Switching from ethylene polymerization to nonselective oligomerization over a homogeneous model catalyst: A triphenylsiloxy complex of chromium(VI)

SiO ₂-supported silyl chromate catalyst (UCC S-2 catalyst) is an important heterogeneous catalyst for the commercial production of polyethylene (PE). However, it has not been fully investigated during the past several decades. A large obstacle has been the lack of homogeneous model systems for this chromium-based heterogeneous catalyst. In this work, bis(triphenylsilyl) chromate (BC), as a homogeneous model for UCC S-2 catalyst, was studied for its polymerization behavior with aluminum alkyl cocatalysts, namely, triisobutylaluminum (TIBA) and methylaluminoxane (MAO). Moreover, activation of BC was investigated by NMR and ESR spectroscopy, radical trapping on fullerene, and MALDI-TOF mass spectroscopy. In the case of TIBA, only an activity of ethylene polymerization were observed; In the case of MAO, BC exhibits an interesting transformation from ethylene polymerization (Al/Cr ≤ 200) to nonselective oligomerization (Al/Cr ≥ 500) with an increase of Al/Cr molar ratio. The obtained PE display a switching from a unimodal molecular weight distribution (MWD) (Al/Cr ≤ 100) to a bimodal MWD (Al/Cr ≥ 200), and the obtained oligomers are main even-numbered linear α-olefins (C ₆-C ₁₆). The butyl radical and [Cr(η ⁶-arene) ₂] ⁺ cation was identified during activation of BC with TIBA, and the [Cr(η ⁶-arene) ₂] ⁺ cation was proved to be inactive for ethylene polymerization by temperature-dependent ESR experiments.