How is polistirex activated?

Okay, so I know that it’s basically an edible plastic that encapsulates its target chemical and releases it over the period of 12 hours. But if there was no activation mechanism, it would just dissociate in the medicine jar!

I couldn’t find out this info (not helped by the multiple spellings of the chemical,) so is it the:

– acid in the stomach?
– some enzyme in the intestines or stomach?
– something completely different?