Mechanistic Insights Into the Thermal Pyrolysis of Branched Polyolefins Using Kinetic Modeling

We present a detailed mechanistic analysis of thermal pyrolysis of linear and branched polyolefins. The pyrolytic pathways in high-density polyethylene (HDPE), polypropylene (PP), and linear low-density polyethylene (LLDPE) are compared. The mechanistic model for LLDPE is newly developed, based on the population balance approach incorporating reactions on the branches as well as the main chain. Specific reactions are unfurled based on a compact number of reaction families, each of which is governed by a structure/reactivity relationship to specify the activation energy of individual reactions and statistical mechanical estimates to quantify the Arrhenius pre-factor. The results from the model for LLDPE are compared to new data collected from micropyrolysis experiments that quantify detailed temporal product distributions. PP is confirmed as the most reactive of the three polymers, making the highest amount of low molecular weight products at a given time. For LLDPE, branches serve as positions for hydrogen abstraction, opening up new pathways for deconstructing the main chain. The length of the branches in LLDPE is shown to influence the degradation rate. Understanding the connection between structure and reactivity can help design polymer topologies and pyrolysis reaction conditions to optimize the recycling process and thus the product yields and selectivities.