer solution should be economically determined, but there are some (let's say) rule of thumb.