Polymers can fundamentally be de-polymerized and then you can reuse the monomers, for example see [1]. They call it "chemical recycling". Questionable whether it will ever be price competitive with direct production from crude oil derivatives.
You can also burn them (pyrolysis), collect the carbon and start from scratch. Of course this is insane (economically and thermodynamically), but hey, you are "recycling" the same carbon.
[1] https://cen.acs.org/environment/recycling/Plastic-problem-ch...