Reactions of a Prototypical Phenolic Antioxidant with\nRadicals in Polyethylene: Insights from Density Functional Theory

Phenolic antioxidants\nare widely used to prevent oxidation, which\nis the main degradation process for many polymers, in particular polyolefins\namong which polyethylene is the most employed one. Although it is\ngenerally understood that one of the main mechanisms by which phenolic\nantioxidants prevent or slow down oxidation is by deactivating radicals\nand preventing the formation of alkyl radicals, detailed understanding\nat the atomic scale of the hierarchy of radical reactions is still\nlacking. Here, we investigate the interaction of a prototypical phenolic\nantioxidant, butylated hydroxytoluene (BHT), with radicals in a polyethylene\nmodel by means of static and dynamic simulations based on density\nfunctional theory. We focus on the H-transfer reactions between BHT\nand radical species by evaluating the associated energy barriers and\nanalyze the conditions in which these relevant reactions occur by\nfirst-principles molecular dynamics simulation. Our polyethylene model\nincludes a realistic surface of a crystalline lamella, thus describing\nthe local atomic environment in which the reactions mainly take place.\nOur results suggest that the H-transfer reaction of the BHT molecule\nwith an alkoxy radical is spontaneous, and the energy barrier is small\n(∼0.1 eV) with a peroxy radical. Conversely, direct scavenging\nof alkyl radicals by BHT seems excluded. Our molecular dynamics simulations\nhighlight the influence of steric hindrance and antioxidant diffusion\nwithin amorphous regions on antioxidant efficiency.