Polymers come in a wide variety of compositions, and this diversity of chemistry results in wide-ranging embrittlement mechanisms. The most common sources of polymer embrittlement include oxygen in the air, water in liquid or vapor form, ultraviolet radiation from the sun, acids, and organic solvents.
One of the ways these sources alter the mechanical properties of polymers is through chain scission and chain cross-linking. Chain scission occurs when atomic bonds are broken in the main chain, so environments with elements such as solar radiation lead to this form of embrittlement. Chain scission reduces the length of the polymer chains in a material, resulting in a reduction of strength. Chain cross-linking has the opposite effect. An increase in the number of cross-links (due to an oxidative environment for example), results in stronger, less ductile material.
The thermal oxidation of polyethylene provides a quality example of chain scission embrittlement. The random chain scission induced a change from ductile to brittle behavior once the average molar mass of the chains dropped below a critical value. For the polyethylene system, embrittlement occurred when the weight average molar mass fell below 90 kg/mol. The reason for this change was hypothesized to be a reduction of entanglement and an increase in crystallinity. The ductility of polymers is typically a result of their amorphous structure, so an increase in crystallinity makes the polymer more brittle. In the case of polyethylene terephthalate, hydrolysis produces chain scission embrittlement. It has been demonstrated that the degradation of the mechanical properties correlates with the reduction of the mobile amorphous fraction (MAF), and that the ductile-to-brittle transition occurs when the minimum MAF is reached. This supports a micromechanical interpretation of the embrittlement mechanism rather than a molecular interpretation.
The embrittlement of silicone rubber is due to an increase in the amount of chain cross-linking. When silicone rubber is exposed to air at temperatures above  oxidative cross-linking reactions occur at methyl side groups along the main chain. These cross-links make the rubber significantly less ductile.
Solvent stress cracking is a significant polymer embrittlement mechanism. It occurs when liquids or gasses are absorbed into the polymer, ultimately swelling the system. The polymer swelling results in less shear flow and an increase in crazing susceptibility. Solvent stress cracking from organic solvents typically results in static fatigue because of the low mobility of fluids. Solvent stress cracking from gasses is more likely to result in greater crazing susceptibility.
Polycarbonate provides a good example of solvent stress cracking. Numerous solvents have been shown to embrittle polycarbonate (i.e. benzene, toluene, acetone) through a similar mechanism. The solvent diffuses into the bulk, swells the polymer, induces crystallization, and ultimately produces interfaces between ordered and disordered regions. These interfaces produce voids and stress fields that can be propagated throughout the material at stresses much lower than the typical tensile strength of the polymer.