Pre-treatment methods for polyolefin-based feed streams before depolymerization are described. Polyolefins are separated from other material in the polyolefin-based feed stream using density differences in an aqueous solution, which allows for a pre-treatment method that does not affect the depolymerization catalyst. By removing the non-polyolefin materials from the feed stream, the depolymerization of the polyolefin material can proceed at lower temperatures for longer cycles. This results in a more efficient process with a smaller carbon footprint.