Solution XAS Analysis for Exploring Active Species in Syndiospecific Styrene Polymerization and 1‑Hexene Polymerization Using Half-Titanocene–MAO Catalysts: Significant Changes in the Oxidation State in the Presence of Styrene

Mechanistic studies
through Ti K-edge XANES and EXAFS spectra of
the catalyst solution for 1-hexene polymerization using Cp*TiX2(O-2,6-iPr2C6H3) [X = Cl (1), Me (2)]–MAO
catalysts and for syndiospecific styrene polymerization using (tBuC5H4)­TiCl2(O-2,6-iPr2C6H3) (3)–MAO catalyst have been explored.
Significant changes in the XANES spectrum (low energy shift in the
edge peak, in addition to decrease in intensities of two pre-edge
peaks) were observed when styrene (200 equiv) was added into a toluene
solution containing 3 and MAO, strongly suggesting that
the complex 3 [Ti­(IV)] was reduced to Ti­(III) by addition
of styrene under these conditions. The EXAFS analysis suggests that
the Ti–O bond (corresponding to coordination of phenoxide)
was preserved along with dissociation of Ti–Cl bonds by treating
with MAO. These XAS analysis data thus suggest that neutral Ti­(III)
species, (tBuC5H4)­Ti­(R)­(OAr), play a role as the active species. The DFT-based computational
analysis on the syndiospecific styrene insertion reaction also revealed
that the neutral Ti­(III) catalyst exhibits the lower activation energy
than the others, strongly supporting the above mechanism. In contrast,
no significant changes (in the oxidation state, basic structure) in
the XANES spectra were observed when the toluene solution containing 1 (or 2) was added MAO and 1-hexene, whereas
preservation of the Ti–O bond along with dissociation of the
Ti–Cl by treating with MAO was suggested through the EXAFS
analysis.