Couldn't you technically synthesize anything from anything as long as you have all the molecules and don't care about yield or cost? Trivially, all organic compounds can be broken down into methane, and then reformed into whatever you want. Polystyrene is all carbon though, so you'll still need a source of nitrogen to get meth.

It's been over a decade since I've taken orgchem, so I don't know if I'm remembering this stuff correctly.