Catalytic post-modification of alkene polymers. Chemistry and kinetics of dehydrogenation of alkene polymers and oligomers with pincer Ir complexes

The paper presents details of catalytic dehydrogenation reactions of two high molecular weight, highly branched hydrocarbons, a polymer of 1-hexene and an oligomer of 1-butene, and describes kinetic analysis of these reactions. The dehydrogenation reactions were catalyzed with two pincer Ir complexes, [4-methoxy-2,6-C₆H₃(CH₂P-tert-Bu₂)₂]IrH₂ and [4-methoxy-2,6-C₆H₂(CH₂P-iso-Pr₂)₂]IrH₂, and were carried out at 150 °C in p-xylene solutions with norbornene as a hydrogen acceptor. The structure of all dehydrogenation reaction products and the Ir species was determined by ¹H and ³¹P NMR. Mechanistically, these reactions are similar to dehydrogenation reactions of low molecular weight alkanes. Kinetic analysis of the reactions yielded the values of effective rate constants for all major reaction steps in the catalytic cycle. The results show that catalytic dehydrogenation of branched polymers in the presence of an alkene hydrogen acceptor is feasible at increased temperatures and represents a viable route to post-synthetic modification of branched polyolefins.