  Understanding the degradation mechanisms of aliphatic polymers by thermal
oxidation and radio-oxidation is very important in order to assess their
lifetime in a variety of industrial applications. We focus here on polyethylene
as a prototypical aliphatic polymer. Kinetic models describing the time
evolution of the concentration of chain defects and radicals species in the
material identify a relevant step in the formation and subsequent decomposition
of transient hydroperoxides species, finally leading to carbonyl defects, in
particular ketones. In this paper we first summarize the most relevant
mechanistic paths proposed in the literature for hydroperoxide formation and
decomposition and, second, revisit them using first principles calculations
based on Density Functional Theory (DFT). We investigate the reaction paths for
several chemical reactions, for both isolated alkane molecules and a
crystalline model of polyethylene, and confirm, in some cases, the accepted
activation energies; in some other cases, we challenge the accepted view
finding alternative, more favourable, reaction paths for which we estimate the
activation energy. We highlight the infuence of the environment -- crystalline
or not -- on the outcome of some of the studied chemical reactions. A
remarkable results of our calculations is that hydroxyl radicals play an
important role in the decomposition of hydroperoxides. Based on our findings,
it should be possible to improve the set of equations and parameters used in
current kinetic simulations of polyethylene radio-oxidation.
