Mass Spectrometric Mechanistic Investigation of Ligand Modification in Hafnocene-Catalyzed Olefin Polymerization

We recently reported evidence that a cyclometalated intermediate can facilitate the polymerization of 1-hexene to append polymer chains to the termini of propyl groups of the Me2Hf(Cpn‑Propyl)2 catalyst precursor. Herein we provide further mechanistic details on the activation of Me2Hf(Cpn‑Propyl)2 by B(C6F5)3 and the polymerization of 1-hexene mainly by applying a battery of mass spectrometry-based techniques. First, a combination of MALDI and CID fragmentation is used to characterize the high molecular mass region (up to 6 kDa) of the isolated poly(1-hexene) material with attached metallocene. The CID fragmentation patterns are explained by relatively low-energy ligand losses and higher energy hydrocarbon chain degradation via C−C bond cleavage and 1,3-hydrogen shift reactions. Further mechanistic insights are gained by investigating 1-hexene polymerization reaction employing a properly 13C-labeled catalyst activated by B(C6F5)3. Mass spectrometry analyses, along with supporting NMR experiments, indicate that polymer chain growth from the propylcyclopentadienyl ligand proceeds via a series of 2,1-insertion ring expansions of the hafnium metallacycle. In contrast, free poly(1-hexene) chains are generated by conventional 1,2-insertions. In addition, six boron-containing species were identified from negative ion mode ESI-QqTOF: [B(C6F5)3]−•, [H −B(C6F5)3]−, [CH3−B(C6F5)3]−, [HO−B(C6F5)3]−, [C 6H13− B(C6F5)3]−, and [B(C6F5)4]−.