Low-Temperature Thermal Pyrolysis of Polypropylene: Tailoring Molecular Weight and Terminal Vinylidene Functionality for Upcycling

Abstract

The low-temperature pyrolysis of polypropylene (PP) (at 300 and 320 °C and reaction times spanning 30–270 min) was explored to tailor molecular weight distribution and terminal vinylidene functionality, yet maintain high solid residue yield for utility in polymer upcycling. At lower pyrolysis temperatures, the solid residue yield was increased up to greater than 90 wt % with an average of two terminal vinylidene (TVD) groups per chain. As the temperature and reaction time were increased from 300 °C–365 °C, the molecular weight of the solid residue decreased due to chain scission. Whereas gas and liquid products were observed at 365 °C, they were reduced to less than 10% of the product yield at 320 °C. The TVD end functionality (two end groups per chain) was retained across pyrolysis temperatures and times. Degradation by random scission was the predominant mechanism of chain breakage, as shown by the unimodal molecular weight distributions for the virgin polymer and all solid residues. Molar TVD concentration correlated linearly with the gas product yield, indicating a relationship between the frequency of scission reactions and the probability that TVD groups will form. The yielded products were consistent with the known degradation pathway of thermal PP pyrolysis, including intermolecular hydrogen abstraction, followed by successive β-scission reactions and intramolecular hydrogen transfers. A decrease in the final melting temperature and an increase in crystallinity were observed for the solid residues, driven by molecular weight changes due to chain scission. This work demonstrates the low-temperature pyrolysis of PP, which retains the value of the polymer in its solid form and imparts end functionality that can enable its reuse and further conversion to new materials.

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