To achieve a sustainable society supported by resource circulation, vinyl monomers that can radically polymerize and be recovered from vinyl polymers (VPs) are desirable. However, the chemical recycling of VPs remains challenging because of the difficulty in quantitative and selective main-chain scission or depolymerization. In this study, VPs of cyclic styrene derivatives, such as 3-methylene phthalide, were investigated to be chemically recyclable. The ring-opening of the pendant groups by saponification enhanced the steric hindrance of the pendants, which resulted in main-chain scission and depolymerization to the monomer precursors. Highly efficient chemical recycling was achieved by suspending the polymer in aqueous KOH. These results facilitate resource circulation toward achieving a sustainable society.