Polish can be used to repair the scratches on the plastic lenses of sunglasses.However,polish will damage your sunglasses because it will remove the UV coating and affect the prescription of the sunglasses lenses.
You'd better not polish the plastic lenses in sunglasses. As we know, the main purpose of the sunglasses is in the coating part which will prevent our eyes from the uv rays and other bad lights. If you polish the plastic lenses in sunglasses, the coating part will be damaged which will affect the practical use. You'd better notice this point.
Why do you want to polish your plastic lenses? Are there some scratches on the lenses? Plastic lenses are easier to get scratches. So we must put them in the case if we don't need to use them. I suggest you to have your plastic lenses coated next time. Opticians will have new glass lenses coated. There is a kind of hard coating which is scratch resistance. It can protect the lenses. If you want to polish your lenses, I suggest you to turn to an optician. Good luck!