The mechanism of autoxidation of organic materials by elemental oxygen is a free radical chain reaction initiated by hydroperoxide decomposition to form alkoxy and peroxy radicals. Chain scission and crosslinking reactions occur, along with the formation of hydroperoxide, resulting in degradation of properties of polymeric materials. The presence of unsaturation in natural and synthetic rubbers makes them particularly vulnerable to autoxidation, and antioxidants are thus essential to provide protection against oxidative degradation. There are two ways in which antioxidants can function to retard autoxidation. Preventive antioxidants act in some way to reduce the rate of initiation, while chain-breaking antioxidants intercept the chain-propagating peroxy free radicals and thus terminate the chain mechanism. Both types include a variety of compounds and possible modes of action as indicated in the following classification: