Cyclopentadienyl Titanium Imido Compounds and Their Ethylene Polymerization Capability: Control of Molecular Weight Distributions by Imido N-Substituents

Reaction of the previously reported Cp*Ti(NtBu)Cl(py) (1) with bulky ortho-substituted anilines ArNH2
afforded the corresponding aryl imido derivatives Cp*Ti(NAr)Cl(py) (Ar = 2,6-C6H3iPr2 (2), 2,6-C6H3Br2 (3), 2-C6H4tBu (4), and 2-C6H4iPr (5)). Reaction of 2 with B(C6F5)3 in C6D6 or heating in vacuo at
200 °C afforded the imido-bridged dimer Cp*2Ti2(μ-N-2,6-C6H3iPr2)2Cl2 (8). Activation of 1−5 with
MAO gave moderately active catalyst systems for the polymerization of ethylene in contrast to the
previously reported, highly active titanium imido systems Ti(Me3[9]aneN3)(NR)Cl2/MAO and Ti{HC(Me2pz)3}(NR)Cl2/MAO, which are isolobal and isoelectronic with 1−5. The Cp*-supported precatalyst
productivities were sensitive to both the imido N-substituents and initial precatalyst/cocatalyst concentrations. Depending upon the imido N-substituents and initial precatalyst/cocatalyst concentrations,
polyethylene with unimodal (either rather low or very high molecular weight) or bimodal molecular
weight distributions can be obtained. Excess AlMe3 suppresses catalyst productivity but does not affect
the overall molecular weight distribution in the system evaluated (1/MAO). Both chain transfer to aluminum
and β-hydrogen transfer appear to be active pathways for formation of the low molecular weight fractions
of the polymers formed with 1−5/MAO. Under otherwise identical polymerization conditions the catalyst
systems 2/MAO and 8/MAO had comparable productivities and gave polyethylene with very similar
molecular weight and molecular weight distributions, suggesting a potential role for binuclear species in
the catalyst systems 1−5/MAO.