The currently available commercial processes for plastics upcycling are either operated at scale well below the large volume of plastic waste generated or are only applicable for specific plastic streams. Thermal processes are predicted to be one of the major players for plastic waste conversion. The success of those processes relies on their flexibility to feedstock composition, the value of the end-products, and efficiency. The optimum balance between feed flexibility, process efficiency and outputs can be achieved through process optimization and simultaneous assessment of lifecycle impact and economics at the process level. Pyrolysis products can be integrated into a modified refinery that upgrades high-value chemical streams, refines complex streams to useful fuels, and generates needed process energy from low-value streams. This project will address a critical barrier to the financial and environmental viability of plastic chemical recycling: an inability to design and operate efficient processes that reliably achieve target material, energy, and emissions benefits despite geo-temporal variation of composition and quality of plastic streams. This barrier stems from a lack of mechanistic understanding of the process components. We will develop a mechanistic understanding of sequential fast pyrolysis and catalytic upgrading of a mixture of polyester, i.e. polyethylene terephthalate (PET), and polyolefin, i.e. polypropylene (PP), to valuable chemicals. PET is the most widely used polyester and is often combined with other polyolefins such as PP in addition to additives and metals dependent upon the target application for carpets, packaging, fibers, sheets, and films. The mechanistic understanding gained in this work will allow the development of microkinetic reaction models, which can be incorporated into reactor simulations for process optimization and translate the knowledge obtained in the laboratory to the manufacturing industry. Our team will utilize experiments, data science and theory to overcome fundamental science and engineering challenges in mixed plastic upcycling.