Free radicals, oxidation chemistry, polymer degradation

In polyethylene, the tertiary carbon atom, which dominated the chemistry of the oxidative degradation of PP, is present only at branch points. This suggests that there may be a difference among LDPE, LLDPE and HDPE in terms of the expected rates of oxidation. This is complicated further by the presence of catalyst residues from the Ziegler-Natta polymerization of HDPE that may be potential free-radical initiators. The polymers also have differences in degree of crystallinity, but these should not impinge on the melt properties at other than low temperatures at which residual structure may prevail in the melt. Also of significance is residual unsaturation such as in-chain tra s-vinylene and vinylidene as well as terminal vinyl, which are defects in the idealized PE strucmre. 

Hydrolysis is the principal degradation mechanism for the condensation polymers. From the point of view of chemistry, the equilibrium molecular weight of these polymers is determined by the H O concentration at given temperature, T. However, owing to the moisture absorption from the air, the reaction equilibrium is shifted toward depolymerization. The rate of hydrolytic depolymerization depends on the moisture content, T and the presence of catalyst. Since these polymers are also subject to free-radical and oxidative processes (that lead to formation of unsaturations, hence the... 

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