Identification of Oxidation Products in Selectively Labeled Polypropylene with Solid-State ¹³C NMR Techniques

Oxidatively degraded polypropylene (PP) samples, with selective 13 C labeling of the three carbon sites on the PP chain (tertiary, secondary, and methyl carbons), have been analyzed with a suite of one- and two-dimensional solid-state 13 C nuclear magnetic resonance (NMR) experiments that have been used to assign several 13 C resonances attributed to oxidation-induced functional groups. These NMR techniques, several of which were recently developed, included dipolar dephasing for MAS speeds ≥10 kHz, chemical shift anisotropy (CSA) filtering, SUPER NMR to separate quasi-static CSA patterns, and 1 H− 13 C heteronuclear correlation (HETCOR). In the course of the study, it has been demonstrated that NMR experiments which utilize the 13 C CSA for resonance identification can be sensitive to sample temperature as a result of molecular motion-induced averaging of the CSA. These experiments have allowed hemiketal groups to be identified for the first time to our knowledge in oxidized PP. Possible mechanisms for the formation of hemiketals and other functional groups have been discussed.