Solid-State ^(13)C NMR Investigation of the Oxidative Degradation of Selectively Labeled Polypropylene by Thermal Aging and γ-Irradiation

Unstabilized polypropylene (PP) films having selective ^(13)C isotopic labeling were subjected to thermal aging at 50, 80, and 109 °C and to γ-irradiation at 24 and 80 °C. The oxidized films were examined using solid-state ^(13)C nuclear magnetic resonance (NMR) spectroscopy. Dramatic differences were found in the type and distribution of oxidation products originating from the three carbon atom sites within the PP macromolecule (tertiary carbon, secondary carbon, and methyl side group). Most of the oxidation products that formed on the polymer chain originated through chemical reactions at the PP tertiary carbons. Under all of the aging conditions examined, tertiary peroxides (from the PP tertiary site) were the most abundant functional group produced. Also originating from the PP tertiary carbon were significant amounts of tertiary alcohols, together with several more minor products that included “chain-end” methyl ketones. No significant amount of peroxides or alcohols associated with the PP secondary carbon sites was detected. A substantial yield of carboxylate groups was identified (acids, esters, etc.). The majority of these originated from the PP secondary carbon site, from which other minor products also formed, including in-chain ketones. We found no measurable yield of oxidation products originating from reaction at the PP methyl group. Remarkably similar distributions of the major oxidation products were obtained for thermal aging at different temperatures, whereas the product distributions obtained for irradiation at the different temperatures exhibited significant differences. Time-dependent concentration plots have been obtained, which show the amounts of the various oxidation products originating at the different PP sites, as a function of the extent of material oxidation

Insights into oxidation mechanisms in gamma-irradiated polypropylene, utilizing selective isotopic labeling with analysis by GC/MS, NMR and FTIR

In an effort to shed additional light on the chemical mechanisms underlying the radiation–oxidation of polypropylene (PP), we are using samples having selective ^(13)C isotopic labeling at the three unique sites within the macromolecular structure. After radiation exposure, we applied GC/MS, solid-state ^(13)C NMR, and FTIR to evaluate the applicability of each technique in identifying the molecular labeling of the oxidation products, with the goal of determining the site of origin of the products with respect to the macromolecule. Using GC/MS, we have identified the position of origin of CO_2 and CO from the polymer. Most of the CO_2 (60%) and CO (>90%) come from the C(1) (methylene) position of PP, with (30%) of the CO_2 originating from the C(3) (methyl) position, and 10% coming from the C(2) (tertiary) position. By GC/MS we have also identified the labeling patterns in four volatile oxidation products (acetone, methylisobutylketone, isobutane, and methyl acetate), and have used this information to map each compound onto the macromolecular framework. Using NMR we have quantified the time-dependent formation of solid-phase degradation products occurring from post-irradiation aging of PP samples held for 28 months at room temperature in air. Most of the solid oxidation products occur at the C(2) (tertiary) site; the predominant species, C(2) peroxides, increase linearly during the first 2 years, after which they plateau at a relatively high concentration