The literature dealing with degradation-induced embrittlement mechanisms in semi-crystalline polymers having their amorphous phase in rubbery state is reviewed. It is first demonstrated that the decrease of molar mass resulting from a quasi-homogeneous chain scission process is responsible for embrittlement. The main specificity of the polymer family under study is that embrittlement occurs at a very low conversion of the degradation process, while the entanglement network in the amorphous phase is slightly damaged. In these polymers, chain scission induces chemicrystallization. The analyses of available data on this process show that it is characterized by a relatively high yield: about one half entanglement strands integrate the crystalline phase after one chain scission. A simple relationship expressing the chemicrystallization yield for a given polymer structure is proposed. Chain scission and chemicrystallization can lead to embrittlement through two possible causal chains: (1) chain scission -> molar mass decrease -> chemicrystallization -> decrease of the interlamellar spacing -> embrittlement. (2) Chain scission -> molar mass decrease -> chemicrystallization -> decrease of the tie-macromolecule concentration -> embrittlement. At this state of our knowledge, both causal chains are almost undistinguishable.