Oxidation of Chain-Growth Polymers
Polymers that lack double bonds, like polyethylene, can be considered high molecular weight paraffin. They are slow to oxidize in the absence of UV light, much like the low molecular weight hydrocarbons. On the other hand, polymeric materials with double bonds oxidize rapidly. Neverthe less, polymers like polyethylene may oxidize rapidly as well when contaminated with metallic ions because such ions catalyze the decomposition of peroxides. The chemical structure of the polyolefins determines their susceptibility to oxidative degradation. Linear polyethylene, in the absence of additives, is more resistant to oxidation that polypropylene that oxidizes rather readily due to the presence of labile tertiary hydrogens. It was demonstrated, for instance, that the molecular weigh of polypropylene sheets in a 138C oven can drop from 250,000 to approximately 10,000 in 3 h [522]. The process of oxidation was shown to take place according to the following scheme [522]:
Further oxidative degradation of fragments leads to formation of a carboxylic acid, an ester, and a g-lactone [522]. It was also found that the main oxidation products of polyethylene are an acid and a ketone. On the other hand, polypropylene yields upon oxidation approximately equal quantities of an acid, a ketone, an aldehyde, an ester, and a g-lactone [522]. In order for a polymer molecule to be attacked by oxygen, it must come in contact with it. This means that oxygen must be able to permeate into the material. Otherwise, all the oxidation will occur only at the surface. It was shown that oxidation occurs more readily in amorphous regions of the polymers where permeation of oxygen is not hindered by the chains being packed tightly together in the crystallites. That is only true, of course, at temperatures below Tm of the polymer. Among the chain-growth polymer, oxidation of polystyrene was investigated thoroughly. It was found that the rate of oxygen absorption and the number of chain scissions remain constant up to a high degree of reaction. There is no evidence of cross-linking under these circumstances [523]. During the degradation process, carbonyl groups accumulate in the polymer [524]. Among the degradation products were identified benzaldehyde [525] and a number of ketones [526]. The primary oxidation and chain scission process in polystyrene at room temperature is as follows [527]:
the reaction can also proceed in this manner:
Continuation of the process reduces the polymer to small fragments that include benzaldehyde and methyl phenyl ketone [527]. When solid polystyrene is subjected to an ozone attack, carbonyl, peroxide, and carboxylic acid groups form on the surface of the polymer [528]. The reaction rate is proportional to the concentration of the ozone and the surface of the sample. As a result of the ozone action, intramolecular cross linking takes place. The reaction mechanism of the ozone attack on polystyrene can be shown as follows [528]:
In poly (vinyl toluene), the initial oxidation steps consist of formations of radicals at the tertiary carbon atoms as they do in polystyrene. The radicals subsequently form peroxides that decompose into ketones and aldehydes.
