Carbonyl group in chain formation and cleavage

Carbonyl Groups in Chain Formation and Cleavage Starting with COj... 

In conclusion, the hght-induced oxidation of polymers results in the formation of hydroperoxide, peroxide, and carbonyl groups. In the case of commercial polymers consisting of linear macromolecules, oxidation leads to main-chain cleavage and causes the deterioration of certain important mechanical properties. Appropriate stabilization measures to avoid these problems are discussed in the following subsection. 

Several of the more common commodity polymers like the polyolefins are susceptible to photo-oxidation. For a polymer like polyethylene, photo-oxidation leads to increasing amounts of carbonyl compounds. In-chain ketone groups act as sensitisers by UV light absorption. Through the well-known Norrish type I and II degradations radicals, end-vinyl and ketone groups are formed. Other products often observed in photo-oxidised low-density polyethylene (LDPE) are esters [5]. Scheme 1 shows one mechanism for abiotic ester formation. By Norrish type I cleavage the radical formed can react with an alkoxyl radical on the polyethylene (PE) chain. 

Ozone cracking is a physicochemical phenomenon. Ozone attack on olefinic double bonds causes chain scission and the formation of decomposition products. The first step in the reaction is the formation of a relatively unstable primary ozonide, which cleaves to an aldehyde or ketone and a carbonyl. Subsequent recombination of the aldehyde and the carbonyl groups produces a second ozonide [58]. Cross-linking products may also be formed, especially with rubbers containing disubstituted carbon-carbon double bonds (e.g. butyl rubber, styrene-butadiene rubber), due to the attack of the carbonyl groups (produced by cleavage of primary ozonides) on the rubber carbon-carbon double bonds. 

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