Abstract
Chemically recycled plastics are indistinguishable from their fossil-derived counterparts, necessitating Chain-of-Custody (CoC) methods like Mass Balance (MB) accounting, to certify recycled content (RC) claims. Although CoC methods determine how RC is attributed to products, their implications for plastic supply chain design remain poorly understood. This work develops a mixed-integer optimization framework for the spatial design of integrated plastic supply chains under recycling policy scenarios that incorporates alternative CoC methods for RC tracking along with competition between fossil and recycling pathways. We compare a range of MB allocation designs, from proportional to non-proportional, against a more flexible CoC method, Book-and-Claim (BC), which decouples the RC attribute from physical flows. Results show that the choice of CoC method is a key determinant of technology deployment and supply chain design. Within MB accounting, more stringent allocation rules drive greater chemical recycling deployment, whereas under more flexible MB allocations and BC methods, the same RC target is met with lower infrastructure investment but a larger share of material sent to landfill. We also find potential trade-offs between system emissions and circularity goals across CoC frameworks, with the largest trade-offs arising under the most stringent MB allocation choices.