3 years ago

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

Mass Spectrometric Mechanistic Investigation of Ligand Modification in Hafnocene-Catalyzed Olefin Polymerization
Richard J. Keaton, Roger L. Kuhlman, Anthony P. Gies, Alceo Macchioni, Cristiano Zuccaccia
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], [C6H13–B(C6F5)3], and [B(C6F5)4].

Publisher URL: http://dx.doi.org/10.1021/acs.organomet.7b00293

DOI: 10.1021/acs.organomet.7b00293

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