Abstract : This work aimed at investigating the degradation pathways of polydimethylsiloxanes through two perspectives. The experimental approach studied bond scission and cross-linking degradation mechanisms when the polymer is exposed to thermo- or photo-oxidation conditions that are similar to that of the natural ageing. In the second part, the experimental results are modeled with a theoretical kinetic model, based on ab initio calculations.The in-vitro study of the polymer degradation showed that formation of carbon monoxide and carbon dioxide were by-products of Si-CH3 bond scissions. However, the degradation of PDMS for longer time periods showed clearly the formation of cyclic oligomers obtained through depolymerization of the macromolecule. In parallel, the photo-oxidation conditions yielded the formation of multidimensional cross-linked networks and formation of formic acid. This study showed that in the natural conditions, the degradation of PDMS proceeded through two opposite mechanisms, one that produced cross-linked networks while in the other, depolymerization was favored. In the second part, we investigated the mechanisms of the formation of cyclic oligomers that were observed experimentally during the degradation of the polymer. A detailed theoretical model was built in order to reproduce the dependence of the experimental cyclization constant with cycle size. This study showed the key role of the coupling in the treatment of hindered rotors. Our theoretical results reproduced accurately the oscillatory behavior of the cyclization constant for cycle sizes less than 20-mers, namely the global maximum for the 4-unit cyclic oligomers, and the local minimum for cycle sizes of 10 units. We have also shown that the origin of the oscillatory behavior of the cyclization constant revealed that some degrees of freedom underwent specific character transformation between a pure vibration for small cycles and hindered rotation for larger ring sizes.