Investigation of oxidative degradation in polymers using O-17 NMR spectroscopy

The thermal oxidation of pentacontane (C50H102), and of the homopolymer pol yisoprene, has been investigated using O-17 NMR spectroscopy. By performing the oxidation using O-17-labeled O-2 gas, it is possible to easily identif y nonvolatile degradation products, even at relatively low concentrations. It is demonstrated that details of the degradation mechanism can be obtaine d from analysis of the O-17 NMR spectra as a function of total oxidation. P entacontane reveals the widest variety of reaction products and exhibits ch anges in the relative product distributions with increasing O-2 consumption . At low levels of oxygen incorporation, peroxides are the major oxidation product, while at later stages of degradation these species are replaced by increasing concentrations of ketones, alcohols, carboxylic acids, and este rs. Analyzing the product distribution can help in identification of the di fferent free-radical decomposition pathways of hydroperoxides, including re combination, proton abstraction, and chain scission, as well as secondary r eactions. The O-17 NMR spectra of thermally oxidized polyisoprene reveal fe wer degradation functionalities but exhibit an increased complexity in the type of observed degradation species due to structural features such as uns aturation and methyl branching. Alcohols and ethers formed from hydrogen ab straction and free radical termination reactions are the dominant oxidation products. In polyisoprene, the formation of esters and carboxylic acids is relatively minor, distinctly different from the oxidation of pentacontane. An approximately linear increase in these degradation functionalities is o bserved with increasing oxidation levels. These results demonstrate the pro mise of O-17 NMR as a new technique for detailed investigation of oxidative polymer degradation.