HDPE is less reactive than iPP, forming a waxy low molecular weight solid (Mn = 1200 g mol -1 , Đ = 2.3) that is freely soluble in hydrocarbon solvents at 25 o C in 21% yield. NMR analysis of the product indicates that this material contains ~90 branches/1000C, consistent with a hyperbranched structure that is more typically encountered in ethylene polymerization reactions using late transition metal catalysts. DFT calculations support the hypothesis that transiently formed SO3 is responsible for the C-H bond activation step in these reactions. Figure shows the calculated reaction energy diagram for the activation of an isotactic propylene tetramer by the pyrosulfate site on the SZO model. The pyrosulfate is maintained when the tetramer is adsorbed onto this model, which is used as a reference point for our reaction energy diagram. However, the barrier to opening the pyrosulfate to form adsorbed SO3 and sulfate is only 26.1 kcal mol -1 . The first intermediates formed after pyrosulfate opening are best described as alkane adducts of SO3. Figure shows the reaction profile for reactivity of a tertiary C-H bond, complementary results for a secondary C-H bond are presented in the supporting information. The stability of the alkane adduct depends on which C-H bond coordinates to sulfur, if a tertiary C-H coordinates the intermediate is slightly favored (DErxn = -0.5 kcal mol -1 ). If a secondary C-H bond coordinates to sulfur the intermediate is endothermic (DErxn = 4.8 kcal mol -1 ). Both intermediates have very similar structure. The dCH is 1.21 Å, significantly longer than a typical sp 3 C-H bond, dSH is 1.81 Å, well within the sum of the van der waal radii, and the S-H-C bond angle is 170 o . Abstraction of the C-H bond as a hydride in both cases has a negligible barrier. The tertiary carbocation formed is stable with respect to the tetramer adsorbate (DErxn = -8.3 kcal mol -1 ), while the secondary carbocation is less stable (DErxn = 13.4 kcal mol -1 ).