The steps involved in loss of diastereoselectivity and incorporation of regioirregular "errors" are also shown in Figure . Epimerization can occur by reversible b-H elimination and unselective olefin insertion through a 3 o Hf-R intermediate, or HfH(olefin) + could also undergo non-dissociative alkene flipping 23 followed by unselective olefin reinsertion. Deuterium incorporation into the isolated alkanes is consistent with either process. These steps also account for the incorporation of deuterium into tertiary positions of residual iPP, though stereoselective reinsertion is likely because high mmmm purity of unreacted polymer is observed in solution NMR. Until this report the most common catalysts for hydrogenolysis of polyolefins were supported noble metal nanoparticles 2 or the supported d 0 metal hydrides shown in Figure . Comparisons between these disparate classes of catalysts are difficult, but 1 does appear to offer some advantage. 1 selectively produces long chain hydrocarbons and avoids significant formation of light gases at prolonged reaction times. We suspect that the selectivity rests on the moderate activity of 1 in the reactions shown in Figure . For example, under essentially identical reaction conditions Ta-H + sites supported on sulfated aluminum oxide convert the same iPP to shorter liquid hydrocarbon fragments and more light gas, indicating that Ta-H + facilitates more chain cleavage events than 1. 3g Typical Ziegler-Natta catalysts that polymerize olefins have been overlooked as catalysts for degradation of fully saturated polyolefin plastics. The reactivity of 1, and inactivity of 2, in iPP hydrogenolysis shows the critical role of forming an organometallic ion-pair in this reaction. Many industrially relevant catalysts for olefin polymerization contain mixtures of metallocene, aluminum alkyl (or methaluminoxane), and an oxide support; and these mixtures likely self-assemble to form ion-pairs similar to the Hf-H + derived from 1. A plausible implication of the results shown here is that the broad portfolio of typical Ziegler-Natta catalysts available for olefin polymerization reactions may also catalyze reactions that degrade the polymers these catalysts produce.