Semiempirical calculation on the oxidative degradation of polypropylene

The key reaction steps during the oxidative degradation of (R,S) and (R,R)-2,4-diethylpentanes, which are model compounds of isotactic (iso-) and syndiotactic (syn-)PPs, were investigated using the semiempirical method, PM3. The following reaction paths were calculated: (I) formation of peroxyl radical, (II) hydrogen atom abstraction by peroxyl radical, (III) decomposition of hydroperoxide, and (IV) chain scission reaction. A significant difference in the activation energy between the (R,R)- and (R,S)-2,4-diethylpentanes was observed during the hydrogen atom abstraction step (II), while the other three steps showed little or no energy differences. The tertiary hydrogen atom abstraction and successive reactions were found to predominantly proceed compared with the reaction path including the secondary hydrogen atom abstraction. In the tertiary hydrogen atom abstraction step, the activation energy for the (R,R)-structure was found to be considerably higher, than that for the (R,S)-structure, suggesting that the hydrogen atom abstraction only slightly proceeds in syn-PP compared to iso-PP. The theoretical calculation of the 2,4-substituted pentanes was also conducted to discuss the electron effect, rotation of the main chain, and steric hindrance. (C) 1999 Elsevier Science Ltd. All rights reserved.