The genesis of CO2 and CO in the thermooxidative degradation of polypropylene

Using a set of three isotactic polypropylene samples that had been individually labeled with carbon-13 at each of the three positions in the monomer unit, we conducted experiments to determine the position of origin of carbon monoxide and carbon dioxide that arise during thermal oxidation of this polymer. By GC-mass-spectral analysis, we find that 2/3 of the CO₂ derives from the C(1) [methylene] carbon and the remaining 1/3 comes from the C(2) [tertiary] carbon, with none coming from the C(3) [methyl group] carbon. The CO also comes mainly from the C(1) [methylene] carbon (≥80%). This is in contrast to the solid-phase oxidation products, which have been found (by C-13 NMR on these same labeled PP materials) to originate predominantly (80-85%) from oxidation at the C(2) [tertiary] carbon. These results can be understood in terms of the free-radical reactions that underlie the polypropylene oxidation chemistry.