How formaldehyde affects the thermo-oxidative and photo-oxidative mechanism of polypropylene: A DFT/TD-DFT study

Abstract

Previous studies have shown that formaldehyde significantly accelerates the thermo-oxidative and photooxidative ageing of polypropylene (PP). However, the related mechanisms have not yet been clarified. Computational simulation can compensate for the limitations of experimental methods to help investigate reaction mechanisms and has been widely applied to study thermal and thermal-oxidative ageing processes. However, there has been no systematic methodology for investigating photooxidative ageing processes by simulation. In this paper, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) are applied to investigate how formaldehyde affects the thermo-oxidative and photooxidative ageing mechanism of PP. Formaldehyde can promote the generation of tertiary carbon radicals by reacting with tertiary hydrogen, causing a significant decrease in the bond energy of the tertiary C-H bond. The relaxed scan shows that when the oxygen atom of formaldehyde approaches the tertiary hydrogen up to 1.20 Å, the transition energy from the HOMO to the LUMO will decrease to 324 kJ/mol, which is equivalent to photons of 361 nm. The charge density difference (CDD) shows that the transfer of electrons from the tertiary carbon‒hydrogen bond to the formaldehyde molecule is followed by photoexcitation, which causes the breaking of the tertiary carbon bond and the formation of hydroxymethyl radicals. The TD-DFT simulation further confirms the above conclusions from the geometry of the molecules. The energy required for each elementary reaction to the formation of the carbonyl group is also calculated.