Autoxidation

The common mechanism is a free radical chain reaction, where the addition of oxygen gives rise to hydroperoxides and their associated peroxy radicals (ROO•). Typically, an induction period is seen at the start where there is little activity; this is followed by a gradually accelerating take-up of oxygen, giving an autocatalytic reaction which can only be kept in check by the use of antioxidants. Unsaturated compounds are the most strongly affected but many organic materials will oxidise in this way given time.

Although autoxidation is usually undesirable, it has been exploited in chemical synthesis. In these cases the term 'autoxidation' is often used more broadly to include spontaneous reactions with oxygen at elevated temperatures, such as in the Cumene process.

Mechanism

The free radical chain reaction is sometimes referred to as the Bolland-Gee mechanism or the basic autoxidation scheme (BAS) and was originally based on the oxidation of rubbers, but remains generally accurate for many materials. It can be divided into three stages: initiation, propagation, and termination. The initiation step is often ill-defined and many agents have been proposed as radical initiators. The autoxidation of unsaturated compounds may be initiated by reactions with singlet oxygen or environmental pollutants such as ozone and NO₂. Saturated polymers, such as polyolefins would be expected to resist autoxidation, however in practise they contain hydroperoxides formed by thermal oxidation during their high temperature moulding and casting, which can act as initiators. In biological systems reactive oxygen species are important. For industrial reactions a radical initiator, such as benzoyl peroxide, will be intentionally added.

All of these processes lead to the generation of carbon centred radicals on the polymer chain (R•), typically by abstraction of H from labile C-H bonds. Once the carbon-centred radical has formed, it reacts rapidly with O₂ to give a peroxy radical (ROO•). This in turn abstracts an H atom from a weak C-H bond give a hydroperoxide (ROOH) and a fresh carbon-centred radical. The hydroperoxides can then undergo a number of possible homolytic reactions to generate more radicals, giving an accelerating reaction. As the concentration of radicals increases chain termination reactions become more important, these reduce the number of radicals by radical disproportionation or combination, leading to a sigmoid reaction plot.

Chain initiation

${\displaystyle \text{Polymer}⟶\text{P}\bullet +\text{P}\bullet }$

Chain propagation

${\displaystyle \text{P}\bullet +{\text{O}}_2^{}⟶\text{POO}\bullet }$

${\displaystyle \text{POO}\bullet +\text{PH}⟶\text{POOH}+\text{P}\bullet }$

Chain branching

${\displaystyle \text{POOH}⟶\text{PO}\bullet +\text{OH}\bullet }$

${\displaystyle \text{PH}+\text{OH}\bullet ⟶\text{P}\bullet +{\text{H}}_2^{}\text{O}}$

${\displaystyle \text{PO}\bullet ⟶\text{Chain}\text{scission}\text{reactions}}$

Termination

${\displaystyle \text{POO}\bullet +\text{POO}\bullet ⟶\text{cross}\text{linking}\text{reaction}\text{to}\text{non}-\text{radical}\text{product}}$

${\displaystyle \text{POO}\bullet +\text{P}\bullet ⟶\text{cross}\text{linking}\text{reaction}\text{to}\text{non}-\text{radical}\text{product}}$

${\displaystyle \text{P}\bullet +\text{P}\bullet ⟶\text{cross}\text{linking}\text{reaction}\text{to}\text{non}-\text{radical}\text{product}}$