Methods of depolymerizing polyolefin-based material into useful petrochemical products using styrene oligomers or polymers, and heat are described. The styrene oligomers or polymers improve the depolymerization reaction by decreasing the halftime for the depolymerization, which results in a higher depolymerization rate and a shorter residence time in the depolymerization unit, allowing for a predictable depolymerization reaction, and decreasing the branching or aromatic formations in the product.