Computational Study of Catalytic Poisoning Mechanisms in Polypropylene Polymerization: The Impact of Dimethylamine and Diethylamine on the Deactivation of Ziegler–Natta Catalysts and Co-Catalysts

In this study, density functional theory (DFT) was used to analyze the processes that govern the interactions among triethylaluminum (TEAL), the Ziegler-Natta (ZN) catalyst, and the inhibitory compounds dimethylamine (DMA) and diethylamine (DEA) during olefin polymerization. The structural and charge characteristics of these inhibitors were examined through steric maps and DFT calculations. Combined DFT calculations (D3-B3LYP/6-311++G(d,p)) and IR spectroscopic analysis show that the most efficient way to deactivate the ZN catalyst is via the initial formation of the TEAL·DMA complex. This step has a kinetic barrier of only 27 kcal mol^-1 and a negative ΔG, in stark contrast to the >120 kcal mol^-1 required to form TEAL·DEA. Once generated, TEAL·DMA adsorbs onto the TiCl₄/MgCl₂ cluster with adsorption energies of -22.9 kcal mol^-1 in the gas phase and -25.4 kcal mol^-1 in n-hexane (SMD model), values 5-10 kcal mol^-1 more favorable than those for TEAL·DEA. This explains why, although dimethylamine is present at only 140 ppm, its impact on productivity (-19.6%) is practically identical to that produced by 170 ppm of diethylamine (-20%). The persistence of the ν(Al-N) band at ~615 cm^-1, along with a >30% decrease in the Al-C/Ti-C bands between 500 and 900 cm^-1, the downward shift of the N-H stretch from ~3300 to 3200 cm^-1, and the +15 cm^-1 increase in ν(C-N) confirm Al←N coordination and blockage of alkyl transfer, establishing the TEAL·DMA → ZN pathway as the dominant catalytic poisoning mechanism.