My friend and I have been struggling with this problem for a couple days. Because polystyrene has a similar structure to many plastics, we assumed that it would dissolve in acetone. However, after running several tests, we found that polystyrene would not dissolve in a measurable quantity even if we let it sit in acetone for a significant amount of time.
Here is the plastic we are attempting to dissolve:
And here is a plastic more easily dissolved:
We understand that most molecular-bonded plastics dissolve via van der Waals dispersion forces, so why isn't polystyrene dissolving? My theory is that the benzene ring is interfering with the dissolving process -- that only half the compound of polystyrene is interacting with the acetone and thus it does not dissolve like a polyethylene (which obviously has twice as much surface area to dissolve).