Free-radical formation in the presence of oxygen

A suitable system to consider initially is polypropylene, which has a hydrogen atom on the tertiary carbon atom that is available for abstraction to form a polymer-backbone radical. This is also of practical significance since controlled scission of polypropylene has been used to produce polymers with controlled molar-mass distribution and rheological properties (Brown, 1992). 

In thermal oxidation, initiation (1) results Irom the thermal dissociation of chemical bonds that may arise Irom intrinsically weak links formed as by-products of the polymerization reaction (e.g. head-to-head links) or impurities formed in the polymerization reactor such as hydroperoxides, POOH, or in-chain peroxides as occur in polystyrene from oxygen scavenging. Reaction (1 ) shows that POOH may produce peroxy and alkoxy radicals that may subsequently form alkyl radicals via reaction (3). 

Termination (leading to non-radical products) POO + POO POOOOP ( POOP + O2) (8) 

Hydrogen abstraction is known to occur from secondary carbon atoms in polyethylene (12) and may also occur in polypropylene, but with lower reaction rates. For polypropylene it was shown that intramolecular hydrogen abstraction in a six-ring favourable stereochemical arrangement will preferentially lead to the formation of sequences of hydroperoxides in close proximity (Scheirs et al, 1995b, Chien et al, 1968, Mayo, 1978). Infrared studies of polypropylene hydroperoxides showed that more than 90% of these groups were intramolecularly 

The cleavage of peracids can lead to polymer carboxy and hydroxyl radicals, reaction (18), with the carboxy radicals again able to abstract hydrogen to form carboxylic groups by reaction (19). 

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