A Deterministic Model to Predict Tacticity Changes During Controlled Degradation of Polypropylene

Abstract

Controlled degradation of polypropylene (CPP) converts high-molecular-weight (Mw) polypropylene (PP) to lower Mw PP using peroxide-induced degradation. A deterministic model is developed to predict tacticity changes in isotactic PP. The model determines instantaneous rates of defective-pentad formation and tracks accumulation of these pentads using dynamic material balances. The model relies on the assumption that lifetimes of polymeric radicals are short compared to extruder residence times. Model predictions agree with previous Monte Carlo simulations that require long simulation times. Two versions of the model are proposed: one assumes that radical migration, also called chain walking, occurs via 1–6 isomerization and one assumes 1–8 isomerization. Chain-transfer-to-polymer and chain-walking parameters are estimated using 13C NMR data obtained using initiator levels between 0.006 and 0.52 wt%. Both 1–6 and 1–8 isomerization models result in better fit to the data.