Mechanistic studies on the switching from ethylene polymerization to nonselective oligomerization over the triphenylsiloxy chromium(II)/methylaluminoxane catalyst

Our previous experimental report showed a switching behavior from ethylene polymerization to nonselective oligomerization by a novel triphenylsiloxy complex of chromium(II) [(Ph₃SiO)Cr·(THF)]₂(μ-OSiPh₃)₂ (1) together with methylaluminoxane (MAO) as a cocatalyst. In this work, combined experimental and computational studies were carried out to shed some light on the nature of the active species and their fascinating switching mechanism. The experimental results and DFT calculations suggested that (i) the chain propagation and chain transfer processes proceed via a Cossee-Arlman mechanism and β-hydrogen transfer to the chromium center, respectively; (ii) the trivalent cationic model [(Ph₃SiO)Cr^IIIMe]⁺ and [(η⁶-toluene)Cr^IIIMe₂]⁺, which could be generated by a disproportionation reaction, are the most plausible active species for ethylene polymerization, and the divalent cationic model [(η⁶-toluene)Cr^IIMe]⁺ might be responsible for ethylene nonselective oligomerization. A switching mechanism from ethylene polymerization to nonselective oligomerization in the 1/MAO catalyst system was proposed on the basis of DFT calculations. These results may have useful implications for studying active species and the mechanism of transition-metal-catalyzed olefin polymerization and oligomerization.