Abstract
<jats:p>Chemical recycling has emerged as a promising pathway for increasing plastic circularity by recovering value from mixed and contaminated waste streams that are unsuitable for mechanical recycling. Because chemically recycled products become indistinguishable from their fossil-derived counterparts, recycled-content (RC) certification relies on Chain-of-Custody (CoC) accounting frameworks, primarily Mass Balance (MB) and Book-and-Claim (BC). While these accounting approaches determine how recycled content is attributed to products, their implications for plastic supply chain design remain poorly understood. This work develops a mixed-integer optimization framework for the design of integrated plastic supply chains that explicitly incorporates alternative MB allocation methods and BC accounting for RC tracking while capturing competition between fossil and recycling pathways. The model simultaneously optimizes technology selection and capacity, facility location, transportation, and material flow across the supply chain, spanning monomer production, resin manufacturing, collection, recycling, and end-of-life management. A Texas case study spanning ethylene and propylene polymer supply chains is used to evaluate MB accounting via proportional (P) and non-proportional (NP) allocation as well as BC accounting under varying recycled-content requirements. Results show that CoC accounting is a key determinant of optimal supply chain design. More stringent MB allocation methods (e.g. P case) require substantially greater deployment of chemical recycling technologies, whereas more flexible accounting approaches (e.g. NP) achieve identical recycled-content targets with lower infrastructure investment. These differences lead to markedly different recycled-content distributions in endproducts, landfill diversion, system costs, and CO2 emissions.</jats:p>