Polymers from Auto-oxidation Products

Both nylon 6 and nylon 6,6 have similar structures with peptide bonds, i.e., -CONH- linkages. However, nylon 6, unlike nylon 6,6, is not a condensation but an addition polymer. The methods of syntheses and the structures of these two polymers are different These differences are shown by reactions 8.4.1 and 8.4.2. 

In nylon 6,6 the peptide bonds are formed from the condensation of adipic acid with hexamethylene diamine. The latter could be made from adiponitrile (see Section 5.6) by catalytic hydrogenation. 

For nylon 6, caprolactam is made from the oxime by acid-catalyzed Beckman rearrangement. As shown by reaction 8.4.2, heating caprolactam under nitrogen breaks the carbon-nitrogen bond and gives the polymer. 

PET is the most widely used polyester. As shown by the first step of reaction 8.4.3, terephthalic acid reacts with ethylene glycol to give bis(hydroxyethyl) terephthalate, 8.17. In the second step, 8.17 is converted to PET by a Sb203/Ti(0R) -catalyzed transesterification or self-condensation reaction. 

The polymer polylactide (PEA) is another polyester. Although not made from any auto-oxidation product, we discuss it here because in recent years it has come under much attention. As mentioned in Section 1.6.3, lactic acid is a fermentation product, and, unlike PET, PEA is biodegradable. 

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Ecostar (St. Lawrence Starch Company). This product associates PE with a mixture of starch and auto-oxidant unsaturated fatty acids. The global content of starch is between 6 and 15%. The degradation process then follows two mechanisms in the first, the starch is fragmented, then assimilated by microorganisms, whereas in the second, the interaction between the auto-oxidants and the metallic complexes from soil or water gives peroxides that attack the synthetic polymer chains. 

The first two steps in the synthesis of melanin are catalyzed by tyrosinase, a copper-containing oxidase, which converts tyrosine to dopaquinone. All subsequent reactions presumably occur through nonenzymatic auto-oxidation, in the presence of zinc, with formation of the black to brown pigment eumelanin. The yellow to reddish brown, high-molecular-weight polymer known as pheomelanin and the low-molecular-weight trichromes result from addition of cysteine to dopaquinone and further modification of the products. Pheome-lanins and trichromes are primarily present in hair and feathers. 

F. Gugumus [21] provides an alternative view of the thermal oxidation reactions in polymers. Various possibilities arising from inter- and intramolecular reactions between hydroperoxide groups, peroxy radicals, and alkoxy radicals are postulated. The author underlines the plausible over-estimation of degradation attributed to -scissions in polypropylene (PP) and offers alternative (non (3-scission) routes that result in formation of 1,2-dioxetane which can account for auto-oxidation, chain scissions and enhanced chemiluminescence of PP oxidation products. An illustration of this proposed scheme is provided in Scheme 6.4. 

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