Field Desorption Mass Spectrometry Studies of the Samarium-Catalyzed Polymerization of Ethylene under Hydrogen

The utility of field desorption mass spectrometry (FD-MS) in evaluating organolanthanide-based polymerizations of ethylene is described. The ethylene polymerization reactivity of (C 5 Me 5 ) 2 Sm, [(C 5 Me 5 ) 2 Sm(μ-H)] 2 , (C 5 Me 5 ) 2 Sm(THF) 2 , and related complexes was studied under hydrogen, and oligomers were analyzed which have molecular weights by FD-MS in the 300-1000 Da range. FD-MS was used with this system to study the effects of solvent, temperature, and catalyst precursor on molecular weight, termination steps, and end group identity. The FD-MS data showed that neither β-hydrogen elimination nor solvent metalation was competitive with hydrogenolysis as a termination step under these conditions. Reactions under D 2 led to oligomers which were identified by FD-MS and NMR spectroscopy to cleanly incorporate only two deuterium atoms per polymer chain (i.e., D-(CH 2 CH 2 ) n -D), and reactions conducted with C 2 D 4 under hydrogen formed H-(CD 2 CD 2 ) n -H. No deuterium incorporation from the solvent, toluene-d 8 , was observed. The similarity of the FD-MS data obtained from different precursors suggests that each of the above organosamarium precursors leads into the same catalytic cycle. Styrene is not polymerized by these samarium compounds, but FD-MS data on styrene and ethylene reactions showed that styrene could be incorporated into the polyethylene.