On the First Insertion of α-Olefins in Hafnium Pyridyl-Amido Polymerization Catalysts

Reactions of [{N−,N,Cnaphthyl −}HfMe][MeB(C6F5)3] (2) precatalyst with a series of α-olefins have been investigated in order to intercept the active polymerization species generated by an in situ modification of the precursor by insertion of a single monomer unit into the Hf−CAryl bond. In all cases the first migratory insertion of monomer occurs into the Hf−CAryl bond rather than the Hf−CAlkyl bond. A low-temperature polymerization with 170 equiv of 1-hexene activated with tris(pentafluorophenyl)borane (FAB) allows for the complete NMR characterization of a Hf−CAlkylaryl methyl cation. This structure agrees with DFT studies of the kinetically favored diastereomer, although a number of other structures are more stable thermodynamically. Attempts to trap an inserted catalyst through the stoichiometric addition of 2-vinylpyridine or 3-ethoxy-1-propylene led to complicated reactions, but in both cases, experimental and computational data suggest that both processes initiate through insertion of the substrate into the Hf−CAryl bond. In addition, an activated Hf-dibutyl complex is studied in an attempt to minimize differences in the rates of initiation and propagation, as a butyl group is a reasonable mimic for a propagating polymer chain. Tritylborate activation of this complex cleanly generates 1 equiv of 1-butene in close proximity to the Hf-butyl cation via β-hydride abstraction. This reaction results in formation of isotactic poly(1-butene) with a fairly high molecular weight rather than butene oligomers. The observed molecular weight is consistent with a small fraction of active species, and quenching studies show a similar fraction of butene-modified ligand. These results are consistent with slow insertion into the Hf−CAryl bond followed by fast polymerization kinetics, with the latter rate constant being 3 orders of magnitude faster than the former.