Mechanochemical degradation of ethylene-propylene copolymers: Characterization of olefin chain ends

We have-examined chain ends in ethylene-propylene (EP) copolymers following high-temperature shearing in an extruder. New chemical structures, not present prior to shearing, corresponding to vinyl, vinylidene, and vinylene chain ends were observed via H-1 and C-13 NMR and FTIR spectroscopy. A compilation of the various types of olefin chain ends possible via main chain scission of EP was made and their C-13 chemical shifts were calculated and compared with experimental results. This comparison, in combination with the results of DEFT experiments, allowed the assignment of the olefin region of the C-13 spectrum. The mechanism of polymer shearing appears to be hydrogen abstraction followed by disproportionation via beta-scission of the main chain radicals to form olefins. A strong preference was shown for abstraction of methine protons, presumably due to the stability of the tertiary radical intermediate, and main chain scission at the position beta to a methyl group.