I don't think there is a good way to do this chemically. I would generally say to try to find a solvent the polymer dissolves in, like DMSO, that the leather would not, but this would take gallons of solvent to accomplish. Could you separate it mechanically?
And furthermore, the solvent would likely damage leather.
Also, have these materials become finely divided, as in pulverized to dust, and you'd like to generate a wide charge to separate them? I don't know if that's possible.
However, if you have large strips of leather, and large chucks of plastic, and you want magic beams of electrostatics to make them fly into separate piles, then you've left behind all modern science. And you're not clear that that's not what you want.