Unravelling the contribution of chain microstructure in the mechanism of the syndiotactic polypropylene pyrolysis

Abstract

The pyrolysis of a highly syndiotactic and high-molecular weight polypropylene (sPP) has been thoroughly studied through preparative TGA experiments, which were performed up to final temperatures between 262 °C and 427 °C. The 13C NMR characterization of the samples shows a two-stage process clearly evidenced by changes in the chain microstructure. In a first period, up to a 22% weight loss, the molecular weight collapses and only vinylidene end-capped chains are produced. In a second stage, over 22% weight loss, a wider variety of groups, including both terminal vinylen and isobutenyl functions as well as n-propyl tails, are also detected. Regardless the type, all these new species are formed at the expense of propylene units belonging to rrrr pentads.The evolution of the Eact predicted by means of analytical TGA allows us to associate the lowest Eact value found with the changes taking place during the very beginning of the process (up to a 3% weight loss), i.e. the molecular weight collapse and a some evidence of configurational stereo-specificity. As regards the further Eact build-up, it can be rationalized in terms of the ability of chains to be rolled in the most stable GGTT conformation, as it is supported by complementary FTIR and DSC characterization of the samples.